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Apollo Discussions => The Hoax Theory => Topic started by: Edwardwb1001 on October 08, 2012, 07:29:49 PM

Title: Why is there no dust on the Lunar Lander's footpads?
Post by: Edwardwb1001 on October 08, 2012, 07:29:49 PM
A straightforward question but one to which I have not yet received a straightforward answer.  Photos of Apollo 11's landing pads whilst resting on the moon show them to completely dust free, the gold foil covering them shimmering and gleaming - with not a speck of lunar dust or soil present.

Even if the LM's exhaust did not produce a strong enough blast to form a crater, and even if the landing site was quite hard and rocky, there surely would have been a certain amount of dust/soil which would have billowed up to float down and settle on the abovementioned landing pads?

Other landers show decidedly dusty footpads whilst resting on the surface of other worlds. The 2008 Phoenix Mars lander of 2008 shows a completely dust-covered footpad in photos taken.  Why would there be this discrepancy?
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Grashtel on October 08, 2012, 07:43:57 PM
A straightforward question but one to which I have not yet received a straightforward answer.  Photos of Apollo 11's landing pads whilst resting on the moon show them to completely dust free, the gold foil covering them shimmering and gleaming - with not a speck of lunar dust or soil present.

Even if the LM's exhaust did not produce a strong enough blast to form a crater, and even if the landing site was quite hard and rocky, there surely would have been a certain amount of dust/soil which would have billowed up to float down and settle on the abovementioned landing pads?

Other landers show decidedly dusty footpads whilst resting on the surface of other worlds. The 2008 Phoenix Mars lander of 2008 shows a completely dust-covered footpad in photos taken.  Why would there be this discrepancy?
The dust would have billowed up and floated down in what?  Remember that the Moon, unlike Mars, doesn't have an atmosphere so there is nothing to cause the dust to billow or for it to float down in, instead it flies away on a ballistic trajectory going a long way before landing.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Nowhere Man on October 08, 2012, 07:44:59 PM
Even if the LM's exhaust did not produce a strong enough blast to form a crater, and even if the landing site was quite hard and rocky, there surely would have been a certain amount of dust/soil which would have billowed up to float down and settle on the above mentioned landing pads?

Other landers show decidedly dusty footpads whilst resting on the surface of other worlds. The 2008 Phoenix Mars lander of 2008 shows a completely dust-covered footpad in photos taken.  Why would there be this discrepancy?

Guiding question:  What is it that allows dust to "billow up" and "float down?"  What is one difference between the Moon and Mars?

Curse you Grashtel for beating me to the punch.

Fred
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: cjameshuff on October 08, 2012, 08:59:39 PM
http://www.lpi.usra.edu/resources/apollo/frame/?AS11-40-5926
http://www.lpi.usra.edu/resources/apollo/frame/?AS11-40-5925

Both images clearly show dust in the little wrinkles of aluminized mylar. Not a lot, but given the airless environment, that's normal. The atmosphere of Mars is quite thin, but it's still enough for dust storms. The moon has nothing of the sort to keep dust suspended. This might even be stuff kicked up by the contact probes rather than the rocket exhaust.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: JayUtah on October 08, 2012, 10:15:24 PM
Even if the LM's exhaust did not produce a strong enough blast to form a crater, and even if the landing site was quite hard and rocky, there surely would have...

Your expectation; your burden of proof.

The exhaust gas velocity was about 2,000 meters per second.  The "dust" was entrained in a gale approximately 200 times faster than the most ferocious hurricane ever seen on Earth.  Throw a handful of gravel at a pie plate and see how much stays.

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...would have billowed up to float down...

Nope.

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Why would there be this discrepancy?

Try to work out which one has an atmosphere to support aerosolization and which does not.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Glom on October 09, 2012, 01:56:41 AM
As mentioned, your talk of dust billowing and floating despite the lack of an atmosphere calls into question how informed your expectations are.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Edwardwb1001 on October 09, 2012, 06:36:27 AM
If dust 'flies away on a ballistic trajectory', and 'go(es) a long way before landing', then how would you explain video footage of dust flying up behind the lunar rover's wheels, and not flying away on a ballistic trajectory or travelling a far distance before landing. It falls virtually straight down, albeit in slow motion.

And Glom, Grashtel speaks of dust 'landing'. Does this not indicate that it did indeed 'float down', even at a distant point? Hence dust does float down - even on the moon. Actually, Glom, you imply from your comment that 'floating', cannot take place on the moon, due to the lack of atmosphere. I said 'float down'. I think you are confusing gravity with atmosphere.  How informed are you? Not very much, it seems. If objects do not float down on the moon (although Grashtel states that they eventually do), why did the astronauts not fly away after each bouncing step on the moon?
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Trebor on October 09, 2012, 06:49:42 AM
And Glom, Grashtel speaks of dust 'landing'. Does this not indicate that it did indeed 'float down', even at a distant point?

No, it indicates it hits the ground.
Please stop with the word games.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: DataCable on October 09, 2012, 06:52:20 AM
then how would you explain video footage of dust flying up behind the lunar rover's wheels
Because it was propelled up by the rover's wheels.

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and not flying away on a ballistic trajectory
Do you know what "ballistic trajectory" means?
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Valis on October 09, 2012, 06:54:55 AM
If dust 'flies away on a ballistic trajectory', and 'go(es) a long way before landing', then how would you explain video footage of dust flying up behind the lunar rover's wheels, and not flying away on a ballistic trajectory or travelling a far distance before landing. It falls virtually straight down, albeit in slow motion.
It all depends on the point where a grain of "dust" (regolith) leaves the wheel. This point defines the horizontal and vertical velocities for the particle. Some particles have mostly vertical velocity, so they'll land near the place they were picked up; others leave the wheel earlier, and travel further. I think the main point is that there is no dust cloud left behind, thus there can't be an atmosphere.
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And Glom, Grashtel speaks of dust 'landing'. Does this not indicate that it did indeed 'float down', even at a distant point? Hence dust does float down - even on the moon.
No. "Floating" implicates a buoyancy of some sort, and you need a medium (air, water, etc.) for that. "Landing" means just touching down on the ground. And would you say that a rock floats down here on Earth when dropped? The little buoyancy the atmosphere gives to the rock is pretty much negligible here. However, dust does float on Earth, and float down eventually.
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Actually, Glom, you imply from your comment that 'floating', cannot take place on the moon, due to the lack of atmosphere. I said 'float down'. I think you are confusing gravity with atmosphere.
No, it's you who's mixing things up. The dust doesn't float down on Moon, it falls down. 
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If objects do not float down on the moon (although Grashtel states that they eventually do), why did the astronauts not fly away after each bouncing step on the moon?
Once again, objects don't float (down or in any other direction) without an atmosphere. The motion is strictly defined by the initial velocity vector and gravity.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Jason Thompson on October 09, 2012, 07:15:25 AM
If dust 'flies away on a ballistic trajectory', and 'go(es) a long way before landing', then how would you explain video footage of dust flying up behind the lunar rover's wheels, and not flying away on a ballistic trajectory or travelling a far distance before landing.

Do you know what a ballistic trajectory is? Or why you can't look at a cloud of fine particles and deduce the behaviour of the individual particles from watching the behaviour of the cloud?

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And Glom, Grashtel speaks of dust 'landing'. Does this not indicate that it did indeed 'float down', even at a distant point? Hence dust does float down - even on the moon.

No, it does not float. Floating requires something for it to float in. The dust falls.

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I said 'float down'. I think you are confusing gravity with atmosphere.  How informed are you? Not very much, it seems.

More so than you if you can't tell the difference between floating and falling.

Can we assume you accept the explanation about the dust being blown away by the rather large and powerful rocket engine next to the footpads, since you declined to even mention it in your response?
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: smartcooky on October 09, 2012, 07:46:45 AM
If dust 'flies away on a ballistic trajectory', and 'go(es) a long way before landing', then how would you explain video footage of dust flying up behind the lunar rover's wheels, and not flying away on a ballistic trajectory or travelling a far distance before landing. It falls virtually straight down, albeit in slow motion.

That is exactly what I would expect in a vacuum UNLESS that dust is thrown upwards (in which case, the ballistic trajectory comes into play)

But that isn't what is happening. The lunar dust is simply being carried upwards on the Lunar Rover's wheels and falling off.

And its NOT falling off in slow-motion (which implies manipulation of the video playback speed, which is the angle you are trying to approach it from). The dust falls at 1.63 m/s. It doesn't matter in a vacuum whether its a dust particle weighing a microgram or a rock weighing a kilogram; BOTH will fall at the same rate because there is no air-resistance.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Echnaton on October 09, 2012, 10:03:07 AM
If dust 'flies away on a ballistic trajectory', and 'go(es) a long way before landing', then how would you explain video footage of dust flying up behind the lunar rover's wheels, and not flying away on a ballistic trajectory or travelling a far distance before landing. It falls virtually straight down, albeit in slow motion.

One of the observed features of motion of the dirt (regolith) ejected by the LR tires is driving is that the dirt falls forward from where it was picked up.  This may appear to be odd or unexpected to some observers but is a perfectly normal motion.  It can be seen in certain clips where the LR brakes quickly and the dirt falls into the back of the wheel.  IIRC, it is most visible in the lunar grand prix but, sorry, I don't have any reference clips.

Some people may have a intuitive notion that a turning wheel should eject the material in the opposite direction of travel of the vehicle.  Seeing cars driving in snow (maintaining traction and not spinning the wheels) with rooster tails coming off the back tires it may look this way, but it is not the case.  The snow travels in the direction of travel of the car, but at a slower speed making it fall behind the tire that kicked it up, but ahead of where it was ejected.  By fixing the observational reference on the ground rather when observing the movement of the snow it becomes apparent that the snow is moving forward in a nearly parabolic arc.  A little thought to imagine the rotation of the tire yields the conclusion that a rolling tire that has traction can only move forward or be still with reference to the ground.   The surface of an ideal circular tire would be still to the ground at the line of contact and move at twice the speed of the vehicle at the line opposite of contact.  Actual tires are not geometrically circular, and make contact over a significant area, but the concept remains.

So on the LR, we should expect the material ejected while driving with traction to move forward of the point which it was ejected.  While this may seem counter intuitive to some, it is an expected result that can easily be observed on earth.  There are certain times that the LR does spin its wheels and send a nice rooster tail out the back but that is also no different than spinning your wheels in a car.  If you don't have any snow handy to try this, you can also see the same effect while four wheeling on a beach.  We do more of that in Houston.   
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: sts60 on October 09, 2012, 10:29:35 AM
Hi, Edwardwb1001.  Welcome to the board.

If dust 'flies away on a ballistic trajectory', and 'go(es) a long way before landing', then how would you explain video footage of dust flying up behind the lunar rover's wheels, and not flying away on a ballistic trajectory or travelling a far distance before landing. It falls virtually straight down, albeit in slow motion.
(Bolding mine.)

You might also want to consider the difference in speeds of the two means of propelling lunar regolith.  As Jay pointed out above, the stuff getting blown out from under the LM at landing is getting pushed by a 2,000 m/s exhaust gas.  I just did a rough estimate in my head for tangential speed of the LM tires and got, order of magnitude, about 5 m/s.  So even a stationary LRV spinning its wheels can't throw surface material nearly as hard as the LM's exhaust.

The exhaust gas velocity was about 2,000 meters per second.  The "dust" was entrained in a gale approximately 200 times faster than the most ferocious hurricane ever seen on Earth...
Jay, I think you mean "approximately 20 times faster".  Hurricane Camille was clocked up to around 190 mph, or about 85 m/s, so if we round up to account for some prehistoric monster storm that's roughly 100 m/s.

Anyway, [ETA: forgot to finish this], the difference in speeds imparted to the lunar material is striking - over two orders of magnitude. 
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: JayUtah on October 09, 2012, 10:47:39 AM
Jay, I think you mean "approximately 20 times faster".

Yes.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Tedward on October 09, 2012, 10:50:40 AM
I seem to remember an article on the Mars rovers, Spirit and Opportunity, with regards the solar cells. This article went to say something along the lines that power was reduced due to Martian dust but then got better when the wind blew it off. The dust cometh and the dust goeth (them proper words?)
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: JayUtah on October 09, 2012, 10:55:23 AM
If dust 'flies away on a ballistic trajectory'

It does not.  It is entrained in the exhaust fluid flow, which, after surface impingement, is roughly coincident with and parallel to the lunar surface.  It does not "billow up" and it is not likely to be trapped in great amounts in an upward-facing cup (i.e., the footpad).  It will certainly not stick to Kapton-covered vertical struts.

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...and 'go(es) a long way before landing', then how would you explain video footage of dust flying up behind the lunar rover's wheels

It's a completely different physical phenomenon.

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It falls virtually straight down, albeit in slow motion.

But it falls; it is not aerosolized as it would be on Mars.   Therein lies the answer to your first question above.

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I think you are confusing gravity with atmosphere.

No, you are.  You are using words that refer to aerosolization and asking why the lunar environment does not behave the same way as Mars in that respect.

Yes, gravity causes particles to fall to the surface in a vacuum at the same rate, plus initial conditions.  However in the case of fluid entrainment, the disturbed (and therefore entrained) particles largely left the vicinity of the lunar module horizontally at very high velocities.

The gold-colored covering on components of the lunar module is very similar to the material small potato chip bags are made out of.  Imagine high-velocity particles hitting it.  How many of them do you think are going to stick?  Do some experiments with a chip wrapper and a handful of dry sand and tell me what happens.

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How informed are you? Not very much, it seems.

It's a little early in your tenure to accuse the regulars of being stupid.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Rob260259 on October 09, 2012, 12:24:51 PM
http://www.youtube.com/watch?v=7o3Oi9JWsyM

Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Glom on October 09, 2012, 01:31:12 PM
If dust 'flies away on a ballistic trajectory', and 'go(es) a long way before landing', then how would you explain video footage of dust flying up behind the lunar rover's wheels, and not flying away on a ballistic trajectory or travelling a far distance before landing. It falls virtually straight down, albeit in slow motion.

Erm, it is.  What do you think ballistic dust plumes should look like?

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And Glom, Grashtel speaks of dust 'landing'. Does this not indicate that it did indeed 'float down', even at a distant point? Hence dust does float down - even on the moon. Actually, Glom, you imply from your comment that 'floating', cannot take place on the moon, due to the lack of atmosphere. I said 'float down'. I think you are confusing gravity with atmosphere.  How informed are you? Not very much, it seems. If objects do not float down on the moon (although Grashtel states that they eventually do), why did the astronauts not fly away after each bouncing step on the moon?

There is no floating on the Moon, only falling.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: gillianren on October 09, 2012, 02:01:23 PM
http://www.youtube.com/watch?v=7o3Oi9JWsyM



No.  No, a single YouTube clip is not an argument.  I don't know what it shows, because I'm not clicking on YouTube links blindly.  (Also, I'm already watching something, and I'm not pausing Oedipus Rex for this!)  The proper etiquette is to give a description of what's on the link and what you are intending to show by it.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Rob260259 on October 09, 2012, 03:11:22 PM
http://www.youtube.com/watch?v=7o3Oi9JWsyM



No.  No, a single YouTube clip is not an argument.  I don't know what it shows, because I'm not clicking on YouTube links blindly.  (Also, I'm already watching something, and I'm not pausing Oedipus Rex for this!)  The proper etiquette is to give a description of what's on the link and what you are intending to show by it.

You're right. I should have quoted Glom and explained that this video is about the Apollo 16 rover 'grand prix' showing ballistic trajectories of the moon dust.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Edwardwb1001 on October 09, 2012, 05:14:45 PM
True. I should not have used the word 'float', as of course dust would only 'fall' on the moon.  Jay, you state that it is "a little early in your tenure to accuse the regulars of being stupid." I was not saying categorically that any member is stupid.  However, Grashtel states that dust "flies away on a ballistic trajectory." You contradict this statement and state that "It does not", thereafter explaining your reason for saying so.  I'm not hereby implying that Grashtel is stupid, incidentally - only that believers seem to differ in their explanations and convictions. 

Jason Thompson refers to the 'rather large and powerful rocket engine' blowing dust away from the lander's footpads.
Yet it is often mentioned that the reason for no crater (or even indentation) having being created below the lander, was due to the exhaust not being powerful enough on descent to blow any dust away - thus not leaving even the slightest crater.  Too weak on descent to form even an indentation in the dust below the lander - yet 'large and powerful' enough to blow the dust away from the footpads. Which is it?
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: cos on October 09, 2012, 05:24:57 PM
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Too weak on descent to form even an indentation in the dust below the lander - yet 'large and powerful' enough to blow the dust away from the footpads. Which is it?

Oh do I get a point for spotting the first false dichotomy?
 
You might want to read this thread -

http://apollohoax.proboards.com/index.cgi?board=theories&action=display&thread=3146&page=5

Is there nothing new in the hoax world?

Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: cjameshuff on October 09, 2012, 05:42:49 PM
There's a wide, wide range between being able to blow dust over a mylar covered footpad and being able to excavate a crater. Film of the landings did show lose dust streaming away (and later photos showed a surface largely cleared of such dust under the lander, with small pockmarks dug out and a blast-scoured appearance), but only fine, loose surface particles were picked up.

http://www.lpi.usra.edu/resources/apollo/frame/?AS11-40-5921
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Trebor on October 09, 2012, 06:16:29 PM
However, Grashtel states that dust "flies away on a ballistic trajectory." You contradict this statement and state that "It does not", thereafter explaining your reason for saying so.  I'm not hereby implying that Grashtel is stupid, incidentally - only that believers seem to differ in their explanations and convictions.

Perhaps the problem is that you are missing something....

Jason Thompson refers to the 'rather large and powerful rocket engine' blowing dust away from the lander's footpads.

Indeed...

Yet it is often mentioned that the reason for no crater (or even indentation) ..

Let me stop you there... there most certainly was a broad shallow crater. And this is visible in the photos taken.

Which is it?

You are mistaken in your assertion, that is what the problem is.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Edwardwb1001 on October 09, 2012, 06:29:09 PM
You're such a tease, Trebor! Tell me, what am I missing? Don't be rude, now.

This is the trouble. You say there was a 'broad, shallow crater'. I have looked at photos of the area below the lander, and I do not see any type of crater - even shallow. You say tomato...
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: nomuse on October 09, 2012, 06:30:27 PM
If dust 'flies away on a ballistic trajectory', and 'go(es) a long way before landing', then how would you explain video footage of dust flying up behind the lunar rover's wheels, and not flying away on a ballistic trajectory or travelling a far distance before landing. It falls virtually straight down, albeit in slow motion.

You may have gone into more detail later (I don't have the memory to read a whole thread before going back through it!) but you are expressing yourself oddly above.  How is falling down NOT ballistic?  It doesn't matter what the ratio of the parabola is; what matters is that the dust behind the Rover doesn't aerosolize.  It doesn't hang, it doesn't disperse, it doesn't show turbulent patterns.

The distance-versus-height is entirely dependent on how the wheel flings it. 

And Glom, Grashtel speaks of dust 'landing'. Does this not indicate that it did indeed 'float down', even at a distant point? Hence dust does float down - even on the moon. Actually, Glom, you imply from your comment that 'floating', cannot take place on the moon, due to the lack of atmosphere. I said 'float down'. I think you are confusing gravity with atmosphere.  How informed are you? Not very much, it seems. If objects do not float down on the moon (although Grashtel states that they eventually do), why did the astronauts not fly away after each bouncing step on the moon?

What, are we in heavy boots territory now?
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: gillianren on October 09, 2012, 07:14:28 PM
You're right. I should have quoted Glom and explained that this video is about the Apollo 16 rover 'grand prix' showing ballistic trajectories of the moon dust.


I'm sorry for being so snippy about it, but it drives me crazy.  I am almost always watching something while I'm posting, and there's this expectation that whatever-it-is I'm watching (or even listening to!) isn't as important as someone's YouTube video.  I'm not saying you had that expectation, but it's the most common explanation as far as I'm concerned for the blind YouTube link.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Trebor on October 09, 2012, 07:31:42 PM
You're such a tease, Trebor! Tell me, what am I missing? Don't be rude, now.

What you are missing:
Jay stated "It [the dust] is entrained in the exhaust fluid flow," and indeed it is, but if you actually think you would realise that the exhaust as you get further from the source would expand very rapidly and the effect on the dust would quickly become insignificant.
After this point the dust particles would follow nicely parabolic trajectories. With a hefty initial velocity...

This is the trouble. You say there was a 'broad, shallow crater'.

Correct.

I have looked at photos of the area below the lander, and I do not see any type of crater - even shallow. You say tomato...

Interesting, because the fluid erosion marks formed by the exhaust are very apparent in them.
AS11-40-5920 is a very good example...
AS14-66-9261 onward are also excellent examples, what were you expecting to see exactly?
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: JayUtah on October 09, 2012, 07:52:50 PM
...only that believers seem to differ in their explanations and convictions.

Yes, that will happen in life.

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Jason Thompson refers to the 'rather large and powerful rocket engine' blowing dust away from the lander's footpads.

Did he quantify "large and powerful?"  Did I?  Did you?

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Too weak on descent to form even an indentation in the dust below the lander - yet 'large and powerful' enough to blow the dust away from the footpads. Which is it?

What's your justification for believing both can't be simultaneously true?

I spoke of exhaust gas velocity.  Is that the only parameter that determines whether a crater will be excavated?  How about exhaust gas density?  How about collimation?  Did you consider those factors, or are you just trying to drive meaningless wedges wherever you think you see a crack?
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: JayUtah on October 09, 2012, 07:53:55 PM
...the exhaust as you get further from the source would expand very rapidly and the effect on the dust would quickly become insignificant.
After this point the dust particles would follow nicely parabolic trajectories. With a hefty initial velocity...

This is a physically correct description of what happens.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Grashtel on October 09, 2012, 08:44:13 PM
AS11-40-5920 is a very good example...
AS14-66-9261 onward are also excellent examples, what were you expecting to see exactly?
I think AS11-40-5921 (http://www.apolloarchive.com/apg_thumbnail.php?ptr=616&imageID=AS11-40-5921) is an even better example, at the right there is a highly visible ray pattern formed by the exhaust (if you look closely it is present in other areas but most visible there due to the angle of the Sun) and to me at least it seems that there is a slight depression under the engine bell itself.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Echnaton on October 09, 2012, 08:52:57 PM
You're such a tease, Trebor! Tell me, what am I missing? Don't be rude, now.

This is the trouble. You say there was a 'broad, shallow crater'. I have looked at photos of the area below the lander, and I do not see any type of crater - even shallow. You say tomato...
You have looked at photos of all the landings and not seen any depression of any kind caused by the LM?  You need to look closer.  There are obvious examples of dust being swept from the surface by the approaching LM and very modest broad and shallow indentations caused by the final LM landing.  Go back and check again. 
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: raven on October 09, 2012, 08:55:17 PM
Also worth noting is that, if it was fake, every little grain, ever little crack and granny, ever exhaust ray, would have to be sculpted and shaped by hand.
So they put all this attention to detail, meticulous to the point of superlative, yet forgot to do something as simple as dig a crater?
<Yoda voice> Much contradiction I see, hmm?</Yoda voice>
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Jason Thompson on October 10, 2012, 03:23:20 AM
True. I should not have used the word 'float', as of course dust would only 'fall' on the moon.

Concession noted. Since you concede that things will not float, will you also concede that there is no way for the dust to be billowing up and settling down on the footpads, which was your original question?

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Jason Thompson refers to the 'rather large and powerful rocket engine' blowing dust away from the lander's footpads.

On the scale of dust grains, the LM engine is rather large and powerful. It's nearly five feet across at the nozzle end, and generates 3-10,000 lb of thrust when it is on, depending on the throttle. In the whole scheme of rocketry it's not that powerful, especially when put next to something like an F-1 engine, but it's still a force to be reckoned with.

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Yet it is often mentioned that the reason for no crater (or even indentation) having being created below the lander, was due to the exhaust not being powerful enough on descent to blow any dust away - thus not leaving even the slightest crater.

It is never stated that the LM engine is not powerful enough to blow away any dust. This is where hoax believers and the rest of us differ: we know how ridiculous it would be to claim that no dust was blown away because it is evident from the film of all the landings and from the pictures that dust was indeed blown away. There is, however, a world of difference between blowing away surface dust and carving a crater in compacted regolith. It's much the same as the difference between a Hawker Harrier landing in desert terrain and blowing up a large cloud of sand and dust and not carving out a huge crater as it lands. (The Harrier, incidentally, generates more thrust than the LM during vertical take off and landing, and yet it doesn't carve craters out of the ground.)

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Too weak on descent to form even an indentation in the dust below the lander - yet 'large and powerful' enough to blow the dust away from the footpads. Which is it?

That's a false dilemma, as I have just explained. It doesn't have to be large and powerful to blow dust away from footpads, does it?
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: smartcooky on October 10, 2012, 05:27:16 AM
On the scale of dust grains, the LM engine is rather large and powerful. It's nearly five feet across at the nozzle end, and generates 3-10,000 lb of thrust when it is on, depending on the throttle. In the whole scheme of rocketry it's not that powerful, especially when put next to something like an F-1 engine, but it's still a force to be reckoned with.
-----
It is never stated that the LM engine is not powerful enough to blow away any dust. This is where hoax believers and the rest of us differ: we know how ridiculous it would be to claim that no dust was blown away because it is evident from the film of all the landings and from the pictures that dust was indeed blown away. There is, however, a world of difference between blowing away surface dust and carving a crater in compacted regolith. It's much the same as the difference between a Hawker Harrier landing in desert terrain and blowing up a large cloud of sand and dust and not carving out a huge crater as it lands. (The Harrier, incidentally, generates more thrust than the LM during vertical take off and landing, and yet it doesn't carve craters out of the ground.)

I agree with all of the above. I have personally watched a Harrier GR3 land on bare ground (i.e. hard dirt with no grass) when I was at RAF Akrotiri in Cyprus back in the mid 1980's. The Pegasus engine generates over 20,000 lb of thrust (more than twice the LM landing motor at full thrust) distributed through four vectored nozzles, all of which are much narrower than the bell on the LM motor.

After it had taxied away, all you could see was a big "blast patch" where the loose dirt had been blown away. There was definitely no crater.

(http://2.bp.blogspot.com/-3CTpiG7TVeg/TntVUMQ-zEI/AAAAAAAADGg/m30ivrn3aPA/s1600/Hawker+Harrier+GR3.jpg)
Photo of a GR9 landing on grass at RAF Wittering. NOTE: It is not making a crater!

Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Rob260259 on October 10, 2012, 11:12:15 AM
You've got a lot to read Edwardwb1001. And some questions to answer as well. However, something tells me you will not answer most comments and remarks. And please don't comment on these few lines.

Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: sts60 on October 10, 2012, 12:04:46 PM
Not to be too nanny-ish - aw, who am I fooling?  to be totally nanny-ish - can we hold off on the snark a little longer?  Let's give Edwardwb1001 a little more chance to study the difference between exhaust stream entrainment and wheel ejection, not to mention examine the clear evidence of dust being blown out from under the DPS engines. 

At best the metacommentary doesn't advance the discussion; at worst it provides an excuse to avoid substantive discussion.  Just because Edwardwb1001 has started out in a familiar way doesn't mean he is bound to follow the familiar path.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Echnaton on October 10, 2012, 01:44:18 PM
Just because Edwardwb1001 has started out in a familiar way doesn't mean he is bound to follow the familiar path.

True!  Perhaps it's been too slow around here and everyone is wanting to get in the beginning of the discussion.  i am happy to see if he can come forward with anything new or interesting.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: theteacher on October 10, 2012, 02:56:48 PM
A straightforward question but one to which I have not yet received a straightforward answer.  Photos of Apollo 11's landing pads whilst resting on the moon show them to completely dust free, the gold foil covering them shimmering and gleaming - with not a speck of lunar dust or soil present.

Even if the LM's exhaust did not produce a strong enough blast to form a crater, and even if the landing site was quite hard and rocky, there surely would have been a certain amount of dust/soil which would have billowed up to float down and settle on the abovementioned landing pads?

Other landers show decidedly dusty footpads whilst resting on the surface of other worlds. The 2008 Phoenix Mars lander of 2008 shows a completely dust-covered footpad in photos taken.  Why would there be this discrepancy?
It is not a discrepancy. It's a difference.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: smartcooky on October 10, 2012, 03:23:33 PM
A straightforward question but one to which I have not yet received a straightforward answer.  Photos of Apollo 11's landing pads whilst resting on the moon show them to completely dust free, the gold foil covering them shimmering and gleaming - with not a speck of lunar dust or soil present.

Even if the LM's exhaust did not produce a strong enough blast to form a crater, and even if the landing site was quite hard and rocky, there surely would have been a certain amount of dust/soil which would have billowed up to float down and settle on the abovementioned landing pads?

Other landers show decidedly dusty footpads whilst resting on the surface of other worlds. The 2008 Phoenix Mars lander of 2008 shows a completely dust-covered footpad in photos taken.  Why would there be this discrepancy?
It is not a discrepancy. It's a difference.

....and its a difference with a reason.

Mars has an atmosphere... Its not much of one, but it is enough to allow the existence of weather events and phenomena such as clouds, a coloured sky, dust storms and dust devils. The very finest particles will billow and float.

It is a BIG mistake to think that Mars is just a "Big Moon", and that any physical phenomena we see on the Moon will automatically be the same on Mars 
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: JayUtah on October 10, 2012, 04:23:52 PM
The Pegasus engine generates over 20,000 lb of thrust (more than twice the LM landing motor at full thrust)...

And close to ten times the thrust of the DPS as throttled for terminal descent.  Total LM thrust just before touchdown is around 2,500 lbf, of which up to 40% is pressure thrust and thus quantitatively unconnected to plume impingement.

In terms of rocket engines for, say, launch vehicles the DPS is small.  It's on par with the shuttle's OMS engines -- the little ones in the pods in back.  We routinely operate jet engines in commercial aviation that, as you say, have an order of magnitude more thrust.

The velocity of the exhaust gas is the key value in the entrainment problem, not the thrust rating or throttle setting of the engine.  However the overall thrust at that moment is just a little less than half an OMS engine and far, far less than the typical jet engine for a large airframe.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Glom on October 10, 2012, 04:42:24 PM
But let's compare like with like. How does the thrust of the DPS compare to a Robinson R-22?
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: smartcooky on October 10, 2012, 05:18:37 PM
The Pegasus engine generates over 20,000 lb of thrust (more than twice the LM landing motor at full thrust)...

And close to ten times the thrust of the DPS as throttled for terminal descent.  Total LM thrust just before touchdown is around 2,500 lbf, of which up to 40% is pressure thrust and thus quantitatively unconnected to plume impingement.

In terms of rocket engines for, say, launch vehicles the DPS is small.  It's on par with the shuttle's OMS engines -- the little ones in the pods in back.  We routinely operate jet engines in commercial aviation that, as you say, have an order of magnitude more thrust.

The velocity of the exhaust gas is the key value in the entrainment problem, not the thrust rating or throttle setting of the engine.  However the overall thrust at that moment is just a little less than half an OMS engine and far, far less than the typical jet engine for a large airframe.

The Rolls Royce Pegasus engine produces between 20,000 and 24,000 lbs of thrust depending on the type of aircraft it is installed in. This is more than many fighter engines even with full afterburners running. Its also equipped with water injection, which gives it the ability to attain peak thrust on hot days or at altitude. The reason it is so much greater is simple; most other aeronautical jet engines are designed the push an aircraft along a runway until aerodynamic lift gets it airborne, but with the Pegasus, we asking the engine to "dead lift" the full weight of the aircraft at takeoff (or more correctly, liftoff) and to support its full weight during descent and landing.

The exhaust velocity also varies between models, but the one in the photo I posted earlier is a GR9 fitted with the Mk.107 engine, and that delivers an exhaust velocity of over 950 fps through the rear nozzles, and somewhat less (about 450fps) through the front nozzles, in full thrust. How does that compare with the LM descent engine?
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: nomuse on October 10, 2012, 05:43:39 PM
The Pegasus engine generates over 20,000 lb of thrust (more than twice the LM landing motor at full thrust)...

And close to ten times the thrust of the DPS as throttled for terminal descent.  Total LM thrust just before touchdown is around 2,500 lbf, of which up to 40% is pressure thrust and thus quantitatively unconnected to plume impingement.

In terms of rocket engines for, say, launch vehicles the DPS is small.  It's on par with the shuttle's OMS engines -- the little ones in the pods in back.  We routinely operate jet engines in commercial aviation that, as you say, have an order of magnitude more thrust.

The velocity of the exhaust gas is the key value in the entrainment problem, not the thrust rating or throttle setting of the engine.  However the overall thrust at that moment is just a little less than half an OMS engine and far, far less than the typical jet engine for a large airframe.

The Rolls Royce Pegasus engine produces between 20,000 and 24,000 lbs of thrust depending on the type of aircraft it is installed in. This is more than many fighter engines even with full afterburners running. Its also equipped with water injection, which gives it the ability to attain peak thrust on hot days or at altitude. The reason it is so much greater is simple; most other aeronautical jet engines are designed the push an aircraft along a runway until aerodynamic lift gets it airborne, but with the Pegasus, we asking the engine to "dead lift" the full weight of the aircraft at takeoff (or more correctly, liftoff) and to support its full weight during descent and landing.

The exhaust velocity also varies between models, but the one in the photo I posted earlier is a GR9 fitted with the Mk.107 engine, and that delivers an exhaust velocity of over 950 fps through the rear nozzles, and somewhat less (about 450fps) through the front nozzles, in full thrust. How does that compare with the LM descent engine?

That sounds like something that should be on the radio show "Says You!"  "Panel, for our next question, define the difference between 'takeoff' and 'liftoff.'"
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Edwardwb1001 on October 10, 2012, 06:25:14 PM
Echnaton and sts60 exhibit good sense and discernment.  I'm certainly going to study the photos of the area under the LM. I'm not here to try and purposefully be difficult, Jay.  So far, much of what you all say makes sense. Unlike the impression some believers are under, not all hoax believers are too stubborn or proud to finally say that they were wrong on certain matters.  To Jason - yes I do concede that 'billowing' would not apply in a vacuum, but I need to study the answers given further before I can accept that no dust at all would have fallen on the footpads.

Thank you for the answers given - I'm one hoax believer who does actually appreciate them!
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Abaddon on October 10, 2012, 07:19:20 PM
Echnaton and sts60 exhibit good sense and discernment.  I'm certainly going to study the photos of the area under the LM. I'm not here to try and purposefully be difficult, Jay.  So far, much of what you all say makes sense. Unlike the impression some believers are under, not all hoax believers are too stubborn or proud to finally say that they were wrong on certain matters.  To Jason - yes I do concede that 'billowing' would not apply in a vacuum, but I need to study the answers given further before I can accept that no dust at all would have fallen on the footpads.

Thank you for the answers given - I'm one hoax believer who does actually appreciate them!

You have a couple of problems.

1. You have drifted from the OP. The OP was about dust on the pads. You have been provided with the reasons why there wasn't much, but the photographic record shows there was at least some. Try AS14-66-9235 for example.

2. You have diverted to the "crater under the LM" argument, without addressing the substantive issue of the OP. This presents you with a twofold problem:
a: You have moved the goalposts vis a vis the OP.
and
b: You must perforce justify why it is that earthly VTOL craft do not gouge out large craters, yet you expect that lunar craft should. Personally, I attribute this belief to Arthur C. Clarkes novella "A Fall of Moondust" which postulated that in some areas the lunar dust might be 30 meters deep. He wrote it in 1960 before any landings proved the regolith depth was mere inches, yet it remains a meme which HB types glom onto.

Finally, the Mars comparison is right out. Mars has an atmosphere which supports weather, the Moon does not.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Edwardwb1001 on October 10, 2012, 08:05:16 PM
There is no 'problem' Abaddon, and neither have I moved the goalposts. The 'crater' question was simply an additional point.  As I said, I am looking at the photos and studying the responses.

I did not say that I expected the LM to gouge out a large crater. I referred to a crater - or indentation. The only problem which is evident, is that you are yet another member who possesses an inflated and unjustifiable opinion of your intellectual ability.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: cos on October 10, 2012, 08:33:05 PM
you are yet another member who possesses an inflated and unjustifiable opinion of your intellectual ability.

From the man who didn't know there was an environmental difference between the moon & mars you might expect a little humility for a while.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: cos on October 10, 2012, 09:03:41 PM
As I said, I am looking at the photos and studying the responses.


The problem is that without some sort of model to determine just how much soil the engine thrust could have moved, it is just idle speculation. Just what is it that informs your opinion and expectation? Certainly not the Harrier jump jet, which is more powerful and doesn't leave a crater.

On page 2 I posted a link to a couple of members attempt to formulate a model based on the known thrust of the engine and the density of the soil. This provides a figure of how much soil could physically have been shifted and it fits remarkably well with what was observed. I think this is an excellent (only sensible) starting point for how deep or otherwise we think the crater might be. If you have a better model or you can spot a flaw in the methodology, we will be happy to discuss it.

Incidentally, the description of the crater is just about the first thing Neil Armstrong describes after stepping off the LM. Why do that if you want to brush over the fact that 'special effects' have forgotten to dig the damn thing?! Makes no sense.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Peter B on October 10, 2012, 09:16:01 PM
Total LM thrust just before touchdown is around 2,500 lbf, of which up to 40% is pressure thrust {my emphasis} and thus quantitatively unconnected to plume impingement.
Apologies for the thread derail, but could you expand on this, please, Jay.

What is pressure thrust?

What does the balance of the thrust consist of?

In what ways do they differ?

Do the proportions change at different levels of thrust? Or for different types of engines?

Thank you.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Echnaton on October 10, 2012, 09:44:41 PM
There is no 'problem' Abaddon, and neither have I moved the goalposts. The 'crater' question was simply an additional point.  As I said, I am looking at the photos and studying the responses.

I did not say that I expected the LM to gouge out a large crater. I referred to a crater - or indentation. The only problem which is evident, is that you are yet another member who possesses an inflated and unjustifiable opinion of your intellectual ability.
More confirmation.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: smartcooky on October 10, 2012, 10:04:05 PM
There is no 'problem' Abaddon, and neither have I moved the goalposts. The 'crater' question was simply an additional point.  As I said, I am looking at the photos and studying the responses.

You may not have moved the goal-posts per se, but you went onto a different subject (crater under the LM) before you acknowledged whether or not you were convinced by the arguments concerning your OP (dust on the lander pads)

The only problem which is evident, is that you are yet another member who possesses an inflated and unjustifiable opinion of your intellectual ability.

And who you are you to judge that? There are a good number of the regular posters in this forum who have clearly been involved in the aerospace industry for many years. I was 20 years in the Air Force involved in the repair, maintenance and refurbishment of Avionics systems, which includes radio, radar, electrical and instrumentation systems.

Incidentally, this constant changing of the subject is typical of HB behaviour, as is "abusing the messenger" when they fail to understand explanations that amount to the most basic principles of high school science.

So far, you have been given a generous benefit of a very large doubt, however, I think I am sense a gradual loss of patience on the part of some.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: ka9q on October 10, 2012, 10:38:57 PM
C'mon guys, relax. When someone new asks a question, why not simply answer it as best you can without assuming some sort of hidden agenda right off the bat? He'll make his agenda, if he has one, obvious soon enough.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Tanalia on October 11, 2012, 01:25:41 AM
But let's compare like with like. How does the thrust of the DPS compare to a Robinson R-22?
Maximum certified (tested and legal) takeoff weight is listed as 1370 lb.  It likely has a safety margin, so maybe a maximum capability around 1500.
Title: Re: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Glom on October 11, 2012, 02:06:02 AM
C'mon guys, relax. When someone new asks a question, why not simply answer it as best you can without assuming some sort of hidden agenda right off the bat? He'll make his agenda, if he has one, obvious soon enough.

Seconded. Regardless of whether we're dealing with typical HB behaviour, we have it within us to be the better men.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Jason Thompson on October 11, 2012, 02:29:53 AM
To Jason - yes I do concede that 'billowing' would not apply in a vacuum, but I need to study the answers given further before I can accept that no dust at all would have fallen on the footpads.

Why is it so hard for you to accept that dust kicked up with a velocity on the order of thousands of miles an hour would not fall on something barely three feet away from the point it was kicked up from, and even if it did it would be going so fast it would simply bounce off it?

Consider also that the engine was either shut down when the contact probes touched the surface, so when the footpads were still about three feet up and therefore above the dust which was entrained in a flow running close to the surface, or else was still running right up to the point the footpads touched the surface, in which case the exhaust flow would blow the dust away from the footpads anyway.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Jason Thompson on October 11, 2012, 02:32:03 AM
The only problem which is evident, is that you are yet another member who possesses an inflated and unjustifiable opinion of your intellectual ability.

OK, comments like this are what is unjustified. People on this forum habve been discussing this subject for years, and many of them, as you will have already seen, actually work in related fields professionally.You have a lot of very knowledgeable people here to answer your questions. Don't throw it back in their faces.
Title: Re: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Glom on October 11, 2012, 07:16:37 AM
The only problem which is evident, is that you are yet another member who possesses an inflated and unjustifiable opinion of your intellectual ability.

OK, comments like this are what is unjustified. People on this forum habve been discussing this subject for years, and many of them, as you will have already seen, actually work in related fields professionally.You have a lot of very knowledgeable people here to answer your questions. Don't throw it back in their faces.

Ooh, I missed that bit. That's a whole new level of obnoxious.

Edward, on what basis do you assume the confidence of members here in their intellectual abilities is not justified? You're the one who struggled to grasp the concept of their being no billowing in a vacuum. You have hardly demonstrated the intellectual ability to pass judgement on others.

I'm sure you'd like to believe you talking with people who are about on your level but you're not.  The history of debate here is like a scrum half raping a dwarf, such is the disparity between the knowledge, intelligence and experience of the people here and the occasional hoax believer.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Abaddon on October 11, 2012, 11:26:41 AM
There is no 'problem' Abaddon, and neither have I moved the goalposts. The 'crater' question was simply an additional point.  As I said, I am looking at the photos and studying the responses.

I did not say that I expected the LM to gouge out a large crater. I referred to a crater - or indentation. The only problem which is evident, is that you are yet another member who possesses an inflated and unjustifiable opinion of your intellectual ability.
You have moved the goalposts, and furthermore, have descended into ad hom more rapidly than I would have thought.

I provided you with an example of dust on the pads. Others have provided you with the reasons why there is not much.
Your response is sly insults.


Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: twik on October 11, 2012, 12:43:59 PM
To Jason - yes I do concede that 'billowing' would not apply in a vacuum, but I need to study the answers given further before I can accept that no dust at all would have fallen on the footpads.

I don't think you have to assume that "no dust at all" fell, just "not enough to make a significant layer, or even to be visible in photographs".
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: raven on October 11, 2012, 01:03:48 PM
No dust at all verses masses is a false dichotomy. There is in fact dust visible, just not much, on the footpads. Not enough for a layer, but enough to be visible in some close up photographs that have already been linked to earlier in this topic.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Glom on October 11, 2012, 02:53:00 PM
It probably got there from being kicked up by the astronauts.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: sts60 on October 11, 2012, 03:52:01 PM
The astronauts kicked up a lot of dirt.  Also, some footpads shoveled dirt onto themselves as the LM drifted slightly at touchdown.  That said, I expect that some dust managed to get onto the footpads as a result of the DPS plume, but not necessarily in amounts visible without very close inspection.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Edwardwb1001 on October 12, 2012, 06:37:03 PM
Thank you Ka9q, for injecting some common sense into this thread. Contrary to what a number of members suppose, I do not have a subversive 'agenda'. The only agenda I do have is a 'truth' agenda. I might be a hoax believer, but I'm a reasonable one! Jason, that is a good point that the LM's engine may have been shut down before the contact probes touched the surface...etc. Twik - good point - as you say some dust may have fallen, but not enough to be visible in photographs.

Glom, I didn't 'struggle' to grasp the concept of their being no billowing in a vacuum - you are exaggerating. My response which WAS rather unkind (perhaps even obnoxious) was however my own response to a member saying "I have a few problems" - which I found rather irritating. Perhaps it wasn't meant as an insult.

I can see that dust may well not have fallen to any appreciable level on the footpads, unlike on those of the Phoenix lander and I accept the reasons given.  I am looking at photographs of the moon's surface under the LM regarding any scouring or removal of regolith.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: gillianren on October 12, 2012, 09:50:27 PM
A reasonable hoax believer would answer questions posed.  A reasonable hoax believer would start with the thing that had convinced them that the Apollo landings were faked.  A reasonable hoax believer . . . does not exist, because there is no reasonable doubt about Apollo.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: ka9q on October 12, 2012, 11:35:26 PM
Edward, another explanation that I don't think has been given yet is that in a vacuum, the exhaust plume from the LM closely hugged the surface as it went out radially in all directions at very high velocity. The dust entrained in the plume would have followed the plume itself; there was simply nothing to cause it to rise off the surface, much less fall into the landing pads.

Look at the flow of water in a sink when you turn the faucet away from the drain. The water hits the sink and flows smoothly outward as a sheet in all directions; it doesn't billow above the sink, right? Water is about 1,000 times as dense as sea level air, so the air has no significant effect on it. On the moon, even the rapidly expanding engine exhaust gas is far more dense than the lunar atmosphere, which for all practical purposes doesn't exist at all. So it too flows outward as a surface-hugging sheet.

Neil Armstrong said he was surprised to see the dust flying away toward the horizon and disappearing almost instantly when he shut off the descent engine. He said that he then realized it was exactly as it should have been, but he was still surprised because he hadn't thought of all the ramifications of the lack of a lunar atmosphere in advance. So if even a guy who went there was surprised by how the lunar dust behaved, it is not at all surprising that someone like you who hasn't been there might at first also think it unusual until the physics are explained.

Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: smartcooky on October 13, 2012, 05:45:36 AM
This is something that I think a lot of people still don't get their heads around.

In a vacuum, even a microscopic dust particle that weighs 0.000000000000000000001 of a gramme will fall at exactly the same rate as a 10lb concrete breeze-block; on the moon that is 1.63 m/s

Another thing that people don't get is that an object on a ballistic trajectory that starts at 'h' height above the ground, a with an initial motion parallel to the ground, will fall to the ground at the same rate and in the same time as an object that fall directly from 'h' height to the ground.

(http://i116.photobucket.com/albums/o35/smartcooky99/trajectory.png)

So in a vacuum, discounting the curvature of the moon, if a bullet leaves the muzzle a gun (the barrel of is which is parallel to the ground) at 'A' and at the same moment, a second bullet is dropped from 'A', the dropped bullet will arrive at 'B' at precisely the same moment that the fired bullet strikes the ground 400 yards away at 'C'. This holds for all muzzle velocities, and it holds for dust particles as well.

Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Andromeda on October 13, 2012, 05:53:25 AM
Smartcooky, did you see the Mythbusters episode where they tested that with a gun?  The air resistance was negligible for the bullet so they got a great result.  It was very well done and a great teaching tool.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: smartcooky on October 13, 2012, 06:21:04 AM
Smartcooky, did you see the Mythbusters episode where they tested that with a gun?  The air resistance was negligible for the bullet so they got a great result.  It was very well done and a great teaching tool.

Yes, I did see it.

It was ka9q's remark about the dust not rising that made me think of it.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Kiwi on October 13, 2012, 08:40:41 AM
Jason, that is a good point that the LM's engine may have been shut down before the contact probes touched the surface...etc.

You have apparently studied Apollo, but how much time have you spent on JayUtah's website Clavius (http://www.clavius.org), where he deals with most of the well-worn hoax claims?

And you don't appear to have spent much time at the online Apollo Lunar Surface Journals and the Apollo Flight Journals, with their stunning abundance of reference materials and links.

If you had read just the first section of the Apollo 11 and Apollo 12 Lunar Surface Journals you would probably have noticed that Eagle touched down with the engine still running (which you can also see in the 16mm landing film), and Intrepid landed with enough of a thump to almost cause Pete Conrad to let out just one more of his umpzillion expletives, as he relates in the journal here:

Quote
110:32:28 Carr: 30 seconds (of fuel remaining).

110:32:29 Bean: 18 feet, coming down at 2. He's got it made! Come on in there. 24 feet.

110:32:35 Bean: Contact Light.

110:32:36 Carr: Roger. Copy Contact.

[Jones - "I gather from the tech debrief that you actually dropped the last two or three feet."]

[Conrad - "You're supposed to."]

[Jones - "And the theory on that was?"]

[Conrad - "Lunar contact light came on and the probes were six feet below the gear. We were supposed to shut the engine off right then because they did worry about the bell mouth too close to the ground."]

[Bean - "Or hitting a rock and denting the bell mouth."]

[Conrad - "And I said, always, 'I'll never do that; who wants to shut off a good engine when you're still in the air?' But we had to train to shut it off. Neil landed with his (engine still) on. And, so, I was going to do the same thing. And, whoever said 'lunar contact light', I went 'bamm' and shut it down. (Laugh) Somewhere in there, I think there's an 'Oh shit'. Or there almost was. But about that time we were on (the Moon), and I didn't have to get it (the 'oh shit') the rest of the way out. I remember that."]

You might have also seen in the Apollo 11 Lunar Surface Journal, among the plentiful details about Eagle's powered descent and landing, Eric Jones's comment about the astronaut's use of "air":

Quote
102:45:43 Armstrong (on-board): Shutdown.

102:45:44 Aldrin: Okay. Engine Stop.

[Neil had planned to shut the engine down when the contact light came on, but didn't manage to do it.]

[Armstrong, from the 1969 Technical Debrief - "I heard Buzz say something about contact, and I was spring-loaded to the stop engine position, but I really don't know...whether the engine-off signal was before (footpad) contact. In any event, the engine shutdown was not very high above the surface."]

[Armstrong - "We actually had the engine running until touchdown. Not that that was intended, necessarily. It was a very gentle touchdown. It was hard to tell when we were on."]

[Aldrin - "You wouldn't describe it as 'rock' (as in, 'dropping like a rock'). It was a sensation of settling."]

[Some of the other crews shut down 'in the air' (meaning 'prior to touchdown') and had a noticeable bump when they hit.]

[Aldrin - (Joking) "Well, they didn't want to jump so far to the ladder."]

[Readers should note that, although the Moon has no atmosphere, many of the astronauts used expression like 'in the air' to mean 'off the ground' and, after some thought, I have decided to follow their usage.]

[Armstrong, from the 1969 Technical Debrief - "The touchdown itself was relatively smooth; there was no tendency toward tipping over that I could feel. It just settled down like a helicopter on the ground, and landed."]

[On a final note about engine shutdown, Ken Glover calls attention to the following from an interview done with Neil on 19 September 2001 by historians Stephen Ambrose and Douglas Brinkley at NASA Johnson.]

[Brinkley: "Was there anything about your Moon walk and collecting of rocks and the like that surprised you at that time when you were on the Moon, like, 'I did not expect to encounter this,' or, 'I did not expect it to look like this'? Or included in that, the view of the rest of space from the Moon must have been quite an awesome thing to experience."]

[Armstrong: "I was surprised by a number of things, and I'm not sure (I can) recall them all now. I was surprised by the apparent closeness of the horizon. I was surprised by the trajectory of dust that you kicked up with your boot, and I was surprised that even though logic would have told me that there shouldn't be any, there was no dust when you kicked. You never had a cloud of dust there. That's a product of having an atmosphere, and when you don't have an atmosphere, you don't have any clouds of dust."]

["I was absolutely dumbfounded when I shut the rocket engine off and the particles that were going out radially from the bottom of the engine fell all the way out over the horizon, and when I shut the engine off, they just raced out over the horizon and instantaneously disappeared, you know, just like it had been shut off for a week. That was remarkable. I'd never seen that. I'd never seen anything like that. And logic says, yes, that's the way it ought to be there, but I hadn't thought about it and I was surprised."]

I'm pasting from the DVD-ROM version, which may differ a little from the more-up-to-date online version.

My response which WAS rather unkind (perhaps even obnoxious) was however my own response to a member saying "I have a few problems" - which I found rather irritating. Perhaps it wasn't meant as an insult.

The quote, "I have a few problems" very clearly means that the person writing it is admitting to having problems, but I cannot find such a quote in this thread.  If the quote was "You have problems" then the writer is clearly referring to the reader(s), but that quote doesn't appear here either.

By my estimation, the words "problem" or "problems" have been used in this thread the following number of times:

Trebor, post 24, 2x
JayUtah, post 41, 1x
Abbadon, post 46, 2x
Edwardwb1001, posts 47 and 64, 3x
Cos, post 49, 1x
Total, nine times.

I certainly wouldn't bother myself over words like, "You have a couple of problems" and I cannot imagine why anyone else would want to. If that were true I'd want to fix them because getting rid of problems is a very good thing. Could you have perhaps been oversensitive and overemotional and imagined slights where there are none?  It has happened here many times before.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Kiwi on October 13, 2012, 09:29:18 AM
The only agenda I do have is a 'truth' agenda.

Then please, pursue that agenda with the utmost vigour (as I and others here do), and don't spend so much time complaining about how others do exactly the same.

In other words, get on with it!
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Abaddon on October 13, 2012, 02:18:06 PM
Glom, I didn't 'struggle' to grasp the concept of their being no billowing in a vacuum - you are exaggerating. My response which WAS rather unkind (perhaps even obnoxious) was however my own response to a member saying "I have a few problems" - which I found rather irritating. Perhaps it wasn't meant as an insult.
Reacting in this way to someone pointing out problems with your hypothesis is not a good sign.

I can see that dust may well not have fallen to any appreciable level on the footpads, unlike on those of the Phoenix lander and I accept the reasons given.  I am looking at photographs of the moon's surface under the LM regarding any scouring or removal of regolith.
Ok, you accept there would be no expectation for much dust to be on the pads.
Moving on to the "crater" try AS11-40-5918 or AS11-40-5921 for starters.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Peter B on October 13, 2012, 07:49:15 PM
This is something that I think a lot of people still don't get their heads around...an object on a ballistic trajectory that starts at 'h' height above the ground, a with an initial motion parallel to the ground, will fall to the ground at the same rate and in the same time as an object that fall directly from 'h' height to the ground.
I remember watching a demonstration of this in about Year 9 physics. The teacher had a simple spring-loaded device which popped one ball-bearing out sideways, while simultaneously dropping another ball bearing vertically onto the ground. The experiment worked really well in the lab with its lino-on-concrete floor, as the ball bearings hit the floor with a very audible click. And only one click, because they hit the ground simultaneously.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: ka9q on October 13, 2012, 07:49:55 PM
So in a vacuum, discounting the curvature of the moon
That's actually not always a realistic thing to do. The moon is so much smaller than the earth, and has such a lower escape velocity (about 2.38 km/sec) that particles moving at high speeds will see quite a bit of surface curvature during their flight. The effective exhaust velocity of a hypergolic engine like that on the LM is about 2940 m/s, above lunar escape velocity, so a particle entrained in that exhaust has a very good chance of escaping the moon entirely, if not flying much of the way around before falling back. This would happen even if the particle's initial trajectory has it leaving the surface at a very low altitude and angle; as long as it doesn't hit a rock or hill right away, the surface will fall away faster than it itself falls, and then it'll just keep going.

This even happens with the ejecta from many large lunar impacts.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: ka9q on October 13, 2012, 10:48:03 PM
What is pressure thrust?What does the balance of the thrust consist of?
I think I can answer this. The purpose of a chemical rocket engine is to convert the heat of combustion into kinetic energy of the exhaust, as that's how you generate thrust. While all heat consists of kinetic energy of the individual gas molecules, they're going in random directions; the purpose of the engine is to convert as much of that energy as possible into smooth linear motion out the nozzle. This makes a rocket a heat engine, just like the one in a gasoline-powered car, that converts heat into useful work. As in any real heat engine, this can only be done with less than 100% efficiency.

The combustion products start very hot and under high pressure in the combustion chamber, but with essentially no linear velocity. It's the nozzle's job to convert this heat to linear motion as efficiently as possible.

The standard nozzle design is a De Laval or converging-diverging nozzle. It brings the gases up to the local speed of sound in the throat (the narrowest part) and then further accelerates them to supersonic velocity as they expand and cool in the divergent part. If the pressure at the mouth of the nozzle is exactly equal to the ambient pressure, then the nozzle is optimally expanded and there is no pressure thrust; all the thrust comes from the momentum of the exhaust gases leaving the nozzle.

But if the nozzle is under-expanded, then the exhaust gases leave the nozzle with greater than ambient pressure. The difference between this local pressure and ambient, times the exit area of the nozzle, is the pressure thrust. That thrust is summed with the momentum thrust. There may or may not be pressure thrust in an atmosphere, and it can even be negative if the nozzle is too long and over-expands the exhaust gases so that they exit at less than ambient pressure. But in a vacuum there is always at least a little pressure thrust.

I think of momentum thrust as that produced by the combustion gases pushing on the forward end of the combustion chamber, while pressure thrust is that component produced by the expanding exhaust gases pushing forward on the inside of the nozzle. Because  pressure thrust results from underexpansion, it takes away even more from the momentum thrust so zero pressure thrust is the optimum condition.

This is why rockets designed for operation at sea level are designed with shorter nozzles than those designed for vacuum operation. This is most easily seen in the 9-SRB versions of the Delta II launch vehicle. Six of the strap-on solid boosters are lit on the ground, and they have short nozzles. Three more strap-on solids are lit at T+60 seconds, and they have noticeably longer nozzles since they operate at higher altitude and lower ambient air pressure.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Andromeda on October 14, 2012, 04:31:38 AM
This is something that I think a lot of people still don't get their heads around...an object on a ballistic trajectory that starts at 'h' height above the ground, a with an initial motion parallel to the ground, will fall to the ground at the same rate and in the same time as an object that fall directly from 'h' height to the ground.
I remember watching a demonstration of this in about Year 9 physics. The teacher had a simple spring-loaded device which popped one ball-bearing out sideways, while simultaneously dropping another ball bearing vertically onto the ground. The experiment worked really well in the lab with its lino-on-concrete floor, as the ball bearings hit the floor with a very audible click. And only one click, because they hit the ground simultaneously.

Yes, the "monkey and the hunter" setup.  What your teacher didnt tell you was that the whole rig was so finely balanced it could take an hour to set up so it actually worked!  I don't miss doing that  :)
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: JayUtah on October 14, 2012, 12:28:00 PM
I think I can answer this.

That's an answer worthy of an introductory textbook:  concise and correct.

Quote
If the pressure at the mouth of the nozzle is exactly equal to the ambient pressure, then the nozzle is optimally expanded and there is no pressure thrust...

True.  And since it is impossible in practice to achieve zero ambient pressure with any achievable nozzle design, there is always pressure thrust in a vacuum.

Quote
There may or may not be pressure thrust in an atmosphere, and it can even be negative if the nozzle is too long and over-expands the exhaust gases so that they exit at less than ambient pressure.

Indeed the design for launch vehicle nozzles is the most demanding because you need knowledge of mission requirements in order to know where to set the sweet spot.  The SSME nozzle is an overexpansion nozzle, hence the shock diamonds and Mach cone seen when the engine reaches steady-state operation at launch.

The cone is actually just a degenerate diamond.  The plume, with its low static pressure, is "squeezed" by the ambient air pressure as it exits the nozzle.  This takes a few milliseconds for a response in the form of increased incandescence due to higher density and temperature, hence its distance from the exit plane.  The cone shape occurs because the center of the plume moves faster than the periphery, and because the elasticity of the gas under compression forces the center of plume to travel farther before undergoing its Mach transition (i.e., to subsonic flow).

That design was chosen to maximize the LOX/LH2 propulsion at high altitude after SRB staging.  For launch vehicles without SRBs, optimal nozzle expansion for the first stage is tuned for sea level.

Quote
I think of momentum thrust as that produced by the combustion gases pushing on the forward end of the combustion chamber, while pressure thrust is that component produced by the expanding exhaust gases pushing forward on the inside of the nozzle.

That's often the way it's depicted in diagrams.  Unfortunately for the momentum case, the "little arrows pressing against all the walls of the thrust chamber and also out the exit plane" is misleading because it conveys the wrong impression that momentum thrust is simply chamber pressure multiplied by exit-plane area.  Momentum thrust is a function of the fluid dynamics in the nozzle.

For practical purposes, the center of thrust is at the throat of the nozzle.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: JayUtah on October 14, 2012, 02:45:37 PM
Apologies for the thread derail, but could you expand on this, please, Jay.

Sorry, just noticed this request after having been directed to it.  KA9Q's answer is as good as you're going to get.

Quote
What is pressure thrust?

Static pressure of the exhaust gas at the exit plane.  It exerts pressure upward (well, in all directions, but only in the upward case is there anything to expand against and thereby convey mechanical force).  You can think about it in terms similar to air-cushion vehicles.

Quote
What does the balance of the thrust consist of?

Depends on operating environment and engine design.  In atmospheric boost operations, only a small percentage of total thrust is contributed by pressure thrust.  For the throttled DPS in a vacuum, up to 40% of total thrust is pressure thrust.

Quote
In what ways do they differ?

In practically all ways, except in their ability to convey mechanical force to the spacecraft.  Momentum thrust is a pure application of Newton's third law.  Pressure thrust is more governed by classical gas laws.

Quote
Do the proportions change at different levels of thrust? Or for different types of engines?

Yes, and more.  In engines that can be throttled, plume expansion in a fixed nozzle and fixed ambient pressure varies, leading to variations in exit-plane pressure.  Since the definition of pressure thrust is the difference between exit pressure and ambient pressure, clearly a change in the ambient changes the proportion.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Tanalia on October 15, 2012, 09:52:42 PM
In a vacuum, even a microscopic dust particle that weighs 0.000000000000000000001 of a gramme will fall at exactly the same rate as a 10lb concrete breeze-block; on the moon that is 1.63 m/s
Minor nitpick -- they will accelerate at 1.63 m/s2.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: smartcooky on October 15, 2012, 11:00:15 PM
In a vacuum, even a microscopic dust particle that weighs 0.000000000000000000001 of a gramme will fall at exactly the same rate as a 10lb concrete breeze-block; on the moon that is 1.63 m/s
Minor nitpick -- they will accelerate at 1.63 m/s2.

Very true

However, when you start talking about metres per second squared to most HBs, the hand waving and glazed expressions begin.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: gillianren on October 15, 2012, 11:11:28 PM
Honestly, I don't myself entirely understand how you square a second.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: JayUtah on October 15, 2012, 11:57:18 PM
Honestly, I don't myself entirely understand how you square a second.

The same way you turn a phrase.  ;D

It's a calculus thing.  The "squared" second is in the denominator, which designates a rate.  In calculus there's always a rate at which the rate-of-change changes -- that is, an endless chain of differentials.  When you unravel the algebra, it's m/s2 or m/s/s, which may be easier thought of as (m/s)/s.  (m/s) is a velocity -- change in position over time.  Acceleration is a change in velocity over time, so (m/s)/s.

It's best not to try to visualize the behavior too concretely from the arithmetic of the units.  Not even we who do this for a living manage to get that right.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: smartcooky on October 16, 2012, 12:40:39 AM
.....when you start talking about metres per second squared to most HBs, the hand waving and glazed expressions begin.

It's a calculus thing.  The "squared" second is in the denominator, which designates a rate.  In calculus there's always a rate at which the rate-of-change changes -- that is, an endless chain of differentials.  When you unravel the algebra, it's m/s2 or m/s/s, which may be easier thought of as (m/s)/s.  (m/s) is a velocity -- change in position over time.  Acceleration is a change in velocity over time, so (m/s)/s

I couldn't have come up with a better demonstration of my point than that!! 8)

All I recall about this what what my physics teacher told me, more years ago that I care to admit.

There is a height above which, on the Moon, an object dropped will impact the surface at a higher speed than one dropped from the same height on the Earth, even though the moon has a much lower gravity.

An object dropped on the Earth accelerates at 9.8 m/s2 until it reaches its "terminal velocity", the speed at which it can no longer fall any faster because of air resistance - i.e. drag. (for a human skydiver, in a standard free-fall attitude, this is about 120 mph)

On the Moon, while the same object only accelerates much more slowly (1.63 m/s2) there is no air, therefore no terminal velocity, it just keeps accelerating until "wham", it hits the lunar surface.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: ka9q on October 16, 2012, 06:34:57 AM
Honestly, I don't myself entirely understand how you square a second.
It's even easier than Jay made it out to be.

On earth, ignoring atmospheric drag, for every second that some object free-falls, it picks up 9.8 m/s of downward velocity. If it falls for 1 second, it picks up 9.8 m/s. If it falls for 2 seconds, it picks up 19.6 m/s, and so on. This is true regardless of starting speed, even upward. If you toss something straight up at 9.8 m/s, then after 1 second it will come to a dead stop, and after 2 seconds it will be coming down at 9.8 m/s (and quite likely hit you in the head). So we say that the acceleration of gravity is 9.8 meters per second, per second. Or just 9.8 m/s2 for short.

On the moon, simply substitute 1.622 everywhere you see 9.8 in the above paragraph.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Zakalwe on October 16, 2012, 10:12:36 AM
On the Moon, while the same object only accelerates much more slowly (1.63 m/s2) there is no air, therefore no terminal velocity, it just keeps accelerating until "wham", it hits the lunar surface.

I'm no mathematician, so please accept my apologies if this is incorrect.
The effects of gravity decreases with the square of the distance, so an object twice as high would "feel" 1/4 of the gravitational attraction when compared to an object at half the height. So it would accelerate more slowly.

With this in mind, does the Moon then have a "terminal" velocity? For example, if a body (with no movement relative to the Moon's surface) appeared at a certain altitude, would it impact at a higher velocity than a similar object appearing at a different altitude? And is there a point where increasing the altitude does not increase the impact velocity?
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Glom on October 16, 2012, 11:00:53 AM
On the Moon, while the same object only accelerates much more slowly (1.63 m/s2) there is no air, therefore no terminal velocity, it just keeps accelerating until "wham", it hits the lunar surface.

I'm no mathematician, so please accept my apologies if this is incorrect.
The effects of gravity decreases with the square of the distance, so an object twice as high would "feel" 1/4 of the gravitational attraction when compared to an object at half the height. So it would accelerate more slowly.

Yes, but be careful.  That distance is from the centre of the attractor's mass, not the surface.  The difference in weight  between an object 100m up above the surface and an object 200m up is only seen deep down in the jillionth decimal place.  For practical purposes, when we're talking about acceleration due to gravity on the surface, it is a constant.

Quote
With this in mind, does the Moon then have a "terminal" velocity? For example, if a body (with no movement relative to the Moon's surface) appeared at a certain altitude, would it impact at a higher velocity than a similar object appearing at a different altitude? And is there a point where increasing the altitude does not increase the impact velocity?

No.  Terminal velocity is due to air resistance, which increases with the square of airspeed.  When falling in an atmosphere, you accelerate due to gravity minus the air resistance until you speed reaches the terminal speed where the air resistance is now equal but opposite to the weight so there is no longer a net force to cause further acceleration.  On the Moon, without an atmosphere, you keep accelerating so the higher you start, the faster you'll be going when you hit the surface.  It's also worth pointing out that terminal speed differs from object to object due to drag coefficient.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Glom on October 16, 2012, 11:03:44 AM
On the Moon, while the same object only accelerates much more slowly (1.63 m/s2) there is no air, therefore no terminal velocity, it just keeps accelerating until "wham", it hits the lunar surface.
With this in mind, does the Moon then have a "terminal" velocity? For example, if a body (with no movement relative to the Moon's surface) appeared at a certain altitude, would it impact at a higher velocity than a similar object appearing at a different altitude? And is there a point where increasing the altitude does not increase the impact velocity?

Rereading, I wondered if you meant a different kind of terminal speed due to the diminishing gravitational force as distance increases.  The answer is no.  The greater the height at the start of the freefall, the faster the speed at impact.  You can apply conservation of energy to see how this is the case.  The higher an object is above the surface, the more gravitational potential energy it has.  In freefall, this energy becomes kinetic energy.  More GPH at the beginning mean more kE at the end hence higher speed.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Valis on October 16, 2012, 11:07:22 AM
With this in mind, does the Moon then have a "terminal" velocity? For example, if a body (with no movement relative to the Moon's surface) appeared at a certain altitude, would it impact at a higher velocity than a similar object appearing at a different altitude? And is there a point where increasing the altitude does not increase the impact velocity?
If an object is falling towards the Moon, its impact velocity increases with increasing altitude. Even though the accelerating force decreases with increasing distance, it still accelerates the object. Think of an example of two objects falling from different heights. Hold the lower object in place until the higher object has reached it. Now, the initially higher object has already gained some velocity (let's say v0), while the lower object starts at zero. As they experience the same gravitational attraction with the Moon from the lower object's initial position onwards, from that point on they gain the same amount of velocity until the impact. Let's call this velocity gain  v.  As v is the only velocity gained by the lower object, it's its impact velocity. However, the higher object's impact velocity is  v0+v.

Terminal velocity comes from a medium's (like air) resistance of the falling motion. In a vacuum, the velocity increases without a limit in Newtonian mechanics (in real life, relativity would kick in at some point).
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: cjameshuff on October 16, 2012, 11:19:29 AM
Rereading, I wondered if you meant a different kind of terminal speed due to the diminishing gravitational force as distance increases.  The answer is no.  The greater the height at the start of the freefall, the faster the speed at impact.  You can apply conservation of energy to see how this is the case.  The higher an object is above the surface, the more gravitational potential energy it has.  In freefall, this energy becomes kinetic energy.  More GPH at the beginning mean more kE at the end hence higher speed.

Actually, there is a limit. An object dropped from an infinite distance would (disregarding the infinite time taken to fall) hit the surface at surface escape velocity. This is quite different from the concept of a terminal velocity, however.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Valis on October 16, 2012, 11:33:48 AM
Actually, there is a limit. An object dropped from an infinite distance would (disregarding the infinite time taken to fall) hit the surface at surface escape velocity. This is quite different from the concept of a terminal velocity, however.
True. My use of the words "without a limit" above is misleading, what I rather meant was that there isn't a "cut-off distance" in the sense Zakalwe suggested, and if you'd increase the mass of the Moon, you'd also increase the impact velocity.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Chew on October 16, 2012, 11:55:32 AM
However, the higher object's impact velocity is  v0+v.

That ain't right.

Consider two objects dropped on the Moon, one at 11 km, the other at 10 km. As the higher object passes the lower object it will be traveling at 57 m/s. But the objects will hit at 189 and 180 m/s.


See the 7th and 8th equations at this link for the equations used where the fall distance is significant to the radius of the Moon: Equations for a falling body (http://en.wikipedia.org/wiki/Equations_for_a_falling_body#Overview)
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Valis on October 16, 2012, 12:02:40 PM
That ain't right.

Consider two objects dropped on the Moon, one at 11 km, the other at 10 km. As the higher object passes the lower object it will be traveling at 57 m/s. But the objects will hit at 189 and 180 m/s.


See the 7th and 8th equations at this link for the equations used where the fall distance is significant to the radius of the Moon: Equations for a falling body (http://en.wikipedia.org/wiki/Equations_for_a_falling_body#Overview)
Again, correct (trying to keep my kid happy while I'm typing, not really good for any coherent thought...). Of course the initial velocity will lower the time the gravitational acceleration has for acting on the falling object, resulting in a smaller difference at impact.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: JayUtah on October 16, 2012, 01:10:47 PM
All good examples and demonstrations.  Not to put too many words in Gillianren's mouth, because her words are so much better than mine, but the sticky wicket for most non-mathematicians and non-physicists is the notion that an abstract arithmetic operation has meaning to a real-world object such as a unit of time.  You can't square a second anymore than you can take the square root of a gallon.  Of course you can in engineering physics, but what does that mean in real-world terms?  Not even engineers typically go there.

The confusion arises from the dual nature of specific measurements as practical aids to daily life -- "Add a teaspoon of vinegar" -- and also as representatives of the underlying quantities and values that play in our investigation of the behavior of the physical world -- "The rate of decay is slowing."  The latter requires reasoning about quantities, rates, and combinations using increasingly more abstract relationships and thus requiring increasingly more formal notation and techniques that end up squaring seconds, square-rooting gallons, "arbitrarily" halving the mass, or similarly confusing calculus fu.  Units jump the shark at that point, when considered as the practical measurements they are to most of us.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: gillianren on October 16, 2012, 01:31:10 PM
No, that's exactly what I meant.  It's like in . . . Swiftly Tilting Planet, I believe, where Mr. Murry and Charles Wallace are building a model of a tesseract.  It's an abstract mathematical concept (and made The Avengers a little weird for me), and I wasn't sure how you would build a model of one any more than I can get a feel for "seconds squared."
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: nomuse on October 16, 2012, 05:07:07 PM
Well, I've seen 3d models of a 4d hypercube.  Sorta like the usual 2d representation of a 3d wireframe cube.  Except, of course, I don't think I've ever seen one of those models in the flesh, so I've only seen a 2d representation or image of a 3d model of a 4d concept...

Anyhow, I sorta figured that's what they meant.  Although that sentence bugged me.  (Not as much as a later one, in the second book...about Charles Wallace just "needing to adapt...!")
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: ka9q on October 16, 2012, 05:31:24 PM
the sticky wicket for most non-mathematicians and non-physicists is the notion that an abstract arithmetic operation has meaning to a real-world object such as a unit of time.
The really interesting thing is how often you can find a real-world meaning to the units on some set of measurements.

Example: the fuel economy of a car. In the USA it's usually measured in miles per gallon. It could also be given as gallons per mile, as in many other countries it is given as litres per 100 km. A gallon is a unit of volume, so it has units of length cubed. (You compute volume by multiplying three length measurements, so the result has units of length cubed).

The mile, of course, is just a unit of length. So if you divide gallons by miles you get units of length squared. Does this have a physical meaning? Actually, it does! It's the cross sectional area of the trough of gasoline the car would have to scoop up to continue moving.

Edited to add: As an example, a car that gets 30 mpg would have to scoop up a trough of gasoline with a cross-sectional area of 0.0784 mm^2. That's a square 0.28 mm on a side. Doesn't seem like much, but it adds up.

The same works for electric vehicles, which are rated in units of miles per kilowatt-hour, or kilowatt-hours per 100 km. A kilowatt-hour is a unit of energy, which has basic units of kg m2/s2, also known as the joule. (1 kWh = 3.6 million joules.) If you divide that by units of distance, you get kg m/s2, which happens to be the newton, the unit of force. In other words, the mileage rating for an electric car is equivalent to the physical force needed to overcome drag and keep the car going. (This also includes some electrical and mechanical losses that appear as "virtual" drag in the final result.)

There are all sorts of other examples like these; physics can be a lot more intuitive than many people think.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: DataCable on October 16, 2012, 05:42:06 PM
Except, of course, I don't think I've ever seen one of those models in the flesh, so I've only seen a 2d representation or image of a 3d model of a 4d concept...
How about 2D photographs of 3D immersions of 4D Klein Bottles (http://kleinbottle.com)?
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: smartcooky on October 16, 2012, 05:55:39 PM
Except, of course, I don't think I've ever seen one of those models in the flesh, so I've only seen a 2d representation or image of a 3d model of a 4d concept...
How about 2D photographs of 3D immersions of 4D Klein Bottles (http://kleinbottle.com)?
Well, I've seen 3d models of a 4d hypercube.  Sorta like the usual 2d representation of a 3d wireframe cube.  Except, of course, I don't think I've ever seen one of those models in the flesh, so I've only seen a 2d representation or image of a 3d model of a 4d concept...

How about an animated 2d image of a 3d model of a 4d concept being rotated in 2 dimensions of a 4D space...?

(http://i116.photobucket.com/albums/o35/smartcooky99/Tesseract.gif)
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: gillianren on October 16, 2012, 06:59:22 PM
Anyhow, I sorta figured that's what they meant.  Although that sentence bugged me.  (Not as much as a later one, in the second book...about Charles Wallace just "needing to adapt...!")

If I'm right and it's Swiftly Tilting Planet, a reference in the second book is earlier.  It's A Wrinkle in Time, A Wind in the Door, and then A Swiftly Tilting Planet.  And then Many Waters and so forth.  I don't actually like A Swiftly Tilting Planet, though, largely because I don't like Charles Wallace all that much.

As to the animation--mind?  Blown.
Title: Re: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Glom on October 16, 2012, 07:31:23 PM
the sticky wicket for most non-mathematicians and non-physicists is the notion that an abstract arithmetic operation has meaning to a real-world object such as a unit of time.
The really interesting thing is how often you can find a real-world meaning to the units on some set of measurements.

Example: the fuel economy of a car. In the USA it's usually measured in miles per gallon. It could also be given as gallons per mile, as in many other countries it is given as litres per 100 km. A gallon is a unit of volume, so it has units of length cubed. (You compute volume by multiplying three length measurements, so the result has units of length cubed).

The mile, of course, is just a unit of length. So if you divide gallons by miles you get units of length squared. Does this have a physical meaning? Actually, it does! It's the cross sectional area of the trough of gasoline the car would have to scoop up to continue moving.

Edited to add: As an example, a car that gets 30 mpg would have to scoop up a trough of gasoline with a cross-sectional area of 0.0784 mm^2. That's a square 0.28 mm on a side. Doesn't seem like much, but it adds up.

The same works for electric vehicles, which are rated in units of miles per kilowatt-hour, or kilowatt-hours per 100 km. A kilowatt-hour is a unit of energy, which has basic units of kg m2/s2, also known as the joule. (1 kWh = 3.6 million joules.) If you divide that by units of distance, you get kg m/s2, which happens to be the newton, the unit of force. In other words, the mileage rating for an electric car is equivalent to the physical force needed to overcome drag and keep the car going. (This also includes some electrical and mechanical losses that appear as "virtual" drag in the final result.)

There are all sorts of other examples like these; physics can be a lot more intuitive than many people think.

I was having trouble understanding what you meant by trough but I think I have it.

Imagine a car is like a train running on a third rail. Except instead of a rail of steel carrying a current, it is a gully filled with petrol that miraculously doesn't evaporate. Instead of a contact shoe, it has a suction inlet that dips into the petrol and is shaped to spade the gully and as the car moves, the inlet sweeps the gully leaving it dry behind and sucks in the petrol it sweeps. The thirstier the car, the bigger this gully needs to be to satisfy the requirements. The cross sectional area of this gully is mathematically equivalent to the consumption.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: JayUtah on October 16, 2012, 07:45:06 PM
It's the cross sectional area of the trough of gasoline the car would have to scoop up to continue moving.

Neat!  And true!
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: JayUtah on October 16, 2012, 07:52:11 PM
As to the animation--mind?  Blown.

The mind-blowy thing is the realization that the spaces between a face on the inner cube and its corresponding face on the outer cube are degenerate cubes.  You can "project" a 4D cube into three dimensions using the same technique as a 3D cube into two dimensions, knowing that some of the square faces will be distorted by the projection (i.e., perspective).

Consider a train track where the ties are spaced along the track the same distance as the width between the rails.  Thus looking straight down, the ties appear to form a line of adjacent squares.  But looking ahead from the locomotive cab, the squares appear to be trapezoids, the rails converging to their vanishing point and the farther ties appearing shorter and more closely spaced.  Now imagine rolling forward and keeping your eye on one set of ties.  It first appears as a trapezoid, but then gradually becomes more square-like as you approach.  Looking through the glass bottom of your magic locomotive, there will be one instant where you see it as the square before it recedes into the distance to become a trapezoid again.

So watch the animation anew and see where the inner cube is briefly a perfect cube before it's distorted into a cube-trapezoid.
Title: Why is there no dust on the Lunar Lander's footpads?
Post by: Sus_pilot on October 17, 2012, 01:24:21 AM
the sticky wicket for most non-mathematicians and non-physicists is the notion that an abstract arithmetic operation has meaning to a real-world object such as a unit of time.
The really interesting thing is how often you can find a real-world meaning to the units on some set of measurements.

Example: the fuel economy of a car. In the USA it's usually measured in miles per gallon. It could also be given as gallons per mile, as in many other countries it is given as litres per 100 km. A gallon is a unit of volume, so it has units of length cubed. (You compute volume by multiplying three length measurements, so the result has units of length cubed).

The mile, of course, is just a unit of length. So if you divide gallons by miles you get units of length squared. Does this have a physical meaning? Actually, it does! It's the cross sectional area of the trough of gasoline the car would have to scoop up to continue moving.

Edited to add: As an example, a car that gets 30 mpg would have to scoop up a trough of gasoline with a cross-sectional area of 0.0784 mm^2. That's a square 0.28 mm on a side. Doesn't seem like much, but it adds up.

The same works for electric vehicles, which are rated in units of miles per kilowatt-hour, or kilowatt-hours per 100 km. A kilowatt-hour is a unit of energy, which has basic units of kg m2/s2, also known as the joule. (1 kWh = 3.6 million joules.) If you divide that by units of distance, you get kg m/s2, which happens to be the newton, the unit of force. In other words, the mileage rating for an electric car is equivalent to the physical force needed to overcome drag and keep the car going. (This also includes some electrical and mechanical losses that appear as "virtual" drag in the final result.)

There are all sorts of other examples like these; physics can be a lot more intuitive than many people think.

I was having trouble understanding what you meant by trough but I think I have it.

Imagine a car is like a train running on a third rail. Except instead of a rail of steel carrying a current, it is a gully filled with petrol that miraculously doesn't evaporate. Instead of a contact shoe, it has a suction inlet that dips into the petrol and is shaped to spade the gully and as the car moves, the inlet sweeps the gully leaving it dry behind and sucks in the petrol it sweeps. The thirstier the car, the bigger this gully needs to be to satisfy the requirements. The cross sectional area of this gully is mathematically equivalent to the consumption.

[Thread hijack]That's one of the ways the New York Central Railroad and the Pennsylvania Railroad replenished the water in steam locomotive tenders (I'm not just a pilot, as Jay can tell you).  [/Thread hijack]
Title: Re: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: ka9q on October 17, 2012, 01:29:07 AM
The cross sectional area of this gully is mathematically equivalent to the consumption.
Exactly! And the small size of this cross section for a typical car demonstrates just how energetic gasoline really is, and why it's so difficult to find something to replace it.

The computer in my Nissan Leaf typically shows around 3.9 miles/kWh. (I'm not sure if this is DC energy from the battery or AC wall socket energy. Probably DC.) This works out to 1.7 mm/joule, or an equivalent drag force of 574 newtons (about 129 pounds force).
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Noldi400 on October 18, 2012, 06:21:04 PM
The whole trough concept reminds me of the "water brakes" they used on Col. Stapp's rocket sled back in the day.

I may not be an aerospace professional or, indeed, an engineer of any sort, but I knew who John P. "Death Wish" Stapp was before I was clear on Captain Kangaroo.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Commander Cody on November 11, 2012, 02:11:25 PM
the amount of blast force onto the moon while the apollo was landing it must of blew the moon dust to the sides. but at the same time some dust might fall down onto the pads, but mabye the image was not zoomed in enough to see the dust on the pads????
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Jason Thompson on November 11, 2012, 03:19:08 PM
How would dust fall 'down' on the pads when it is being blown away by the engine? Remember it doesn't billow in clouds or linger around. It is blasted off at very high speeds laterally.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: cjameshuff on November 11, 2012, 06:46:39 PM
And as the images I linked to show, some bits of dust did end up on the pads, probably bouncing off the leg and getting trapped in folds of mylar, or the trailing wisps of dust as the engine shut down.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: ka9q on November 12, 2012, 12:23:33 AM
the amount of blast force onto the moon while the apollo was landing it must of blew the moon dust to the sides. but at the same time some dust might fall down onto the pads, but mabye the image was not zoomed in enough to see the dust on the pads????
You are basing this on your intuition, but your intuition is based on a lifetime of living on the earth, with 1 g gravity and 1 atmosphere pressure. Neither is true on the moon, so some things behave very differently there.

Title: Re: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Britmax on January 04, 2020, 06:20:20 PM
the sticky wicket for most non-mathematicians and non-physicists is the notion that an abstract arithmetic operation has meaning to a real-world object such as a unit of time.
The really interesting thing is how often you can find a real-world meaning to the units on some set of measurements.

Example: the fuel economy of a car. In the USA it's usually measured in miles per gallon. It could also be given as gallons per mile, as in many other countries it is given as litres per 100 km. A gallon is a unit of volume, so it has units of length cubed. (You compute volume by multiplying three length measurements, so the result has units of length cubed).

The mile, of course, is just a unit of length. So if you divide gallons by miles you get units of length squared. Does this have a physical meaning? Actually, it does! It's the cross sectional area of the trough of gasoline the car would have to scoop up to continue moving.

Edited to add: As an example, a car that gets 30 mpg would have to scoop up a trough of gasoline with a cross-sectional area of 0.0784 mm^2. That's a square 0.28 mm on a side. Doesn't seem like much, but it adds up.

The same works for electric vehicles, which are rated in units of miles per kilowatt-hour, or kilowatt-hours per 100 km. A kilowatt-hour is a unit of energy, which has basic units of kg m2/s2, also known as the joule. (1 kWh = 3.6 million joules.) If you divide that by units of distance, you get kg m/s2, which happens to be the newton, the unit of force. In other words, the mileage rating for an electric car is equivalent to the physical force needed to overcome drag and keep the car going. (This also includes some electrical and mechanical losses that appear as "virtual" drag in the final result.)

There are all sorts of other examples like these; physics can be a lot more intuitive than many people think.

I was having trouble understanding what you meant by trough but I think I have it.

Imagine a car is like a train running on a third rail. Except instead of a rail of steel carrying a current, it is a gully filled with petrol that miraculously doesn't evaporate. Instead of a contact shoe, it has a suction inlet that dips into the petrol and is shaped to spade the gully and as the car moves, the inlet sweeps the gully leaving it dry behind and sucks in the petrol it sweeps. The thirstier the car, the bigger this gully needs to be to satisfy the requirements. The cross sectional area of this gully is mathematically equivalent to the consumption.

In England, steam locomotives on express trains used to have troughs like that to reduce the number of stops on long runs and thus keep the speed up. The fireman would lower a scoop at a set place into a trough of water set between the rails and the momentum of the train would force the water into the tender.  Two things you wouldn't want to happen were to have track workers nearby or to mistime lowering the scoop and knocking the end off the trough. The first three rows will get wet, as they say at SeaWorld.
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Jason Thompson on January 05, 2020, 06:21:41 AM
In England, steam locomotives on express trains used to have troughs like that to reduce the number of stops on long runs and thus keep the speed up. The fireman would lower a scoop at a set place into a trough of water set between the rails and the momentum of the train would force the water into the tender.  Two things you wouldn't want to happen were to have track workers nearby or to mistime lowering the scoop and knocking the end off the trough. The first three rows will get wet, as they say at SeaWorld.

Used to great comic effect in an episode of Dad's Army, where the end of the episode sees the entire platoon (after a series of comic mishaps, of course) line up trackside to salute a train carrying the King. The train used the scoop and trough system, and of course the final shot shows the whole lot of them getting absolutely drenched as they attempt to smartly salute the monarch....
Title: Re: Why is there no dust on the Lunar Lander's footpads?
Post by: Britmax on January 05, 2020, 07:09:54 AM
In England, steam locomotives on express trains used to have troughs like that to reduce the number of stops on long runs and thus keep the speed up. The fireman would lower a scoop at a set place into a trough of water set between the rails and the momentum of the train would force the water into the tender.  Two things you wouldn't want to happen were to have track workers nearby or to mistime lowering the scoop and knocking the end off the trough. The first three rows will get wet, as they say at SeaWorld.

Used to great comic effect in an episode of Dad's Army, where the end of the episode sees the entire platoon (after a series of comic mishaps, of course) line up trackside to salute a train carrying the King. The train used the scoop and trough system, and of course the final shot shows the whole lot of them getting absolutely drenched as they attempt to smartly salute the monarch....

I remember that episode but haven't seen it lately. Those looking for a "return to topic" will enjoy the thought I had that that according to hoaxer logic, Jimmy Perry and David Croft must have made it all up because one of them wasn't in the real Home Guard during the war, as obviously that "conflict" was made up by the Jewish Lizards. Or the military industrial complex. And that.