ApolloHoax.net

Apollo Discussions => The Reality of Apollo => Topic started by: Allan F on November 05, 2014, 11:54:11 AM

Title: Saturn V acceleration
Post by: Allan F on November 05, 2014, 11:54:11 AM
How fast was the Saturn V accelerating from the launch pad and to orbit? What was it's velocity at "tower clear"?
Title: Re: Saturn V acceleration
Post by: Zakalwe on November 05, 2014, 12:06:15 PM
Apollo by the Numbers gives a lot of this info:

http://history.nasa.gov/SP-4029/Apollo_18-20_Ascent_Data.htm

And Bob's excellent page will also help:
http://www.braeunig.us/apollo/saturnV.htm
Title: Re: Saturn V acceleration
Post by: Bob B. on November 05, 2014, 12:54:38 PM
The following is from a simulation, but it mirrors almost exactly what the real graph looks like:

(http://www.braeunig.us/apollo/pics/SVsim-fig3.gif)

It was roughly accelerating between about 1.2 and 1.3 g during the time that you ask about.

 
Title: Re: Saturn V acceleration
Post by: Allan F on November 05, 2014, 02:14:14 PM
Someone somewhere quoted a speed of 100 km/h at tower clear, which I don't think the Saturn V did.
Title: Re: Saturn V acceleration
Post by: Glom on November 05, 2014, 02:46:28 PM
What's EMR shift?
Title: Re: Saturn V acceleration
Post by: Bob B. on November 05, 2014, 03:03:01 PM
What's EMR shift?

EMR = Engine mixture ratio.  The J-2 engine had a valve that could control the flow of LOX to the combustion chamber.  By closing down the value and reducing the flow of LOX, the mixture ratio could be changed from 5.5:1 to 4.5:1.  Since this reduced the mass flow rate to the engine, it also reduced the thrust.  This valve was part of a propellant utilization subsystem, the purpose of which was to monitor and control the flow rates to make sure the fuel and LOX supplies where depleted simultaneously.  The "EMR shift" was the moment this value changed positions.  It shows up on the acceleration graph because of the decrease in thrust that resulted.
Title: Re: Saturn V acceleration
Post by: Allan F on November 05, 2014, 03:03:31 PM
What's EMR shift?

They changed the fuel/oxidizer ratio.
Title: Re: Saturn V acceleration
Post by: Bob B. on November 05, 2014, 03:12:34 PM
Someone somewhere quoted a speed of 100 km/h at tower clear, which I don't think the Saturn V did.

That sounds pretty close.  According to my simulation, it would be traveling about 25 m/s, or 90 km/h.
Title: Re: Saturn V acceleration
Post by: Allan F on November 05, 2014, 03:27:24 PM
The faster the better. Didn't look that fast on the little screen though. Must be because it is mentally difficult to grasp the SIZE of that machine.
Title: Re: Saturn V acceleration
Post by: AstroBrant on November 05, 2014, 10:02:19 PM
Someone somewhere quoted a speed of 100 km/h at tower clear, which I don't think the Saturn V did.

I also saw somewhere that it was 60 mph when it cleared the tower. So that's about the same. One is intuitively struck by how slow it appears to be moving, but that is an illusion based on failing to sense the enormity of the craft. I have noticed with ships and large planes how they always seem to be moving slower than they are.
Title: Re: Saturn V acceleration
Post by: Bryanpoprobson on November 06, 2014, 01:58:49 PM
Just by eye, one thing I noticed in being an avid launch watcher, The Shuttles acceleration from the launch pad seemed noticeably quicker than the Saturn V. Is this just my eye or a fact? I should look back over the launch videos and time it. :)
Title: Re: Saturn V acceleration
Post by: JayUtah on November 06, 2014, 02:37:49 PM
Just by eye, one thing I noticed in being an avid launch watcher, The Shuttles acceleration from the launch pad seemed noticeably quicker than the Saturn V. Is this just my eye or a fact? I should look back over the launch videos and time it. :)

I don't believe that's just your eye.  The shuttle fairly leaps off the pad and is generally going in excess of 140 km/h when it clears the tower, according to one of the press kits.
Title: Re: Saturn V acceleration
Post by: Echnaton on November 06, 2014, 02:45:41 PM
Watching the films from the highest tower cameras give and idea of how fast the S5 was going.   A few seconds after the slow start, those fins at the business end of the rocket just zoom by.
Title: Re: Saturn V acceleration
Post by: Bob B. on November 06, 2014, 03:28:02 PM
The Shuttles acceleration from the launch pad seemed noticeably quicker than the Saturn V. Is this just my eye or a fact?

It's a fact.  The Shuttle's thrust-to-weight ratio at liftoff was considerably more than the Saturn V -- about 1.7 for the Shuttle vs. 1.2 for Saturn V.
Title: Re: Saturn V acceleration
Post by: Allan F on November 06, 2014, 04:07:50 PM
The tower for the shuttle launch was much shorter too.
Title: Re: Saturn V acceleration
Post by: ka9q on November 06, 2014, 07:44:59 PM
EMR = Engine mixture ratio.  The J-2 engine had a valve that could control the flow of LOX to the combustion chamber.
The interesting thing about the EMR shift is that even though it took the engines even further from stoichiometric (which would be an 8:1 oxidizer:fuel mass ratio) the specific impulse actually increased a few seconds. The amount of unburned hydrogen in the exhaust increased, decreasing its average molecular weight and the efficiency with which the engine converted combustion energy into the kinetic energy of the exhaust. That more than made up for the lower combustion power.

Why not just run with the richer mixture? Two reasons I can see. First, the EMR shift was, as Bob says, timed to cause the two propellants to deplete simultaneously; this time was calculated on the fly by the IU based on measured consumption rates.

Second, while high specific impulse is generally a good thing, high thrust is even more important during the early stages of flight when the rocket is pitched up and overcoming gravity losses. As it pitches down toward the horizontal, gravity losses decrease and thrust can be decreased in favor of specific impulse since the rocket is at that time building up horizontal velocity to make orbit. Also, decreasing thrust toward the end of a stage burn can help limit peak acceleration stresses.
Title: Re: Saturn V acceleration
Post by: Allan F on November 06, 2014, 08:47:37 PM
Impressive. I ask a simple question, which leads to a whole new line of information I didn't even considered existed. Layer upon layer of complexity.

Thank you for your information.
Title: Re: Saturn V acceleration
Post by: AstroBrant on November 06, 2014, 11:33:57 PM
In addition to what's already been said about the shuttle acceleration, it must also be due to the two-part firing of the engines. By the time the SRBs were lit up, much of the craft's weight was already canceled out by the orbiter's main engines.
(IMHO)
Title: Re: Saturn V acceleration
Post by: Bob B. on November 07, 2014, 01:06:57 AM
The interesting thing about the EMR shift is that even though it took the engines even further from stoichiometric (which would be an 8:1 oxidizer:fuel mass ratio) the specific impulse actually increased a few seconds. The amount of unburned hydrogen in the exhaust increased, decreasing its average molecular weight and the efficiency with which the engine converted combustion energy into the kinetic energy of the exhaust. That more than made up for the lower combustion power.

The mixture ratio of most engines is at or near that that produces the highest specific impulse.  Hydrogen burning engines are an exception.  Most operate at mixtures ratios between 5:1 and 6:1, while highest specific impulse occurs closer to 4:1 or even lower.  The reason for using the higher mixture ratio is to lower the overall density of the propellant and decrease the size of the propellant tanks.  For example the total volume occupied by the LOX+LH2 at 4:1 is 27% greater than at 6:1 for the same mass.  Smaller tanks mean less weight and less drag, though this comes at the expense of slightly lower specific impulse.  It works out the maximum delta-v is achieved with mixtures ratios in that 5:1 to 6:1 range.

Second, while high specific impulse is generally a good thing, high thrust is even more important during the early stages of flight when the rocket is pitched up and overcoming gravity losses. As it pitches down toward the horizontal, gravity losses decrease and thrust can be decreased in favor of specific impulse since the rocket is at that time building up horizontal velocity to make orbit. Also, decreasing thrust toward the end of a stage burn can help limit peak acceleration stresses.

While the S-II (second stage) operated mostly at the high-mixture ratio setting, the S-IVB (third stage) operated at the intermediate- and low-mixture ratio settings.  The S-IVB operated at a time when gravity loses were inconsequential, so they could sacrifice some thrust in exchange for higher specific impulse.
Title: Re: Saturn V acceleration
Post by: Abaddon on November 14, 2014, 07:57:49 PM
Just by eye, one thing I noticed in being an avid launch watcher, The Shuttles acceleration from the launch pad seemed noticeably quicker than the Saturn V. Is this just my eye or a fact? I should look back over the launch videos and time it. :)
I don't believe that's just your eye.  The shuttle fairly leaps off the pad and is generally going in excess of 140 km/h when it clears the tower, according to one of the press kits.
How much does the prevalence of playing back high speed camera footage at lower speed contribute to the impression of slow motion? It's pretty commonplace in documentaries and so forth to do so.
Title: Re: Saturn V acceleration
Post by: Glom on November 14, 2014, 11:58:46 PM
Sounds like engineering thought went into this. Most disturbing.

Immediate thought is that it seems weird that reducing the molecular weight of the exhaust would increase specific impulse. I would have thought it would be the other way round as higher molecular weight gives more impulse for kinetic energy. But maybe I actually need to review the maths.
Title: Re: Saturn V acceleration
Post by: Bob B. on November 15, 2014, 01:07:47 AM
Immediate thought is that it seems weird that reducing the molecular weight of the exhaust would increase specific impulse. I would have thought it would be the other way round as higher molecular weight gives more impulse for kinetic energy. But maybe I actually need to review the maths.

Lightweight molecules can be accelerated to higher velocity.  Heavy molecules have more momentum per molecule, but light molecules have more momentum per unit mass.
Title: Re: Saturn V acceleration
Post by: gwiz on November 15, 2014, 06:19:27 AM
Temperature of the gas in the combustion chamber is a measure of the average kinetic energy of the molecules, proportional to mass times velocity squared.  For a given maximum temperature that you can get your rocket to withstand, low mass molecules have greater velocity than high mass ones.
Title: Re: Saturn V acceleration
Post by: cjameshuff on November 15, 2014, 10:14:00 AM
Sounds like engineering thought went into this. Most disturbing.

Immediate thought is that it seems weird that reducing the molecular weight of the exhaust would increase specific impulse. I would have thought it would be the other way round as higher molecular weight gives more impulse for kinetic energy. But maybe I actually need to review the maths.

More impulse for a given amount of kinetic energy, but specific impulse is a measure of impulse per fuel mass. Specific impulse is a measure of mass efficiency, not energy efficiency: the faster you eject a given mass, the greater the total impulse you get from it, at the cost of energy requirements increasing with the square of exhaust velocity.
Title: Re: Saturn V acceleration
Post by: JayUtah on November 16, 2014, 12:52:46 PM
This is one of the reasons why solid fuel rocketry is still a thing.  While solid fuels generally have lower specific impulse, they have much greater mass density and invoke the tradeoffs involved with building the rocket casings and fuselages.  Aside from the qualitative advantages of solid fuels in some application, you can cram more propellant mass into a smaller fuselage that way.  Fuselage and casing structural requirements scale generally according to the cube of the rocket size, affecting the mass ratio.
Title: Re: Saturn V acceleration
Post by: AstroBrant on November 17, 2014, 09:21:32 AM
This is one of the reasons why solid fuel rocketry is still a thing.  While solid fuels generally have lower specific impulse, they have much greater mass density and invoke the tradeoffs involved with building the rocket casings and fuselages.  Aside from the qualitative advantages of solid fuels in some application, you can cram more propellant mass into a smaller fuselage that way.  Fuselage and casing structural requirements scale generally according to the cube of the rocket size, affecting the mass ratio.
I would think another effective advantage of solid fuel is that it eliminates the need for weighty fuel/oxidizer pumping systems.
Title: Re: Saturn V acceleration
Post by: Allan F on November 17, 2014, 10:43:17 AM
This is one of the reasons why solid fuel rocketry is still a thing.  While solid fuels generally have lower specific impulse, they have much greater mass density and invoke the tradeoffs involved with building the rocket casings and fuselages.  Aside from the qualitative advantages of solid fuels in some application, you can cram more propellant mass into a smaller fuselage that way.  Fuselage and casing structural requirements scale generally according to the cube of the rocket size, affecting the mass ratio.
I would think another effective advantage of solid fuel is that it eliminates the need for weighty fuel/oxidizer pumping systems.

In order to get maximum take-off thrust, you need to have a complex hollow shape in the middle of your solid fueled rocket, otherwise you'd get a very slow start, and an undesireable power ramp-up till burn-out. So a lot of your solid fueled rocket will be empty space.
Title: Re: Saturn V acceleration
Post by: Echnaton on November 17, 2014, 11:15:03 AM
Do this elimination of certain problems lead to a cost advantage?  Or does it just introduce other problems that offset the advantages and eat up cost savings elsewhere?  I remember reading around here in the past that solid boosters introduced a significant vibration due to uneven levels of thrust. 

I would think that the transportation costs for the "fueled" solid rockets needed for a heavy lift booster would start to offset many other advantages.  Vs Lox and H2 produced on site at the KSC. 
Title: Re: Saturn V acceleration
Post by: Bob B. on November 17, 2014, 12:58:52 PM
In order to get maximum take-off thrust, you need to have a complex hollow shape in the middle of your solid fueled rocket, otherwise you'd get a very slow start, and an undesireable power ramp-up till burn-out. So a lot of your solid fueled rocket will be empty space.

From photos and diagrams I've seen, the center channel of at least some fuel blocks looks to be about 1/3 of the diameter.  That would make the block about 1/9th empty space.

It should also be noted that it's possible to control the thrust profile by using center channels with different shapes.  For example, a smooth cylindrical channel results in a progressively greater thrust because the exposed surface area of the block increases as the fuel burns away.  With a larger burn surface the fuel burns at a greater rate, producing more gas and more thrust.  Another common type is a "star" shaped channel.  This produces relatively constant thrust because the surface area doesn't changed much as the fuel burns away.  It's possible to produce many different thrust profiles simply by changing the solid fuel geometry.

Title: Re: Saturn V acceleration
Post by: Bob B. on November 17, 2014, 01:03:08 PM
I would think that the transportation costs for the "fueled" solid rockets needed for a heavy lift booster would start to offset many other advantages.  Vs Lox and H2 produced on site at the KSC.

I believe the Space Shuttle SRBs where shipped to KSC via barge from Louisiana.   It's may understanding that water is the least expansive mode of transportation.
Title: Re: Saturn V acceleration
Post by: Sus_pilot on November 17, 2014, 01:47:38 PM
This is one of the few times I can speak with authority - they were shipped by rail from Utah to Florida.
Title: Re: Saturn V acceleration
Post by: Bob B. on November 17, 2014, 02:49:22 PM
This is one of the few times I can speak with authority - they were shipped by rail from Utah to Florida.

I should have known better, I know Thiokol (presently ATK) is in Utah.  I was thinking of the external tanks, which were made at Lockheed Martin's Michoud Assembly Facility in New Orleans.
Title: Re: Saturn V acceleration
Post by: JayUtah on November 23, 2014, 06:15:05 PM
I should have known better, I know Thiokol (presently ATK) is in Utah.  I was thinking of the external tanks, which were made at Lockheed Martin's Michoud Assembly Facility in New Orleans.

Officially, ATK Wasatch Propulsion Division.  The shipping containers are still at the siding in the Corinne, Utah rail depot.  They are the long, white rectangular objects seen in this aerial view.

https://www.google.com/maps/place/Corinne,+UT/@41.5622323,-112.1320478,326m/data=!3m1!1e3!4m2!3m1!1s0x8753639505cb521f:0xee6c1390c36ddc66

The segments were transported from the vast ATK complex by highway to Corinne and placed on rail cars there.  Here is the main ATK site.

https://www.google.com/maps/place/Corinne,+UT/@41.6586852,-112.4276487,5205m/data=!3m1!1e3!4m2!3m1!1s0x8753639505cb521f:0xee6c1390c36ddc66

The roadway between the remote Thiokol site and Corinne is very smooth and flat for that purpose, and very infrequently patrolled by highway patrol.  Hence the speed limit is rarely obeyed.

Here's someone else's photo page of the ATK rocket garden in Utah.  You can see the grain of the Minuteman first stage with its central cavity.
http://www.ahpra.org/thio.htm
Title: Re: Saturn V acceleration
Post by: Bob B. on November 23, 2014, 11:38:00 PM
Cool!  Thanks.

I see the Minuteman uses a 6-pointed star.  A few years ago I did some calculations with some different shapes to see what kind of thrust profile they gave.  I concluded that a 6-pointed star produced the most even thrust over the duration of the burn.
Title: Re: Saturn V acceleration
Post by: JayUtah on November 24, 2014, 12:19:12 AM
The STS SRM used a grain that altered the thrust to match the programmed SSME throttling profile for dealing with max Q, etc.
Title: Re: Saturn V acceleration
Post by: Bob B. on November 24, 2014, 04:54:43 AM
The STS SRM used a grain that altered the thrust to match the programmed SSME throttling profile for dealing with max Q, etc.

Yes, I've seen of the SRB thrust versus time (see link below) and it definitely drops around max Q and then rises again.  It's my understanding that they used a variety of geometries to get it to do that.

http://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster#mediaviewer/File:Srbthrust2.svg

Title: Re: Saturn V acceleration
Post by: JayUtah on November 24, 2014, 10:54:08 AM
Yes, it's a star pattern with a variable taper on the star points.  Plus, the forward and aft sections of the motor have different cavity shapes.
Title: Re: Saturn V acceleration
Post by: ka9q on December 04, 2014, 04:38:20 AM
The reduced thrust period around max-Q in the shuttle SRBs was called the "thrust bucket".

I'm old enough that I still got a lump in my throat every time I heard the Capcom call out "go at throttle up" at the end of the thrust bucket during a shuttle launch.

My understanding is that solid rocket boosters are still popular because they can provide a lot of thrust (as opposed to specific impulse) fairly cheaply. High thrust is important early in a launch when gravity losses are greatest (because the launcher is still pointed mostly up) and the stack is still pretty heavy. I still don't like to see them launching humans, though.

Gravity losses decrease as the launcher pitches down and mass quickly declines to only a small fraction of that at liftoff. High specific impulse then becomes more important than high thrust as the launcher builds most of its velocity. This is why high performance liquid fueled engines (especially those burning H2) are popular in upper stages, though there are many exceptions.
Title: Re: Saturn V acceleration
Post by: Noldi400 on December 08, 2014, 10:52:28 AM
I'm old enough that I still got a lump in my throat every time I heard the Capcom call out "go at throttle up" at the end of the thrust bucket during a shuttle launch.
I hear you, brother. I can't hear the phrase without getting a mental flash of that fireball and the Y shaped cloud that followed.