Author Topic: Quick question about rocket engines  (Read 21111 times)

Offline mako88sb

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Quick question about rocket engines
« on: February 12, 2015, 02:11:25 PM »
I finally managed to track down a copy of Virtual LEM a while ago for a reasonable price. One thing mentioned in it is the 400 to 1 ratio for how many lbs of fuel required to land 1 lb on the moon and return it back to Earth. I believe I also read the same in Tom Kelly's book but I can't seem to locate it right now. Anyway, I've been wondering if this ratio has been improved upon over the years seeing as what I can make sense about rocket engines, they have improved in efficiency much like everything else nowadays. I imagine the use of SRB's probably helps as well. I'm sure there's not a clear cut answer as it all depends on the different configurations but which one would improve upon the 400 to 1 ratio the most?

Offline Bob B.

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Re: Quick question about rocket engines
« Reply #1 on: February 12, 2015, 03:33:16 PM »
Any maneuver that one needs to perform on a space mission requires a certain change in velocity, called ΔV ("delta vee").  To complete a mission you need to add up all the ΔV to produce a ΔV budget.  For instance, the ΔV budget for Apollo looked something like this (rough numbers):

Ascent to LEO = 9000 m/s
Tranlunar injection = 3200 m/s
Lunar orbit insertion = 900 m/s
Lunar descent = 2100 m/s
Lunar ascent = 1900 m/s
Transearth injection = 1000 m/s

There's not a lot that technology can do to change the ΔV budget, so it's just a matter of how the ΔV can be produced.  There's really only two ways that I can see to do this:  (1) use more efficient staging, or (2) use more efficient propellants/engines.  As far as staging goes, Apollo was pretty efficient - this was the purpose of adopting the lunar orbit rendezvous method.  There might be some ways to squeeze a little more efficiency out of it, but I suspect those improvements would be small.  That leaves us with propellants/engines.

The Saturn V was pretty efficient as far as propellant went - it used LOX/RP-1 in the first stage and LOX/LH2 in the upper stages.  It is, however, possible to use more efficient engines.  Both the F-1 and J-2 where open cycle engines and, therefore, had lower specific impulse than is possible with closed cycle engines (i.e. staged combustion).  Although closed cycle is more efficient, it is also more complicated and expensive.  There was at one time talk of adapting the space shuttle main engines (LOX/LH2 staged combustion) for use in future heavy launch vehicles, but I'm not certain of the current status of that.  I think the plan was dropped because of the cost, instead favoring an upgraded version of the J-2.  Staged combustion has never been used on anything approaching the size of the F-1, so we'd be getting into uncharted territory.  Future heavy launchers are likely to make extensive use of SRBs, which are powerful and relatively cheap, but have lower specific impulse than LOX/RP-1.

The Apollo spacecraft itself used hypergolic propellants, which aren't especially efficient in terms of specific impulse.  Back when the Constellation project was being considered, the plan was to use LOX/LH2 in the lunar lander.  The Orion service module would use hypergols just as Apollo did, though LOX/methane was considered at one time.  The staging was also going to be modified.  The lander's descent stage would be used to perform lunar orbit insertion rather than the service module.  The only major maneuver that the service module would perform was transearth injection.  I don't remember if the lander was to use LOX/LH2 in both the descent and ascent stages or just the descent stage.  Regardless, this meant that somewhere between 3 km/s and 5 km/s ΔV would be switched from low-efficiency hypergols to high-efficiency LOX/LH2.  This would certainly lower the amount of propellant needed to produce the required ΔV.

I don't know exactly how the 400:1 ratio was derived (see below), but there are certainly some things that can be done with propellants and engines to be more fuel-efficient than Apollo.  However, with current technology, I think the improvements would be marginally small.
 
(ETA 1)

The entire Apollo-Saturn vehicle contained about 2700 metric tons of propellant, thus 1/400th of that is 6.75 t.  The total mass recovered at splashdown was no more than 5.5 t, which is closer to a 500:1 ratio.  More importantly, the entire 5.5 t did not "land on the moon".  The only portion of that to have actually been on the lunar surface was two astronauts, the lunar samples, and some other odds and ends.  Probably not more than about 300 kg made its way from the lunar surface back to Earth, which is more like a 9000:1 ratio.

(ETA 2)
 
OK, I just read the quote from Virtual LM and is says that "the fuel-to-payload ratio was 400 to 1, meaning that for every pound of payload required for the trip to and from the Moon it would require some 400 pounds of fuel to get the job done."  The quote doesn't specifically mention landing on the moon and is non-specific at to exactly what payload is.  If we're talking about something like an Apollo 8 mission, then 400:1 sounds about right.
« Last Edit: February 12, 2015, 04:35:19 PM by Bob B. »

Offline Allan F

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Re: Quick question about rocket engines
« Reply #2 on: February 12, 2015, 03:50:24 PM »
Switching from hypergols to cryos has some problems too - larger tanks needed since the fuel is less dense - and the need for an ignition system.
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Offline mako88sb

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Re: Quick question about rocket engines
« Reply #3 on: February 12, 2015, 04:35:52 PM »
Thanks for the help guys. Much appreciated. Lots of info that I'm not up to speed on but should make for interesting reading when I get home from work tonight. Bob, the last bit you mention in your edit, is quite an eye opener.

Just seen your second edit. I wish I had Kelly's book handy because he explained it better if I recall right. It's been awhile since I read it though so maybe I'm off about that to.

Offline Luke Pemberton

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Re: Quick question about rocket engines
« Reply #4 on: February 12, 2015, 04:44:38 PM »
Bob, the last bit you mention in your edit, is quite an eye opener.

I have to agree. I used to do Educational Outreach visits a long time ago. I was asked to help with a rocket building session so took in my model of the Saturn. I explained to the kids thay only the Command Module came back to Earth (OK, that's not quite the truth in the strictest sense). It amazed the kids as I took the model apart to demostrate each part of the missions, finally finishing with the CM sat next to all the other parts.
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Offline Bob B.

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Re: Quick question about rocket engines
« Reply #5 on: February 12, 2015, 04:58:57 PM »
Switching from hypergols to cryos has some problems too - larger tanks needed since the fuel is less dense - and the need for an ignition system.

Yes, I was going to mention that but it was starting to get to be a bit too much for one post.  A pressure-fed hypergolic system is relatively simple, compact, and reliable.  Sometimes that is more important than a high specific impulse.  I think much of it depends on how much ΔV is needed.  For instance, if all one needs is 1000 m/s, then that can be achieved using hypergols with a mass ratio of about 1.4, which isn't all that bad.  In that case I would favor using tried and true hypergols.  On the other hand, when we need a much higher ΔV and a large mass ratio, it becomes more advantageous to use more efficient propellants.


Offline Bob B.

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Re: Quick question about rocket engines
« Reply #6 on: February 12, 2015, 05:04:10 PM »
Just seen your second edit. I wish I had Kelly's book handy because he explained it better if I recall right. It's been awhile since I read it though so maybe I'm off about that to.

I bought Tom Kelly's book but I still haven't had the chance to read it.  I certainly plan to get to it.

Offline Bob B.

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Re: Quick question about rocket engines
« Reply #7 on: February 12, 2015, 05:28:33 PM »
It amazed the kids as I took the model apart to demostrate each part of the missions, finally finishing with the CM sat next to all the other parts.

It can sometimes seem incomprehensible that one F-1 engine could be considerably larger and heavier than the CM.

Offline JayUtah

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Re: Quick question about rocket engines
« Reply #8 on: February 12, 2015, 06:26:52 PM »
It can sometimes seem incomprehensible that one F-1 engine could be considerably larger and heavier than the CM.

That feeling goes away when you take apart the turbopump or try to lift the injector plate.
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Offline ka9q

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Re: Quick question about rocket engines
« Reply #9 on: February 12, 2015, 07:53:20 PM »
There haven't been any major breakthroughs in chemical rockets since Apollo but there certainly have been in avionics. The Saturn's guidance system, the IU, was about 2 tonnes. Since it sat on the S-IVB, those two tonnes came directly out of its lunar payload capacity. Today's guidance systems can be vastly smaller and lighter.

The avionics on the CSM and LM were also 1960s technology, of course, and they could be much smaller, lighter and less power-hungry today. Lower power also means the electrical power system could be lighter and smaller. Solar arrays can replace fuel cells as the primary power source, as they will on Orion. All these weight savings can be applied to more useful things.

A lot of the mass in an Apollo/Saturn stack was structure, and materials have also improved a fair bit since the 1960s. It might also be possible to redesign the stack to reduce the amount of structure needed, or to get rid of it as soon as possible. For example, the SLA that enclosed the LM was another 1200 kg that came directly out of the Saturn's useful lunar payload. The counterpart on modern launchers is the payload shroud typically dropped shortly after staging so its dead weight need not be carried all the way to orbit.

There are also system-level approaches, particularly multiple launches, to reduce the total amount of mass and fuel that need to be launched. Equipment and supplies could be sent ahead of a crew on an unmanned launch vehicle taking a slow but energy-efficient route to the moon via a Lagrange point. It would not need a launch escape system or fly a sub-optimal trajectory to maximize the chances of one working.

Offline Allan F

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Re: Quick question about rocket engines
« Reply #10 on: February 12, 2015, 08:19:50 PM »
Didn't the SLA carry the CSM? That's 40+ tonnes you need to carry at 4+ G.
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Offline ka9q

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Re: Quick question about rocket engines
« Reply #11 on: February 12, 2015, 08:34:55 PM »
Yes, and that's why it had to be carried through TLI. The Apollo/Saturn stack is probably about as good a design as there is for a single launch using the LOR mission mode.

I gave the SLA as an example of how a different mission mode might eliminate some of the structural overhead. E.g., if the LM and CM were launched separately and joined in earth orbit, the shroud protecting the LM could be dropped earlier in the ascent.  It would also allow the use of smaller launch vehicles that already exist instead of developing a new, large one that might have no other uses.

Offline JayUtah

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Re: Quick question about rocket engines
« Reply #12 on: February 12, 2015, 10:50:22 PM »
Conversely, the thin-shelled truncated cone is still the structural basis of about half the available payload adapters for the Delta family.  It's stronger than it looks.
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Offline smartcooky

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Re: Quick question about rocket engines
« Reply #13 on: February 13, 2015, 12:47:21 AM »
It seems there is no such thing as a "quick question about rocket engines" !!

Just looking through some of the information in this thread, such as the massive size of the F1 engines, and the complexity of spacecraft design and engineering, brings home something I feel is really important to keep in mind about the Apollo hoax theories and those who believe them. No matter what particular angle or aspect they are coming from, whether it is shadows on photos, van Allen belts, no stars in the lunar sky, etc, their whole expostulation boils down to one, gigantic Argument from Incredulity. They are, simply put, completely unable to understand, conceive of or imagine how it was possible to send people to walk on the Moon and return them to the Earth. Therefore they conclude that it cannot have happened. All evidence to the contrary is either handwaved away or dismissed as shillspeak. They would rather invent or believe in a massive, extremely complex, ongoing and ever expanding conspiracy to explain what they are either incapable of understanding or unwilling to try understanding.

To some extent, this makes me feel somewhat sorry for them.
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Offline ka9q

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Re: Quick question about rocket engines
« Reply #14 on: February 13, 2015, 06:55:21 AM »
Exactly my observation too.