Author Topic: Apollo LEM  (Read 23892 times)

Offline HarryM

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Re: Apollo LEM
« Reply #20 on: 08/05/2009 08:45 PM »
Good photo of descent stage (-) ascent stage.

http://www.myspacemuseum.com/lm7ds.jpg

Online the_other_Doug

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Re: Apollo LEM
« Reply #21 on: 08/06/2009 12:48 AM »
The ascent engine ignition on top of the descent stage was referred to as a "fire in the hole" or FITH ignition.  While it was listed as a requirement to demonstrate the FITH could be done safely and successfully before a landing could be attempted, in actuality Apollo 11 was the first flight during which an actual FITH ignition of the ascent engine was accomplished.  (It was not accomplished as planned during the unmanned Apollo 5 test, nor was it attempted during Apollos 9 or 10.)

The portion of the descent stage directly underneath the ascent engine was covered with a break-away layer of kapton/mylar thermal shield.  At ignition, the exhaust from the ascent engine would blow the thermal shield out of the way and the exhaust would pressurize the descent stage.  Portions of the descent stage base were also covered by similar blow-away thermal shields, and so most of the ascent engine exhaust flowed through the descent stage and out onto the ground below.  In several ascent films, you can see dust blowing out from below the ascending ascent stage before it rose high enough for the exhaust to clear the descent stage; this illustrates how the exhaust was vented through the descent stage.  (You can also see various pieces of discarded equipment, especially large thermal blankets, which had been tucked out of the way underneath the LM go flying out along the ground just after liftoff, in one case [Apollo 15] nearly wiping out the ALSEP central station.)

So -- to answer the main question, the descent stage was designed to allow the ascent engine plume to flow through its structure and avoid damaging back-pressure waves.  That doesn't change the fact that the FITH maneuver was never tested in flight until the point at which it was required for the first lunar lift-off.  For that matter, the LM's descent engine had never been fired for more than a couple of minutes at a time in flight prior to its first 12-plus-minute PDI burn on Apollo 11.  Engines had been tested on the ground, to be sure, but the propellants were so corrosive that every descent engine, ascent engine and CSM SPS engine that flew in space were pristine, and had never before been fired.  Ever.

Amazing, ain't it?

-the other Doug
« Last Edit: 08/06/2009 12:49 AM by the_other_Doug »
-Doug  (With my shield, not yet upon it)

Offline Jim

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Re: Apollo LEM
« Reply #22 on: 08/06/2009 12:54 AM »
Engines had been tested on the ground, to be sure, but the propellants were so corrosive that every descent engine, ascent engine and CSM SPS engine that flew in space were pristine, and had never before been fired.  Ever.

Not true.   They were all acceptance fired.   Just that either ablative nozzles or nozzle extensions were not installed.    Hypergol prop engine can and are cleaned up after acceptance firing.
« Last Edit: 08/06/2009 12:55 AM by Jim »

Offline Proponent

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Re: Apollo LEM
« Reply #23 on: 08/06/2009 01:21 AM »
Was there ever a serious thought to testing just the ascent stage of the LM?

Early in the program when the spacecraft were expected to be lighter than the turned out to be, there were plans to fly the LM ascent stage together with the CSM on a single Saturn IB . I presume that such an ascent stage, like the CSM, would have been partially fueled. Have a look at the attached documents from NTRS.

EDIT: Added second document.
« Last Edit: 08/06/2009 01:26 AM by Proponent »

Offline Proponent

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Re: Apollo LEM
« Reply #24 on: 08/06/2009 03:39 AM »
Partially fill tanks are big deal for launch integration

Why?

Offline Analyst

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Re: Apollo LEM
« Reply #25 on: 08/06/2009 06:59 AM »
I have another, probably stupid, question. Why not to use just one engine? Both engines used same fuel and descent engine was 3x more powerful, so it could lift-off just fine. Plus, they could throttle it. It would save development costs and lunar module mass.

Mass. You don't have enough prop to lift the whole descent stage. The same reason why rockets have several stages.

Analyst

Offline yinzer

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Re: Apollo LEM
« Reply #26 on: 08/06/2009 07:03 AM »
Partially fill tanks are big deal for launch integration

Why?

Slosh?
California 2008 - taking rights from people and giving rights to chickens.

Offline Jim

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Re: Apollo LEM
« Reply #27 on: 08/06/2009 12:08 PM »

Offline mrbliss

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Re: Apollo LEM
« Reply #28 on: 08/06/2009 02:53 PM »

I have another, probably stupid, question. Why not to use just one engine? Both engines used same fuel and descent engine was 3x more powerful, so it could lift-off just fine. Plus, they could throttle it. It would save development costs and lunar module mass.

Reliability

Wow, yeah.  A single engined LM would basically have no abort mode in case of engine failure during descent.  That would be scary for mission planning.  Although it would simplify the abort mode tree...

Plus, that single engine would have to successfully restart and safely run a second firing to get the astronauts off the surface.

Ouch.

Steve



Offline relyon

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Re: Apollo LEM
« Reply #29 on: 08/06/2009 04:34 PM »
Wow, yeah.  A single engined LM would basically have no abort mode in case of engine failure during descent.  That would be scary for mission planning.  Although it would simplify the abort mode tree...

An ascent engine failure carried much of the same risk.

Quote
Plus, that single engine would have to successfully restart and safely run a second firing to get the astronauts off the surface.

The CSM SPS engine had to do that for TEI.

Bob
Bob

Online the_other_Doug

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Re: Apollo LEM
« Reply #30 on: 08/06/2009 05:41 PM »
First, to Jim -- I have read in any number of books (particularly, IIRC, "Chariots for Apollo" by Pellegrino and Stoff) that the LM descent and ascent engines were never acceptance-fired, that they could not be successfully refurbished after firing.  Also, that the LM and CSM propellants, once loaded, set a "last date" by which the mission must be flown, after which the lines, gaskets, etc., in the propellant system had to be completely refurbished.  After having the hypergolics loaded, the spacecraft was only "good" on the pad for about a month before it would have to be taken down and the lines refurbished.

As for restarting engines, all of the Apollo spacecraft engines were capable of being restarted, but the APS was not cleared for restart until the program was in mid-stride.  Seems that the first-orbit rendezvous scheme that was used for Apollos 14-17 required an APS restart, and there was a fair amount of resistance to actually trying to restart it in flight.  It took a fair amount of work to convince the engineers that the APS could be used for a first-orbit TPI burn.

-the other Doug
-Doug  (With my shield, not yet upon it)

Offline Jim

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Re: Apollo LEM
« Reply #31 on: 08/06/2009 05:59 PM »
First, to Jim -- I have read in any number of books (particularly, IIRC, "Chariots for Apollo" by Pellegrino and Stoff) that the LM descent and ascent engines were never acceptance-fired, that they could not be successfully refurbished after firing. 


2.  Also, that the LM and CSM propellants, once loaded, set a "last date" by which the mission must be flown, after which the lines, gaskets, etc., in the propellant system had to be completely refurbished.  After having the hypergolics loaded, the spacecraft was only "good" on the pad for about a month before it would have to be taken down and the lines refurbished.


1.  Show me an actual quote.  I have read the same books.  That is just not done. 

2.  SOP for hypergolic stages (Delta, Agena, etc)

Offline Proponent

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Re: Apollo LEM
« Reply #32 on: 08/07/2009 01:03 AM »
...  A single engined LM would basically have no abort mode in case of engine failure during descent.

An ascent engine failure carried much of the same risk.

But the ascent engine was particularly simple and reliable, which I believe is what Jim alluded to in his one-liner "Reliability" earlier in this thread. This simplicity was possible because, unlike the descent engine, the ascent engine did not need to be throttled.

Quote
The CSM SPS engine had to do that [restart] for TEI.

Still no reason not to minimize the risk of LM failure. And the SPS was almost two engines in one in that many of its components had back-ups.

Offline rsp1202

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Re: Apollo LEM
« Reply #33 on: 08/07/2009 01:32 AM »
Not to take this too far OT, but when Martin Caiden wrote "Marooned," the plot of which was triggered by the failure of the SPS engine to light -- despite all the best efforts of ground support, vendors and the astronauts to fix it -- Wally Schirra told him there was another method to get the thing to work that Caiden had overlooked, but he was going to keep it to himself so he could remain "one up" on the author. Never did find out what that was.

Offline Jorge

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Re: Apollo LEM
« Reply #34 on: 08/07/2009 01:46 AM »
...  A single engined LM would basically have no abort mode in case of engine failure during descent.

An ascent engine failure carried much of the same risk.

But the ascent engine was particularly simple and reliable, which I believe is what Jim alluded to in his one-liner "Reliability" earlier in this thread. This simplicity was possible because, unlike the descent engine, the ascent engine did not need to be throttled.

Or gimballed.
JRF

Offline mike robel

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Re: Apollo LEM
« Reply #35 on: 08/07/2009 03:11 AM »
Not to take this too far OT, but when Martin Caiden wrote "Marooned," the plot of which was triggered by the failure of the SPS engine to light -- despite all the best efforts of ground support, vendors and the astronauts to fix it -- Wally Schirra told him there was another method to get the thing to work that Caiden had overlooked, but he was going to keep it to himself so he could remain "one up" on the author. Never did find out what that was.

Actually, I think that quote refers to the 1st edition version of Marooned which involved Mercury, Gemini, and Vostok.  I will have to dig the book out and check.  Although it might be that the quote was in the 2nd books forward, refering to the 1st book.

Offline rsp1202

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Re: Apollo LEM
« Reply #36 on: 08/07/2009 03:43 AM »
I hope you can find it and that I remembered it correctly. :) That would have to be where I saw it, but it's been years and yet it's stuck in my mind all this time. I also have to imagine that Schirra finally came clean, but have never heard the answer.

Online the_other_Doug

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Re: Apollo LEM
« Reply #37 on: 08/11/2009 01:32 AM »
Jim, as far as whether Apollo hypergol engines were fired prior to flight, I don't have the time to go hunting through my library at the moment.  I have run across this reference in more than one source, though.  If it's not true, then multiple people documenting the Apollo era have the fact wrong.

Do you (or Blackstar, or anyone else here) have any documentation of acceptance testing of Apollo flight DPS, APS or SPS engines?  That would also settle the question... all I have to go on are multiple reports in mainstream Apollo books of a "fact" that some seminal source may have gotten wrong.

It is true, though, that no LM DPS engine was ever fired *in flight* for its rated 12-plus-minute PDI lifetime prior to Apollo 11.  I'm sure many engines were fired through PDI profiles in ground testing, but in flight?  LM-5's DPS was the first.

-the other Doug
-Doug  (With my shield, not yet upon it)

Online the_other_Doug

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Re: Apollo LEM
« Reply #38 on: 08/11/2009 01:43 AM »
...  A single engined LM would basically have no abort mode in case of engine failure during descent.

An ascent engine failure carried much of the same risk.

But the ascent engine was particularly simple and reliable, which I believe is what Jim alluded to in his one-liner "Reliability" earlier in this thread. This simplicity was possible because, unlike the descent engine, the ascent engine did not need to be throttled.

Or gimballed.

The LM APS was as simple and reliable as it could possibly be made.  It used pressure-fed propellants (no pumps), and there were a good half-dozen ways you could ignite the engine from the control panel, the PGNCS and/or the AGS.  And if all else failed, a piece of the ascent engine cover was removable and a special tool included in the LM manifest on each flight could be used to manually open the propellant feed lines and thus start the engine.

I also seem to recall there was an emergency procedure for using a PLSS battery to activate the APS and/or the guillotines that separated the ascent and descent stages at liftoff.  The idea was that, even if you drastically lost electrical power in the LM, you had a fighting chance of getting off the Moon and back into an orbit from which the CSM could rescue you.  (I realize you'd also have to have an operational RCS to maintain control during ascent, but there were a lot of ways to make the RCS fire, too...)

-the other Doug
-Doug  (With my shield, not yet upon it)

Offline Jim

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Re: Apollo LEM
« Reply #39 on: 08/11/2009 02:13 AM »
Jim, as far as whether Apollo hypergol engines were fired prior to flight, I don't have the time to go hunting through my library at the moment.  I have run across this reference in more than one source, though.  If it's not true, then multiple people documenting the Apollo era have the fact wrong.

Do you (or Blackstar, or anyone else here) have any documentation of acceptance testing of Apollo flight DPS, APS or SPS engines?  That would also settle the question... all I have to go on are multiple reports in mainstream Apollo books of a "fact" that some seminal source may have gotten wrong.

It is true, though, that no LM DPS engine was ever fired *in flight* for its rated 12-plus-minute PDI lifetime prior to Apollo 11.  I'm sure many engines were fired through PDI profiles in ground testing, but in flight?  LM-5's DPS was the first.

-the other Doug

All engines are acceptance tested, especially manrated ones.

The SPS, DPS and APS weren't acceptance tested, but the engines were.

The LMDE

http://cbtsresidents.com/Spectrum/donharvey1.htm

http://pdf.aiaa.org/preview/CDReadyMJPC06_1178/PV2006_5220.pdf


Ascent engine

http://www.astronautix.com/details/fir17720.htm

SPS engine

http://books.google.com/books?id=y4jik85mL8EC&pg=PA157&lpg=PA157&dq=sps+engine+acceptance&source=bl&ots=JMIofOcJlm&sig=Bpbrz_tKqqZd5TvDza4RgXzPE_g&hl=en&ei=0NWAStqfKNK3twfE16DNCg&sa=X&oi=book_result&ct=result&resnum=7#v=onepage&q=sps%20engine%20acceptance&f=false
« Last Edit: 08/11/2009 02:25 AM by Jim »

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