We all know the 2 stages LM design used by Grumman was intended to discard the mass of the landing gear (+ other components) at the moment of launching off the Moon surface to reach back the Service module. But was it really necessarily for the LM to have two stages? The reason I wonder is that, when Armstrong landed, there was fuel left for about 25 seconds - however, this was actually 25 s before aborting the landing, not before running out of fuel. After these 25 seconds of burning fuel, the complete LM still had enough fuel to ascend with both of its stages right back to the Service module. In other words, the LM was designed to be able to take off from the Moon surface with BOTH stages, even right after touching the surface, in case something would have gone wrong. Then, why did it use two stages? It surely added complexity, weight and a second engine.

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    $\begingroup$ Do you have a reference for the ascent stage having enough fuel to return to orbit? Every source I have seen talks about the criticality of the ascent stage working because there were no other options. including sub optimal performance choices for better reliability and design of this en.wikipedia.org/wiki/Lunar_escape_systems. AFAIK the abort at 25 seconds involved firing the separation bolts and dumping the descent stage. $\endgroup$ – GremlinWranger Apr 28 '19 at 2:23
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    $\begingroup$ related ahttps://space.stackexchange.com/questions/2493/how-was-reserve-fuel-calculated-for-the-apollo-missions/30208#30208. Looks like descent module was designed to land with about 1.8% of the fuel it started out with. $\endgroup$ – GremlinWranger Apr 28 '19 at 2:37
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    $\begingroup$ See this previous question for clarity about LEM abort modes. space.stackexchange.com/questions/21686 There’s more than one, but none of them get back to orbit on descent stage engine only $\endgroup$ – Bob Jacobsen Apr 28 '19 at 2:58
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    $\begingroup$ "So aborting the landing would have meant going back to the service module with the LM in its complete configuration." -- why do you think that? $\endgroup$ – Russell Borogove Apr 28 '19 at 3:46
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    $\begingroup$ I've tried this in KPS; combining the LEM and CSM, losing the escape system and the need for any rendezvous. It was a +300 ton 500 billion dollar cake dream of imaginary structural engineering that should not have survived reentry. $\endgroup$ – Mazura Apr 28 '19 at 22:57

After these 25 second would have ended, the LM still had enough fuel to ascend with both of its stages right back to the Service module. In other words, the LM was designed to be able to take off from the Moon surface with BOTH stages, even right after touching the surface, in case something would have gone wrong. Then, why using two stages which surely added complexity, weight and a second engine?

Your assumption is not correct. Aborting from the "bingo" (low fuel) call would have required the ascent stage to be used. The stages can be separated, and the ascent engine fired, while in flight; this was demonstrated on Apollo 9 and Apollo 10.

Because there would be a brief delay between staging and the ascent stage coming up to full thrust, the safest way to abort in this case would be to take the descent stage to full thrust to gain altitude and vertical speed, then stage and activate the ascent stage engine once the descent stage fuel was exhausted.

The ascent from lunar surface to rendezvous orbit took about 7 minutes on the ascent stage; there was nowhere near enough fuel in the descent stage to do that.

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    $\begingroup$ Thanks, I've now read a little about the Abort Stage sequence which describes this procedure, with the descent stage being jettisoned near the Moon surface, then ascent stage would ignite while in flight (surely very challenging) and ascent back. Very interesting. The Apollo 10 did just that - only from much high above Moon's surface. Also interesting there was a 'dead man's zone' at low altitude where this procedure would have not been possible - so there were only land or crash alternatives at such altitude. Very interesting, if someone has links with this procedure please share. $\endgroup$ – Mathias Apr 28 '19 at 9:32
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    $\begingroup$ In comparison, it seems the Russian lunar module, the LK, used a single engine/fuel tanks configuration for descent and ascent, while still leaving on the Moon surface the landing gear assembly. Basically, it was a single stage module design. Perhaps a simpler design, compared with the LM who had dual engines/tanks and systems, one for each of the two stages. en.wikipedia.org/wiki/LK_(spacecraft) $\endgroup$ – Mathias Apr 28 '19 at 10:17
  • $\begingroup$ @Mathias: The LK design requires the ascent to carry all the batteries, oxygen and water tanks, propellant tankage needed only for descent, etc. This is OK for short lunar stays, but it wouldn't scale well to the 3-day stays of the later Apollo flights. Being able to leave behind equipment needed to support the surface stay was a huge advantage for the two-stage LM. (Note that LK did have two engines -- one was a pure redundant backup.) $\endgroup$ – Russell Borogove Apr 28 '19 at 14:50
  • $\begingroup$ Indeed, there were major differences for the mission profiles between the LK and LM, plus the LK was designed to carry very little scientific experiments on the Moon surface. Thanks for the great explanations Russel. $\endgroup$ – Mathias Apr 28 '19 at 18:08
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    $\begingroup$ And even if it was possible to get back with both stages, there would have been no way for the descent stage to survive re-entry. So there would have been no point in doing so. $\endgroup$ – vsz Apr 30 '19 at 20:16

Early conceptual designs suggested that two stages would save weight. Another issue came up that made a single-stage lander unreliable.

Pressurizing the fuel and oxidizer tanks of the lunar module was a considerable engineering challenge. Because of the temperature changes that happen during launch and Earth orbit, the tanks were kept unpressurized until right before they were needed. At that time, the astronauts would fire some explosive valves (i.e. you can open them but never close them again) which would release a small amount of supercritical helium to pressurize the tanks.

Development and testing of the LM pressurization system took 6 years and had a lot of problems. The fuel froze, so they added a heat exchanger. A test article exploded, so they had to change the alloy used in the heat exchanger. The pressure regulator cracked. You were supposed to be able to raise or lower the thrust of the descent engine; the pressurization system wasn't compatible with that, so there was another re-design. Another heat exchanger was added. The descent engine shut down prematurely on Apollo 5. It was under-pressured on Apollo 9. When Apollo 11 dumped their excess descent fuel, the fuel lines froze and over-pressurized.

Suffice to say, once you've popped the valves on the LM pressurization system, you have a few hours to use the engine before it becomes unreliable. This was enough time to land on the moon, but not reliable enough to last several days on the moon. A one-stage craft is just too risky. Instead, the ascent stage had its own fuel and pressurization system. (Apollo 13 took the risk of reusing the descent engine, and was lucky.)

Update: The existance of time limits on pressurization of the LM propulsion systems is confirmed by the Apollo Operations Handbook: Lunar Module vol. 1, section 2.8.6:

  • The ASCENT He PRESS switch should not be actuated longer than 24 hours before termination of ascent engine operation. The ascent pressurization valves are designed to operate for only 24 hours after exposure to propellant vapors. Ex­ceeding this limit may cause ascent valve failure.

  • The DES START He PRESS and DES PRPLNT ISOL VLV switches should not be actuated longer than 3.5 days before termination of descent engine operation. The descent pressurization valves are designed to operate for only 3.5 days after exposure to propellant vapors. Exceeding this limit may cause descent valve failure.

The Soviets designed a manned lunar lander, the LK. To save weight, it would have had a single engine for descent and ascent. However, it still would have left some parts behind on the moon.

Soviet LK

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    $\begingroup$ These are many interesting technical details. We don't have much information on the LM design and testing challenges, but I'm sure there was some unprecedented engineering efforts put into this space ship. On the risk factor, you are suggesting that one of the reasons for a two stages LM was reducing the risk of failure (increasing reliability). From an engineering point of view, I feel that adding complementary instead of redundancy systems, actually increases the odds of system and consequently mission failure. Of course, only my suppositions. $\endgroup$ – Mathias Apr 28 '19 at 9:09
  • $\begingroup$ Also worth adding that the Russian lunar module (acronym-ed NK) was initially designed to carry only one astronaut, at 1/3 the LM total weight. Several NK were built, there are 5 still existing. There are several very interesting design/concept differences between the NK and the LM, on the Wiki page at the bottom en.wikipedia.org/wiki/LK_(spacecraft) $\endgroup$ – Mathias Apr 28 '19 at 9:46
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    $\begingroup$ Worth mentioning? Apollo/Saturn only had two engines for which mission profiles required restarts: the Saturn 3rd stage (S-IVB) and the Apollo SM. The 3rd stage fired initially to complete LEO insertion, fired again for TLI. The SM engine fired for lunar orbit insertion, again to depart lunar orbit for Earth return, but also fired en-route to/from the Moon for course corrections. Among numerous technical challenges would have been: short-term storability of the 3rd stage LH/LOX propellants, an ignition system to effect 3rd stage restart, and ullage for all those zero-g starts. $\endgroup$ – Anthony X Apr 28 '19 at 16:11
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    $\begingroup$ @AnthonyX LM descent engine also fired twice normally — a 30-second burn (“descent orbit insertion”) to lower pericynthion, then the ~12 minute powered descent burn beginning almost an hour later. $\endgroup$ – Russell Borogove Apr 28 '19 at 18:31
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    $\begingroup$ @Hohmannfan It actually was done that way, sort of, on the later Apollo missions, to accommodate the weight of the extra equipment like the Lunar Roving Vehicle. The CSM would burn the first descent orbit insertion burn, the LM would separate, and the CSM would then recircularize. The CSM had way more capability than it really needed for the moon landings, because it had been designed for direct ascent. $\endgroup$ – Russell Borogove Apr 29 '19 at 2:10

You all seem to have far greater scientific background than I, so please forgive my input if it's too basic. My father was one of the people responsible for the electrical systems (life support), and from what I remember him telling me the reason for the two stage system really came down to two things: minimizing weight on ascension (meaning less fuel was needed) and the fact that there were just too many uncertainties about what they would find out about the moons surface once they got there. They needed to account for excessive sinking/tilt whatever. The best way to overcome this was to bring your own launch platform with you that could compensate for this. Finally it was the simple fact that that whole landing system was not needed after liftoff. Jettisoning almost half the mass of the ship gave them the extra room for ascent and rondevius. As a personal observation I would wonder if the top of the ship would have had the integrity to survive liftoff with the gear still attached. Those walls were paper thin.

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    $\begingroup$ If it could survive landing, it could survive take off. $\endgroup$ – user20636 Apr 30 '19 at 15:50

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