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When Apollo 13 had most of its internal systems destroyed by an explosion, NASA decided that the best course of action would be to fire the main engine on the dark side of the moon to send the capsule back to Earth.

However, many sources mention that there was great apprehension as to whether the engine would successfully start and sustain a burn. From what I can tell, this apprehension was tied to the low supply of electrical power.

What about the engine made the engineers distrust it so? Doesn't an engine keep burning until you shut off it's fuel, no electrical power required?

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    $\begingroup$ Without electrical power, it was not possible to start and control the engine. To avoid rotation of the spaceship, the thrust direction should be continously controlled. A certain time and direction of the thrust should be achieved. $\endgroup$
    – Uwe
    Mar 2, 2017 at 9:44
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    $\begingroup$ Would you trust an engine mounted on a part of your spacecraft that's been damaged by an explosion severe enough to cause multiple systems failures? $\endgroup$
    – Vikki
    Mar 29, 2020 at 16:45

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I think you are incorrect on your question. There were two main burns done using the Lunar Module descent engine. One was before reaching the moon, to position them into a free return trajectory, the other was about 2 hours after passing the moon, to allow them to land in the Atlantic Ocean instead of the Indian Ocean, a difference of about 19 hours.

From the Apollo 13 Mission Report the reason they couldn't initiate a immediate turn around using the Service Module propulsion system was because they would need to jettison the Lunar Module. It was determined that the Lunar Module was necessary to serve as a life boat for the astronauts, and a direct abort was not an option. (page 23)

Using the SPS was only an option with the removal of the Lunar Module (page 43)

It was obvious that we were committed to going around the moon rather than performing a direct abort because the large [vector deviation] could have been supplied by the SPS only if the LM were jettisoned, but that was out of the question.

Because the damage to the Service Module was uncertain, all systems relating to the Service Module, including the propulsion system, was determined to be unreliable and only to be used as a last resort. pages 40 and 49

At this time the spacecraft was on a non-free-return trajectory with a 62 n.m. pericynthion. The crew had an abort pad onboard which required an SPS burn of 6079 fps at 60+00 to land in the Pacific Ocean at 118 hours.

At 56+48, the Flight Dynamics team was advised to initiate all of the return-to-earth planning based on going around the moon, and assuming use of the LM descent engine and LM RCS, and that we would not use the SPS except as a last ditch effort.

Perform a PC+2 hour abort burn with the Service Module Propulsion System for a landing at 118+00 in the Pacific Ocean. This option was rejected unanimously as being too risky since no data was available as to the structural integrity of the Service Module. This option was put into the category of "last ditch".

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  • $\begingroup$ The Apollo 13 Mission Report is in fact incorrect, although this wouldn't become generally known until this century. If the power to ignite the SPS engine could have been found (the fuel pumps consume power), direct abort was possible by burning the SPS engine until fuel exhaustion, jettisoning the now useless service module, and finishing the direct abort burn with the DPS engine. $\endgroup$
    – Joshua
    Jul 7, 2018 at 16:10
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    $\begingroup$ @Joshua: there were no fuel pumps, pressure feed was used. Igniting the engine was not necessary due to the hypergolic fuel/oxidzer combination used. Electric power was necessary to open the propellants valves. $\endgroup$
    – Uwe
    Oct 31, 2018 at 10:59
  • $\begingroup$ @Uwe: You need electrical power to pump fuel from the storage tanks to the sump tanks or you can only use half your fuel. $\endgroup$
    – Joshua
    Oct 31, 2018 at 16:46
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    $\begingroup$ I agree there are sump tanks, but haven't found any reference to pumps. wikipedia says "The propellants were pressure-fed to the (SPS) engine" $\endgroup$
    – user8269
    Jan 5, 2019 at 2:06
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    $\begingroup$ @Joshua, discarding the service module early was a really last-resort option. They wanted to keep it attached as long as possible to protect the heat shield from space debris and temperature fluctuations. $\endgroup$
    – Mark
    May 2, 2019 at 1:45

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