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I see in the accounts of the Apollo 13 mission that the descent stage engine of the LM were used to propel the joined spacecraft into a return trajectory to earth and that the crew remained in the LM ascent stage until shortly before reentry and return to Earth.

Here’s my question. Can we assume that the descent stage remained attached to ‘Aquarius’ all the way and so was burned up on reentry to the atmosphere with the ascent stage?

I have not seen any report indicating the descent stage was jettisoned after the engines of the descent stage were fired and its fuel supply exhausted.

Why would it not be jettisoned, if it wasn’t? It had served its purpose and would be in the way were there need to use the ascent stage engine. It would be dead weight.

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    $\begingroup$ Dead weight isn't a problem if you're not manoeuvring. $\endgroup$ – JCRM Feb 13 at 19:05
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    $\begingroup$ In fact, if your intent is to not maneuver, keeping dead weight is ideal. Remember that the whole problem started with a pressure tank failing. Such an event is basically an unintended burn.The more weight you have, the smaller your speed change will be due to such an event; a=F/m. $\endgroup$ – MSalters Feb 14 at 15:33
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    $\begingroup$ The headline question is different to the question in the last paragraph. The latter seems to be the one that people have focused on. What is your actual, single question? $\endgroup$ – Dave Gremlin Feb 14 at 23:06
  • $\begingroup$ Doesn't jettisoning anything from something the has already been accelerated actually mean wasting perfectly good momentum (unless momentum is exactly what you don't want anymore)? $\endgroup$ – rackandboneman Feb 15 at 14:10
  • $\begingroup$ @rackandboneman When you jettison you get a shove in the opposite direction. So if you point your ship right you get a little help, not much but every little helps. $\endgroup$ – Diego Sánchez Feb 15 at 19:19
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Lets have a look into Apollo By The Numbers:

enter image description here

There was a lot of unused fuel (Aerozin 50) and oxidizer (nitrogen tetroxide) remaining in the descent stage of the Apollo 13 LM, 55 %. No need to use the fuel in the ascent stage. The LM fuel was not used for a single burn, five burns were neccessary, the last about five hours before splash down.

enter image description here

DPS– Descent Propulsion System, RCS– Reaction Control System. From Apollo 13 Guidance, Navigation, and Control Challenges Table 1.

But the other supplies of oxygen, water and battery power were much bigger in the descent stage than in the ascent stage. It was therefore neccessary to keep the descent stage as long as possible, not only for fuel.

There was a big tank for 48 pounds of oxygen at 3000 psi in the descent stage but only two small tanks, each with 2.4 pounds of oxygen at 900 psi in the ascent stage. 90 % of the available oxygen for breathing was stored in the descent stage. See the APOLLO EXPERIENCE REPORT - LUNAR MODULE ENVIRONMENTAL CONTROL SUBSYSTEM on page 8.

The LM was built for only a 45-hour lifetime, and it needed to be stretch to 90. Oxygen wasn't a problem. The full LM descent tank alone would suffice, and in addition, there were two ascent-engine oxygen tanks, and two backpacks whose oxygen supply would never be used on the lunar surface. Two emergency bottles on top of those packs had six or seven pounds each in them. (At LM jettison, just before reentry, 28.5 pounds of oxygen remained, more than half of what was available after the explosion)

Six batteries in the LM delivered electrical power, four in the descent stage with 400 Ah each and two in the ascent stage with 296 Ah each. So 73 % of the available battery energy was located in the descent stage. As long as the descent stage was present, all six batteries could be switched to both DC buses individually or disconnected as neccessary.

All non-critical systems were turned off and energy consumption was reduced to a fifth of normal, which resulted in having 20 percent of our LM electrical power left when Aquarius was jettisoned.

The LM water was stored in a large descent stage tank (332 pounds) and in two small ascent stage tanks (42 pounds each), see page 5 of the report. The water was not only needed for drinking and preparation of dehydrated food, most of it was needed to cool the electronics. Even the batteries in ascent and descent stage were water cooled to remove excess heat. The water was evaporated in the sublimators to remove excess heat from the coolant loops.

The crew conserved water. They cut down to six ounces each per day, a fifth of normal intake, and used fruit juices; they ate hot dogs and other wet-pack foods when they ate at all. The crew became dehydrated throughout the flight and set a record that stood up throughout Apollo: Lovell lost fourteen pounds, and the crew lost a total of 31.5 pounds, nearly 50 percent more than any other crew. Those stringent measures resulted in the crew finishing with 28.2 pounds of water, about 9 percent of the total.

Blockquotes from this NASA page.

The descent stage with all its supplies was not at all dead weight.

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    $\begingroup$ @JCRM The LM used hypergolic fuel, no cryogenic oxygen. But oxygen was needed for breathing by the astronauts. $\endgroup$ – Uwe Feb 13 at 20:07
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    $\begingroup$ @JCRM I believe the point of listing the propellant information was to convey that there was more than enough fuel left in the Descent Stage, so the OP's assertions that the fuel supply was exhausted, and that the Ascent Stage Engine might have been needed, were flawed. $\endgroup$ – Kamil Drakari Feb 14 at 14:19
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Regardless of consumables, the main concern with the descent module was the RTG.

Each Apollo LM carried a small nuclear device containing nearly 4 Kg of plutonium that was to be left on the moon, a compact nuclear generator that would power the experiments left on the moon for years. See PAGE 67 of the original press release at NASA

The reentry of Apollo 13 was timed so that any surviving parts of the descent module of Aquarius ended up in the Tonga trench in the southern Pacific Ocean. The plutonium casket was designed to survive re-entry and was indeed confirmed to splash down at the expected location.

Nasa's own FAQ on RPS mentions this in their "Have there been any previous failures with space nuclear systems?" section

Modified mission plan mentions LM Jettison at 141h into the mission, even after the SM

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  • $\begingroup$ What was this nuclear device? $\endgroup$ – Thorbjørn Ravn Andersen Feb 14 at 7:13
  • $\begingroup$ @ThorbjørnRavnAndersen: The RTG, as the answer says. While some links for background info would indeed be handy, just Googling for apollo rtg gives plenty of results, including this magazine article, this page from nasa.gov and even this Q&A right here on space.SE. $\endgroup$ – Ilmari Karonen Feb 14 at 8:17
  • $\begingroup$ looks just great! $\endgroup$ – uhoh Feb 14 at 9:02
  • $\begingroup$ I never realised the Apollo missions had RTGs, somehow I always assumed they were a more recent development. TIL. $\endgroup$ – Ruadhan2300 Feb 14 at 9:27
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    $\begingroup$ @Ruadhan2300 the core 'generation of electricity by heat difference of metal junctions' goes back to the 1800s and was an option for powering things like radios in the narrow time interval between household piped gas and domestic mains power. RTG is certainly an ideal use case for the concept though. $\endgroup$ – GremlinWranger Feb 14 at 12:11
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Aquarius' ascent and descent stages stayed together until reentry.

As described in this QA, the descent stage carried the majority of battery power, oxygen, and water supply. Battery power in particular was the limiting factor for Apollo 13's survival, so the descent stage had to be retained until the very last moment.

As Uwe notes, over half the descent stage fuel remained (using more could have shortened the return flight further, but would have landed the command module in the wrong ocean), so there was no need for the ascent stage engine.

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    $\begingroup$ "shortened the flight further" ? $\endgroup$ – JCRM Feb 14 at 16:00
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    $\begingroup$ @JCRM: There was a solution for taking a whole day and a half of the return trip by using all the fuel in the descent stage and half the ascent stage. With the time cut, the ascent stage resources would have lasted. They didn't do it because they didn't want to expose the heatshield to the cold. $\endgroup$ – Joshua Feb 14 at 16:29

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