SPS failure to fire for trans-earth injection

[Everett]

Well, if the SPS on the SM malfunctioned somehow for the trans earth injection, what would be the plan?

It seems that, in my uneducated opinion, that, since the SPS and the RCS use the same propellents, a crossfeed valve between the two would enable the burn to be done with the RCS thrusters. It would take a lot longer, and the gravity losses would be higher, but it's the same amount of delta-V either way, whether it's delivered fast or slow (ignoring for the moment that the RCS thrusters have a slightly lower specific impulse, but I doubt that would make enough difference).

The big question is whether the designers of the service module actually installed such a valve? Anybody know?

[Nowhere Man]

They would have been hosed.  From Wikipedia's Apollo 8 article:

The only task left for the crew at this point was to perform the Trans-Earth Injection (TEI), which was scheduled for 2½ hours after the end of the television transmission. The TEI was the most critical burn of the flight, as any failure of the SPS to ignite would strand the crew in lunar orbit, with little hope of escape. As with the previous burn, the crew had to perform the maneuver above the far side of the Moon, out of contact with Earth.

On top of that, per WP's Service Module article, while both systems used the same oxidizer (nitrogen tetroxide), the SPS used Areozine-50 for fuel and the RCS used mono-methyl hydrazine.

Fred

[raven]

Any reason for the difference?
Hmm, looking up mono-methyl hydrazine and Aerozine-50, the former has a far lower freezing point (or melting point if you look at it from the opposite direction) and a 20 degree Celsius higher boiling point. I am guessing being out in the RCS quads mean they were more exposed to the extremes of space.
That would be my layman's guess though.

[cjameshuff]

Perhaps MMH ignites more quickly on contact with the oxidizer. For RCS, it's more important that the burns are predictable and reproducible.

[raven]

Perhaps MMH ignites more quickly on contact with the oxidizer. For RCS, it's more important that the burns are predictable and reproducible.

Perhaps, hence why I asked.

[Allan F]

The RCS system had 4 set of nozzles. Each nozzle had 445 newtons of thrust. The SPS had 91.000. That means they would have had to fire the RCS 51 times longer to get the same dV. The SPS was fired for around 203 seconds to gain around 1076 m/s of extra speed. The same dV from the RCS would take nearly 3 hours, or 1.5 orbit around the moon at 100 km. Perhaps it was possible, my orbital mechanics aren't up to the math. But the SPS was designed to be very reliable, with no ignition system and hypergolic propellants.

[Noldi400]

The RCS system had 4 set of nozzles. Each nozzle had 445 newtons of thrust. The SPS had 91.000. That means they would have had to fire the RCS 51 times longer to get the same dV. The SPS was fired for around 203 seconds to gain around 1076 m/s of extra speed. The same dV from the RCS would take nearly 3 hours, or 1.5 orbit around the moon at 100 km. Perhaps it was possible, my orbital mechanics aren't up to the math. But the SPS was designed to be very reliable, with no ignition system and hypergolic propellants.

Don't forget that, even in it were possible, the couldn't have just done a 3-hour burn.  There would only be certain points in the orbit where adding dV would put them on a trajectory toward Earth. They would probably run out of Life Support consumables long before they could accomplish anything useful.

There was a reason Mike Collins caller TEI "the get-us-home burn, the save-our-ass burn, the we-don't-want-to-be-a-permanent-moon-satellite burn".

[Everett]

Ok, looks like my source was wrong on the propellents. I guess that leads to the question of "how fast can you get another saturn V on the pad" and "how land can you make the life support last?"

As for "how long can the life support last" question...

Probably the first limiting consumable would be Lithium Hydroxide canisters for CO2 scrubbing. Those wouldn't last long. But, since there's plenty of extra oxygen, what about say, letting the CO2 build up to the maximum possible tolerable level, have the crew get in their suits, depressurize the cabin, then repressurize with fresh oxygen from the tanks. That's what they did for repeated EVA's on the LM, and it only took 3-4 pounds of O2, so it shouldn't take much more then that on the CM. They actually did depressurize the CM for EVA's on the last few missions, so it could be done. After Apollo 13, there was also an extra  O2 tank, so oxygen would be in a surplus.

Then there's power. Shutting down 2 of the 3 fuel cells is obvious, and powering down anything you can as well. Did the fuel cells produce power at fixed rate, and consume H2 and O2 at a fixed rate, or could they be set to produce less power, and consume less? Since the fuel cells produce an excess of water, I would guess that one fuel cell would be able to provide enough water for the crew to drink. Did the CSM use water for cooling electronics?
On the 'after Apollo 13' missions, the SM had a "5-10 hour battery." I guess that means enough for all power needs, so how long would it power a two-way radio, and the occasional O2 tank heater (see above)?

With food, they'll probably die of something else before they'll die of starvation. They may be emaciated, but if the other consumables can be made to last long enough, they'll be alive.

Anybody know the answer to "minimum time to get a Saturn V set up on the pad" question?"

This actually seems like an interesting research question.

[smartcooky]

Anybody know the answer to "minimum time to get a Saturn V set up on the pad" question?"

This actually seems like an interesting research question.

Even we assume there was Saturn V "ready to go", there are questions that would need answering. What is the minimum crew number to fly a CSM on a rescue mission? I think it would probably be two, which means the crew module would have to re-enter with five crew members on board; is that even possible?

[raven]

The Skylab Rescue CSM comes to mind.

[Allan F]

If they burned the RCS at the far side of the moon, for 15 minutes each orbit, they would gradually raise their orbit at the near side. Firing at the same point behind the moon would ensure maximum efficiency - If the 3 hour total was achieved, it was only 24 hours extra they had to spend in space. I believe the oxygen and other consumables were up to this - there must have been a safety margin. Once reaching the point where Earth's gravity is stronger, it's all freefall, and they could use the remaining propellant to adjust or speed up as needed.

Or is this wrong?

[Nowhere Man]

But did the RCS have the fuel & oxidizer for 3+ hours of burn, continuous or otherwise?

Fred

[Allan F]

The idea for this to work is, that the fuel/oxidizer for the SPS could be piped to the RCS. The RCS alone did not have anywhere near that amount of fuel.

[ka9q]

The orbital mechanics of lunar orbit are much more complicated than those of low earth orbit. The earth, being 81x more massive than the moon, exerts far more perturbing effect even on objects in a low lunar orbit. For example, there was a window of time during the LOI burn during which a premature shutdown would have resulted in a lunar impact after another orbit. This wouldn't happen without the earth perturbing the orbit during the nearside apocynthion after the aborted burn.

So while it might have been possible to escape lunar orbit with the RCS (presuming a way to crossfeed propellants) the burn planning would have been quite hairy.

[VQ]

If they burned the RCS at the far side of the moon, for 15 minutes each orbit, they would gradually raise their orbit at the near side. Firing at the same point behind the moon would ensure maximum efficiency - If the 3 hour total was achieved, it was only 24 hours extra they had to spend in space. I believe the oxygen and other consumables were up to this - there must have been a safety margin. Once reaching the point where Earth's gravity is stronger, it's all freefall, and they could use the remaining propellant to adjust or speed up as needed.

Or is this wrong?

The "out of the moon's gravity well, fall back to earth" thinking is unfortunately not correct. As the spacecraft raised its apoapsis, perturbation of the lunar orbit by the earth would lower its periapsis to below the moon's surface causing a crash.

Also, I would imagine the specific impulse of the RCS was significantly lower than that of the main engine so even if they were cross-plumbed, the same delta-v would not be possible. I wonder though if there were any plausible, survivable scenarios in which a partial failure of the TEI burn, resulting in a "near miss" of earth on a highly elliptical trajectory, could be corrected after half an orbit to a successful reentry by stretching consumables and adjusting course with the RCS?

Incidentally, the Apollo manned Venus flyby would have used a modified CSM that had two smaller engines rather than the large SPS. As I recall this was to save space for consumables in the launch stack for the much lengthier voyage, but it would have added redundancy as well.