SPS failure to fire for trans-earth injection

[ka9q]

N₂O₄ freezing point: -11.2 C. Boiling: +21 C. Density: 1.45
Aerozine-50 freezing point: -7 C. Boiling: +70 C. Density: 0.903
UDMH freezing point: -57 C. Boiling: +64 C. Density: 0.79
MMH freezing point: -52 C. Boiling: +91 C. Density: 0.875.
N₂H₄ freezing point: +2 C. Boiling: +114 C. Density: 1.021

As you can see, none of the hydrazines combine the lowest freezing point with the highest density, so you have to compromise. AZ-50 is one such compromise, and it's stable enough to be used in regenerative cooling while straight hydrazine is not.

MMH provides a much lower freezing point than AZ-50 at only a small decrease in density. This is probably what makes it popular in maneuvering thrusters because the tanks are small anyway. My guess is that large engines use AZ-50 because the slightly higher density gives smaller tanks, and it's easier to control the temperature of a single very large tank than a bunch of small ones on different spacecraft surfaces.
 

[ka9q]

How would the two engines save space for consumables?

Cassini carried two main engines, probably for redundancy.

[Noldi400]

How would the two engines save space for consumables?

Cassini carried two main engines, probably for redundancy.

I think the idea was to use two of the (appx) 2.5 meter tall descent engines to replace the, what, 5 meter or so tall AJ10-137.  Between this and not carrying a LM, the SLMA (instead of being jettisoned) could have a docking fitting attached to provide access to the "wet workshop" SIV-B as well as space for supplies:



Larger Image: http://upload.wikimedia.org/wikipedia/en/4/49/VenusFlybyCutaway.jpg

Part of the plan was to test the design with a year-long mission in a 25,000 mile orbit; basically a deep space Skylab, sounds like.

Sure hope they could find a compatible crew for that one...
"Where's Bob?"
"Gee, I dunno, he was here a minute ago." 

[Peter B]

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.

Why not add a short prograde burn at apoapsis to raise the periapsis? Or wouldn't there be enough fuel for all those burns?