Urine cooled heat shield. | Command module design | Forum

Hear me out on this one, I know the idea sounds strange.

We will need to either store or vent any urine that the astronaut produces during the flight. My proposal is to store the urine in a cavity between the heat shield and the bottom of the capsule's structure. During reentry the heat shield will, by design, not transfer much heat to the capsule structure but some heat transfer is bound to happen. When heat does begin to transfer the water will absorb the energy and it will be vented off as steam. 

The cavity would need to have some pressure in it prior to reentry so that it doesn't boil off in the vacuum.

I can see that ice formation will be an issue which could result in the blockage of the drain or it could force the heat shield away from the capsule. A solution might be to keep the heat shield pointed toward the Sun but I doubt that is possible for the entire duration of the mission.

Crazy idea? Probably yes.

If we're trying to reduce heat transfer, there is no better insulator than hard vacuum. Normally, that's not a very easy thing to come by, but space happens to be full of it. If transfer of heat to the capsule structure is the problem, we could perhaps have an air tight, structurally strong cavity between the heat shield and the main part of the capsule and vent it to space, then close off the opening with a valve before re-entry.

If heat dissipation, rather than transfer, is the concern, then your method might work reasonably well, though any cryogenic propellants would probably work considerably better and allowing left over compressed life support gasses to expand and escape could also work. In any case, a good ablative heat sheild ought to take care of most of the heat.