In article ,
says...

I suppose they could still go for propulsive landing over water. At sea
and then move to a large fresh water tank on dry ground? Or if
up-welling steam is a problem maybe a high boiling point oil?

When Soyuz lands on ground, are the engines that fire at last second
roughly equivalent to Draco or Super Draco in terms of power ?

Not really. Soyuz uses solid rocket engines for its propulsive
"landing". These engines are triggered by radiation detectors which
have a corresponding radiation source. The sensors detect radiation
reflecting off of the ground (or water) as the capsule comes close to
landing. It's essentially a switch activated by radiation.

So, the system is designed to reduce the velocity at landing. It
doesn't reduce the velocity to zero for a truly "soft" landing because
that would require an active control system (otherwise it might thrust
too much, making the craft go back up a bit).

If Dragon were to land propulsively in water, are there things it could
bring back that it can't bring back now because of the impact on water?
If not, is there any point in firing engines to land?

NASA doesn't care, so it doesn't really matter.

If He2 enters tanks from the top, then as long as you are on earth,
there is no reason for NTO or MMH to flow up through a faulty backflow
valve and onto titanium tubing to the He tank.

But once that capsule travels to space, in 0G, any deceleration would
make either NTO or MMH want to flow "up" and a faulty backflow valvce
would then let it pass and move towards the He tabnk in tha titanium tubing.

This might explain why all the tests on ground had worked well, but the
one test after the capsule had flown went "kaboom".

No, SpaceX has already said the NTO got into the helium line during
ground (re)processing of the system.

Jeff
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