In article , says...

On 7/16/2019 7:41 PM, Fred J. McCall wrote:
David Spain wrote on Tue, 16 Jul 2019 13:58:46
-0400:


On Rand Simberg's blog, George Turner postulated they (titanium values)
were to allow engine restarts back in the days when Dragon V2 was
supposed to use propulsive landing. With burst disks you don't get that
capability but don't need it because Dragon V2 will use its chutes and
ocean landings only. I'd have to study it more myself to know for a fact
if that is true...


That sounds wrong to me. These engines are throttleable and it
shouldn't matter if the propellant system is pressurized. Set
throttles to zero and the engine shuts off. Open the throttles and
the hypergolics hit the combustion chamber again and the thing lights.



Yeah unclear to me as well. Why would these check valves be used for any
purpose other that to close in order to refill helium tanks between
flights?

The abort system's Super Dracos require *much* higher chamber pressure
than the Dracos, so the system is not pressurized until an abort is
initiated. Why they chose to do it this way, I'm not sure. Perhaps
NASA didn't want super high pressure hypergolic propellant tanks docked
to ISS.

For it to effect propulsive landing you have to postulate a scenario
whereby the helium gets past the propellants and is expelled out the
engine thus allowing the helium supply to get below the propellant
supply. But I don't see how the helium gets past the liquid propellant
being throttled. Physical chemistry is not my forte. Am I missing
something here?

Also what bursts the burst disks? I assume something will be used to
over pressurize the helium? Or will they burst when the helium tank
itself is pressurized? Thus the fueling operation would require
hypergolic propellant loading before helium tank pressurization where
that wasn't the case before. Correct?

See above. They burst when the hypergolic system for the Super Dracos
is pressurized, which is only when an abort is initiated.

Jeff
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