Wouldn't that militate against rapid turnaround, though? That's probably
why they didn't want to use burst disks (even though they'd given up on
rapid turnaround for Crew Dragon, from the standpoint of NASA
requirements...)
On 2019-07-15 19:56, Henry Spencer wrote:
On Mon, 15 Jul 2019, Paul Breed wrote:
How big is the penalty for completely separate pressurization systems.....
By the sounds of it, this wasn't a case of oxidizer-fuel mixing --
just a slug of liquid in part of the plumbing where only gas was
expected, leading to severe water hammer when that section got
pressurized suddenly.
The problem is that check valves don't reliably block slow reverse
flow of *gas*, and so a volatile propellant can seep up past the check
valve and condense in colder plumbing upstream. This is a known
problem, and has been for decades! In the case of N2O4, such seepage
can also corrode upstream components. (This is almost certainly what
really happened to Mars Observer, whose helium pressure regulators
were *not* rated for N2O4 exposure -- when the pressurization system
was activated, the corroded regulators failed to control the helium
flow, and the propellant tanks burst. Once this possibility was
noticed, the regulator failure was successfully duplicated in the
lab.) So just taking it slow on the pressurization is not sufficient.
The fix is, *don't* rely on check valves to block volatile liquids
from getting up into the pressurization system(s). For one-shot
systems, burst disks will do. For multi-burn systems where you want
to turn off active pressurization between uses, use actuated shutoff
valves to positively, hermetically close the pressurization path.
Henry
