Dragon 2 Draco OMS/RCS thrusters were stated as running at a lower
pressure. Implicitly, also pressurized at different times than the
SuperDraco escape thrusters. Assuming regulators, then definitely
separate regulators and separate systems downstream of the regulators.
Upstream, no telling whether the high pressure He storage is common or not.
Though, speculative, if the SuperDraco system is treated as a one-shot,
you could save a regulator by just sizing a separate He bottle to do the
job once. Or use pressure sensors plus a fast valve to regulate bang-bang.
Henry
On 7/15/2019 8:28 PM, George Herbert wrote:
It's only for the one shot escape system, I think. I thought the OMS/RCS were separately pressurized?...
On Mon, Jul 15, 2019 at 8:05 PM Rand Simberg <simberg@xxxxxxxxxxxxxxx <mailto:simberg@xxxxxxxxxxxxxxx>> wrote:
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
--
-george william herbert
george.herbert@xxxxxxxxx <mailto:george.herbert@xxxxxxxxx>
