[AR] Re: What blew up Crew Dragon...
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: arocket <arocket@xxxxxxxxxxxxx>
- Date: Mon, 15 Jul 2019 22:56:54 -0400 (EDT)
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
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