On 7/15/2019 7:56 PM, Henry Spencer wrote:
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.
