On 7/15/2019 10:03 PM, Henry Vanderbilt wrote:
On 7/15/2019 7:56 PM, Henry Spencer wrote:Note that the propellants don't have to condense as liquids to do bad things. Vapor-phase reaction of hydrazine and nitrogen tetroxide produces a solid residue that precipitates on the nearest convenient surface. Which will very often be just inside one of your check valves, where the leaking vapor of one component has its first opportunity to meet a pre-existing saturated vapor of the other. The precipitate is a detonable high explosive, but you'll probably get a boring failure before you ever see that exciting one - because the precipitate is also either a gummy or hard crystalline solid depending on e.g. how methylated your hydrazine was, but either one in the working parts of a check valve will tend to make the valve stick. If the check valve sticks closed, you probably can't pressurize your feed system and you can't run your engines properly.
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 one time I was involved in an incident investigation where fuel and oxidizer had met illicitly and noisily, I ended up spending some considerable time testing out that migrate-as-vapor-then-condense-in-a-bad-place possibility. FWIW, in the real world it's quite difficult to make that happen.
Check valves are prone enough to just flat-out sticking and leaking fluids that more exotic explanations aren't often needed.
