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Old July 18th 19, 01:39 PM posted to sci.space.policy
David Spain
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Default SpaceX Capsule Explosion

On 7/16/2019 7:33 PM, Fred J. McCall wrote:

One story I read indicated (by an ex-SpaceX engineer) that it was for
reusability, as burst valves would have to be replaced (with some
difficulty) after any pressurization of the escape system. Some
reports also made it sound as if the titanium parts were only used on
the fuel side and if there had been no backflow everything would have

^^^^^^^^^^^^^

been fine (and we know they've successfully done this before). This
is sort of supported by reported SpaceX comments that they had no
reason to suspect this could happen.


Fred just a nit, but I think you meant to write on the helium side...

As for back flow: here's an excerpt from Henry (Spencer) from the
Arocket mailing list on how that could happen and a possible fix for a
reusable system with pressurization shut off between uses:

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


Dave