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Old July 17th 19, 12:15 PM posted to sci.space.policy
Jeff Findley[_6_]
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Default SpaceX Capsule Explosion

In article , says...

On 7/16/2019 7:03 AM, Jeff Findley wrote:

The space people I follow on Twitter are already questioning why a
titanium check valve was used in the first place (posting a link to an
old paper on the fact that NTO can cause a fire with titanium parts if
enough energy is present). I don't know how common titanium check
valves would be in aerospace NTO plumbing (some on Twitter were saying
it's common). I also don't know if this was such a big issue, why
didn't NASA oversight catch this?

Right now, I'm more confused than anything.

To correct the situation, the titanium check valves will be replaced
with 'burst valves'; essentially a pressure plate that breaks when
full helium pressure hits it, which prevents the backflow problem.
Another case of 'simpler is better'.


This sounds like a sane solution, so that NTO will never get into the
helium plumbing by mistake.


On Rand Simberg's blog, George Turner postulated they (titanium values)
were to allow engine restarts back in the days when Dragon V2 was
supposed to use propulsive landing. With burst disks you don't get that
capability but don't need it because Dragon V2 will use its chutes and
ocean landings only. I'd have to study it more myself to know for a fact
if that is true...


Good point which I saw several times on the Internet yesterday. Also,
SpaceX has an aversion to single use hardware simply because you can't
test it before flight. And, at the very least, if you use the system,
you now have parts to replace rather than just refilling the system and
going again. So SpaceX dislikes single use hardware in general.

On the other hand, a burst disk is about as simple as it can get. And
you can make several off a single sheet of metal and test several of the
ones on the sheet, leaving the ones you're going to use. So they're
about as reliable as you can get. And replacing one shouldn't be that
hard to do, but you'll need someone to check off that it was done right
(lest we have another DC-XA type failure).

So, after a day of seeing posts on this, I'm not as confused as I was
yesterday morning while drinking my first cup of coffee. ;-)

Jeff
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