[AR] Re: SpaceX failure update
- From: Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Sat, 29 Oct 2016 09:43:16 -0700
On 10/28/2016 10:50 PM, David Weinshenker wrote:
On 10/28/2016 03:35 PM, Ian Woollard wrote:
Looks like they've got trouble all right. There's been an update as of
October 28, 4:00pm EDT:
http://www.spacex.com/news/2016/09/01/anomaly-updates
"The investigation team has made significant progress on the fault tree.
Previously, we announced the investigation was focusing on a breach in
the cryogenic helium system of the second stage liquid oxygen tank. The
root cause of the breach has not yet been confirmed, but attention has
continued to narrow to one of the three composite overwrapped pressure
vessels (COPVs) inside the LOX tank. Through extensive testing in Texas,
SpaceX has shown that it can re-create a COPV failure entirely through
helium loading conditions. These conditions are mainly affected by the
temperature and pressure of the helium being loaded."
Apparently Armadillo had similar problems back in 2009 (fortunately
with externally mounted helium tanks) - tank went "pop" and sprung
a leak during pressurization; subsequent examination showed heat
damage to the tank wall.
http://spacefellowship.com/news/art13045/armadillo-aerospace-2009-lunar-lander-challenge-level-2-done-.html
You'd think that in the SpaceX case having the helium tank submerged in
LOX would tend to keep the wall cool despite compression-heating of the
incoming helium, though...
Assuming the obvious case, overheating of the COPV He bottle via
too-rapidly compressing too-warm He into it,
1) they may have overheated the inside of the COPV bottle despite it
being immersed in LOX, as there would be some thermal gradient across
the bottle's wall thickness. Heat the inner wall rapidly enough and
there might not have been enough conductivity to prevent overheating.
2) they may have begun filling the COPV He bottles before they were
immersed in LOX, in a parallel-filling process.
Or, possibly a combination of the two.
Far more intriguing is the other possibility that Elon suggested early
on: That LOX freezing in the COPV bottle overwrap led to mechanical
failure as the bottle stretched under increasing pressure.
This would require some combination of expansion-cooling of He as it was
released from high-pressure bulk storage into the at-start low-pressure
bottle, possibly with help from the non-intuitive thermal properties of
He under certain conditions someone mentioned here recently.
Speculative: if they filled the bottles in sequence (perhaps
overlapping) rather than in parallel, the last bottle filled might not
have benefited much from ambient heat in the fill plumbing countering
expansion cooling - that could have been largely used up already.
How much would He cool down from ambient temperature when expanded from,
say, 6000 psi bulk storage to 14.7 psi? Anyone who knows how to run
that number care to? Overnight low at Canaveral was 73F that day, high
87, and the explosion was at 9 am - 80F is probably a reasonable guess
at the bulk temp of the stored He.
Henry V
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