[AR] Re: SpaceX failure update
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Sat, 15 Oct 2016 00:54:49 -0400 (EDT)
On Sat, 15 Oct 2016, John Dom wrote:
So WRONG voltage *was* the cause.
No, it was *part* of the ultimate cause -- another part was a handling
accident that had damaged the tank slightly, misaligning a dip tube that
then didn't function as it should, which was the only reason anybody tried
to empty that tank using the heaters in the first place. Still another
part was that none of the guys doing the emptying knew that the tank
temperature gauge could read only up to 85deg.
Note that the wrong voltage was used only on the ground. The flight power
system supplied the proper voltage. (Which didn't matter, since the
thermostat was never called on to open in flight.)
The partly incinerated PTFE (is not equal to PTFE) ...
Actually, even normal PTFE will ignite in supercritical oxygen -- they
tried that during the investigation, and were startled at how well it
burned. The deterioration from overheating just made it easier, and
(probably) messed up the insulation enough that two of the wires could
touch and cause a spark. (That wasn't the only possible source of a
spark, as it turns out -- the tank-stir motors were unreliable and known
for electrical shorts, and there were sharp edges in places on the in-tank
hardware that could have cut through wiring insulation.)
... ignited as a result, not as the cause of the overvoltage.
The burning of the degraded Teflon was the *immediate* cause of the
accident. (Immediate and ultimate causes are both important.) None of
the other stuff would have mattered if the Teflon hadn't caught fire.
Henry
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