[AR] Re: J-2 igniter failures on Apollo 6 \ was: clusters and reliability
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sat, 23 Jan 2016 19:13:51 -0500 (EST)
On Sat, 23 Jan 2016, David Weinshenker wrote:
... the cited report, dated 25 June 1968, just says that fixes were
being incorporated, not many months before the Apollo 7 flight in
October (same S-IVB with a J-2), and not that many months more until
December's Apollo 8 (S-II and S-IVB...). Don't overthink it, they seem
to be saying, just fix it and carry on.
So what was the actual problem with the igniter fuel line, and how
was it eventually fixed? (It sounds like it must have been a rather
straightforward update that didn't require more than a localized
redesign of the affected feature...)
Correct. As originally designed/tested/flown, the igniter fuel line had
two or three little bellows segments in it (each wrapped in metal braid
for physical protection) to accommodate the thermal contraction when LH2
started flowing through it. Turned out that the bellows were too
flexible, so when the engine started firing and the vibration environment
got vigorous, the line segments would shake around too much and sometimes
cause fatigue failures in the bellows.
This effect was not found in ground testing because most J-2 testing was
done in the open air (with various measures taken to avoid problems with
flow separation in the nozzle). When a J-2 ran in air, liquid air
condensed on uninsulated bits of LH2 plumbing, including the igniter line.
So inside the braid, the folds of the bellows were full of liquid air,
which provided quite a bit of damping. Only tested in *vacuum* -- not
easy for a 230klbf engine, and so it wasn't done much -- did the problem
show up. But of course, in flight the J-2s were in vacuum throughout
their runs. Like fatigue failures in general, the timing wasn't very
predictable, which is why actual line breaks showed up in only two of the
first sixteen flown J-2s.
The fix was a redesigned igniter fuel line, eliminating the bellows and
replacing them with a bunch of bends in the line, giving it some
flexibility but not too much.
I think the line investigation/redesign probably wasn't entirely complete
at the time of the Flight Evaluation Report, less than three months after
the flight. The FER does point to the report with more details on the
line work, but that one doesn't seem to exist online yet.
Clearing the fixed J-2 to fly on manned missions was definitely a bold
thing to do, in the circumstances. A cautious man would have wanted at
least one more Saturn V unmanned test first, especially given the other
fixes. (The Russians probably considered Apollo 8 in particular grossly
reckless -- their rule was two full successes unmanned before a manned
flight.) But the end of the decade was looming, and in particular there
was real concern that the Russians might send a manned Zond around the
Moon before the first lunar Apollo, so it was time to get moving.
Henry
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