[AR] shuttle SRBs (was Re: Re: Phenolic regression rate)
- From: "John Stoffel" <john@xxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Sun, 4 Feb 2018 13:53:38 -0500
"Henry" == Henry Spencer <hspencer@xxxxxxxxxxxxx> writes:
Henry> On Sat, 3 Feb 2018, William Claybaugh wrote:
Any Idea
if the "leak test points" ( between first and secondary O-ring )
monitored online during the burn?
No; the leak test was--as I recall--first to "set" the o-rings in the
correct position w/i the grove and then to verify same by lack of
leakage. It was strictly an assembly aid.
Henry> Bill is correct that it was strictly an assembly aid, to verify that the
Henry> O-rings were more-or-less undamaged after segment mating. That's why
the
Henry> second diagram shows a plug in the leak-test port. There was no
real-time
Henry> monitoring in flight.
Henry> However, no, "setting" the O-rings wasn't part of the intent -- that
Henry> wasn't recognized as an issue, and in fact the leak test probably made
the
Henry> situation *worse*. Because the test was done by pressurizing the volume
Henry> between the O-rings and monitoring how well it held pressure, it pushed
Henry> the outer ring outward *and the inner ring inward*. The former was
okay;
Henry> the latter was not, because the inner ring -- the first line of defence
Henry> against leakage -- was pushed *away* from where it would have to seat to
Henry> hold combustion pressure. It had to move back to the outward end of its
Henry> groove before it could re-seat and seal. That's why it was very
Henry> significant that the O-ring rubber was much stiffer at low temperatures:
Henry> that slowed down the movement and re-seating.
Henry> The leak tests also tended to send enough air inward, past the inner
Henry> O-ring, to produce blow holes in the sealing putty, whose purpose was to
Henry> keep combustion gas away from the O-rings.
Henry> Most everybody's seen the plots of how pre-Challenger O-ring
Henry> erosion varied with temperature -- much more likely when cold.
Henry> But there's another such plot: how erosion varied with the
Henry> pressure used for the assembly leak test (Rogers Commission
Henry> report, p. 133). The leak tests started out at 50psi, and then
Henry> were raised to 100 and soon to 200 -- and the frequency of
Henry> erosion got much worse. The inner O-ring was being pushed into
Henry> the wrong position more and more strongly, and was having more
Henry> and more trouble moving and re-seating promptly when combustion
Henry> started, and there were more and bigger blowholes in the putty.
Henry> Despite the low launch temperatures, the Challenger crew might
Henry> have lived if their O-rings had been leak-tested at 50psi
Henry> instead of 200psi.
This is really interesting to know, I've never heard this before. I
really should read the report from start to finish.
Henry> One of the changes made after Challenger was the addition of an
Henry> auxiliary third O-ring, farther inside the joint, so that if
Henry> you leak-tested first the volume between first and second, and
Henry> then the volume between second and third, *both* primary
Henry> O-rings would end up pushed outward, seated in their desired
Henry> flight positions. And the putty was eliminated entirely, in
Henry> favor of a flap of solid insulation glued in place.
So how well did this all work afterwards? Were the SRBs now
considered safe again, and that burn throughs were acceptably unlikely
to happen in the future?
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