[AR] shuttle SRBs (was Re: Re: Phenolic regression rate)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sat, 3 Feb 2018 20:12:47 -0500 (EST)
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.
Bill is correct that it was strictly an assembly aid, to verify that the
O-rings were more-or-less undamaged after segment mating. That's why the
second diagram shows a plug in the leak-test port. There was no real-time
monitoring in flight.
However, no, "setting" the O-rings wasn't part of the intent -- that
wasn't recognized as an issue, and in fact the leak test probably made the
situation *worse*. Because the test was done by pressurizing the volume
between the O-rings and monitoring how well it held pressure, it pushed
the outer ring outward *and the inner ring inward*. The former was okay;
the latter was not, because the inner ring -- the first line of defence
against leakage -- was pushed *away* from where it would have to seat to
hold combustion pressure. It had to move back to the outward end of its
groove before it could re-seat and seal. That's why it was very
significant that the O-ring rubber was much stiffer at low temperatures:
that slowed down the movement and re-seating.
The leak tests also tended to send enough air inward, past the inner
O-ring, to produce blow holes in the sealing putty, whose purpose was to
keep combustion gas away from the O-rings.
Most everybody's seen the plots of how pre-Challenger O-ring erosion
varied with temperature -- much more likely when cold. But there's
another such plot: how erosion varied with the pressure used for the
assembly leak test (Rogers Commission report, p. 133). The leak tests
started out at 50psi, and then were raised to 100 and soon to 200 -- and
the frequency of erosion got much worse. The inner O-ring was being
pushed into the wrong position more and more strongly, and was having more
and more trouble moving and re-seating promptly when combustion started,
and there were more and bigger blowholes in the putty. Despite the low
launch temperatures, the Challenger crew might have lived if their O-rings
had been leak-tested at 50psi instead of 200psi.
One of the changes made after Challenger was the addition of an auxiliary
third O-ring, farther inside the joint, so that if you leak-tested first
the volume between first and second, and then the volume between second
and third, *both* primary O-rings would end up pushed outward, seated in
their desired flight positions. And the putty was eliminated entirely, in
favor of a flap of solid insulation glued in place.
Henry
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