atkins@opus.UUCP (Brian Atkins) (02/12/86)
For those of you that don't know, C-SPAN is televising the hearings
of the President's Blue Ribbon Commission on the Challenger Accident
(Sally Ride, Neil Armstrong, etc.).
Tuesday's presentation was about the SRB motor (SRM) joints and O-rings,
which have been a trouble spot in the past. (Tuesday's presentation was
aired late Tuesday night (w.r.t MST) and again around 6:00am MST Wednesday.)
There are only three such joints per SRB. The joint between the lowest
SRM section and the nozzle (which was also discussed) and the joint
between the top most SRM section and whatever comes above it (which was
not discussed in the part of the presentation I saw) are different in
build, and stress. The focus of the discussion that I saw was on the
three SRM joints, which are all the same.
Apparently there have been cases of erosion of the primary O-ring in
some of the recovered motors. I believe the number seen was something
on the order of 6 cases of hot gas blow-by in 170 or so motors over the
course of SRB flights and tests (the actual numbers are close, I didn't
write the facts down as I should).
There was quite a bit of emphases on the fact that in all cases, the
primary O-rings in question sealed successfully and performed their
assigned function after the blow-by during the initial 0 to ~900PSI pressure
build up which occurs in the first 1/10th second after ignition of
the SRBs. It was said repeatedly that at no time has the system been
forced to rely on the redundant secondary O-ring.
When asked where the plume seen in the video tapes was, the
speaker (head of the SRB project, I believe) could only estimate
that it was near the lower SRB/ET mount point, which is 12" from
the nearest SRM casting joint.
The putty used to seal the SRM castings together at KSC also came under
examination, as did the joint inspection ports and spacing shims.
Three SRM castings are sent to KSC for final assembly. Each of these
castings is made up of two SRM sections, which are assembled in
Utah. Insulation is laied over the joint assembled at the
manufacturer in Utah, but not across the joints assembled at KSC (this
is the insulation between the grain and the casting). The putty mentioned
above is used to seal those joints at KSC. The shims in question are
places at each joint pin point to assure minimum and uniform compression
of the O-rings prior to ignition. There are something like 170 such pins.
Most of this section of the presentation was an attempt to explain to the
commission the need for the putty and shims, and the forces acting on each
of component of the joint during flight and recovery.
Mr. Cook (first name eludes me) who's memo concerning the O-rings,
dated last July, made it's way to The New York Times, was also given
an opportunity to speak. My impression was that he was being grilled
a little by the commission which wanted to find out 1) how the memo
got to the Times and 2) did Mr Cook consider himself qualified to
comment on certain aspects of the budget/safety trade offs, and/or
did he consider those assigned to making those trade offs qualified.
(I have not read the memo or the Times' article.)
Alas, I could not watch any more of the presentation, which continued
with Mr Cook. After Mr. Cooks "grilling", the SRB project head
was to return and comment on Mr. Cooks memo and discussion.
The chairman of the commission called for a full day presentation
on the possible effects of the weather prior to 51L on the O-rings,
putty, and other joint components. No date was specified.
My impression was that this was a fact finding presentation only.
At no time was there any conclusions drawn between the joints, SRB/ET
mount points, or any other aspect of the SRBs and the 51L accident.
As of that presentation, none of the SRM joints had been recovered from 51L.
Brian Atkins ...{attunix, hao, allegra, ucbvax}!nbires!atkins
NBI Inc., P.O. Box 9001, Boulder CO 80301 (303) 444-5710