Early Evidence of a Temperature Effect
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The final flight readiness assessment chart read as
follows:
Conclusion: STS-51C consistent with erosion data base. Low
temperature enhanced probability of blow-by.
STS-51C experienced worst case temperature change in Florida
history. STS-51E could exhibit same behavior. Condition is
acceptable.
STS-51E field joints are acceptable for flight.
These conclusions were accepted and the flight was certified
ready for launch.
Later, I met with Arnie Thompson to discuss the blow-by
scenario and the effect of cold temperature on O-ring
resiliency, which is the ability of the seal to restore itself
to a round shape when the squeeze on the seal is removed. Arnie
proposed that subscale lab tests be conducted which would
provide us with assessment data. The resiliency testing was
performed in March and showed that low temperature was a
problem. The results indicated that the seals would lift off
their sealing surfaces for several seconds at 75 degrees
Fahrenheit and in excess of 10 minutes at 50 degrees
Fahrenheit. This data was discused with Morton Thiokol
engineering management but was thought to be too sensitive by
them to release.
Another post flight inspection occurred in June 1985 at
Morton Thiokol in Utah. This time a nozzle joint from Flight
51B, which flew on April 29, 1985, was found to have a primary
seal eroded in three places over a 1.3 inch length up to a
maximum depth of 0.171 inches, and the secondary seal in the
same joint was eroded 0.032 inches. It was postulated that this
primary seal had never sealed during the full two minutes of
flight.
My former concerns now escalated because if this same
scenario happened in a field joint, the secondary seal could
also be compromised especially during a low temperature launch.
A Flight Readiness Review presentation was prepared for Flight
51F, which was scheduled for launch on July 29, 1985. The
presentation was given to NASA at MSFC on July 1, 1985, with an
additional presentation on the overall status of the booster
seals given the next day. The preliminary results of the O-ring
resilience testing in March were presented for the first time
during this meeting. All O-ring test samples were 0.280 inch
diameter and compressed to 0.040 inches with a decompression
distance of 0.030 inches at a 2-inch-per-minute rate as
compared with a flight rate of approximately 3.2 inches per
minute. The results showed that the seals did not lose contact
at 100 degrees Fahrenheit; lost contact for 2.4 seconds at 75
degrees, and lost contact in excess of 10 minutes at 50
degrees. Test results also indicated that a 0.295-inch diameter
seal lost contact for 2 to 3 seconds at 50 degrees, which meant
that the 0.295-inch diameter seal performance at 50 degrees was
similar to the performance of a 0.28-inch diameter seal at 75
degrees. Everyone on the program for the first time was now
aware of the influence of low temperature on the joint
seals.
My concern increased once again due to the lack of attention
being given to this problem.
Discussion Questions
The bench tests showed that temperature can adversely affect
the resiliency, and therefore the effectiveness, of the
O-rings, yet management at Thiokol and NASA shows no interest
in planning a design change. What general courses of action are
reasonable for an engineer in this sort of situation?