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Subject:Quoting The Findings Correctly From:George Mena <George -dot- Mena -at- esstech -dot- com> To:"TECHWR-L" <techwr-l -at- lists -dot- raycomm -dot- com> Date:Thu, 13 Nov 2003 10:09:18 -0800
List:
I suspect the "urban legend" became that more because of distortion that
occurred in re-telling the story of the ignored warnings about the O-rings
over the years more than anything else. It's not unlike any other kind of
gossip in that regard.
Peter Neilson qouted one part of the Challenger report that said nobody
listened to "internal warnings about the faulty seal design."
The engineer most ignored at Morton Thiokol was one Allan MacDonald, the man
who generated those internal warnings ad nauseum.
After Thiokol upper management realized MacDonald had tried to warn the
company, they finally offered him a raise and the opportunity to manage the
booster program.
He told them to stick it.
The findings that describe the O-ring failure itself are quoted here:
=========
FINDINGS
1. A combustion gas leak through the right Solid Rocket Motor aft
field joint initiated at or shortly after ignition eventually weakened
and/or penetrated the External Tank initiating vehicle structural
breakup and loss of the Space Shuttle Challenger during STS Mission
51-L.
5. Launch site records show that the right Solid Rocket Motor segments
were assembled using approved procedures. However, significant
out-of-round conditions existed between the two segments joined at the
right Solid Rocket Motor aft field joint (the joint that failed).
b. The diameters of the two Solid Rocket Motor segments had grown as
a result of prior use.
c. The growth resulted in a condition at time of launch wherein the
maximum gap between the tang and clevis in the region of the joint's
O-rings was no more than .008 inches and the average gap would have
been .004 inches.
e. The lack of roundness of the segments was such that the smallest
tang-to-clevis clearance occurred at the initiation of the assembly
operation at positions of 120 degrees and 300 degrees around the
circumference of the aft field joint. It is uncertain if this tight
condition and the resultant greater compression of the O-rings at
these points persisted to the time of launch.
6. The ambient temperature at time of launch was 36 degrees
Fahrenheit, or 15 degrees lower than the next coldest previous launch.
a. The temperature at the 300 degree position on the right aft
field joint circumference was estimated to be 28 degrees plus or minus
5 degrees Fahrenheit. This was the coldest point on the joint.
b. Temperature on the opposite side of the right Solid Rocket
Booster facing the sun was estimated to be about 50 degrees
Fahrenheit.
7. Other joints on the left and right Solid Rocket Boosters
experienced similar combinations of tang-to-clevis gap clearance and
temperature. It is not known whether these joints experienced
distress during the flight of 51-L.
8. Experimental evidence indicates that due to several effects
associated with the Solid Rocket Booster's ignition and combustion
pressures and associated vehicle motions, the gap between the tang and
the clevis will open as much as .017 and .029 inches at the secondary
and primary O-rings, respectively.
a. This opening begins upon ignition, reaches its maximum rate of
opening at about 200-300 milliseconds, and is essentially complete at
600 milliseconds when the Solid Rocket Booster reaches its operating
pressure.
=======
>From a test engineering standpoint, a multi-compartment type of temperature
test chamber probably should have been created as part of the temperature
test engineering plan for the booster. This chamber would simulate the
environmental conditions present at pad 39B on January 28, 1986. Such
chambers tend to be costly from the standpoint of test system maintenance,
mostly because of the variations of temperature in the different parts of
the chamber. But, such a testing system would also have shown how the
O-rings might experience out-of-round failure conditions caused by
variations in temperature found at different points on the booster.
When gases transition from a compressed smaller space to a larger, less
compressed space, the gases travel at Mach numbers, as in the speed of sound
type of Mach numbers. This is commonly referred to as a reverse venturi
effect. Automobile carburetors operate on a type of venturi effect. Outside
air is sucked into the venturi chamber of a carburetor during engine
cranking by the starter motor. As the starter cranks the engine, a partial
vacuum is created by the movement of the pistons and crankshaft in order to
draw air into the piston chambers to attain combustion and engine startup.
Most do-it-yourself shadetree mechanics think of brake drums when they hear
the term "out-of-round" because they know brake drums can be resurfaced to
restore the roundness on the interior walls of brake drums. Whether Thiokol
even had the capacity to modify an O-ring for the booster is a question only
they could answer. In all likelihood, a quicker and cheaper fix would
probably have been to cover the areas where the O-rings met the booster
walls with a lot of epoxy. A concurrent effort on designing a new O-ring
could also have been in place, and probably should have been in place long
before 1986.
George Mena
Sr. Technical Writer
ESS Technology, Inc.
48401 Fremont Blvd.
Fremont, CA USA 94538
510-492-1763
e-mail: George -dot- Mena -at- esstech -dot- com
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