Challenger Tragedy: Presidential Report Continued
The Cause of the Accident
The consensus of the Commission and participating investigative
agencies is that the loss of the Space Shuttle Challenger was caused by a
failure in the joint between the two lower segments of the right Solid
Rocket Motor. The specific failure was the destruction of the seals
that are intended to prevent hot gases from leaking through the joint
during the propellant burn of the rocket motor. The evidence assembled by
the Commission indicates that no other element of the Space Shuttle system
contributed to this failure.
In arriving at this conclusion, the Commission reviewed in detail all
available data, reports and records; directed and supervised numerous
tests, analyses, and experiments by NASA, civilian contractors and various
government agencies; and then developed specific scenarios and the range
of most probable causative factors.
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.
2. The evidence shows that no other STS 51-L Shuttle element or the
payload contributed to the causes of the right Solid Rocket Motor aft
field joint combustion gas leak. Sabotage was not a factor.
3. Evidence examined in the review of Space Shuttle material,
manufacturing, assembly, quality control, and processing on
non-conformance reports found no flight hardware shipped to the launch
site that fell outside the limits of Shuttle design specifications.
4. Launch site activities, including assembly and preparation, from
receipt of the flight hardware to launch were generally in accord with
established procedures and were not considered a factor in the accident.
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).
a. While the assembly conditions had the potential of generating
debris or damage that could cause O-ring seal failure, these were not
considered factors in this accident.
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.
d. With a tang-to-clevis gap of .004 inches, the O-ring in the joint
would be compressed to the extent that it pressed against all three
walls of the O-ring retaining channel.
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.
b. The External Tank and right Solid Rocket Booster are connected by
several struts, including one at 310 degrees near the aft field joint
that failed. This strut's effect on the joint dynamics is to enhance the
opening of the gap between the tang and clevis by about 10-20 percent in
the region of 300-320 degrees.
9. O-ring resiliency is directly related to its temperature.
a. A warm O-ring that has been compressed will return to its original
shape much quicker than will a cold O-ring when compression is relieved.
Thus, a warm O-ring will follow the opening of the tang-to-clevis gap. A
cold O-ring may not.
b. A compressed O-ring at 75 degrees Fahrenheit is five times more
responsive in returning to its uncompressed shape than a cold O-ring at
30 degrees Fahrenheit.
c. As a result it is probable that the O-rings in the right solid
booster aft field joint were not following the opening of the gap
between the tang and cleavis at time of ignition.
10. Experiments indicate that the primary mechanism that actuates
O-ring sealing is the application of gas pressure to the upstream
(high-pressure) side of the O-ring as it sits in its groove or channel.
a. For this pressure actuation to work most effectively, a space
between the O-ring and its upstream channel wall should exist during
pressurization.
b. A tang-to-clevis gap of .004 inches, as probably existed in the
failed joint, would have initially compressed the O-ring to the degree
that no clearance existed between the O-ring and its upstream channel
wall and the other two surfaces of the channel.
c. At the cold launch temperature experienced, the O-ring would be
very slow in returning to its normal rounded shape. It would not follow
the opening of the tang-to-clevis gap. It would remain in its compressed
position in the O-ring channel and not provide a space between itself
and the upstream channel wall. Thus, it is probable the O-ring would not
be pressure actuated to seal the gap in time to preclude joint failure
due to blow-by and erosion from hot combustion gases.
11. The sealing characteristics of the Solid Rocket Booster O-rings are
enhanced by timely application of motor pressure.
a. Ideally, motor pressure should be applied to actuate the O-ring
and seal the joint prior to significant opening of the tang-to-clevis
gap (100 to 200 milliseconds after motor ignition).
b. Experimental evidence indicates that temperature, humidity and
other variables in the putty compound used to seal the joint can delay
pressure application to the joint by 500 milliseconds or more.
c. This delay in pressure could be a factor in initial joint failure.
12. Of 21 launches with ambient temperatures of 61 degrees Fahrenheit
or greater, only four showed signs of O-ring thermal distress; i.e.,
erosion or blow-by and soot. Each of the launches below 61 degrees
Fahrenheit resulted in one or more O-rings showing signs of thermal
distress.
a. Of these improper joint sealing actions, one-half occurred in the
aft field joints, 20 percent in the center field joints, and 30 percent
in the upper field joints. The division between left and right Solid
Rocket Boosters was roughly equal.
b. Each instance of thermal O-ring distress was accompanied by a leak
path in the insulating putty. The leak path connects the rocket's
combustion chamber with the O-ring region of the tang and clevis. Joints
that actuated without incident may also have had these leak paths.
13. There is a possibility that there was water in the clevis of the
STS 51-L joints since water was found in the STS-9 joints during a destack
operation after exposure to less rainfall than STS 51-L. At time of
launch, it was cold enough that water present in the joint would freeze.
Tests show that ice in the joint can inhibit proper secondary seal
performance.
14. A series of puffs of smoke were observed emanating from the 51-L
aft field joint area of the right Solid Rocket Booster between 0.678 and
2.500 seconds after ignition of the Shuttle Solid Rocket Motors.
a. The puffs appeared at a frequency of about three puffs per second.
This roughly matches the natural structural frequency of the solids at
lift off and is reflected in slight cyclic changes of the tang-to-clevis
gap opening.
b. The puffs were seen to be moving upward along the surface of the
booster above the aft field joint.
c. The smoke was estimated to originate at a circumferential position
of between 270 degrees and 315 degrees on the booster aft field joint,
emerging from the top of the joint.
15. This smoke from the aft field joint at Shuttle lift off was the
first sign of the failure of the Solid Rocket Booster O-ring seals on STS
51-L.
16. The leak was again clearly evident as a flame at approximately 58
seconds into the flight. It is possible that the leak was continuous but
unobservable or non-existent in portions of the intervening period. It is
possible in either case that thrust vectoring and normal vehicle response
to wind shear as well as planned maneuvers reinitiated or magnified the
leakage from a degraded seal in the period preceding the observed flames.
The estimated position of the flame, centered at a point 307 degrees
around the circumference of the aft field joint, was confirmed by the
recovery of two fragments of the right Solid Rocket Booster.
a. A small leak could have been present that may have grown to breach
the joint in flame at a time on the order of 58 to 60 seconds after lift
off.
b. Alternatively, the O-ring gap could have been resealed by
deposition of a fragile buildup of aluminum oxide and other combustion
debris. This resealed section of the joint could have been disturbed by
thrust vectoring, Space Shuttle motion and flight loads inducted by
changing winds aloft.
c. The winds aloft caused control actions in the time interval of 32
seconds to 62 seconds into the flight that were typical of the largest
values experienced on previous missions.
Conclusion
In view of the findings, the Commission concluded that the cause of the
Challenger accident was the failure of the pressure seal in the aft field
joint of the right Solid Rocket Booster. The failure was due to a faulty
design unacceptably sensitive to a number of factors. These factors were
the effects of temperature, physical dimensions, the character of
materials, the effects of reusability, processing and the reaction of the
joint to dynamic loading.
The Contributing Cause of the Accident
The decision to launch the Challenger was flawed. Those who made that
decision were unaware of the recent history of problems concerning the
O-rings and the joint and were unaware of the initial written
recommendation of the contractor advising against the launch at
temperatures below 53 degrees Fahrenheit and the continuing opposition of
the engineers at Thiokol after the management reversed its position. They
did not have a clear understanding of Rockwell's concern that it was not
safe to launch because of ice on the pad. If the decision makers had known
all of the facts, it is highly unlikely that they would have decided to
launch 51-L on January 28, 1986.
Findings
1. The Commission concluded that there was a serious flaw in the
decision making process leading up to the launch of flight 51-L. A well
structured and managed system emphasizing safety would have flagged the
rising doubts about the Solid Rocket Booster joint seal. Had these matters
been clearly stated and emphasized in the flight readiness process in
terms reflecting the views of most of the Thiokol engineers and at least
some of the Marshall engineers, it seems likely that the launch of 51-L
might not have occurred when it did.
2. The waiving of launch constraints appears to have been at the
expense of flight safety. There was no system which made it imperative
that launch constraints and waivers of launch constraints be considered by
all levels of management.
3. The Commission is troubled by what appears to be a propensity of
management at Marshall to contain potentially serious problems and to
attempt to resolve them internally rather than communicate them forward.
This tendency is altogether at odds with the need for Marshall to function
as part of a system working toward successful flight missions, interfacing
and communicating with the other parts of the system that work to the same
end.
4. The Commission concluded that the Thiokol Management reversed its
position and recommended the launch of 51-L, at the urging of Marshall and
contrary to the views of its engineers in order to accommodate a major
customer.
Findings The Commission is concerned about three aspects of the
ice-on-the-pad issue.
1. An Analysis of all of the testimony and interviews establishes that
Rockwell's recommendation on launch was ambiguous. The Commission finds it
difficult, as did Mr. Aldrich, to conclude that there was a no-launch
recommendation. Moreover, all parties were asked specifically to contact
Aldrich or other NASA officials after the 9:00 Mission Management Team
meeting and subsequent to the resumption of the countdown.
2. The Commission is also concerned about the NASA response to the
Rockwell position at the 9:00 a.m. meeting. While it is understood that
decisions have to be made in launching a Shuttle, the Commission is not
convinced Levels I and II appropriately considered Rockwell's concern
about the ice. However ambiguous Rockwell's position was, it is clear that
they did tell NASA that the ice was an unknown condition. Given the extent
of the ice on the pad, the admitted unknown effect of the Solid Rocket
Motor and Space Shuttle Main Engines ignition on the ice, as well as the
fact that debris striking the Orbiter was a potential flight safety
hazard, the Commission finds the decision to launch questionable under
those circumstances. In this situation, NASA appeared to be requiring a
contractor to prove that it was not safe to launch, rather than proving it
was safe. Nevertheless, the Commission has determined that the ice was not
a cause of the 51-L accident and does not conclude that NASA's decision to
launch specifically overrode a no-launch recommendation by an element
contractor.
3. The Commission concluded that the freeze protection plan for launch
pad 39B was inadequate. The Commission believes that the severe cold and
presence of so much ice on the fixed service structure made it inadvisable
to launch on the morning of January 28, and that margins of safety were
whittled down too far.
Additionally, access to the crew emergency slide wire baskets was
hazardous due to ice conditions. Had the crew been required to evacuate
the Orbiter on the launch pad, they would have been running on an icy
surface. The Commission believes the crew should have been made aware of
the condition, greater consideration should have been given to delaying
the launch.
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Source: NASA. |
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