◆Part 1: The Launch
Flight of the Space Shuttle Challenger on Mission STS 51 L began at 11:38 a.m. Eastern Standard Time on 28 January 1986. It ended 73 seconds later. All seven crew members perished.
The crew were Commander Francis R. Scobee, Pilot Michael J. Smith, Mission Specialists Ellison S. Onizuka, Judith A. Resnik and Ronald E. McNair, and Payload Specialists Gregory B. Jarvis and Christa McAuliffe.
The ambient air temperature at launch was 36 degrees Fahrenheit measured at ground level about 1000 feet from Launch Pad 39B. That temperature was 15 degrees colder than that of any previous launch.
Just after liftoff, at 0.678 seconds into the flight, photographic data show a strong puff of gray smoke spurting from the vicinity of the aft field joint on the right Solid Rocket Booster. Eight more distinctive puffs of increasingly blacker smoke were recorded between 0.836 and 2.500 seconds. The last telemetry data from Challenger was received 73.618 seconds after launch.
◆Part 2: The Joint
The Presidential Commission on the Space Shuttle Challenger Accident concluded that the cause of the accident was the failure of the pressure seal in the aft field joint of the right Solid Rocket Motor. The failure was due to a faulty design unacceptably sensitive to a number of factors. Those 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.
Joint sealing was provided by two rubber O rings. Zinc chromate putty was applied to the insulation face prior to assembly. The putty was intended to act as a thermal barrier. The O rings were intended to be actuated and sealed by combustion gas pressure.
O ring resiliency is directly related to its temperature. 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. The Commission found it 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 clevis at time of ignition.
The temperature at the 300 degree position on the right aft field joint circumference was estimated to be 28 degrees Fahrenheit, plus or minus 5 degrees. That was the coldest point on the joint.
◆Part 3: Criticality 1
On 24 November 1980 the Solid Rocket Booster joint was classified on the Solid Rocket Booster Critical Items List as Criticality 1R. NASA defined Criticality 1R as any subsystem that contains redundant hardware, total element failure of which could cause loss of life or vehicle. The R meant NASA believed the secondary O ring would pressurize and seal if the primary O ring did not.
The 1980 Critical Items List already stated that redundancy of the secondary field joint seal cannot be verified after motor case pressure reaches approximately 40 percent of maximum expected operating pressure. Joint rotation at that pressure enlarged the extrusion gap and caused the secondary O ring to lose compression as a seal.
On 17 December 1982 Marshall reclassified the joint from Criticality 1R to Criticality 1. The revised Critical Items List stated that leakage of the primary O ring seal is classified as a single failure point due to possibility of loss of sealing at the secondary O ring because of joint rotation after motor pressurization. Failure Effect Summary: Actual Loss. Loss of mission, vehicle and crew due to metal erosion, burn through, and probable case burst resulting in fire and deflagration.
On 21 January 1983 the Marshall Configuration Control Board, chaired by Lawrence Mulloy, approved the change. On 28 March 1983 L. Michael Weeks approved for Level I a waiver of the fail safe design requirement for the field joint.
◆Part 4: The Warnings
Flight history already showed O ring thermal distress. Of 21 launches with ambient temperatures of 61 degrees Fahrenheit or greater, only four showed signs of O ring thermal distress, meaning 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.
The worst blow by recorded before Challenger occurred on SRM 15, flight STS 51 C, in January 1985, with O ring temperatures of 53 degrees Fahrenheit.
On 31 July 1985 Roger Boisjoly of Morton Thiokol wrote a memorandum titled O ring Erosion Potential Failure Criticality to R. K. Lund, Thiokol Vice President of Engineering. He stated that if a scenario like the nozzle joint erosion on flight STS 51 B occurred in a field joint, it was a jump ball as to success or failure of the joint because the secondary O ring cannot respond to the clevis opening rate and may not be capable of pressurization. The result would be a catastrophe of the highest order, loss of human life.
Boisjoly concluded that if Thiokol did not take immediate action to dedicate a team to solve the problem, with the field joint having the number one priority, then the company stood in jeopardy of losing a flight along with all the launch pad facilities.
In the Flight Readiness Review chain for STS 51 L, no mention of the O ring problems in the Solid Rocket Booster joint appeared in the Certification of Flight Readiness signed for Thiokol on 9 January 1986 by Joseph Kilminster. No mention appeared in the certification endorsement signed on 15 January 1986 by Kilminster and by Mulloy.
◆Part 5: The Night
On the evening of 27 January 1986 Morton Thiokol engineers raised concerns about launching in the forecast cold. A teleconference linked Thiokol at Wasatch in Utah, Marshall Space Flight Center, and Kennedy Space Center.
The second phase of the teleconference began at about 8:45 p.m. Eastern Standard Time. Thiokol charts presented a history of O ring erosion and blow by on previous flights, subscale testing, and static tests of Solid Rocket Motors.
Robert K. Lund, Vice President and Director of Engineering at Thiokol, presented the conclusions and recommendations. The engineering recommendation was that STS 51 L not be launched if the O ring temperatures predicted at launch time would be lower than any previous launch. That previous low was 53 degrees Fahrenheit.
Lawrence B. Mulloy, Manager of the Solid Rocket Booster Project at Marshall, asked Joseph C. Kilminster, Vice President for Space Booster Programs at Thiokol, for his recommendation. Kilminster stated that based on the engineering recommendation he could not recommend launch.
George Hardy, Deputy Director of Science and Engineering at Marshall, was reported by Allan McDonald and by Boisjoly to have said he was appalled by the Thiokol recommendation. Hardy also stated on the teleconference that he would not recommend launching if Thiokol did not. Mulloy challenged the temperature rationale and noted there were currently no Launch Commit Criteria for joint temperature.
◆Part 6: The Reversal
Kilminster asked for a five minute off net caucus. The caucus lasted about 30 minutes at Thiokol in Utah. About 10 engineers participated along with Jerald Mason, Senior Vice President for Wasatch Operations; Calvin Wiggins, Vice President and General Manager, Space Division; Kilminster; and Lund.
Arnold Thompson and Boisjoly voiced very strong objections to launch. Boisjoly testified that there was never one comment in favor of launching by any engineer or other nonmanagement person in the room before or after the caucus.
Mason said a management decision was necessary. He turned to Lund and asked him to take off his engineering hat and put on his management hat. A final management review was then conducted by Mason, Lund, Kilminster, and Wiggins.
At about 11:00 p.m. Eastern Standard Time the teleconference resumed. Thiokol management stated that temperature effects were a concern but that the data were admittedly inconclusive. Kilminster read the rationale recommending launch and stated that that was Morton Thiokol recommendation. Hardy requested that it be sent in writing by telefax to Kennedy and to Marshall. The fax was signed by Kilminster.
The Commission concluded that Thiokol Management reversed its position and recommended the launch of STS 51 L, at the urging of Marshall and contrary to the views of its engineers in order to accommodate a major customer.
◆Part 7: The Silence
Crucial information about the O ring damage in prior flights and about the Thiokol engineers argument with the NASA teleconference participants never reached Jesse Moore or Arnold Aldrich, the Level I and Level II program officials, or J. A. Gene Thomas, the Launch Director for STS 51 L.
Stanley R. Reinartz, Manager of the Shuttle Projects Office at Marshall, told the Commission he was the person who made the decision not to escalate the temperature concern to a Level II item.
Thomas testified that if he had had the information that the temperature according to Launch Commit Criteria should have been 53 degrees rather than 31, they would not have launched if it had not been 53 degrees.
The Commission concluded that 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.
The Commission found a serious flaw in the decision making process leading up to the launch of flight STS 51 L. Testimony revealed failures in communication that resulted in a decision to launch based on incomplete and sometimes misleading information, a conflict between engineering data and management judgments, and a NASA management structure that permitted internal flight safety problems to bypass key Shuttle managers.
◆Part 8: The Record
A combustion gas leak through the right Solid Rocket Motor aft field joint initiated at or shortly after ignition. It eventually weakened or penetrated the External Tank, initiating vehicle structural breakup and loss of the Space Shuttle Challenger during STS Mission 51 L.
The Commission examined the chain of decisions that culminated in approval of the launch. It concluded that the decision making process was flawed in several ways, including failure in communication resulting in a launch decision based on incomplete and misleading information, a conflict between engineering data and management judgments, and a NASA management structure that permitted flight safety problems to bypass key Shuttle managers.
The report records the Criticality 1 redesignation, the 28 March 1983 waiver, the prior flight erosion and blow by, the Boisjoly memorandum of 31 July 1985, the 27 January 1986 teleconference, the engineering recommendation against launch below 53 degrees Fahrenheit, the management reversal, the Kilminster signature on the launch recommendation, and the fact that Levels I and II and the Launch Director were not told of the dissent.
Seven crew members died. The documents show what was known. The Commission report shows what was overridden.
Report of the Presidential Commission on the Space Shuttle Challenger Accident. Volume I. Washington, D.C. 6 June 1986. Especially Chapter IV (The Cause of the Accident), Chapter V (The Contributing Cause of the Accident), Chapter VI (An Accident Rooted in History), and Appendix D (Supporting Charts and Documents).