Space Shuttle Challenger disaster
2007 Schools Wikipedia Selection. Related subjects: Engineering; Space transport
The Space Shuttle Challenger disaster was a space disaster that occurred at 11:39 a.m. EST on January 28, 1986, when the NASA Space Shuttle Challenger disintegrated 73 seconds into its flight after an O-ring seal in its right solid rocket booster (SRB) failed. The seal failure caused a flame leak from the solid rocket booster that impinged upon the adjacent external propellant tank. Within seconds, the flame caused structural failure of the external tank, and the orbiter broke up abruptly due to aerodynamic forces. The shuttle was destroyed and all seven crew members were killed, probably when the crew compartment hit the surface of the ocean. The crew compartment and many other vehicle fragments were eventually recovered from the ocean floor after a lengthy search and rescue operation.
The disaster resulted in a 32-month hiatus in the shuttle program and the formation of the Rogers Commission, a special commission appointed by President Ronald Reagan to investigate the accident. The Rogers Commission found that NASA's organizational culture and decision-making processes had been a key contributing factor to the accident. NASA managers had failed to deal with the flawed design of the O-rings, had ignored warnings from engineers about the dangers of launching on such a cold day, and had failed to adequately report these technical concerns to their superiors. The Rogers Commission offered NASA nine recommendations that were to be implemented before shuttle flights resumed.
Many schoolchildren saw the launch live due to the presence on the crew of Christa McAuliffe, the first member of the Teacher in Space Project. Media coverage of the accident was extensive: one study reported that 85 percent of Americans surveyed in a poll had heard the news within an hour of the accident. The Challenger disaster has been used as a case study in many discussions of engineering safety and workplace ethics and has inspired a television movie, Challenger, which was made in 1990.
Pre-launch conditions and delays
Challenger's launch was originally set for 2:43 pm Eastern Standard Time (EST) on January 22. However, delays suffered by the previous mission, STS-61-C, caused the launch date to be pushed back to the 23rd and then to the 24th. Launch was then rescheduled for the 25th due to bad weather at the Transoceanic Abort Landing (TAL) site in Dakar, Senegal. NASA decided to use Casablanca as the TAL site, but because it was not equipped for night landings, the launch had to be moved to the morning ( Florida time). Predictions of unacceptable weather at Kennedy Space Centre (KSC) caused the launch to be rescheduled for 9:37 am EST on the 27th. Launch was then delayed 24 hours when the pad technicians could not remove a closing fixture from the orbiter's hatch. When the fixture was finally sawn off, cross winds at the Shuttle Landing Facility exceeded the limits for a Return to Launch Site (RTLS) abort, and thus the launch was delayed once again.
The morning of the 28th dawned unusually cold, with temperatures having dropped into the low twenties overnight. The low temperatures had prompted concern from engineers at Morton Thiokol, the contractor responsible for the construction and maintenance of the shuttle's solid rocket boosters. At a teleconference which took place on the evening of the 27th, Thiokol engineers and managers discussed the weather conditions with NASA managers from Kennedy Space Centre and Marshall Space Flight Centre. Although several engineers—most notably Roger Boisjoly—expressed their concern about the effect of the temperature on the resilience of the rubber O-rings that sealed the joints of the solid rocket boosters, Morton Thiokol managers in the end decided to recommend that the launch should go ahead.
Due to the low temperatures, a significant amount of ice built up on the fixed service structure that stood beside the shuttle. Although the KSC ice team had worked through the night removing ice, engineers at Rockwell International, the shuttle's prime contractor, still expressed concern. They warned that ice might be shaken loose during launch, and might either strike the shuttle or be aspirated by its main engines. Managers at Rockwell told shuttle program manager Arnold Aldrich that they could not completely assure that the shuttle was safe to launch, but failed to communicate a firm recommendation against launching. As a result of these discussions, Aldrich decided to postpone the shuttle launch by an hour in order to give the ice team the time to perform another inspection. After that last inspection, during which the ice appeared to be melting, Challenger was finally cleared to launch at 11:38 am EST.
January 28 launch and failure
Liftoff and initial ascent
The following account of the accident is derived from real time telemetry data and photographic analysis, as well as from transcripts of air-to-ground and mission control voice communications.All times are given in seconds after launch and correspond to the telemetry time-codes from the closest instrumented event to each described event.
At 6.6 seconds before liftoff, the three space shuttle main engines (SSME) ignited. Until liftoff actually occurs, the SSMEs can be safely shut down and the launch aborted if necessary. At liftoff time (T=0, which was at 11:38:00.010 EST), the three SSMEs were at 100% of their original rated performance, and began throttling up to 104% under computer control. At this moment, the two SRBs were ignited and hold-down bolts were released with explosives, freeing the vehicle from the pad. With the first vertical motion of the vehicle, the gaseous hydrogen vent arm retracted from the External Tank (ET) but failed to latch back. Review of film shot by pad cameras showed that the arm did not re-contact the vehicle, and thus it was ruled out as a contributing factor in the accident. The post-launch inspection of the pad also revealed that kick springs on four of the hold-down bolts were missing, but they were similarly ruled out as a possible cause. Later review of launch film showed that at T+0.678, strong puffs of dark grey smoke were emitted from the right-hand SRB near the aft strut that attaches the booster to the ET. The last smoke puff occurred at about T+2.733. The last view of smoke around the strut was at T+3.375.
As the vehicle cleared the tower, the SSMEs were operating at 104% their rated maximum thrust, and control switched from the Launch Control Centre (LCC) at KSC to the Mission Control Centre (MCC) in Houston, Texas. To prevent aerodynamic forces from tearing the shuttle apart, at T+28 the SSMEs began throttling down to limit the velocity of the shuttle in the dense lower atmosphere. At T+35.379, the SSMEs throttled back further to the planned 65%. Five seconds later, at about 19,000 feet, Challenger passed through Mach 1. At T+51.860, the SSMEs began throttling back up to 104% as the vehicle approached Max Q, the period of maximum aerodynamic pressure on the vehicle.
Plume
At T+58.788, a tracking film camera captured the beginnings of a plume near the aft attach strut on the right SRB. Unknown to those on Challenger or in Houston, ignited gas had begun to leak through a growing hole in one of the right-hand SRB's joints. Within a second, the plume became well defined and intense. Internal pressure in the right SRB began to drop because of the rapidly enlarging hole in the failed joint, and at T+60.238 there was evidence of flame through the rupture impinging on the external tank.
At T+64.660, the plume suddenly changed shape, indicating that a leak had begun in the liquid hydrogen tank, located in the aft portion of the external tank. The nozzles of the main engines pivoted under computer control to compensate for the unbalanced thrust produced by the booster burn-through. The pressure in the shuttle's external liquid hydrogen tank began to drop at T+66.764, indicating the effect of the leak.
At this stage the situation still seemed normal both to the astronauts and to flight controllers. At T+68, the CAPCOM informed the crew that they were "go at throttle up," and Commander Dick Scobee confirmed the call. His response, "Roger, go at throttle up," was the last communication from Challenger on the air-to-ground loop.
Vehicle breakup
At T+72.284, the right SRB apparently pulled away from the aft strut attaching it to the external tank. Later analysis of telemetry data showed a sudden lateral acceleration to the right at T+72.525, which may have been felt by the crew. The last statement captured by the crew cabin recorder came just half a second after this acceleration, when Pilot Michael J. Smith said "Uh oh". Smith may also have been responding to onboard indications of main engine performance, or to falling pressures in the external fuel tank.
At T+73.124, the aft dome of the liquid hydrogen tank failed, producing a propulsive force that pushed the hydrogen tank into the liquid oxygen tank in the forward part of the ET. At the same time, the right SRB rotated about the forward attach strut, and struck the intertank structure.
The breakup of the vehicle began at T+73.162 seconds and at an altitude of 48,000 feet (15 km). With the external tank disintegrating, Challenger veered from its correct attitude with respect to the local air flow and was immediately torn apart by aerodynamic forces. The two SRBs, which can withstand greater aerodynamic loads, separated from the ET and continued in uncontrolled powered flight for another 37 seconds. The SRB casings were one-half-inch thick steel and much stronger than the orbiter and ET; thus, both SRBs survived the breakup of the space shuttle stack, even though the right SRB was still suffering the effects of the joint burn-through that had set the destruction of Challenger in motion.
Post-breakup flight controller dialog
In Mission Control, there was silence for a few seconds after the accident. Television screens showed a cloud of smoke and vapor where Challenger had been, with pieces of debris falling toward the ocean. At about T+89, flight director Jay Greene prompted his flight dynamics officer for information. The response was that "filters [radar] got discreting sources," a further indication that Challenger had broken into multiple pieces. The ground controller reported "negative contact, loss of downlink" of radio and telemetry data from Challenger.
At T+110.250, the Range Safety Officer (RSO) at the Cape Canaveral Air Force Station sent radio signals that activated the range safety system's "destruct" packages on board both solid rocket boosters. This was a normal contingency procedure, undertaken because the RSO judged the free-flying SRBs a possible threat to land or sea. The same destruct signal would have destroyed the External Tank had it not already disintegrated.
"Flight controllers here are looking very carefully at the situation," reported the public affairs officer. "Obviously a major malfunction." Shortly afterwards, the flight dynamics officer relayed the range safety officer's report that the vehicle had exploded. Flight director Jay Greene then ordered that contingency procedures be put into effect at Mission Control; these procedures included locking the doors of the control centre, shutting down telephone communications with the outside world, and following checklists that ensured that the relevant data was correctly recorded and preserved.
No "explosion"
Contrary to the flight dynamics officer’s initial statement, the shuttle and external tank did not actually " explode"; there was no detonation. Instead they rapidly disintegrated under tremendous aerodynamic forces, since the shuttle was near " Max Q", or maximum aerodynamic pressure. When the external tank disintegrated, the fuel and oxidizer stored within it were released, producing the appearance of a massive fireball. However, according to the NASA team that analyzed imagery after the accident, there was only "localized combustion" of propellant. Instead, the visible cloud was primarily composed of vapor and gases resulting from the release of the shuttle's liquid oxygen and liquid hydrogen propellant. Stored in cryogenic conditions, the liquid hydrogen could not have ignited rapidly enough to trigger an "explosion" in the traditional sense. Had there been a true explosion, the entire shuttle would have been instantly destroyed, killing the crew at that moment. The more robustly constructed crew cabin and SRBs survived the breakup of the launch vehicle; while the SRBs were subsequently detonated remotely, the detached cabin continued along a ballistic trajectory, and was observed exiting the cloud of gases at T+75.237. Twenty-five seconds after the breakup of the vehicle, the trajectory of the crew compartment peaked at a height of 65,000 feet (21.5 km); the breakup had occurred at only 48,000 feet (16 km).
Cause and time of death
During vehicle breakup, the robustly constructed crew cabin detached in one piece and slowly tumbled. NASA estimated separation forces at about 12 to 20 times the force of gravity (g) very briefly; however, within two seconds, the magnitude of the forces on the cabin had already dropped to below 4 g, and within ten seconds the cabin was undergoing free fall. These forces were likely insufficient to cause major injury. At least some of the astronauts were likely alive and briefly conscious after the breakup, because three of the four personal egress air packs (PEAPs) on the flight deck were found to have been activated. Investigators found their remaining unused air supply roughly consistent with the expected consumption during the 2 minute 45 second post-breakup trajectory. Whether the astronauts remained conscious long after the breakup is unknown, and largely depends on whether the detached crew cabin maintained pressure integrity. If it did not, time of useful consciousness at that altitude is just a few seconds; the PEAPs supplied only unpressurized air, and hence would not have helped the crew retain consciousness. The crew cabin impacted the ocean surface at roughly 207 miles per hour (333 km/hour), causing an instantaneous deceleration of over 200 g, far beyond the structural limits of the crew compartment or crew survivability levels.
On July 28, 1986, Rear Admiral Richard H. Truly, NASA's Associate Administrator for Space Flight and a former astronaut, released a report from Joseph P. Kerwin, biomedical specialist from the Johnson Space Centre in Houston, Texas, relating to the deaths of the astronauts in the accident. Dr. Kerwin, a veteran of the Skylab 2 mission, had been commissioned to undertake the study soon after the accident. According to the Kerwin Report:
The findings are inconclusive. The impact of the crew compartment with the ocean surface was so violent that evidence of damage occurring in the seconds which followed the explosion was masked. Our final conclusions are:
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Crew escape was not possible
During powered flight of the space shuttle, crew escape was not possible. While launch escape systems were considered several times during shuttle development, NASA's conclusion was that the shuttle's expected high reliability would preclude the need for one. Modified SR-71 Blackbird ejection seats and full pressure suits were used on the first four shuttle orbital missions, which were considered test flights, but they were removed for the operational missions that followed. Providing a launch escape system for larger crews was considered undesirable due to "limited utility, technical complexity and excessive cost in dollars, weight or schedule delays."
After the loss of Challenger, the question was re-opened, and NASA considered several different options, including ejector seats, tractor rockets and bailing out through the bottom of the orbiter. However, NASA once again concluded that all of the launch escape systems considered would be impractical due to the sweeping vehicle modifications that would have been necessary and the resultant limitations on crew size. A bail-out system was designed to give the crew the option to leave the shuttle during gliding flight; however, this system would not have been available in the Challenger scenario.
Aftermath
Tributes
On the night of the disaster, President Ronald Reagan had been scheduled to give his annual State of the Union address. He initially announced that the address would go on as scheduled, but under mounting pressure he postponed the State of the Union address for a week and gave a national address on the Challenger disaster from the Oval Office of the White House. It was written by Peggy Noonan, and finished with the following statement, which quoted from the poem "High Flight" by John Gillespie Magee, Jr.:
| We will never forget them, nor the last time we saw them, this morning, as they prepared for their journey and waved goodbye and 'slipped the surly bonds of earth' to 'touch the face of God.' |
Three days later, Reagan and his wife Nancy traveled to the Johnson Space Centre to be present at a memorial service honoring the astronauts. It was attended by six thousand NASA employees, as well as by the families of the crew.
Funeral ceremonies
The remains of the crew that were identifiable were returned to their families on April 29, 1986. Two of the crewmembers, Dick Scobee and Michael Smith, were buried by their families at Arlington National Cemetery at individual grave sites. Other crew remains were buried at the Space Shuttle Challenger Memorial in Arlington on May 20, 1986.
Recovery of debris
In the first minutes after the accident, recovery efforts were begun by NASA's Launch Recovery Director, who ordered the ships used by NASA for recovery of the solid rocket boosters to be sent to the location of the water impact. Search and rescue aircraft were also dispatched. At this stage, however, debris was still falling, and the Range Safety Officer (RSO) held both aircraft and ships out of the impact area until it was safe for them to enter. It was about an hour until the RSO allowed the recovery forces to begin their work.
The search and rescue operations which took place in the first week after the Challenger accident were managed by the Department of Defense on behalf of NASA, with assistance from the United States Coast Guard, and mostly involved surface searches. According to the Coast Guard, "the operation was the largest surface search in which they had participated." This phase of operations lasted until February 7. Thereafter, recovery efforts were managed by a Search, Recovery and Reconstruction team; its aim was to salvage debris that would help in determining the cause of the accident. Sonar, divers, remotely-operated submersibles and manned submersibles were all used during the search, which covered an area of 480 square nautical miles, and took place at depths of up to 1200 feet. By May 1, enough of the right solid rocket booster had been recovered to determine the original cause of the accident, and the major salvage operations were concluded. While some shallow-water recovery efforts continued, this was unconnected with the accident investigation; it aimed to recover debris for use in NASA's studies of the properties of materials used in spacecraft and launch vehicles.
On board Challenger was an American flag that was sponsored by Boy Scout Troop 514 of Monument, Colorado. It was recovered intact, still sealed in its cargo bag. Debris from Challenger washed up on Florida beaches for years after the incident. On December 17, 1996, ten years after the incident, two large pieces of the shuttle were found at Cocoa Beach.
Investigation
In the aftermath of the accident, NASA was criticized for its lack of openness with the press. The New York Times noted on the day after the accident that "neither Jay Greene, flight director for the ascent, nor any other person in the control room, was made available to the press by the space agency". In the absence of reliable sources, the press turned to speculation; both the New York Times and United Press International ran stories suggesting that a fault with the space shuttle external tank had caused the accident, despite the fact that NASA's internal investigation had quickly focused in on the solid rocket boosters. "The space agency," wrote space reporter William Harwood, "stuck to its policy of strict secrecy about the details of the investigation, an uncharacteristic stance for an agency that long prided itself on openness."
Rogers Commission
The Presidential Commission on the Space Shuttle Challenger Accident, also known as the Rogers Commission (after its chairman), was formed to investigate the disaster. The commission members were Chairman William P. Rogers, Vice Chairman Neil Armstrong, David Acheson, Eugene Covert, Richard Feynman, Robert Hotz, Donald Kutyna, Sally Ride, Robert Rummel, Joseph Sutter, Arthur Walker, Albert Wheelon, and Chuck Yeager. The commission worked for several months and published a report of its findings. It found that the Challenger accident was caused by a failure in the O-rings sealing a joint on the right solid rocket booster, which allowed pressurized hot gases and eventually flame to "blow by" the O-ring and make contact with the adjacent external tank, causing structural failure. The failure of the O-rings was attributed to a faulty design, whose performance could be too easily compromised by factors including the low temperature on the day of launch.
More broadly, the report also considered the contributing causes of the accident. Most salient was the failure of both NASA and its contractor Morton Thiokol to respond adequately to the danger posed by the deficient joint design; rather than redesigning the joint, they came to define the problem as an acceptable flight risk. The report also strongly criticized the decision making process that led to the launch of Challenger, saying that it was seriously flawed.
| ...failures in communication... resulted in a decision to launch 51-L 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. |
One of the commission's most well-known members was theoretical physicist Richard Feynman. During a televised hearing, he famously demonstrated how the O-rings became less resilient and subject to seal failures at ice-cold temperatures by immersing a sample of the material in a glass of ice water. He was so critical of flaws in NASA's "safety culture" that he threatened to remove his name from the report unless it included his personal observations on the reliability of the shuttle, which appeared as Appendix F. In the appendix, he argued that the estimates of reliability offered by NASA management were wildly unrealistic, differing as much as a thousandfold from the estimates of working engineers. "For a successful technology," he concluded, "reality must take precedence over public relations, for nature cannot be fooled."
U.S. House Committee hearings
The U.S. House Committee on Science and Technology also conducted hearings, and on October 29, 1986 released its own report on the Challenger accident. The committee reviewed the findings of the Rogers Commission as part of its investigation, and agreed with the Rogers Commission as to the technical causes of the accident. However, it differed from the committee in its assessment of the accident's contributing causes.
| ...the Committee feels that the underlying problem which led to the Challenger accident was not poor communication or underlying procedures as implied by the Rogers Commission conclusion. Rather, the fundamental problem was poor technical decision-making over a period of several years by top NASA and contractor personnel, who failed to act decisively to solve the increasingly serious anomalies in the Solid Rocket Booster joints. |
NASA response
After the Challenger accident, further shuttle flights were suspended, pending the results of the Rogers Commission investigation. Whereas NASA had held an internal inquiry into the Apollo 1 fire, its actions after Challenger were more constrained by the judgments of outside bodies. The Rogers Commission offered nine recommendations on improving safety in the space shuttle program, and NASA was directed by President Ronald Reagan to report back within thirty days as to how it planned to implement those recommendations.
In response to the commission's recommendation, NASA initiated a total redesign of the space shuttle's solid rocket boosters, which was watched over by an independent oversight group as stipulated by the commission. NASA's contract with Morton Thiokol, the contractor responsible for the solid rocket boosters, included a clause stating that in the event of a failure leading to "loss of life or mission," Thiokol would forfeit $10 million of its incentive fee and formally accept legal liability for the failure. After the Challenger accident, Thiokol agreed to "voluntarily accept" the monetary penalty in exchange for not being forced to accept liability.
NASA also created a new Office of Safety, Reliability and Quality Assurance, headed as the commission had specified by a NASA associate administrator who reported directly to the NASA administrator. George Martin, formerly of Martin Marietta, was appointed to this position. Former Challenger flight director Jay Greene became chief of the Safety Division of the directorate.
The unrealistically optimistic launch schedule pursued by NASA had been criticized by the Rogers Commission as a possible contributing cause to the accident. After the accident, NASA attempted to aim at a more realistic shuttle flight rate: it added another orbiter, Endeavour, to the space shuttle fleet in order to replace Challenger, and it worked with the Department of Defense in order to put more satellites in orbit using expendable launch vehicles rather than the shuttle. In August 1986, President Reagan also announced that the shuttle would no longer carry commercial satellite payloads. After a 32-month hiatus, the next shuttle mission, STS-26, was launched on September 29, 1988.
Although significant changes were made by NASA after the Challenger accident, many commentators have argued that the changes in its management structure and organizational culture were neither deep nor long-lasting. After the Space Shuttle Columbia disaster in 2003, attention once again focused on the attitude of NASA management towards safety issues. The Columbia Accident Investigation Board (CAIB) concluded that NASA had failed to learn many of the lessons of Challenger. In particular, the agency had not set up a truly independent office for safety oversight; the CAIB felt that in this area, "NASA's response to the Rogers Commission did not meet the Commission's intent". The CAIB believed that "the causes of the institutional failure responsible for Challenger have not been fixed," saying that the same "flawed decision making process" that had resulted in the Challenger accident was responsible for Columbia's destruction seventeen years later.
Popular impact
Use as case study
The Challenger accident has frequently been used as a case study in the study of subjects such as engineering safety, the ethics of whistleblowing, communications and group decision-making. Roger Boisjoly, the engineer who had warned about the effect of cold weather on the O-rings, left his job at Morton Thiokol and became a speaker on workplace ethics. He argues that the caucus called by Morton Thiokol managers, which resulted in a recommendation to launch, "constituted the unethical decision-making forum resulting from intense customer intimidation." Universities such as Texas A&M and the University of Texas have also used the accident in classes on the ethics of engineering.
The graphic designer Edward Tufte has used the Challenger accident as an example of the problems that can occur when information is incorrectly presented. He argues that if Morton Thiokol engineers had more clearly presented the data that they had on the relationship between cold temperatures and burn-through in the solid rocket booster joints, they might have succeeded in persuading NASA managers to cancel the launch.