Report on the Challenger accident

On 28 January 1986, the Space Shuttle Challenger exploded 73 seconds after launching from Kennedy Space Center. After the explosion, the crew module separated intact from the fireball before going into a two‑minute free fall from 50,000 feet and plunging into the sea. A gas leak in the right booster rocket was blamed for the explosion.

Challenger was carrying seven crew members, including a New Hampshire schoolteacher. They had no parachutes and no way to jettison the hatch. All seven members of the crew were killed. They were Francis R. Scobee, commander; Michael J. Smith, pilot; three mission specialists, Judith A. Resnik, Ellison Onizuka and Ronald E. McNair; payload specialist, Gregory Jarvis of Hughes Aircraft; and payload specialist, S. Christa McAuliffe. McAuliffe, a New Hampshire teacher, was the first Space Shuttle passenger/observer to participate in the NASA Teacher in Space Program and had intended to teach planned lessons during live television transmissions.

McAuliffe was selected by NASA in 1984 from among more than 11,000 teachers who applied for the Challenger mission and took a leave of absence that fall to train for it.

It was the 25th mission in the Space Shuttle program and was designated mission 51‑L.

NASA managers had been anxious to launch the Challenger for several reasons, including economic considerations, political pressures, and scheduling backlogs. The previous shuttle mission had been delayed a record number of times due to foul weather and mechanical factors. NASA wanted to launch the Challenger without any delays so the launch pad could be refurbished in time for the next mission, which would be carrying a probe that would examine Halley’s Comet. If launched on time, this probe would have collected data a few days before a similar Russian probe would be launched.

There was probably also pressure to launch Challenger so it could be in space when President Reagan gave his State of the Union address. Ronald Reagan’s main topic was to be education, and he was expected to mention the shuttle and the first teacher in space, Christa McAuliffe.

On 3 February 1986 President Reagan ordered a Commission to report on the Space Shuttle Accident (Executive Order 12546). The Commission reported:

The shuttle’s solid rocket boosters are key elements in the operation of the shuttle. Without the boosters, the shuttle cannot produce enough thrust to overcome the earth’s gravitational pull and achieve orbit. There is a solid rocket booster attached to each side of the external fuel tank. Each booster is 149 feet long and 12 feet in diameter. Before ignition, each booster weighs 2 million pounds. Solid rockets in general produce much more thrust per pound than their liquid fuel counterparts. The problem is that once the solid rocket fuel has been ignited, it cannot be turned off or even controlled. So it was extremely important that the shuttle’s solid rocket boosters were properly designed. Morton Thiokol was awarded the contract to design and build the solid rocket boosters in 1974. Thiokol’s design is a scaled‑up version of a Titan missile which had been used successfully for years. NASA accepted the design in 1976.

The booster is comprised of seven hollow metal cylinders. The solid rocket fuel is cast into the cylinders at the Thiokol plant in Utah, and the cylinders are assembled into pairs for transport to Kennedy Space Center in Florida. At Kennedy Space Center, the four booster segments are assembled into a completed booster rocket. The joints where the segments are joined together at Kennedy Space Center are known as field joints. These field joints consist of a tang and clevis joint. The tang and clevis are held together by 177 clevis pins. Each joint is sealed by two O‑rings, the bottom ring known as the primary O‑ring, and the top known as the secondary O‑ring. (The Titan booster had only one O‑ring. The second ring was added as a measure of redundancy since the boosters would be lifting humans into orbit. Except for the increased scale of the rocket’s diameter, this was the only major difference between the shuttle booster and the Titan booster.) The purpose of the O‑rings is to prevent hot combustion gases from escaping from the inside of the motor. To provide a barrier between the rubber O‑rings and the combustion gases, a heat‑resistant putty is applied to the inner section of the joint prior to assembly. The gap between the tang and the clevis determines the amount of compression on the O‑ring. To minimize the gap and increase the squeeze on the O‑ring, shims are inserted between the tang and the outside leg of the clevis.

During the night, temperatures dropped to as low as 8°F. This was much lower than had been expected. Safety showers and fire hoses were turned on to keep the water pipes in the launch platform from freezing. Some of this water had accumulated, and ice had formed all over the platform. There was some concern that the ice would fall off of the platform during launch and might damage the heat‑resistant tiles on the shuttle. The ice inspection team thought the situation was of great concern, but the launch director decided to go ahead with the countdown. Safety limitations on low temperature launching had to be checked and authorized by key personnel several times during the final countdown. These key personnel were not aware of the teleconference about the solid rocket boosters that had taken place the night before. At launch, the impact of ignition broke loose a shower of ice from the launch platform. Some of the ice struck the left‑hand booster, and some ice was actually sucked into the booster nozzle itself by an objective effect. Although there was no evidence of any ice damage to the Orbiter itself, NASA analysis of the ice problem was wrong. The booster ignition transient started six hundredths of a second after the igniter fired. The aft field joint on the right‑hand booster was the coldest spot on the booster: about 28°F. The booster’s segmented steel casing ballooned and the joint rotated, expanding inward as it had on all other shuttle lights. The primary O‑ring was too cold to seal properly, the cold‑stiffened heat‑resistant putty that protected the rubber O‑rings from the fuel collapsed, and gases at over 5000°F burned past both O‑rings across seventy degrees of arc. Eight hundredths of a second after ignition, the shuttle lifted off. Engineering cameras focused on the right‑hand booster showed about nine smoke puffs coming from the booster aft field joint. Before the shuttle cleared the tower, oxides from the burnt propellant temporarily sealed the field joint before flames could escape. Fifty‑nine seconds into the flight, Challenger experienced the most violent wind shear ever encountered on a shuttle mission. The glassy oxides that sealed the field joint were shattered by the stresses of the wind shear, and within seconds flames from the field joint burned through the external fuel tank. Hundreds of tons of propellant ignited, tearing apart the shuttle. One hundred seconds into the flight, the last bit of telemetry data was transmitted from the Challenger.

The primary cargo was the second Tracking and Data Relay Satellite (TDRS). Also on board was another Spartan free‑flying module which was to observe Halley’s Comet.

The Commission tried to be both open and to take account of measures which might be recommended to ensure the safety of future missions. In the words of the report: “the Commission focused its attention on the safety aspects of future flights based on the lessons learned from the investigation with the objective being to return to safe flight.”

After the first few weeks NASA began to co‑operate fully so that the Commission was able to report:

The result has been a comprehensive and complete investigation.

The report described the accident:

Just after liftoff at .678 seconds into the flight, photographic data show a strong puff of gray smoke was spurting from the vicinity of the aft field joint on the right Solid Rocket Booster. The two pad 39B cameras that would have recorded the precise location of the puff were inoperative. Computer graphic analysis of film from other cameras indicated the initial smoke came from the 270‑ to 310‑degree sector of the circumference of the aft field joint of the right Solid Rocket Booster. This area of the solid booster faces the External Tank. The vaporized material streaming from the joint indicated there was not complete sealing action within the joint.

Eight more distinctive puffs of increasingly blacker smoke were recorded between .836 and 2.500 seconds. The smoke appeared to puff upwards from the joint. While each smoke puff was being left behind by the upward flight of the Shuttle, the next fresh puff could be seen near the level of the joint. The multiple smoke puffs in this sequence occurred at about four times per second, approximating the frequency of the structural load dynamics and resultant joint flexing. Computer graphics applied to NASA photos from a variety of cameras in this sequence again placed the smoke puffs’ origin in the 270‑ to 310‑degree sector of the original smoke spurt.

As the Shuttle increased its upward velocity, it flew past the emerging and expanding smoke puffs. The last smoke was seen above the field joint at 2.733 seconds.

The black color and dense composition of the smoke puffs suggest that the grease, joint insulation and rubber O‑rings in the joint seal were being burned and eroded by the hot propellant gases.

At approximately 37 seconds, Challenger encountered the first of several high‑altitude wind shear conditions, which lasted until about 64 seconds. The wind shear created forces on the vehicle with relatively large fluctuations. These were immediately sensed and countered by the guidance, navigation and control system.

The steering system (thrust vector control) of the Solid Rocket Booster responded to all commands and wind shear effects. The wind shear caused the steering system to be more active than on any previous flight.

Both the Shuttle main engines and the solid rockets operated at reduced thrust approaching and passing through the area of maximum dynamic pressure of 720 pounds per square foot. Main engines had been throttled up to 104 percent thrust and the Solid Rocket Boosters were increasing their thrust when the first flickering flame appeared on the right Solid Rocket Booster in the area of the aft field joint. This first very small flame was detected on image‑enhanced film at 58.788 seconds into the flight. It appeared to originate at about 305 degrees around the booster circumference at or near the aft field joint.

One film frame later from the same camera, the flame was visible without image enhancement. It grew into a continuous, well‑defined plume at 59.262 seconds. At about the same time (60 seconds), telemetry showed a pressure differential between the chamber pressures in the right and left boosters. The right booster chamber pressure was lower, confirming the growing leak in the area of the field joint.

As the flame plume increased in size, it was deflected rearward by the aerodynamic slipstream and circumferentially by the protruding structure of the upper ring attaching the booster to the External Tank. These deflections directed the flame plume onto the surface of the External Tank. This sequence of flame spreading is confirmed by analysis of the recovered wreckage. The growing flame also impinged on the strut attaching the Solid Rocket Booster to the External Tank.

The first visual indication that swirling flame from the right Solid Rocket Booster breached the External Tank was at 64.660 seconds when there was an abrupt change in the shape and color of the plume. This indicated that it was mixing with leaking hydrogen from the External Tank. Telemetered changes in the hydrogen tank pressurization confirmed the leak. Within 45 milliseconds of the breach of the External Tank, a bright sustained glow developed on the black‑tiled underside of the Challenger between it and the External Tank.

Beginning at about 72 seconds, a series of events occurred extremely rapidly that terminated the flight. Telemetered data indicate a wide variety of flight system actions that support the visual evidence of the photos as the Shuttle struggled futilely against the forces that were destroying it.

At about 72.20 seconds the lower strut linking the Solid Rocket Booster and the External Tank was severed or pulled away from the weakened hydrogen tank permitting the right Solid Rocket Booster to rotate around the upper attachment strut. This rotation is indicated by divergent yaw and pitch rates between the left and right Solid Rocket Boosters.

At 73.124 seconds, a circumferential white vapor pattern was observed blooming from the side of the External Tank bottom dome. This was the beginning of the structural failure of the hydrogen tank that culminated in the entire aft dome dropping away. This released massive amounts of liquid hydrogen from the tank and created a sudden forward thrust of about 2.8 million pounds, pushing the hydrogen tank upward into the intertank structure. At about the same time, the rotating right Solid Rocket Booster impacted the intertank structure and the lower part of the liquid oxygen tank. These structures failed at 73.137 seconds as evidenced by the white vapors appearing in the intertank region.

Within milliseconds there was massive, almost explosive, burning of the hydrogen streaming from the failed tank bottom and liquid oxygen breach in the area of the intertank.

At this point in its trajectory, while travelling at a Mach number of 1.92 at an altitude of 46,000 feet, the Challenger was totally enveloped in the explosive burn. The Challenger’s reaction control system ruptured and a hypergolic burn of its propellants occurred as it exited the oxygen‑hydrogen flames. The reddish brown colors of the hypergolic fuel burn are visible on the edge of the main fireball.

The Orbiter, under severe aerodynamic loads, broke into several large sections which emerged from the fireball. Separate sections that can be identified on film include the main engine/tail section with the engines still burning, one wing of the Orbiter, and the forward fuselage trailing a mass of umbilical lines pulled loose from the payload bay.

The Commission concluded that the cause of the accident was:

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.

The Commission concluded that other factors contributed to 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.

The Commission made the following 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.