STS-51-L

Launch

Challenger launched January 28, 1986, at 11:38:00 a.m. EST, the first Shuttle to launch from pad 39-B (and the first launch of any sort from this facility since the Apollo-Soyuz Test Project mission in 1975). Launch was originally set for 3:43 p.m. EST, Jan. 22; it slipped to Jan. 23, then Jan. 24, due to delays in mission STS-61-C. Launch was reset for Jan. 25 because of bad weather at transoceanic abort landing (TAL) site in Dakar, Senegal. To utilize Casablanca (not equipped for night landings) as alternate TAL site, T-zero moved to a morning liftoff time. Launch was postponed a day when launch processing was unable to meet the new morning liftoff time. A prediction of unacceptable weather at Kennedy Space Center (KSC) led managers to reschedule launch for 9:37 a.m. EST, January 27. Launch was delayed 24 hours again when a ground servicing equipment hatch closing fixture could not be removed from the orbiter hatch. Maintenance crews sawed off and drilled out the attaching bolt before completing closeout. During the delay, cross winds exceeded return-to-launch-site limits at KSC's Shuttle Landing Facility. Launch January 28 was delayed two hours when hardware interface module in the launch processing system, which monitors fire detection system, failed during liquid hydrogen tanking procedures.

Related Topics:
January 28 - 1986 - STS-61-C - Dakar - Senegal - Casablanca - Kennedy Space Center - January 27 - Hydrogen

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Just after liftoff, at 0.678 seconds into the flight, photographic data show a large puff of gray smoke spurted from the vicinity of the aft field joint on the right Solid Rocket Booster (SRB). Computer graphic analysis of film from pad cameras indicated the initial smoke came from the 270- to 310-degree sector of the circumference of the aft field joint of the right SRB. This area of the SRB faces the Space Shuttle External Tank. The vaporized material streaming from the joint indicated there was incomplete sealing action within the joint.

Related Topics:
Solid Rocket Booster - Space Shuttle External Tank

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Eight more distinctive puffs of increasingly blacker smoke were recorded between 0.836 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. 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.

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It was later determined that an O-ring seal in the right SRB had failed, losing pliability due to the cold weather. As the O-ring could not flex and expand, a gap opened in the SRB which could not be sealed by the O-ring, and superheated gases were able to escape for 2.5 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. The gap was blocked by aluminium oxide particles generated by the burning fuel.

Related Topics:
Pliability - O-ring - Aluminium oxide

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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 SRBs responded to all commands and wind shear effects. The wind shear caused the steering system to be more active than on any previous flight.

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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 lbf/ft² (34 kPa). Main engines had been throttled up to 104 percent thrust and the SRBs were increasing their thrust. At this point, the aluminium oxide particles plugging the gap in the right SRB were jarred loose by the high winds, and the first flicker of flame appeared on the right SRB 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.

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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.

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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.

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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 hydrogen was now leaking from the External Fuel Tank (due to the flame) and was mixing with it. 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.

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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.

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At about 72.20 seconds the hydrogen tank was weakening and the lower strut linking the Solid Rocket Booster and the External Tank was severed or pulled away, 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.

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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 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 force (12 MN), pushing the hydrogen tank upward into the intertank structure. At about the same time, the rotating right SRB 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.

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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.

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At this point in its trajectory, while traveling at a Mach number of 1.92 at an altitude of 46,000 ft (14 km), 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.

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The explosion 73 seconds after liftoff destroyed the vehicle. The still-burning SRBs, separated from the tank in the explosion, were destroyed with on-board explosives designed for this emergency purpose (the Range Safety System) by remote command from NASA.

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The crew cabin, separated from the orbiter, survived the explosion intact and fell into the ocean. Whether the crew survived the breakup is unclear, the official report by Joseph P. Kerwin states that:

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• "the forces to which the crew were exposed during Orbiter breakup were probably not sufficient to cause death or serious injury and"
• "the crew possibly, but not certainly, lost consciousness in the seconds following Orbiter breakup due to in-flight loss of crew module pressure".

Recovered debris show that at least two of the astronauts survived long enough to turn on their Personal Egress Air Packs (PEAPS, designed for on-pad emergencies). Three PEAPS were activated, however: Resnik's, Onizuka's, and Smith's. Being that Smith's activation switch was located on the back of his seat, it is likely that either Resnik or Onizuka activated Smith's pack in an attempt to save his life. It is important to note that PEAPS would not have provided life-sustaining air for the crew, as they only provide unpressurized air. They are intended to simply give astronauts breathing air on the launch pad should noxious gases flood the cabin. It is highly likely that the cabin depressurized due to the blast, and that all of the astronauts were unconscious at the time of splashdown, though there continues to be debate regarding this subject. Astronaut Robert Crippen theorized that had the cabin not depressurized, there would have been no reason to active the PEAPS, thus solidifying the possibility that the astronauts were making a "desperate" attempt to save their own lives, and the chances that any were conscious at splashdown (due to the complete loss of cabin pressure) are nonexistant.

Related Topics:
Personal Egress Air Packs - Robert Crippen

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Regardless of the crew's consciousness during the descent, the splashdown would have generated at least 200 times the force of gravity, instantly killing any of the astronauts that may have been alive (conscious or not). http://www.aerospaceweb.org/question/investigations/q0122.shtml

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Orbiter launch weight was 268,829 lb (121,939 kg).

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