STS-51-L Mission timeline

STS-51-L Mission timeline is a detailed timeline of events from the ignition of "Challenger"s main engines to the remote destruction of the two Solid Rocket Boosters, and includes a transcript of crew conversations from the Cockpit Voice Recorder on board the Orbiter.

STS-51-L was the twenty-fifth flight of the American Space Shuttle program, which marked the first time a civilian had flown aboard the Space Shuttle. The mission used Space Shuttle "Challenger", which lifted off from launch pad 39B in 1986 from Kennedy Space Centre, Florida. The mission ended in disaster following the destruction of "Challenger" 73 seconds after lift-off because of the failure of an O-ring seal on "Challenger"'s right solid rocket booster, which led to the rapid disintegration of the shuttle stack due to overwhelming aerodynamic pressures. The seven-member crew was killed some time after the breakup of the vehicle.

ummary timeline

Following several days of lengthy delays, "Challenger" finally lifted off at 16:38:00 UTC on 28 January 1986. Her three main engines were ignited at T-6.6 seconds and at T-0 the solid rocket boosters were ignited, lifting the shuttle stack off launchpad 39-B at Kennedy Space Centre. Almost immediately, cameras recording the launch registered the presence of smoke at the field joint next to the attachment strut on the right-hand SRB, indicating the failure of the O-rings that were supposed to seal the joint against the 'blow-by' of hot gases from the boosters. However, sometime at around T+2 seconds, a piece of solid fuel from inside the booster moved inside the joint and provided a temporary seal against the blow-by, and, as such, the launch proceeded normally for around forty seconds after launch.

However, at around T+36 seconds and an altitude of just over 10,000 feet, "Challenger" experienced the strongest wind shear ever felt by a Space Shuttle launch. The pitch and yaw commanded by the shuttle's computers in order to counter this wind caused the solid fuel plug to become dislodged from the field joint on the right SRB, and from that moment on the fate of both "Challenger" and her crew was sealed.

At around T+58, cameras noted the creation of a plume on the aft attachment strut on the right-hand SRB - ignited gas had begun to force itself through a rapidly growing hole in the field joint. 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 there was visual evidence of flame coming through the joint and impinging on the external tank. As the mission clock passed up through T+64 seconds, the plume suddenly changed shape, showing that it had burned a hole in the liquid hydrogen tank in "Challenger"'s ET, causing the tank to leak. The pressure in the tank began to drop, whilst meanwhile "Challenger"'s onboard computers began to pivot the nozzles of the SSMEs in order to counter the now-unbalanced thrust between the two SRBs.

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.

At around T+72, 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 external tank. 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 (14.6 km).cite web| author=Kerwin, Joseph P.| title="Challenger" crew cause and time of death| year=1986| url=http://history.nasa.gov/kerwin.html| accessmonthday=July 4 | accessyear=2006] With the external tank disintegrating, "Challenger" veered from its correct attitude with respect to the local air flow and was immediately torn apart by abnormal aerodynamic forces resulting in a load factor of up to 20"g" – well over its design limit. 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 made of half an inch (12.7 mm) thick steel and were 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.cite video | people= Photo and TV Analysis Team Report| title = [http://www.archive.org/details/ChallengerAccidentandInvestigation Space Shuttle Challenger Accident Investigation] | publisher = STS-51L Data and Analysis Task Force | date = 1986]

Detailed timeline and transcript

The following timeline provides a detailed list of the major events of the launch of STS-51-L, culminating in the destruction of "Challenger". The list also contains a transcript from the shuttle's Cockpit Voice Recorder (CVR), from ignition of the main engines to T+73 seconds. Acronyms used in the timeline are as follows:

*APU - Auxilixary Power Unit
*ET - External Tank
*GPC - General Purpose Computer
*HPFT - High Pressure Fuel Turbopump
*LH2 - Liquid Hydrogen
*LO2 - Liquid Oxygen (same as LOX)
*MEC - Main Engine Controller
*PIC - Pyrotechnics Initiator Controller
*psf - Pounds per square foot
*psi - Pounds per square inch
*RCS - Reaction Control System
*CDR - Commander
*PLT - Pilot
*MS1/MS2 - Mission specialist

References

External links

* Spaceflight Now. [http://www.spaceflightnow.com/challenger/timeline The Challenger Accident Timeline] .