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Mission ProfileCountdown to LaunchCountdown begins three days before launch when the Launch Control Center (LCC) gives the "call to stations" and begins to activate the Shuttle for flight. Marshall propulsion experts monitor Shuttle performance from the Huntsville Operations Support Center (HOSC). Lasting about three hours, the final countdown begins after the crew boards the Shuttle. Planned 10-minute holds at T -20 and T -9 minutes allow launch crews to make up for any last minute delays in the launch. At T -9 minutes, the countdown is resumed and the Launch Control Center makes final preparations for the launch. At T -5 and counting, the launch controllers, arm the Shuttle's solid rocket boosters. The next few minutes pass quickly as the Shuttle is readied for liftoff. Countdown to Launch (min:sec)
T -0:31.0 Auto Sequence StartThe final 31 seconds of countdown are the busiest, as the Shuttle's on-board computers take over the count, and every system on the vehicle operates on internal power. The Shuttle's three main engines (SSMEs) are sequentially started at approximately the T -7 second mark. When the engine controllers indicate that they are all running normally, the twin solid rocket boosters (SRBs) are ignited at the T -0 mark.
T -0 Ignition & LiftoffAt the T -0:00:0 mark, the SRBs are ignited and the eight giant hold-down posts on the SRBs are released. At that instant, liftoff occurs. At
liftoff, the Shuttle's two SRBs and three SSMEs produce more than seven million
pounds of thrust - more thrust than the Saturn V moon rocket! Liftoff to Tank SeparationAbout one-half minute into flight, the increasing speed of the Shuttle versus the thinning atmosphere produces maximum pressure on the vehicle, known as MAX-Q.
As the SSMEs throttle down, the shape of the boosters' propellant causes the solid rocket motors to produce less thrust, reducing stress on the vehicle and crew. At one minute into the flight, the SSMEs throttle up to 104 percent and the Shuttle continues its flight upward. Two minutes after ignition, with the Shuttle moving over 3,000
miles per hour, the SRBs use up their 2.2 million pounds of propellant. Explosive
bolts
separate the boosters from the external tank, and booster separation motors
push them
away from the vehicle. The boosters continue to rise to an altitude of 41
miles before parachuting back into the ocean.
Press to MECOThe three main engines continue to fire for another six minutes after booster separation - propelling the orbiter to 17,500 miles per hour - and then shut down. This event is known as MECO - main engine cutoff. About 10 seconds later, the external tank, its fuel consumed, is jettisoned to fall and burn up over a deserted stretch of ocean. Meanwhile, small liquid fuel rocket engines on the orbiter maneuver it into orbit.
The Cycle ContinuesLanding is assisted by a parachute that quickly slows the orbiter. A few hours later, the vehicle is towed into a special hangar so workers can begin on the next mission.
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| Last Updated:
July 20, 2006
Responsible NASA Official: Yolanda B. Harris Curator: Shuttle Curator |
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