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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Space Shuttle Solid Rocket Booster | 4/9 | https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster | reference | science, encyclopedia | 2026-05-05T13:22:36.055022+00:00 | kb-cron |
=== Ignition === SRB ignition can occur only when a manual lock pin from each SRB safe and arm device has been removed. The ground crew removes the pin during prelaunch activities. At T−5:00, the SRB safe and arm device is rotated to the arm position. The solid rocket motor ignition commands are issued when the three Space Shuttle Main Engines (SSMEs) are at or above 90% of rated thrust, no SSME fail and/or SRB ignition Pyrotechnic Initiator Controller (PIC) low voltage is indicated and there are no holds from the Launch Processing System (LPS). The solid rocket motor ignition commands are sent by the orbiter computers through the Master Events Controllers (MECs) to the safe and arm device NASA standard detonators (NSDs) in each SRB. A PIC single-channel capacitor discharge device controls the firing of each pyrotechnic device. Three signals must be present simultaneously for the PIC to generate the pyro firing output. These signals, arm, fire 1 and fire 2, originate in the orbiter general-purpose computers (GPCs) and are transmitted to the MECs. The MECs reformat them to 28 volt DC signals for the PICs. The arm signal charges the PIC capacitor to 40 volts DC (minimum of 20 volts DC). The GPC launch sequence also controls certain critical main propulsion system valves and monitors the engine ready indications from the SSMEs. The MPS start commands are issued by the onboard computers at T−6.6 seconds (staggered start engine three, engine two, engine one all approximately within 0.25 of a second), and the sequence monitors the thrust buildup of each engine. All three SSMEs must reach the required 90% thrust within three seconds; otherwise, an orderly shutdown is commanded and safing functions are initiated. Normal thrust buildup to the required 90% thrust level will result in the SSMEs being commanded to the lift off position at T−3 seconds as well as the fire 1 command being issued to arm the SRBs. At T−3 seconds, the vehicle base bending load modes are allowed to initialize (referred to as the "twang", movement of approximately 25.5 in (650 mm) measured at the tip of the external tank, with movement towards the external tank). The fire 2 commands cause the redundant NSDs to fire through a thin barrier seal down a flame tunnel. This ignites a pyro. booster charge, which is retained in the safe and arm device behind a perforated plate. The booster charge ignites the propellant in the igniter initiator; and combustion products of this propellant ignite the solid rocket motor initiator, which fires down the entire vertical length of the solid rocket motor igniting the solid rocket motor propellant along its entire surface area instantaneously. At T−0, the two SRBs are ignited, under command of the four onboard computers; separation of the four explosive bolts on each SRB is initiated; the two T-0 umbilicals (one on each side of the spacecraft) are retracted; the onboard master timing unit, event timer and mission event timers are started; the three SSMEs are at 100%; and the ground launch sequence is terminated.
=== Lift-off and ascent ===
Timing sequence referencing in ignition is critical for a successful liftoff and ascent flight. The explosive hold-down bolts relieve (through the launch support pedestals and pad structure) the asymmetric vehicle dynamic loads caused by the SSME ignition and thrust buildup, and applied thrust bearing loads. Without the hold-down bolts the SSMEs would violently tip the flight stack (orbiter, external tank, SRBs) over onto the external tank. That rotating moment is initially countered by the hold-bolts. Prior to release of the vehicle stack for liftoff, the SRBs must simultaneously ignite and pressurize their combustion chambers and exhaust nozzles to produce a thrust-derived, net counter-rotating moment exactly equal to the SSME's rotating moment. With the SRBs reaching full thrust, the hold-down bolts are blown, releasing the vehicle stack, the net rotating moment is zero, and the net vehicle thrust (opposing gravity) is positive, lifting the orbiter stack vertically from the launch pedestal, controllable through the coordinated gimbal movements of the SSMEs and the SRB exhaust nozzles. During ascent, multiple all-axis accelerometers detect and report the vehicle's flight and orientation (referencing the flight deck aboard the orbiter), as the flight reference computers translate navigation commands (steering to a particular waypoint in space, and at a particular time) into engine and motor nozzle gimbal commands, which orient the vehicle about its center of mass. As the forces on the vehicle change due to propellant consumption, increasing speed, changes in aerodynamic drag, and other factors, the vehicle automatically adjusts its orientation in response to its dynamic control command inputs.
=== Separation ===