Editing Space Shuttle (section)

===Launch===
[[File:ShuttleLaunch.gif|thumb|Early ignition and lift-off view of main-engines and SRB (ground-camera view)]]
The mission crew and the Launch Control Center (LCC) personnel completed systems checks throughout the countdown. Two built-in holds at T−20&nbsp;minutes and T−9&nbsp;minutes provided scheduled breaks to address any issues and additional preparation.<ref name=jenkins2016 />{{rp|III–8}} After the built-in hold at T−9&nbsp;minutes, the countdown was automatically controlled by the Ground Launch Sequencer (GLS) at the LCC, which stopped the countdown if it sensed a critical problem with any of the Space Shuttle's onboard systems.<ref name="sts121_blog" /> At T−3&nbsp;minutes&nbsp;45&nbsp;seconds, the engines began conducting gimbal tests, which were concluded at T−2&nbsp;minutes&nbsp;15&nbsp;seconds. The ground [[Launch Processing System]] handed off the control to the orbiter vehicle's GPCs at T−31&nbsp;seconds. At T−16&nbsp;seconds, the GPCs armed the SRBs, the sound suppression system (SPS) began to drench the MLP and SRB trenches with {{convert|300000|USgal|L|sigfig=2|order=flip|sp=us|abbr=on}} of water to protect the orbiter vehicle from damage by [[acoustical]] energy and rocket exhaust reflected from the flame trench and MLP during lift-off.<ref name="sound_suppression">{{cite web |url=http://www.nasa.gov/mission_pages/shuttle/launch/sound-suppression-system.html |title=Sound Suppression System |date=November 23, 2007 |last=Ryba |first=Jeanne |publisher=NASA |access-date=March 22, 2020 |archive-date=June 29, 2011 |archive-url=https://web.archive.org/web/20110629143632/http://www.nasa.gov/mission_pages/shuttle/launch/sound-suppression-system.html |url-status=live}}</ref><ref name="sps">{{cite web |title=Sound Suppression Water System |publisher=NASA |date=August 28, 2000 |last=Grinter |first=Kay |url=http://www-pao.ksc.nasa.gov/kscpao/nasafact/count4ssws.htm |archive-url=https://web.archive.org/web/20140313042409/http://www-pao.ksc.nasa.gov/kscpao/nasafact/count4ssws.htm |archive-date=March 13, 2014 |url-status=dead |access-date=April 9, 2020}}</ref> At T−10&nbsp;seconds, hydrogen igniters were activated under each engine bell to quell the stagnant gas inside the cones before ignition. Failure to burn these gases could trip the onboard sensors and create the possibility of an overpressure and explosion of the vehicle during the firing phase. The hydrogen tank's prevalves were opened at T−9.5&nbsp;seconds in preparation for engine start.<ref name=jenkins2016 />{{rp|II–186}}

[[File:STS 135 Launch.gif|thumb|Shuttle lift-off via on-board camera view.]]
Beginning at T−6.6&nbsp;seconds, the main engines were ignited sequentially at 120-millisecond intervals. All three RS-25 engines were required to reach 90% rated thrust by T−3&nbsp;seconds, otherwise the GPCs would initiate an [[RSLS Abort|RSLS abort]]. If all three engines indicated nominal performance by T−3&nbsp;seconds, they were commanded to gimbal to liftoff configuration and the command would be issued to arm the SRBs for ignition at T−0.<ref name="countdown101">{{cite web |last=Ryba |first=Jeanne |title=Countdown 101 |publisher=NASA |date=September 17, 2009 |url=http://www.nasa.gov/mission_pages/shuttle/launch/countdown101.html |access-date=March 22, 2020 |archive-date=January 26, 2020 |archive-url=https://web.archive.org/web/20200126124224/https://www.nasa.gov/mission_pages/shuttle/launch/countdown101.html |url-status=live}}</ref> Between T−6.6&nbsp;seconds and T−3&nbsp;seconds, while the RS-25 engines were firing but the SRBs were still bolted to the pad, the offset thrust would cause the Space Shuttle to pitch down {{convert|25.5|in|abbr=on|order=flip}} measured at the tip of the external tank; the 3-second delay allowed the stack to return to nearly vertical before SRB ignition. This movement was nicknamed the "twang." At T−0, the eight [[Pyrotechnic fastener|frangible nuts]] holding the SRBs to the pad were detonated, the final umbilicals were disconnected, the SSMEs were commanded to 100% throttle, and the SRBs were ignited.<ref name=nuts0>{{cite web |url=http://www.nasa.gov/centers/marshall/pdf/290339main_8-388221J.pdf |title=Space Shuttle Solid Rocket Booster |access-date=March 22, 2020 |publisher=NASA |date=November 2008 |last=Roy |first=Steve |archive-date=November 13, 2018 |archive-url=https://web.archive.org/web/20181113090531/https://www.nasa.gov/centers/marshall/pdf/290339main_8-388221J.pdf |url-status=live}}</ref><ref name="frang_nut_liftoff">{{cite web |title=Solid Rocket Boosters |date=August 31, 2000 |last=Dumoulin |first=Jim |url=http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html |publisher=NASA |access-date=March 22, 2020 |archive-date=February 16, 2012 |archive-url=https://web.archive.org/web/20120216005534/http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html |url-status=live}}</ref> By T+0.23&nbsp;seconds, the SRBs built up enough thrust for liftoff to commence, and reached maximum chamber pressure by T+0.6&nbsp;seconds.<ref>{{cite web |title=Shuttle Crew Operations Manual |url=http://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf |publisher=NASA |access-date=May 4, 2018 |archive-date=December 16, 2017 |archive-url=https://web.archive.org/web/20171216034929/https://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf |url-status=live}}</ref><ref name=jenkins2016 />{{rp|II–186}} At T−0, the JSC [[Mission Control Center]] assumed control of the flight from the LCC.<ref name=jenkins2016 />{{rp|III–9}}

[[File:STS135 SRB Separation.gif|thumb|On-board camera-view of SRB separation.]]
At T+4&nbsp;seconds, when the Space Shuttle reached an altitude of {{convert|73|ft|m|sigfig=2|sp=us|order=flip|adj=off}}, the RS-25 engines were throttled up to 104.5%. At approximately T+7&nbsp;seconds, the Space Shuttle rolled to a heads-down orientation at an altitude of {{convert|350|ft|m|sigfig=2|sp=us|order=flip|adj=off}}, which reduced aerodynamic stress and provided an improved communication and navigation orientation. Approximately 20–30&nbsp;seconds into ascent and an altitude of {{convert|9000|ft|m|sigfig=2|sp=us|order=flip|adj=off}}, the RS-25 engines were throttled down to 65–72% to reduce the maximum aerodynamic forces at [[Max Q]].<ref name=jenkins2016 />{{rp|III–8–9}} Additionally, the shape of the SRB propellant was designed to cause thrust to decrease at the time of Max Q.<ref name=jenkins />{{rp|427}}  The GPCs could dynamically control the throttle of the RS-25 engines based upon the performance of the SRBs.<ref name=jenkins2016 />{{rp|II–187}}

[[File:Shuttle ET Separation STS-135.gif|thumb|On-board camera-view of external-tank separation]]
At approximately T+123&nbsp;seconds and an altitude of {{convert|150000|ft|m|sigfig=2|sp=us|order=flip|adj=off}}, pyrotechnic fasteners released the SRBs, which reached an [[apogee]] of {{convert|220000|ft|m|sigfig=2|sp=us|order=flip|adj=off}} before parachuting into the [[Atlantic Ocean]]. The Space Shuttle continued its ascent using only the RS-25 engines. On earlier missions, the Space Shuttle remained in the heads-down orientation to maintain communications with the [[tracking station]] in [[Cooper's Island, Bermuda|Bermuda]], but later missions, beginning with [[STS-87]], rolled to a heads-up orientation at T+6&nbsp;minutes for communication with the [[tracking and data relay satellite]] constellation. The RS-25 engines were throttled at T+7&nbsp;minutes&nbsp;30&nbsp;seconds to limit vehicle acceleration to 3 ''g''. At 6&nbsp;seconds prior to main engine cutoff (MECO), which occurred at T+8&nbsp;minutes&nbsp;30&nbsp;seconds, the RS-25 engines were throttled down to 67%. The GPCs controlled ET separation and dumped the remaining liquid oxygen and hydrogen to prevent outgassing while in orbit. The ET continued on a ballistic trajectory and broke up during reentry, with some small pieces landing in the Indian or Pacific Ocean.<ref name=jenkins2016 />{{rp|III–9–10}}

Early missions used two firings of the OMS to achieve orbit; the first firing raised the apogee while the second circularized the orbit. Missions after [[STS-38]] used the RS-25 engines to achieve the optimal apogee, and used the OMS engines to circularize the orbit. The orbital altitude and inclination were mission-dependent, and the Space Shuttle's orbits varied from {{convert|120|to|335|nmi|km|sigfig=2|sp=us|adj=off|abbr=on|order=flip}}.<ref name=jenkins2016 />{{rp|III–10}}