Space Shuttle abort modes
Space Shuttle abort modes were procedures by which the nominal launch of the NASA Space Shuttle could be terminated. A pad abort occurred after ignition of the shuttle's main engines but prior to liftoff. An abort during ascent that would result in the orbiter returning to a runway or to an orbit lower than planned was called an "intact abort", while an abort in which the orbiter would be unable to reach a runway, or any abort involving the failure of more than one main engine, was called a "contingency abort". Crew bailout was still possible in some situations in which the orbiter could not land on a runway.
Redundant set launch sequencer abort
The three Space Shuttle main engines were ignited roughly 6.6 seconds before liftoff, and computers monitored their performance as they increased thrust. If an anomaly was detected, the engines would be shut down automatically and the countdown terminated before ignition of the solid rocket boosters at T = 0 seconds. This was called a "redundant set launch sequencer abort", and occurred five times: STS-41-D, STS-51-F, STS-55, STS-51, and STS-68.Ascent abort modes
Once the shuttle's SRBs were ignited, the vehicle was committed to liftoff. If an event requiring an abort happened after SRB ignition, it was not possible to begin the abort until after SRB burnout and separation, about two minutes after launch. There were five abort modes available during ascent, divided into the categories of intact aborts and contingency aborts.The choice of abort mode depended on how urgent the situation was and what emergency landing site could be reached.
The abort modes covered a wide range of potential problems, but the most commonly expected problem was a main engine failure, causing the vehicle to have insufficient thrust to achieve its planned orbit. Other possible failures not involving the engines but necessitating an abort included a multiple auxiliary power unit failure, a progressive hydraulic failure, a cabin leak, and an external tank leak.
Intact abort modes
There were four intact abort modes for the Space Shuttle. Intact aborts were designed to provide a safe return of the orbiter to a planned landing site or to a lower orbit than that which had been planned for the mission.Return to launch site
Return to launch site was the first abort mode available and could be selected just after SRB jettison. The shuttle would continue downrange to burn excess propellant, as well as pitch up to maintain vertical speed in aborts with a main-engine failure. After burning sufficient propellant, the vehicle would be pitched all the way around and begin thrusting back towards the launch site. This maneuver was called the "powered pitcharound" and was timed to ensure that less than 2% propellant remained in the external tank by the time the shuttle's trajectory brought it back to the Kennedy Space Center. Additionally, the shuttle's OMS and reaction control system motors would continuously thrust to burn off excess OMS propellant to reduce landing weight and adjust the orbiter's center of gravity.Just before main engine cutoff, the orbiter would be commanded to pitch nose-down to ensure proper orientation for external tank jettison, since aerodynamic forces would otherwise cause the tank to collide with the orbiter. The main engines would cut off, and the tank would be jettisoned, as the orbiter used its RCS to increase separation.
Cutoff and separation would occur effectively inside the upper atmosphere at an altitude of about 230,000 ft, high enough to avoid subjecting the external tank to excessive aerodynamic stress and heating. The cutoff velocity would depend on the distance still to be traveled to reach the landing site and would increase based on the distance of the orbiter at cutoff. In any case, the orbiter would be flying too slowly to glide gently at such high altitude, and would start descending rapidly. A series of maneuvers in quick succession would pitch the orbiter's nose up to level off the orbiter once it reached thicker air, while at the same time ensuring that the structural limits of the vehicle were not exceeded.
Once this phase was complete, the orbiter would be about from the landing site and in a stable glide, proceeding to make a normal landing about 25 minutes after liftoff.
If a second main engine failed at any point during PPA, the shuttle would not be able to reach the runway at KSC, and the crew would have to bail out. A failure of a third engine during PPA would lead to loss of control and subsequent loss of crew and vehicle. Failure of all three engines as horizontal velocity approached zero or just before external tank jettison would also result in LOCV.
The capsule communicator would call out the point in the ascent at which an RTLS was no longer possible as "negative return", approximately four minutes after liftoff, at which point the vehicle would be unable to safely bleed off the velocity that it had gained in the distance between its position downrange and the launch site.
The RTLS abort mode was never needed in the history of the shuttle program. It was considered the most difficult and dangerous abort, but also among the most unlikely to occur as only a very narrow range of probable failures existed that were survivable but nevertheless so time-critical as to rule out more time-consuming abort modes. Astronaut Mike Mullane referred to the RTLS abort as an "unnatural act of physics", and many pilot astronauts hoped that they would not have to perform such an abort because of its difficulty.
Transoceanic abort landing
A transoceanic abort landing involved landing at a predetermined location in Africa, Western Europe, or the Atlantic Ocean about 25 to 30 minutes after liftoff. It was to be used when velocity, altitude, and distance downrange did not allow return to the launch point by Return To Launch Site. It was also to be used when a less time-critical failure did not require the faster but more dangerous RTLS abort.For performance issues such as engine failure, a TAL abort would have been declared between roughly T+2:30 and about T+5:00, after which the abort mode changed to Abort Once Around followed by Abort To Orbit. However, in the event of a time-critical failure, or one that would jeopardize crew safety such as a cabin leak or cooling failure, TAL could be called until shortly before main engine cutoff or even after MECO for severe underspeed conditions. The shuttle would then have landed at a predesignated airstrip across the Atlantic. The last four TAL sites were Istres Air Base in France, Zaragoza and Morón air bases in Spain, and RAF Fairford in England. Prior to a shuttle launch, two sites would be selected based on the flight plan and were staffed with standby personnel in case they were used. The list of TAL sites changed over time because of geopolitical factors. The exact sites were determined from launch to launch depending on orbital inclination.
Preparations of TAL sites took four to five days and began one week before launch, with the majority of personnel from NASA, the Department of Defense and contractors arriving 48 hours before launch. Additionally, two C-130 aircraft from the space flight support office from the adjacent Patrick Space Force Base would deliver eight crew members, nine pararescuemen, two flight surgeons, a nurse and medical technician, and of medical equipment to Zaragoza, Istres, or both. One or more C-21S or C-12S aircraft would also be deployed to provide weather reconnaissance in the event of an abort with a TALCOM, or astronaut flight controller aboard for communications with the shuttle pilot and commander.
This abort mode was never used during the entire history of the Space Shuttle program.
Abort once around
An abort once around was available if the shuttle was unable to reach a stable orbit but had sufficient velocity to circle Earth once and land at around 90 minutes after liftoff. Around five minutes after liftoff, the shuttle reaches a velocity and altitude sufficient for a single orbit around Earth. The orbiter would then proceed into re-entry; NASA could choose to have the orbiter land at Edwards Air Force Base, White Sands Space Harbor, or Kennedy Space Center. The time window for using the AOA abort was very short, just a few seconds between the TAL and ATO abort opportunities. Therefore, taking this option because of a technical malfunction was very unlikely, although a medical emergency on board could have necessitated an AOA abort.This abort mode was never needed during the entire history of the Space Shuttle program.
Abort to orbit
An abort to orbit was available when the intended orbit could not be reached but a lower stable orbit above above Earth's surface was possible. This occurred during mission STS-51-F, when ChallengerThe moment at which an ATO became possible was referred to as the "press to ATO" moment. In an ATO situation, the spacecraft commander rotated the cockpit abort mode switch to the ATO position and depressed the abort push button. This initiated the flight-control software routines that handled the abort. In the event of a loss of communication, the spacecraft commander could have made the abort decision and taken action independently.
A hydrogen fuel leak in one of the SSMEs during the STS-93 mission resulted in a slight underspeed at main engine cutoff but did not necessitate an ATO, and Columbia achieved its planned orbit; if the leak had been more severe, it might have necessitated one of the earlier abort options.