ASpace Shuttle abort was an emergency procedure due to equipment failure on NASA's Space Shuttle, most commonly during ascent. A main engine failure was a typical abort scenario. There were fewer abort options during reentry and descent. For example, the Columbia disaster happened during reentry, and there were no alternatives in that portion of flight.
Later in descent, certain failures were survivable, although not usually classified as an abort. For example, a flight control system problem or multiple auxiliary power unit failure would make reaching a landing site impossible, thus requiring the astronautstobail out.
There were five abort modes available during ascent, in addition to pad (RSLS) aborts. These were divided into the categories of intact aborts and contingency aborts.[1] 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 Space Shuttle Main Engine (SSME) failure, causing inability either to cross the Atlantic or to achieve orbit, depending on timing and number of failed engines. Other possible non-engine failures necessitating an abort included multiple auxiliary power unit (APU) failure, cabin leak, and external tank leak (ullage leak).
The main engines were ignited roughly 6.6 seconds before liftoff. From that point to ignition of the Solid Rocket Boosters at T - 0 seconds, the main engines could be shut down. This was called a "Redundant Set Launch Sequencer Abort", and happened five times, on STS-41-D, STS-51-F, STS-51, STS-55, and STS-68. It always happened under computer (not human) control, caused by computers sensing a problem with the main engines after starting but before the SRBs ignited. The SRBs could not be turned off once ignited, and after which the shuttle was committed to take off. If an event such as an SSME failure requiring an abort happened after SRB ignition, acting on the abort would have to wait until SRB burnout 123 seconds after launch. No abort options existed if that wait was not possible.[2]
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 planned for the mission.
In a Return To Launch Site (RTLS) abort, the Shuttle would have continued downrange until the solid rocket boosters were jettisoned. It would then pitch around, so the SSMEs fired retrograde. This maneuver would have occurred in a near-vacuum above the appreciable atmosphere and was conceptually no different from the OMS engines firing retrograde to de-orbit. The main engines continued burning until downrange velocity was killed and the vehicle began heading back toward the launch site at sufficient velocity to reach a runway. Afterwards the SSMEs were stopped, the external tank was jettisoned, and the orbiter made a normal gliding landing on the runway at Kennedy Space Center about 25 minutes after lift-off. The CAPCOM would call out the point in the ascent at which an RTLS was no longer possible as "negative return", approximately four minutes after lift-off.
Should all three SSMEs have failed, the shuttle would not have been able to make it back to the runway at KSC, forcing the crew to bail out. While this would have resulted in the loss of the Shuttle, the crew could escape safely and then be recovered by the SRB recovery ships.
This abort mode was never needed in the history of the Shuttle program. 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 due to its difficulty.[3]
A Transoceanic Abort Landing (TAL) involved landing at a predetermined location in Africa or western Europe about 25 to 30 minutes after lift-off.[4] It was used when velocity, altitude, and distance downrange did not allow return to the launch point via RTLS. It was also used when a less time-critical failure did not require the faster but possibly more stressful RTLS abort.
A TAL abort would be declared between roughly T+2:30 minutes (2 minutes and 30 seconds after liftoff) and Main Engine Cutoff (MECO), about T+8:30 minutes. The Shuttle would then land at a predesignated friendly airstrip in Europe. The last four TAL sites until the Shuttle's retirement were Istres Air Base in France, Zaragoza and Morón air bases in Spain, and RAF Fairford in England. Prior to a Shuttle launch, two of them were selected depending on the flight plan, and staffed with standby personnel in case they were used. The list of TAL sites changed over time; most recently Ben Guerir Air BaseinMorocco (TAL site from July 1988–June 2002) was eliminated due to terrorist attack concerns. Other previous TAL sites included Lajes Air Base, Terceira, Azores, Mallam Aminu Kano International Airport, Kano, Nigeria; Mataveri International Airport, Easter Island, Chile (for Vandenberg launches); Rota, Spain; Casablanca, Morocco; Banjul, Gambia; and Dakar, Senegal.
Preparations of TAL sites took 4 to 5 days and began a week before a 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 Manned Space Flight support office from the adjacent Patrick Air Force Base including eight crew members, nine pararescuemen, two flight surgeons, a nurse and medical technician, along with 2,500 pounds of medical equipment were deployed to either Zaragoza, Istres, or both. One or more C-21 or a C-12 aircraft were also 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.[4]
This abort mode was never needed during the entire history of the space shuttle program.
An Abort Once Around (AOA) was available when the shuttle could not reach a stable orbit but had sufficient velocity to circle the earth once and land, about 90 minutes after lift-off. 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 was very unlikely.
This abort mode was never needed during the entire history of the space shuttle program.
An Abort to Orbit (ATO) was available when the intended orbit could not be reached but a lower stable orbit was possible. This occurred on mission STS-51-F, which continued despite the abort to a lower orbit. The Mission Control Center in Houston (located at Lyndon B. Johnson Space Center) observed an SSME failure and called "Challenger--Houston, Abort ATO. Abort ATO".
The 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 which handled the abort. In the event of lost communications, the spacecraft commander could have made the abort decision and taken action independently.
A hydrogen fuel leak in one of the SSMEsonSTS-93 resulted in a slightly lower orbit than anticipated, but was not an ATO; if the leak had been more severe, it might have necessitated an ATO, RTLS, or TAL abort.
Pre-determined emergency landing sites for the Orbiter were determined on a mission-by-mission basis according to the mission profile, weather and regional political situations. Emergency landing sites during the shuttle program included:[5][6]
Sites in which an Orbiter has landed are listed in bold, but none is an emergency landing.
British Overseas Territories
Democratic Republic of the Congo
In the event of an emergency deorbit that would bring the Orbiter down in an area not within range of a designated emergency landing site, the Orbiter was theoretically capable of landing on any paved runway that was at least 3 km (9,800 ft) long, which included the majority of large commercial airports. In practice, a US or allied military airfield would have been preferred for reasons of security arrangements and minimizing the disruption of commercial air traffic.
There was an order of preference for abort modes:
Unlike all previous U.S. manned launch vehicles, the shuttle never flew unmanned test flights. To provide an incremental non-orbital manned test, NASA considered making the first mission an RTLS abort. However, STS-1 commander John Young declined, saying, "let's not practice Russian roulette."[13]
Contingency aborts were designed to permit flight crew survival following more severe failures when an intact abort is not possible. A contingency abort would have generally resulted in a ditch operation.
Were the Orbiter unable to reach a runway, it could ditch in water, or could land on terrain other than a landing site. It would have been unlikely for the flight crew still on board to survive. However, for ascent abort scenarios where controlled gliding flight is achievable, a bailout was possible. For more details, see "Post-Challenger abort enhancements" below.
In the two disasters, things went wrong so fast that little could be done. In the case of Challenger, the Space Shuttle Solid Rocket Boosters were still burning as they tore free from the rest of the stack, with the right-hand booster impacting the external tank. The orbiter disintegrated almost instantly from aerodynamic stresses as the stack broke up. The Columbia disaster occurred high in the atmosphere during reentry.
Before the Challenger disaster during STS-51-L, very limited ascent abort options existed. Failure of only a single SSME was survivable prior to about 350 seconds into the ascent. Two or three failed SSMEs prior to that point would mean loss of crew and vehicle (LOCV), since no bailout option existed. Two or three failed SSMEs while the SRBs were firing would probably have overstressed the struts attaching the orbiter to the external tank, causing vehicle breakup. For that reason, a Return To Launch Site (RTLS) abort was not possible in the event of two or three failed SSMEs. Studies showed an ocean ditching was not survivable. Furthermore, the loss of a second or third SSME at almost any time during an RTLS abort would have caused a LOCV.
After the loss of Challenger in STS-51-L, numerous abort enhancements were added. With those enhancements, the loss of two SSMEs was now survivable for the crew throughout the entire ascent, and the vehicle could survive and land for large portions of the ascent. Loss of three SSMEs was survivable for the crew for most of the ascent, although survival in the event of three failed SSMEs before T+90 seconds is questionable. However, it was conceivable that failure of three SSMEs just after liftoff might be survivable, since the SRBs provided enough thrust and steering authority to continue the ascent until a bailout or RTLS. The struts attaching the orbiter to the external tank were strengthened to better endure a multiple SSME failure.
A particular significant enhancement was bailout capability. This is not ejection as with a fighter plane, but an Inflight Crew Escape System[14] (ICES). The vehicle was put in a stable glide on autopilot, the hatch was blown, and the crew slid out a pole to clear the orbiter's left wing. They would then parachute to earth or the sea. While this may at first appear only usable under rare conditions, there were many failure modes where reaching an emergency landing site was not possible yet the vehicle was still intact and under control. Before the Challenger disaster, this almost happened on STS-51-F, when a single SSME failed at about T+345 seconds. The orbiter in that case was also Challenger. A second SSME almost failed due to a spurious temperature reading; fortunately the engine shutdown was inhibited by a quick-thinking flight controller. If the second SSME failed within about 69 seconds of the first, there would have been insufficient energy to cross the Atlantic. Without bailout capability the entire crew would be lost. After the loss of Challenger, those types of failures were made survivable. To facilitate high altitude bailouts, the crew began wearing the Launch Entry Suit and later the Advanced Crew Escape Suit during ascent and descent. Before the Challenger disaster, crews for operational missions wore only fabric flight suits.
Another post-Challenger enhancement was the addition of East Coast Abort Landings (ECAL). High-inclination launches (including all ISS missions) were now able to reach an emergency runway on the East Coast of the United States under certain conditions.
An ECAL abort was similar to RTLS, but instead of landing at the Kennedy Space Center, the orbiter would attempt to land at another site along the east coast of North America. Various emergency landing sites extended from South Carolina and Bermuda up into Newfoundland, Canada. ECAL was a contingency abort that was less desirable than an intact abort, primarily because there was so little time to choose the landing site and prepare for the orbiter's arrival. The ECAL emergency sites were not as well equipped to accommodate an orbiter landing as those prepared for an RTLS abort.[15]
Numerous other abort refinements were added, mainly involving improved software for managing vehicle energy in various abort scenarios. These enabled a greater chance of reaching an emergency runway for various SSME failure scenarios.
An ejection escape system, sometimes called a launch escape system, had been discussed many times for the shuttle. After the Challenger and Columbia losses, great interest was expressed in this. All previous US manned space vehicles had launch escape systems, although none was ever used.
Modified Lockheed SR-71 ejection seats were installed on the first four shuttle flights (all two-man missions aboard Columbia) and removed afterwards. Ejection seats were not further developed for the shuttle for several reasons:
[I]n truth, if you had to use them while the solids were there, I don’t believe you’d—if you popped out and then went down through the fire trail that’s behind the solids, that you would have ever survived, or if you did, you wouldn't have a parachute, because it would have been burned up in the process. But by the time the solids had burned out, you were up to too high an altitude to use it. ... So I personally didn't feel that the ejection seats were really going to help us out if we really ran into a contingency.[16]
The Soviet shuttle Buran was planned to be fitted with the crew emergency escape system, which would have included K-36RB (K-36M-11F35) seats and the Strizh full-pressure suit, qualified for altitudes up to 30,000 m and speeds up to Mach 3.[17] Buran flew only once in fully automated mode without a crew, thus the seats were never installed and were never tested in real human space flight.
An alternative to ejection seats was an escape crew capsule or cabin escape system where the crew ejected in protective capsules, or the entire cabin is ejected. Such systems have been used on several military aircraft. The B-58 Hustler, XB-70 Valkyrie, General Dynamics F-111 and early prototypes of the Rockwell B-1 Lancer used cabin ejection.
Like ejection seats, capsule ejection for the shuttle would have been difficult because no easy way existed to exit the vehicle. Several crewmembers sat in the middeck, surrounded by substantial vehicle structure.
Cabin ejection would work for a much larger portion of the flight envelope than ejection seats, as the crew would be protected from temperature, wind blast, and lack of oxygen or vacuum. In theory an ejection cabin could have been designed to withstand reentry, although that would entail additional cost, weight and complexity. Cabin ejection was not pursued for several reasons:
Source:[18]
| Date | Orbiter | Mission | Type of Abort | Time of Abort | Description |
|---|---|---|---|---|---|
| 1984-06-26 | Discovery | STS-41-D | RSLS | T-4 seconds | Sluggish valve detected in Space shuttle main engine (SSME) #3. Discovery rolled back to VAB for engine replacement. |
| 1985-07-12 | Challenger | STS-51-F | RSLS | T-3 seconds | Coolant valve problem with SSME #2. Valve was replaced on launch pad. |
| 1985-07-29 | Challenger | STS-51-F | ATO | T+5 minutes, 45 seconds | Sensor problem shutdown SSME #1. Mission continued in lower than planned orbit. |
| 1993-03-22 | Columbia | STS-55 | RSLS | T-3 seconds | Problem with purge pressure readings in the oxidizer preburner on SSME #2. All engines replaced on pad. |
| 1993-08-12 | Discovery | STS-51 | RSLS | T-3 seconds | Sensor that monitors flow of hydrogen fuel in SSME #2 failed. All engines replaced on launch pad. |
| 1994-08-18 | Endeavour | STS-68 | RSLS | T-1 second | Sensor detected higher than acceptable readings of the discharge temperature of the high pressure oxidizer turbopump in SSME #3. Endeavour rolled back to VAB to replace all 3 engines. A test firing at Stennis Space Center confirmed a drift in the fuel flow meter which resulted in a slower start in the engine which caused the higher temperatures. |
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