Although the shuttle is long dead, from time to time some fascinating documents are digged and published. The last in a serie is a Houston memo on shuttle abort modes.
The defunct space shuttle was not as lucky as Apollo, and in fact couldn't really ditch in water without tearing itself apart. NASA really didn't wanted astronauts swimming or rafting the icy and tormented North Atlantic waves.
It seems there were three intact abort modes planned 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.
Note: it seems that memo come late in the shuttle program, perhaps mid-1971 or so, only weeks before the program cancellation. At the time the orbiter had switched to an external, expendable tank.
Return To Launch Site: The Greater of Four Evils
An RTLS abort could be declared if a failure happened during the first four minutes of the launch. Relatively speaking, the shuttle was still quite close to KSC during this period. In order to land, the shuttle had to convert from a fire-belching rocket ship into an oversized glider. Unfortunately, limits within each of the main propulsion systems prevented this transition from happening until the shuttle was much higher, faster, and further down range.
During ascent it was theoretically possible to shut down the orbiter main engines (although they could not be restarted) and jettison the ET. The tank, however, was still quite full of fuel at this point. The primary concern was that firing the explosive bolts to jettison a heavy ET would cause the fuel to slosh and possibly steer the tank back into a collision course with the orbiter. The consequences of that scenario are obvious. The unintuitive, yet safer option was to continue burning fuel while heading away from the place you planned to end up.
Then in order to go back to KSC the shuttle would have swapped ends while still heading spaceward at about Mach 5.
The RTLS isn’t quite so easy to rationalize. While this abort shares the eastbound powered ascent, most of the descent phase would have been westbound.
Tying those two events together was an element that was unique to RTLS: the Powered Pitch Around (PPA). The shuttle would have swapped ends while still heading spaceward (at about Mach 5) and never easing off of the throttle. With the SSMEs now facing forward, their retrograde thrust would slow the orbiter’s eastward progress. Eventually, the shuttle would stop in midair (albeit very thin, high-altitude air) and begin heading westward back to the runway at KSC.
The PPA would have begun at an altitude somewhere around 400,000 feet, where there is insufficient air to cause much concern about the aerodynamic effects of tumbling the shuttle. The topic of debate was whether a handicapped propulsion system would have sufficient control authority to whip the orbiter/ET combination around at the 10-degrees per second rate required for the PPA.
A further concern with the PPA was the rate at which the shuttle/ET would descend. As the shuttle’s eastward progress began to slow after the PPA, it would begin losing altitude at an ever increasing rate. At the point where its eastward velocity was zero and it began to accelerate westward, it would have been falling vertically at greater than Mach 1. Such a fast descent rate would have subjected the orbiter and ET to significant and ever-increasing dynamic pressure and heat loading. The descent rate would have been gradually arrested as the shuttle picked up westbound velocity (about 200,000 feet of altitude is lost during the PPA), but the air would get thicker and cause more friction with each foot of altitude lost.
For the sake of argument, let’s say that all aspects of the PPA went perfectly and the shuttle was zooming back towards KSC (called the “flyback phase”). The crew would have no time for relieve as they now would have to negotiate what many consider to be the most difficult and risky portion of an RTLS: the Powered Pitch Down (PPD). With the orbiter main engines still burning, but the external tank nearly empty, the shuttle would be placed in a slight nose-down attitude. This attitude would be held through MECO and the firing of the explosive bolts that jettison the external tank. A few bursts of the shuttle’s downward-firing reaction control thrusters would have put elbow room between the orbiter and the freefalling external tank.
The point of the PPD was to maximize the shuttle’s odds of making a clean getaway from the jettisoned external tank. What made it difficult was the precision and timing with which it had to be executed. Immediately after clearing the external tank, the shuttle’s nose would have to be raised to a specific positive angle. This move was necessary to position the belly-mounted heat shields to do their job through reentry. Just like the PPA, the PPD was not really a question of piloting skill (most of the maneuver could be flown by auto pilot), but whether the damage already absorbed by the shuttle would allow it to perform these precise and critical maneuvers.
The second shuttle abort mode was the so-called Transoceanic Abort Landing (TAL). Very ironically it was designed to face issues somewhat similar to the current Helios controversy, that is, unlike Apollo, the shuttle can't abort in the North Atlantic – it has to reach an airport.
A Transoceanic Abort Landing (TAL) involved landing at a predetermined location in Africa or western Europe about 25 to 30 minutes after lift-off. It was to be used when velocity, altitude, and distance downrange did not allow return to the launch point via RTLS. It was also to be used when a less time-critical failure did not require the faster but more dangerous RTLS abort.
A TAL abort would have been 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 have landed at a predesignated airstrip across the Atlantic. TAL sites depended on orbital inclination. A guess can be made that Skylab and Liberty inclinations would result in European aboart landing sites. Low inclination orbits would abort on the West coast of Africa.
Prior to a Shuttle launch, two sites would be selected based on the flight plan, and would be staffed with standby personnel in case they were used.
The last abort mode was the so-called Abort to Orbit (ATO). It was available when the intended orbit could not be reached but a lower stable orbit was possible. The moment at which an ATO became possible was referred to as the "press to ATO" moment.
January 23, 1986
Dakar, Africa
Today a Titan III will launch an unmanned Big Gemini stack into a low inclination orbit. It will be a milestone into the return to flight process.
The recent Titan mishap illustrated how astronauts have little interest in swimming in the North Atlantic icy and tormented waters. Apollo had no such issues: the large SPS engine in the back ensured the CSM would safely jump over North Atlantic. Even if such thing happened, however, Apollo capsules could land with parachutes and float in the water waiting for rescue ships, aircrafts and helicopters.
Big Gemini lacks the big SPS on its back to safely push it out of North Atlantic deadly embrace. Ideally, the powerful Titan stage 2 was tasked with the job... but in this case its LR-91 rocket engine failed.
NASA has now to take into account that major issue. Congress has made a lot of noise about astronauts chance of survival, not only in Northern Atlantic, but also in unhospitable places like deserts or rain forests. Today's flight will try landing in the Sahara desert, testing a fast-reaction recovery force – essentially an Atlantic ocean landing turning into a desert landing at the last moment.
There will be a large rescue force in Dakar airport and harbor – notably a Navy LPH helicopter carrier and a P-3 maritime patrol aircraft. Preparations of the Dakar sites took 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 Manned Space Flight support office from the adjacent Patrick Air Force Base, Florida, including eight crew members, nine pararescuemen, two flight surgeons, a nurse and medical technician, along with 2,500 pounds (1,100 kg) of medical equipment will be deployed to Dakar. One or more C-21 or a C-12 aircraft would also be deployed to provide weather reconnaissance in the event of an abort.
Dakar would be used on non-Liberty missions, albeit they are rare. A decade ago NASA selected Dakar for satellite inspection missions such as Pegasus 1, Pegasus 2, OSO-8 and Apollo 9 "Spider" LM. All were launched in orbit inclined by 28 to 33 degree. After 1982 the agreement with Senegal was put on hold as most missions went to Liberty. After the July 1985 mishap the agreement was hastily renewed in October, if only for one test flight.
Since December Senegalese officials have warned NASA that Dakar airport might be overcrowded, since it also support the Paris – Dakar rally, which needs heavy logistic support from the air. That year Dakar was the 8th running of that event, which ended January 22. The event was overshadowed by the death of the event organiser, Thierry Sabine, and five others in a helicopter crash in Mali. The Dakar airlift usually flies no less than twenty or even thirty cargo aircrafts – a rag-tag fleet made of Dakotas, Twin Otters, Fokker F-27s and even L-100 civilian Hercules, plus a boatload of helicopters.
Another major issue with Helios is that, once the launch escape system (LES) is jettisoned, the crewed reentry module can't detach from the large cylindrical cargo section until in orbit. NASA is currently correcting that mistake by keeping the LES farther into the ascent, long enough that a Big Gemini reentry module could detach and land at a runway under its parasail.
Other landing sites are being considered further north, in Great Britain, France and Spain, to cover Big Gemini majority of flights, that is to space station Liberty 51.6 degree inclination. There the US Navy ensured the help of British, French, and Spanish navies. Senegal however doesn't have a blue water navy.
Congress forced NASA to a deep review of Big Gemini landing zones and safety. One test will land in the Sahara, the other (in February or March) will test North Atlantic emergency landings. Consideration has been given to try snatching the Big Gemini crew module midair (as done with Corona capsules). But C-130 Hercules can't do the trick, and were replaced by C-141 Starlifters.
In the days of Corona, Hercules recovery aircrafts were manned by a crew of 10 personnel. The crew consisted of two pilots, one flight engineer, two telemetry operators, one winch operator, and four riggers. The telemetry operators would acquire the location of the satellite and relay the info to the pilots. Once visually acquired the pilots would head on course to the capsule descending towards the Atlantic Ocean. One could visually acquire the capsule and its parachute at an altitude of approximately 50,000 ft. The winch operator and the riggers would deploy the retrieving apparatus called the "Loop", which consisted of high quality nylon rope with a series of brass hooks spliced into the apparatus. The whole snatching operation by the pilots was done visually. The winch operator and the four riggers would deploy the loop. Once contact was made between the parachute and the loop the winch line would pay out and stop. The winch then was put into gear and the retrieval process commenced.
Retrieval of Big Gemini crew module would be very similar to that process. C-141s would be on alert in Great Britain, France and Spain, together with airborne C-141s across the Atlantic, eventually supported by aerial refueling.
