Abort modes
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[edit] Launch Abort Modes
While the establishment of the abort advisory system went smoothly, the matter of who held abort authority during the first ten seconds of flight (until tower clearance) proved more troublesome. KSC officials believed the launch operations director was in the best-position to command an abort. The astronauts objected, arguing that the launch director might abort the mission at an undesirable moment for them or the spacecraft. Eventually the astronauts won the argument. As information came to the launch director during the first seconds of flight, he would assess the situation. If an abort appeared necessary, the director could trigger the "Abort Light" on the flight panel in the spacecraft. If the "Thrust O.K." light indicated a malfunction or if the astronauts sensed a problem, the crew could manually activate the launch escape system.[1]
[edit] Apollo 15
Scott, from 1998 correspondence - "The 'abort' function was so very critical in terms of success/failure that many people thought there should be no crew function, and it should all be automatic (which in turn would introduce other more consequential failure modes). The most difficult simulations during the entire training process were 'launch aborts' - even more so than lunar landings (the landing itself was more difficult than launch, but not for 'aborts'). More crews 'bought it' during launch sims than any other area, by far!" (ap15fj)
[edit] Abort mode IA
The initial 42 seconds, to an altitude of about 3,000 metres (10,000 feet) are flown in abort Mode IA (one alpha). If a dangerous situation occurs within this period, the CM would separate from the SM, and the LET (Launch Escape Tower, or just 'tower'), which is the solid-fuelled rocket mounted on top of the CM, would carry it up from the wayward launch vehicle while a small 'pitch control' motor at the top of the LET steers the assembly east out over the ocean and away from a possibly exploding booster below. The tower would be jettisoned only 14 seconds after the initiation of the abort. While this is going on, the highly dangerous hypergolic propellants of the Command Module's RCS would quickly and automatically be dumped overboard as they would be harmful to the recovery forces. The CM would then descend on parachutes to a normal splashdown. (ap15fj)
[edit] Abort mode IB
Abort Mode IB extends from 42 seconds into the flight to an altitude of 30.5 km (16.5 nautical miles) as defined by the abort checklist. With the vehicle being further downrange and tilted over, the pitch control motor would not be required in the event of a IB abort. However, it had been discovered during hypersonic testing, that the CM/LET stack could be aerodynamically stable in a tower-first as well as a base-first attitude so a pair of canards were added which would be deployed automatically to force the combination into an attitude where the base of the CM is facing the direction of travel, ready for the safe deployment of the drogue and main parachutes. While the canards have little effect in a low altitude abort, they become increasingly important as the Saturn V gains speed through the IB mode. (ap15fj)
[edit] Abort mode IC
Mode IC is used for aborts occurring between 30.5 km (16.5 nautical miles) and the jettison of the tower. As the air is now very thin, the airflow around the pair of canards at the top of the tower would have little aerodynamic effect during an abort, so the Command Module's RCS would be used to control the orientation of the spacecraft until they become effective. The safe range of vehicle motion rates are now defined as not exceeding ±9° per second in pitch and yaw, ±20° per second in roll. (ap15fj)
[edit] Abort mode II
Abort Mode II lasts from the jettisoning of the tower to the decision to stage from the S-II to the S-IVB. In a Mode II abort, the Command and the Service Modules will separate from the launch vehicle and the SM main engine or its RCS engines will be used to get the spacecraft away from the launch vehicle. Then the CM and SM will separate before the CM completes a normal splashdown on the ocean. (ap15fj)
[edit] Abort mode III (COI)
COI stands for Contingency Orbit Insertion. This is another way of saying "abort Mode III". The S-IVB now has the capability to take the stack to a point where the Service Module's large SPS engine can ignite and place the CSM into Earth orbit. However, in the event of such an abort, and without the S-IVB, the spacecraft would not be able to depart for the Moon, instead embarking on a planned for, but hopefully unrequired Earth orbit mission. (ap15fj)
[edit] Abort mode IV
Mode IV is the abort mode where the crew have been given a Go decision to continue to orbit using the S-IVB, and should that stage deviate from its allowed limits, the CSM will separate from the Saturn and use the SPS (Service Propulsion System) to continue into Earth orbit.
The sequence of events for the first ignition of the single J-2 engine in the third stage is essentially the same as for the engines in the S-II. The main change is that the supercold fuel is allowed to flow through the walls of the thrust chamber to condition it for three seconds, instead of one, before the Start Tank discharges through the turbines, spinning them up in preparation for operation. (ap15fj)
[edit] Range safety ordnance
Each stage of the Saturn V launch vehicle has shaped explosive charges attached to its outer surface which, in the event of an abort, rupture the fuel and oxidiser tanks, dispersing their contents into the atmosphere rather than allow them to impact the Earth with dangerous loads still on board. The charge for the S-IC (the designation of the first stage) cuts a longitudinal breach in the fuel tank on the opposite side of the vehicle from that for the oxidiser tank so as to minimise their mixing during dispersion. Charges for the S-II (second stage) cut a 9 metre longitudinal opening in the hydrogen fuel tank and a series of lateral 4 metre ruptures in the squat LOX (liquid oxygen) tank. Those for the S-IVB (third stage) make two parallel 6 metre openings in the fuel tank and a 1.2 metre diameter hole in the LOX tank. These charges are fired only after the Command Module has separated from the launch vehicle. During a normal ascent, the destruct system is safed soon after the Launch Escape Tower is jettisoned; after about 3½ minutes of flight.
Early in the Apollo program, the Air Force had insisted that the LM, a vehicle from which every ounce of unneeded weight had been trimmed and which would be a home to men on the lunar surface, also had to have destruct ordnance attached. This was based on the premise that, in the event of a launch abort, it was better for propellants to be consumed before reaching the ground. After all, the LM would be unmanned at this stage. NASA pointed out the weight and safety penalties of this arrangement and eventually won what was a substantial argument by pointing out that the LM was hardly likely to survive the destruction of the S-IVB stage anyway. A similar conflict occurred over demands by the Air Force that the Service Module also carry destruct charges. (ap15fj)
- ↑ Charles D. Benson, William Barnaby Faherty, Moonport: A History of Apollo Launch Facilities and Operations, 9-6 Range Safety, NASA Special Publication - 4204 in the NASA History Series, 1978