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Apollo abort modes

If the rocket failed during the first phases of the flight, the Emergency Detection System (EDS) would automatically give the command to abort. The reason is that life-threatening situations can develop too fast for humans to discuss and react to. In the later, less violent phases of the ascent, the EDS was turned off and an abort would have to be initiated manually.

Overview

Of the five abort modes, the modes up to two (II) are variations of jettisoning the entire rocket followed by an immediate landing in the sea (splashdown). Mode three (III) and up are variations of jettisoning only the failing rocket stage, using the other stages to contine into Earth orbit. Once there, a backup Earth orbit mission could be performed so that the flight was not entirely in vain. In all cases, the Command Module (CM) with the astronauts performs a splashdown by

• dumping the hypergolic fuel overboard since the toxic substance would be an unnecessary risk to recovery personnel
• if high enough, deploying high-speed parachutes (drogue parachutes)
• jettisoning the drogues and deploying the main parachutes
• splashing down in the sea and waiting for recovery forces to arrive.

Detailed discussion

Apollo's planned-for abort modes were, in chronological order:

• Pad abort--If the rocket failed in the last five minutes before launch, the CM would separate from the rest of the rocket and the launch escape system (LES, see figure) would propel the CM upwards. The pitch control motor, a small solid-fuelled engine at the top of the launch escape tower, steered the CM eastwards over the sea. The launch escape tower would then be jettisoned in anticipation of the parachute deployment, and the CM would splash down. (Preparation for a pad abort can be witnessed in Apollo video footage: five minutes before launch, the umbilical arm connecting to the CM retracts, swinging clear of the rocket. It does that because the EDS is being armed at that time, meaning the swingarm has to be out of the way in case the EDS decides to abort.)
• Mode I
  • Mode IA (one alpha)--During the first 42 seconds of flight (up to 3000 metres), the rocket is still relatively upright and an abort is much like a pad abort. The main and pitch control motors move the CM out of the flight path of the possibly exploding rocket. 14 seconds into the abort, the LES tower is jettisoned, leading to splashdown.
  • Mode IB (one bravo)--From 3000 metres to 30.5 km, the rocket is tilted eastwards far enough that firing the pitch control motor is unnecessary. After the LES main motor has moved the CM away from the rocket, the tower would deploy canards (small wings at the tip). They force the CM-LES combination to fly with the CM bottom forward (blunt-end forward or BEF attitute). That is necessary because the parachutes stowed at the CM top can only be deployed in downwind direction.
  • Mode IC (one charlie)--From 30.5 km until the LES is jettisoned, turning the CM-LES combination around into the CM-forward position would still be necessary, but in the now thin air the canards are useless. Instead, the small engines of the CM's reaction control system (RCS) would do the job. During One-Charlie, the first staging occurs, that is the jettisoning of the spent first stage (S-IC) and ignition of the second stage (S-II). One-Charlie ceases about 30 seconds after the staging when the LES is jettisoned.
• Mode II--With the LES gone, the Command/Service Module (CSM) would be separate as a whole from the rocket and use its large engine and RCS engines to move clear of the rocket and align itself. The CM would then separate from the SM and splash down.
• Mode III, also known as Contingency Orbit Insertion (COI) or S-IVB to COI--In case of an S-II failure, it would simply be jettisoned early. For the first time in the flight, the rocket is now high and fast enough that the third stage (S-IVB) engine, followed by the Service Module (SM) engine, have enough power to place the spacecraft in Earth orbit. The S-IVB, the spacecraft's means to reach the Moon, would be spent, meaning that only an Earth orbit mission can be performed.
• S-IVB to orbit--Like in Mode III, the failing S-II would be jettisoned early, but the rocket is now high and fast enough that the S-IVB can place itself, the CSM and LM in Earth orbit. The difference to Mode III is that the SM engine is not required in this mode, but the S-IVB still wouldn't have enough fuel left to travel to the Moon. Therefore, an Earth orbit mission would follow.
• Mode IV--Starts with the second normal staging, when the burnt-out S-II is jettisoned and the S-IVB ignites. Should the S-IVB fail, the Service Module engine can now place the CSM in Earth orbit to perform an Earth orbit mission.

References

• Apollo 15 Flight Journal