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 are being checked and abort readiness maintained. An Emergency Detection System is provided onboard for sensing various mal­ functions of the launch vehicle and displaying this information to the crew, who can then initiate an abort, if necessary. Automatic abort capability is maintained to nearly the end of first stage operations to allow for extremely time critical situations.

Following cutoff of the outboard engines of the first stage, thrust decay to lo% occurs in about one-half second, at which time the second stage ullage rockets and first stage retro­ rockets are fired, and S-IC/S-II separation occurs ( Figure 16).

The S-II stage is powered by five J-2 engines, each having 200,000 pounds of thrust. Thrust buildup to the rated value occurs rapidly and at 163 seconds after liftoff, the second stage has reached full thrust.

At this point, the switchover is made from the programmed-pitch guidance scheme, used during first stage operations, to a path­ adaptive scheme used during second and third stage operations. All guidance equipment for the launch vehicle is contained in the Instrument Unit located between the S-IVB stage and the spacecraft adapter. As stated previously, all guidance opera­tions during ascent are performed automatically by the launch vehicle. However, from this point on in the ascent, the capa­bility exists onboard the spacecraft to take over the guidance function in the event of a failure of the launch vehicle inertial platform.

Approximately 2 5 seconds after S-II ignition, the S-IC/ S-II forward interstage is jettisoned, and this is followed by the Launch Escape System jettison five seconds later ( Figure 17). Up until this point, the Launch Escape System has been the means of safely removing the CM and crew away from a malfunc­tioning vehicle in the event an abort was necessary. The high thrust and acceleration capability of the Launch Escape System motors was required to accomplish a safe abort during the atmospheric portion of the flight. At this point in the mission, however, the Service Propulsion System has the capability to abort the spacecraft off the launch vehicle, so the LES is jettisoned.

The Boost Protective Cover is attached to the LES and is jettisoned at the same time. The BPC is a semi-soft fiber­ glass construction, and its function has been to absorb the CM aerodynamic heating during boost and to provide a protective