Aborting before LOI

If the mission had to be aborted before the LOI burn, Retro's response attempted to achieve two things. It ensured that the spacecraft was set on an accurate path, not only to Earth, but to a designated landing site, usually in the mid-Pacific Ocean, at which there was a recovery fleet on station. It also strived to increase the speed of the spacecraft slightly, so that its coast to Earth would bring it to the landing site 24 hours (one Earth axial rotation) earlier than would occur without intervention. On Apollo 8, Retro had planned to use two separate burns to achieve these goals: a flyby manoeuvre that would have been carried out just before the spacecraft disappeared around the far side of the Moon; and a pericynthion plus 2 manoeuvre, or 'PC + 2' for short, to be made 2 hours after their closest approach to the Moon. On later missions, the functions of both were combined into a single planned abort contingency.

The PC + 2 abort burn was actually used on one occasion as part of the effort to get the ailing Apollo 13 back to Earth. On this mission, Retro had to calculate the burn using the descent engine of the lunar module for the PC + 2 burn, the SPS engine being considered unusable owing to the loss of power that the CSM had suffered.

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