After waking up on the final day of their coast to the Moon, a crew would set about their usual post-sleep chores of reporting their condition to mission control and preparing their breakfast. Normally the spacecraft was slowly turning around in its barbecue roll, spreading the heat of the Sun across its surface. While the crew slept, engineers at the ground stations on Earth had taken precise measurements of the spacecraft's position and velocity to accurately monitor their trajectory. Using this trajectory data, FIDO, the flight dynamics flight controller in the MOCR, calculated the amount by which their approach to the Moon needed to be adjusted, if at all. Was the spacecraft coming in too quickly or too slowly to pass around the far side at the correct altitude? Was it staying within the correct orbital plane to pass over the landscape they expected it to? Applying the results of overnight radio tracking, and with the help of the big computers in the real-time computer complex (RTCC), FIDO calculated the details of a burn to be carried out at the fourth opportunity for a mid-course correction, which was usually timed to occur 5 hours before entry into lunar orbit. The details of this corrective burn were read up to the crew, along with the results of calculations by the Retro flight controller.
While FIDO had been deciding where they wanted the spacecraft to fly, Retro was busily working out where they should go if something went wrong. He had two scenarios to consider: the first was if something were to happen to stop the LOI burn from occurring; the second was for the situation in which the LOI burn had been completed successfully, but the crew were required to return to Earth at the earliest opportunity. Having decided the manoeuvres the crew should make in these scenarios, it was then important that the details be passed up to the spacecraft while it was still in communication with Earth. The mantra was that they should always have the data necessary to get home without further assistance from mission control, in case communications are lost.
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