Having established the spacecraft in its initial trajectory around the Moon. FIDO in mission control could begin working on his next move: a short burn by the SPS that would be carried out about four hours later, after two complete orbits, in order to get the Apollo stack into its operational orbit. This burn would be very carefully monitored to ensure that it had exactly the required effect.

The details of this burn depended on the flight in question. For Apollos 8 to 12. a relatively short burn, known as LOI-2, brought the apolune down from 300 kilometres to 110 kilometres and made the orbit circular. Apollo 8. without the mass of a lunar module, required only a 9-second burn to achieve this. On the next three flights, the extra 16 tonnes or more of the LM meant that their burns had to be somewhat longer. On Apollo 10 the lunar module lacked a full propellant load and the LOI-2 burn was 14 seconds, but the full LM tanks on the next two flights extended the burn to 17 seconds. On these early flights, the CSM never left its 110- kilometre circular orbit, and come landing day, the LM would have to do all the work of manoeuvring down to the surface. The first part of this would be the descent orbit insertion (DOI) burn to place the LM in an orbit with a low point of only 15.000 metres. This was the descent orbit, so called because this perilune was the point from which the LM would begin its final descent to the surface.

After Apollo 12 the strategy changed. Planners were keen to increase the capability of the Apollo system, and analysis had shown that the LM’s payload capacity to the lunar surface would be maximised by having the CSM make the DOI burn to take the LM into the descent orbit, thereby saving its propellant for the Final descent. Later, once the LM was released and inspected, the CSM would make another burn to circularise its orbit at 110 kilometres to undertake a programme of lunar reconnaissance while the LM was on the surface. This also placed the CSM in a suitable orbit for the rendezvous when the LM returned.

Owing to its unforeseen circumstances, Apollo 13 never got as far as carrying out this burn. On Apollo 14, which was first to perform this manoeuvre, the burn Look 21 seconds, which was four seconds longer than Apollo 12’s LOI-2 burn, reflecting the fact that, in this case, the near-side altitude was being dropped all the way down to 17 kilometres. As the mass of the stack increased for the final three J-missions. the duration of the DOI burn stretched to 24 seconds.

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