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What was the plan if the flight controller received a no-go prior to Apollo 8's traverse of the far side of the Moon? How would they have returned to Earth without the gravity assist from the Moon?

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Apollo 8 was on a free-return trajectory when approaching the moon. If there was a no-go for the lunar orbit insertion, or if the engine failed to fire, they would have returned to Earth needing only minor course correction (doable with the RCS thrusters, which had a terrific amount of redundancy) to get home.

Via Americaspace.com:

During the translunar coast, the large Service Propulsion System (SPS) engine in the service module would be test-fired for a few seconds. If it refused to work, the astronauts could still be brought home safely, thanks to a safety feature built into Apollo 8’s trajectory design. Known as the “free return,” it would allow the crew to essentially loop around the Moon and use its gravitational influence to slingshot them back to Earth without using the SPS. In fact, if Borman, Lovell, and Anders did find themselves with a useless engine, they would only need to perform a couple of mid-course correction burns, using the service module’s thruster quads, to keep them on track for home.

Your question is a bit confused, though: the free return trajectory is a gravity assist trajectory, using the moon's gravity to sling the ship back towards Earth. It's simply set up long before the ship reaches the moon.

This page has a nice animation of the trajectory - after the ship leaves Earth, gravity is doing all the work. The Apollo missions after 11 switched to a different profile, starting with a free return trajectory falling well short of the moon, then switching to a non-free-return with a midcourse correction.

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  • $\begingroup$ So they would have still gone round the moon regardless? $\endgroup$ – Burgi Dec 20 '15 at 18:24
  • $\begingroup$ In my head I kind of just had them off to one side of the Moon. They got the no-go and just turned around and came home. $\endgroup$ – Burgi Dec 20 '15 at 18:32
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    $\begingroup$ In space you are always in some kind of orbit, there is no "turning around". Take a look at this image. $\endgroup$ – Brian Lynch Dec 20 '15 at 18:57
  • $\begingroup$ I think that diagram is for the landing missions after A11 - I believe Apollo 8's free-return trajectory went behind the moon. blogs.scientificamerican.com/life-unbounded/… $\endgroup$ – Russell Borogove Dec 21 '15 at 2:06
  • $\begingroup$ Interesting; I seemed to recall that Apollo 12 also used a free-return trajectory, but apparently I was half wrong. Apollo 12 was the first Apollo craft to use what NASA termed a "hybrid non-free-return trajectory". In their words: "The hybrid profile was constrained so that a safe return using the descent propulsion system could be made following a failure to enter lunar orbit." $\endgroup$ – a CVn Jan 10 '16 at 19:47

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