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Just what the title states.

I read that LOI was used by the Apollo program (and possibly others) to achieve escape velocity. How does it work? Is it merely a matter of centrifugal/centripetal force?

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up vote 4 down vote accepted

The trans-lunar injection doesn't achieve escape velocity. Instead it changes the orbit of the spaceship to an elliptical orbit that intersects the Moon's orbit.

The Wikipedia article on Hohmann transfers explains this very nicely. The spaceship starts out in a circular orbit around the Earth. The Moon is also in a circular orbit, except of course the radius of the Moon's orbit is a lot greater. If you suddenly accelerate the spaceship by firing the motors then the spaceship is moving too fast to stay in the original circular orbit. In fact you've put it into a highly elliptical orbit with the apogee near the Earth and the perigee near the Moon's orbit. You time this orbit so that as the spaceship reaches perigee it hits (not literally!) the Moon.

At perigee the spaceship is moving too slowly to stay in the Moon's orbit, so you fire the engines to accelerate it again. It may seem odd that you're accelerating instead of braking, but because your spaceship has lost lots of speed as it drew away from the Earth it would pass the Moon and fall back towards the Earth. Firing the motor gives it the extra speed to keep up with the Moon.

When you want to return to earth you fire the motors to decelerate. This puts the spaceship back into an elliptical orbit with the apogee near the Earth. As you reach the Earth one last burn puts the spaceship in an orbit that takes it into the atmosphere, and atmospheric friction slows it down enough to parachute into the sea.

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Beautifully explained! Thank you! – Everyone Jun 9 '12 at 11:34
The Apollo missions didn't do a burn before reentry - they did course corrections a ways out and hit the atmosphere at full speed. This orbital profile worked very well on Apollo 13. – paul Sep 10 '14 at 5:20
@paul: ah, OK, thanks Paul. – John Rennie Sep 10 '14 at 5:29
@JohnRennie: Did you mean to say "you've it into a highly elliptical orbit with the apogee near the Moon's orbit and the perigee near the Earth's orbit"? I may be confusing things here but I thought the apogee being the furthest point from Earth would be the closest to the Moon. – xci13 Jul 10 '15 at 17:54

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