It is said that Apollo was set to the free return trajectory while leaving earths gravity, heading to moon.Now the Free return trajectory is the one in which satellite uses moon gravity to return back to the earth.But somewhere there is mentioned that the crue while coming back, fired the rocket of the service module to reach the earth.Then what was the purpose of that initial so called free return trajectory. Can anybody explain why was this?
For the Apollo to orbit the Moon, a capture maneuver had to be performed at arrival, to change from the trans-lunar trajectory to an orbit around the Moon. This maneuver had to be performed in the opposite side of the Moon, without possibility of communications with the Earth. If a minimum energy trans-lunar trajectory was used, in the case of capture maneuver failure the astronauts would be lost in space.
The free return trajectory was used for the astronauts to return to the Earth in the case of the capture maneuver failure only. In the case of success the return to the Earth would have to be achieved using rockets. In the latter Apollo missions the free return trajectory was abandoned, as NASA became confident that the capture wouldn't fail and other transfer orbits would allow transporting more mass e.g. support for a longer mission and the Rover.