25 December 1968: During the 10th orbit of the Moon, the crew of Apollo 8 fired the Service Propulsion System (SPS) of the Command Service Module for the Trans Earth Injection (TEI) maneuver that would send them home.
TEI was a critical maneuver which had to be timed perfectly. It occurred while the spacecraft was on the side of the Moon away from Earth, and so the crew was out of radio communication with Mission Control in Houston, Texas. If initiated too soon, the Apollo capsule would miss Earth, or ricochet off the atmosphere. Too late and the capsule would re-enter too steeply and burn up.
The engine had to burn for precisely the correct amount of time to accelerate the space craft out of lunar orbit and to arrive at Earth at exactly the correct point in space where where our home planet would be 57 hours, 26 minutes, 56.2 seconds later, as it traveled in its orbit around the Sun.
The SPS engine was an AJ10-137, built by Aerojet General Corporation of Azusa, California. It burned a hypergolic fuel combination of Aerozine 50 and nitrogen tetraoxide, producing 20,500 pounds of thrust (91.19 kilonewtons). It was designed for a 750 second burn, or 50 restarts during a flight. The SPS engine had already been used for the Trans Lunar Injection maneuver, sending Apollo 8 from Earth orbit to the moon, and now served the same function in reverse.
The SPS started at mission time T+089:19:16.6 and cut off at T+089:22:40.3, a burn duration of 3 minutes, 23.97 seconds, increasing the velocity (Δv, or “delta–v”) 3,531 feet per second (1,076 meters per second).
© 2018, Bryan R. Swopesby