Free Genesis: The Story of Apollo 8 by Robert Zimmerman Page A

able to generate 94 pounds of thrust. One set of jets would be used for orientating the capsule as it reentered the atmosphere, the other reserved as a backup should the first fail.
Attached to die command module's base was the service module. In this thirteen foot long cylinder were oxygen tanks for supplying the astronauts with air, as well as three fuel cells, which combined the oxygen with hydrogen to generate electricity and drinking water. 1 On the service module's outside surface were four clusters of four additional rocket engines, used to adjust the spacecraft's orientation in space. Each one of these sixteen engines produced 100 pounds of thrust.
The spaceship's main engine, called the Service Propulsion System, or S.P.S. for short, was also part of the service module. The S.P.S., generating 20,500 pounds of thrust, was the rocket engine that would put the astronauts into lunar orbit in three days and, more importantly, blast them back to the earth when it was time to leave.
In this tiny spacecraft three men now drifted towards the moon. While Anders focused on photographing the earth, Borman piloted the spacecraft. Unlike driving a car, steering in space required more than left or right turns. Borman used two hand controls, resembling many of today's popular computer joysticks. One control accelerated the spacecraft in the desired direction, while the other merely pivoted the spacecraft around its center of mass. For example, by moving this second joystick backward or forward, Borman pitched the spacecraft's nose up or down. Tilted left or right, and the spacecraft rolled to the left or right. And twisting the joystick caused the whole spacecraft to yaw, a term borrowed from nautical dictionaries. Here the spacecraft was like a bottle lying on its side, and the pilot a teenager spinning it one way or the other, depending on the direction he turned the hand control.
As Borman maneuvered the spacecraft the abandoned third stage was causing him a lot of aggravation, trailing behind them in its own independent
     

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C.G. stands for "center of gravity."
path to the moon. "The damned S4B was uncomfortably close, its nose wandering within 500 feet." 2 Soon he was forced to turn the capsule away from the earth in order to keep watch on the booster.
Nor did he like how the S4B was venting fuel. "It's spewing out from all sides like a huge water sprinkler," he told the ground. "I believe we're going to have to vent or thrust away from this thing. We seem to be getting closer."
Moving away from the booster wasn't going to be as simple as Borman would have liked. The computers on the ground had calculated Apollo 8's heading, and determined that it was so accurate that the next mid-course correction was hardly needed. But mission control also wanted to fire the S.P.S. engine before the spacecraft got too far from earth. Like Borman's maneuvering controls, the S.P.S. was quite different from most earthbound engines, and was one of the reasons that NASA had gambled on sending Apollo 8 to the moon. The S.P.S. used hypergolic chemicals, meaning that when the fuel, a mixture of hydrazine and unsymmetrical dimethyl-hydrazine, made contact with the oxidizer, nitrogen tetroxide, the chemicals instantly ignited, producing thrust. No spark was needed. Without a complicated ignition system, the engine was simpler, and hopefully more reliable.
Two days before launch, however, engineers doing ground tests on another S.P.S. engine noticed an anomaly that posed a possible hazard. The engineers found that unless the combustion chamber of each new S.P.S. engine was primed, "wetted" with a small amount of fuel, there was a chance
     

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that the engine might explode the first time it was turned up to full thrust. The solution, not complicated, required giving the S.P.S. a single very short burst. The first mid-course correction, scheduled about eleven hours into the flight, would be the ideal opportunity to do this.
First,