Throttle down

Six minutes into Apollo ll’s descent. Duke came up with a time for the crew. "Six plus 25. throttle dowm.”

“Roger. Copy,” said Aldrin.

‘’Six plus 25.’’ reiterated Armstrong.

Houston had calculated that they could expect the engine to start to throttle 6 minutes 25 seconds into the descent. It was part of a clever strategy that engineers had come up with to w’ork around the engine’s forbidden throttle settings in its high thrust range. They arranged that P63 should compute a course to a spot 4.5 kilometres short of the landing site. In some cases, strangely enough, this point could be below’ the surface but this didn’t matter since P63 never took the LM anywhere near it. This profile had been chosen to achieve two goals. First, it protected the engine from the erosion that would result from the forbidden throttle settings. Second, it yielded nearly optimal efficiency. The profile called for an initial thrust level that w as higher than the engine could achieve, to which the engine responded with its constant high thrust setting that is, 92.5 per cent of maximum, referred to as the fixed throttle position. For about 6.5 minutes of the burn, the spacecraft continued to lose speed and gently arc towards the surface while the engine continued in its high thrust setting. Hvenlually, the thrust required to achieve the profile fell below 57 per cent of maximum whereupon the program’s logic permitted the engine to move into its allowable throttle range which lay between 65 and 10 per cent of maximum. For the remaining 2.5 minutes of P63’s work, the computer could control the throttle and adjust it as necessary to compensate for any errors and drive the spacecraft’s trajectory towards an optimal flight path.

"Wow! Throttle down," called Aldrin. joyfully.

“Throttle down on time,” said Armstrong.

“Roger." said Duke. “We copy throttle down."

“You can feel it in here when it throttles down," noted Aldrin. "Better than the simulator." The crews had intensively simulated the descent, but the one thing the simulators could not provide was the g-force provided by the engine.