Gauging the propellant

In view’ of the severe weight restrictions imposed on the LM. and given that propellant w-as a major fraction of the spacecraft’s weight, typically about 70 per cent, it wras vital that the tanks, which were somew hat larger than they wrere required to be, were loaded with only as much fuel and oxidiser as would ensure a safe landing. It was equally vital that a system be in place that would allow’ the crew and flight controllers to monitor the remaining quantity, especially because the levels would get low just when the commander w-as likely to be hovering, looking for a safe place to set down.

The tanks of the DPS had two independent systems for measuring propellant quantity, either of which could be monitored by a gauge in the cabin and both of which could be monitored from Earth. As the descent progressed, flight controllers closely watched how each system responded to the falling propellant levels and decided which one seemed to be more trustworthy and appropriate for the crew to monitor.

”Eagle, Houston,” said Duke from his Capcom seat. “It’s ‘Descent two’ fuel to monitor. Over."