LM DPS low level light fixing

I think this will amuse you. It’s something that came up the other day during a Descent Abort Mission Techniques meeting.

As you know, there is a light on the LM dashboard that comes on when there is about two minutes worth of propellant remaining in the DPS tanks with the engine operating at quarter thrust. This is to give the crew an indication of how much time they have left to perform the landing or to abort out of there. It compliments the propellant gauges. The present LM weight and descent trajectory is such that this light will always come on prior to touch down. This signal, it turns out, is connected to the master alarm - how about that! In other words, just at the most critical time in the most critical operation of a perfectly nominal lunar landing mission, the master alarm with all its lights, bells, and whistles will go off. This sounds right lousy to me. In fact, Pete Conrad tells me he labeled it completely unacceptable four or five years ago, but he was probably just an Ensign at the time and apparently no one paid any attention. If this is not fixed, I predict the first words uttered by the first astronaut to land on the moon will be “Gee whiz, that master alarm certainly startled me.”

As I understand it, cutting the wire to the master alarm eliminates the low level sensor light too. If nothing else can be done, this should be and we’ll get along just using the propellant gauges without the light. If possible, a better fix would be to cut the wire on both sides of the master alarm and jumper the signal to the light only.

Incidentally, on the D mission the propellent levels will be low enough when we get to the DPS rendezvous maneuvers - Phasing and Insertion - that if this system is activated prior to ullage, the master alarm will likely go off. I guess it will be standard procedure to punch it off if that happens. But, where this is just an annoyance on D, it is dangerous on G.

Terms & Abbreviations

DPS

Descent Propulsion System.

LM

Lunar Module. Earlier it was known as the Lunar Excursion Module and abbreviated “LEM.” Even after the name change, it continued to be pronounced “lem.”

Ullage

Ullage is the empty space in a liquid rocket’s fuel tank. In zero-gravity, the fuel of course tended to float around inside the tank. Before many maneuvers, rockets were gently fired for a short time “for ullage,” that is, to push all the fuel to the back of the tank so that calculations about how to maneuver the spacecraft would be simpler and more accurate.