See list belowOCT 18 196767-FM-T-86FM/Deputy ChiefSome things about the Descent Propulsion system as controlled by the LGC
1. I guess I would have realized this if I had thought about it, but it was a surprise to me (and others) to learn that the Descent Propul- sion System (DPS) engine in the LM is not aligned through the cg prior to ignition. I suppose I had assumed that it would be handled the same way as the SPS engine on the command module, but it is not for several reasons.
2. For one thing, it is possible for the cg to move substantially be- tween one maneuver and the next since propellant transfer from one tank to another is not inhibited or controlled. Therefore, cg location is really not known prior to ignition. In addition, actual engine align- ment is not known prior to ignition since there is no measurement and feedback of gimbal position. As I understand it, the way the DPS is aligned prior to the maneuver is as follows. The engine is moved to its maximum possible deflection with the drive motors running against the stops until it has surely gotten there. It is then driven back the length of time necessary to align it with the spacecraft X-axis based on an assumed engine drive rate. Of course, it is almost certain that thrust vector alignment obtained in this manner will not pass through the cg and will result in attitude disturbance torques. How- even, since the DPS is started at only 10% thrust, it is well within the capability of the RCS jets to hold attitude until the DPS has been realigned by the DAP through the cg.
3. Another thing I have learned about the DPS is that maneuvers which are shorter than 10 seconds never have closed loop steering other than attitude hold control. (This is equivalent to the 7 or 8 second ?PS burn duration without steering.) Of course, there is no minimum im- pulse DPS programmed in the LGC but there is nothing in the spacecraft computer program to prevent calling for an extremely short burn. How- ever, the shortest that could be achieved is in the order of 2 seconds duration at 10% thrust. This duration would occur even if a shorter burn, say ½ second duration, were requested.