See list attachedJuly 1, 196969-PA-T-102APA/Chief, Apollo Data Priority CoordinationNew DPS GDA Descent mission rule is imminent if not now
A lot of activity has been going on lately regarding the manner in which the DPS gimbal drive actuator (GDA) is managed during descent. This memo is to make sure everyone knows this business is going on – and producing dramatic changes – and it is not finished yet.
The basic question, of course, is, What should be done if the GDA caution and warning alarm goes off during descent? Until a few weeks ago it was planned to ignore it until some secondary cue appeared to backup the alarm since it was felt a properly operating GDA was manda- tory for descent. A number of new factors have appeared on the scene recently, which almost certainly changes this procedure. The first and most significant was the addition of the RCS plume deflectors which apparently have all but eliminated RCS impingement as an operational constraint. GAEC's analysis is continuing and unless we have some sort of duty cycle limitation, it appears we can tolerate as much activity as is required for total attitude control by the RCS during a complete lunar descent. Incidentally, RCS propellant quantities also appear adequate for this purpose.
Some secondary factors which support this technique are the uncertainty of whether or not the crew can sense a build-up in RCS activity when wearing helmets and gloves. Another interesting factor is that during normal descent, apparently the GDA doesn't move the DPS engine more than about 0.1° to account for c.g. shift during the entire descent. Apparently, the main excuse for even activating the GDA is to guard against unsymmetrical DPS throat erosion and engine compliance changes when throttling. It appears a final mission rule will be to turn off the GDA as quickly as possible if a GDA caution and warning alarm occurs and complete the descent using RCS attitude control unless something unexpected appears in the analysis going on now and between the flight.