See list attachedApril 10, 196969-PA-T-58APA/Chief, Apollo Data Priority CoordinationDescent monitoring at MCC
We have reached a plateau in our work on Descent Monitoring, perhaps making it worthwhile to send out this memo. First of all, I don't think there is any question that Descent is the thing that requires most of our attention between now and the G mission, at least in the empire of Mission Techniques. There are still a lot of things to do and so starting about a month ago we have been having one full day meeting per week, which will probably continue for another month. I think we have pretty well established what the MCC has to do and how they do it during Descent. That's really the subject of this memo. Our job is to work over the onboard techniques and integrate them with the ground monitoring to make sure everything is complete and consistent.
After considerable discussion, we have established that the ground's job during Descent is to attempt to do the following things (not necessarily in order of importance!):
a. Detect DPS malfunctions and excessive RCS plume impingement.
b. Predict that adequate propellent margins are available to permit landing.
c. Detect impending PGNCS failures.
d. Make sure PGNCS guidance is not diverging.
e. Make sure trajectory constraints of some sort or other are not being violated.
As far as we can tell, all of the necessary telemetry and tracking data programs have been identified and are being implemented in the RTCC; all necessary display formats have also been provided in the MCC. There are a couple of items associated with this which I would like to mention:
a. We are on the verge of assuming that RCS plume impingement is a honest-to-God constraint which must not be violated. Choke! The LM systems guys have a display which processes telemetry data yielding the cumulative plume impingement from each of the downward firing jets. They subtract this from the value GAEC has established as the total allowed duration and display the results. That is, it is a display of permissible time remaining. It is proposed that when this parameter reaches zero, indicating we have violated the plume impingement constraint, they will recommend that the crew “Abort Stage” out of there!!!
b. Another interesting computation and display that the CSM people have provided themselves is a prediction of DPS propellent margin at touchdown. This is an especially sophisticated processor which utilizes a number of PGNCS guidance parameters obtained by telemetry to predict the amount of DPS propellant required to fly the remainder of the descent trajectory. They subtract this propellent requirement from the measured propellant still remaining obtained from telemetry data, to obtain the predicted margin at touchdown. This parameter is plotted vs. horizontal velocity on an analog display. It is proposed that if the prediction of propellant crosses “zero,” the crew should be advised to “Abort.” It has been stated there is no question, when this prediction reaches zero, that propellent depletion will occur before landing and so abort- ing is the thing to do. It is not safe to assume the converse – that is, it does not always accurately predict that sufficient propellant is available to complete the Descent. We're going to check this program thoroughly to see if it really does that.
c. Impending PGNCS failure will be detected from strip charts dis- playing guidance system differences, very much the same as during the launch phase. That is, differences between the AGS and PGNCS and differ- ences between MSFN and PGNCS will be displayed on the strip charts. Abort limit lines will be provided upon which that action will be recommended. Other displays are used in conjunction with these strip charts to positively ascertain that the PGNCS is the errant system.
d. There was a somewhat surprising outcome from our discussion of trajectory constraints. Unlike launch, we were basically unable to find any “hard” descent trajectory constraints with a possible exception of the APS abort line (previously callously referenced as the “Dead Man” curve). That is, there appears to be no reason we could identify which would prevent the LM from flying all over the sky, if that is what you call it at the moon. As a result, it seems as though we have two options- either provide no trajectory abort limits or alternatively select dispersion limits (for example, 3 sigma, 6 sigma, or 9 sigma) beyond which we will arbitrarily not allow the trajectory to diverge from nominal. This cur- rently is my personal preference, mostly based on intuition and no data. There is by no means a general agreement on that yet.
And that's our plateau.