See list attachedApril 15, 196969-PA-T-63APA/Chief, Apollo Data Priority CoordinationSome things about Descent
This memo is to list a few odds and ends dribbling out of our latest Descent Monitoring clambake.
1. We have identified a new entry for the PDI pad message voiced to the crew before DOI. Just prior to PDI the crew makes a crude estimate of their altitude above the lunar surface by measuring the time it takes for a lunar landmark to move from one end to the other of their LPD line on the LM window. (I believe it normally takes about 20 seconds and therefore two seconds is equivalent to about a mile accuracy in altitude.) The new pad entry is the time at which the altitude check landmark should appear at the lower end of the LPD line. It is currently proposed that the landmark to be used will be the same one the crew performs their on-the-job training sextant tracking on LOI day. This has the additional benefit of providing the MCC with data for determining its location with some precision before the altitude check.
2. During powered descent the crew monitors their various data sources to ascertain whether or not the DPS is producing an acceptable thrust. If there is thrust degradation of a fairly small amount, they are supposed to exercise established malfunction procedures in an attempt to improve DPS performance. If the degradation is more severe, malfunction procedures will not help and the crew should abort. LM systems flight controllers were requested to establish the amount of thrust degradation which the crew should tolerate before beginning the malfunction procedures and what amount they should use to decide on an immediate abort.
3. There has been a great deal of discussion over the merit of the crew observing the lunar landscape during the early part of powered descent. There are some benefits the crew is supposed to obtain from this but it is important that it not be carried on so long that landing radar data is lost as a result. Since it is possible to start getting landing radar data as early as two minutes after PDI, if altitude is dispersed low by one mile, it is proposed that the crew yaw the spacecraft from its face down attitude no later the PDI + 2 minutes. Yawing sooner would be fine.
4. The attitude the crew should hold after yawing to acquire landing radar is 6° off the principle axis in order to give symmetrical landing radar antenna coverage. This, of course, provides greater probability of acquisition and “data good.” (Incidentally, a possible candidate for future spacecraft computer program change is to have the automatic system also control to this attitude, compensating for the landing radar antenna offset.)
5. It has been said that the hi-gain S-band pointing angles during the braking phase of powered descent are more or less constant once the spacecraft has been yawed for landing radar acquisition. It would be very useful for the crew to have these pointing angles in their onboard data for use in manual acquisition during this period if the S-band were to lose lock. Who figures out what these angles are – Rocky Duncan is that you?