See list attachedSeptember 23, 196868-PA-T-202APA/Chief, Apollo Data Priority CoordinationG Rendezvous Mission Techniques
If you can stand it, I would like to announce another change in the G mission lunar rendezvous timeline. In order to provide more tracking which will hopefully improve CSI targeting and to avoid bothersome real time variations of time between CSI and CDH which foul up the plane change scheduling, we propose:
a. Move CSI five minutes later – to 55 minutes after insertion which is nominal apogee. This is primarily to avoid a rather large radial ΔV at CDH.
b. Always schedule CDH one half a revolution (180°) after CSI.
c. Schedule plane changes 30 minutes prior to CDH and at CDH, as before. The LM should use the Z-axis RCS LM thrusts for the CDH maneuver (by yawing if necessary) to avoid losing RR acquisition.
d. The LM may include a plane change at CST if the CSM has adequate sextant tracking for targeting it. Rendezvous radar only is not considered adequate.
The new timeline looks like this:
55 | 27 | 30 | 33 0 55 82 112 145 INS. CSI P.C. CDH & P.C. TPI
The only disadvantage we currently see is that it reduces the time between CDH and TPI to about 33 minutes. However, 33 minutes should be adequate even with dispersions and the advantages of a relatively fixed maneuver schedule and better navigation before CSI seem well worth it. It should be noted that a (hopefully small) change in the CSI targeting programs (P32 and P72) would be required to force the computer to use the 180° spacing between CSI and CDH. This can be done in either of two ways. Our preference would be to provide the crew control probably by modifying the second P32 DSKY display format to utilize the third register which is currently blank as option code. [The other two displays in this format are apsidal crossing (N) and TPI elevation angle (E).] The simplier but less flexible way of doing this job is to increase the magnitude of the parameter currently stored in fixed memory which is used in the CSI Ṙ test, which forces the logic to use a 180° transfer when the pre-CSI orbit is found to be essentially circular and apsidal crossings become ill-defined. Ed Lineberry will submit a PCR for this.
Several action items came out of our meeting as follows:
a. MPAD – It is necessary to develop a rule governing the use of the VHF data in the event no sextant data is being obtained. It is our under- standing that VHF data by itself is not only inadequate, but could actually degrade the processing. If this is so, we need to establish procedures whereby the crew inhibits VHF into the CMC when sextant data is not avail- able.
b. MPAD – It is our proposal that the CSM be the prime source of targeting the plane change maneuver regardless of which spacecraft executes it. This is because the sextant is potentially more accurate than the rendezvous radar for this particular purpose. Here again a rule is needed to define how much sextant data is needed to target the plane change maneuver as opposed to using the rendezvous radar solution.
c. MPAD – We came to the conclusion at the last meeting that it was not possible to use the same maneuver solution for CSM mirror image targeting as the LM uses for burn execution. This meant the crew would have to cycle through two programs rather than just one. On further thought, it seems as though we can avoid this extra complexity, which is really rather serious. I am sure we can for the CDH burn and it seems probable that something can be done for the CSI burn too, particu- larly since it's constrained to be horizontal. Accordingly, we have requested OMAB to re-examine this procedure to see if we can't clean it up. We must also determine whether one minute delay in the mirror image targeting is really a requirement since these are RCS burns and problems at TIG don't appear to be too likely.
d. ASPO – Milt Contella repeated a rumor that the rendezvous radar may have random error in the shaft angle measurement when the line-of- sight from LM to CSM is close to the lunar surface. We must find out what the true situation is as quickly as possible and start figuring out some workaround procedure to be added to all the other ones.
Odds and Ends
We are assuming that the CSM will backup the LM CSI and CDH maneuvers using the SPS; it is probable, however, as on the D mission, that it will backup TPI with RCS. We have also concluded that the CSM should not backup the plane change since that requires yawing out-of-plane and disrupts tracking between CSI and CDH. Of course, if it is known that the LM will not be able to perform the plane change maneuver, the CSM will do it at that time. If the LM and CSM both fail to perform the plane change 30 minutes before CDH, the CDH plane change will force the node near TPI and so in that event the plane change will be taken out during the TPI burn targeted with R-36 to force a new node 90° after TPI time. This, of course, is a departure from the nominal TPI plan which calls for forcing the node at intercept (TPF).