External Delta V for LOI

See list belowJAN 16 196868-PA-T-11APA/Chief, Apollo Data Priority CoordinationExternal Delta V for LOI

1. In a December 15 memo (67-PA-T-121A), I indicated that we were going to perform LOI using two maneuvers. I also stated that pending results of some studies then underway we expected to use the External Delta V guidance mode for these maneuvers rather than Lambert steering. The primary reason for doing this was to make the crew's monitoring easier since External Delta V provides a known constant attitude, whereas it was anticipated the Lambert steering would be neither constant attitude nor of a known profile until final targeting of the maneuver. However, the studies have now shown that the Lambert mode can also be targeted to give a constant attitude burn, and since it apparently has some other rather marginal benefits we have decided to select its use for the first stage of the LOI maneuver rather than External Delta V. (The second stage of the LOI will still be done using External Delta V.) The marginal benefits to which I refer are a) slightly lower sensitivity to dispersions in spacecraft weight, ensing I_sp, and navigation state vector, and b) reduced time we will spend in arguing about and justifying use of External Delta V since there are a number of groups, both at MSC and MIT, who are convinced the Lambert mode is superior.

2. Bob Ernull, please take note. It seems perfectly reasonable to me that the RTCC LOI targeting program should be configured in accordance with the above. That is, if significant effort can be saved by not providing a choice of either Lambert or External Delta V in the programs. I would recommend you save that effort and design the system in accordance with our latest recent decision which I am now relatively confident is firm.