Tweak burns

See list attachedJuly 10, 196969-PA-T-105APA/Chief, Apollo Data Priority CoordinationTweak burns

If you can stand it I would like for you to hear the latest on tweak burns – the trim maneuvers made after LM insertion from a descent abort. I thought we had this settled and on ice a couple of months ago but some things have happened which probably make it logical to revise the tweak rules. The things that have happened are:

a. The LM RCS plume impingement constraints have been substan- tially reduced.

b. Simulations have shown that the Flight Dynamics Officer (FDO)/ RTCC capability of computing the tweak maneuvers on a timely basis is much better than anticipated.

Some FCSD, FCD, and MPAD guys got together July 8 and came up with the following:

a. Our previous rule was quite simple; if the LM inserted into orbit with the DPS attached, the command module would make the burn; if the LM had staged, the LM would make the maneuver. Now that the LM has been modified with plume deflectors and additional thermal protection, it has the capability of performing any tweak maneuver we foresee. Accordingly, the rule is being modified to say that for all descent aborts prior to PDI + 10 minutes the LM will perform the tweak provided it is within the RCS plume impingement constraint, regardless of whether the LM has staged or not. If for some abnormal reason the LM capability is exceeded, the CSM will perform it; the LM should not stage the DPS just to provide a greater RCS capability. Also, the LM should not trim insertion condi- tions.

b. As you recall, aborts after PDI + 10 minutes require an extra rev in addition to a phasing maneuver, which makes the tweak burn unneces- sary. We have also stated that trimming the insertion conditions is nec- essary. However, if the crew wishes to trim +x there is no objection to that and obviously if the +x required is large, there is no choice. It must be trimmed.

c. I would like to emphasize another rule which has been on the books for a long time but which may not have been clear to the crew. Namely, if the DPS shuts down with a ΔV required to reach the insertion conditions greater than 30 fps, the crew should utilize the APS and P71 to achieve orbit. We have recommended that automatic Abort Stage sequence to achieve this.