FA/Chairman, Apollo Spacecraft Software Configuration Control BoardJAN 11 196868-FM-T-5FM/Deputy ChiefAGS/PNGCS incompatability review – Chapter One
1. At the January 8 meeting of your CCB I was given the action item of reviewing AGS SCP's Nos. 34 and 35. Specifically, I was to determine our position on the advisability of making these changes which you tentatively approved at that meeting, and, in particular, we are to satisfy ourselves that the AGS and PNCGS were sufficiently compatible. This memo is to report the results of that review.
2. AGS SCP No. 34 dealt with changing the limit on r⃛d; as a function of vehicle configuration. This parameter, known as the “Jerk” (that is, rate of change of acceleration), influences attitude control of the spacecraft in the event of switching over to the AGS during a descent abort or during ascent. This change makes the AGS compatible with the PGNCS for monitoring purposes as well is minimizing attitude changes in the event of a switchover after 150 seconds into the powered descent maneuver. More important then that, it provides a safer insertion perilune if the AGS is utilized. Specifically, it would always aim for a value of 60,000 feet. This change does make the AGS somewhat incom- patible with the PGNCS during the first 150 seconds of this maneuver which in a small disadvantage to the change. However, it was the consensus that the advantages far outweigh the disadvantages and we agree with your direction to TRW regardless of the outcome of the following proposal. An additional factor that came out during our meeting was that it is quite desirable to change the PGNCS to operate in the same way as the AGS and a PCR is being prepared by Floyd Bennett's people for the consideration of your panel. Although this certainly cannot be considered a mandatory PGNCS program modification, it provides three significant improvements. Specifically, it would make the AGS compatible with the PGNCS throughout the entire descent phase. More important, it would cause the PGNCS to target for a safer perilune. And most important of all, we would be able to move the mode change limit from 150 seconds into the powered descent to a much earlier time—probably to a value of 50 seconds of less; this reduces the probability of having to stop and restart the DPS by that amount. These final remarks are to make you aware of the status and are, by no means, meant to convince you of the desirability of this program change.
3. AGS SCP No. 35 is a small change to compensate for dispersions in cutoff altitude by adjusting the cutoff velocity. I believe the number proposed is 7 fps for each 10,000 feet altitude error. It was our opinion that it was a desirable change in the AGS since it will provide a somewhat better insertion orbit with no apparent disadvantage. The resulting incom- patibility with the PGNCS is trivial with regard to guidance system monitor- ing and certainly does not necessitate a like change in the PGNCS. In fact, we do not propose that that even be considered.
4. The above discussion deals with our specific action item, but implicitly there was a much larger one, namely, review the overall AGS/PGNCS compatibil- ity situation. Actually, Floyd Bennett had personnel of his branch already engaged in this activity and a memo outlining all of the identified differ- ences is in final stages of preparation at this time. Both of the items discussed above, of course, were included on this list. Once they have satisfied themselves that the list is complete based on discussions with all interested parties, we will review them in detail and prepare a presenta- tion for your panel describing the situation and recommending alternate courses of action for consideration.