Downthread: Lunar Reentry Mission Techniques meeting (Feb 06, 1968)
See list belowJAN 17 196868-PA-T-14APA/Chief, Apollo Data Priority CoordinationReentry from lunar missions
1. On January 12 a group of Flight Crew, Flight Control and Mission Planning guys got together to talk about the lunar reentry and some rather interesting things came out which I am recording here for my records and your amusement.
2. Apparently, the last midcourse correction before reentry on a lunar mission occurs about one hour prior to reaching 400,000 feet altitude. It is probable this maneuver will be made with RCS, although I suppose if it is big enough it may be necessary to use the SPS. It is currently planned to use the External Delta V guidance mode. If SPS is used it is expected that we will have to trim residuals with the RCS. There was a question, however, as to whether this last midcourse maneuver should include an out-of-plane component. Claude Graves seemed to think it worthwhile but was not sure; he took the action item of determining the advantages and disadvantages of this. Since the same platform orientation will probably be used for both the final midcourse correction and the reentry, it is anticipated that the most desirable REFSMMAT will be chosen based on reentry considerations making the position on the ball relatively random during the midcourse correction. John Llewellyn intends to propose what REFSMMAT to use. It may be necessary to add a capability for computing it in the RTCC if that does not already exist. It is expected that the platform alignment performed prior to the midcourse maneuver will be adequate for the reentry, that is, it will not be necessary to redo it.
3. We next dealt with the question of the most desirable spacecraft attitude during service module separation. As you know, the PNGCS automatically maneuvers the spacecraft to a preferred attitude which is stored in the Colossus reentry program (P-61). We were not sure how that value was selected and it is not certain it is the one we will ultimately want to use. We are not even sure we will want to use an automatic mode in the first place. One matter which it seems should he given consideration is use of an alignment providing an out-of-plane component while avoiding platform gimbal lock in the same way as will be done on earth orbital missions. I expect to request MPAS (Carl Huss) to resolve this overall preferred attitude question if it has not been done already. And we are reviewing the desirability of eliminating the auto attitude maneuver from the P-61 program since MIT says this would be a good time to do it if we're not going to use it.
4. If and how the ORDEAL will be incorporated into the operation must also be examined, as usual.
5. There was a lengthy and emotional discussion concerning the overall reentry trajectory philosophy. Within the past year the MPAD reentry people have developed what appears to be a rather good way to back up the PNGCS reentry guidance in the event it fails. Early in their investigation it was found that earth orbital backup techniques, that is, constant bank angle reentries, were completely unacceptable and they developed a technique involving use of a constant acceleration which is expected to be both safe and easily performed by the crew. Of course, simulations are necessary to verify that. Unfortunately, after about 2 minutes into the reentry phase, which starts at 400,000 feet, the backup technique is not compatible with the primary guidance resulting is a significant difference in where the spacecraft will land, depending on which is used. That is, during the first 2 minutes of reentry the automatic guidance system will control the spacecraft during which time the crew will evaluate its performance leading to a GO/NO GO decision on whether or not it should be used. If they stick with the PNGCS, and it works, they will land at the targeted landing point. If they declare it NO GO, they will take over manually, fly a constant acceleration profile and land at a substantially shorter range. The difference between these two landing points is currently about 800 miles, although it can probably be reduced somewhat without changing procedures very much.
6. There are two schools of thought though. John Llewellyn would prefer to make the closed loop guidance more compatible with this shorter range backup mode for obvious reasons. It is Claude Graves' argument that to do so will severely cut into the ranging control needed to avoid bad weather which has been stated as some sort of a requirement from the beginning of Apollo. The issues are not black and white and probably do not require resolution immediately. Several things are going on which will help to make this decision. For example, Claude Graves' people are looking into how much the difference in ranging with the primary and backup modes can be reduced while still retaining the weather avoidance capability and reasonable landing point accuracy. Mike Collins offered to look into the delay we are currently experiencing in FCOD's response to our query about preferable reentry g profiles. This is of interest since the backup mode results in some 250 seconds spent in excess of 3 or 4 g's while the primary mode gives two 50 second spikes up to 5 g's each separated by about 3½ minutes of low g's. Apparently, the onboard programs as they are currently designed cannot be made completely compatible with a short range backup mode, although they will handle trajectories much closer to it than we are currently proposing. It was stated that complexity of procedures for each of the approaches are relatively equivalent.
7. An apparent discrepancy was uncovered regarding the EMS initialization. Specifically, prior to reaching 400,000 feet the primary guidance system will orient the spacecraft to one of four possible roll attitudes: 15°, 165°, 195° or 345°, that is, 15° to one side or the other of straight up or straight down. It cannot be predicted which of these will be commanded. Unfortunately, it is necessary that the EMS roll attitude indicator and the spacecraft attitude be the same when it is started, and as I understand it, this occurs substantially after PNGCS takes control. Actually, those of us present were not real sure how it all works, and Claude Graves took the action item of finding out what the score is.
8. We'll work on all this some more and within two months we'll get together with NR and MIT too.