Downthread: April 9 Lunar Reentry Mission Techniques meeting (Apr 17, 1968)
See list belowFEB 6 196868-PA-T-26APA/Chief, Apollo Data Priority CoordinationLunar Reentry Mission Techniques meeting
1. On February 1 we had another meeting on lunar reentry mission techniques. Almost all of our discussion dealt with the final midcourse maneuver prior to entry. As you know, midcourse maneuvers are currently planned to occur approximately 12 hours after TEI which is near the sphere of influence of the moon and about 15 hours prior to reaching the Entry Interface (400,000 feet altitude). Analyses have shown it is highly probable that these maneuvers will have to be made and propellant is budgeted for them. Planning has also included a third midcourse maneuver just prior to reentry, the need for which is nowhere near as certain. Of course, it must be included in the timeline regardless of that. It is this midcourse maneuver we discussed.
2. When should the maneuver be scheduled? Ron Berry stated that, according to their studies, the magnitude of dispersions at Entry Interface (EI) are relatively insensitive to the time at which the third midcourse maneuver is made as long as it is no earlier than about 5 hours before (EI). Therefore, this consideration puts an upper bound on the time at which this maneuver mist be made. Paul Pixley states that for the cases they have examined it is always possible for the MSFN to obtain a good state vector for entry initial- ization provided the final midcourse maneuver occurs no later than 2 hours before EI. This MSFN tracking limitation establishes the lower bound. Selection of the actual time the maneuver should be made between these bounds is primarily based on operational considera- tions. That is, we would like to make sure the crew timeline following the maneuver is not unduly hurried and will be very much interested in the flight planning people's input on this (Tom Holloway please note). Until something comes along to change it, we propose for now to schedule the third midcourse maneuver 2 hours prior to 400,000 feet and all mission planning and analysis activity should be based on that.
3. We also established a criteria upon which it will be possible for the flight controllers to establish the need for this maneuver in real time. Based on the work of Claude Graves' group, it was stated that flight path angle dispersion at EI of .38° is considered acceptable. According to Paul Pixley, the MSFN is capable of determining that parameter to within 0.02°, given 30 minutes of tracking within 2 hours of EI. By subtracting this we established a flight path angle dispersion limit of .36° as the GO/NO GO criteria for whether or not to make the midcourse maneuver. That is, if the predicted flight path angle at EI differs from the desired value by more than .36°, the third midcourse maneuver will be executed. According to Pete Frank, this value is sufficiently large that the likelihood of the third midcourse maneuver is very low.
4. It was decided that the midcourse maneuver, if necessary, will be entirely in plane. This ground rule was established based on an under- standing that very little lateral landing point adjustment is available without very large out-of-plane maneuvers. Nor is it needed since the lifting reentry footprint should provide more than enough lateral landing point control.
5. Another ground rule we established was that there would be no comparison of onboard navigation to MSFN navigation associated with the third midcourse maneuver. This is a necessary constraint since onboard navigation changes the CMC spacecraft state vector, which is an unaccept- able thing to do just prior to entry. Furthermore, it is unnecessary anyway, since by that time in the mission we should have sufficient faith in the one which has been uplink from the ground without that coarse comparison.
6. This ruling poses the question as to how long before entry the ground determined state vectors propagated to EI are of equal accuracy to that determined onboard since, given communication loss, at some point the crew should abandon the MSFN state vector and start navigation and maneuver targeting onboard. The Mathematical Physics Branch and Orbital Mission Analysis Branch people were given the action item of determining this crossover point which is anticipated to be well before the second midcourse maneuver. In other words, I expect that once we have comitted the spacecraft to executing the ground computed second midcourse maneuver utilizing a MSFN state vector update, there should be no further star landmark/star horizon exercises carried out onboard the spacecraft.
7. As a side issue, it may be desirable to include in the lunar mission plan some sort of “onboard Navigation and Return-to-Earth targeting” exercise as a systems test either on the translunar phase of the mission, or more reasonably, early in the transearth phase to evaluate that capability. But it is to be emphasized that it is a systems test only and that navigation and targeting of all these maneuvers should be based on ground computations given adequate communications.
8. Another question which Must be answered dealt with how soon before EI it is reasonable for the CMC Average g program to start running. Of particular concern is the effect of approximations on the accuracy of the average g integrator when computing the influence of just the gravitation the spacecraft is experiencing. Guidance and Performance Branch is to answer that.
9. In the current flight plan we propose that platform alignments be carried out based on a ground computed REFSMMAT at 3 hours and 1 hour prior to EI. (We still haven't pinned down its specific orientation. ) In addition to the ground transmission of this REFSMMAT, it is necessary to send up the spacecraft state vectors and External Delta V targeting parameters for the third midcourse maneuver if it is needed. Also the state vector for entry initialization must be sent sometime during the last hour before entry with its time tag close to the predicted EI time.
10. There was considerable discussion regarding the spacecraft computer entry programs. Several modifications have been proposed, but it was evident from our discussion that we didn't know enough about the current definition of these programs to do anything. We also inconclusively discussed initialization of the EMS again. Accordingly, it was decided that our next meeting should include participation by MIT and North American personnel.