Upthread: Lunar Reentry Mission Techniques meeting (Feb 06, 1968)
Downthread: No special visual reference will be provided for the final transearth midcourse correction maneuver (May 10, 1968)
See list belowAPR 17 196868-PA-T-78APA/Chief, Apollo Data Priority CoordinationApril 9 Lunar Reentry Mission Techniques meeting
1. Almost the entire April 9 Lunar Reentry Mission Techniques meet- ing was spent in discussion of the second and third midcourse correction maneuvers made on the way back from the moon. Ron Berry presented some really interesting data which I think gives us an insight into the character of these maneuvers needed to assure the techniques we are developing will be effective.
2. It was noted that there are four modes for computing midcourse corrections in the Mission Control Center. All are designed to place the spacecraft on a trajectory which hits the center of the reentry corridor. They differ by the additional contracts they are designed to meet.
a. The first and most sophisticated determines the maneuver which will cause the spacecraft to land at a specified landing site (PLA).
b. The second relieves the constraint for latitude but will cause the spacecraft to land at a specified longitude (CLA).
c. The third mode does not constrain reentry to occur at any specific place but rather determines the minimum delta V maneuver required to assure a safe reentry.
d. The fourth mode does not aim for a specific landing site either but determines the optimum maneuver based on delta V available to get the spacecraft back to earth in the least time.
Of these four modes, we are primarily concerned here with the first and the third. That is, we intend to use the PLA mode for determining the first and second midcourse corrections coming back from the moon (MCC 1 and MCC 2) provided the delta V cost is reasonable. The third midcourse correction (MCC 3) which occurs two hours before reentry will utilize the minimum delta V mode since effective landing point control is best achieved at that point with the guided lifting reentry and we want to make this maneuver as small as possible for safety reasons.
3. The MCC 2 will be executed utilizing the External Delta V spacecraft computer program. Right now ???? has the maneuver scheduled at about 16 or 17 hours before entry, which is good for both trajectory and crew timeline reasons. According to Ron Berry the only component which has a significant effect on the trajectory is horizontal (i.e., perpendicular) to the radial vector. A dispersion of 0.8 feet per second (fps) in the horizontal direction would cause the spacecraft to miss the entry corridor. That is, it will cause ????? degree dispersion on flight path angle at the reentry interface. It is anticipated that MSFN accuracy will contri- bute approximately one half of that dispersion leaving only 0.4 fps for maneuver targeting and execution error. This sort of dispersion is about what we would expect, which means the likelihood of having to make MCC 3 much greater than we had been previously been led to believe.
4. Ron Berry reported that the MCC 3 also is most sensitive in a hori- zontal direction. Four fps dispersion in that direction will cause the flight path angle at the entry interface to change .36 degrees. After considerable discussion, the consensus was that we should modify the criteria we had previously established for determining whether or not to execute MCC 3. We had stated the maneuver would only be made if dispersion in flight path angle was anticipated to be in excess of .36 degrees which we now know to be equivalent to a 4 fps horizontal maneuver. There really seemed to be little reason for not reducing this limit. Accordingly, we have now modified the criteria such that whenever the maneuver required to bring the spacecraft trajectory back to the center of the entry corridor is greater than 4 fps is should be made. Making the decision based on delta V rather than flight path angle dispersion simplified the flight controller's job considerably since that parameter is readily available to him. One other thing that needs to be emphasized is that it is a “safe” burn to make in the sense that it is relatively insensitive in direction. That is, errors in the direction we apply the maneuver only tend to reduce its effectiveness and it is not until a 90 degree error is reached that any adverse effect would be felt. As a matter of fact, since only the horizontal component of the maneuver is effective in influencing the entry interface conditions, consideration is being give to orienting the maneuver such that a good visual reference is available when the maneuver is made. This would introduce delta V components lateral to the desired (horizontal) delta V, and that has no significant effect except to increase the magnitude of the burn.
5. Everyone was requested to see if there was any reason for not making this maneuver when it is not absolutely necessary. Otherwise, we will proceed as noted above, which I should think would make the probability of making this maneuver something like three out of four.
6. It is intended that our next meeting, currently scheduled for April 30, to review the mission technique flow chars that TRW are preparing base on our meeting up to this time.