“C” rendezvous open item clean up

See list belowMAY 10 196868-PA-T-99APA/Chief, Apollo Data Priority Coordination“C” rendezvous open item clean up

1. Paul Kramer, Phil Shaffer, Duane Mosel, Ed Lineberry, and myself spent the morning of May 7 trying to close our major open items remaining on the “C” mission rendezvous. These items were:

a. How to handle an excessive slip in TPI time.

b. What kind of cross checking and backup modes should be used for the TPI maneuver.

This memorandum briefly summarizes the results of our discussion.

2.First of all let me point out that without radar, it is important that the CSM does not approach the S-IVB while in darkness since range information is only obtained visually. Also, the sun must not be too near the line-of-sight – i.e., in back of the CSM – during braking for the same reason. These two constraints can be used to establish a “window” of acceptable TPI times to provide optimum lighting during the braking phase.

a. At this meeting we concluded that it is still best to locate TPI at the midpoint of darkness nominally.

b. In addition, we have specified that tolerable slip in TPI time is from 12 minutes early to 18 minutes late about that nominal time. That is, if the onboard solution for TPI time, based on the first sextant rendezvous tracking period following NSR falls within that period, no steps will be taken to change it. (It is currently estimated that the 3s uncertainty of the onboard computation of TPI time at that point in the mission is 4 minutes. Exceeding the bounds listed above by 4 minutes is not unacceptable.)

c. On the other hand, if the predicted TPI time slips earlier than 12 minutes or later than 18 minutes, the TPI elevation angle will be adjusted as necessary to bring the TPI time back to the closest bound. This is done as follows. Let us assume that at the end of the first tracking period the TPI time is found to be more than 12 minutes early by having run through the TPI program (P34) using the “elevation angle option.” P34 would be recalled using the “TPI time option” and the crew will input a TPI time exactly 12 minutes early in order to determine the elevation angle which will exist at that time. They will then recall P34 using the “elevation angle option” and will input that elevation angle. They will also relay this elevation angle to the ground in order that the MCC may determine a backup solution for TPI.

3. Associated with this, we have made a small change to the crew timeline. Based on latest trajectory information, it has been determined that the first sextant tracking period can be carried out about 16 minutes earlier than previously planned, even under the worst possible conditions. We are rescheduling it earlier in order to insure adequate time before the second sextant rendezvous tracking period to carry out the operation noted above if it is necessary. Specifically, the first tracking period will now begin 64 minutes before nominal TPI. This will occur no earlier than about eight minutes after local sunrise.

4. Deciding how to handle TPI comparison and backup solutions was a much more difficult task. The following is our proposal which we would be hard pressed to defend. Specifically, there are four parameters associated with TPI, any one of which could create problems if excessively in error. Therefore, it is our proposal that each of these parameters, as computed by the G&N, be compared with the best alternate source. If the difference does not exceed established limits on any of these parameters, the G&N will have passed the test and its maneuver will be executed. On the other hand, if any one of the four fails the test, a hybrid maneuver will be executed, each of its components being determined by the best alternate source. In that event, the average “G” program (P47) will be used to permit the G&N to navigate through the maneuver, and sextant tracking and G&N navigation would be continued after the burn in order to obtain as much data as possible for post-flight G&N performance analysis. However, if the failure exceeds twice the limit, we are arbitrarily recommending that the G&N be abandoned completely. That is, no more sextant observation would be taken if the indicated failure is that gross.

5. The four G&N evaluation parameters are TPI time, and the three components of delta V measured in “line-of-sight” coordinates, i.e., along the LOS and perpendicular to it both up (or down) and left (or right). The follow- ing table lists what we felt to be the best alternate source for each of these parameters and given our current guess of the expected 3σ accuracies for both that system and the G&N.

PARAMETER BEST ALTERNATE SOURCE 3σ BAS 3σ G&N TEST VALUE (BAS) Time of TPI FDAI 8 Ball 2 min 1½ min 3½ min LOS ΔV MSFN 1 fps 4 fps 5 fps Up/Down ΔV Backup Chart 3 fps 4 fps 7 fps Left/Right ΔV Backup Chart 3 fps 4 fps 7 fps

Of course it is intended to replace the estimated 3σ values listed with those obtained by up-to-date analysis.

6. It is readily apparent that all we did was to add the largest anti- cipated G&N error to the largest anticipated error of the comparison source in order to obtain the test limit. This rather simple-minded approach should make abandoning the PGNCS quite unlikely. On the other hand, it is felt that maneuver errors of that magnitude are not intolerable for continuing the rendezvous exercise.

7. The procedures and technique noted above will be incorporated in all official controlled documents, unless someone objects. Of course, they will then be subject to the same change control as everything else.

8. One other “C” rendezvous item worth noting is with regard to the NCC2 maneuver. Recent analysis has shown that it is not as likely to require an SPS burn as was previously thought. However, since RCS propellant is precious, I understand it is currently the flight controller's intention to reschedule it – delay it about two or three minutes – to force its growth to SPS size (15 fps) if it “naturally” falls in the region between 10 and 15 fps.