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tindallgram
May 10 1968
PA/Chief, Apollo Data Priority Coordination
68-PA-T-99A
"C" rendezvous open item clean up
rendezvous
TPI

  1. Paul Kramer, Phil Shaffer, Duane Mosel, Ed Lineberry, and myself spent the morning of May 7 trying to close out 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.

  1. 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 contraints 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 3° 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 trackign 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

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