See list attachedDecember 13, 196868-PA-T-271APA/Chief, Apollo Data Priority CoordinationD Rendezvous Mission Techniques
This memo is to let you know about some things on the D Rendezvous that have been giving us a lot of trouble. The problem we have been having is associated with the football trajectory and how to exit from it gracefully. It seems like most of the significant mission techniques open items deal with this subject. In fact, we submitted a Trajectory Change Request in an attempt to relieve this problem area a little bit. It was disapproved – rightfully, I think.
The thing that is beginning to bother me is the realization that the probability of aborting the D Rendezvous from the football trajectory is rather great. This is due to a center wide feeling that a rendezvous from the football accomplishes almost everything we want and going through the CDI/CDH does not offer enough benefit to justify the addi- tional risk of two or three extra hours of LM operation unless all systems are operating. That is, even failure of equipment like the rendezvous radar and the AGS currently appear to be justification not to exit the football. The other thing that I am slowly beginning to realize is that the football rendezvous is by no means simple. In many ways it is a lot more difficult than the standard coelliptic rendezvous. Not only are many special procedures required for it but the TPI maneuver is very sensitive to small dispersions in the relative trajectory of the two spacecraft. By the same token, small errors in the MSFN state vectors will cause the ground computed solution to differ significantly from the onboard. These things have led us to propose a basic ground rule – namely that TPI₀ should never be executed on the first opportunity except in a time critical situation. Furthermore, we could define no single guidance, navigation, or control system problem which we consider time critical. That is, time critical situations must arise from some serious environmental or electrical problem or something like that. By going an extra revolution in the football we give both the crew and the flight controllers an opportunity to get squared away before going into the critical terminal phase. We should have considerably more confidence in the MSFN state vectors too since we would have a sustained period of unper- turbed radar tracking. Unfortunately, spending an extra revolution in the football for this purpose aggravates another problem. Small dispersions prior to and during the phasing burn can cause a situation wherein the spacecraft never arrives at a 27.5 degree elevation angle for execution of TPI. Going the extra revolution makes us even more susceptible to this. It was due to this that we proposed a trajectory change. Specifically, by reversing the direction of the CSM 5 fps Separation burn from radially down to radially up, we become tolerant of much larger dispersions. How- ever, the impact on other things at this late a date was considered unac- ceptable.
It is recognized by everyone that we still do not have TPI₀ procedures worked out yet and that by disapproving the trajectory change we were buying additional complexity in them. We are also making the probability greater for having to do TPI₀ at some angle smaller than the nominal (27.5°).
We have initiated an analysis to determine if it is possible for the ground to give useful assistance for TPI₀ at the first opportunity. There is a feeling on the part of some of us that the ground solution for the first TPI₀ could be substantially in error making it useless both for comparison with the onboard system and for backup in the event of an onboard failure. The point is we may have to establish a technique whereby the rendezvous must be carried out independent of the ground in the time critical case.
1. It is obvious that we must have well thought-out procedures and thorough training to handle the football rendezvous since the probability of doing it is very great (e.g., 5 or 10 percent, I would guess).
2. The football rendezvous is significantly more difficult to perform than intuition leads you to believe. Accordingly, we are proposing to always spend one extra revolution in the football prior to attempting the rendezvous if it is at all possible to do so.
3. The crew procedures will be developed to make sure they serve well for initiation of rendezvous on all revolutions in the football not just the first.
In attempt to finally clean up those darned TPI₀ mission techniques prior to start of MCC-H/crew simulations, we will probably get together over the Christmas Holidays – whatever that is.