Flyby solutions in the RTCC Midcourse program will not be absolutely optimum

See list belowAPR 8 196868-FM-T-72FM/Deputy ChiefFlyby solutions in the RTCC Midcourse program will not be absolutely optimum

This memo is to inform you of a simplification in RTCC program requirements I recently approved. As noted below, the capability we are providing appears to be adequate and the cost of the optimization is incompatible with the benefit to be gained. The rest of this memo is lifted almost verbatim from one Bob Ernull wrote to me.

Quite a few months ago, it was agreed by MPAD, FCD, and FSD that a circum- lunar (flyby) mode would be included in the RTCC midcourse program for alternate missions and circumlunar aborts. One problem we were particularly concerned about was the case where we have to get back home with the RCS only; this implies both a SPS failure and DPS failure, or failure to extract the LM, after TLI. Because of the limited delta V available from the RCS, approximately 150 fps for translation, the guideline established was to develop a program logic which would provide the absolute minimum delta V solution to insure safe entry.

In trying to develop a program which would compute the “optimum” solution, we ran into many problems. We have reached a point now where even though program development is not complete, we probably know how to build the program required; however, the running time on the RTCC computers ranges from 20-40 minutes per solution. We have examined ways of reducing this time and do not see any possibilities which would effect any significant reduction. Although this might be acceptable during an operation, imagine the computer time and effort required to check it all out.

During the evaluation of computation techniques for the “optimum” solution it was found that a very near optimum solution could be found using a simple computation procedure based on a “return-to-nominal” concept. This concept simply takes advantage of the fact that the nominal pericynthion conditions which were optimized pre-flight, will still be very near optimum for any small midcourse maneuver. Since for the RCS problem we are by definition considering for the flyby solutions, get an answer which is near optimum and avoid the iterative search for optimized pericynthion conditions. This reduces the run time from 20-40 minutes for the “optimum” solution to 1-2 minutes for the “return-to-nominal” solution.

The next question is how much delta V penalty is incurred if we decide to implement the simple and faster computation technique in the RTCC. It can be shown that the “optimum” solution will cover S-IVB injection errors 50-100% larger than the return-to-nominal. However, these dispersions must be compared with the expected S-IVB 3σ dispersions to get a true picture of the situation. This comparison shows that with the return-to-nominal we can cover S-IVB injection errors twice as large as the 3σ errors. This is based on the assumption that up to 100 fps is available for the first maneuver, the additional 50 fps is reserved for subsequent corrections.

Summarizing, in order for the return-to-nominal solution to be inadequate we have to have an SPS failure, a failure of the DPS (or no extraction) and a S-IVB dispersion twice as large as the predicted 3σ dispersions.

On this basis, and considering the major impact of developing, checking out and verifying a program where each run takes 20 minutes or more, the decision was made to delete the requirement for computing an optimum flyby solution and use the return to nominal technique. I hope you agree.