The Apollo rendezvous can be shortened by 2 hours
- Date: Jan 20 1970
- From: PA/Chief, Apollo Data Priority Coordination
- Subject: The Apollo rendezvous can be shortened by 2 hours
As you no doubt are aware, there is a movement afoot to shorten the Apollo rendezvous by 2 hours. This would be done by eliminating the CSI and CDH maneuvers and executing TPI about ½ hour after insertion. I thought the reason this was being considered was to reduce the crew’s workday, which has been pretty long. Apparently it is also to permit more EVA time on the lunar surface. In any case, a gang of us got together January 14 to talk it over. We were interested in hearing about what work has gone on, what the feasibility of doing this is, and to decide where to go from there. This memo is to briefly describe the technique (Ed Lineberry’s people are documenting this in detail and if you are interested you should call him) and to let you know that it does appear feasible. I will also note what has to be done now - the first thing being, to obtain MSC management approval to go on with it.
Following is a brief description of what the technique is:
a. Both the CSM and LM platform are aligned prior to LM lift-off. They are not ordinarily realigned during the rendezvous.
b. The CSM orbital should be 60 n. mi. circular as before. The LM insertion orbit will be 10 x 48 n. mi., instead of 10 x 45 n. mi. This small change will cause the post-TPI trajectory to be virtually identical to that utilized in the past.
c. Lift-off will be timed to provide the proper relative position of the LM to the CSM at the time of TPI execution which will occur 38 minutes after insertion. Thus, lift-off would be about 2½ minutes earlier than on previous missions.
d. It should be possible to obtain at least 25 marks by each spacecraft for their rendezvous navigation. Since we intend to always use the time option of the TPI targeting program, it should be possible to continue navigation significantly later than in the past. It can’t slip early on us.
e. The TPI maneuver is significantly different than before. It is about 85 fps and rather than along the line-of-sight, it is almost perpendicular to it (i.e., pitched down about 45°). Also, in order to provide an in-plane braking, the TPI maneuver will be made to force a node 90° later, that is, at the second midcourse maneuver.
f. We concluded that, since the LM TPI maneuver is RCS, the probability of an unexpected LM inability to execute the maneuver is almost zero. Accordingly there should be no requirement for the CSM to prepare to execute a mirror image TPI maneuver. Of course, if a LM failure has occurred which would preclude its performing TPI, the CSM would do it. It was noted that, since a CSM TPI would result in a very low orbit, it must also be active for braking.
Although we probed all related areas, we could find very little adverse impact by going to this plan. Certainly we have not changed the descent aborts and their associated rendezvous techniques - that is, one and two rev plans, including the CSI and CDH would still be utilized exactly as before and, of course, the crew and ground control must be trained and prepared to do them. This plan essentially consists of eliminating part of that standard rendezvous and, therefore crew training is unaffected. One area that FCSD will probably look into is the provision of TPI charts for the crew to backup the PGNCS and AGS. If these are required, they must be substantially different from the current ones.
The only other open area deals with changes to the RTCC. Only two were identified - the lift-off time computation and a program to determine a trim maneuver after LM insertion into orbit. The former should be extremely simple, if it is required at all. The need for the latter will depend to some extent on the sensitivity of the rendezvous to small errors in actual LM lift-off time and other insertion dispersions. Ed Lineberry’s people will continue their work in pinning down this sensitivity. The three involved FOD divisions will then establish whatever new RTCC requirements are really needed. This should be done within a week or so.
One pseudo-mission rule we agreed on was that this rendezvous approach should only be used in the nominal case when all important systems and trajectory conditions are as they should be. That is, if things like the rendezvous radar, the tracking light, or any of the other systems used for rendezvous are known to be broken, or if we have targeting problems, such as poor definition of the LM’s position, or of the CSM orbital elements we would, in real time, switch from this quick rendezvous to the standard approach used on all previous flights. Of course, this switchover must be made before lift-off since after that time we will have created a phasing situation that pretty well commits us to go on with the shortened plan.
In summary, a simple approach to shortening the Apollo rendezvous by 2 hours was agreed upon by just about everyone interested in this subject. The impact seems quite limited and, to me, well worth paying for the rather attractive benefit. I would be surprised if we have overlooked anything that would change this picture although, of course, it is possible, I suppose. Accordingly, we will continue working on this approach - cleaning up the loose ends noted above and will approach our leaders to see if it should be incorporated into the Apollo 14 mission. Essentially what we are offering is an increased capability which can be used either to extend the lunar surface work or to just shorten a long, tough day.