Tindallgrams

Topics:

I really blew it at the June 5 Apollo Spacecraft Software Configuration Control Board meeting. Although dozens of rather minor changes were approved, the one I was most concerned about wasn’t even discussed and I completely forgot it. This memo is to inform you that we are now desperately trying to include a capability in the LM computer program for a lunar landing flight in November which substantially improves descent abort targeting and procedures. Currently the LM descent abort programs target the spacecraft to insertion conditions which is not entirely accurate. This is because the more sophisticated equations required to do the job right were too complicated to get in the program for the G mission and we settled for some approximations that only do a pretty good job. Unfortunately, if we have a descent abort this makes it necessary to trim the insertion conditions based on ground targeting. This is the so-called “tweak” maneuver you’ve heard so much about which either the LM or command module must execute shortly after LM insertion into orbit. It is a messy procedure and the program change proposed will eliminate the need. Furthermore, for aborts late in powered descent (that is, after PDI + 10 minutes) it is necessary for the LM to execute a phasing maneuver approximately one-half rev after insertion to set up the proper rendezvous conditions. This, too, is a messy ground targeted procedure which will be eliminated if this program change is implemented.

Although I wanted to tell you about that, my main purpose in writing this memo was to inform you that in order to get this program change in we have to sacrifice some other things and I thought you should have an opportunity to complain if you wanted to. First of all, storage has again become a problem and so we propose that, if necessary, MIT should delete the two Stable Orbit Rendezvous targeting program (P38 and P39) from the LM program. We have never discovered an operational use for these programs but maybe this deletion may bug somebody. (Incidentally, in order to provide more room for the dozen or so other changes already approved, the externally targeted Lambert pre-thrust program (P31) has already been deleted.) The other capability which may have to be dropped is the rendezvous radar automatic acquisition provided by the PGNCS during the Descent Abort programs (P70 and P71). Disabling this capability (R29), may be required to avoid a computer cycle problem. That is, obviously the computer can only do so much in a given period of time and it is MIT’s option that adding the proposed sophistication in the guidance may cause us to exceed that limitation. This in turn forces us to give up another task and we have chosen the so-called Rendezvous Radar Designate Routine.

This final paragraph is on another subject, but I thought I would point out that one of the more significant capabilities added last Thursday was the capability for the crew to readout raw rendezvous radar range and range rate data on the DSKY during the operation of the Rendezvous Navigation program (P20). This capability had been requested several times previously but never made it in to the program due to scheduling problems. It is a real nice thing to have.

Terms & Abbreviations

LM

Lunar Module. Earlier it was known as the Lunar Excursion Module and abbreviated “LEM.” Even after the name change, it continued to be pronounced “lem.”

MIT

Massachussets Institute of Technology. In these memos, MIT is shorthand for the MIT Instrumentation Laboratory, created and led by avionics pioneer Charles Stark Draper. It is now known as the Charles Stark Draper Laboratory and became independent of MIT in 1973.

PGNCS

Primary Guidance, Navigation and Control System (pronounced “pings”). The inertial guidance system in both the Command Module and the Lunar Module.