Tindallgrams

As you know, the rendezvous orbit navigation process involves updating the spacecraft state vector based on the spacecraft radar data. However, the radar apparently has unacceptably large angular bias errors for some reason. Instead of fixing the damn radar, someone decided to include in the LEM spacecraft computer program the capability of computing these radar angle biases at the same time the spacecraft state vector is updated. Once these biases have been determined to the computer’s satisfaction, they are not updated further; that is, they are assumed to remain unchanged thereafter.

There is a contingency, however, which would cause them to change, so I’m told, and that is if the LEM were to undergo loss of pressurization. It had been MIT’s intention to provide an option in their rendezvous orbit determination program to reinitialize the computer such that it would redetermine the radar angle biases in this event. However, in line with our campaign to simplify the computer program, this option is being deleted, which means that, in the event of spacecraft pressurization loss, the radar angle bias may be in error by some fraction of a degree. This does not disable the rendezvous guidance system, but rather may cause some loss of efficiency in the use of propulsion fuel. Just how much depends on when it happens, of course, but the maximum extra cost is not expected to exceed about 50 ft/sec. We’ll get a better estimate of this cost and make sure it’s acceptable.

Ed Lineberry, how about you getting that done. In the meantime, we’re telling MIT to take this option out unless we’re directed to do otherwise.

Terms & Abbreviations

LEM

see LM

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.