There are differences in the descent guidance programs on AS-503 and AS-504

It is currently intended to include some sort of tests in the LM descent propulsion and guidance on the AS-503 mission. However, it is not possible to use the same guidance equations on AS-503 as will be used on the AS-504 lunar descent. This is due to obvious differences of an earth orbital mission compared to an actual descent on the moon’s surface. The gravitational potential is different; the objective of the maneuver is different; there is nothing for the lunar landing radar to bounce signals off of, etc. Accordingly it is MIT’s intention, with our concurrence, to omit certain vital parts of the AS-504 descent guidance program from the AS-503 mission. The purpose of this memo is to make sure that you all know this.

It is my understanding that there are four main processors of the AS-504 descent guidance program which are not to be included in the AS-503 program:

  1. processing of the landing radar data
  2. landing point designation
  3. x-axis override
  4. automatic rate of descent control

In addition, there is likely to be a change in the coordinate system of some sort needed.

I certainly do not claim to be an expert in descent guidance; in fact, quite the opposite. If anything above interests you, I would suggest you contact someone who really knows what they are talking about like Don Cheatham, Floyd Bennett or Tom Price.

Incidentally, we are currently in the midst of an exercise designed to make the AS-278 spacecraft computer programs identical to those for AS-503. Although I don’t expect this to have any influence on the descent guidance, I just mentioned it here to cover that possibility.

Terms & Abbreviations


see AS-207/208


see AS-503


see AS-504


AS-207/208 (also known as AS-278) was to have been the first test of the LM in Earth orbit. It was also to have be a dual mission with the command and lunar modules launched on separate Saturn 1Bs. The mission was cancelled after the Apollo 1 fire and the Saturn 1Bs were used to launch Skylab 3 (AS-207) on July 28, 1973 and Skylab 4 (AS-208) on November 16, 1978. The LM was first tested by Apollo 5 in January 1968.


see AS-207/208


Before the Apollo 1 fire, the mission referred to as AS-503 was an unmanned Earth orbit test flight of the LM and CM scheduled for October 1967. The launch vehicle, SA-503 was used for Apollo 8, December 23 1968.


Before the Apollo 1 fire, the mission referred to as AS-504 was originally scheduled for December 1967. AS-504 eventually launched as Apollo 9, March 3 1969.


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.”


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.