AS-206 LGC program status

On Octber 6th at MIT, we held a review of the AS-206 computer program which they have romantically christened “Sunburst”. Our primary objectives were twofold; first, to make certain that the formulation of the program was consistent with the way we intend to fly the mission, and second, to determine the current status of the program development. Generally speaking, I would say we are in good shape on the AS-206. We appear to be on schedule with some tolerance for problems of a nature you ordinarily expect to encounter in this type of work, and with a few exceptions, the program as currently defined should be entirely adequate to support the mission.

It is still planned to release this program for rope manufacture on December 26th. This date has held firm for a number of months now; and Jim Miller, who has taken over direction of this program at MIT in the absence of hospitalized George Cherry, presented fairly detailed program development plans upon which he based his confidence of staying on that schedule. He identified as the two most critical items:

a) The descent guidance for Mission Phase 2 (i.e., the second DPS maneuver), and

b) The digital auto pilot which is also the major processor remaining to be completed. Jim pointed out that a number of processors have been coded and unit tested which are now awaiting the availability of the DAP for integrated systems tests.

Jim Miller has beefed up the manpower in both of these areas recently.

There was one item requiring immediate attention if anything is to be done about it. This involves the manner in which the LGC is set into action at Saturn launch. Apparently, the program is started by the receipt of a guidance reference release (GRR) signal sent some 3 minutes and 10 seconds prior to liftoff by the Houston MCC command system. There are two things that sound kind of lousy to me; one is the desirability of having to send a command from a remote site to start the system working, and the second is the fact that it is currently planned to send this signal so long prior to liftoff. Obviously, the problem here is that if a hold in the countdown is encountered after it is sent, it is necessary to recycle the launch countdown back as much as 2 or 3 hours which sounds completely unacceptable. I wouldn’t be surprised if I have this all confused. If you are concerned with this type of thing, I suggest you get ahold of someone who knows what they are talking about for a precise description of the situation. Incidentally, if program changes are required associated with this GRR problem, there is a possibility of schedule implications.

Another item on which we spent a considerable amount of time had to do with the implementation of two jet ullage, which is desired on AS-206 in order to make the DAP for that mission consistent with the DAP for AS-208. A question arose as to whether there should be some sort of interlock to inhibit the main engine start signal based on onboard sensing of ullage, or rather lack of it. It was finally decided that we should leave the program essentially as it is with a fixed duration of ullage and an engine start signal issued by the LGC at a particular time in the sequence. This was primarily to insure that the tests performed on AS-206 are applicable to AS-208. MIT did request that we direct Grumman to provide RCS jet fail indications to the LGC for use in their automatic jet select logic in a somewhat different way than is currently planned.

Other matters receiving consideration at this meeting were:

a) The possibility of utilizing the LGC to keep track of RCS fuel used and remaining – a job which apparently can not be done accurately in any other manner. MIT expressed reservations that the LGC would be able to do this accurately either. In addition, there may be computer cycle time problems since this processor would have to operate simultaneously with the DAP which is already heavily loading the computer.

b) The matter of increasing the size of the downlink lists from 100 to 200 words on the AS-206 program since FCD expects that this will be necessary for the later LM’s.

MIT indicated that they intend to issue a new, complete GSOP around the end of November. The last one, R-527, was dated June 1966. MIT intends to release certain parts of this earlier since it is badly needed by some parties right now.

Terms & Abbreviations


Originally scheduled as the first unmanned flight of the LM, it was cancelled after the Apollo 1 fire. The AS-206 launch vehicle, a Saturn 1B, was used to launch Skylab 2 on May 25, 1973.


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


Digital Autopilot.


Descent Propulsion System.


Flight Control Division.


see Guidance System Operations Plan

Guidance System Operations Plan

The GSOP was essentially the specification for how the guidance computer and its software where required to work for a specific mission. Many of GSOP’s are available online including the GSOP for the cancelled AS-207/208 mission


Lunar Module Guidance Computer.


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.”


Mission Control Center. Popularly known as “Houston” (as in “Houston, we have a problem”)


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.


Reaction Control System.


Ullage is the empty space in a liquid rocket’s fuel tank. In zero-gravity, the fuel of course tended to float around inside the tank. Before many maneuvers, rockets were gently fired for a short time “for ullage,” that is, to push all the fuel to the back of the tank so that calculations about how to maneuver the spacecraft would be simpler and more accurate.