Spacecraft computer program requirements for AS-207/208, AS-503, and AS-504
- Date: May 12 1966
- From: FM1/Assistant Chief, Mission Planning and Analysis Division
- Subject: Spacecraft computer program requirements for AS-207/208, AS-503, and AS-504
On May 13 and 14, 1966, a flock of MSC people met with MIT people in Boston to discuss the spacecraft computer program requirements for missions AS-207/208, AS-503, and AS-504. This memorandum is probably one of several on the subject you will be getting in the near future. My main purpose is to describe the situation as it exists on these important programs; it is not altogether a happy one.
Our basic problem seems to center on the time available to prepare the computer program for these flights and on the fact that the computer is not big enough to contain all of the programs which appear to be either required or highly desirable for the mission. According to MIT estimates, the programs which had been identified as needed for the CSM on the AS-504 mission are in the order of 15,000 words in excess of the 36,000 word computer. The LEM computer storage capacity was exceeded by about 6,000 words for the LORS configuration and 4,500 words for the radar configuration.
Since we have assumed a basic ground rule that no routines would be included in the AS-207/208 programs which are not in the AS-504 program, our first task was to reduce the AS-504 program requirements to a point where the CSM and LEM programs would fit within the computer storage available. After doing this, we went through the AS-504 programs and determined which processors could be omitted from the AS-207/208 programs if the overall schedule situation would be improved by their deletion. Accordingly, our task at this meeting was to identify the lower priority routines with an understanding that no further work would be done on them and they would not be included in the computer programs for the AS-207/208, AS-503, and AS-504 flights. It was evident from the start that there were very few programs which could be easily deleted. In fact, it was a very painful process. For the most part, “requirements” could only be dropped at some cost in probability of mission success or by putting a greater workload on the crew or reliance on ground support. We did adopt a basic ground rule that obviously flight crew safety could not be impaired.
We were successful in our task to the extent that the program requirements were reduced to a point wherein a reasonable chance of their fitting into the computer storage was assured. In addition, we identified the next computer routines which would be deleted in the event storage was ultimately exceeded, forcing the removal of more routines. I would like to list a few of the programs which were deleted to give you a feel for the situation. For example, the following routines were removed from the command module computer program:
Concentric flight plan routines, which provide an onboard capability for computing the first two maneuvers of the coelliptic flight plan, setting up proper initial conditions for terminal phase, were deleted. Since flight crew safety is implicit in the rendezvous capability, we (MPAD) have accepted a task of identifying the various failure situations in which the command module must rescue the LEM to assure ourselves that this capability will still exist with these programs not available to the crew. This is not certain at this time.
Programs used for computing attitude maneuvers were deleted. These programs were used in the guidance system for automatically making minimum fuel attitude change maneuvers while avoiding gimbal lock. It is obvious that these routines would be used even in a nominal mission, but it is felt the pilot could do the job instead of the computer, although probably at some extra cost in our precious RCS fuel.
It was identified that deletion of the capability to take over launch guidance of the S-II and/or S-IVB stages of the Saturn by the command module guidance system would considerably relieve our computer storage problem. However, it has been directed by NASA Headquarters that this capability be provided. Accordingly, steps are being taken within MSC in an attempt to relieve this Headquarters requirement, which is primarily one of improving probability of mission success with indirect and hopeful acceptable implications on flight crew safety.
Programs were being supplied to enable guidance system to execute maneuvers necessary for inserting the spacecraft into orbit or for landing in a preselected launch abort area by use of a spacecraft SPS maneuver in the event of a late launch abort. These routines, which were originally scheduled for AS-204 but were deleted from that flight due to schedule problems, were also deleted from the AS-504 program. This is more serious, however, since ground support of Saturn V aborts is more limited than for the 200 series missions.
Several other actions were taken to relieve the storage problems, such as deleting some programs from the flight ropes which support preflight pad tests. (It is not intended to delete the tests but rather to support them in another way.) In addition, action designed to streamline the program was initiated.
Identified as the next programs to be deleted, if it turns out to be necessary, are the stored star catalog and the automatic star selection routines which the pilot would use routinely even during a nominal mission for platform alignment. Deletion of these routines would force manual selection of which stars to use for this purpose and would require that their characteristics be manually keyed into the computer.
It is evident that the above programs would be extremely valuable during the lunar and AS-207 rendezvous missions, and the necessity of deleting these programs is probably the best indication of how critical the computer storage problem is.
Deletions in the LEM program were similar. It was interesting to note that the LORS configuration requires about 1,500 more storage locations than the radar. Thus, if the radar wins the guidance system olympics, we will recover this nice bonus. Generally speaking, however, it appeared that the computer storage problem was more severe on the command module computer than on the LORS at this date.
I would like to include a couple of remarks here regarding the programs for the AS-207/208 mission. Since it is intended to use only AS-504 programs and since it is possible to fly the AS-207/208 mission with a number of the CSM AS-504 programs omitted, by definition we do not have a storage problem for that mission. Our problem here–and it is a serious one–is that MIT maintains that we are considerably behind schedule. Although we intend to initiate action designed to improve this situation, it bas been recommended by MIT that a number of the AS-504 programs be deleted which are not essential for the AS-207/208 mission. Some of us at MSC are concerned that, although this may improve the schedule situation for AS-207/208, it may damage the schedule for AS-504, which is probably even more undesirable. Accordingly, we intend to review very carefully the overall schedule situation before any of the AS-504 programs are omitted from the AS-207/208 programs. At the very least, it is intended that all internal program interfaces be provided to insure the maximum similarity between the AS-207/208 and AS-504 programs. In fact, it may even prove desirable to substitute dummy programs for each of those deleted from the AS-504 program. MIT was in complete accord with us on this matter.
MIT is still expressing concern over their ability to define, design, and implement the concentric flight plan routines in time for including them in the AS-207/208 LEM program. However, they indicated that they could continue with development of the Guidance System Operations Plan (GSOP) for the AS-207/208 with those capabilities included for at least six weeks without any schedule impact. Thereafter they feel that if they have not arrived at an acceptable solution, it may be necessary to drop these routines, which are considered mandatory by MSC, from the AS-207/208 program. I personally have every intention of making sure that they are not dropped, but there seemed to be no need to argue this point at this time since it has no influence on the current course of action.
At the conclusion of the discussions of the AS-504 programs, MIT agreed that there was nothing more MSC could do to enhance the schedule situation for the AS-504 program. That is, further deletions of the program requirements would not help in any way. This was stated and restated several times to insure that MSC would not subsequently be notified that schedules could not be met as a result of excessive demands by MSC in the area of program requirements.