Comments on the AS-207/208 Preliminary Spacecraft Reference Trajectory


TRW Systems released the AS-207/208 Preliminary Spacecraft Reference Trajectory during the first week of May. This report was put together on a compressed schedule, starting from rather hastily defined mission requirements. As a result, there are a number of things about it which were recognized as being in error even before release; however, since we have started so late in the development of this mission plan, it was felt the release of a rough cut such as this was better than to delay for a more polished one. This is not to criticize the TRW report; considering the conditions they did a good job. However, since a large number of directly concerned people were interested in learning about this mission plan in detail and since it was desirable to identify as many corrections as possible right away, I set up an informal presentation by TRW on May 11, to be attended by whoever was interested. At this meeting TRW reviewed the trajectory-oriented aspects of the AS-207/208 mission plan with primary emphasis on the four rendezvous exercises currently scheduled. The purpose of this memorandum is to document the discrepancies and open items discussed during the meeting. Assignment of action items was not the objective of this meeting and none were assigned.

I would like to start out with a personal observation about this mission: Beyond a doubt, this mission plan is presently at least an order of magnitude more complex than any mission we will have flown before it. It was designed in an attempt to satisfy an overwhelming list of mission objectives established to test out spacecraft systems and crew procedures, both for nominal and for contingency situations. It is my feeling that, unless these mission objectives can be considerably cut back, we may be embarking on an unrealistic undertaking, including the development of a nominal mission plan which can really satisfy all of these objectives, the development of complex crew procedures, both to carry out that plan and to handle contingency-contingencies, and, perhaps most significant, the dumping of an overwhelming, if not impossible, load on the flight crew, not only for preparation and training for the mission, but also its actual execution. This crew will be expected to check out the first Block II CSM, man and check out a LEM for the first time, perform three or four completely different rendezvous exercises with different guidance systems and procedures, carry out two EVA exercises, perform a number of contingency operations, such as switching over from one guidance system to another during primary engine burns, simulate crew rescue in terms of both rendezvous and crew transfer from one spacecraft to another by EVA, and so forth. It is to be noted that all of this is supposed to be done with spacecraft which have been designed for a specific mission–the lunar landing. That is, they have not been developed with operational flexibility as a design criteria.

And so with that introduction, I would like to record here a number of the specific comments of this discussion:

  1. CSM/S-IVB Separation: The Preliminary Spacecraft Reference Trajectory has the command module separating from the S-IVB after 1 hour and 41 minutes of mission time. We were informed that agreements currently in effect with MSFC call for the CSM to stay with the S-IVB for at least two orbits and unless there is some problem associated with this, it would probably be preferable to retain that procedure.

  2. S-IVB Venting: There was some question as to how we would handle the problem.of spurious S-IVB venting in the event rendezvous is not carried out at the time scheduled. Of particular concern was the possibility of venting during the latter part of the rendezvous, with the problem becoming more critical during the braking and docking maneuvers. According to the Apollo Spacecraft Program Office (ASPO), MSFC is waiting for a set of ground rules from MSC defining how the venting situation should be handled.

  3. Braking Gates: Based on mission requirements established by ASPO, TRW showed a maneuver being made at the first braking gate to reduce the closing velocity to 20 ft/sec. The consensus shows that this magnitude is somewhat too low in that it tends to undesirably stretch out the terminal phase, which increases the possibility of the situation deteriorating, as well as possibly costing more fuel.

  4. Priority of Mission Objectives: Repeatedly throughout the meeting we came upon situations in which mission objectives were in conflict with each other and/or were undesirable in terms of excessive consumable usage or mission complexity. Accordingly, it seems highly desirable that the ASPO review the mission objectives and assign priorities defining the relative importance of the various mission objectives in order that meaningful mission planning can be carried out both in advance of the mission and in real time.

  5. Recontact: Another problem area reidentified at this meeting dealt with the possibility of recontact of the spacecraft with either of the S-IVB’s Or the LEM nose cone. Obviously, attention must be given to the relative motion of all the many orbiting objects associated with this mission.

  6. Stroking: When and how the stroking tests are to be carried out still remains ill defined with regard to such questions as the necessary propellant loading in the LEM at the time of the test, nature of network coverage required, etc.

  7. Crew Rest: Ground rules associated with the crew rest periods, such as whether or not it is permissible or necessary that all crew members do sleep or do not sleep at the same time, has a heavy impact on the scheduling of the various activities. Accordingly, it is necessary that ground rules associated with crew rest be established at the earliest possible time.

  8. CSM/LEM Separation for Re-rendezvous: In each of the re-rendezvous exercises: TRW included a considerable period of time between actual disconnection of the two spacecraft and the time at which the first major maneuver is made to establish the desired conditions for carrying out the terminal phase of each of the re-rendezvouses. It was agreed that the procedure TRW had included in the Preliminary Reference Trajectory seemed as good as any; however, prior to development of the follow-on documentation, it seems advisable to give further consideration to how we actually want to set up this procedure.

  9. Minimum SPS Maneuver: A rather lengthy, but inconclusive, discussion centered on defining the minimum SPS maneuver which could be carried out. This has particular influence on RCS propellant usage in that the larger this minimum SPS maneuver is set, the more likely it will be necessary to carry out maneuvers with the RCS. On the other hand, it was noted that the capability of controlling the SPS engines for these small maneuvers leaves something to be desired in that large residual tumbling rates can result if the SPS thrust vector is not directed through the spacecraft e.g. and sufficient time is not given for the guidance system to compensate for it. RCS fuel would then be required to stop the rates.

  10. Extra-vehicular Activity (EVA): The situation regarding EVA is still badly clouded. This is the case in terms of how many EVA exercises should be carried out, when they should be scheduled in the mission, whether the spacecraft should be docked or undocked, and, in fact, even includes what appears to be a need for re-evaluating the associated mission objectives. One thing that was clear, however, was that not enough time had been included for these exercises. TRW had provided about 1½ hours, whereas the Flight Crew Support Division (FCSD) feels that 4 to 5 hours would be a more accurate estimate. It was also noted that, as scheduled by TRW, ground coverage was inadequate particularly considering the fact that this will be the first EVA carried out in the Apollo Program.

  11. Spacecraft Guidance Switchover: Mission objectives have been established which call for switchover from the primary to the backup LEM guidance system during powered maneuvers. In order to provide maneuvers of sufficient magnitude to evaluate this procedure, it was necessary to orient them such that much of the energy is dissipated out-of-plane. Simultaneously, an in-plane component is provided for establishing the initial conditions for the re-rendezvous terminal phase for each of the LEM active re-rendezvous exercises. This whole activity seems highly undesirable in that it increases the complexity of the mission to a great extent, has a good chance of fouling up the re-rendezvous exercise, and presents serious operational problems. For example, the platform alignment must be in an attitude different than would be used in an actual lunar mission in order to avoid gimbal lock. In addition to perturbing the navigation carried out by the primary guidance, it presents special problems with initialization of the abort guidance system which is programmed to assume that the primary inertial reference is aligned in the orbital plane. This is one example referred to in the previous note regarding relative priorities of the various mission objectives.

  12. Ground Coverage Versus Lighting: The Preliminary Reference Trajectory was prepared such that all maneuvers were scheduled to occur over ground stations to the greatest possible extent. No consideration was really given to the lighting conditions for the rendezvous. This was intentionally done since the Preliminary Reference Trajectory was needed to supply the necessary information to make reasonable tradeoffs prior to preparation of the Reference Trajectory. It is obvious that there will be a direct conflict between station coverage and lighting which must be resolved prior to preparation of the Reference Trajectory. Flight crew requirements associated with this are urgently needed.

  13. LEM RCS Usage: It was noted by several of the participants at this meeting that the Preliminary Reference Trajectory as presented exceeds the LEM RCS capability in that ullage is only available when the down-firing jets are used since there is no planned interconnect on this flight. Did I say that right?

  14. Docked DPS Burn: There was considerable discussion regarding the LEM Descent Propulsion System (DPS) maneuver in the docked configuration. In particular, there was concern as to whether it should be carried out as scheduled early in the mission or as part of one of the re-rendezvous exercises. Although there were problems associated with both, the consensus was to leave it as scheduled; that is, one of the Hohmann, transfer maneuvers to place the CSM/LEM in the 180 n.m. circular orbit prior to the first re-rendezvous exercise.

  15. Fire-in-the-Hole: It appears that requirements associated with camera coverage of the FITH should be established-as soon as possible.

There are undoubtedly other items I should have included here that I either missed or forgot. At least they won’t make this memorandum any longer than it is.

Terms & Abbreviations


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.


Apollo Spacecraft Program Office.


Command-Service Module.


Descent Propulsion System.


Extra-vehicular activity, such as space-walks and moon-walks.


Flight Crew Support Division.


see Fire-in-the-hole


When the LM’s Ascent Propulsion System was fired with the descent stage still attached, for instance if the mission were aborted during lunar landing, this was called ”Fire in the hole. ” The first test of this maneuver was planned for AS-207/208. After that mission was cancelled, it was ultimately tested during the unmanned Apollo 5 mission in January 1968. Nasa produced a short film about Apollo 5 that briefly explains FITH.

Gimbal Lock

The loss of one degree of freedom in a three-dimensional space that occurs when the axes of two of the three gimbals are driven into a parallel configuration, “locking” the system into rotation in a degenerate two-dimensional space. More at Wikipedia.


see 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.”


Manned Spacecraft Center. Now known as Johnson Space Center.


Marshall Space Flight Center


Reaction Control System.


The second stage of a Saturn IB or the third stage of a Saturn V.


Service Propulsion System, the large engine of the Service Module that was used to enter and exit lunar orbit, as well as make course corrections while going to and from the moon.


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.