Alternate rendezvous technique - mission planning status
- Date: Sep 19 1966
- From: FM/Deputy Chief, Mission Planning and Analysis Division
- Subject: Alternate rendezvous technique - mission planning status
Since our meeting in your office on the stable orbit rendezvous, Ed Lineberry and I have met on a number of occasions with other interested people in an attempt to lay out a schedule of work needed to arrive at the decision as to how to go on 278 and subsequent missions. This note is to let you know the things we (MSC) intend to do and when we expect to get them done. As you will see, most of the work is being done by the Flight Crew Support Division and Guidance Control Division since the most significant difference from the old Concentric Flight Plan (CFP) involves the terminal phase of the rendezvous.
Paul Kramer, FCSD, has assigned a task to McDonnell Aircraft Co. to perform man-in-the-loop simulations of both the stable orbit rendezvous technique and the CFP with comparable approach velocities. Simulation of both systems will be initiated with the transfer maneuver. The approach velocities will be equivalent to the CFP with differential altitude varying from about 5 to 15 nautical miles. All failures modes will be investigated. It is intended to start this three-week effort on about September 19.
GCD has two studies going. The first is an evaluation of the CSM optical rendezvous guidance system to determine its accuracies and performance when operating in a stable orbit type rendezvous. The prime objective of this study is to determine the magnitude of the dispersions to be expected in the on-board computed maneuvers starting with the transfer from the stable orbit point. It is anticipated that this analysis will be completed by mid-October.
The second GCD study concerns the braking phase. Ron Simpson is in charge of this investigation which is primarily an expansion of one previously carried out for higher closing rates. He intends to start with conditions corresponding to CFP differential altitudes of between 5 and 15 miles. As I understand it, his man-in-the-loop simulations are usually initiated at about a l0 mile range. The purpose, of course, is to determine if there is some preferred closing rate going into the braking maneuver. These runs will be performed both with and without a cockpit display of range and range rate. He expects to start this analysis in mid-September with the results to be available early in October.
We are doing some things in MPAD too, of course, but they are not as extensive as I indicated they might be during our meeting. Ed Lineberry’s people are carrying out analyses aimed at selection of the optimum transfer angle (ωt) and trailing displacement for the stable orbit rendezvous technique. These two parameters are probably interrelated growing larger together to keep the closing rate meaningful in the face of dispersions. We still expect the preferred trailing displacement to be in the order of 15 to 20 nautical miles. At present the two prime candidates for ωt are 292° and 330°. 292°, you recall, has the advantage of providing the same approach conditions - primarily minimum inertial line of sight rates - as the CFP. This was the transfer angle used on the Gemini XI re-rendezvous which, in effect, checked out a ground controlled (perfectly!) CFP with braking without a radar simulating a differential altitude of 5 nautical miles. The 330° figure was jointly selected by MSC and MAC for the Gemini XII re-rendezvous based on lighting considerations and time available to make mid-course corrections. The objective was to provide as large a value of ωt as possible while avoiding the unique problems associated with a 360° transfer in the presence of out-of-plan and altitude dispersions. (Incidentally, McDonnell is carrying out a considerable amount of work both in terms of dispersion analysis and the preparation of flight crew rendezvous charts. Much of this work will be applicable to our effort.)
In addition, the mission planning for the Gemini XII re-rendezvous is being constrained as much as possible to duplicate the AS-278 initial CSM rendezvous conditions. In particular, we are trying to duplicate the ground tracking orbit determination capabilities as well as the relative motion trajectories to simulate the stable orbit rendezvous technique.
As you may recall, we originally estimated development of ten reference trajectories was required to provide information for the big decision. We have concluded that it is virtually impossible to do that much work in a reasonable length of time, regardless of how we redistribute. our manpower. However, RAB is developing a reference trajectory for the nominal lunar rendezvous assuming an on-time LM lift off. It will be a two-impulse, minimum ΔV trajectory to the stable orbit position. Once this is completed they intend to perturb the LM insertion conditions up to the 3 Sigma performance of the Abort Guidance System and the Ascent Propulsion System in order to determine the effects of these dispersions on the Delta V. Their work will be based on the assumption that there is a perfect knowledge of the situation at the time of the maneuvers and that they are executed perfectly. Ed anticipates that this work will be completed around the middle of October.
Finally, we are issuing a program change notice to MIT to provide an offset rendezvous target capability - trailing position only. I should point out that some resistance is expected to this program change, primarily from the FCOD since there are many other capabilities they give much higher priority which we have not agreed to implement so far. I am not certain how ASPO will react either since, as I understand it, TRW has reported to Joe Shea that they see no significant advantage to this technique.
Also associated with all this, the AS-207/208 Reference Trajectory is to be issued on about September 23. As you are probably aware, there are a large number of unresolved areas on this complex mission primarily due to the uncertainty associated with the AS-206 mission; thus, the quality of this Reference Trajectory is going to be questionable in any case. Unfortunately it will continue to show the initial CSM active rendezvous as a CFP type with a differential altitude of 20 nautical miles. Although it does not correspond to the planned documentation schedule, I really expect another Reference Trajectory will have to be issued prior to the release of the Operational Trajectory. Therefore, if we change to the stable orbit rendezvous, that will either be reflected in the new Reference Trajectory, or we will issue an addendum of some sort such as an internal note documenting the change.
Chris, this has been a tough problem and, believe it or not, we have spent a lot of time developing this plan for getting the answers you and Sig want. If there is something else you think we should be doing, please let me know.