Downthread: Recommendation to retain the Two-Stage Lunar Orbit Insertion (LOI) Maneuver (Aug 05, 1968)
See list belowDEC 15 196767-PA-T-121APA/Chief, Apollo Data Priority CoordinationIt is proposed that we plan a two step LOI maneuver
1. For the past several months there has been considerable discussion regarding the question of whether we should perform the lunar orbit insertion (LOI) maneuver as a single burn or if it should be made in two steps. This is an important matter to resolve since it has impact on RTCC program development, guidance systems targeting, crew monitor- ing procedures, mission timeline, the mission plan, the flight plan, and mission rules. Accordingly, the entire December 12 meeting of the Midcourse Phase Working Group of the Data Priority Coordination effort was devoted to this subject. Detailed minutes are being prepared for distribution by Mr. Ronald Berry, the chairman of that working group. However, the significance of the decisions reached were such that a more immediate, wider dissemination of them warranted writing this note. Specifically, it was concluded that we should utilize a two step LOI maneuver and that it should be performed utilizing the External Delta V guidance mode. The entire justification for this mission plan modifica- tion is bared on the need to provide an adequate flight crew monitoring procedure, although it is anticipated that further advantage may be obtained by incorporating dispersion killing altitude and plane change adjustments in the second LOI burn which will provide a saving of both propellant and a reduction of timeline activity.
2. Considerable analyses, thought and discussion have been devoted to how the 3,200 fps, 380 sec. LOI maneuver should be monitored. That this maneuver is performed behind the moon and in its later stages has serious time critical, flight crew safety implications presents sort special problems. The most serious consideration is the need to assure in the later stages of the LOI maneuver that the SPS engine is not shut down too late. For example, an overburn of approximately 8½ seconds would result in lunar surface impact. To avoid this with certainty the crew will be observing the duration of the maneuver (a clock) and the delta V counter and will be forced to manually shut down the engine when one or the other, or perhaps both, indicates a hazardous situation is imminent. Taking into account reasonable dispersions in those monitoring systems and the duration of overburn considered acceptable, it has been determined that the proba- bility the crew would manually take over and shut down the engine before a satisfactorily operating primary guidance system gets the chance is more than 50%. This action, of course, creates a two stage LOI maneuver in itself, but with a variable second stage magnitude. And the mission planner must provide for it in the lunar timeline. Since the probability of this occurring is so great, it was concluded the nominal mission plan should include a controlled, two step LOI.
3. Given this decision, it was determined that the first maneuver should be intentionally targeted to terminate early enough to insure that the primary guidance system has every opportunity to perform its function and to insure that the second phase of LOI is of sufficient duration to give good guidance system performance. A value of about 15 seconds was selected since it met both of these criteria and, therefore, it is proposed to tar- get the initial LOI maneuver to provide a lunar orbit of 60 X 170 nautical miles. The second maneuver would occur approximately two revolutions later and would have a duration of approximately 15 seconds.
4. Another matter of significance was also resolved. Namely, it has been determined that there is very little to choose between use of Lambert steering or external Delta V steering modes with regard to delta V costs and guidance accuracy. On the other hand, the use of External Delta V does provide a real advantage for crew monitoring of spacecraft attitude during the burn which is also very important. Whereas, Lambert steering results in a continuously varying attitude with a profile which cannot be established preflight, External Delta V provides a constant inertial attitude, substantially simplifying the procedures, the training and the operation. Therefore, pending confirmation by some simple, short studies it is proposed that we establish External Delta V as the guidance mode and concentrate all effort accordingly. For example, RTCC programs associated with Lambert targeting the LOI maneuver may be deleted.
5. In summary then, given CCB approval (if that's how this sort of thing is done!) I anticipate all future lunar mission work will be based on a two stage LOI maneuver performed utilizing external Delta V with the second maneuver being in the order of 15 seconds duration.