LM propulsion of the LM/CSM configuration as an SPS backup technique

PA/Manager, Apollo Spacecraft ProgramMAY 1 1968EG23-88-68-486EA/Director of Engineering and DevelopmentLM propulsion of the LM/CSM configuration as an SPS backup technique

Reference is made to the following documents:

1. MSC memorandum, “Mission Planning Systems Meeting No. 4,” October 10 1963.

2. MSC memorandum, “Use of the Apollo LM propulsion system as backup to the service propulsion system,” November 10, 1963.

3. GAEC memorandum, IMO-500-112, “Dynamic and static stability analysis of a combined LM/CSM propelled by LM thrusting,” May 28, 1964.

4. GAEC memorandum, IMO-50-192, “Stability of combined LM/CSM with LM thrusting including guidance loop,” July 22, 1964.

5. Apollo Mission Planning Task Force (AMPTF) report, IED-540-8, July 27, 1964.

6. MSC memorandum, EG23, April 9, 1965, “Preliminary results of analog simulation of CSM/LM transearth injection using LM propulsion.”

7. MSC Internal Note, EG-65-26, “Analog Simulation Study of LM Descent Engine Propulsion Backup to the SM Service Propulsion System,” June 16, 1965.

8. MSC memorandum, 66-FM11-30, “DPS backup of SPS in the earth's sphere of influence,” October 19, 1966.

9. MSC memorandum, “EG27-128-66-1178, “Manual mode DPS for SPS backup,” November 10, 1966.

10. MSC Internal Note, 66-EG-48, “Limit cycle bounds for docked con- figuration,” November 22, 1966.

11. MSC memorandum, EA2 to EG2, “Non-free return trajectories,” October 5, 1967.

12. MSC memorandum, EG23-68-41, “Proposed extension of the LM DAP manual modes evaluation at GAEC,” February 19, 1968.

The question of using LM propulsion in a LM/CSM configuration has recently been the subject of detailed review. Control system studies, both inhouse and at GAEC, have contributed to that review. Because of the current interest in the LM propelled docked configuration and its guidance and control systems, a review of the studies performed in sup- port of the system development effort has been made. The present memo- randum is intended to summarize the results of those studies.

Use of the LM ascent and/or descent propulsion system to propel the LM/CSM configuration (as a backup to the SPS) was proposed by NR in late 1963 (reference 1). Shortly thereafter, a static feasibility study of such backup was made at MSC (reference 2). A dynamic sta- bility analysis of the proposed backup control system was completed at GAEC in the Spring of 1964 (reference 3), and extended to include guid- ance loop dynamics two months later (reference 4). With the completion of single-axis static and dynamic guidance and control studies of the LM propelled configuration, a joint MSC/���/NAA/GAEC commission (the Apollo Mission Planning Task Force, AMPTF), recommended adoption of a LM DPS backup to the SPS (reference 5). Adoption of a control mode which would employ the LM APS to propel the LM/CSM docked configuration was not recommended because of the requirement to contain large APS/CG moment unbalances with the RCS.

The GAEC study described in reference 3 showed that adequate closed loop attitude control of the docked configuration during a DPS firing required a large gain increase (over the undocked LM configuration) in the pitch and yaw rate feedback channels of the LM analog attitude con- trol system. This requirement was corroborated by a three-axis rota- tional and translational guidance and control evaluation of the docked configuration completed by the Guidance and Control Division early in 1965 (references 6 and 7). The effects of body bending and propellant sloshing were included in the evaluation, and the feasibility of both manual and automatic modes of control was demonstrated. The ASPO, in order to avoid the �������� change ������ in the LM analog autopilot to accommodate both CSM on and CSM off configurations, decided to imple- ment control of the DPS-propelled docked configuration in the LM digi- tal autopilot only.

Attention was again called to the subject of LM DPS backup to the SPS in late 1966. The Mission Planning and Analysis Division pointed out that the LGC ������������ requirements could be relieved by omitting a return-to-earth program in the LGC. Mission Planning and Analysis Division requested ����������������� the feasibility of a LM propelled fixed attitude burn for transearth injection of the docked configu- ration (reference 8). The Guidance and Control Division agreed that the proposed fixed attitude burn ����������������� (reference 9) and determined the ������������������ of the required attitude hold under the ��������� of LM/CSM bending (reference 10). These considerations strengthened the position of the Guidance Software Control Panel, which held that the LGC Should not include a return-to-earth program, but should rely upon the CMC return-to-earth program and the MSFN.

In the fall of 1967, the question of LM propulsion of the LM/CSM docked configuration was raised in connection with non-free return trajectories (reference 11). The subject was reviewed by the Guidance and Control Division and the control status of the configuration presented to the Directorate. Most recently, the Astronauts expressed interest in the one time rejected APS-propelled LM/CSM. Control of this configuration (rotated at the docked interface to minimize APS/CG moment unbalance) was investigated at GAEC (reference 12). Inasmuch as the GAEC investi- gation did not include the dynamic effects of structural bending and propellant sloshing, it could not be considered conclusive. Nevertheless, there is indication that control of the described configuration may be feasible. Whether it is feasible to rotate the docking interface in the manner assumed is also a matter that needs verification.

According to present mission planning, there is only one control mode specifically intended for LM backup propulsion of the docked LM/CSM con- figuration. That mode employs LM DPS propulsion, DAP attitude control in “auto” mode, external delta V targeting, and cross-produce steering. The Guidance and Control Division does not plan to conduct further studies in the depth needed to verify the design of docked APS mode unless a mission requirement is established.