The LM4 RR/LGC interface may be broken, but that’s okay – sorta

See list attachedMay 8, 196969-PA-T-74APA/Chief, Apollo Data Priority CoordinationThe LM4 RR/LGC interface may be broken, but that’s okay – sorta

This memo is to document the Data Priority position regarding a recent LM4 systems problem. To wit, it is considered acceptable to proceed with the nominal F mission with a questionable or known interface failure between the rendezvous radar (RR) and the LM spacecraft com- puter (LGC). It should be emphasized that a properly operating ren- dezvous radar with crew readout is still considered mandatory for DOI. Also, this recommendation does not necessarily apply to the G mission.

Justification for this position is based on the unique character of the F mission and on the availability of three adequate alternate data sources. The F mission rendezvous starts with precisely controlled, known initial conditions since one spacecraft separates from the other in orbit; furthermore, consumables – particularly, propellant – are abundant. The alternate data sources which can be used for rendezvous navigation and maneuver targeting in the event of an RR/LGC interface failure are:

a. The crew backup charts using raw RR data as displayed on the tape meter and/or DSKY

Error analysis by FCSD has proven the crew backup chart solution to the rendezvous problem is competitive with the PGNCS. These charts are utilized in the nominal crew procedures. The tape meter is the primary source of input data, however, it is also possible to obtain raw RR data by use of the RR Self Test routine (R04) with the RR test switch set to the “Off” position. Incidentally, the crew already uses this routine periodically to check and calibrate the tape meter. It should be noted, however, that R04 cannot be used simultaneously with the rendezvous navigation program (P20) nor if the RR/LGC interface is totally broken rather than intermittently malfunctioning.

b. The CSM using sextant and/or VHF ranging data

This solution is also routinely available and competitive with the PGNCS. It should be noted, however, that the VHF ranging system has never been flight tested and there is certainly no great confidence in the high intensity tracking light on the LM. It failed on D! However, either of these data sources is adequate for successful operation of the system.

c. The MSFN solution based on pre-separation tracking and PGNCS navigation through LM maneuvers

This solution is also comparable in accuracy to the PGNCS and, in fact, is the real foundation upon which we are able to base our case for this recommendation. It assumes, of course, that the PGNCS is operating nominally – controlling and navigating through the maneuvers. It should be noted that if it is known the interface has failed and PGNCS rendezvous radar navigation cannot be carried out, it is possible for the MCC to update state vectors to the LGC enabling it to obtain its own targeting more-or-less equivalent to the MCC. Procedures for doing this are well known to the flight controllers.

d. It is important to emphasize that AGS rendezvous navigation and maneuver targeting should not be utilized on the F mission due to computer program limitations which result in unacceptable errors. The AGS can be used for maneuver execution, of course.

If an RR/LGC interface failure occurs but is not detected by some other means, it is quite possible that the LGC LM state vectors could be damaged by acceptance of bum RR data – that is, crew editing is not infallible by a long shot. However, special rendezvous solution com- parison and AGS state vector update procedures are not required since current mission techniques were developed especially to prevent execution of wrong maneuvers. Failures of this type are the reason for the very existence of Mission Techniques!! The specific situation under discussion here is not unique except that preflight concern makes everyone alert for this specific problem. (I am assuming that the crew will be adequately briefed, although, I am not sure when and by whom at this time.)

This paragraph is to present the other side of the coin. Our only real concern is the added vulnerability to failures of other systems which can force switching the mission to a rendezvous abort sequence (such as an APS failure at the insertion maneuver). Crew backup charts are not available for these high ellipse cases (except for a CDH chart for the PDI abort situation). Multiple failure cases leave us dependent upon the CSM solution, item”b” above, plus the PGNCS solution noted in item “c” above, which should be adequate for a safe return without RR data, although probably dispersed and perhaps costly.

This recommendation has been coordinated with authoritative representatives of FCD, FCSD, and MPAD, who all agree with it. No crew input has been obtained, however, I would be amazed if they do not also agree. Assuming Stafford's vote, I assume this matter is settled. The mission rules do not specifically address this interface problem and require no change unless it is desirable to add this.