Crew monitoring of the LOI maneuver

See list belowAUG 14 196767-FM-T-60FM/Deputy ChiefCrew monitoring of the LOI maneuver

1. On August 3 we had an informal meeting to talk about crew moni- toring of the Lunar Orbit Insertion (LOI) maneuver. The subject came up in connection with Jim McDivitt's preparation for the STAC presentation. I'm writing this note because we tentatively agreed on some fairly basic points with regard to how we might use the various systems. These preliminary conclusions, if they hold up, could have application on some of the other maneuvers, not just LOI.

2. I am sure you are all aware of the slow response of the thrust vector control digital autopilot (DAP) in the Command Module when docked with the LM. In order to avoid exciting the low structural frequency of this configuration (about 1 cps), it has been necessary to reduce the response of the DAP to a very large degree. As a re- sult, if there is an offset in the alignment of the initial thrust vector from the spacecraft e.g., turning moments will exist at the beginning of the maneuver causing large spacecraft attitude excur- sions which take a couple of long period oscillations to damp out. Our current estimate of the maximum excursion for LOI is about 8° based on the assumption of fully loaded propellant tanks and ini- tial thrust misalignment of 1°. The period of oscillation, as I recall, is in the order of 20 seconds for the half cycle in which the greatest excursion occurs and, unless the crew were prepared for it, it could create considerable concern on whether or not the guidance system was working properly. In the case of the LOI maneuver, which has a nominal duration of about 370 seconds, it is probable that the transverse velocity increments accumulated during this period should not jeopardize the crew. If this is true, the consensus is that the crew would be willing to passively ride out this perturbation.

3. Crew monitoring of the rest of the maneuver must be provided for two characteristics: duration of the burn itself and attitude error. With regard to the former, it was readily apparent that the only danger to the crew occurs from an overburn, that is, failure of the engine to shut down in time. There are three devices which can be used to monitor and cross check against overburn: the PNGS, the Δ V counter on the EMS based on acceleration measured along the longitude spacecraft axis, and the clock which can be used to com- pare against the anticipated duration of the nominal burn. An over- burn of about 110 fps would result in lunar impact. This is equiv- alent to about 10 seconds of extra burn duration out of a total 370 second maneuver. (Acceleration level at burnout is approximately 1/3 g.) A 3 σ low performance engine would extend the burn time just about 10 seconds which makes monitoring with the clock somewhat marginal. The EMS longitude accelerometer is said to have an accu- racy of approximately 1.3 percent which is equivalent to about 40 fps for the LOI maneuver. It should provide a suitable cross check. In addition, lunar impact resulting from overburn, of course, occurs as much as 180° from LOI, thus, MSFN should have a good capability of predicting this event as soon as the spacecraft appears from behind the moon with sufficient time for the crew to respond following advice from the ground.

4. Monitoring attitude error is somewhat more difficult. It appears that a constant pitchdown error of less than 5° throughout the maneu- ver would result in a radial, Δ V downward causing lunar impact ap- proximately 90° orbital travel following LOI, that is, at approximate- ly first appearance of the spacecraft from behind the moon. It was proposed that the FDAI's be set up with one driven by the PNGS and the other by the SCS for attitude comparison purposes once the initial attitude transients noted above have ceased. In addition, it is nec- essary that the attitude time history compare favorably with a nominal determined preflight. The comparison against the preflight nominal is to protect against a degraded Z-axis accelerometer which could cause the guidance to deviate dangerously but would not be apparent from a com- parison of the two FDAI's with each other. Differences in the FDAI's, of course, would indicate that one of the two systems was in error. Since there is no capability for vote breaking with a third source, there would be little option but to shut down when either of the two systems indicate a dangerous condition is impending. It should be noted, though, that attitude dispersions in only one direction, namely in the direction causing a radial velocity increment downward, creates a crew safety problem. In all other cases, it would not be necessary to shut down the engine. Critical downward incremental velocity is approximately 440 fps.

5. I guess to sum it up, even without ground monitoring and without very much onboard redundancy, it looks like given some ingenuity ways can be found to assure crew safety. However, they may require a willingness to have “blind” faith for a considerable time in a system that might be malfunctioning and may require an action that could prevent mission success, that is, premature manual shutdown of a perfectly performing system. Probably most of this is old stuff, but I thought it might be worthwhile to write it down.