Powered descent throttle logic correction

See list attachedJUL 17 196868-PA-T-156APA/Chief, Apollo Data Priority CoordinationPowered descent throttle logic correction

On July 2 I sent you a note regarding the way the DPS is throttled up after the gimbal trim phase during the powered descent maneuver. There were a couple of errors in that memo which are too significant to be left uncorrected.

I pointed out that MIT has programmed the LM computer so that the throttle up time was a fixed number of seconds after the targeted time of ignition (TIG). To illustrate how important it is that the engine be throttled up to the FTP at that time, I pointed out that for each second delay in throttling we lose 12 seconds of “hover time.” This was my first error since it is not hover time that is lost but rather “throttle recovery time.” Throttle recovery time is that period which has been allotted in the powered descent maneuver for the guidance system to regulate the thrust such that it can achieve the hi-gate targeting conditions. Failure to provide a sufficient period of throttling will jeopardize meeting those conditions and can result in a fouled up descent.

I went on to say that if the engine failed to start when it was supposed to, the crew could recycle to TIG minus five seconds and the PGNCS would countdown to ignition again with a delay of about 13 seconds from TIG (all true) and that the trim time would be reduced by that amount since the throttle up time was maintained as originally set. George Cherry informs me that this is not true since in the event of a recycle to TIG minus five seconds the throttle up time is redesignated. Accordingly, the recycle capability is really not an acceptable thing to use on the powered descent maneuver. I do not believe that the program has been designed improperly. It is just that the capability, as I described it, does not really exist.

MIT is submitting a PCN describing how the program has actually been coded since it is different than documented in the GSOP.