"C" Mission Retrofire and Reentry Mission Techniques Meeting

On Friday morning, July 19, we had a "C" Mission Retrofire and Reentry Mission Techniques meeting to clean up some open items. It is evident that a distribution of correction pages to our previously distributed Mission Techniques document will be inadequate and it is our current plan to republish the whole book. Some of the most significant items resolved at this meeting are described in this memo.

  1. It has been established that the G&N guidance system will be used in the event of a hybrid RCS deorbit. (A hybrid deorbit is one in which both the command module and service module RCS jets are used.) The retrofire will be targeted for a half-lift reentry.

  2. It has been established that the G&N is mandatory for performing a hybrid deorbit; thus, if the G&N has failed and the service module RCS remaining has fallen below the return-line limits, the only remaining system for retrofire is SPS using SCS control. Accordingly, there is a mission rule that retrofire will be performed to land in the next best planned recovery area (PLA).

  3. It has been established that if insufficient time is available for a fine alignment prior to retrofire, the G&N will be used with a coarse alignment if that can be done. Current estimate is that a coarse alignment will be to within 2° on all axis, which can result in as much as a 30 mile landing point miss.

  4. In the absence of response to our request for better numbers, we have established the following limits beyond which the G&N will be declared No Go for reentry and the backup system will be used. The DSKY VG displays must be within 1 fps and the gimbal angles must be within . Guidance and Control Division and MIT people please pay particular note.

  5. Apparently the procedure has been established that command module separation from the service module will be performed following retrofire while still in the SPS thrust program (P40). This is to keep Average G on during the separation maneuver without having to wait one minute as the entry programs are currently coded. The entry programs (P61, P62, etc.) will be sequenced after separation. Thus, these programs are being used in a completely different way than they were designed.

  6. IMU PIPA and gyro drift compensation values are monitored continuously by MCC-H. It has been established that if the values currently loaded in the G&N are in error by more than .003 ft/sec² and .075°/hr, they will be updated in the CMC.

Terms & Abbreviations


Command Module Computer.


The "Display and Keyboard" interface through which the astronauts controlled their guidance computers.


Guidance and Navigation.


Intertial Measurement Unit


Mission Control Center. Popularly known as “Houston” (as in “Houston, we have a problem”)


Massachussets Institute of Technology. In these memos, MIT is shorthand for the MIT Instrumentation Laboratory, created and led by avionics pioneer Charles Stark Draper. It is now known as the Charles Stark Draper Laboratory and became independent of MIT in 1973.


Reaction Control System.


Service Propulsion System, the large engine of the Service Module that was used to enter and exit lunar orbit, as well as make course corrections while going to and from the moon.