Tindallgrams

We ran into a little snag on confirming the CSM DOI maneuver which has forced us to change the mission technique a little bit and I think you should know about it.

The CSM DOI burn brings perigee to about 8-miles altitude and it only takes an overspeed of 10 fps to cause an impact. Accordingly, we must have absolute confidence that such an overspeed has not occurred. On the other hand, we strongly desire to give the G&N every chance to do its job since it almost certainly will do it right. For this reason we have retained the simple crew technique for protecting against a malfunctioning G&N by manually shutting down the engine if the predicted burn time is exceeded by 1 second, and we are not including the EMS in the logic. If at the conclusion of the maneuver the EMS confirms that the G&N did right, we should have confidence that everything is okay since that has got to be more than just coincidence. Our only problem occurs if both the G&N and EMS appear to be operating properly, but the EMS indicates an overspeed. Then something must be done to determine which of the two systems is correct. If the G&N proves to be correct, we should press on with the mission. If the EMS is right, an emergency maneuver must be executed within ½ hour to get out of there and, since the G&N must be broken, the landing will probably have to be abandoned. Originally we intended to solve this dilemma in the unlikely event it occurs by having the crew note the time of earth rise. It was originally felt that this observation would provide the crew an absolutely dependable, simple onboard technique for making this critical decision. We have since found that that is not so dependable and have chosen to use an alternate procedure. Namely, we have been unable to find dependable onboard techniques and have decided to depend on the MSFN tracking and MCC processing to determine which of the sources is correct if the G&N and EMS disagree with each other. This can be done dependably to inform the crew in time for them to execute the bail-out maneuver. This procedure has been agreed to over the phone by key flight controllers and the prime Apollo 13 crew, and it will be used during the simulations starting this week. Work on earth-rise procedures is being terminated.

Terms & Abbreviations

CSM

Command-Service Module.

DOI

Descent Orbit Insertion. A LM maneuver performed to reduce the altitude of its orbit in preparation for landing.

EMS

Entry Monitoring System. A CM system used during re-entry into the Earth's atmosphere.

G&N

Guidance and Navigation.

MCC

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

MSFN

Manned Space Flight Network (pronounced "misfin").

Perigee

The point in an Earth orbit where the orbiting object is closest to the Earth's center.