Pulse Torquing to Achieve IMU Realignments

See list attachedJUL 30 196868-PA-T-173APA/Chief, Apollo Data Priority CoordinationPulse Torquing to Achieve IMU Realignments

This memo is to describe the gyro pulse torque realign capability being added to the IMU Realign Program in Luminary and Colossus, Jr. Most of it is quoted word for word from a memo Steve Copps (MIT) wrote last February proposing it.

“The purpose of the program is to provide the capability of moving the stable member from one orientation to another without losing inertial reference. The actual program change is an addition to the IMU Realign Program (P52). Presently a display comes on showing V06N22 and the gimbal angles which will be achieved by coarse aligning the gimbals. This display is being changed to provide the navigator the option of achieving the new orientation by coarse aligning or by pulse torquing ('enter' achieves one and 'proceed' the other).

“Obviously the most accurate method of realigning the IMU is to use star sightings, and if star sightings will be taken there is probably not much advantage to pulse torquing. However, if there is some doubt as to one's ability to acquire and mark on stars, or the inertial reference accuracy required in the next orientation is less than the error induced by pulse torquing, then this option has great value.

“The time to pulse torque to a new orientation is a consideration. The maximum time to coarse align is 15 seconds. The time to pulse torque is much longer. Since only one gyro is torqued at a time, the total changes in angle for each axis is summed together and that total angle is multiplied by 2 (torquing rate is approximately 1/2 degree per second) to obtain an estimate of realignment time.

“The induced error is directly proportional to the sum of the angles that each gyro is pulse torqued through. An estimate of the error induced is obtained by multiplying the sum total of change in angle by .002.

“So a single 90° yaw reorientation would take three minutes and would induce an error of .180 degrees. The time to pulse torque is alleviated by the fact that no star sightings are required following the alignment.

“It should be noted that during pulse torquing there is no need to hold the spacecraft in a fixed orientation since the IMU is always inertial. However, there is a possibility of pulse torquing the middle gimbal into gimbal lock. It was decided to do nothing about this problem and leave it to the astronaut to monitor the FDAI or N20 and maneuver if required.”

The significant point to be made is that the change is being mechanized as an option in P52 – the IMU Realignment Program – and so the controls for achieving the new alignment are the same as exist for that program. That is, there is no direct way for the crew to tell the system to move 90°. Of course, he can probably fake it out by targeting an External ΔV maneuver he has no intention of making – say out-of-plane to get a preferred REFSMMAT and then go into P52 to realign the IMU to an out-of-plane orientation. This last paragraph is my comment. Don't call Steve if its nutty – or me either for that matter.