GETTING CLOSER TO THE PICKEL BARREL


There was a period of time when I was assigned to work with the Group that had design responsibility for the G6 gyro. Ray Noar, the Manger of  the group, was an expert on the subject of the G6 gyro having been the Responsible Engineer for the gyro prior to his becoming a Manager. The G6 gyro was used on the Minuteman I, II,and III missile. I believe it is still in use on the MM III missile. We were to consider ways to improve the G6 gyro’s performance. I have a vague recollection of a larger effort to improve the accuracy of the Minuteman III missiles and we were part of that. We used the Minuteman INS error budget as a guide to rank possible improvements in gyro performance and consequently we were looking at ways to reduce gyro ‘random drift’ and ‘gas bearing model’ errors. The ‘random drift’ of the G6 gyro ensemble was well known because the random drift rate of each gyro produced was measured during the acceptance testing of the gyro, but we realized a reduction of the INS error due to gyro random drift would require a much better understanding of the mechanism of random drift within the gyro beyond a suspicion it was somehow related to gas flow inside the gyro.

The speed of the gas flowing inside the gyro was in excess of 100 mph due to the friction between the gas and the spinning rotor. Ray believed that if we could learn how this gas flow was related to the random drift rate, we could find a way to reduce the random drift rate of the gyro. The big question before him was how to the visualize and measure the turbulent effects of a gas moving ever 100 mph. Ray’s approach was simple; he slowed the process down by substituting a fluid for the gas. This fluid would have a viscosity such that when rotated at a much slower speed than 100 mph, the Reynold’s number would be the same as that of the gyro fill gas. The turbulence of the fluid was made visible by mixing powdered aluminum with the fluid. This was all done inside a clear plexiglass full scale mockup of the gyro that permitted high speed photography as the means to record the fluid flow dynamics. Using this apparatus, Ray was able to “see” the formation and subsequent history of vortices in the fluid. By studying the patterns of turbulence in the fluid, he came to the belief that controlling these vortices was the key to reducing the random drift rate. His idea for vortex control was to place channels adjacent to the locations where the vortices started forming and thus contain them. He was instrumental in the design of the channel inserts that became part of the improved gyro. I believe the addition of these channels reduced the random drift rate of the gyro, but I do not recall by how much. It was amazing to watch a vortex form, detach and dissipate adjacent to the rotor surface. As I recall, he also devised a better way to calculate random drift from the raw test data.

The drift rate of the gyro changed as the acceleration of missile launch loaded the gas bearing of the gyro. A mathematical model, based on an earlier analysis by Stan Cogan, was used to predict the acceleration dependant drift rate changes and inflight real time corrections were made to the computed flight path of the missile being launched. The G6 gyro ensemble averages of the gyro’s sensitivities to acceleration and acceleration squared were the basis for the calculation of the corrections to the flight path. It was thought that if a practical method could be found to additionally measure the individual gyro’s sensitivity to acceleration to the fourth power, and if this and the individual, not the ensemble average, gyro’s measured sensitivities were used, that improvements in missile accuracy would be realized.

The gyro acceleration and acceleration squared sensitivities were already being measured during acceptance testing of each gyro. No comparable tests for higher order gyro sensitivities was available. I was working in the test laboratory developing new G6 gyro test methods that used a “linear vibrator” machine. This was a machine that imparted horizontal motion to the test gyro without rotation input. For these tests the gyro was mounted on the machine with the gyro spin axis in the horizontal plane. The angle between the test motion input axis and the rotor spin axis could be set at a predetermined  fixed angle. Gyro drift rates over a range of acceleration inputs were then measured for each of several fixture angles. These data were then used to calculate the gyro model coefficients. I had barely started to acquire test data when I was offered a position with the Group developing a new strapdown system that was based on the Autonetics Electromagnetically Suspended Gyro (ESG). I quickly accepted the offer and I turned over my G6 gyro work to others to complete. The development of test methods to measure the higher order model coefficients continued and an improved version of the G6 gyro became part of the Minuteman III INS. I was in effect starting a new career as a test engineer for strapdown systems.

 

 

 

 

 

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