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The Case of the Gallivanting Gyros

The Case of the Gallivanting Gyros

Delivering a missile to a target thousands or more miles away requires some sort of guidance. So-called inertial guidance was pioneered by Anschutz-Kaempfe in 1908 and further developed by Sperry, Schuler, and Draper. The guidance system is based on accelerometers and gyroscopes; the former measure changes in velocity and the latter detect attitude. The information gained from these devices is converted to position and velocity through Newton's laws of motion. Knowledge of position and velocity are required in planning course changes needed to get the missile to its intended target.

The gyroscopes and accelerometers are maintained in precise orientations with respect to one another by a so-called "stable member." In this case the stable member was a lightweight magnesium-alloy casting that resembled the metal web for the exterior of a basketball-sized geodesic dome. The gyroscopes and accelerometers were affixed into the various openings in the member.

This guidance system in this case was for submarine-launched, long-range nuclear-tipped missiles. The missiles had been in place for some years when the U.S. developed a new generation and prepared to pass the old missiles off to a NATO partner. Before the transfer someone noted that the stable member wasn't stable; it was deforming while in the missile. Consternation ensued!

I got this case when the cold war was raging. When telling the story I would (jokingly) illustrate the consequences of the instability by remarking that a missile fired at Pinsk might come down on Paris. What with the recent shift in geopolitics, it is not entirely clear which of the two is now considered the most hostile to U.S. interests.

Slow, Continued Precipitation the Suspect

The casting alloy contained 9 percent of aluminum that was supposed to precipitate in the solid as small particles and give strengthening. The alloy was aged at about 200C to accelerate atomic diffusion in the crystal lattice.

Solubilities mostly decrease with decreasing temperature, e.g., salt in water, so not all of the Al was removed from solid solution at the aging temperature. It was initially thought that slow, continued precipitation at the missile tube temperature (some 40C above room temperature) was occurring. Precipitates have a different volume than the base material so that the precipitation would have caused distortion.

Someone involved with the guidance system found that I was doing research on the effects of radiation on microstructures of alloys and figured that irradiation might stop the instability by pinning the atoms in place. I was then hired as a consultant. As it turned out, irradiation would have produced a highly radioactive but still unstable member.

The continued precipitation scenario was in fact inconsistent with the distortion not starting until the stable member had been in service for several years. Continued precipitation should have started right away and slowed with time. I picked every brain I could find, both inside and outside MIT, for suggestions as to the cause of the instability but came up with very little.

High Temperature the Culprit

Finally, my client and I did elevated temperature-aging tests on samples of both new and used stable members. We conducted one series of tests in wet air to better match humid submarine conditions but got no distortion. Then we realized that air saturated with water at room temperature (as we had done) was not saturated at higher test temperatures.

We did the saturation at the higher temperatures and the members started to deform. The instability caused water vapor getting into pores in the castings and giving accelerated oxidation of the magnesium. The crumbly magnesium oxide (or hydroxide) filled the pores in the casting and continued oxidation caused the distortion.

Some porosity is inevitable in castings. The porosity in the present case was high enough that the water vapor could percolate through from the surface to the inside of the casting. Going back and producing new, less porous castings was out of the question.

My client ended the project at this point. I submitted my final report and bill, was paid, and that was the end of it for me. I still wonder if the missiles were transferred to our ally, and if so, whether or not the ally was advised to put dehumidifiers in the missile storage areas of the submarines.

Recently, I ran into the now-retired former president of my client company. He remembered the case. The company replaced offending magnesium castings with a machined beryllium structure. Beryllium is certainly strong enough and light enough to do the job, but is hellishly toxic. The company took huge pains to comply with environmental regulations regarding beryllium processing, but the expensive facility now sits idle for lack of demand for its capabilities.

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