THE CRISPY CRITTER AFFAIR


For about ten years I was what was called “The Senior Responsible Engineer For The G9 Gyro”. The engineer working in that capacity was responsible for all engineering work that was pertinent to that gyro. His approval was required for literally every thing. It was a position of great responsibility and it required an in depth knowledge of the G9 gyro design. I was never bored in the years I held that position. Prior to becoming the Senior Responsible Engineer, I was the Senior Test engineer for the G9 gyro and had held that position since the first G9 gyros were powered up. Several hundreds of G9 gyros were produced for the N16 Inertial Navigation System (INS), used on the FB-111 strategic bomber, and the N16 Minisins INS used on the USS Los Angeles class submarines. The performance requirements for the two applications of the gyro were fundamentally different. The aircraft INS was required to be ready to navigate eight minutes after power was applied and the length of the mission was less than ten hours. The submarine INS on the other hand was used in a much more permissive environment and the length of the mission was measured in weeks. The gyro was first produced for the aircraft INS and later for the submarine INS. As a consequence of being the first INS produced, all of the hard won experience we had accrued during the production of the G9 was biased toward the short term requirements of the aircraft mission. We began the production of the G9 gyro for the Minisins with little practical knowledge of what problems would arise as a result of the lengthy submarine mission. It turned out we had some interesting challenges that increased running time brought us.

I remember that I was somewhat confused by the phone call I received from someone in the the project office. The caller was trying to describe to me what had happened when a Minisins N16 failed during an acceptance test. I was told of a strong burned smell that was noticed after the INS system was opened up. I remember wondering why they were calling me. I could not recall any gyro failure that had produced smoke. The caller finally told me that he could see what appeared to be heat damage at the module end of the gyro. I agreed to the removal of the gyro and its return so that we could look at it. When we looked at the gyro we saw what could only be described as damage due to overheating. I remember trying to make sense of what I saw and coming up blank. The only possible source of the energy, required to produce that much damage, was the power source for the rotor spin motor. How could a short circuit in the motor circuit have resulted in the damage we plainly were seeing. Someone, during one of our gatherings to discuss the problem, used the term “crispy critter” with reference to the damaged gyro. The name stuck! We now had to solve the mystery of the crispy critter affair.

We were astounded at the severity of the damage we found when we disassembled the gyro. It looked like an explosion had occurred within the electronic module. The question in our minds was: what had exploded and how? The G9 gyro is a smaller version of the G6 gyro and it uses case rotation to improve gyro performance. Sliprings are an essential part of the G9 gyro due to the case rotation.  The gyro is designed such that all of the power and signal circuits, except the torquer coil circuits, pass through the  electronics module. The slipring is part of the electronics module. We were at a loss as to what was the root cause of the explosive damage to the electronic module. We did speculate the causal event was a short circuit in the spin motor circuit as only that power supply seemed robust enough to delivery the burst of energy needed for an explosion. Meanwhile, we had more crispy critter type failures. We were on our way to a serious problem.

All programs fall behind schedule sooner or later. The Minisins program was already behind schedule when we had our first crispy critter failure. Dr. Pickrell, our VP & Gen. Mgr., believed that any large program that fell behind schedule could only benefit from his personal attention. We, all the responsible engineers and their support staff , project office staff, and factory managers, were required to meet with Dr. Pickrell at 0700 hrs every day and report to him verbally about any problems that were delaying the program. I had to make a report on our crispy critter problem and we were flat on our ass. Not good! Dr. Pickrell was a person who did not suffer fools well. As long as he felt you were working the problem and not feeding him a line of B.S., you were treated OK. But, if he ever thought you were feeding him B.S., he could be harsh towards you. I survived the encounter, but he made it plain he was not happy.

We examined the damaged modules carefully. We noticed what appeared to be some kind of metallic debris on the slipring rotors. It looked as if it was wear debris from the sliprings. We were uncertain of this as the slipring was a disaster zone. The slipring wipers in many places had been melted back to their base. We began think along the lines of a short between wipers caused by slipring wear debris. It seemed far fetched, but this was our only theory. We disassembled a module removed from a gyro that had accumulated more than a few hours of running time and inspected the slipring rotor, looking for wear debris. We found what we were looking for. The sliprings were wearing out and the debris was piling up on the lands between the rings. We decided to propose a slipring design change to the slipring vendor. The proposed change would be to add a raised barrier between each ring which would keep the wear debris from building up and possibly shorting to the adjacent ring. The vendor agreed to make the change as soon as possible. This change in slipring design did not not provide a solution to or explanation of our crispy critter problem. We, however, thought we were on the right track towards an explanation and a solution for the problem.

I do not remember who reminded me that the photography dept. had the capability to do highspeed photography. It did not take long for me to arrange to have high a speed movie made of an exploding slipring. But I did not know how to cause a slipring to explode. We gradually made a plan that seemed to make sense. We made a precise mockup of the gyro’s module interface and that had provision  for camera access. All the module circuits were energized and loaded such that the current in each circuit was  the same as during gyro operation. We tried to make the test parameters as close to the operation of a gyro as possible. We setup the test, readied the high speed camera, and did several dry runs. When we felt we had done all we could do to make the test a success, we went for it. We energized the circuits, turned case rotation on, started the camera and immediately introduced a piece of debris harvested from a slipring using a pair of tweezers. The slipring exploded! 

We had to wait for the film to be developed. When the film came back we viewed it at normal projection speed. What we saw on the screen was amazing. One saw the debris being introduced at very slow motion speed and soon as it made contact with the wipers, the wipers melted and the molten metal sprayed onto the adjacent wipers which promptly melted and the damage spread to the whole of the slipring. We had replicated a crispy critter failure under controlled conditions. The fix for the problem was already in work with the the addition of a raised barrier between the rings of the slipring rotor to keep wear debris from shorting over to adjacent rings.

I do not recall how long it took fully implement the new design sliprings but it must have been quick as I do not recall the problem being a discussion item at the 0700 hrs meeting. If it were otherwise I would have the scars left by DR. Pickrell’s bite.

   

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