THE FINE LINE BETWEEN SUCCESS AND “CRASH AND BURN”


I was a laboratory technician, and later a test engineer, working in the Instrument Test Laboratory. I worked the swing shift as I was spending my daylight hours studying Physics. Working the night shift in the test laboratory meant I was able to get my homework in on time. Nearly everyone on the night shift was in school in the daytime studying something.

Conducting tests on gyroscopes and accelerometers was often boring work. Most of the time the work was repetitive and slow paced  – but not always. Occasionally, we were asked to perform a test which was actually exciting and interesting to perform. Our definition of “exciting and interesting” included an element of “crash and burn”.

Our laboratory was part of the engineering group that was tasked to develop new and novel inertial instruments. It was the responsibility of our laboratory to conduct the tests that were thought an essential part of the development of new instruments. The art of designing inertial instruments was relatively  new in those days; we had a lot to learn – more than we ever imagined. A lot of what we learned was from the “cut and try” approach” to testing which sometimes involved a lot of “crash and burn” during the learning process.

The success of the Autonetics free rotor gyro was entirely dependent on the success of the spherical gas bearing. To a large part, it was the free rotor gyro, the G6b gyro in particular, that got Autonetics the contract to develop the Minuteman missile IMU. Questions about the performance of the spherical gas bearing had not been satisfactorily answered apparently and our laboratory was tasked to conduct tests concerning the radial load capacity of  the G6B spherical gas bearing.

Understanding how the the radial load bearing capacity was determined in our laboratory requires a basic understanding of the G6B gyro layout and how it all works. The story goes like this. The two rotor halves are joined at the equatorial plane of each rotor half with the the gas bearing ball/shaft assembly inside the hemispherical cavities of the gas bearing. The ball/shaft assembly is fixed to the gyro case. The rotor is supported by the gas bearing. The gas bearing constrains the rotor against linear radial and axial movements except for the small deflections resulting from radial and axial loading of the gas bearing. The rotor can move angularly to such an extent as is allowed by the rotor as it contacts the stop bearing. The stop bearing rotates only when in contact with the rotor. The stop bearing is fixed to the shaft of the gas bearing ball/shaft assembly. The stop bearing is located within the axial hole in the rotor half. The extent of the maximum angular movement of the rotor is set by the geometrics of the stop bearing placement. The gyro rotor is “free” to move angularly, pivoting on the gas bearing. Two pickoffs are located in the gyro which provide signals proportional to the angles between the rotor and the case of the gyro. The pickoff excitation voltage is supplied via the ball/shaft assembly. The ball/shaft assembly is electrically insulated from the gyro case. The gyro can be operated with external servo mechanisms that supplies current to the gyro torquers to precess the spinning rotor to keep the pickoff signals at their null values. The gyro is fixed to the Earth for this mode of operation. In another mode of operation, the gyro is fixed to a gimbal set that is free to move about axes fixed to the Earth. Servo mechanisms causes the gimbals to rotate such that the gyro pickoffs are held at the null values.  The gyro pickoff signals are kept at the null values in either mode of operation.

It is the basic rule of gas bearings: Do not cause the spinning part of the bearing to touch the stationary part. Ignoring this rule provides good examples of “crash and burn”.

Begin the test setup by mounting the gyro on a rate table such that the spin axis of the gyro is perpendicular to the rate table axis. Next spin the rotor up to operating speed. Activate the servo mechanisms to apply torques the rotor to keep the gyro pickoff signals at their null values. Now cause the rate table to rotate. It will be quickly discovered that the servo mechanisms are not capable of applying sufficient torque to the rotor to keep the pickoff signals at their null value once a small threshold value of rate table input is reached. The rotor will make contact with the stop bearing and the rotor then moves with the case; it is precessing at a rate equal to the rate of the rate table. How is the torque necessary to precess the rotor at this rate generated?

The rotor will remain fixed in inertial space. When the stop bearing makes contact, it exerts a force on the rotor at the point of contact. The line of action of this force does not intersect the bearing center of support of the rotor. The magnitude of the contact force multiplied by the normal distance  from the point of contact to the center of support of the gas bearing is equal to the magnitude of the torque that causes the rotor to precesss at the rate table rate. The gas bearing is thus subjected to a radial loading force equal to the contact force acting on the stop bearing. The gas bearing deflects under the action of this loading force until bearing equilibrium is established or, if the load is greater than the bearing can withstand, the rotor “crashes and burns”.  

The goal of this test is to determine the radial load necessary to cause the gas bearing to fail. Failure was when the moving bearing surface of the rotor contacted the stationary surface of the ball. What was needed was a circuit that provided an output signal that was proportional to the gas bearing gap. The circuit, by which the pickoff excitation is supplied to the gyro, is characterized by the capacitances of the gas bearing gap and the pickoff gap. A circuit was designed that exploited the change in the capacitance of the bearing gap when the gap changed as the bearing  was loaded. The circuit had as its output a voltage proportional to the bearing deflection under load. This use of this circuit made it possible to load the gas bearing near the point of failure without actual failure. “Calibration” of the “crash and burn” voltage was done with the rotor at rest. This circuit was also used during vibration testing of the gas bearing. It was close to an initiation rite for technicians and find out who had the nerve to get close to the “crash and burn” limit without actually destroying a rotor.

My experience with “riding the stop bearing” was very useful to me when I went to Concord, Massachusetts to supervise G6B tests using the very large MIT owned centrifuge located there. The engineers at the Draper Lab were vocal in their belief the G6B gas bearing would not survive the firing of the second stage of the Minuteman missile. To mute this chorus, we conducted tests of the gyro that used this massive machine. Due to failure of some of the centrifuge’s wiring, I was asked to authorize gyro test modifications that required us to “ride the stop bearing” for an extended period of time. Based on my “crash and burn” experience, I knew we would have no problems in doing so. We completed the test without incident. We never had problems associated with the gas bearing during missile launch.

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