BEING “SQUARE WITH THE WORLD” CAN BE COSTLY


The North American Aviation (NAA) machine shop in Downey, in which I worked when I first hired in as a Research Machinist, was located a long walk inside the old, WW II era aircraft manufacturing type buildings. Adding to the walk time was the requirement that I had to pass though three check points, each of which marked the outer boundary of areas of increasing security levels. Until I received my security clearance I was escorted in and out of the plant. I got my green “football” shaped sticker on my badge quickly because I was too young to have a lengthy record. The shop was located within a group of rooms which were clearly not designed to be used as a machine shop. The shop was doing work in support of some of the most sensitive projects of the Defense Dept. so we were at the “cutting edge” of new technology. But you would never have guessed this based on seeing the old style machines in the shop. My first assignment was to make several small shafts using an old 9″ ,,flat belt driven, South Bend lathe. Not exactly the machine I was expecting but it did the job.

In a few months the shop was relocated to a building located on the North side of Imperial Hwy. We were treated to a shop full of new machine tools that were more in keeping with the loftiness of our mission. I was unfamiliar with some of the machines in the new shop. I had no idea why the shop had several “jig boring” mills. I resorted to my custom of asking lots of questions. I learned the purpose of the “jig borers” was to machine the “inertial instrument” mounting surfaces located on the “stable platform” of the “inertial measuring unit” to as close to being orthogonal as possible. Full understanding of this would have to await the answers to more of my questions. As learned more about the capabilities of the machines, I began to understand why these machines had special operators and were not used for ordinary shopwork. I was allowed to look but not touch! I do not know what circumstance prompted the summoning of a technician from Switzerland, the country in which the machines were made, to rescrape the ways of these machines but he did just that. I cannot help but speculate it had something to do with some engineer trying to push the boundaries of what was then possible towards “better”. It took a long time to complete the work but it must have been successful because he never came back. As time went on, I began to understand more fully why the “jig borers” were necessary.

In the NAA inertial navigation world I lived in, the “stable element” referred to the innermost structure of the gimbal set. The gimbal set decoupled vehicle motion from a local level, North pointing, frame of reference. In other words, the vehicle could do a loop maneuver and the stable platform would remain North pointing and level throughout. The “inertial instruments” were the three gyros and three accelerometers necessary for navigation purposes. These instruments were mounted on the stable element such that the control and/or measuring axes of the instruments were mutually orthogonal. The function of a gyro was to provide the means by which a navigation reference axis could be defined. Three gyros and you get three reference axes. If the three gyros are mounted such that their control axes are mutually orthogonal, the control axes can become the basis for the three mutually orthogonal axes assumed in the navigation equation solutions. The function of an accelerometer is to provide a signal proportional to the acceleration of the instrument. If each of the three accelerometers are mounted on the stable element such that the sensitive axis of each accelerometer is coincident with a gyro control axis, then a frame of reference in which vehicle position can be computed from the vehicle’s accelerations is formed. An assumption implicit in the statement that “the computer solves the navigation equations and thus knows where it is” is that the frame of reference used in the computations is orthogonal. The computing capacity of “computers” was very limited in those days and it was not possible to perform the complicated calibration procedures that came later. This fact of life made it necessary that the gimbals and stable element be machined to as close to perfect as possible. It seems reasonable that the best possible machines be used for this work. This is my simple explanation for the presence of the jig boring machines in our shop. 

As the computing power increased over the years, the complexity of the IMU calibration procedures became more complex but this increased complexity enabled a great relaxation in gyro test requirements. By the time of the  N16 IMU only nominal tolerances governed the placement of the G9 gyro on the stable element. Perhaps the greatest level of complexity came into play with the advent of the “strap down” system. The “strap down” system replaced the actual gimbals of the 1950s inertial navigation system with virtual gimbals that existed only as mathematical equations in the computer. The Autonetics N73 was a pioneer strap down system and the strap down mechanization worked flawlessly. I wonder what became of the shop’s jig boring machines?

  

 

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