I AM A SILVER EAGLE

Silver Eagle was the name given to those who signed up for a lifetime subscription to the “North American Aviation Retirees Bulletin”. I am a “silver eagle” and have been for years.

On to other matters. Tony, you asked me some questions about Beryllium (Be) and what I knew about machining it. Although I am a machinist by trade, I did not work as such in the NAA shop after I became a Laboratory Technician in the late 50s. I never had the experience of machining Beryllium. I maintained my contacts with the shop people and after I became a member of the technical staff in 1961, these contacts became very helpful to me as my responsibilities expanded. (The leadman in my first job at NAA became the shop Manager.) Over the years my work took me into the shops from time to time. It was advantageous to me that I spoke as a fellow machinist. It is from my memories of these times that I draw upon as I discuss Be metal and its importance to Autonetics.

Beryllium is toxic to humans when it is in the form of small particles or as a gas. It is gets into the body through the skin by abrasion or into the lungs by breathing vapors produced when hot working the metal. Such exposure results in Berylliosis, a serious ulcerating disease of the skin. The machining of Be was done in a NAA shop built to safely machine it. History validates the shop design as there has been only one documented case of Berylliosis to my knowledge and this case was reason enough to rebuild the shop to a higher standard. The normal handling of Be metal when it is in the solid form is safe. I have personally handled solid Be metal parts many times since the first use of Be metal at NAA in 1960 without any ill effects whatsoever.

There are compelling reasons why Be metal was chosen as the metal used for the improved G6 gyro. When compared to Aluminium, its dimensional stability is better, its density is less, has higher heat capacity, lower thermal coefficient of expansion, lower corrosion potential, the resistance to distortion under load is greater, and the thermal coefficient of expansion is virtually the same as nickel metal. However, the use of Be in the improved G6 was not without some risk. Material costs were  greater, it is toxic under certain conditions, it cannot be cast, welded, or forged, and it requires costly machine shop infrastructure.

It was decided that the stationary part of  gas bearings would be hard chrome plated and the rotating part would be electroless nickel plated. It is better to use Be metal as the spherical bearing rotor half material because the thermal coefficient of Ni metal is the same as that of Be metal. This would ensure that rotor distortion due to any bi-metal type thermal effects would be small. Also, any gyro part, not required to be Be metal, could be made of Ni metal with no bi-metal effects. ( The G9 gyro, a 3/5 scale G6 gyro with case rotation, used a Ni metal inertia ring cemented to each Be metal rotor half to reduce the gyro bias rate by increasing the rotor moment of inertia.)

The Autonetics spherical gas design rotors distort significantly when subjected to stress induced by the spinning of the rotor at normal speed. All things being equal except  for the rotor material, a rotor made of Be metal will distort less than Aluminum. Less distortion at normal rotor speed means the actual geometry of the gas bearing cavity is closer to the theoretical shape assumed in bearing calculations. Less spinning induced rotor distortion means improved stability of the IMU azimuth drift rate due to reduced spin induced creep in the rotor mass unbalance along the spin axis of the gyro.

Pressure to improve IMU performance increases relentlessly as the visionaries, that inhabit the think tank world, dream up new and better ways to make war. Reaction to this pressure to improve performance usually results in new technology being developed and the new technology becomes the new “whatever” of the time. At Autonetics the ball bearing was replaced by the gas bearing and it was replaced by the ElectroStatic Gyro (ESG). The Mass Unbalance Modulation (MUM) pickoff signal invention was the key to the success of the ESG. The ESG design is wholly dependant on the use of Be metal. The ESG rotor design makes virtuous use of the otherwise deleterious properties of Be metal. I do not believe the ESG would have been possible if Be metal had not been available. Be metal is furnished as a sintered billet made from Be powder. The sintering process uses high heat and pressure applied to the Be metal powder contained in a mold to create a billet of solid metal. The mechanical properties of the metal are not uniform throughout the billet. The values of these properties are dependant on the location and orientation of the measured billet sample within the billet. The nonuniform mechanical properties characteristic of the sintered metal billet are altered for the better by a hot extrusion process. This extrusion process yields metal for which the mechanical properties are readily identified as being either the radial or longitudinal value. This extrusion process also leaves the metal with a precisely located higher density metal inclusion that is necessary for the MUM signal mass unbalance of the rotor. ESG rotors are subsequently lapped round at a temperature such that when the rotor is spinning at normal speed and temperature, the spin induced rotor distortion is compensated by a rotor distortion induced by operating the rotor at a temperature higher than the lapping temperature and on the difference in the values of the thermal coefficient of expansion in the radial and longitudinal directions.

The selection of Be metal, the metal from which the G6B gyro was made, had ramifications far beyond those considered in the original choice. It was a fortuitous choice indeed!