Navigation: Bottled Star

Deep in a dial-studded cabinet on the Navy’s test ship Compass Island lies a hollow sphere of beryllium no bigger than a baseball. It has no visible means of support, yet it spins at 30,000 r.p.m. Awed naval technicians call it a “star in a bottle,” and they count on that man-made star to tell nuclear submarines exactly where they are, even after months of cruising in black ocean deeps.

Developed by Minneapolis-Honeywell on theories worked out at the University of Illinois, the bottled star is officially named ESG (Electrically Suspended Gyro). Like all the gyroscopic equipment that guides modern missiles, ships, aircraft and spacecraft, ESG’s performance depends on the fact that a rapidly spinning rotor tends to maintain an unchanging attitude in space; it sticks to its stance regardless of the movement of the vessel on which it is mounted. Gyros that can do this job accurately for short periods are not too hard to build. But when a gyro is used steadily for days or weeks at a time, it tends to drift from its proper direction, usually because of friction in its bearings and other supporting parts. Even though that friction can be reduced almost to the vanishing point, the least trace of a rub can make the gyro drift.

Electrical Support. For all practical purposes Honeywell’s ESG has no friction at all. The beryllium sphere that is its rotor is enclosed in a ceramic case lined with copper electrodes that do not quite touch the sphere’s surface. The electrodes carry powerful electric charges so that each of them tugs at the sphere. Whenever the tug gets uneven, a quick and intricate electronic circuit adjusts the charges so that the beryllium ball remains precisely in the center of the cavity, supported by nothing but electrical force.

Before the suspended sphere can work as a gyroscope, almost every trace of air must be pumped out of the cavity. Then a set of coils creates a rotating magnetic field that spins the sphere like an electric motor. When the rotor reaches a speed of 30,000 r.p.m., the power is shut off and the sphere spins on for weeks or months without appreciable slowing.

Lemon Shape. The sphere itself must be machined with incredible accuracy: no more than five millionths of an inch of error can be tolerated. When it is not spinning, the sphere is not exactly spherical; it is slightly prolate (lemon-shaped), with its equatorial radius (where the metal is thickest) .000226 in. shorter than the polar radius. When it is spinning at 30,000 r.p.m., though, centrifugal force makes the equator bulge just enough to form a perfect sphere.

Since no mechanical probes can touch the bottled star without disturbing it, parts of its polished surface are treated chemically to make them reflect less light. These parts are observed by photoelectric instruments that report the exact direction in which the axis is pointing.

The accuracy of ESG is a closely guarded Navy secret, but Honeywell confidently claims that it outclasses all other gyros. It will first be put to use on nuclear submarines, where it will serve as a monitor to check the guidance apparatus that is already installed for the sub and its Polaris missiles. With a bottled star on board, subs need never come near the dangerous surface to check their positions by celestial sights.