On The Drawing board     

By this time, 1937, a new type of gyroscope had been developed by Orland E. Esval, one of Sperry's foremost electrical engineers. Since the gyroscopic effect is due to the moment of inertia of the wheel, the  greatest effect is obtained by a massive gyro spinning fast. Esval's new gyro had twice the mass of the one used in the then-current Sperry O-1 bombsight, and about the same weight as the vertical gyro in the Norden Mark XV. However, Esval's gyro was designed to spin at 30 000 rpm nearly four times faster than the Norden's gyros. The increased gyroscopic effect overcame friction in the gimbal bearings that was a source of precession (a slow gyration of the rotation axis) and failure.    

Carl Frische, then a young development engineer who years later became Sperry's president, assisted Esval in developing the first self-erecting system for the new vertical gyro. When engaged, the self-erecting system would automatically find the exact vertical, eliminating the necessity for a pilot and bombardier to spend time in a bombing run aligning liquid levels. Esval and Frische designed the self-erecting system so that it could be turned off during banking maneuvers, so as not to precess the gyro to a false vertical; when switched on again after the aircraft returned to level flight, it would again automatically seek the true vertical.     

Esval's high-speed gyro and Frische's self-erecting system, along with an optical gyro-balancing machine that speeded manufacture, dramatically improved the vertical tracking accuracy of Sperry's O-1 bombsight, later designated the S-1. Next, they turned a second gyro wheel assembly on its side to make an azimuth gyro.     

Esval and Frische also decided to treat the azimuth gyro as a sensor only, to eliminate the physical linkage that in the Norden bombsight was a source of friction. To do this, they mounted an electromagnetic pickoff on a nonspinning ring that was centered on the spinning rotor and was controlled by the azimuth servo motor. When aircraft movements caused the slightest angular deviation of the gyro's from the plane's axes, the E-pickoff generated electric signals that, when amplified, controlled a servomechanism that compensated for the plane's movement and thus stabilized the bombsight optics in azimuth. This may have been the first use of closed-loop amplifiers.     

Esval's new gyros were self-lubricating and induction-powered, eliminating the dc brushes that caused carbon dust. This innovation, however, required the new gyro to have its own ac power source, because in the late 1930s U.S. airborne instrumentation ran only on dc power or on vacuum suction generated through venturi tubes mounted outside the cockpit. The Army Air Corps was so inspired by the performance of the Sperry bombsight that it soon adopted induction electrical systems for aircraft, which later facilitated radio instrumentation. The Air Corps settled on a 400-hertz electrical system that, accordingly, spun the new gyros at a somewhat reduced rate of 24 000 rpm. Although there was some loss in gyroscopic momentum, the instrument still spun more than three times faster than the Norden Mark XV's gyros.     

In 1940 and 1941, the Norden XV bombsight was installed in Air Corps B-17 bombers. The Sperry S-1 was installed in B24Es used by the 15th Air Force in the Mediterranean area and in lendlease B-24s supplied to the British Royal Air Force (RAF), since the Navy refused to release Norden sights to foreign governments.     

A modified Sperry O-1 bombsight first saw combat on April 30, 1941 from a British bomber, more than six months before the United States entered the war with its Norden-equipped planes.

"The target was a heavily armed yet small Nazi supply ship of 700-800 tons" near Tyboron, Denmark, recalled John Mallinson, a former RAF wing commander who flew on that first mission. "Our squadron was the 220 Coastal Command based at Thornaby, Yorkshire. The Sperry had been installed in a Lockheed Hudson Mk V, and we made our approach at 8000 ft [2.4 km]. The German supply ship looked like  a tiny speck from 8000 ft, but with the Sperry bombsight, our bombardier and Wing Commander Charles Dann dropped only one salvo, and our bombs hit squarely across the ship's stern on the first pass."