The Sperry S-1 bombsight was designed by Elmer Sperry, Carl Norden's former employer. It had several distinctions from the Norden, and some advantages. The Sperry gyroscopes spun at 24,000 rpm, compared to the Norden gyros which ran at approximately 8,000 rpm. This higher rate of rotation overcame the tendency to precess, so there was no leveling mechanism needed, or a requirement for leveling bubbles. In addition, the gyros ran off of induction rather than brushes; the carbon brushes in the Norden gyros tended to produce dust which fouled the bearings.

The Sperry ran off of 110V AC current compared to the 26V DC current used by the Norden. Whereas the Norden had all of its controls mounted on the right, the Sperry had the knobs for range located on the left and for azimuth located on the right, so that the bombardier could use both hands to adjust them simultaneously. The Norden used a manual geared system to correct for crosswind and drift, but the Sperry did this electromechanically.

The Navy would not allow the Norden to be exported, but did allow the Sperry to go to other countries such as England. The B-24 Liberator was being sent overseas (as the LB-30), and the Sperry was allowed to go with it. This led to a natural pairing of the S-1 and B-24, so almost all of the Sperry S-1's were in B-24 's. Approximately 5,000 S-1's were made, but the Norden dominated the bombsight market, and production of the S-1 ceased in 1943. Here is a video of what I believe to be is the only working Sperry S-1 in existence. The video seems long, but the second half is a tutorial on the bombing problem and solution:


The Estoppey bombsight, designed by Georges Estoppey, used a clock timer mechanism to compute the drop angle. This was a much simpler and cheaper device than the Norden or Sperry sights. There was a severe shortage of Norden bombsights just before WWII and in the early years, so Estoppeys were called into heavy production beginning in 1938. The bombardiers would wind the timer up with a crank, set the time of fall information into the front dial, then rotate the crank in the opposite direction to activate the timer. He would then look from the top wire down to the front wire, and rotate the crank so as to keep the front wire on the target. When the timer ran out, the crank was locked out, fixing the wire position. He would then shift to looking from the top wire down to the rear wire. When the target came into view, he would release the bomb. There were approximately 9,000 Estoppey bombsights produced. Here is a video of what I believe to be is the only operational Estoppey D-8 Bombsight:


The Sperry T-1 was the American version of the British Mark XIV bombsight. The British did not have enough manufacturing capacity for wartime needs, and approached the American Sperry corporation for assistance; approximately 23,000 were made. It was composed of a computer and a sighting head, connected by rotary drive shafts. It used 60 PSI air pressure and 27V DC to drive a series of bellows and gears/levers to compute the dropping and drift angles. Prior to the mission, the bombardier would dial in the bomb terminal velocity and target height above sea level. The computer would use pitot-static input to determine airspeed and altitude. When the bombardier entered the wind speed and direction, the computer would solve for the angles and adjust the sighting head. A few of these ended up in American bombers. Here is a video of what I believe to be is the only operational Sperry T-1 Bombsight:


WWI bombsights could not correct for drift due to crosswind. Harry Wimperis began designing a line of bombsights in 1916 to solve this problem. Because they could solve the wind vector issue, these belonged to the class known as "vector" bombsights. The Wimperis bombsight, also known as the Course Setting Bombsight, had several models. The Mark IX was used early in WWII, and is a forerunner of the Mark XIV/T-1 shown above. Here is a video demonstration of a Wimperis Mark IXc:


Although the Norden bombsight was our second most highly guarded secret during WWII after the atomic bomb, Carl Norden had a Nazi spy in his New York factory, Herman Lang, that gave the design of the Norden bombsight to the Nazis prior to the War. The Nazis incorporated this information into their own gyroscopically stabilized bombsight, the Lotfernrohr 7, or “Lotfe 7”. Ultimately, the Germans did not have heavy, high altitude bombers or the need for precision bombing. Here is a video demonstration of the Lotfernrohr 7-D: