MHQ Winter 2101 First Lieutenant R
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AMERICAN AIR MUSEUM IN BRITAIN, IMPERIAL WAR MUSEUMS WAR IMPERIAL BRITAIN, IN MUSEUM AIR AMERICAN 62 MHQ Winter 2101 First Lieutenant R. F. Brubaker, a bombardier with the 91st Bomb Group of the U.S. Army Air Forces, uses a Norden bombsight from inside the nose gun of his Boeing B-17 Flying Fortress in June 1943. EYE IN THE SKY The Norden bombsight achieved legendary status in World War II but never lived up to its promised precision. By Robert O. Harder AMERICAN AIR MUSEUM IN BRITAIN, IMPERIAL WAR MUSEUMS WAR IMPERIAL BRITAIN, IN MUSEUM AIR AMERICAN MHQ Winter 2101 63 NORDEN BOMBSIGHT t 9:10 a.m. on Monday, August 6, 1945, Thomas Italo-Turkish War, when Italian Sub-Lieutenant Giulio Ga- Ferebee was hunched over his Norden M-9B votti, flying an Austrian-built Etrich Taube, bombed Turk- bombsight, serial number V-4120. Ferebee, a ish forces at the oases of Ain Zara and Tajura in western 24-year-old major in the U.S. Army Air Forces, Libya. Gavotti simply tossed 17 Cipelli hand grenades from was the bombardier aboard Enola Gay, a specially the cockpit as he passed over the enemy troops. modified Boeing B-29 Superfortress that was When the world exploded into flames three years later, Acoasting in on Honshu, Japan’s largest island. It was a beauti- it became clear that a more scientific approach to aerial ful, clear day—what flyers called CAVU, or ceiling and visi- bombardment was needed. Further, it quickly became ap- bility unlimited. The plane, piloted by Colonel Paul W. parent that gravity-propelled bombs also faced formidable Tibbets Jr., was still 9 miles south of its initial reference point, atmospheric problems—the air beneath a bomber can be a the city of Mahara. Ferebee, temporarily configuring his virtual layer cake of wind patterns. It wasn’t until 1915, the bombsight into extended vision mode, had already peered second year of World War I, that a British professor turned through his 3.2X power telescope and acquired his aiming artillery lieutenant, Henry Tizard, developed a simple me- point: the Aioi Bridge over the Ota River at Hiroshima. chanical bombsight that used two rigid aiming bars Human history was about to change forever. mounted on the side of the plane’s fuselage. The pilot Extended vision—a feature of the X-1 Reflex Sight—was sighted along them until they lined up with the target, con- one of the more useful innovations in the final versions of sulted his stopwatch, and then tripped the release lever at the Norden Bombing System. By engaging the X-1 sight, the appropriate moment. While this was an advance over Ferebee had raised the forward vision angle by another 20 the previous eyeball and windage guesswork methods of degrees, enabling him to pick out the target from an as- 1914, the Tizard CFS-4 was still inadequate. But it was the tounding 75 miles away. The bombardier would stay in ex- most sophisticated option at the time, and several thou- tended vision mode through the plane’s inbound turn before sand of Tizard’s bombsights were manufactured. switching back to the normal viewing configuration.Enola The Tizard was followed by the Mark I, the first reason- Gay’s three-minute bomb run was a complete success—“the ably effective mechanical bombsight. Developed by de- easiest I ever made,” Ferebee signer Harry Wimperis for the Royal Navy in 1917 and would recall. Mild winds, a subsequently adopted by the U.S. Army Air Service in The U.S. Navy well-defined aiming point, 1919, it marked the first time a bomb-aiming designer had aimed to build and the bombardier’s skillful seriously tried to compensate for groundspeed and drift. synchronizing of the cross- Unfortunately, the advance came with a serious drawback: a precision hairs and accurate calculation Pilots had to crisscross the target at 90-degree angles to of the airplane’s 31,060-foot obtain usable readings. (Navigators later called the proce- bombsight that true altitude above sea level dure a double drift.) Flying a warplane back and forth could target resulted in the bomb missing across a heavily defended target was not practical, to say ground zero by 800 feet, a the least. Further, it often took a great deal of ordnance to enemy ships. meaningless deviation. accomplish the job. In a 1921 test bombing, it took 65 “Little Boy” was the bombs for American airmen, under General Billy Mitchell, world’s first atomic bomb. The strikes at Hiroshima and to sink a stationary, captured German battleship. three days later at Nagasaki, which effectively ended World Engineers made significant advances in bombsight de- War II, would not have been possible without decades of velopment in the 1920s. Taking the lead was Carl L. work on sophisticated new bombsights. Much of that work Norden, a Dutch engineer who had immigrated to the was conducted in private and acquired legendary status United States in 1904 and worked for the Sperry Gyro- before the first new, improved bombsight was ever installed scope Company in New York before becoming a consul- in American warplanes. tant to the U.S. Navy in 1915. The navy asked Norden, who was known as “Old Man Dynamite” because of his explo- The idea of dropping ordnance from flying machines came sive temper, to develop a precision bombsight capable of relatively late in their development. The mindset of army targeting enemy ships. Working alongside Norden were general staffs had long been that the airplane was useful former U.S. Army colonel Theodore Barth, who had been only for reconnaissance—a better method than observa- in charge of gas mask production in World War I, and U.S. tion balloons. The first time a heavier-than-air flying- ma Navy lieutenant (later captain) Frederick I. Entwistle, the chine dropped high explosives was in 1911, during the assistant research chief at the navy’s Bureau of Ordnance. FROM LEFT: NATIONAL AIR AND SPACE MUSEUM, SMITHSONIAN INSTITUTION; TED H. LAMBERT, CC BY-SA 3.0; HISTORYNET ARCHIVES 64 MHQ Winter 2101 From left: It took 65 bombs for American airmen to sink a captured German battleship in 1921; Enola Gay bombardier Thomas Ferebee with the Norden bombsight after the dropping of “Little Boy”; Carl Norden in 1944. Before Norden’s work, the vexing trigonometric diffi- crank the crosshairs until they were atop the aiming point. culties in obtaining consistent bombing accuracy had If his precomputation estimates (especially for wind) were defied solutions. How did one accurately place varying off a little—they always were—he could “kill” the actual types of gravity-propelled weapons on specific earthbound drift and speed differences by repeatedly dragging the hairs targets when releasing them from fast-moving airborne ve- back to the aiming point, allowing the Norden’s analog hicles operating in three dimensions? As any experienced computer to neutralize the variations, thus stabilizing the bombardier could testify, it was a lot tougher than it looked crosshairs. This process of coordinating a vertical crosshair on paper. The long list of variables included the size, (course line—the plane’s heading) with a horizontal cross- weight, and shape of the bomb; the manner of releasing the hair (release line—perpendicular to course line) came to bomb; malfunctioning bombs; broken bomb fins; defective be called the “synchronous” method of bomb aiming. Con- bomb-aiming equipment; turbulence and wind drift; the siderable manual dexterity (all the synchronizing was per- aircraft’s altitude and speed; ballistics computation errors; formed with the right hand) and finesse were essential. aiming errors; flak; and, of course, Murphy’s Law. Once the bombardier was satisfied that he had acquired the target, he and the pilot would “clutch in,” slaving the Norden’s answer was to design a completely new bomb- Norden bombsight to the C-1 autopilot and fully uniting aiming system. It was built around an autopilot, a revolu- the Norden Bombing System. The bombardier would con- tionary concept in itself, and what today would be charac- tinue to synchronize the two crosshairs on the target all the terized as an optical-mechanical analog computer. It had way through release, with the autopilot turning the airplane two primary components: the removable sight head— accordingly. Meanwhile, the To Go meter ticked down nicknamed “the Football” (because of its shape) in Europe toward Bombs Away, which occurred when the Norden and “the Blue Ox” (after its code name) in the Pacific—and analog computer sensed that the sighting angle had inter- a stabilizer bolted to the airplane. Spinning gyros kept the sected the dropping angle—about 70 degrees from the ver- Norden in three-dimensional equilibrium. These major tical at 30,000 feet, when the bombs automatically released. hardware elements were supported by a complex array of (As a frame of reference, it took 43 seconds after release knobs, dials, levers, cranks, cams, mirrors, tiny motors, from 31,060 feet for Ferebee’s “Little Boy” uranium bomb to and a multipower telescope. reach its planned airburst altitude of 1,890 feet, which After manually entering his ballistic calculations and caused more widespread destruction than a ground blast.) other data into the Norden Bombing System and having Initial testing of the Norden prototype Mark XI was con- departed the bomb run’s Initial Point (IP), the bombardier ducted at the U.S. Navy’s proving ground in Dahlgren, Vir- would position himself over the bombsight and begin ginia. Pilots and bombardiers were unimpressed, aligning the separately controlled crosshairs on the aiming complaining that the new device “demanded both hands, point.