Info: Bomb Sights and (Im)precision

[Witch-King of Angmar]

Norden Bombsight

A bombsight is a device that is used to drop bombs accurately from aircraft, and the Norden bombsight was perhaps one of the few scientific instruments, along with radar, ever credited with playing such a large role in the Allied victory in WW II. Prior to the Manhattan Project, the Norden bombsight was America's most closely guarded secret, an optical device that allowed the U.S. Army Air Force to carry out daytime strategic bombing with high precision. The bombsight allowed a bomber's deadly payload to be dropped at exactly the right time needed to hit the target. It used a mechanical analog computer consisting of a system of gyros, motors, gears, mirrors, levels, and a telescope. The bombardier would provide the computer with the air speed, wind speed and direction, altitude, and angle of drift. With this information, the bombsight would calculate the trajectory of the bomb. As the airplane approached the target, the pilot would turn the plane over to the autopilot that would fly the plane to the precise location and release the bomb over the target. Supposedly, use of the bombsight could place a bomb side a 100-foot (30-meter) circle from four miles (six kilometers) high.

The bombsight was so secret that it would be loaded onto the plane under armed guard just before it took off and removed as soon as the plane landed (in the photo at left, a Norden bombsight is lifted into an AT-11 prior to night practice mission). Crew members serving on planes with the bombsight had to take an oath to protect the bombsight with their lives if necessary:

Mindful of the secret trust about to be placed in me by my Commander in Chief, the President of the United States, by whose direction I have been chosen for bombardier training ... and mindful of the fact that I am to become guardian of one of my country's most priceless military assets, the American bombsight ... I do here, in the presence of Almighty God, swear by the Bombardier's Code of Honor to keep inviolate the secrecy of any and all confidential information revealed to me, and further to uphold the honor and integrity of the Army Air Forces, if need be, with my life itself.

Owing to this secrecy, apocryphal tales about the device were told all during the war. It was said that bombardiers had to carry a pistol to shoot the device like a lame horse if they crash-landed in enemy territory, though there is no incident of this ever actually occurring.

Carl Norden, a Dutch engineer educated in Switzerland who came to the United States in 1904, designed the bombsight for use on U.S. naval aircraft. Norden had been raised in Java and educated in Switzerland. First, he worked with Elmer Sperry on gyro-stabilizers. He and Sperry were a pair of temperamental geniuses, and Norden left in 1913 to set up his own business. He began designing the bombsight for the Navy in 1920. He finally had a pretty good version in 1928. At the same time, his old boss, Sperry, was developing the Army's bombsight. (Read more about the Norden-Sperry conflict in The Bombsight War.) It was 1932 before the Army compared notes with the Navy and found that Norden's instrument was far better. But Norden regarded the Army as plebian and he wouldn't deal with them. To get his bombsight, the Army had to buy it from the Navy. And it was worth it; it was a superb piece of design — an analog computer that calculated the trajectory of the bomb, given crosswind, altitude, and airspeed. It also released the bomb. In a plane moving 300 feet per second, the bombardier's reaction time was too slow to tolerate.

John Lienhard of the University of Houston reveals some facts about the Norden story that run contra to popular history:

A Norden worker named Lang had stolen the plans in 1937 for the Germans. Reichsmarshall Herman Göring, leader of the German Luftwaffe (Air Force) gave him $3,000 — a huge sum in those days. Later, Lang was caught and sent to jail for 18 years. The irony is, Germany never used the device. The Germans committed to dive-bombing, while we pursued high-altitude precision bombing. (The trouble was, precision was another Norden myth. From 20,000 feet, two-thirds of American bombs fell 1/5 of a mile or more from their targets — even with the best of bombsights. Meanwhile, the bombsight itself had been reclassified from secret to merely confidential two years before Lang's infamy. In 1942 it was downgraded to restricted, the lowest classification. By then we were switching to the English tactic of saturation bombing. A bomber armada flew over a city. The lead plane signaled the drop and they pulverized everything below -- hoping to catch occasional military targets in the general carnage. It was a nasty war, and, I suppose, we needed to make heroes of machines as well as people. Norden give us our mechanical hero.

How did it work? Through crosshairs made of spider's webbing, the Norden bombsight determined, with precision, the exact moment bombs were to be released to reach their target. At its heart, the sight was a mechanical analog computer, not even as powerful as a pocket calculator today. Programmed by the bombardier, the sight compensated for such factors as wind and drift. It was coupled with the autopilot to fly the bomber to the target, at which point it released the bombs. It was often said that with the Norden bombsight, bombardiers could drop a bomb into a pickle barrel from 20,000 feet. In reality, the sight's precision wasn't quite that dramatic. When testing the bombsight, half of the bombs dropped landed within 75 feet of the target. In actual wartime conditions of industrial haze, cloud coverage, and having to fly higher to evade enemy flak, the accuracy of the sight was considerably challenged. Prior to a run, the bombardier checked the sight out of its guarded area, and mounted it in the plane. He connected the equipment and energized the gyros. He programmed the bomb's actual time of fall, and its trail. Crouched in the Plexiglas nose of the plane and breathing pure oxygen, the bombardier set the target under the horizontal crosshair of the sight. He had to wear silk gloves to keep his skin from freezing to the metal on the sight, due to temperatures of 40 below. As the craft withstood heavy flak and attacks from fighters, the sight compensated for crosswind, and the bombardier felt the weight of the mission on his shoulders. Should his plane be shot down, he was responsible for destroying the bombsight in order to save its secret. Initially this was done by firing his pistol into the bombsight.

In 1943 the Norden M-series was delivered to the USAAF. It is estimated that this version was 6 to 8 times more precise than the RAF Mk XIV bombsight. It is estimated that the RAF was capable of putting only 5% of its ordinance within a mile of their aiming point under combat conditions. In contrast, the 8th Air Force was believed to be able to put 24% of their bombs to within 1,000 yds of their targets. By 1944 this figure would rise to 40% to within 500 yds. The Norden bombsight enabled Forts to fly above the flak and still hit their target with reasonable accuracy. The daylight bombing strategy became a viable option to take the war to Germany and bring the war to a quick end. The Norden bombsight was also used on the B-29 Superfortress Enola Gay on August 6, 1945, to drop the atomic bomb on Hiroshima. Considered a work in progress even at war's end, numerous versions of the sight were created. By the end of the war, nearly 90,000 had been made.

Article source:

http://www.skylighters.org/encyclopedia/norden.html

Question:

The Germans had developed before 1940 their own level bombsights, the Lofte 7 series. How good(or bad) did they work, and what was the design difference, in comparison to the Norden model?

~Regards,

The Witch King of Angmar

[Xavier]

as a fact:

One of the german spies in the NY area got a set of blueprints for the norden sight....too bad could not get the manual.

the germans duplicated everything, just could not get to discover how it worked in conjunction with a plane (mounting)

Xavier
the link scrounger

[gabriel pagliarani]

Norden sight was unefficient simply because allied bombs had the worst aerodynamical shape possible. The desired and forecasted cynematical trajectory required by Norden was affected by the dynamical behaviours of such free-falling bombs. Simply any bomb was dragged by high resistances increasing with speed, relative humidity, pressure and wind: too many parameters to control. These problems (..typical of any free-falling bomb: modern bombs are only a bit more aerodynamically efficient than WW2s) had been solved only recently with GPS controlled bombs.
"Reasonable accuracy" in 1945? 5% on target? or less?