Norden bombsight
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The Norden bombsight was a bombsight used by the United States Army Air Force during World War II, the Korean War, and the Vietnam War to aid the crew of bomber aircraft in dropping bombs accurately. Its operation was a closely guarded secret of World War II.
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[edit] Design and operation
The Norden sight was designed for use on US Navy aircraft by Carl Norden, a Dutch engineer educated in Switzerland who emigrated to the US in 1904 and worked on bombsights at the Sperry Corporation before starting his own company. The Norden was later adopted by the USAAF. The Norden was initially built at the Norden plant in New York City before the start of WWII and then at several other companies during the war, with a wide variety of different versions being built, all with minor differences.
The complete Norden system consisted of two primary parts, the stabilizer and the sight head. The stabilizer was a gyroscopically leveled platform that gave the sight head a stable base from which to work. The stabilizer was also normally attached to the aircraft's autopilot, allowing it to direct the aircraft back to the same level point as the sight head. The sight head had to be carefully aligned to the stabilizer in order to ensure it was looking in the same direction as the aircraft heading.
The sight head contained the main operational portions of the bombsight. It consisted primarily of three parts, a mechanical analog computer that calculated the impact point of the bombs relative to the aircraft as an angle, a small telescope used as the primary sight, and a system of electric motors and gyros that moved the telescope so a single point on the ground remained stationary in the sight. Early examples of the Norden included an "Automatic Gyro Leveling Device" to keep the sight head level with the stabilizer, but this proved to be difficult to maintain and was removed from most examples in the field, replaced by a simple bubble level.
The system was operated by pointing the telescope out in front of the aircraft in order to acquire the target while still approaching it. Many Nordens were equipped with a reflector sight to aid in this step. Once turned on, the motors in the sight head would attempt to keep the telescope pointed directly at the selected target, slowly rotating the telescope towards the vertical as the aircraft approached the target. Since the rate of change in the angle depended on the distance to the target and the speed of approach, the bombardier dialed in estimates for airspeed and altitude, which could be read fairly accurately from the aircraft's instruments.
Providing an accurate groundspeed was one of the sources of improved accuracy the Norden provided compared to contemporary instruments. Groundspeed cannot be measured directly from the aircraft, at least not until the introduction of various radar devices, so it had to be calculated by measuring the known airspeed and using a calculated windspeed. The Norden itself was used to make a direct measurement of the groundspeed. During the final approach to target the bombardier would select an easily visible test target and turn the bombsight on with its default settings. Since the aircraft had no information about the windspeed, the test target would drift across the eyepiece as the aircraft moved away from the calculated point. The bombardier then adjusted the sight using a separate set of fine-tuning dials that could be operated by feel while looking through the eyepiece, using them to adjust the drift rates until the target stopped moving in the eyepiece. The dials then held an accurate measurement of windspeed, which could be used for the rest of the bomb run. This was a much more accurate measurement than could be provided by the navigator's drift telescope or dead reckoning.
Once the bombsight was readied and the aircraft was on final approach, the system was turned on and took control of the aircraft's autopilot. From that point on the bombsight actually flew the aircraft, attempting to keep it on the chosen path and correcting for any last-minute adjustments provided by the bombardier. At the proper moment it automatically dropped the bombs; the aircraft was moving over 100 feet per second (30 m/s), so even minor interruptions in timing could dramatically affect aim (a problem common in Bomber Command).
In the European theater, the US introduced an Automatic Flight Control Equipment (AFCE) and a radar system called the H2X (Mickey), which were used directly with the Norden bombsight. The AFCE served as the mechanical computer “autopilot” of the plane. The radar proved most accurate in coastal regions, as the water surface and the coastline produced a distinctive radar echo.
[edit] Operational efficiency
The Norden was developed during a period of United States non-interventionism when the dominant US military strategy was the defense of the United States and its possessions. A considerable amount of the US's strategy was based on stopping attempted attacks at sea, both with direct Naval power, and starting in the 1930s, with US Army Air Force airpower.
Airpower had been coming into its own as an anti-shipping weapon, but hitting a moving ship at sea was a difficult task. Most forces of the era invested heavily in dive bombers or torpedo bombers, but these generally had limited range and were only suitable in a strategic sense for carrier basing. The Army instead invested in the combination of the Norden and B-17, which it was believed would have enough accuracy to allow formations of B-17s to successfully attack shipping at long distances from the USAAF's land bases. Using the Norden, bombardiers could, in theory, drop their bombs within a 100 foot (ca 30 m) circle from an altitude of well over 20,000 feet (ca. 7 km). The high altitude would allow for long cruising ranges and keep them out of range of most ship-borne anti-aircraft fire while the bomb pattern would still give an acceptable probability of a "hit". The Norden was marketed as the tool to win the war; and it was often claimed that the bombsight could drop bombs into pickle barrels.
In practice the Norden never managed to produce accuracies remotely like those it was theoretically capable of. The RAF were the first to use the B-17 in combat, and reported extremely poor results, eventually converting their aircraft to other duties. USAAF anti-shipping operations in the far east were likewise generally unsuccessful, and although there were numerous claims of sinkings, the only confirmed successful action was during the Battle of the Philippines when B-17s damaged two Japanese transports, the cruiser Naka, and the destroyer Murasame, and sank one minesweeper. However these successes were the exception to the rule; actions during the Battle of Coral Sea or Battle of Midway, for instance, were entirely unsuccessful. The USAAF eventually replaced all of their B-17s with other aircraft, and came to use the skip bombing technique in direct low-level attacks.
In Europe the Norden likewise demonstrated a poor real-world accuracy. Under perfect conditions only 50 percent of American bombs fell within a quarter of a mile of the target, and American flyers estimated that as many as 90 percent of bombs could miss their targets.[1][2][3] Nevertheless many veteran B-17 and B-24 bombardiers swore by the Norden.
Many factors have been put forth to explain the Norden's poor performance. Over Europe the cloud cover was a common explanation, although performance did not improve even in favorable conditions. Accuracy did improve with the introduction of the "master bomber" concept, under which only a single aircraft would actually use the Norden while the rest simply dropped on their command. This suggests that much of the problem is attributable to the bombardier. Over Japan, bomber crews soon discovered strong winds at high altitudes, the so-called jetstreams, but the Norden bombsight worked only for wind speeds with minimal wind shear. Additionally, the bombing altitude over Japan reached up to 30,000 feet (9,100 m), but most of the testing had been done well below 20,000 ft (6,100 m) An additional factor was that the shape and even the paint of the bomb mantle greatly changed the aerodynamic properties of the weapon; and, at that time, nobody knew how to calculate the trajectory of bombs that reached supersonic speeds during their fall.
In both theaters of war, one vulnerability was that when the bombardier auto-piloted the aircraft using the bombsight, the aircraft was more susceptible to anti-aircraft fire and collisions with other allied aircraft.
As a mechanical device, the Norden bombsight used complex machinery consisting of many gearwheels and ball bearings, which were prone to produce inaccuracies if not properly maintained. In fact, many bombsights were rushed to war use without thorough testing. Often the bombardier had to oil and repair failures himself. For some time into the war equipped and qualified groundcrew technical staff were simply not available in sufficient numbers (see below).
[edit] Wartime security
Since the Norden was considered a critical wartime instrument, bombardiers were required to take an oath during their training stating that they would defend its secret with their own life if necessary. In case the bomber plane should make an emergency landing on enemy territory, the bombardier would have to shoot the important parts of the Norden with a gun to disable it. As this method still would leave a nearly intact apparatus to the enemy, a thermite gun was installed; the heat of the chemical reaction would melt the Norden into a lump of metal.
After each completed mission, bomber crews left the aircraft with a bag which they deposited in a safe ("the Bomb Vault"). This secure facility ("the AFCE and Bombsight Shop") was typically in one of the base's Nissen hut (Quonset hut) support buildings. The Bombsight Shop was manned by enlisted men who were members of a Supply Depot Service Group ("Sub Depot") attached to each USAAF bombardment group. These shops not only guarded the bombsights but performed critical maintenance on the Norden and related control equipment. This was probably the most technically skilled ground-echelon job, and certainly the most secret, of all the work performed by Sub Depot personnel. The non-commissioned officer in charge and his staff had to have a high aptitude for understanding and working with mechanical devices.
As the end of World War II neared, the bombsight was gradually downgraded in its secrecy; however, it was not until 1944 that the first public display of the instrument occurred.
Its last use in combat was by the Naval Air Observation Squadron Sixty-Seven (VO-67), during the Vietnam War. The bombsights were used in Operation Igloo White for implanting Air-Delivered Seismic Intrusion Detectors (ADSID) along the Ho Chi Minh Trail.
[edit] Espionage
Herman W. Lang had been employed by the Carl L. Norden Company. During a visit to Germany in 1938, Lang conferred with German military authorities and reconstructed plans of the confidential materials from memory. In 1941, Lang, along with the 32 other German agents of the Duquesne Spy Ring, was arrested by the FBI in the largest espionage conviction in U.S. History. He received a sentence of 18 years in prison on espionage charges and a 2-year concurrent sentence under the Foreign Agents Registration Act.
German instruments were actually fairly similar to the Norden, even before the war. A similar set of gyroscopes provided a stabilized platform for the bombardier to sight through, although the more complex interaction between the bombsight and autopilot was not used.
[edit] See also
- Stabilizing Automatic Bomb Sight (SABS) a Royal Air Force bomb sight introduced for precision bombing with similar capabilities to the Norden bomb sight.
- Lotfernrohr 7, a similar German bombsight.
[edit] References
This article incorporates text from http://foia.fbi.gov/foiaindex/duquesne_frederick_interesting.htm Federal Bureau of Investigation: Frederick Duquesne Interesting Case Write-up (publicly released on March 12, 1985 under the Freedom of Information Act), a public domain work of the United States Government. Accessed: 2007–05–12
[edit] Further reading
[edit] Books
- Stewart Halsey Ross: "Strategic Bombing by the United States in World War II"
Features a chapter about the bombsights used by the USA. It describes the development, the fallacies, its use and how Germany soon learned about the Norden. - Albert L. Pardini: "The Legendary Norden Bombsight"
The first detailed volume to cover the Norden Bombsight. 352 pages with over 70 b/w photographs plus line art. ISBN 0-7643-0723-1, Schiffer Publishing, 1999. - "The Norden Bombsight"
A classified manual during WW II, this is a complete guide to maintenance and calibration of the Norden bombsight. 200+ pages. - "Bombing – Students' Manual"
This WW II bombardiers instruction book includes Norden bombsight instruction and more. 250+ pages, illustrated. - "Bombardier's Information File"
This manual features instructions for the use of the Norden Bombsight, and other bomb sights as well. Illustrated with photos of bombs, cameras, various equipment, etc. - Loyd Searle: "The Bombsight War: Norden vs. Sperry"
Gives many details of the history and behind-the-scenes development of the NBS and its competitor. Spectrum IEEE #0018-92235/89/0900-0060, September 1989, http://www.thevaluesell.com/images/LSearle_bombsight.pdf
[edit] Articles
- Norden Bombsight from the Hill Aerospace Museum
- Last Combat Use Observation Squadron Sixty-Seven ( VO-67 )
- U.S. Centennial of Flight Commission
- Additional images and detailed information
- WWII AAF collection (Bombardiers Information File)
- (Movies of the Norden and Sperry Bombsights in operation)
