Norden Bombsight

Norden Bombsight

The Norden Mk. XV, known as the Norden M series in Army service, was a bombsight used by the United States Army Air Forces (USAAF) and the United States Navy during World War II, and the United States Air Force in the Korean and the Vietnam Wars. It was the canonical tachometric design, a system that allowed it to directly measure the aircraft’s ground speed and direction, which older bombsights could only measure inaccurately with lengthy in-flight procedures. The Norden Bombsight further improved on older designs by using an analog computer that constantly calculated the bomb’s impact point based on current flight conditions and an autopilot that let it react quickly and accurately to changes in the wind or other effects.
The Norden bombsight

The Norden bombsight

“There is a Norden Bombsight on display in our Museum”

Together, these features seemed to promise unprecedented accuracy in day bombing from high altitudes; in peacetime testing, the Norden Bombsight demonstrated a circular error probable (CEP) of 23 meters (75 ft), an astonishing performance for the era. This accuracy allowed direct attacks on ships, factories, and other point targets. Both the Navy and the AAF saw this as a means to achieve war aims through high-altitude bombing; for instance, destroying an invasion fleet by air long before it could reach US shores. To achieve these aims, the Norden Bombsight was granted the utmost secrecy well into the war and was part of a then-unprecedented production effort on the same scale as the Manhattan Project. Carl L. Norden, Inc. ranked 46th among United States corporations in the value of World War II military production contracts.

Norden Bombsight

Enola Gay bombardier Thomas Ferebee with the Norden Bombsight on Tinian after the dropping of Little Boy

In practice, it was not possible to achieve the expected accuracy in combat conditions, with the average CEP in 1943 of 370 meters (1,200 ft) being similar to Allied and German results. Both the Navy and Air Forces had to give up on the idea of pinpoint attacks during the war. The Navy turned to dive bombing and skip bombing to attack ships, while the Air Forces developed the lead bomber concept to improve accuracy while adopting area bombing techniques by ever larger groups of aircraft. Nevertheless, the Norden’s reputation as a pin-point device lived on, due in no small part to Norden’s own advertising of the device after secrecy was reduced late in the war.

The Norden Bombsight saw some use in the post-World War II era, especially during the Korean War. Post-war use was greatly reduced due to the introduction of radar-based systems, but the need for accurate daytime attacks kept it in service for some time. The last combat use of the Norden bombsight was in the US Navy’s VO-67 squadron, which used them to drop sensors onto the Ho Chi Minh Trail as late as 1967. The Norden remains one of the best-known bombsights of all time.

The Norden bombsight consisted of two primary parts, the gyroscopic stabilization platform on the left side, and the mechanical calculator and sighting head on the right side. They were essentially separate instruments, connecting through the sighting prism. The sighting eyepiece was located in the middle, between the two, in a less than convenient location that required some dexterity to use.

Before use, the Norden’s stabilization platform had to be righted, as it slowly drifted over time and no longer kept the sight pointed vertically. Righting was accomplished through a time-consuming process of comparing the platform’s attitude to small spirit levels seen through a glass window on the front of the stabilizer. In practice, this could take as long as eight and a half minutes. This problem was made worse by the fact that the platform’s range of motion was limited, and could be tumbled even by strong turbulence, requiring it to be reset again. This problem seriously upset the usefulness of the Norden and led the RAF to reject it once they received examples in 1942. Some versions included a system that quickly righted the platform, but this “Automatic Gyro Leveling Device” proved to be a maintenance problem, and was removed from later examples.

Norden Bombsight

A page from the Bombardier’s Information File (BIF) that describes the components and controls of the Norden Bombsight. The separation of the stabilizer and sight head is evident.

Once the stabilizer was righted, the bombardier would then dial in the initial setup for altitude, speed, and direction. The prism would then be “clutched out” of the computer, allowing it to be moved rapidly to search for the target on the ground. Later Nordens were equipped with a reflector sight to aid in this step. Once the target was located the computer was clutched in and started moving the prism to follow the target. The bombardier would begin making adjustments to the aim. As all of the controls were located on the right and had to be operated while sighting through the telescope, another problem with the Norden is that the bombardier could only adjust either the vertical or horizontal aim at a given time, his other arm was normally busy holding himself up above the telescope.

On top of the device, to the right of the sight, were two final controls. The first was the setting for “trail”, which was pre-set at the start of the mission for the type of bombs being used. The second was the “index window” which displayed the aim point in numerical form. The bombsight calculated the current aim point internally and displayed this as a sliding pointer on the index. The current sighting point, where the prism was aimed, was also displayed against the same scale. In operation, the sight would be set far in advance of the aim point, and as the bomber approached the target the sighting point indicator would slowly slide toward the aim point. When the two met, the bombs were automatically released. The aircraft was moving over 110 meters per second (350 ft/s), so even minor interruptions in timing could dramatically affect aim.

Early examples, and most used by the Navy, had an output that directly drove a Pilot Direction Indicator meter in the cockpit. This eliminated the need to manually signal the pilot, as well as eliminating the possibility of error.

In U.S. Army Air Forces use, the Norden bombsight was attached to its autopilot base, which was in turn connected with the aircraft’s autopilot. The Honeywell C-1 autopilot could be used as an autopilot by the flight crew during the journey to the target area through a control panel in the cockpit but was more commonly used under the direct command of the bombardier. The Norden’s box-like autopilot unit sat behind and below the sight and attached to it at a single rotating pivot. After control of the aircraft was passed to the bombardier during the bomb run, he would first rotate the entire Norden so the vertical line in the sight passed through the target. From that point on, the autopilot would attempt to guide the bomber so it followed the course of the bombsight, and pointed the heading to zero out the drift rate, fed to it through a coupling. As the aircraft turned onto the correct angle, a belt and pulley system rotated the sight back to match the changing heading. The autopilot was another reason for the Norden’s accuracy, as it ensured the aircraft quickly followed the correct course and kept it on that course much more accurately than the pilots could.

Later in the war, the Norden was combined with other systems to widen the conditions for a successful bombing. Notable among these was the radar system called the H2X (Mickey), which were used directly with the Norden bombsight. The radar proved most accurate in coastal regions, as the water surface and the coastline produced a distinctive radar echo.

 

The Naples Museum Of Military History

 

Norden Bombsite

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