The Norden Mk. XV, known as the Norden M series in US Army service, is a bombsight that was 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 an early tachometric design, which, by incorporating optics, a mechanical computer, and an autopilot for the first time, could not only identify a target but also fly the airplane to it. The bombsight directly measured the aircraft's ground speed and direction, which older types could only estimate with lengthy manual procedures. The Norden further improved on older designs by using an analog computer that continuously recalculated the bomb's impact point based on changing flight conditions, and an autopilot that reacted quickly and accurately to changes in the wind or other effects.
Together, these features promised unprecedented accuracy for daytime bombing from high altitudes. During prewar testing the Norden demonstrated a circular error probable (CEP), an astonishing performance for that period. This precision would enable direct attacks on ships, factories, and other point targets. Both the Navy and the USAAF saw it as a means to conduct successful high-altitude bombing. For example, an invasion fleet could be destroyed long before it could reach US shores.
To protect these advantages, the Norden was the subject of utmost secrecy well into the war and of a production effort on a scale similar to that of the Manhattan Project: the overall cost (both R&D and production) was $1.1 billion (equivalent to $ billion in ), as much as 2/3 of the latter or over a quarter of the production cost of all B-17 bombers. The Norden was not as secret as believed; both the British SABS and German Lotfernrohr 7 worked on similar principles, and details of the Norden had been passed to Germany even before the war started.
Under combat conditions the Norden did not achieve its expected precision, yielding an average CEP in 1943 of , similar to other Allied and German results. Both the Navy and Air Forces had to give up using pinpoint attacks. The Navy turned to dive bombing and skip bombing to attack ships, while the Air Forces developed the lead bomber procedure to improve accuracy, and adopted area bombing techniques for ever-larger groups of aircraft. Nevertheless, the Norden's reputation as a pin-point device endured, due in no small part to Norden's own advertising of the device after secrecy was reduced late in the war.
The Norden saw reduced use in the post–World War II period after radar-based targeting was introduced, but the need for accurate daytime attacks kept it in service, especially during the Korean War. The last combat use of the Norden was in the US Navy's VO-67 squadron, which used it to drop sensors onto the Ho Chi Minh trail in 1967. The Norden remains one of the best-known bombsights.
History and development
Early work The Norden sight was designed by Carl Norden, a Dutch engineer educated in Switzerland who immigrated to the US in 1904. In 1911, Norden joined Sperry Gyroscope to work on ship gyrostabilizers, and then moved to work directly for the US Navy as a consultant. At the Navy, Norden worked on a catapult system for a proposed flying bomb that was never fully developed, but this work introduced various Navy personnel to Norden's expertise with gyro stabilization.
World War I bomb sight designs had improved rapidly, with the ultimate development being the Course Setting Bomb Sight, or CSBS. This was essentially a large mechanical calculator that directly represented the wind triangle using three long pieces of metal in a triangular arrangement. The hypotenuse of the triangle was the line the aircraft needed to fly along in order to arrive over the target in the presence of wind, which, before the CSBS, was an intractable problem. Almost all air forces adopted some variation of the CSBS as their standard inter-war bomb sight, including the US Navy, who used a modified version known as the Mark III.
It was already realized that one major source of error in bombing was levelling the aircraft enough so the bombsight pointed straight down, even small errors in levelling could produce dramatic errors in accuracy. The US Army did not adopt the CSBS and instead used a simpler design, the Estoppey D-series, as it automatically levelled the sight during use. Navy experiments showed these roughly doubled accuracy, so they began a series of developments to add a gyroscopic stabilizer to their bombsights. In addition to new designs like the Inglis (working with Sperry) and Seversky, the Navy also asked Norden to provide an external stabilizer for the existing Mark III designs.
Mark III-A
Although the CSBS and similar designs allowed the calculation of the proper flight angle needed to correct for windage, they did so by looking downward out of the aircraft. Very simple bombsights could be operated by the pilot, but as their sophistication grew they demanded full-time operators. This task was often given to the front or rear gunner. In Army aircraft they would sit near enough to the pilot to indicate any required directional adjustments using hand signals, or if they sat behind the pilot, using strings attached to the pilot's jacket.
The Navy's first bombers were large flying boats, where the pilot sat well away from the front of the fuselage, and one could not simply cut a hole through the hull for the bombsight to view through. Instead, the bombs were normally aimed by an observer in the nose of the aircraft. This made communications with the pilot very difficult. To address this, the Navy developed the concept of the pilot direction indicator, or PDI, an electrically-driven pointer that the observer used to indicate which direction to turn. The bombardier used switches to move the pointer on his unit to indicate the direction of the target, which was duplicated on the unit in front of the pilot so he could maneuver the aircraft to follow suit.
Norden's attempt to fit a stabilizer to the Mark III, the Mark III-A, also included a separate contract to develop a new automatic PDI. Norden proposed removing the electrical switches used to move the pointer and using the entire bombsight itself as the indicator. Instead of the thin metal wires that formed the sights on the Mark III, a small low-power telescope would be used in its place. The bombardier would rotate the telescope left or right to follow the target. This motion would cause the gyros to precess, and this signal would drive the PDI automatically. The pilot would follow the PDI as before.
Norden initially delivered three prototypes of the stabilized bombsight without the automatic PDI. In testing, the Navy found that while the system did improve accuracy when it worked, it was complicated to use and often failed, leaving the real-world accuracy no better than before. They asked Norden for suggestions on ways to improve this. They were still interested in the PDI work, and the contract was allowed to continue.
Mark XI
Norden suggested that the only solution to improve accuracy would be to directly measure the ground speed, as opposed to calculating it using the CSBS's wind triangle. To time the drop, Norden used an idea already in use on other bombsights, the "equal distance" concept. This was based on the observation that the time needed to travel a certain distance over the ground would remain relatively constant during the bomb run, as the wind would not be expected to change dramatically over a short period of time. If you could accurately mark out a distance on the ground, or in practice, an angle in the sky, timing the passage over that distance would give you all the information needed to time the drop.
Norden's version of the system was very similar to the Army's Estoppey D-4 of the same era, differing largely in the physical details of the actual sights. The D-4 used thin wires as the sights, while Norden's would use the small telescope of the Mark III-A. To use the system, the bombardier looked up the expected time it would take for the bombs to fall from the current altitude. This time was set into a countdown stopwatch, and the sights were set to the angle that the bombs would fall if there was no wind. The bombardier waited for the target to line up with a crosshair in the telescope. When it did, the timer was started, and the bombardier rotated the telescope around its vertical axis to track the target as they flew toward it. This movement was linked to a second crosshair through a gearing system. The bombardier continued moving the telescope until the timer ran out. The second crosshair was now at the correct aiming angle, or range angle, after accounting for any difference between groundspeed and airspeed. The bombardier then waited for the target to pass through the second crosshair to time the drop.
In 1924, the first prototype of this design, known to the Navy as the Mark XI, was delivered to the Navy's proving grounds in Virginia. In testing, the system proved disappointing. The circular error probable (CEP), a circle into which 50% of the bombs would fall, was wide from only altitude. This was an error of over 3.6%, somewhat worse than existing systems. Moreover, bombardiers universally complained that the device was far too hard to use. Norden worked tirelessly on the design, and by 1928, the accuracy had improved to 2% of altitude. This was enough for the Navy's Bureau of Ordnance to place a US$348,000 contract (equivalent to $ million in ) for 80 production examples.
Norden was known for his confrontational and volatile nature. He often worked 16-hour days and thought little of anyone who did not. Navy officers began to refer to him as "Old Man Dynamite". During development, the Navy asked Norden to consider taking on a partner to handle the business and leave Norden free to develop the engineering side. They recommended former Army colonel Theodore Barth, an engineer who had been in charge of gas mask production during World War I. The match-up was excellent, as Barth had the qualities Norden lacked: charm, diplomacy, and a head for business. The two became close friends.
Initial US Army interest In the December of 1927, the United States Department of War was granted permission to use a bridge over the Pee Dee River in North Carolina for target practice, as it would soon be sunk in the waters of a new dam. The 1st Provisional Bombardment Squadron, equipped with Keystone LB-5 bombers, attacked the bridge over a period of five days, flying 20 missions a day in perfect weather and attacking at altitudes from . After this massive effort, the middle section of the bridge finally fell on the last day. However, the effort as a whole was clearly a failure in any practical sense.
About the same time as the operation was being carried out, General James Fechet replaced General Mason Patrick as commander of the USAAC. He received a report on the results of the test, and on 6 January 1928 sent out a lengthy memo to Brigadier General William Gillmore, chief of the Material Division at Wright Field, stating:
He went on to request information on every bombsight then used at Wright, as well as "the Navy's newest design". However, the Mark XI was so secret that Gillmore was not aware Fechet was referring to the Norden. Gilmore produced contracts for twenty-five examples of an improved version of the Seversky C-1, the C-3, and six prototypes of a new design known as the Inglis L-1. The L-1 never matured, and Inglis later helped Seversky to design the improved C-4.