Norden bombsight - AbsoluteAstronomy.com

The Norden bombsight was a tachometric bombsight

Bombsight

A bombsight is a device used by bomber aircraft to accurately drop bombs. In order to do this, the bombsight has to estimate the path the bomb will take after release from the aircraft. The two primary forces during its fall are gravity and air drag, which makes the path of the bomb through the air...

used by the United States Army Air Forces

United States Army Air Forces

The United States Army Air Forces was the military aviation arm of the United States of America during and immediately after World War II, and the direct predecessor of the United States Air Force....

(USAAF) and the United States Navy

United States Navy

The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...

during World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, and the United States Air Force

United States Air Force

The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...

in the Korean

Korean War

The Korean War was a conventional war between South Korea, supported by the United Nations, and North Korea, supported by the People's Republic of China , with military material aid from the Soviet Union...

and the Vietnam War

Vietnam War

The Vietnam War was a Cold War-era military conflict that occurred in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. This war followed the First Indochina War and was fought between North Vietnam, supported by its communist allies, and the government of...

s to aid the crew of bomber

Bomber

A bomber is a military aircraft designed to attack ground and sea targets, by dropping bombs on them, or – in recent years – by launching cruise missiles at them.-Classifications of bombers:...

aircraft in dropping bomb

Bomb

A bomb is any of a range of explosive weapons that only rely on the exothermic reaction of an explosive material to provide an extremely sudden and violent release of energy...

s accurately. Key to the operation of the Norden were two features; a mechanical computer

Mechanical computer

A mechanical computer is built from mechanical components such as levers and gears, rather than electronic components. The most common examples are adding machines and mechanical counters, which use the turning of gears to increment output displays...

that calculated the bomb's trajectory based on current flight conditions, and a linkage to the bomber's autopilot

Autopilot

An autopilot is a mechanical, electrical, or hydraulic system used to guide a vehicle without assistance from a human being. An autopilot can refer specifically to aircraft, self-steering gear for boats, or auto guidance of space craft and missiles...

that let it react quickly and accurately to changes in the wind or other effects. Together, they allowed for unprecedented accuracy in day bombing from high altitudes; in testing the Norden demonstrated a circular error probable

Circular error probable

In the military science of ballistics, circular error probable is an intuitive measure of a weapon system's precision...

(CEP) of 75 feet, an astonishing performance for the era.

The Norden was initially developed by the Navy in order to provide a system capable of bombing ship

Ship

Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

s from outside the range of their defensive guns, and the Army took up the design for similar missions. Hitting small targets of this sort demanded high accuracy, and the Norden company joked that they could hit a pickle barrel from 30,000 feet. In operations, the Norden proved to be far less accurate than in testing, and early attacks by the US Navy and USAAF in the Pacific failed to hit their targets. The USAAF continued using the Norden for point attacks during the initial operations over Germany in 1943, but post-raid reconnaissance demonstrated the bombers were putting only 24% of their bombs to within 1,000 yards of their targets. This was not due specifically to problems in the Norden, but due largely to its accuracy requiring proper setup and conditions that rarely held in practice.

Given this poor performance, the USAAF placed its most experienced and accurate bombariers aboard each formation's lead bombers (primary and deputy lead) and had the remaining airplanes open their bomb bays and drop their bombs when the leader did. This practice, which greatly increased circular-error accuracy, saw the replacement later in the war of many combat crews' commissioned-officer bombardier with a technical-sergeant "toggleer."

The supposed great secrecy of the device was also widely promoted, to some degree to invent a mythology to boost its reputation. In fact, the Norden received reduced classification long before the war, was declassified in 1942, widely written about in the press in 1943, and shown publicly in 1944. The basic concepts were well known even earlier, and other air forces had been working on similar designs around the same time, notably the RAF's Automatic Bomb Sight. It was also known that Herman Lang, a German spy working at the factory, leaked the design to the Abwehr
Abwehr
The Abwehr was a German military intelligence organisation from 1921 to 1944. The term Abwehr was used as a concession to Allied demands that Germany's post-World War I intelligence activities be for "defensive" purposes only...

in 1938. The Carl Zeiss Lotfernrohr 7

Lotfernrohr 7

The Carl Zeiss Lotfernrohr 7, or Lotfe 7, was the primary bombsight used in most Luftwaffe level bombers, similar to the United States' Norden bombsight, but much simpler to operate and maintain. Several models were produced and eventually completely replaced the simpler Lotfernrohr 3 and BZG 2...

, widely deployed by the Luftwaffe
Luftwaffe
Luftwaffe is a generic German term for an air force. It is also the official name for two of the four historic German air forces, the Wehrmacht air arm founded in 1935 and disbanded in 1946; and the current Bundeswehr air arm founded in 1956....

, was an advanced mechanical system technically similar to the original Norden bombsight.

The Norden's reputation was further enhanced after the war, when it was repeatedly pressed into service over the following decades. Bombing had moved to nuclear weapon

Nuclear weapon

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...

s, where a 3,000 foot CEP was considered good enough, and the development of precision bombsights had largely been ignored. Invariably, when tactical bombing was required in the non-nuclear conflicts that followed, the Norden was returned to service time and time again. The last combat use of the Norden was in US Navy's VO-67 squadron, which used them to drop sensors onto the Ho Chi Minh Trail

Ho Chi Minh trail

The Ho Chi Minh trail was a logistical system that ran from the Democratic Republic of Vietnam to the Republic of Vietnam through the neighboring kingdoms of Laos and Cambodia...

as late as 1967. The Norden remains one of the best known bombsights of all time.

Early work

The Norden sight was designed by Carl Norden

Carl Norden

Carl Lucas Norden , born Carel Lucas van Norden, was an American engineer widely known for having invented the Norden bombsight....

, a Dutch engineer educated in Switzerland

Switzerland

Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....

who emigrated to the U.S. in 1904. In 1911, Norden joined Sperry Gyroscope to work on ship gyrostabilizers,Different sources disagree on Norden's time at Sperry. Most place him there between 1911 and 1915, Moy and Sherman state he left in 1913, and Moy implies he worked there since 1904. 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

Flying bomb

A flying bomb is a manned or unmanned aerial vehicle or aircraft carrying a large explosive warhead, a precursor to contemporary cruise missiles...

that was never fully developed, but this work introduced various Navy personnel to Norden's expertise with gyro stabilization.

During World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

it was already realized that one major source of error in bombing was levelling the aircraft enough so the bombsight pointed the direction it should. Even small errors in levelling could produce dramatic errors, so the Navy had Norden design a gyro platform for their existing Mark III bombsight (a copy of the RAF's Course Setting Bomb Sight

Course Setting Bomb Sight

The Course Setting Bomb Sight is the canonical "vector" bombsight, the first practical system for properly accounting for the effects of wind during the dropping of bombs...

) to eliminate this source of error. At the time, one of the primary sources of error, outside levelling, was the accurate measurement of the wind and its effects on the flight path. Many systems, including the Course Setting Bomb Sight, invested considerable effort in calculating the wind's effect on timing the drop. The Course Setting and similar sights helped direct the aircraft toward the proper point in space to drop the bombs, although flying to that spot was by no means a simple process.

First bombsight design

Norden came up with the idea of improving bombing accuracy through the use of an automated pilot direction indicator

Pilot direction indicator

A pilot direction indicator, or PDI, is an aircraft instrument used by bombardiers to indicate heading changes to the pilot in order to direct them to the proper location to drop bombs...

(PDI). PDIs were relatively new devices, but already widely used by the 1920s. They normally consisted of a set of two pointer indicators, one for the pilot and one for the bombardier, and button that allow the bombardier to move the pointers left or right. During the bomb run, the bombardier would used the buttons to move the pointer to direct the pilot to turn in the right direction to bring the bomber over the drop point. Norden's concept was much more advanced; he proposed attaching a low-power sighting telescope to a gyro platform that would keep the telescope pointed at the same azimuth in spite of the aircraft's movements. The bombardier would simply 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.

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. In Norden's version of the system, the bombardier first 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

Stopwatch

A stopwatch is a handheld timepiece designed to measure the amount of time elapsed from a particular time when activated to when the piece is deactivated. A large digital version of a stopwatch designed for viewing at a distance, as in a sports stadium, is called a stopclock.The timing functions...

, and the bombardier waited for the target to line up with a set of iron sight

Iron sight

Iron sights are a system of shaped alignment markers used as a sighting device to assist in the aiming of a device such as a firearm, crossbow, or telescope, and exclude the use of optics as in telescopic sights or reflector sights...

s at the front of the bombsight. When the target passed through the sights, the timer was started, which started another sight rotating forward at twice the angular speed of the aircraft. When the timer ran to zero, the sight stopped, and was in the correct position for aiming. The bombardier then switched to this second set of sights for the drop.

The first of these Mark XI bombsights was delivered to the Navy's proving grounds in Virginia in 1924. In testing, the system proved disappointing. The circular error probable

Circular error probable

In the military science of ballistics, circular error probable is an intuitive measure of a weapon system's precision...

(CEP), a circle into which 50% of the bombs would fall, was 110 ft wide from only 3,000 ft altitude, an error of over 3.6% and 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, enough that the Navy's Bureau of Ordnance placed a $348,000 contract for the devices. The US Army heard of the system in 1929 and was eventually able to buy an example in 1931. Their testing mirrored the Navy's experience, the gyro stabilization worked and the sight was accurate, but it was also "entirely too complicated" to use.

During development, the Navy suggested that Norden consider taking on a partner to handle the business and leave Norden free to develop on 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 proved to be an excellent one, as Barth had the qualities Norden lacked; charm, diplomacy, and a head for business. The two became close friends.

Fully automatic bombsight

While the Mk. XI was reaching its final design, Norden was already considering an improved version. However, by this point the US Army Air Corps was working with Sperry Gyroscope on an entirely new style of bombsight using the "synchronous" method to measure the ground speed. Norden was initially unconvinced this was workable, but eventually convinced by the Navy to try to build a synchronous design of his own, and a development contract was offered in June 1929. Norden retreated to his mother's house in Zurich

Zürich

Zurich is the largest city in Switzerland and the capital of the canton of Zurich. It is located in central Switzerland at the northwestern tip of Lake Zurich...

and returned in 1930 with a working prototype. Lieutenant Frederick Entwistle, the Navy's chief of bombsight development, judged it revolutionary.

The new design, the Mark XV, was delivered in production quality in the summer of 1931. In testing it proved to eliminate all of the problems of the earlier Mk. XI design. From 4,000 ft altitude the CEP was down to only 35 ft, while even the latest Mk. XI's were 55 ft. At higher altitudes, a series of 80 bomb runs demonstrated a CEP of 75 feet. In a test on 7 October 1931, the Mk. XV dropped 50% of its bombs on a state target, the USS Pennsylvania

USS Pennsylvania (ACR-4)

The second USS Pennsylvania , also referred to "Armored Cruiser No. 4", and later renamed Pittsburgh and numbered CA-4, was a United States Navy armored cruiser, the lead ship of her class....

, while a similar aircraft with the Mk. XI had only 20% of its bombs hit.

Moreover, the new system was dramatically simpler to use. After locating the target in the sighting system, the bombardier simply made fine adjustments throughout the bomb run on two wheels. There was no need for external calculation, lookup tables or pre-run measurements - everything was carried out automatically through the internal mechanical calculator

Mechanical calculator

A mechanical calculator is a device used to perform the basic operations of arithmetic. Mechanical calculators are comparable in size to small desktop computers and have been rendered obsolete by the advent of the electronic calculator....

s. The calculator took a short time to settle on a solution, with setups as short as 6 seconds, down from 50 for the Mk. XI. In general, the bomb run needed to be only 30 seconds long.

In spite of the success, the design also demonstrated several serious problems. In particular, the gyroscopic platform had to be levelled out before use using several spirit level

Spirit level

A spirit level or bubble level is an instrument designed to indicate whether a surface ishorizontal or vertical . Different types of spirit levels may be used by carpenters, stonemasons, bricklayers, other building trades workers, surveyors, millwrights and other metalworkers, and in some...

s, and then checked and repeatedly re-set for accuracy. This procedure could take as long as 8 and ½ minutes. Worse, the gyros had a limited degree of movement, and if the plane banked far enough the gyro would reach its limit and have to be re-set from scratch - something that could happen even in strong turbulence

Turbulence

In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...

. More minor problems were the electric motors which drove the gyroscopes, whose brushes wore down quickly and left carbon dust throughout the interior of the device, and the positioning of the control knobs, which meant the bombardier could only adjust side-to-side or up-and-down aim at a time, not both. But in spite of all of these problems, the Mark XV was so superior to any other design that the Navy ordered it into production.

Carl L. Norden Company incorporated in 1931, supplying the sights under a dedicated source contract. In effect, the company was owned by the Navy. In 1934 the newly-forming GHQ Air Force, the purchasing arm of the US Army Air Corps, selected the Norden for their bombers as well, as the M-1. However, due to the dedicated source contract, the Army had to buy the sights through the Navy. This was not only annoying for inter-service rivalry reasons, but the Air Corps' higher-speed bombers demanded several changes to the design, notably the ability to aim the sighting telescope further forward to give the bombardier more time to set up. The Navy was not interested in these changes, and would not promise to work them in. Worse, Norden's factories were having serious problems keeping up with demand for the Navy alone, and in January 1936, the Navy suspended all shipments to the Army.

Autopilot, production problems

Mk. XV's were initially installed with the same automatic PDI as the earlier Mk. XI. In practice, it was found that the pilots had a very difficult time keeping the aircraft stable enough to match the accuracy of the bombsight. Starting in 1932 and proceeding in fits and starts for the next six years, Norden developed the Stabilized Bombing Approach Equipment (SBAE), a mechanical autopilot that attached to the bombsight. By rotating the bombsight in relationship to the SBAE, the SBAE could calculate the directional changes needed to bring the aircraft onto the bomb run. It was able to correct for wind and turbulence much more quickly than the pilot, and thereby ensure much greater accuracy on the drop. The minor adaptations needed on the bombsight itself produced what the Army referred to as the M-4 model.

In 1937 the Army, faced with the continuing supply problems with the Norden, once again turned to Sperry Gyroscope to see if they could come up with a solution. Their earlier models had all proved unreliable, but they had continued working with the designs throughout this period and had addressed many of the problems. By 1937, Orland Esval had introduced a new AC-powered electrical gyroscope that spun at 30,000 RPM, compared to the Norden's 7,200, which dramatically improved the performance of the inertial platform. The use of three-phase AC power and inductive pickup eliminated the brushes, and further simplified the design. Carl Frische had developed a new system to automatically level the platform, eliminating the time-consuming process needed on the Norden. The two collaborated on a new design, adding a second gyro to handle heading changes, and named the result the Sperry S-1. Existing supplies of Nordens continued to be supplied to the USAAC's B-17s, while the S-1 equipped the B-24Es being sent to the 15th Air Force.

Some B-17s had been equipped with a simple heading-only autopilot, the Sperry A-3, since the late 1920s. The company had also been working on an all-electronic model, the A-5, which stabilized in all three directions, and by the early 1930s it was being used in a variety of Navy aircraft to excellent reviews. By connecting the outputs of the S-1 bombsight to the A-5 autopilot, Sperry produced a system similar to the M-4/SBAE, but one that was much more fast acting. The combination of the S-1 and A-5 so impressed the Army that on 17 June 1941 they authorized the construction of a 186,000 square foot factory and noted that "in the future all production models of bombardment airplanes be equipped with the A-5 Automatic Pilot and have provisions permitting the installation of either the M-Series [Norden] Bombsight or the S-l Bombsight".

When they heard of the new contract, Norden's Barth called a meeting with the Army and Navy at their factory in New York City. Barth offered to build an entirely new factory just to supply the AAC, but the Navy refused this. Instead, the Army suggested that Norden adapt their sight to work with Sperry's A-5, which Barth refused. Norden actively attempted to make the bombsight incompatible with the A-5, and it was not until 1942 that the impasse was finally solved by farming out autopilot production to Honeywell Regulator

Honeywell

Honeywell International, Inc. is a major conglomerate company that produces a variety of consumer products, engineering services, and aerospace systems for a wide variety of customers, from private consumers to major corporations and governments....

, who combined features of the SBAE with the A-5 to produce the C-1. These were mated to sight heads supplied by Norden, and the production problems were solved.

British interest, Tizard mission

By mid-1938 information about the Norden had worked its way up the Royal Air Force

Royal Air Force

The Royal Air Force is the aerial warfare service branch of the British Armed Forces. Formed on 1 April 1918, it is the oldest independent air force in the world...

chain of command and was well known within that organization. The British were in the midst of developing their own Stabilized Automatic Bomb Sight (SABS), but it would not be available until 1940 at the earliest, and likely later. Even then, it did not feature the autopilot linkage of the Norden, and would thus find it difficult to match the Norden's performance in anything but smooth air. Acquiring the Norden became a major goal.

Their first attempt, in the spring of 1938, was rebuffed by the Navy. Sir Edgar Ludlow-Hewitt

Edgar Ludlow-Hewitt

Air Chief Marshal Sir Edgar Rainey Ludlow-Hewitt GCB, GBE, CMG, DSO, MC, DL was a senior Royal Air Force commander.-World War I:...

, Chief Marshal of Bomber Command

Bomber Command

Bomber Command is an organizational military unit, generally subordinate to the air force of a country. Many countries have a "Bomber Command", although the most famous ones were in Britain and the United States. A Bomber Command is generally used for Strategic bombing , and is composed of bombers...

, demanded Air Ministry

Air Ministry

The Air Ministry was a department of the British Government with the responsibility of managing the affairs of the Royal Air Force, that existed from 1918 to 1964...

action, and they wrote to George Pirie, the British air attaché in Washington, suggesting he approach the Army with an offer of an information exchange with their own SABS. Pirie replied that he had already looked into this, and was told that the Army had no licensing rights to the device. The matter was not helped by a minor diplomatic issue that flared up in July when a French air observer was found to be onboard a crashed Douglas Aircraft Company

Douglas Aircraft Company

The Douglas Aircraft Company was an American aerospace manufacturer, based in Long Beach, California. It was founded in 1921 by Donald Wills Douglas, Sr. and later merged with McDonnell Aircraft in 1967 to form McDonnell Douglas...

bomber, forcing President Roosevelt

President Roosevelt

President Roosevelt can refer to two different people who were President of the United States:*Theodore Roosevelt , 26th President from 1901 to 1909, see Presidency of Theodore Roosevelt...

to promise no information exchanges with foreign powers. Six months later, after a change of leadership within the Navy's Bureau of Aeronautics

Bureau of Aeronautics

The Bureau of Aeronautics was the U.S. Navy's material-support organization for Naval Aviation from 1921 to 1959. The bureau had "cognizance" for the design, procurement, and support of Naval aircraft and related systems...

, on 8 March 1939 Pirie was once again instructed to ask the Navy about the Norden, this time sweetening the deal with offers of British power-operated turrets. However, Pirie expressed concern as he noted the Norden had become as much political as technical, and its relative merits were being publicly debated in Congress weekly while the Navy continued to say the Norden was "the United States' most closely guarded secret".

The RAF's desires were only further goaded on 13 April 1939, when Pirie was invited to watch an air demonstration at Fort Benning

Fort Benning

Fort Benning is a United States Army post located southeast of the city of Columbus in Muscogee and Chattahoochee counties in Georgia and Russell County, Alabama...

where the painted outline of a battleship was the target. "At 1:27 while everyone was still searching [the sky for the B-17s] six 300-pound bombs suddenly burst at split second intervals on the deck of the battleship, and it was at least 30 seconds later before someone spotted the B-17 at 12,000 feet." The three following B-17s also hit the target, and then a flight of a dozen Douglas B-18 Bolos placed most of their bombs in a separate 600 by 600 yard square outlined on the ground.

A change of management within the Bureau of Aeronautics had the effect of making the Navy more friendly to British overtures, but no one was willing to fight the political battle needed to release the design. The Navy brass was concerned that giving the Norden to the RAF would increase its chances of falling into German hands, which could put the US's own fleet at risk. The Air Ministry continued increasing pressure on Pirie, who eventually stated there was simply no way for him to succeed, and suggested the only way forward would be through the highest diplomatic channels in the Foreign Ministry. Initial probes in this direction were also rebuffed. When a report stated that the Norden's results were three to four times as good as their own bombsights, the Air Ministry decided to sweeten the pot again, and suggested they offer information on radar

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

in exchange. This too was rebuffed.

The matter eventually worked its way to Prime Minister Neville Chamberlain

Neville Chamberlain

Arthur Neville Chamberlain FRS was a British Conservative politician who served as Prime Minister of the United Kingdom from May 1937 to May 1940. Chamberlain is best known for his appeasement foreign policy, and in particular for his signing of the Munich Agreement in 1938, conceding the...

, who wrote personally to President Roosevelt asking for the Norden, but even this was rejected. The reason for these rejections more political than technical, but the Navy's demands for secrecy were certainly important. They repeated that the design would be released only if the British could demonstrate the basic concept was common knowledge, and therefore not a concern if it fell into German hands. The British failed to convince them, even after offering to equip their examples with a variety of self-destruct devices.

This may have been ameliorated by the winter of 1939, at which point a number of articles about the Norden appeared in the US popular press with reasonably accurate descriptions of its basic workings. But when these were traced back to the press corps at the Army Air Corps, the Navy was apoplectic. Instead of accepting it was now in the public domain, any discussion about the Norden was immediately shut down. This drove both the Air Ministry and Royal Navy

Royal Navy

The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

to increasingly anti-American attitudes when they considered sharing their own developments, notably newer ASDIC systems. By 1940 the situation on scientific exchange was entirely deadlocked as a result.

Looking for ways around the deadlock, Henry Tizard

Henry Tizard

Sir Henry Thomas Tizard FRS was an English chemist and inventor and past Rector of Imperial College....

sent Archibald Vivian Hill to the US to take a survey of US technical capability in order to better assess what technologies the US would be willing to exchange. This effort was the start on the path that led to the famous Tizard Mission

Tizard Mission

The Tizard Mission officially the British Technical and Scientific Mission was a British delegation that visited the United States during the Second World War in order to obtain the industrial resources to exploit the military potential of the research and development work completed by the UK up...

. Ironically, by the time the Mission was being planned, the Norden had been removed from the list of items to be discussed, and Roosevelt personally noted this was due largely to political reasons.

Entering combat

The Norden bombsight was developed during a period of United States non-interventionism

United States non-interventionism

Non-interventionism, the diplomatic policy whereby a nation seeks to avoid alliances with other nations in order to avoid being drawn into wars not related to direct territorial self-defense, has had a long history in the United States...

when the dominant U.S. military strategy was the defense of the U.S. and its possessions. A considerable amount of this strategy was based on stopping attempted attacks at sea, both with direct naval power, and starting in the 1930s, with USAAF airpower. Most air forces of the era invested heavily in dive bomber

Dive bomber

A dive bomber is a bomber aircraft that dives directly at its targets in order to provide greater accuracy for the bomb it drops. Diving towards the target reduces the distance the bomb has to fall, which is the primary factor in determining the accuracy of the drop...

s or torpedo bomber

Torpedo bomber

A torpedo bomber is a bomber aircraft designed primarily to attack ships with aerial torpedoes which could also carry out conventional bombings. Torpedo bombers existed almost exclusively prior to and during World War II when they were an important element in many famous battles, notably the...

s for these roles, but these aircraft generally had limited range, so long-range strategic reach would require the use of an aircraft carrier

Aircraft carrier

An aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers thus allow a naval force to project air power worldwide without having to depend on local bases for staging aircraft operations...

. The Army felt the combination of the Norden and B-17 Flying Fortress presented an alternate solution, believing that small formations of B-17s could successfully attack shipping at long distances from the USAAC's widespread bases. The high altitudes the Norden allowed would help increase the range of the aircraft, especially if equipped with a turbocharger

Turbocharger

A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...

, as in the B-17.

In 1940, Barth, president of Carl L. Norden Inc., said that "we do not regard a 15-foot square ... as being a very difficult target to hit from an altitude of 30,000 feet". At some point the company started using the pickle barrel imagery, to re-enforce the bombsight's reputation. In 1943 the Norden company rented Madison Square Garden

Madison Square Garden

Madison Square Garden, often abbreviated as MSG and known colloquially as The Garden, is a multi-purpose indoor arena in the New York City borough of Manhattan and located at 8th Avenue, between 31st and 33rd Streets, situated on top of Pennsylvania Station.Opened on February 11, 1968, it is the...

and folded their own show in between the presentations of the Ringling Bros. and Barnum & Bailey Circus

Ringling Bros. and Barnum & Bailey Circus

Ringling Bros. and Barnum & Bailey Circus is an American circus company. The company was started when the circus created by James Anthony Bailey and P. T. Barnum was merged with the Ringling Brothers Circus. The Ringling brothers purchased the Barnum & Bailey Circus in 1907, but ran the circuses...

. Their show involved dropping a wooden "bomb" into a pickle barrel, at which point a pickle popped out. However, these claims were greatly exaggerated; in 1940 the average score for an Air Corps bombardier was a circular error of 400 feet, from 15,000 feet instead of 30,000.

Real-world performance was poor enough that the Navy started de-emphasizing level attacks in favour of dive bombing almost immediately. The TBF Avenger

TBF Avenger

The Grumman TBF Avenger was a torpedo bomber developed initially for the United States Navy and Marine Corps, and eventually used by several air or naval arms around the world....

included the ability to mount the Norden, like the TBD Devastator

TBD Devastator

The Douglas TBD Devastator was a torpedo bomber of the United States Navy, ordered in 1934, first flying in 1935 and entering service in 1937. At that point, it was the most advanced aircraft flying for the USN and possibly for any navy in the world...

before it, but combat use was disappointing and eventually described as "hopeless" during the Guadalcanal Campaign

Guadalcanal campaign

The Guadalcanal Campaign, also known as the Battle of Guadalcanal and codenamed Operation Watchtower by Allied forces, was a military campaign fought between August 7, 1942 and February 9, 1943 on and around the island of Guadalcanal in the Pacific theatre of World War II...

. In spite of giving up on the device in 1942, bureaucratic inertia meant they were supplied as standard equipment until 1944.

USAAF anti-shipping operations in the Far East were 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 sank one minesweeper and "damaged" two Japanese transports, the cruiser , and the destroyer . However these successes were the exception to the rule; actions during the Battle of Coral Sea or Battle of Midway

Battle of Midway

The Battle of Midway is widely regarded as the most important naval battle of the Pacific Campaign of World War II. Between 4 and 7 June 1942, approximately one month after the Battle of the Coral Sea and six months after Japan's attack on Pearl Harbor, the United States Navy decisively defeated...

, for instance, were entirely unsuccessful. The USAAF eventually replaced all of their anti-shipping B-17s with other aircraft, and came to use the skip bombing

Skip bombing

Skip bombing was a low-level bombing technique developed by Italian pilot Giuseppe Cenni flying German Junkers Ju 87 Stuka aircraft during attacks on Allied ships off the coast of North Africa, between May and October of 1941...

technique in direct low-level attacks.

Air war in Europe

As U.S. participation in the war started, the USAAF drew up widespread and comprehensive bombing plans based on the Norden. They believed the B-17 had a 1.2% probability of hitting a 100 foot target from 20,000 feet, meaning that 220 bombers would be needed to ensure a target's destruction. This was not considered a problem, and the AAF forecast the need for 251 combat groups to provide enough bombers to fulfill their comprehensive pre-war plans.

However, as at sea, in early missions over Europe the Norden likewise demonstrated widely varied results. Over Bremen-Vegesack

Bremen-Vegesack

-Geography:Vegesack is located at the mouth of the river Lesum, beside the Weser River . Abutting the district of Vegesack to the northwest is the district of Blumenthal, in the southeast the district of Burglesum...

on 19 March 1943, the 303d Bombardment Group

303d Bombardment Group

The 303d Bombardment Group is an inactive United States Air Force unit. Its last assignment was to the 303d Bombardment Wing, being stationed at Davis-Monthan Air Force Base, Arizona. It was de-activated on 16 June 1952....

dropped 76 per cent of its load within a 1,000 ft ring, representing a CEP well under 1,000 ft. But on wider inspection, only 50 percent of American bombs fell within a quarter of a mile of the target (1,320 ft), and American flyers estimated that as many as 90 percent of bombs could miss their targets. The average CEP in 1943 was 1,200 feet, meaning that only 16 percent of the bombs fell within 1,000 feet of the aiming point. A 500-pound bomb, standard for precision missions after 1943, had a lethal radius of only 60 to 90 feet.

Faced with these poor results, Curtis LeMay

Curtis LeMay

Curtis Emerson LeMay was a general in the United States Air Force and the vice presidential running mate of American Independent Party candidate George Wallace in 1968....

started a series of reforms in an effort to address the problems. In particular, he introduced the "combat box" formation in order to provide maximum defensive firepower by densely packing the bombers. As part of this change, he identified the best bombardiers in his command and assigned them to the lead bomber of each box. Instead of every bomber in the box using their Norden individually, the lead bombardiers were the only ones actively using the Norden, and the rest of the box followed them in formation and then dropped their bombs when they saw the lead's leaving his aircraft. Although this spread the bombs over the area of the combat box, this could still improve accuracy over individual efforts. It also helped stop a problem where various aircraft, all slaved to their autopilots on the same target, would drift into each other. This did improve accuracy, which suggests that much of the problem is attributable to the bombardier. However, precision attacks still proved difficult or impossible.

When Jimmy Doolittle

Jimmy Doolittle

General James Harold "Jimmy" Doolittle, USAF was an American aviation pioneer. Doolittle served as a brigadier general, major general and lieutenant general in the United States Army Air Forces during the Second World War...

took over command of the 8th Air Force from Ira Eaker in early 1944, precision bombing attempts were dropped. Area bombing, like the RAF efforts, were widely used with 750 and then 1000 bomber raids against large targets. The main targets were railroad marshaling yards (27.4 percent of the bomb tonnage dropped), airfields (11.6 percent), oil refineries (9.5 percent), and military installations (8.8 percent). To some degree the targets were secondary missions; Doolittle used the bombers as an irresistible target to draw up Luftwaffe

Luftwaffe

Luftwaffe is a generic German term for an air force. It is also the official name for two of the four historic German air forces, the Wehrmacht air arm founded in 1935 and disbanded in 1946; and the current Bundeswehr air arm founded in 1956....

fighters into the ever-increasing swarms of Allied long-distance fighters. As these missions broke the Luftwaffe, missions were able to be carried out at lower altitudes, especially in bad weather when the H2X radar

H2X radar

H2X radar was an American development of the British H2S radar, the first ground mapping radar to be used in combat. It was used by the USAAF during World War II as a navigation system for daylight overcast and nighttime operations...

could be used. In spite of abandoning precision attacks, accuracy nevertheless improved. By 1945, the 8th was putting up to 60 percent of its bombs within 1,000 feet, a CEP of about 900 feet.

Postwar analysis placed the overall accuracy of daylight precision attacks with the Norden at about the same level as radar bombing efforts. The 8th Air Force put 31.8 percent of its bombs within 1,000 feet from an average altitude of 21,000 feet, the 15th Air Force averaged 30.78 percent from 20,500 feet, and he 20th Air Force against Japan averaged 31 percent 16,500 feet.

Many factors have been put forth to explain the Norden's poor real-world performance. Over Europe, the cloud cover was a common explanation, although performance did not improve even in favorable conditions. Over Japan, bomber crews soon discovered strong winds at high altitudes, the so-called jet stream

Jet stream

Jet streams are fast flowing, narrow air currents found in the atmospheres of some planets, including Earth. The main jet streams are located near the tropopause, the transition between the troposphere and the stratosphere . The major jet streams on Earth are westerly winds...

s, 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

Trajectory

A trajectory is the path that a moving object follows through space as a function of time. The object might be a projectile or a satellite, for example. It thus includes the meaning of orbit—the path of a planet, an asteroid or a comet as it travels around a central mass...

of bombs that reached supersonic speeds during their fall.

Still pursuing precision attack, various remotely guided weapons were developed, notably the AZON

Azon

AZON was one of the world's first smart bombs, deployed by the Allies and contemporary with the German Fritz X.Officially designated VB-1 , it was invented by Major Henry J. Rand and Thomas J...

and RAZON bombs and similar weapons.

Production, Army standardization

The conversion of the company's New York City engineering lab to a production factory was a long process. Before the war, skilled craftsmen, most of them German or Italian immigrants, hand-made almost every part of the 2,000-part machine. Between 1932 and 1938, the company produced only 121 bombsights per year. During the first year after the Attack on Pearl Harbor

Attack on Pearl Harbor

The attack on Pearl Harbor was a surprise military strike conducted by the Imperial Japanese Navy against the United States naval base at Pearl Harbor, Hawaii, on the morning of December 7, 1941...

, Norden produced 6,900 bombsights, three-quarters of which went to the Navy.

By May 1943 the Navy was complaining that they had a surplus of devices, and full production was turned over to the Army Air Forces. After investing more than $100 million in Sperry bombsight manufacturing plants, the AAF concluded that the Norden M-series was far superior in accuracy, dependability, and design. Sperry contracts were canceled in November 1943. When production ended a few months later, 5,563 Sperry bombsight-autopilot combinations had been built, most of which were installed in B-24 Liberator bombers.

Expansion of the production to a final total of six factories took several years. The Army Air Forces demanded additional production to meet their needs, and eventually arranged for the Victor Adding Machine company to gain a manufacturing license, and then Remington Rand

Remington Rand

Remington Rand was an early American business machines manufacturer, best known originally as a typewriter manufacturer and in a later incarnation as the manufacturer of the UNIVAC line of mainframe computers but with antecedents in Remington Arms in the early nineteenth century. For a time, the...

. Ironically, during this period the Navy abandoned the Norden in favour of dive bombing, reducing the demand. By the end of the war, Norden and its subcontractors had produced 72,000 M-9 bombsights for the Army Air Force along, costing $8,800 each.

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

Thermite

Thermite is a pyrotechnic composition of a metal powder and a metal oxide that produces an exothermic oxidation-reduction reaction known as a thermite reaction. If aluminium is the reducing agent it is called an aluminothermic reaction...

grenade was installed; the heat of the chemical reaction would melt the Norden into a lump of metal. The Douglas TBD Devastator torpedo bomber

Torpedo bomber

was originally equipped with flotation bags in the wings to aid the aircrew's escape after ditching

Water landing

A water landing is, in the broadest sense, any landing on a body of water. All waterfowl, those seabirds capable of flight, and some human-built vehicles are capable of landing in water as a matter of course....

, but they were removed once the Pacific War

Pacific War

The Pacific War, also sometimes called the Asia-Pacific War refers broadly to the parts of World War II that took place in the Pacific Ocean, its islands, and in East Asia, then called the Far East...

began; this ensured that the aircraft would sink, taking the Norden with it.

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

Nissen hut

A Nissen hut is a prefabricated steel structure made from a half-cylindrical skin of corrugated steel, a variant of which was used extensively during World War II.-Description:...

(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

USAAF bombardment group

A bombardment group or bomb group was a group of bomber aircraft the United States Army Air Forces during World War II. It was the equivalent of an infantry regiment in the Army Ground Forces, or a bomber wing in the British Commonwealth air forces...

. 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

Non-commissioned officer

A non-commissioned officer , called a sub-officer in some countries, is a military officer who has not been given a commission...

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.

Espionage

In spite of the security precautions, the entire Norden system had been passed to the Germans before the war started. Herman W. Lang, a German spy, 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

Duquesne Spy Ring

The Duquesne Spy Ring is the largest espionage case in United States history that ended in convictions. A total of thirty-three members of a German espionage network headed by Frederick "Fritz" Joubert Duquesne were convicted after a lengthy espionage investigation by the Federal Bureau of...

, was arrested by the FBI

Federal Bureau of Investigation

The Federal Bureau of Investigation is an agency of the United States Department of Justice that serves as both a federal criminal investigative body and an internal intelligence agency . The FBI has investigative jurisdiction over violations of more than 200 categories of federal crime...

and convicted in the largest espionage

Espionage

Espionage or spying involves an individual obtaining information that is considered secret or confidential without the permission of the holder of the information. Espionage is inherently clandestine, lest the legitimate holder of the information change plans or take other countermeasures once it...

prosecution in U.S. history. He received a sentence of 18 years in prison on espionage charges and a two-year concurrent sentence under the Foreign Agents Registration Act

Foreign Agents Registration Act

The Foreign Agents Registration Act is a United States law passed in 1938 requiring that agents representing the interests of foreign powers be properly identified to the American public. The act was passed in response to German propaganda in the lead-up to World War II...

.

German instruments were actually fairly similar to the Norden, even before World War II. 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. The Carl Zeiss Lotfernrohr 7

Lotfernrohr 7

, or Lotfe 7, was an advanced mechanical system similar to the U.S' Norden bombsight, or in form to the Sperry S-1. It started replacing the simpler Lotfernrohr 3 and BZG 2 in 1942, and emerged as the primary late-war bombsight used in most Luftwaffe
Luftwaffe
Luftwaffe is a generic German term for an air force. It is also the official name for two of the four historic German air forces, the Wehrmacht air arm founded in 1935 and disbanded in 1946; and the current Bundeswehr air arm founded in 1956....

level bombers. The use of the autopilot allowed single-handed operation, and was key to bombing use of the single-crewed Arado Ar 234

Arado Ar 234

The Arado Ar 234 was the world's first operational jet-powered bomber, built by the German Arado company in the closing stages of World War II. Produced in very limited numbers, it was used almost entirely in the reconnaissance role, but in its few uses as a bomber it proved to be nearly impossible...

Postwar use

In the postwar era the development of new precision bombsights essentially ended. At first this was due to the military drawdown, but as budgets increased again during the opening of the Cold War

Cold War

The Cold War was the continuing state from roughly 1946 to 1991 of political conflict, military tension, proxy wars, and economic competition between the Communist World—primarily the Soviet Union and its satellite states and allies—and the powers of the Western world, primarily the United States...

, the bomber mission had passed to nuclear weapons. These required accuracies on the order of 3,000 yards, well within the capabilities of existing radar bombing systems. Only one major bombsight of note was developed, the Y-4 developed on the B-47 Stratojet

B-47 Stratojet

The Boeing Model 450 B-47 Stratojet was a long-range, six-engined, jet-powered medium bomber built to fly at high subsonic speeds and at high altitudes. It was primarily designed to drop nuclear bombs on the Soviet Union...

. This sight combined the images of the radar and a lens system in front of the aircraft, allowing them to be directly compared at once through a binocular eyepiece.

Bombsights on older aircraft, like the B-29 Superfortress

B-29 Superfortress

The B-29 Superfortress is a four-engine propeller-driven heavy bomber designed by Boeing that was flown primarily by the United States Air Forces in late-World War II and through the Korean War. The B-29 was one of the largest aircraft to see service during World War II...

and the later B-50

Boeing B-50 Superfortress

The Boeing B-50 Superfortress strategic bomber was a post-World War II revision of the Boeing B-29 Superfortress, fitted with more powerful Pratt & Whitney R-4360 radial engines, stronger structure, a taller fin, and other improvements. It was the last piston-engined bomber designed by Boeing for...

, were left in their wartime state. When the Korean War

Korean War

opened these aircraft were pressed into service and the Norden once again became the USAF's primary bombsight. This occurred again when the Vietnam War

Vietnam War

started, in this case retired WWII technicians had to be called up in order to get the bombsights operational. Its last use in combat was by the Naval Air Observation Squadron Sixty-Seven (VO-67

VO-67

Observation Squadron 67 was a secret United States Navy military intelligence aircraft squadron based in Thailand during the Vietnam War. Created in February 1967, the unit was deactivated in July 1968...

), during the Vietnam War. The bombsights were used in Operation Igloo White

Operation Igloo White

Operation Igloo White was a covert United States Air Force electronic warfare operation conducted from late January 1968 until February 1973, during the Vietnam War. This state-of-the-art operation utilized electronic sensors, computers, and communications relay aircraft in an attempt to automate...

for implanting Air-Delivered Seismic Intrusion Detectors (ADSID) along the Ho Chi Minh Trail

Ho Chi Minh trail

The Ho Chi Minh trail was a logistical system that ran from the Democratic Republic of Vietnam to the Republic of Vietnam through the neighboring kingdoms of Laos and Cambodia...

Description and operation

Background

Typical bombsights of the pre-war era worked on the "vector bombsight" principle introduced with the World War I

World War I

Course Setting Bomb Sight

The Course Setting Bomb Sight is the canonical "vector" bombsight, the first practical system for properly accounting for the effects of wind during the dropping of bombs...

. These systems consisted of a slide rule

Slide rule

The slide rule, also known colloquially as a slipstick, is a mechanical analog computer. The slide rule is used primarily for multiplication and division, and also for functions such as roots, logarithms and trigonometry, but is not normally used for addition or subtraction.Slide rules come in a...

-type calculator that was used to calculate the effects of the wind on the bomb based on simple vector arithmetic. The principles are identical to those on the E6B

E6B

The E6B Flight Computer, or simply the "whiz wheel", is a form of circular slide rule used in aviation. They are mostly used in flight training, but many professional and even airline pilots still carry and use these flight computers...

calculator used to this day. In operation, the bombardier would first take a measurement of the wind speed by various methods, and then use that speed and direction to calculate the effects of the wind on the bomb after it was dropped. The aircraft's speed and direction were then used to set up an iron sight

Iron sight

system, then adjusted for the wind effects.

These systems had two primary problems in terms of accuracy. The first was that there were several steps that had to be carried out in sequence in order to set up the bombsight correctly, and there was limited time to do all of this during the bomb run. As a result, the accuracy of the wind measurement was always limited, and errors in setting the equipment or making the calculations were common. The second problem was that the sight was attached to the aircraft, and thus moved about during manoeuvres, during which time the bombsight would not point at the target and could not be used. As the aircraft had to manoeuvre in order to make the proper approach to the target, this limited the time allowed to accurately make corrections. This combination of issues demanded a long bomb run.

Experiments had shown that adding a stabilizer system to a vector bombsight would roughly double the accuracy of the system. This would allow the bombsight to remain level while the aircraft manoeuvred, giving the bombardier more time to make their adjustments, as well as reducing or eliminating mis-measurements when sighting off of non-level sights. However, this would not have any effect on the accuracy of the wind measurements, nor the calculation of the vectors. The Norden attacked all of these problems.

Basic operation

To improve the calculation time, the Norden used a mechanical computer

Mechanical computer

inside the bombsight to calculate the range angle of the bombs. By simply dialling in the aircraft's altitude and heading, along with estimates of the wind speed and direction (in relation to the aircraft), the computer would automatically, and quickly, calculate the aim point. This not only reduced the time needed for the bombsight setup, but also dramatically reduced the chance for errors. This attack on the accuracy problem was by no means unique, several other bombsights of the era used similar calculators. It was the way the Norden used these calculations that was different.

Conventional bombsights are set up pointing at a fixed angle, the range angle, which accounts for the various effects on the trajectory of the bomb. Looking through the sights, its crosshairs indicate the location on the ground where the bombs would impact if released at that instant. As the aircraft moves forward, the target approaches the crosshairs from the top, and the bombardier releases the bombs as it passes through them. One example of a highly automated system of this type was the RAF's Mark XIV bomb sight

Mark XIV bomb sight

The Mark XIV Computing Bomb Sight is a vector bombsight developed and used by the Royal Air Force's Bomber Command during World War II. The bombsight was also known as the Blackett sight after its primary inventor, P.M.S. Blackett...

.

The Norden worked in an entirely different fashion, the "synchronous" or "tachometric" method. Internally, the calculator continually computed the impact point, as was the case for previous systems. However, the resulting range angle was not displayed directly to the bombardier or dialled into the sights. Instead, the bombardier used the sighting telescope to locate the target long in advance of the drop point. The calculator used the inputs for altitude and airspeed to determine the angular velocity of the target, and then used a rotating prism

Prism

-Science and mathematics:* Prism , a transparent object which refracts light** Dispersive prism, the most familiar type of optical prism* Prism , a kind of polyhedron* Prism , a type of sedimentary deposit-Books, comics and magazines:...

to attempt to keep the target centred. So while conventional bombsights waited for the visibly moving target to approach the fixed range angle, with the Norden, one waited for the visibly fixed target to approach the (hidden) moving range angle.

It was the difference between the two calculated values, the current range angle (to the impact point) and the current angle to the target measured from the telescope, that determined the direction and distance the aircraft had to travel in order to take it over the proper drop point. As the bomber approached the target, the difference between the range and target angles would be continually reduced, eventually to zero (within the accuracy of the mechanisms). At this moment the Norden automatically dropped the bombs.

The reason for this additional complexity was that the target generally did not stay centred in the sighting telescope when it was first set up. Instead, due to inaccuracies in the estimated wind speed and direction, the target would drift out of the crosshairs in the sight. To correct for this, the bombardier would use fine-tuning controls to slowly cancel out any motion through trial and error

Trial and error

Trial and error, or trial by error, is a general method of problem solving, fixing things, or for obtaining knowledge."Learning doesn't happen from failure itself but rather from analyzing the failure, making a change, and then trying again."...

. These adjustments had the effect of updating the measured ground speed used to calculate the motion of the prisms, slowing the visible drift. Over a short period of time of continual adjustments, the drift would stop, and the bombsight would now hold an extremely accurate measurement of the exact groundspeed and heading. Better yet, these measurements were being carried out on the bomb run, not before it, it helped eliminate inaccuracies due to changes in the conditions as the aircraft moved. And by eliminating the manual calculations, the bombardier were left with much more time to adjust their measurements, and thus settle at a much more accurate result. Conventional systems would estimate wind speed using a drift telescope or dead reckoning

Dead reckoning

In navigation, dead reckoning is the process of calculating one's current position by using a previously determined position, or fix, and advancing that position based upon known or estimated speeds over elapsed time, and course...

, but this was time consuming to calculate, and did not offer anywhere near the accuracy of the Norden.

The angular speed of the prism changes with the range of the target; consider the reverse situation, the apparent high angular speed of an aircraft passing overhead compared to its apparent speed when it is seen at longer distance. In order to properly account for this non-linear effect, the Norden used a system of slip-disks similar to those used in differential analyser

Differential analyser

The differential analyser is a mechanical analogue computer designed to solve differential equations by integration, using wheel-and-disc mechanisms to perform the integration...

s. However, this slow change at long distances made it difficult to fine tune the drift early in the bomb run. In practice, bombardiers would often set up their groundspeed measurements in advance of approaching the target area by selecting a convenient "target" on the ground that was closer to the bomber and thus had more obvious motion in the sight. These values would then be used as the initial setting when the target was later sighted.

System description

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 largely 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 "up". This was accomplished in a time consuming process of comparing the platform's attitude to small spirit level

Spirit level

s 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.

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

Reflector sight

A reflector or reflex sight is a generally non-magnifying optical device that allows the user to look through a partially reflecting glass element and see an illuminated projection of an aiming point or some other image superimposed on the field of view...

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, their other arm was normally busy holding them 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 350 feet per second (106.7 m/s), so even minor interruptions in timing could dramatically affect aim.

Early examples, and those in Navy use, 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 USAAC use, the entire bombsight was attached to a second device, the "Automatic Flight Control Equipment" (AFCE), an autopilot

Autopilot

system. The AFCE could be used during the flight to the target area through a control panel in the cockpit, but was more common used under direct command of the bombardier. The AFCE sat behind and below the Norden and attached to it at a single rotating point. On the bomb run, the bombardier would first rotate the entire Norden so the vertical line in the sight passed through the target, and then clutched in the AFCE. From that point on, the AFCE 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. The AFCE 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 that the pilots could.

Later in the war the Norden was combined with other systems to widen the conditions for successful bombing. Notable among these as the radar

Radar

system called the H2X (Mickey)

H2X radar

, 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.

External links

Flight 1945 Norden Bomb Sight
How the Norden Bombsight Does Its Job by V. Torrey, June 1945 Popular Science
Popular Science
Popular Science is an American monthly magazine founded in 1872 carrying articles for the general reader on science and technology subjects. Popular Science has won over 58 awards, including the ASME awards for its journalistic excellence in both 2003 and 2004...
"The Bombsight That Thinks." Popular Mechanics, February 1945, p.7-10.
Norden Bombsight from the Hill Aerospace Museum
U.S. Centennial of Flight Commission
images and information from twinbeech.com
WWII AAF collection (Bombardiers Information File)
463rd Sub Depot, 389th Bomb Group
(Movies of the Norden and Sperry Bombsights in operation)
Articles and Links from Norden Club website
Item on use of Norden Bombsight in WW II
Malcolm Gladwell: The strange tale of the Norden bombsight at TED

Accessed: 2007–05–12

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Early work

First bombsight design

Fully automatic bombsight

Autopilot, production problems

British interest, Tizard mission

Entering combat

Air war in Europe

Production, Army standardization

Wartime security

Espionage

Postwar use

Description and operation

Background

Basic operation

System description

See also

Further reading

External links