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The Norden bombsight is arguably one of the most important technological advancements to come out of WWII. Even though Carl Lucas Norden had built a prototype and was developing his bombsight well before, it was its use during WWII which made it famous. Even today people still marvel at the super secret Norden Bombsight. 

The development and use of this great computer is a fascinating story. In time, I will present that story on this web site. If you have any information, photos or stories that you would like to contribute please send it to me as I would like to have as much information posted here as possible.

 

 

This Is The Norden Bombsight

 

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In fact this is the probably the most famous Norden Bomb Sight of them all. Victor 4120, an M9B, is the sight that was used by Tom Ferebee to drop the first atomic bomb on Hiroshima on August 6th, 1945. This sight is presently installed in the nose of the most famous B-29; the Enola Gay, which resides in the National Air and Space Museum at Dulles Airport in Washington DC.

 

 

 

 

 

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This particular version was typical of what was used toward the end of WWII and throughout the Korean Conflict. This is an M-9B bombsight head (L-9772) manufactured by Lucas Harold Incorporated. The sight head has a Maxon X-1 Reflex Sight and the tachometer adapter for the Glide Bombing Attachment. The sight head is mounted on top of a Lucas Harold Stabilizer (L-9404). The stabilizer has a Directional Panel and a Directional Arm lock both of which are used with the C-1 autopilot. On top of the stabilizer is an Automatic Bombing Computer with the HH-1 Tangent Scale Kit. The stabilizer is mounted to a B-17 bombsight mount.

 

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This is a close up of the bombsight head pictured above. The sight head contains the computer which calculates the point in space where the bombs will be released. Below the rubber eyepiece is the optics cradle which the bombardier sights through. The optics cradle is connected to a gyro which keeps the optics stabilized with reference to the ground.

 

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Here is an excerpt from the Bombardier's Information File (BIF) that describes the components and controls of the Norden Bombsight.

The Norden bomb sight is made up of two principal parts; the sight head (pictured above) and the stabilizer (pictured below) which the sight head mounts on top of. The stabilizer is also part of the autopilot (SBAE, AFCE and C-1type auto pilot) and has a gyro that senses deviation about the yaw, or vertical axis of the airplane. The stabilizer is an essential part of the autopilot and will function without the sight  head installed. 

Many people know about the bomb sight head but few seem know that the stabilizer is required for the bomb sight to function and to be complete. You may have a sight head but in order to have a complete bomb sight you will need the stabilizer. The stabilizer was usually mounted to some form of vibration isolating bomb sight mount. Sometimes the bombsight mount was made by the aircraft manufacturer such as in the PBY Catalina, TBM Avenger and other Navy aircraft. The Army seems to have used a standardized mount in its aircraft. The early war mount was usually some form of the B-6 mount, which was later replaced by the B-7. The B-7 mount is the typical mount used in all of the mid to late war Army aircraft like the B-17G, B-24, B-25,  A-26 and the B-29.

 

 

HIROSHIMA

 

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This is the A-6 bombsight trainer that was used to train student bombardiers on the ground. This was a highly advanced trainer as compared to the earlier A-2 system.

At the time of its bombing, Hiroshima was a city of some industrial and military significance. A number of military camps were located nearby, including the headquarters of the Fifth Division and Field Marshal Shunroku Hata's 2nd General Army Headquarters, which commanded the defense of all of southern Japan. Hiroshima was a minor supply and logistics base for the Japanese military. The city was a communications center, a storage point, and an assembly area for troops. It was one of several Japanese cities left deliberately untouched by American bombing, allowing an ideal environment to measure the damage caused by the atomic bomb. Another account stresses that after General Spaatz reported that Hiroshima was the only targeted city without prisoner of war (POW) camps, Washington decided to assign it highest priority.

The center of the city contained several reinforced concrete buildings and lighter structures. Outside the center, the area was congested by a dense collection of small wooden workshops set among Japanese houses. A few larger industrial plants lay near the outskirts of the city. The houses were of wooden construction with tile roofs, and many of the industrial buildings also were of wood frame construction. The city as a whole was highly susceptible to fire damage.

The population of Hiroshima had reached a peak of over 381,000 earlier in the war, but prior to the atomic bombing the population had steadily decreased because of a systematic evacuation ordered by the Japanese government. At the time of the attack the population was approximately 255,000. This figure is based on the registered population used by the Japanese in computing ration quantities, and the estimates of additional workers and troops who were brought into the city may be inaccurate.

 

NAGASAKI

 

On the morning of August 9, 1945, the U.S. B-29 Superfortress Bockscar, flown by the crew of 393rd Squadron commander Major Charles W. Sweeney, carried the nuclear bomb code-named "Fat Man", with Kokura as the primary target and Nagasaki the secondary target. The mission plan for the second attack was nearly identical to that of the Hiroshima mission, with two B-29's flying an hour ahead as weather scouts and two additional B-29's in Sweeney's flight for instrumentation and photographic support of the mission. Sweeney took off with his weapon already armed but with the electrical safety plugs still engaged.

Observers aboard the weather planes reported both targets clear. When Sweeney's aircraft arrived at the assembly point for his flight off the coast of Japan, the third plane (flown by the group's Operations Officer, Lt. Col. James I. Hopkins, Jr.) failed to make the rendezvous. Bockscar and the instrumentation plane circled for forty minutes without locating Hopkins. Already thirty minutes behind schedule, Sweeney decided to fly on without Hopkins.

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By the time they reached Kokura a half hour later, a 7/10 cloud cover had obscured the city, prohibiting the visual attack required by orders. After three runs over the city, and with fuel running low because a transfer pump on a reserve tank had failed before take-off, they headed for their secondary target, Nagasaki. Fuel consumption calculations made en route indicated that Bockscar had insufficient fuel to reach Iwo Jima and they would be forced to divert to Okinawa. After initially deciding that if Nagasaki were obscured on their arrival they would carry the bomb to Okinawa and dispose of it in the ocean if necessary, the weaponeer Navy Commander Frederick Ashworth decided that a radar approach would be used if the target was obscured

At about 07:50 Japanese time, an air raid alert was sounded in Nagasaki, but the "all clear" signal was given at 08:30. When only two B-29 Superfortresses were sighted at 10:53, the Japanese apparently assumed that the planes were only on reconnaissance and no further alarm was given.

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Here is a cut away of a Norden sight head.

A few minutes later, at 11:00, the support B-29 flown by Captain Frederick C. Bock dropped instruments attached to three parachutes.

At 11:01, a last minute break in the clouds over Nagasaki allowed Bockscar's bombardier, Captain Kermit Beahan, to visually sight the target as ordered. The "Fat Man" weapon, containing a core of ~6.4 kg (14.1 lb) of plutonium-239, was dropped over the city's industrial valley. 43 seconds later it exploded 469 meters (1,540 ft) above the ground exactly halfway between the Mitsubishi Steel and Arms Works in the south and the Mitsubishi-Urakami Ordnance Works (Torpedo Works) in the north. This was nearly 3 kilometers (2 mi) northwest of the planned hypocenter; the blast was confined to the Urakami Valley and a major portion of the city was protected by the intervening hills.

On August 12, the Emperor informed the imperial family of his decision to surrender.

 

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This is the Bomb Sight Stabilizer. The Bomb Sight Head mounts to the stainless steel gear and brass bushing on the upper right end of the stabilizer. The sight head will pivot about the yaw axis on the Stabilizer. The large gyro inside the Stabilizer will keep the sight pointed at the target should the aircraft deviate off course. The Stabilizer, being part of the autopilot, will turn the airplane back on course. The Bombardier controls the aircraft through the autopilot and will direct the aircraft on the bomb run. The connection to the C-1 autopilot is made through the Directional Panel which mounts on the left side of the Stabilizer as shown in the diagram below. The solenoid plate on the back of the Stabilizer is called the Directional Arm Lock and is a part of the C-1 Autopilot installation. It locks the directional arm during turns.

 

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This is a rare early Air Corps M-7 sight head. This M-7 has an Automatic Gyro Leveling Device (AGLD) mounted on the top of the gyro. One of the reasons the M-7 is a rarity is that most of these were overhauled and modified during WWII and the AGLD was removed because of difficulties in maintaining the system in the field. If you can zoom in you will see the AGLD mounted on the top of the gyro inside the circular window.

   

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This is a Mark 15 Mod 5 Norden manufactured sight which has the Automatic Erecting system installed. This sight head and stabilizer are one of the rare sets that have matching serial numbers. Norden sights were shipped as matching pairs that were adjusted at the factory. It is extremely rare these days to find sight heads that match their stabilizers. I know of two, this one and the one installed in the B-29 the Enola Gay at the National Air and Space Museum at their Dulles facility. If you know of any other sets please let me know as I would like to document them. This particular sight was installed in an Army Air Force Beech AT-11 Kansan bombardier trainer. I have added a different directional panel that was used in the US Navy's Stabilized Approach Bombing Equipment or SBAE autopilot. This sight was manufactured in April of 1942 and still has the date on the data plate. There was a technical directive that had the manufacturing dates removed from equipment so it wouldn't help the enemy to determine what improvements may have been made by the dates. You can see that the directional panel has two large knobs on it and an electrical connector to the stabilizer. These knobs are actually pulleys for metal bands that were used for a mechanical follow up system for the autopilot. These bands would connect to the vertical gyro. The early Automatic Flight Control System (or AFCE) as used by the Army Air Corps with the Norden M series sight used a mechanical follow up system as well. I understand that Minneapolis Honeywell developed an electrical follow up system that was incorporated into the later C-1 autopilot system. Also of note is the Automatic Bombing Computer (or ABC) that was installed on the top of the stabilizer. This computer allowed lateral evasive maneuvering on the bomb run without the bombardier having to re do his calculations. The greenish canvas bag held the scales for the ABC and would snap onto the rear of the stabilizer as shown.

 

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This is a close up view of the top of the gyro on the Mk15Mod5 sight. This sight was equivalent to the M-7 Army sight as shown above. This photo shows the Automatic Erecting System or AES. The Army called it the Automatic Gyro Leveling Device or AGLD. Very few surviving sights have this modification as most of them were removed, by technical order at the depot level, due to the complex maintenance required to keep them operating. Keeping the gyro level on the bomb run was critical; if the gyro was just a half a bubble out it would throw the bombs off the target an appreciable amount. In this example the leveling bubbles have been removed and would have been installed in the lower left corner. This system is very similar to the erecting system use in the vertical gyro of the C-1 autopilot which leads me to believe that it was designed by Minneapolis Honeywell.

 

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Here is a great rare wartime shot of two bombsight mechanics working on a Lucas Harold built sight head serial number L-581 and a Lucas stabilizer serial L-5037. Bombsights and stabilizers were originally issued as a matched set from the factory. They were intended to remain as a set but that practice soon became too difficult to maintain. A technical order was issued on how to match sights and stabilizers with different serial numbers. The main issue being the fit of the drift worm gear with the sector arm. A fascinating thing to take a look at is the board on the right side of the photo. It shows the various sights and stabs and what ship they were installed in. Of the thirteen sets on the board all have matching serials except the very set that they are working on. This photo is supposedly from the Burma Theatre of operations. It would be interesting to try and correlate the ship numbers (possibly the last three digits of the serial number) with the aircraft serial numbers to determine what type of aircraft these sights were in. It would be quite difficult but it would sure be interesting to figure it out.

 

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Another item that I am looking for is the Glide Bombing Attachment. This device was used to allow the bombardier to perform vertical evasive maneuvers while on the bomb run. This box controlled the disc speed inside the sight head and it was also connected to the pitot system of the aircraft. Inside the GBA was a sensitive bellows that would detect changes in altitude. As the aircraft would climb or descend the internal motor would drive the disc in the sight head through a flexible shaft connected to the tachometer adapter. If the aircraft was climbing the motor would turn slower and visa versa for a decent.  Normally the GBA was mounted on the back of the stabilizer over the gyro cover plate. If the C-1 autopilot was used the directional arm lock would prevent this installation and it would have to be mounted remotely. This remote installation was typical for almost all Army Air Force aircraft as it was a standard practice to have the C-1 installed whenever the Norden M series sight was used.

 

 

The Norden Bombsight

 

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.

 

Its Design & Operation

 

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The Norden bombsight

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

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 initially built at the Norden plant in New York City before the start of WWII and then at several other companies during the war. The device used a mechanical analog computer comprising electric motors, gyros, mirrors, bubble levels, gears and a small telescope.

The bombardier would input the necessary information, such as airspeed and altitude, and the bombsight would calculate the trajectory of the bomb being dropped. Near the target the aircraft would fly on autopilot to a precise position calculated by the bombsight and release the ordnance. Using this device, bombardiers could, in theory, drop their bombs within a 100-foot (ca 30m) circle from an altitude of well over 20,000 feet (ca. 7km). In combat, this accuracy was never achieved—because the Norden had been tested under "artificial conditions" at the US proving grounds, for example in the absence of anti-aircraft fire or adverse weather. 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.

 

 

The Norden's Operational Efficiency

 

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This view of a Norden bombsight shows the front of the device, with the (functional) gyroscope assembly visible through the plexiglass window on the left. The telescope for the bombsight is toward the upper center of the bombsight. To the right are five manual controls for altitude and airspeed.

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. As noted above, the bombsight's effectiveness is debated. Some argue that over typically cloud-covered Europe the Norden was nearly useless, and the British Bomber Command focused on nighttime bombing anyway. However, the United States Army Air Forces (USAAF) did both day and night bombing runs, depending on the distance; and many veteran B-17 and B-24 bombardiers swore by the Norden.

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A page from the Bombardier's Information File (BIF) that describes the components and controls of the Norden Bombsight.

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.

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, under which testing had been done. Additionally, the bombing altitude over Japan reached up to 30,000 feet — but most of the testing had been done well below 20,000 ft.

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 just not available in sufficient numbers (see below).

 

 

Wartime Security

 

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Photo of the AFCE and Bombsight shop ground crew in the 463rd Sub Depot affiliated with the USAAF 389th Bomb Group based at Hethel, Norfolk, England

As a critical wartime instrument, bombardiers were required to take an oath during their training stating that they would defend the Norden 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, disabling it; but as this method still would leave a nearly intact apparatus to the enemy, something like a thermite gun was installed - the sheer 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 dropping Air-Delivered Seismic Intrusion Detectors (ADSID) along the Ho Chi Minh Trail.

 

 

Espionage

 

FBI file photo

Herman W. Lang, a Nazi 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, 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.

 

 

 

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

 Wikipedia,

 

 

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