Norden Bombsight

“It is not very difficult to hit a target from an altitude of 30,000 feet.”

Theodore H. Barth, Norden Bombsight Co.

Great advances are seldom the products of a single mind; rather they arise from lore and facts previously known. Antonie van Leeuwenhoek, a Dutch draper in the seventeenth century was working in his father’s shop when he wanted a better way of seeing the quality of threads they were using than the then current magnifying lenses. His curiosity led to one of the most significant and technical developments in the history of science: the microscope with its magic enlargements. Countless scientists have profited by van Leeuwenhoek’s curiosity and subsequent knowledge.

Two hundred years after Leeuwenhoek, Joseph Lister, a professor of surgery at Glasgow University, learned more about the French chemist Louis Pasteur’s experiments which showed that fermentation and food spoilage could occur under anaerobic conditions if micro-organisms were present. Aware that an aerobe was an organism that required oxygen to live, Lister undertook experiments on his own to confirm the findings reported by Pasteur. It was a time when wounds or injuries of all sorts could be fatal, for often openings into the skin would become infected, swell horribly, and lead to death.

Lister learned that deaths following a wound came from infections which would start soon after a wound was inflicted, swell, and become increasingly sore. The infection seemed unstoppable as it spread throughout the body. The challenge, he decided, lay not in treating the infection but to stop it from occurring.

Upon learning that carbolic acid was used to keep wood in ships and railway ties from rotting, he decided to try the substance on treatment of wounds; there ought to be a way to use “anti-septic” treatments for wounds. By spraying a solution of carbolic acid on surgical instruments and incisions, Lister found the incidence of gangrene remarkably reduced. Other doctors and scientists quickly picked up what he had learned. In the centenary of his death in 2012, Joseph Lister was hailed by most persons in the medical field as “the father of modern surgery.”

Before the Wright brothers made their epochal flight at Kitty Hawk, North Carolina, both had studied pamphlets sent them upon from the Smithsonian Institute in Washington. The Institute had amassed reports on balloons, dirigibles, and whatever previous efforts to fly had been made. Thus, Orville and Wilbur didn’t start entirely afresh; they used knowledge gained by predecessors.

In another instance, Guglielmo Marconi, a young man near Bologna, Italy, had an insatiable interest in science and electricity. A scientific finding known then had come from Heinrich Hertz, a German physicist who in 1888 demonstrated that it was possible to produce and detect magnetic radiation — generally known as radio waves and commonly called Hertzian waves. Marconi, using knowledge he had gained about these radio waves, built his own equipment and began experimenting. His goal was to use the radio waves in creating a practical system of “wireless telegraphy.” It wasn’t long before he had gained enough understanding and had built suitable equipment to transmit signals a distance of 1.5 miles. Using knowledge from Hertz and others, Marconi had constructed new devices capable of spanning great distances — devices which would be invaluable both commercially and militarily.

James Watt recognized the latent power of steam, Robert Fulton showed how steam power could move a boat, and the Wright brothers, using knowledge others had given them about gasoline engines, could propel an airplane through the sky. And so it goes; a scientist stands on the shoulders of predecessors, who give him enough working facts that he can add another step to the scientific ladder.

History will tell us of the role numbers play in our lives, and we will find that a great deal of our knowledge about numbers — numbers which underlie radio, television, compasses, shopping malls, medicines, computers — the list is inexhaustible — goes back to the theories of Pythagoras, a somewhat mysterious figure who lived 450 years before Christ. We remember Pythagoras particularly for his theories and teachings about ratios — the groundwork for modern trigonometry. His most fundamental theory is one asserting that in a right angle triangle, the square of the hypotenuse is equal to the square of the other two sides, and it is this tenet that makes high level precision bombing possible.

In 1939, U. S. diplomats declared the nation’s goal would be non-intervention and its primary military objective would be defense of the U.S. and possessions in the Western Hemisphere. The rationale encouraged aircraft manufacturers to put more efforts in producing fighter planes than bigger ones for bombing or transports. To fulfill the ambitions of visionaries who argued for longer range bombardment, aircraft for that purpose would be a more expensive project in both time and money.

Confronting these hurdles was a batch of brash devotees such as Henry Arnold, Curtis LeMay, and James Doolittle, along with other officers of their ilk who argued for alternate plans. According to these young Turks, Flying Fortresses and better bombsights were near at hand and would make air bombardment invaluable to whatever defensive or offensive strategy the U.S. might choose.

Understandably, the Navy had responsibility for sea-based protection of the U.S. coastlines, while the Army had the responsibility for land-based defenses. The increasing range of aircraft upset this tidy division because the Navy put aircraft aboard ships and the Army extended its operations well beyond the shores. The expansions led to rivalry and duplication between the two services. Yet the rupture also encouraged developing new technology for both aircraft and equipment.

The Army Air Corps was able to obtain the Norden Bombsight — an instrument which, along with the four-engine Flying Fortresses, would revolutionize high level precision bombing. Equipped with a correctly calibrated Norden Bombsight, a high-flying plane leveled in smooth air could put a bomb precisely on a target below. Even though that accuracy was much greater than any hitherto known, Air Corps devotees exaggerated it, and it was in this connection with the Norden Sight that the phrase “pickle barrel” was coined. General Henry “Hap” Arnold, highest commanding officer in the U.S. Army Air Corps during WW II, said it was like “tossing it {the bomb} into a pickle barrel.” The analogy got a big boost when the Ringling Bros. and Barnum and Bailey Circus rented Madison Square Garden to put on their show. One of the most popular skits was to have two clowns on a low trapeze approach a barrel on which they dropped a huge air-filled balloon shaped like a bomb onto the barrel. Out popped a huge green pickle!

Yet even with such ballyhoo, the Norden Bombsight would remain one of the best-kept secrets of World War II — second only to the Manhattan Project.

To protect its secrecy, the U.S. refused to share the Norden sight with the British for fear it might fall into enemy hands. The instrument was so critical that on bases where aspiring U.S. cadets were being trained as bombardiers, each was required to swear an oath stating he would defend the bombsight’s secret with his own life if necessary. In case the plane had to make an emergency landing in enemy territory, the bombardier was to fire bullets from his .45 caliber handgun into the rate end of the Norden.

While in bombardier training, before each flight a U.S. cadet first went to a bomb vault, typically a pit alongside a Nissen or Quonset hut serving as a maintenance shop, where he was given a canvas bag containing the Norden Bombsight. The cadet would sign an acceptance sheet and carry bag and sight to his assigned aircraft. There he would connect the sight to a stabilizer left mounted and ready in the plexi-glass nose of the ship, most often a Beechcraft AT-11.

A bombsight shop was staffed and operated 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 of them and related control equipment. It was probably the most technically skilled ground-echelon job, and one of the most secret, of all work performed by Sub Depot personnel.

Carl Lukas Norden was a Netherlands-born engineer who had emigrated to America and worked for the Sperry Gyroscope Company before WW I. Recognizing his expertise with gyroscopes — instruments fundamental to any high level precision bombing — U.S. Navy officials indirectly guided his work and impeded efforts by the Army Air Corps to share in it.

Navy officials rationalized their arguments by recounting their experience in launching bombs from a sea going ship. True, there were similarities between bombardments sent from a naval vessel and those dropped from an airplane, but there were differences which the unlearned might consider small but to those with more understanding were critical. In both cases, bombs once launched were unguided except for minor effects coming from air currents, and size, shape, and weight of the projectile.

A major difference between the two circumstances was the propelling force for the projectiles. From a vessel on the water, the force comes from power exerted by a big gun and the ammunition it fires to blast the projectile onto its trajectory. Bombs dropped from an airplane have no such explosive power behind them and are subject to the pull of gravity on their size, weight, and shape as well as to the heading and speed of the aircraft delivering them.

Air bombardment crews found it necessary to use special vocabularies, and to understand bombing theory some of these key terms should be explained. Some persons think a plane flies directly over a target if the bomb drop is to be accurate. This is entirely wrong. In order to release a bomb so it will hit a chosen target, the exact point of release must be determined, but one must also remember that a bomb dropped from an aircraft doesn’t fall in a vacuum. There is always air current or wind to consider as well as gravity and true air speed. To drop unguided bombs accurately, two problems must be solved: course and range.

Course or track as it sometimes is called is not the direction the aircraft is pointed in flight but is the actual path on the ground over which the plane flies. As everyone who has sailed or rowed a boat knows, a boat must be turned into the wind to reach a given destination. Likewise, an aircraft must head or crab into the wind to establish a desired course. In air terminology, such crabbing is known as its heading, and one must understand that a dropped bomb falls directly behind the heading of the plane or its fuselage, irrespective of the course or track it is following.

Long before aerial bombing got underway, navies, by using compasses and gyroscopes, had pretty well solved the problems of course; solving problems of range were a different challenge.

A primary factor in finding the actual range, defined as the horizontal distance along the ground a bomb travels from the point of its release to its point of impact, is the matter of gravity. Gravity pulls the bomb toward the earth at a continuously increasing speed. A falling bomb, or any missile for that matter, is subject to the attraction of gravity and of the amount of time or the distance through which it falls. This increase of speed is called the acceleration of gravity, approximately 32.2 ft. per second, meaning that during each second the falling body will increase its speed by 32.2 ft.

In order to hit a selected aiming point, a bomb must be released an exact distance back from the target so that it will not fall short or over. In bombing lexicon, distances on the ground are measured in feet or angular measurements expressed in mils; a mil is the angle subtended by an arc whose length is one-thousandth of the distance from the observer to the object.

The first step in a bomb run is for the operator of the bombsight to find true levels of the aircraft, both vertical and horizontal. As the operator faces the Norden, on its left side is a circular glass pane about three inches in diameter. Through this glass can be seen two small tubes with a bubble in each to represent true horizontal and vertical levels. The bubbles reflect small gyros which can hold a given position. By centering each bubble, the bombardier establishes true levels of the aircraft — both horizontal and vertical — regardless of its yaw or pitch. An error either way will cause an unguided bomb to fall short, over, or wide to the right or left.

A tenet of physics reminds us that a body in motion tends to remain in motion, and we should remember that a bomb carried by an airplane in flight has the same forward velocity as the plane. A bomb is moving forward when it leaves the plane, and for a split second the bomb travels forward beneath the belly of the ship before gravity begins to change the forward motion and pull the missile toward the earth at a continuously increasing speed downward — 32 ft. per second until terminal velocity is reached.

A third force, air resistance, affecting a dropped bomb acts against the first two. Airspeed of the plane moves the bomb inside it forward with the same speed. The horizontal distance on the ground over which a plane flies between the time a bomb is released and the time it hits the earth constitutes its whole range; the distance on the ground between the vertical line marking plane and earth and the point of impact is its actual range. The distance between the point of impact and where the airplane is at the time marks the difference between actual and whole range, and in bombing jargon is known as trail.

Whole and Actual Range

These three factors were the primary ones in determining whether an unguided bomb would hit short or beyond the operator’s aiming point, and there are other factors determining whether the bomb would hit wide of the target. Since an aircraft does not fly in a vacuum, the wind around it will have an effect, and certain terminology must be understood. The difference between a plane’s heading and its course or track over the ground is called its drift — right or left — the true angle between heading and course.

Another aspect of the air’s effect is the crosswind. If winds are from any direction except from dead ahead or directly behind the aircraft, a drift angle enters the bombing problem. If the wind comes from the right, the plane also heads or crabs to the right and vice versa if the wind comes from the left. Thus, the bombing aircraft is turned into the wind, and WW II bombardiers learned quickly that a dropped bomb struck the ground behind the plane along its longitudinal axis and downwind of its true course.

Wind and Crosswind

For five or more years prior to the outbreak of WW II, the U.S. Navy and U.S. Air Forces vied for control of whatever bombsight was to be used. The Navy had the advantages of a longer history along with contracts for the Sperry bombsight and its gyroscopes. The Navy also had more high-ranking officers among militarists in Washington, D. C. than did the younger Air Corps.

The Sperry bombsight required the same data such as altitude, azimuth, level, true air speed, and adjustments to measure ground speed, but in size was bulkier than the Norden. Sperry sights were installed in many early versions of B24s, and some bombardiers who used them claimed they could synchronize with it faster than they could on the Norden; however, the Norden, once leveled and synchronized on a target, was far more accurate, particularly at higher altitudes. Sperry bombsights also had a tiny hi-lo switch almost hidden, and if this switch was in the wrong position for the actual bombing altitude, the bombs would hit either long or short of the target.

Navy brass kept repeating that bombs dropped from the air could best serve as a tactical arm for seagoing vessels or ground attacks by infantry; air bombardment, they argued, could not be a decisive offensive factor. The Air Corps confident with the Norden sight, challenged such denigration and in 1937 succeeded in getting permission to conduct bombing demonstrations. Rules for the exercises were simple; the Air Corps was given twenty-four hours to locate and drop water bombs on a battleship, in this case the U.S.S. Utah, which would be sailing off the coast of California between Los Angeles and San Francisco — roughly 120,000 square miles.

In the first two of three trials, B-17s equipped with Nordens were given erroneous position reports by the Navy and hence were unable to find the vessel they were supposed to bomb. In a third test, again spotters from the Navy gave wrong position reports to the fleet of B17s searching for the U.S.S. Utah. The 17s were led by Colonel Robert Olds who had selected Lt. Curtis LeMay as his navigator.

Young LeMay was a pilot who had chosen also to go through enough training to qualify as a navigator, and Col. Olds had chosen him as chief navigator for the fleet of 17s on that mission. Fortunately for the air crews, a few minutes before the deadline, the battleship came in sight.

When LeMay asked Olds for permission to drop the water bombs, Olds gave the OK signal. In the simulated “attack,” the B17s scored three direct hits and several near misses.

The test supported the Air Corps contentions, but the Navy immediately declared that in the interests of national security, both tests and results must be kept secret. Thus, the public never learned about them until well after the war ended.

Not immune to rivalries, Curtis LeMay, in part relative to those tests, held negative feelings toward the Navy that persisted throughout his life. “The whole thing was too utterly damning,” he would remember.

By 1940, the Norden bombsight under trained operators was replacing Sperry bombsights; nearly every Flying Fortress used for bombing was equipped with a Norden. As with most secrets, fantastic stories about it sprouted and were repeated everywhere. Bombardiers, it was said, had to have special diets because acids from ordinary foods would filter down into their fingers and through them contaminate the delicate inner workings of the sight. A few wiseacres said crosshairs in the Norden were strands from webs of the black widow spider. Other soldiers insisted the crosshairs in the sight’s telescope were really dyed tresses from blond Betty Grable — favorite pin-up girl for thousands of men wearing khaki. In latrines and elsewhere at other bases, soldiers swore they had it on the very best authority that the crosshairs were from the other Hollywood siren, Rita Hayworth.

Neither anecdote was accurate, for in truth the Norden Bombsight never had hair or spider webs for crosshairs; instead, the crosshairs were etched on the glass in the telescope and then painted. With this action, the “hairs” contrasted more sharply with the field around them in the telescope and were better illuminated for night bombing, if that were used.

Legends and exaggerations about the Norden’s accuracy, its structure, and capabilities would flourish, but despite such embellishments, there could be no denying that it was an instrument that made possible the precision bombing and strategic warfare conducted by U.S. Air Forces throughout WW II. The original order for B17s in 1934 had been thirteen; by the end of the war more than 12,000 of the Flying Fortresses would be built.

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