Bombsight - AbsoluteAstronomy.com

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 is roughly parabolic. However, there are additional factors such as changes in air density and wind

Wind

Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...

that may be considered, but these forces take some time to build up to a measurable effect, and are only a concern for bombs that spend a significant portion of a minute falling through the air. These effects can be minimized by reducing the fall time though low-level bombing or by increasing the speed of the bombs, both of which are combined in the 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...

. However, this also increases the danger to the bomber from ground-based defences, and accurate bombing from higher altitudes has always been desired. This has led to a series of increasingly sophisticated bombsight designs dedicated to high-altitude level bombing.

Since their first application prior to the First World War, bombsights have gone though several major revisions. The earliest systems were iron sights

Iron sights

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...

that were pre-set to an estimated fall angle. In some cases these consisted of nothing more than a series of nails hammered into a convenient spar, lines drawn on the aircraft, or alignments of certain parts of the structure. These were replaced by the earliest custom-designed systems, normally iron sights that could be set based on the aircraft's airspeed and altitude. These early systems were replaced by the vector bombsights, which added the ability to measure and adjust for winds. Vector bombsights were useful for altitudes up to about 3,000 m and speeds up to about 300 km/h. Starting in the 1930s, 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...

s with the performance needed to "solve" the equations of motion were incorporated into the new tachometric bombsights, the most famous being the Norden

Norden bombsight

The Norden bombsight was a tachometric bombsight used by the United States Army Air Forces and the United States Navy during World War II, and the United States Air Force in the Korean and the Vietnam Wars to aid the crew of bomber aircraft in dropping bombs accurately...

. In the postwar era, tachometric bombsights were often combined with 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...

systems to allow accurate bombing through clouds or at night. When studies demonstrated that bomb accuracy was roughly equal in these two systems, optical bombsights were generally removed and the role passed to dedicated radar bombsights. Finally, especially from the 1960s on, fully computerized bombsights were introduced, which combined the bombing calculations with navigation and mapping.

Modern aircraft do not have a bombsight, per se, featuring highly computerized systems that combine bombing, gunnery, missile fire and navigation into a single heads up display. These systems have the performance to calculate the bomb trajectory in real time

Real-time data

Real-time data denotes information that is delivered immediately after collection. There is no delay in the timeliness of the information provided. Real-time data is often used for navigation or tracking....

as the aircraft manoeuvres, and add the ability to adjust for weather, relative altitude, relative speeds for moving targets, and climb or dive angle. This makes them useful for both level bombing, as in earlier generations, as well as tactical missions formerly bombed by eye.

Forces on a bomb

It is a basic outcome of Newtonian mechanics that vertical and horizontal motion of a bomb can be considered separately. This makes understanding the motion of a bomb through the air much simpler. There are some effects that operate along the changing path of the bomb, but they are relatively minor and can be considered separately for a basic study.

To start with, consider just the vertical motion of a bomb. The bomb will be subject to two primary forces, gravity and drag

Drag (physics)

In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

, the first constant, and the second varying with the square of velocity. In the first instant the vertical motion is zero for an aircraft flying straight and level. In this case, the drag is zero, and the bomb accelerates downward due to gravity. As it falls the force of gravity remains constant, but as this leads to an increasing velocity, the drag force rapidly increases as well. At some point the force of drag will become equal to the force of gravity, and the bomb will reach terminal velocity

Terminal velocity

In fluid dynamics an object is moving at its terminal velocity if its speed is constant due to the restraining force exerted by the fluid through which it is moving....

. As the air drag will also vary with air density, and thus altitude, the terminal velocity will vary as the bomb falls. Generally the bomb will slow as it reaches lower altitudes where the air is denser, but the relationship is complex.

Now consider the horizontal motion. At the instant it leaves the shackles, the bomb carries the forward speed of the aircraft with it. This motion is countered by drag, which slows it down. Drag is at its greatest when the bomb first leaves the aircraft, and as it rapidly slows. As the forward motion slows, the drag force drops and the rate of slowing diminishes. The forward speed is never entirely bled off. If the bomb was not subject to drag its path would be purely ballistic and it would impact at an easily calculable point, the "vacuum range". In practice, the impact point is short of the vacuum range, and this real-world distance is known simply as the "range". The difference between the vacuum range and actual range is known as the "trail". The trail and range differ for different bombs due to their individual aerodynamics, and typically have to be measured on a bombing range.

The main complexity in completely separating the motion into vertical and horizontal is the terminal velocity. Bombs are designed to fly "nose forward", typically through the use of fins at the back of the bomb. It is the wind acting on the nose that causes drag, so the velocity that defines this is a factor of the angle of the bomb at any given instant. If the bomb is released at low speeds from the aircraft it will quickly turn vertical and the velocity will be defined largely by how long the bomb has been falling. However, if it is released at high speed it may already be close to terminal velocity, and can only accelerate vertically after bleeding off horizontal speed.Reaching terminal velocity tends to "flatten" the trajectory, keeping the bomb in the air longer, and thereby allowing the horizontal speed to act over a longer time, extending the range.

Finally, consider the effects of wind. Wind acts on the bomb through drag, and is thus a function of the wind speed. This is typically a fraction of the speed of the bomber or the terminal velocity, so it only becomes a factor at high altitudes where this small force has enough time to build up to a measurable value. However, as the bomber itself is also effected by the wind, the bomb will leave the aircraft with both the forward airspeed of the bomber, as well as any additional motion due to the wind. Even if the wind drops to zero immediately below the aircraft, this initial speed will be carried by the bomb during its fall. As the wind rarely does drop to zero in this fashion, in practice the effects of wind will continue to grow though the bomb's flight. The difference between the impact point and where it would have fallen if there had been no wind is known as "drift", or "cross trail".

The "bombsight problem"

In ballistics terms, it is traditional to talk of the calculation of aiming of ordinance as the "solution". The "bombsight problem" is the calculation of the location in space where the bombs should be dropped in order to hit the target given all of the effects noted above. Bombsights need to produce the solution to this problem.

In the absence of wind, the bombsight problem is fairly simple. The impact point is a function of three factors, the aircraft's altitude, its forward speed, and the terminal velocity of the bomb. In many early bombsights, these two inputs were adjusted by separately setting the front and back sights of an iron sight, one for the altitude and the other for the speed. Terminal velocity, which extends the fall time, can be accounted for by raising the effective altitude by an amount that is based on the bomb's ballistics.

When windage is accounted for the calculations become more complex. As the wind can operate in any direction, bombsights generally re-calculate the windage by converting it into the portions that act along the flight path, and across it. If the aircraft will carry some residual sideways speed over the drop point, this too has to be added to the sideways component. In practice, it was generally simpler to have the aircraft fly in such a way to zero out any sideways motion and eliminate this factor.

Bombsights are sighting devices that are pointed in a particular direction, or "aimed". Although the solution outlined above theoretically returns a point in space, simple trigonometry

Trigonometry

Trigonometry is a branch of mathematics that studies triangles and the relationships between their sides and the angles between these sides. Trigonometry defines the trigonometric functions, which describe those relationships and have applicability to cyclical phenomena, such as waves...

can be used to convert this point into an angle relative to the ground. The drop point is indicated when the target appears at that relative angle to the aircraft. The distance between the aircraft and target at that moment is the "range", so this angle is often referred to as the "range angle", although "dropping angle", "aiming angle", "bombing angle" and similar terms are often used as well. In practice, some or all of these calculations are carried out using angles and not points in space, skipping the final conversion.

Accuracy

The accuracy of the drop is effected both by inherent problems like the randomness of the atmosphere, as well as more practical problems like how close to flat and level the aircraft is flying or the accuracy of its instruments. This inaccuracies compound over time, so increasing the altitude of the bomb run, and thus increasing the fall time, has a significant impact on the final accuracy of the drop.

For this reason, it is useful to consider a single example of a typical bomb being dropped on a typical mission. In this case we will consider the AN-M65 500 lbs General Purpose Bomb, widely used by the USAAF and RAF during WWII, and with direct counterparts in the armoires of most forces involved. It will be dropped from a Boeing B-17 flying at 200 mph at an altitude of 20,000 feet in a 25 mph wind. Data on this bomb can be found in "Terminal Ballistic Data, Volume 1: Bombing". Given these conditions, the M64 would travel approximately 6,500 feet forward before impact, for a trail of about 1000 feet from the vacuum range, and impact with a velocity of 1150 fps at an angle of about 77 degrees from horizontal. A 25 mph wind would be expected to move the bomb about 300 feet during that time. The time to fall is about 37 seconds. Against men standing in the open, the 500 lbs has a lethal radius of about 350 feet, but much less than that against buildings, perhaps 90 feet.

Consider errors of 5% in every major measurement, and consider their effects based on the methodology and tables in the guide. A 5% error in altitude would be 1,000 feet, which would result in an error around 10 to 15 feet. A 5% error in airspeed, 10 mph, would likewise cause an error of about 15 to 20 feet. In terms of drop timing, errors of 5% are likely far too low for manual release, where times on the order of 1/10th of a second are reasonable. In this case the error is simply the ground speed of the aircraft over this time, or about 30 feet. All of these are well within the lethal radius of the bomb.

The wind effects the accuracy of the bomb in two ways, pushing directly on the bomb while it falls, as well as changing the ground speed of the aircraft before the drop. In the case of the direct effects on the bomb, a measurement that has a 5% error, 1.25 mph, that would cause a 5% error in the drift, which would be 17.5 feet. However, that 1.25 mph error, or 1.8 fps, would also be added to the aircraft's velocity. Over the time of the fall, 37 seconds, that would result in an error of 68 feet, which is at the outside limit of the bomb's performance. Wind speed is generally measured using a 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...

procedure that compares measured movement over the ground with the calculated movement using the aircraft instruments. The Federal Aviation Administration

Federal Aviation Administration

The Federal Aviation Administration is the national aviation authority of the United States. An agency of the United States Department of Transportation, it has authority to regulate and oversee all aspects of civil aviation in the U.S...

's FAR Part 63 suggests 5 to 10% accuracy of these calculations, the US Air Force's AFM 51-40 gives 10%, and the US Navy's H.O. 216 at a fixed 20 miles or greater. Compounding this inaccuracy is the fact that it is made using the instrument's airspeed indication, and as the airspeed in this example is about 10 times that of the wind speed, its 5% error can led to great inaccuracies in wind speed calculations. Eliminating this error through the direct measurement of ground speed (instead of calculating it) was a major advance in the "tachometric" bombsights of the 1930s and 40s.

Finally, consider errors of the same 5% in the equipment itself, that is, an error of 5% in the setting of the range angle, or a similar 5% error in the levelling of the aircraft or bombsight. For simplicity, consider that 5% to be a 5 degree angle. Using simple trigonometry, 5 degrees at 20,000 feet is approximately 1,750 feet, an error that would place the bombs well outside their lethal radius. In tests, accuracies of 3 to 4 degrees were considered standard, and angles as high as 15 degrees were not uncommon. Even the smaller errors would produce misses of 1000 feet, still well outside the lethal radius. This was one of the major reasons that "stabilization" equipment was added, which automatically levelled the bombsight to help eliminate this error. However, pilots were generally unable to correct quickly enough to match this accuracy, which led to the introduction of autopilots as standard equipment.

Early systems

All of the calculations needed to predict the path of a bomb can be carried out by hand, with the aid of calculated tables of the bomb ballistics. However, the time to carry out these calculations is not trivial. Using visual sighting the range at which the target is sighted remains fixed, based on eyesight, so as aircraft speeds increase, there is less time available to carry out the calculations. During the early stages of bombsight development, the issue was always reducing the allowable engagement envelop to reduce the need to calculate marginal effects. For instance, when dropped from very low altitudes, the effects of drag and wind during the fall will be so small that they can be ignored. In this case only the forward speed and altitude have any measurable effect.

One of the earliest recorded examples of a true bombsight was built in 1911 by Lieutenant Riley E. Scott, of the American Coast Artillery. This was a simple device with inputs for airspeed and altitude which was hand-held while lying prone on the wing of the aircraft. After considerable testing, he was able to build a table of settings to use with these inputs. In testing at College Park, Maryland

College Park, Maryland

College Park is a city in Prince George's County, Maryland, USA. The population was 30,413 at the 2010 census. It is best known as the home of the University of Maryland, College Park, and since 1994 the city has also been home to the "Archives II" facility of the U.S...

, Scott was able to place two 18-pound bombs within 10 feet of a 4-by-5-foot target from a height of 400 feet. In January 1912, Scott won $5,000 for first place in the Michelin bombing competition at Villacoublay Airdrome in France, scoring 12 hits on a 125-by-375-foot target with 15 bombs dropped from 800 meters.

In spite of early examples like Scott's prior to the war, during the opening stages of the First World War bombing was almost always carried out "by eye", dropping the small bombs by hand when the conditions looked right. As the use and roles for aircraft increased during the war, the need for better accuracy became pressing. At first this was accomplished by sighting off parts of the aircraft, such as struts and engine cylinders, or drawing lines on the side of the aircraft after test drops on a bombing range. These were useful for low altitudes and stationary targets, but as the nature of the air war expanded, the needs quickly outgrew these solutions as well.

One of the earliest fully developed bombsights to see combat was the German Görtz bombsight, developed for the Gotha heavy bombers. The Görtz used a telescope with 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:...

at the bottom which was pre-set to an angle read from a table of speed vs. altitude. Similar bombsights were developed in France and England, notably the Michelin and Central Flying School Number Seven bombsight. All of these shared the problem that they had no way to account for windage across the aircraft's path, and required the aircraft to fly directly along the wind line in order to be accurate. Even then, the adjustment for drift in setting the trail was normally estimated using a 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 manually timing the flight of the aircraft over the ground, a time-consuming and error-prone process. A simpler solution was needed.

The first successful attack on the windage problem was made by Harry Wimperis

Harry Wimperis

Harry Egerton Wimperis was an aeronautical engineer who acted as the Director of Scientific Research at the UK's Air Ministry prior to World War II. He is best known for his role in setting up the Committee for the Scientific Survey of Air Defence under Henry Tizard, which led directly to the...

, better known for his later role in the development of radar

Radar

in England

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

. In 1916 he introduced the Drift Sight

Drift Sight

The Drift Sight was a bombsight developed by Harry Wimperis in 1916 for the Royal Naval Air Service . It used a simple mechanical device to measure the wind speed from the air, and used that measurement to calculate the wind's effects on the trajectory of the bombs...

that added a simple system for directly measuring the wind speed. Prior to the bomb run, the bomber would fly at right angles to the bomb line, and the bomb aimer would line up a scale on the sight with the motion of objects on the ground. Using the known airspeed and the measured angle, the wind speed could then be calculated with some degree of accuracy. Simply measuring the speed using this device also adjusted the sights to the proper sighting angles, eliminating the need for separate calculations. A later modification was added to calculate the difference between true and indicated airspeed, which grows with altitude. This version was the Drift Sight Mk. 1A, introduced on the Handley Page O/400

Handley Page Type O

The Handley Page Type O was an early biplane bomber used by Britain during the First World War. At the time, it was the largest aircraft that had been built in the UK and one of the largest in the world...

heavy bomber. Variations on the design were common, like the US Estoppey bombsight.

All of these bombsights shared the problem that they were unable to deal with wind in any direction other than along the path of travel. That made them effectively useless against moving targets, like submarine

Submarine

A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

s and 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, as these targets would normally maneuver as soon as an attack was spotted, leading the bomber away from the wind line. Additionally, as anti-aircraft artillery grew more effective, they would often pre-sight their guns along the wind line of the targets they were protecting, knowing that attacks would come from those directions. A solution for attacking cross-wind was sorely needed.

Vector bombsights

Calculating the effects of an arbitrary wind on the path of an aircraft was already a well-understood problem in air navigation

Air navigation

The basic principles of air navigation are identical to general navigation, which includes the process of planning, recording, and controlling the movement of a craft from one place to another....

, one requiring basic vector mathematics. Wimperis was very familiar with these techniques, and would go on to write a seminal introductory text on the topic. The same calculations would work just as well for bomb trajectories, with some minor adjustments to account for the changing velocities as the bombs fell. Even as the Drift Sight was being introduced, Wimperis was working on a new bombsight that helped solve these calculations and allow the effects of wind to be considered no matter the direction of the wind or the bomb run.

The result was the 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...

(CSBS), called "the most important bomb sight of the war". Dialling in the values for altitude, airspeed and the speed and direction of the wind rotated and slid various mechanical devices that solved the same vector problem. Once set up, the bomb aimer would watch objects on the ground and compare their path to thin wires on either side of the sight. If there was any sideways motion, the pilot could slip-turn to a new heading in an effort to cancel out the drift. A few attempts were typically all that was needed, at which point the aircraft was flying in the right direction to take it directly over the drop point, with zero sideways velocity. The bomb aimer (or pilot in some aircraft) then sighted through the iron sights to time the drop.

The CSBS was introduced into service in 1917 and quickly replaced earlier sights on aircraft that had room for the CSBS. Versions for different speeds, altitudes and bombs were introduced as the war progressed. After the war, the CSBS continued to be the main bombsight in British use, thousands were sold to foreign air forces, and numerous versions were created for production around the world. A number of experimental devices based on a variation of the CSBS were also used, notably the US's Estoppey D-1 sight, developed shortly after the war, and similar versions from many other nations. These "vector bombsights" all shared the basic vector calculator system and drift wires.

As bombers grew and multi-place aircraft became common, it was no longer possible for the pilot and bomb aimer to share the same instrument. The bomb aimer had to take over the duty of adjusting for drift, relaying commands to the pilot. In a noisy aircraft this was not trivial. A variety of solutions using dual optics or similar systems were suggested in the post-war era, but none of these became widely used. This led to the introduction of the "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...

", a pointer which the bomb aimer could move to indicate corrections from a remote location in the aircraft.

Vector bombsights remained the standard by most forces well into the Second World War, and was the main sight in British service until 1942. This was in spite of the introduction of newer sighting systems with great advantages over the CSBS, and even newer versions of the CSBS that failed to be used for a variety of reasons. The later versions of the CSBS included adjustments for different bombs and ways to attack moving targets.

Tachometric bombsights

One of the main problems using vector bombsights was the long straight run needed before dropping the bombs. This was needed so the pilot would have enough time to accurately account for the effects of wind, and get the proper angles set up with some level of accuracy. If anything changed during the bomb run, especially if the aircraft had to maneuver in order to avoid defences, everything had to be set up again. Additionally, the introduction of monoplane bombers made the adjustment of the angles more difficult, because they were not able to slip-turn like their earlier biplane counterparts. They suffered from an effect known as "Dutch roll

Dutch roll

Dutch roll is a type of aircraft motion, consisting of an out-of-phase combination of "tail-wagging" and rocking from side to side. This yaw-roll coupling is one of the basic flight dynamic modes...

" that made the more difficult to turn and tended to "hunt" after levelling. This further reduced the time the bomb aimer had to adjust the path.

One solution to this later problem had already been used for some time, the use of some sort of gimbal

Gimbal

A gimbal is a pivoted support that allows the rotation of an object about a single axis. A set of two gimbals, one mounted on the other with pivot axes orthogonal, may be used to allow an object mounted on the innermost gimbal to remain immobile regardless of the motion of its support...

system to keep the bombsight pointed roughly downward during maneuvering or being blown around by wind gusts. Experiments as early as the 1920s had demonstrated that this could roughly double the accuracy of bombing. The US carried out an active program in this area, including Estoppey sights mounted to weighted gimbal

Gimbal

s and Sperry Gyroscope's experiments with US CSBS's mounted to what would today be called an inertial platform. These same developments led to the introduction of the first really useful 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...

s, which could be used to directly dial in the required path and have the aircraft fly to that heading with no further input. A variety of bombing systems using one or both of these systems were considered throughout the 1920s and 30s.

During the same period, a separate line of development was leading to the first reliable mechanical computer

Mechanical computer

s. These could be used to replace a complex table of numbers with a carefully shaped cam

Cam

A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-versa. It is often a part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path...

-like device, and the manual calculation though a series of gears or slip wheels. Originally limited to fairly simple calculations consisting of additions and subtractions of the numbers, by the 1930s they had progressed to the point where they were being used to solve differential equations. For bombsight use, such a calculator would allow the bomb aimer to dial in the basic aircraft parameters - speed, altitude, direction, and known atmospheric conditions, density, wind speed and direction, and automatically calculate the proper aim point in a few moments. Even the relatively complex calculations needed to adjust for different bomb's trajectories could be reduced to a single cam. Some of the traditional inputs could even be taken directly from the aircraft instruments, eliminating errors.

Although these developments were well known within the industry, only the US Army Air Corps and US Navy put any concerted effort into development. During the 1920s, the Navy funded development of the Norden bombsight

Norden bombsight

while the Army funded development of the Sperry O-1. Both systems were generally similar; a bomb sight consisting of a small telescope was mounted on a stabilizing platform to keep the sighting head stable. A separate mechanical computer was used to calculate the aim point. This was fed back to the sight, which automatically rotated it to the right angle and tried to keep the target still in the telescope view. When the bomb aimer sighted through the telescope, he could see any drift and relay this to the pilot, or later, feed that information directly into the autopilot

Autopilot

. Watching the target in the sight, the windage calculations could be finely corrected. For a variety of reasons, the Army dropped their interest in the Sperry and features from both bombsights were folded into the Norden. The Norden equipped almost all US high-level bombers, most notably the B-17 Flying Fortress. In tests, these bombsights were able to generate fantastic accuracy. In practice, however, operational factors seriously upset them, to the point that pinpoint bombing using the Norden was eventually abandoned.

Although the US put the most effort into these development, they were also being studied elsewhere. In the UK, work on the Automatic Bomb Sight (ABS) had been carried on since the mid-30s in an effort to replace the CSBS. However, the ABS did not include stabilization of the sighting system, nor the Norden's autopilot system. In testing the ABS proved to be too difficult to use, requiring long bomb runs to allow the computer to "solve" the aim point. When the 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...

complained that even the CSBS had too long a run-in to the target, efforts to replace it with the ABS ended. For their needs they developed a new bombsight, the Mk. XIV

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 Mk. XIV featured a stabilizing platform and aiming computer, but worked more like the CSBS in overall functionality - the bomb aimer would set the computer to move the sighting system to the proper angle, but the bombsight did not "track" the target or attempt to correct the aircraft path. The advantage of this system was that it was dramatically faster to use, and could be used even while the aircraft was manoeuvring. Facing a lack of production capability, Sperry was contracted to produce the Mk. XIV, calling it the Sperry T-1.

Both the British and Germans would later introduce Norden-like sights of their own. Based at least partially on information about the Norden passed to them through 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...

, 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....

developed the 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...

. This was operationally almost identical to the Norden, but much smaller. In certain applications the Lotfernrohr 7 could be used by a single crewed aircraft, as was the case for the 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...

, the world's first operational jet bomber. Late in the war the RAF had the need for accurate high-altitude bombing and introduced a stabilized version of the earlier ABS, the hand-built Stabilized Automatic Bomb Sight (SABS). It was produced in such limited numbers that it was at first used only by the famed No. 617 Squadron RAF

No. 617 Squadron RAF

No. 617 Squadron is a Royal Air Force aircraft squadron based at RAF Lossiemouth in Scotland. It currently operates the Tornado GR4 in the ground attack and reconnaissance role...

, The Dambusters.

All of these designs collectively became known as "tachometric sights", "tachometric" referring to the timing mechanisms which counted the rotations of a screw or gear that ran as a specified speed.

Radar bombing and integrated systems

As radar systems dramatically improved through the war, it was found that it made an effective system for mapping the ground at night or through clouds. This led to experiments to combine a radar display with a bombsight to allow direct attack via radar. Operational use soon followed with the British H2S radar

H2S radar

H2S was the first airborne, ground scanning radar system. It was developed in Britain in World War II for the Royal Air Force and was used in various RAF bomber aircraft from 1943 to the 1990s. It was designed to identify targets on the ground for night and all-weather bombing...

, soon copied in the US as the H2X

H2S radar

and then followed by a series of improved versions like the AN/APQ-13

AN/APQ-13

AN/APQ-13 radars were a ground scanning radar developed by Bell Laboratories, Western Electric, and MIT as an improved model of the airborne H2X radar, itself developed from the first ground scanning radar, the British H2S radar. They were used on B-29s during World War II in the Pacific theater...

and AN/APQ-7 used on the Boeing B-29 Superfortress. These early systems operated independently of any existing optical bombsight, but conversions and adaptations were quickly made to repeat the radar signal in the existing bombsights. The AN/APA-47 was used to combine the output from the AN/APQ-7 with the Norden, allowing the bomb aimer to easily check both images to compare the aim point.

The strategic bombing role was following an evolution over time to ever-higher, ever-faster, ever-longer-ranged missions with ever-more-powerful weapons. Although the tachometric bombsights provided most of the features needed for accurate bombing, they were complex, slow, and limited to straight-line and level attacks. In 1946 the US Army Air Force asked the Army Air Forces Scientific Advisory Group to study the problem of bombing from jet aircraft that would soon be entering service. They concluded that at speeds over 1,000 knots, optical systems would be useless as they would not give the bomb aimer enough time to find the target given the limited sighting ranges they offered, ranges that would be the same or even shorter as the range of the bomb being dropped at high speed.

Only a radar system would have the required range, and existing radar systems did not offer anywhere near the performance needed. At the stratospheric altitudes and long ranges being considered, the radar antenna would need to be very large to offer the required resolution, yet this ran counter for the need to develop an antenna that was as small as possible in order to reduce drag. They also pointed out that many targets would not show up directly on the radar, so the bombsight would need the ability to drop at points relative to some landmark that did appear, the so-called "offset aiming points". Finally, the group noted that many of the functions in such a system would overlap formerly separate tools like the navigation systems. They proposed a single system that would offer mapping, navigation, autopilot and bomb aiming, thereby reducing complexity, and especially the needed space. Such a machine first emerged in the form of the AN/APQ-24, and later the "K-System", the AN/APA-59.

Through the 1950s and 1960s, radar bombing of this sort was common and the accuracy of the systems were limited to what was needed to support attacks by 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 - a Circular Probable Error (CEP) of about 3,000 feet was considered adequate. As mission range extended to thousands of miles, bombers started incorporating inertial guidance and star trackers to allow accurate navigation when far from land. These systems quickly improved in accuracy, and eventually became accurate enough to handle the bomb dropping as well without the need for a separate bombsight. This was the case for the 1,500 foot accuracy demanded of the B-70 Valkyrie.

Modern systems

During 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 weapon of choice was a nuclear one, and accuracy needs were limited. Development of tactical bombing systems, notably the ability to attack point targets with conventional weapons that had been the original goal of the Norden, was not considered seriously. Thus when the US entered 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...

, their weapon of choice was the Douglas A-26 Invader equipped with the Norden. Such a solution was inadequate.

At the same time, the ever-increasing power levels of new jet engines led to fighter aircraft

Fighter aircraft

A fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets...

with bomb loads similar to heavy bombers of a generation earlier. This generated demand for a new generation of greatly improved bombsights that could be used by a single-crew aircraft and employed in fighter-like tactics, whether high-level, low-level, in a dive towards the target, or during hard maneuvering. A specialist capability for toss bombing

Toss bombing

Toss bombing is a method of bombing where the attacking aircraft pulls upwards when releasing its bomb load, giving the bomb additional time of flight by starting its ballistic path with an upward vector.The purpose of toss bombing is to compensate for the gravity drop of the...

also developed in order to allow aircraft to escape the blast radius of their own nuclear weapon

Nuclear weapon

s, something that required only middling accuracy but a very different trajectory that initially required a dedicated bombsight.

As electronics improved, these systems were able to be combined together, and then eventually with systems for aiming other weapons. They may be controlled by the pilot directly and provide information through the heads-up display or a video display on the instrument panel. The definition of bombsight is becoming blurred as "smart" bombs with in-flight guidance

Precision-guided munition

A precision-guided munition is a guided munition intended to precisely hit a specific target, and to minimize damage to things other than the target....

, such as laser-guided bomb

Laser-guided bomb

A laser-guided bomb is a guided bomb that uses semi-active laser homing to strike a designated target with greater accuracy than an unguided bomb. LGBs are one of the most common and widespread guided bombs, used by a large number of the world's air forces.- Overview :Laser-guided munitions use a...

s or those using GPS replace "dumb" gravity bomb

Gravity bomb

An unguided bomb, also known as a free-fall bomb, gravity bomb, dumb bomb, or iron bomb, is a conventional aircraft-delivered bomb that does not contain a guidance system and hence, simply follows a ballistic trajectory....