Bombsight

A bombsight is a device used by military aircraft to drop

A bombsight has to estimate the path the bomb will take after release from the aircraft. The two primary forces during its fall are

Bombsights were first used before World War I and have since gone through several major revisions. The earliest systems were iron sights, which were pre-set to an estimated fall angle. In some cases, they consisted of nothing more than a series of nails hammered into a convenient spar, lines drawn on the aircraft, or visual alignments of certain parts of the structure. They 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.

In the 1930s, mechanical computers with the performance needed to solve the equations of motion started to be incorporated into the new tachometric bombsights, the most famous of which is the Norden. Then, in World War II, tachometric bombsights were often combined with radar systems to allow accurate bombing through clouds or at night. When postwar studies demonstrated that bomb accuracy was roughly equal either optically or radar-guided, optical bombsights were generally removed and the role passed to dedicated radar bombsights.

Finally, especially since the 1960s, fully computerized bombsights were introduced, which combined the bombing with long-range navigation and mapping.

Modern aircraft do not have a bombsight but use highly computerized systems that combine bombing, gunnery, missile fire and navigation into a single head-up display. The systems have the performance to calculate the bomb trajectory in real time, as the aircraft manoeuvres, and add the ability to adjust for weather, relative altitude, relative speeds for moving targets and climb or dive angle. That makes them useful both for level bombing, as in earlier generations, and tactical missions, which used to bomb by eye.

Theory

Forces on a bomb

The drag on a bomb for a given air density and angle of attack is proportional to the relative air speed squared. If the vertical component of the velocity is denoted by ${\displaystyle v_{v}}$ and the horizontal component by ${\displaystyle v_{h}}$ then the speed is ${\displaystyle {\sqrt {v_{v}^{2}+v_{h}^{2}}}}$ and the vertical and horizontal components of the drag are:

${\displaystyle {\begin{aligned}d_{v}&=CA\rho {\frac {v_{v}}{\sqrt {v_{v}^{2}+v_{h}^{2}}}}(v_{v}^{2}+v_{h}^{2})\\&=CA\rho v_{v}{\sqrt {v_{v}^{2}+v_{h}^{2}}}\end{aligned}}}$

${\displaystyle {\begin{aligned}d_{h}&=CA\rho {\frac {v_{h}}{\sqrt {v_{v}^{2}+v_{h}^{2}}}}(v_{v}^{2}+v_{h}^{2})\\&=CA\rho v_{h}{\sqrt {v_{v}^{2}+v_{h}^{2}}}\end{aligned}}}$

where C is the coefficient of drag, A is the cross-sectional area, and ρ is the air density. These equations show that horizontal velocity increases vertical drag and vertical velocity increases horizontal drag. These effects are ignored in the following discussion.

To start with, consider only the vertical motion of a bomb. In this direction, the bomb will be subject to two primary forces, gravity and drag, the first constant, and the second varying with the square of velocity. For an aircraft flying straight and level, the initial vertical velocity of the bomb will be zero, which means it will also have zero vertical drag. Gravity will accelerate the bomb downwards, and as its velocity increases so does the drag force. At some point (as speed and air density increase), the force of drag will become equal to the force of gravity, and the bomb will reach terminal velocity. As the air drag varies with air density, and thus altitude, the terminal velocity will decrease as the bomb falls. Generally, the bomb will slow as it reaches lower altitudes where the air is denser, but the relationship is complex.^[1]

Now consider the horizontal motion. At the instant it leaves the shackles, the bomb carries the forward speed of the aircraft with it. This momentum is countered solely by drag, which starts to slow the forward motion. As the forward motion slows, the drag force drops and this deceleration diminishes. The forward speed is never reduced entirely to zero.^[1] If the bomb were not subject to drag, its path would be purely ballistic and it would impact at an easily calculable point, the vacuum range. In practice, drag means that the impact point is short of the vacuum range, and this real-world distance between dropping and impact is known simply as the range. The difference between the vacuum range and actual range is known as the trail because the bomb appears to trail behind the aircraft as it falls. The trail and range differ for different bombs due to their individual aerodynamics and typically have to be measured on a bombing range.^[1]

The main problem in completely separating the motion into vertical and horizontal components is the terminal velocity. Bombs are designed to fly with the nose pointed forward into the relative wind, normally through the use of fins at the back of the bomb. The drag depends on the angle of attack of the bomb at any given instant. If the bomb is released at low altitudes and speeds the bomb will not reach terminal velocity and its speed will be defined largely by how long the bomb has been falling.

Finally, consider the effects of wind. The wind acts on the bomb through drag and is thus a function of the wind speed. This is typically only a fraction of the speed of the bomber or the terminal velocity, so it only becomes a factor if the bomb is dropped from altitudes high enough for this small influence to noticeably affect the bomb's path. 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.^[1][2]

The bombsight problem

In ballistics terms, it is traditional to talk of the calculation of aiming of ordnance 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 when all of the effects noted above are taken into account.^[2]

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, the first 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 measured ballistics.^[3]

When windage is accounted for, the calculations become more complex. As the wind can operate in any direction, bombsights generally break the windage into the portions that act along the flight path and across it. In practice, it was generally simpler to have the aircraft fly in such a way to zero out any sideways motion before the drop, and thereby eliminate this factor.^[4] This is normally accomplished using a common flying techniques known as crabbing or sideslip.

Bombsights are sighting devices that are pointed in a particular direction, or aimed. Although the solution outlined above returns a point in space, simple trigonometry can be used to convert this point into an angle relative to the ground. The bombsight is then set to indicate that angle. The bombs are dropped when the target passes through the sights. 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.^[3]

Accuracy

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

It is useful to consider a single example of a bomb being dropped on a typical mission. In this case we will consider the AN-M64 500 lbs General-Purpose Bomb, widely used by the USAAF and RAF during World War II, with direct counterparts in the armouries of most forces involved. Ballistic data on this bomb can be found in "Terminal Ballistic Data, Volume 1: Bombing".^[5] Against men standing in the open, the 500 lbs has a lethal radius of about 107 m (351 ft),^[6] but much less than that against buildings, perhaps 27 m (89 ft).^[7]

The M64 will be dropped from a

Assuming errors of 5% in every major measurement, one can estimate those effects on accuracy based on the methodology and tables in the guide.[5] A 5% error in altitude at 20,000 feet would be 1,000 feet, so the aircraft might be anywhere from 19 to 21,000 feet. According to the table, this would result in an error around 10 to 15 feet. A 5% error in airspeed, 10 mph, would cause an error of about 15 to 20 feet. In terms of drop timing, errors on the order of one-tenth of a second might be considered the best possible. 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 affects 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.^[5]

The measurement of the wind speed is a more serious concern. Early navigation systems generally measured it using a dead reckoning procedure that compares measured movement over the ground with the calculated movement using the aircraft instruments. The Federal Aviation Administration's FAR Part 63 suggests 5 to 10% accuracy of these calculations,^[12] the US Air Force's AFM 51-40 gives 10%,^[13] and the US Navy's H.O. 216 at a fixed 20 miles or greater.^[14] Compounding this inaccuracy is 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 lead 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 far 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.^[11] Given the seriousness of the problem, systems for automatic levelling of bombsights was a major area of study before World War II, especially in the US.^[15]

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 first sighted remains fixed, based on eyesight. As aircraft speeds increase, there is less time available after the initial spotting to carry out the calculations and correct the aircraft's flight path to bring it over the proper drop point. During the early stages of bombsight development, the problem was addressed by reducing the allowable engagement envelope, thereby reducing 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.^[3]

One of the earliest recorded examples of such a bombsight was built in 1911 by Lieutenant Riley E. Scott, of the

For higher altitude drops, the effect of wind and bomb trajectory could no longer be ignored. One important simplification was to ignore the terminal velocity of the bomb, and calculate its average speed as the square root of the altitude measured in feet. For instance, a bomb dropped from 10,000 feet would fall at an average rate of 400 fps, allowing easy calculation of the time to fall. Now all that remained was a measurement of the wind speed, or more generally the ground speed. Normally this was accomplished by flying the aircraft into the general direction of the wind and then observing motion of objects on the ground and adjusting the flight path side to side until any remaining sideways drift due to wind was eliminated. The speed over the ground was then measured by timing the motion of objects between two given angles as seen through the sight.[17]

One of the most fully developed examples of such a sight to see combat was the German Görtz bombsight, developed for the Gotha heavy bombers. The Görtz used a telescope with a rotating prism at the bottom that allowed the sight to be rotated fore and aft. After zeroing out sideways motion the sight was set to a pre-set angle and then an object was timed with a stopwatch until it was directly below the aircraft. This revealed the ground speed, which was multiplied by the time taken to hit the ground, and then a pointer in the sight was set to an angle looked up on a table. The bomb aimer then watched the target in the sight until it crossed the pointer, and dropped the bombs. Similar bombsights were developed in France and England, notably the Michelin and Central Flying School Number Seven bombsight. While useful, these sights required a time-consuming setup period while the movement was timed.^[16]

A great upgrade to the basic concept was introduced by

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

Calculating the effects of an arbitrary wind on the path of an aircraft was already a well-understood problem in air navigation, 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.^[20] 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.^[18]

The result was the Course Setting Bomb Sight (CSBS), called "the most important bomb sight of the war".^[18] Dialling in the values for altitude, airspeed and the speed and direction of the wind rotated and slid various mechanical devices that solved the 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 attached iron sights to time the drop.^[21]

The CSBS was introduced into service in 1917 and quickly replaced earlier sights on aircraft that had enough room – the CSBS was fairly large. Versions for different speeds, altitudes and bomb types 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 developed, notably the US's Estoppey D-1 sight,^[22] 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, differing primarily in form and optics.

As bombers grew and multi-place aircraft became common, it was no longer possible for the pilot and bombardier to share the same instrument, and hand signals were no longer visible if the bombardier was below the pilot in the nose. A variety of solutions using dual optics or similar systems were suggested in the post-war era, but none of these became widely used.^[23][24][25] This led to the introduction of the pilot direction indicator, an electrically driven pointer which the bomb aimer used to indicate corrections from a remote location in the aircraft.^[26]

Vector bombsights remained the standard by most forces well into the

The limitations of vector bombsights (which required a long straight run before dropping the bombs to accommodate windage) led to the development of bombsights based on the field of

Once the operator of a Norden M-1, the most famous of all tachometric bombsights, was able to identify a target the bombsight was able, in perfect conditions, to fly the plane to it. In battle, complicated by anti-aircraft defenses, crosswinds and clouds, and the need for aircraft to stay in formation to avoid collisions, results were less ideal but as good as could be achieved with the technology under the circumstances.

Two real-world considerations accelerated the development of tachometric bombsights: the introduction of monoplane bombers made manual adjustments to keep a plane on target more difficult. Large monoplanes suffered from an effect known as "Dutch roll", and were not able to slip-turn to correct for it as easily as their biplane predecessors. Also, intense ground-based anti-aircraft defenses and improved interceptors made it impossible to sustain long straight-and-level bombing runs without excessive loss of aircraft and their valuable crews.

If anything changed the aircraft's path during the bomb run, whether due to difficulty turning and a tendency to oscillate after levelling induced by the Dutch roll or evasive maneuvers forced by the enemy, bomb trajectory calculations had to be set up again. There simply was not time to do so with the bombsights and mechanical computers of the day.

One solution was to stabilize the bombsight independent of the aircraft. The use of a

Although the US put the most effort into development of the tachometric concept, they were also being studied elsewhere. In the UK, work on the Automatic Bomb Sight (ABS) had been carried on since the mid-1930s 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 time to solve the aim point. When RAF Bomber Command complained that even the CSBS had too long a run-in to the target, efforts to deploy the ABS ended. For their needs they developed a new vector bombsight, the Mk. XIV. 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, only a few seconds of straight-line flying were needed before the drop. Facing a lack of production capability, Sperry was contracted to produce the Mk. XIV in the US, calling it the Sperry T-1.^[35]

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

In the pre-World War II era there had been a long debate about the relative merits of daylight versus night-time bombing. At night the bomber is virtually invulnerable (until the introduction of

During the early 1930s the debate had been won by the night-bombing supporters, and the RAF and Luftwaffe started construction of large fleets of aircraft dedicated to the night mission. As "the bomber will always get through", these forces were strategic in nature, largely a deterrent to the other force's own bombers. However, new engines introduced in the mid-1930s led to much larger bombers that were able to carry greatly improved defensive suites, while their higher operational altitudes and speeds would render them less vulnerable to the defences on the ground. Policy once again changed in favour of daylight attacks against military targets and factories, abandoning what was considered a cowardly and defeatist night-bombing policy.

In spite of this change, the Luftwaffe continued to put some effort into solving the problem of accurate navigation at night. This led to the

Analysis of the results of bombing attacks carried out using radio navigation or radar techniques demonstrated accuracy was essentially equal for the two systems – night time attacks with Oboe were able to hit targets that the Norden could not during the day. With the exception of operational considerations – limited resolution of the radar and limited range of the navigation systems – the need for visual bombsights quickly disappeared. Designs of the late-war era, like the Boeing B-47 Stratojet and English Electric Canberra retained their optical systems, but these were often considered secondary to the radar and radio systems. In the case of the Canberra, the optical system only existed due to delays in the radar system becoming available.^[39][40]

Postwar developments

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

At the attack ranges being considered, thousands of miles, radio navigation systems would not be able to offer both the range and the accuracy needed. This demanded radar bombing systems, but existing examples did not offer anywhere near the required performance. At the stratospheric altitudes and long "sighting" 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.^[38]

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

Modern systems

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During the Cold War 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, 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 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 also developed in order to allow aircraft to escape the blast radius of their own nuclear weapons, 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, and then eventually with systems for aiming other weapons. They may be controlled by the pilot directly and provide information through the

See also

• Norden bombsight (USAAF)
• Stabilized Automatic Bomb Sight
  (RAF)
• Mark XIV bomb sight (RAF) less accurate, for area bombing
• Lotfernrohr 7 (Luftwaffe)
• SVP-24

References

Bibliography

• Terminal Ballistic Data, Volume I: Bombing (Technical report). US Army Office of the Chief of Ordnance. August 1944.
• Terminal Ballistic Data, Volume III: Effects (Technical report). US Army Office of the Chief of Ordnance. 1944. Archived from the original on 7 August 2010.
• United States Naval Academy, Department of Ordnance and Gunnery (1958). "Chapter 23: Aircraft Fire Control". Naval Ordnance and Gunnery. Vol. 2. U.S. Government Printing Office.
• Perry, Robert (1961). Development of Airborne Armament 1910 - 1961. Vol. 1 - Bombing Systems.
• Raymond, Allan (December 1943). "How Our Bombsight Solves Problems". Popular Science. Bonnier Corporation: 116–119, 212, 214.
• Volta Torrey, "How the Norden Bombsight Does Its Job", Popular Science, June 1945, pp. 70–73, 220, 224, 228, 232
• Goulter, Christina J. M. (1995). A Forgotten Offensive: Royal Air Force Coastal Command's Anti-shipping Campaign, 1940-1945. Routledge.
  ISBN 978-0-7146-4617-6
  .
• Searle, L. (September 1989). "The bombsight war: Norden vs. Sperry" (PDF). IEEE Spectrum. 26 (9): 60–64.
  S2CID 22392603. Archived from the original
  (PDF) on 30 September 2011.

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