Bombsight

A bombsight is a device used by military aircraft to drop bombs accurately. Bombsights, a feature of combat aircraft since World War I, were first found on purpose-designed bomber aircraft and then moved to fighter-bombers and modern tactical aircraft as those aircraft took up the brunt of the bombing role.

A 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 make the path of the bomb through the air roughly parabolic. There are additional factors such as changes in air density and wind that may be considered, but they are concerns only for bombs that spend a significant portion of a minute falling through the air. Those effects can be minimized by reducing the fall time by low-level bombing or by increasing the speed of the bombs. Those effects are combined in the dive bomber.

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

Since their first application prior to World War I, bombsights have 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 being 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.

Bombsight
An early bombsight, 1910s.
1923 Norden MK XI Bombsight Prototype
1923 Norden MK XI Bombsight Prototype

Bombsight concepts

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 we denote the vertical component of the velocity by and the horizontal component by then the speed is and the vertical and horizontal components of the drag are:

where C is the coefficient of drag, A is the cross-sectional area, and ρ is the air density. Thus we see that horizontal velocity increases vertical drag and vertical velocity increases horizontal drag. We ignore these effects in the following.

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]

387th-b262
The way the line of bombs falling from this B-26 goes toward the rear is due to drag. The aircraft's engines keep it moving forward at a constant speed, while the bombs slow down. From the bomber's perspective, the bombs trail behind the aircraft.

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 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 very high altitudes so this small influence has enough time to act as the bomb falls. 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 re-calculate the windage by converting it 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-M65 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 (350 feet),[6] but much less than that against buildings, perhaps 27 m (90 feet).[7]

The M65 will be dropped from a Boeing B-17 flying at 322 km/h (200 mph) at an altitude of 6096 m (20,000 feet) in a 42 km/h (25 mph) wind. Given these conditions, the M65 would travel approximately 1981 m (6,500 feet) forward before impact,[8] for a trail of about 305 m (1000 feet) from the vacuum range,[9] and impact with a velocity of 351 m/s (1150 fps) at an angle of about 77 degrees from horizontal.[10] A 42 km/h (25 mph) wind would be expected to move the bomb about 91 m (300 feet) during that time.[11] The time to fall is about 37 seconds.[12]

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,[13] the US Air Force's AFM 51-40 gives 10%,[14] and the US Navy's H.O. 216 at a fixed 20 miles or greater.[15] 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.[12] 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.[16]

Early systems

Drift Sight Mk. I on DH4.jpeg
A Mk. I Drift Sight mounted on the side of an Airco DH.4. The lever just in front of the bomb aimer's fingertips sets the altitude, the wheels near his knuckles set the wind and airspeed.

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.[17]

One of the earliest recorded examples of such a bombsight was built in 1911 by Lieutenant Riley E. Scott, of the U.S. Army Coast Artillery Corps. 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, 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.[18]

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.[18]

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.[19]

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.[18]

A great upgrade to the basic concept was introduced by Harry Wimperis, better known for his later role in the development of radar in England. In 1916 he introduced the Drift Sight, which added a simple system for directly measuring the wind speed. The bomb aimer would first dial in the altitude and airspeed of the aircraft. Doing so rotated a metal bar on the right side of the bombsight so it pointed out from the fuselage. Prior to the bomb run, the bomber would fly at right angles to the bomb line, and the bomb aimer would look past the rod to watch the motion of objects on the ground. He would then adjust the wind speed setting until the motion was directly along the rod. This action measured the wind speed, and moved the sights to the proper angle to account for it, eliminating the need for separate calculations.[20] A later modification was added to calculate the difference between true and indicated airspeed, which grows with altitude.[20] This version was the Drift Sight Mk. 1A, introduced on the Handley Page O/400 heavy bomber.[21] 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 submarines and ships. Unless the target just happened to be travelling directly in line with the wind, their motion would carry the bomber away from the wind line as they approached. 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.[18]

Vector bombsights

Course Setting Bomb Sight Mk IA
The CSBS Mk. IA, the first widely produced vector bombsight. The drift wires are visible on the right, the windage calculator on the left, and the altitude scale in the middle (vertical). The actual sights are the white rings near the top of the altitude slider and while dots mid-way along the drift wires. The drift wires are normally taut, this example is almost a century old.

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.[22] 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.[23]

The result was the Course Setting Bomb Sight (CSBS), called "the most important bomb sight of the war".[23] 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.[24]

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 used, notably the US's Estoppey D-1 sight,[25] 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.[26][27][28] 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.[29]

Vector bombsights remained the standard by most forces well into the Second World War, and was the main sight in British service until 1942.[30] 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, eventually reaching the Mark X, included adjustments for different bombs, ways to attack moving targets, systems for more easily measuring winds, and a host of other options.

Tachometric bombsights

Norden M1 bombsight
The Norden M-1 is the canonical tachometric bombsight. The bombsight proper is at the top of the image, mounted on top of the autopilot system at the bottom. The bombsight is slightly rotated to the right; in action the autopilot would turn the aircraft to reduce this angle back to zero.
Bomb Aimer Window-Avro Shackleton-001
Bomb aimer's window and bomb sight in the nose of an Avro Shackleton.

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 flight angle 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 as easily as their earlier biplane counterparts. They suffered from an effect known as "Dutch roll" that made them more difficult to turn and tended to oscillate 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 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 gimbals and Sperry Gyroscope's experiments with US versions of the CSBS mounted to what would today be called an inertial platform.[18] These same developments led to the introduction of the first useful autopilots, 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.[31]

During the same period, a separate line of development was leading to the first reliable mechanical computers. These could be used to replace a complex table of numbers with a carefully shaped cam-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, by the 1930s they had progressed to the point where they were being used to solve differential equations.[32] 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 - and the bomb sight would automatically calculate the proper aim point in a few moments. Some of the traditional inputs, like airspeed and altitude, could even be taken directly from the aircraft instruments, eliminating operational 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 while the Army funded development of the Sperry O-1.[33] 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. The aim point was fed back to the sight, which automatically rotated the telescope to the correct angle to account for drift and aircraft movement, keeping the target still in the view. When the bomb aimer sighted through the telescope, he could see any residual drift and relay this to the pilot, or later, feed that information directly into the autopilot. Simply moving the telescope to keep the target in view had the side effect of fine-tuning the windage calculations continuously, and thereby greatly increasing their accuracy. For a variety of reasons, the Army dropped their interest in the Sperry, and features from the Sperry and Norden bombsights were folded into new models of the Norden.[34] The Norden then 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.[35]

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.[36]

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, the Luftwaffe developed the Lotfernrohr 7.[37] The basic mechanism was almost identical to the Norden, but much smaller. In certain applications the Lotfernrohr 7 could be used by a single-crew aircraft, as was the case for the Arado Ar 234, 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, The Dambusters.[38]

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 at a specified speed.

Radar bombing and integrated systems

H2Xradar
The AN/APS-15 radar bombing system, a US version of the British H2S.

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 radar) but finding its target was a major problem. In practice, only large targets such as cities could be attacked. During the day the bomber could use its bombsights to attack point targets, but only at the risk of being attacked by enemy fighters and anti-aircraft artillery.

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 Battle of the Beams during the opening stages of the war. The RAF returned in force in early 1942 with similar systems of their own, and from that point on, radio navigation systems of increasing accuracy allowed bombing in any weather or operational conditions. The Oboe system, first used operationally in early 1943, offered real-world accuracies on the order of 35 yards, much better than any optical bombsight. The introduction of the British H2S radar further improved the bomber's abilities, allowing direct attack of targets without the need of remote radio transmitters, which had range limited to the line-of-sight. By 1943 these techniques were in widespread use by both the RAF and USAAF, leading to the H2X and then a series of improved versions like the AN/APQ-13 and AN/APQ-7 used on the Boeing B-29 Superfortress.

These early systems operated independently of any existing optical bombsight, but this presented the problem of having to separately calculate the trajectory of the bomb. In the case of Oboe, these calculations were carried out before the mission at the ground bases. But as daylight visual bombing was still widely used, conversions and adaptations were quickly made to repeat the radar signal in the existing bombsights, allowing the bombsight calculator to solve the radar bombing problem. For instance, 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.[39]

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.[40][41]

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 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 - the visual range to the target would be less than the range of a bomb being dropped at high altitudes and speeds.[39]

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.[39]

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 weapons - a circular error probable (CEP) of about 3,000 feet was considered adequate.[39] 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 without the need for a separate bombsight. This was the case for the 1,500 foot accuracy demanded of the B-70 Valkyrie, which lacked any sort of conventional bombsight.[42]

Modern systems

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 together, and then eventually with systems for aiming other weapons. They may be controlled by the pilot directly and provide information through the head-up display or a video display on the instrument panel. The definition of bombsight is becoming blurred as "smart" bombs with in-flight guidance, such as laser-guided bombs or those using GPS, replace "dumb" gravity bombs.

See also

References

  1. ^ a b c d See diagrams, Torrey p. 70
  2. ^ a b Fire Control 1958.
  3. ^ a b Fire Control 1958, p. 23D2.
  4. ^ Fire Control 1958, p. 23D3.
  5. ^ a b c Bombing 1944.
  6. ^ Effects 1944, p. 13.
  7. ^ John Correll, "Daylight Precision Bombing", Air Force Magazine, October 2008, pg. 61
  8. ^ Bombing 1944, p. 10.
  9. ^ Ordnance 1944, p. 47.
  10. ^ Bombing 1944, p. 39.
  11. ^ Bombing 1944, p. 23.
  12. ^ a b Raymond 1943, p. 119.
  13. ^ "Federal Aviation Regulations, Navigator Flight Test"
  14. ^ "Precision Dead Reckoning Procedure"
  15. ^ "Visual Flight Planning and Procedure"
  16. ^ All of the USAAC's pre-war bombsights featured some system for automatically levelling the sight; the Estopery D-series used pendulums, Sperry designs used gyroscopes to stabilize the entire sight, and the Norden used gyroscopes to stabilize the optics. See Interwar for examples.
  17. ^ Fire Control & 23D2.
  18. ^ a b c d e Perry 1961, Chapter I.
  19. ^ "Bomb Dropping". Society of the Automotive Engineers: 63–64. January 1922.
  20. ^ a b Goulter 1995, p. 27.
  21. ^ The Encyclopedia of Military Aircraft, 2006 Edition, Jackson, Robert ISBN 1-4054-2465-6 Parragon Publishing 2002
  22. ^ Harry Egerton Wimperis, "A Primer of Air Navigation", Van Nostrand, 1920
  23. ^ a b Goulter 1996, p. 27.
  24. ^ Ian Thirsk, "De Havilland Mosquito: An Illustrated History", MBI Publishing Company, 2006, pg. 68
  25. ^ "Interwar Development of Bombsights" Archived 11 January 2012 at the Wayback Machine, US Air Force Museum, 19 June 2006
  26. ^ "Target Following Bomb Sight", US Patent 1,389,555
  27. ^ "Pilot Direction Instrument and Bomb Dropping Sight for Aircraft", US Patent 1,510,975
  28. ^ "Airplane Bomb Sight", US Patent 1,360,735
  29. ^ Torrey p. 72
  30. ^ Sir Arthur Travers Harris, "Despatch on war operations, 23rd February, 1942, to 8th May, 1945", Routledge, 1995. See Appendix C, Section VII
  31. ^ Searle 1989, p. 60.
  32. ^ William Irwin, "The Differential Analyser Explained", Auckland Meccano Guild, July 2009
  33. ^ Searle 1989, p. 61.
  34. ^ Searle 1989, p. 63.
  35. ^ Geoffery Perrett, "There's a War to Be Won: The United States Army in World War II", Random House, 1991, p. 405
  36. ^ Henry Black, "The T-1 Bombsight Story", 26 July 2001
  37. ^ "The Duquesne Spy Ring" Archived 30 September 2013 at the Wayback Machine, FBI
  38. ^ "Royal Air Force Bomber Command 60th Anniversary, Campaign Diary November 1943" Archived 11 June 2007 at the Wayback Machine, Royal Air Force, 6 April 2005
  39. ^ a b c d Perry 1961, Chapter II.
  40. ^ "Biographical memoirs of fellows of the Royal Society", Royal Society, Volume 52, p. 234
  41. ^ Robert Jackson, "BAe (English Electric) Canberra", 101 Great Bombers, Rosen Publishing Group, 2010, p. 80
  42. ^ Perry 1961, Chapter VI.

Bibliography

Army Air Forces Bombardier School

A Bombardier School was a United States Army Air Forces facility that used bombing ranges for training aircrew. After ground simulator training with the Norden bombsight, the 12- to 18-week course recorded each student's scores for approximately 160 practice bomb drops of "Bomb Dummy Units" (BDU), both in daytime and at night. The elimination rate was 12%, and graduates transferred to a Second or Third Air Force training unit to join a crew being trained for overseas duty. The bombardier trainer used was the Beech AT-11 Kansan. With the Bradley Plan increase in Eighth Air Force aircrews needed for the Combined Bomber Offensive, the 17 Army Air Forces Bombardier Schools graduated 47,236.

Beef Manhattan

Beef Manhattan is a dish consisting of roast beef and gravy. It is often served with mashed potatoes either on top of the steak or on the side of the plate. A variation on this dish is Turkey Manhattan, which substitutes turkey for the roast beef. The term "Manhattan" is a misnomer as the beef and turkey variants are usually referred to as

"open-face sandwiches" in New York City and much of the eastern United States and the term "Manhattan" is limited to the Midwest, the South, and parts of the western United States. It is unlikely that restaurants in the New York City area would understand what a customer was asking for if the diner used the "Manhattan" phrase.

The dish was first served in a restaurant under the name "Beef Manhattan" in a now-defunct Indianapolis deli in the late 1940s where it gained traction as a Hoosier staple. The dish was named by Naval Ordnance Plant Indianapolis (NOPI) workers who were trained on a fabrication of the Norden Bombsight in Manhattan during World War II. They enjoyed the open-faced sandwich they had in Manhattan and brought it back to their cafeteria as the "Beef Manhattan". In Indiana, it is served on bread. The roast beef is sliced and put on the bread like a sandwich, then cut corner to corner and plated in a V shape. Mashed potatoes are served between the two halves, and the whole is covered in gravy.

Bombardier (aircrew)

A bombardier or bomb aimer is the crew member of a bomber aircraft responsible for the targeting of aerial bombs. "Bomb aimer" was the preferred term in the military forces of the Commonwealth, while "bombardier" (from the French word for "bomb thrower" and similar in meaning to "grenadier") was the equivalent position in the United States Armed Forces.

In many planes, the bombardier took control of the airplane during the bombing run, using a bombsight such as the Norden bombsight which was connected to the autopilot of the plane. Often stationed in the extreme front of the aircraft, on the way to the target and after releasing the bombs, he could also serve as the front gunner in aircraft that had a front turret.

In the latter part of the 20th century, the title of bombardier fell into disuse, due largely to changes in technology, emanating from the replacement of this manual function with the development of computerized technology and smart bombs, that has given rise to terms like weapons systems officer or combat systems officer (CSO) to describe the modern role. The equivalent in the US Navy and US Marine Corps is the Naval Flight Officer.

In the United States, the position of bombardier was originally held by a sergeant, but they were commissioned as officers in 1941. In the Commonwealth, a bomb aimer could be an officer or (more frequently) a senior non-commissioned officer (sergeant or flight sergeant) or warrant officer; like wireless operators, air engineers and air gunners, all officer bomb aimers were commissioned from the ranks after non-commissioned aircrew service, unlike pilots and navigators who could also join directly as commissioned officers.

During World War II, US Army Air Forces bombardiers were recognized with the award of the Bombardier Badge. With the establishment of an independent US Air Force in 1947, USAF bombardiers were awarded the wings known as the Navigator badge, now known as the Combat Systems Officer badge. Commonwealth bomb aimers wore a single-wing aircrew brevet with the letter "B".

The aircraft of the United Kingdom's V bomber force carried two navigators, one of whom acted as bomb aimer, although having the official title of "Navigator Radar".

Carl Norden

Carl Lucas Norden (April 23, 1880 – June 14, 1965), born Carel Lucas van Norden, was a Dutch engineer widely known for having invented the Norden bombsight.

Norden was born in Semarang, Java. After attending a boarding school in Barneveld, Netherlands, he was educated at the ETH Zürich in Switzerland. He emigrated to the United States in 1904.

Along with Elmer Sperry, Norden worked on the first gyrostabilizing equipment for United States ships, and became known for his contributions to military hardware. In 1913, he left Sperry and formed his own company. In 1920, he started work on the Norden bombsight for the United States Navy. The first bombsight was produced in 1927. It was essentially an analog computer, and bombardiers were trained in great secrecy on how to use it. The device was used to drop bombs accurately from an aircraft, supposedly accurate enough to hit a 100-foot (30 m) circle from an altitude of 21,000 feet (6,400 m), but such an accuracy was never achieved under actual combat situations.

Norden died in Zürich, Switzerland in 1965.

He was enshrined in the National Aviation Hall of Fame in July 1994.

Course Setting Bomb Sight

The Course Setting Bomb Sight (CSBS) is the canonical vector bombsight, the first practical system for properly accounting for the effects of wind when dropping bombs. It is also widely referred to as the Wimperis sight after its inventor, Harry Wimperis.

The CSBS was originally developed for the Royal Naval Air Service (RNAS) in order to attack submarines and ships. It was first introduced in 1917, and was such a great advance over earlier designs that it was quickly adopted by the Royal Flying Corps, and the Independent Air Force. It has been called "the most important bomb sight of the war".After the war the design found widespread use around the world. A US version of the CSBS was used by Billy Mitchell on his famous attack on the Ostfriesland. The basic design was adapted by almost all air forces and used well into World War II. It was eventually replaced in British service by the more advanced designs like the Mark XIV bomb sight and the Stabilized Automatic Bomb Sight. Other services used vector bombsights throughout the war.

Desmond Paul Henry

Desmond Paul Henry (1921–2004) was a Manchester University Lecturer and Reader in Philosophy (1949–82). He was one of the first British artists to experiment with machine-generated visual effects at the time of the emerging global computer art movement of the 1960s (The Cambridge Encyclopaedia 1990 p. 289; Levy 2006 pp. 178–180). During this period, Henry constructed a succession of three drawing machines from modified bombsight analogue computers which were employed in World War II bombers to calculate the accurate release of bombs onto their targets (O'Hanrahan 2005). Henry's machine-generated effects resemble complex versions of the abstract, curvilinear graphics which accompany Microsoft's Windows Media Player. Henry's machine-generated effects may therefore also be said to represent early examples of computer graphics: "the making of line drawings with the aid of computers and drawing machines". (Franke 1971, p. 41)

During the 1970s Henry focussed on further developing his own unique photo-chemical techniques for the production of original visual effects. He went on to make a fourth and a fifth drawing machine in 1984 and 2002 respectively. These later machines however, were based on a mechanical pendulum design and not bombsight computers. (O'Hanrahan 2005)

Drift Sight

The Drift Sight was a bombsight developed by Harry Wimperis in 1916 for the Royal Naval Air Service (RNAS). 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. The Drift Sight eliminated the need for a stopwatch to perform this calculation, as on earlier devices, and greatly eased the bomb aimer's workload.

The Drift Sight was quickly introduced into RNAS service and quickly thereafter by the Royal Flying Corps (RFC) as well. In British service, Wimperis' Course Setting Bomb Sight (CSBS) started replacing the Drift Sight in 1917, but it remained in widespread use in the US Army Air Service into the 1920s. In US use the Drift Sight is often referred to as the Wimperis sight, but this name is more commonly applied to the CSBS, especially in Commonwealth air forces.

Firepower (video game)

For the similarly named tank game, see Fire Power (video game).Firepower is a video game of the combat flight simulation genre released in 2004 as an add on to enhance Microsoft Combat Flight Simulator 3: Battle for Europe. Firepower adds 18 new aircraft bringing the total of 36 military aircraft (or 56 with variants) to Microsoft Combat Flight Simulator 3: Battle for Europe. Firepower also adds 50 new historical type missions, new ordnance and improved graphic effects to Microsoft's combat flight simulation.

The aircraft in Firepower include the B-17 Flying Fortress models F and G, B-29A Superfortress, Arado 234B Blitz, Dewoitine D.520 fighter, Dornier 217M medium bomber, Dornier 217N night fighter, He-162A Salamander, Me334, Me410A Schnellbomber, Me410B-2 R-3 zerstorer, Me410B-2 U-2 R-4 zerstorer, Me410B-2 U-4 zerstorer, P-40N Warhawk, Ta-154 Moskito, Ta-152C1 R31, Ta-152H-1, Ta-183 Huckebein (Raven) Interceptor, Avro Lancaster Mk III heavy bomber, Ho-229 V5 Supersonic Fighter /bomber, Avro Lancaster GS heavy bomber, and a B-24J Liberator.

What's interesting in Shockwave Production's add on to Microsoft Combat Flight Simulator 3: Battle for Europe is the vast increase in the amount of available World War II military aircraft in the European Theater of Operations. Also there are 4(or 5 with a free patch) new bombers in which the player can man different aircrew positions.

It can be very interesting to use the bombsight to drop newly added ordnance onto a target like the Little Boy, Fat Man, German radiological dirty bombs or the Tallboy bomb. However some of these aircraft are hypothetical especially the German designs some of which never saw aerial combat during World War II. So is the German ordnance hypothetical as well. Another feature in this add on is that you can fly bombers in a formation, with FLAK effects when under enemy anti aircraft fire. Twenty-five missions of the Memphis Belle B-17 Flying Fortress are included in this add on. One thing that's lacking though is a sophisticated simulation of the Norden Bombsight which B-17 Flying Fortress the Mighty 8th has a decent representation of albeit not fully emulating the Norden computer, it's gyrostabilizer and ancillary equipment. Instead the Bombsight in Firepower is fixed but it is functional. Even so, this is workable in the simulation add on with a fixed sight but it takes a lot of practice. Firepower is one of the most popular add ons to Microsoft Combat Flight Simulator 3.

Glasgow Army Airfield Norden Bombsight Storage Vault

The Glasgow Army Airfield Norden Bombsight Vault was listed on the National Register of Historic Places in 2011.It is a one-story 16.5 by 13 feet (5.0 m × 4.0 m) concrete shed with a 11 by 6 feet (3.4 m × 1.8 m) projection that has 8 inches (0.20 m)-thick walls and a poured concrete floor. It protected Norden bombsights at the Glasgow Army Airfield during World War II. The bombsights were top-secret and were used on B-29 bombers. It had steel vault doors which have been removed.

Gyroscopic autopilot

The gyroscopic autopilot was a type of autopilot system developed primarily for aviation uses in the early 20th century. Since then, the principles of this autopilot has been the basis of many different aircraft control systems, both military and civilian.

Joe Smith, American

Joe Smith, American is a 1942 American spy film directed by Richard Thorpe and stars Robert Young and Marsha Hunt. The film, loosely based on the story of Herman W. Lang, and the theft of plans of a top-secret bombsight, is the account of a worker at an aviation factory who is kidnapped by enemy spies. The opening credits contained the following written prologue: "This story is about a man who defended his country. His name is Joe Smith. He is an American. This picture is a tribute to all Joe Smiths."

Joe Smith, American was the first in a series of B films made at MGM under the supervision of Dore Schary who also wrote the initial treatment, based on "his own yarn". His story was later adapted to a postwar setting and new characters to become The Big Operator (1959).

Lotfernrohr 7

The Carl Zeiss Lotfernrohr 7 (Lot meant "Vertical" and Fernrohr meant "Telescope"), or Lotfe 7, was the primary series of bombsights 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 bombsights. The Lotfe 7C, appearing in January 1941, was the first one to have gyroscopic stabilization.

Low Level Bombsight, Mark III

The Low Level Bombsight, Mark III, sometimes known as the Angular Velocity Sight, was a Royal Air Force (RAF) bombsight designed for attacks by aircraft flying below 1,000 feet (300 m) altitude. It combined components of the Mark XIV bomb sight with a new mechanical computer. It featured a unique solution for timing the drop, projecting a moving display onto a reflector sight that matched the apparent motion of the target at the right instant.

The Mk. III was designed for, and mostly used by, Coastal Command aircraft in order to attack submarines. In this role, it was found to increase the chance of destroying a U-Boat by 35%, and damaging it by 60%. It also saw some use in Bomber Command on the De Havilland Mosquito in the tactical role, and in a single case, on the Avro Lancaster. It remained in use in the post-war era, equipping the Avro Shackleton throughout that aircraft's lifetime until 1991.

Mark XIV bomb sight

The Mark XIV Computing Bomb Sight was a bombsight developed by Royal Air Force (RAF) Bomber Command during the Second World War. The bombsight was also known as the Blackett sight after its primary inventor, P. M. S. Blackett. Production of a slightly modified version was also undertaken in the United States as the Sperry T-1, which was interchangeable with UK-built version. It was the RAF's standard bombsight for the second half of the War.

Developed starting in 1939, the Mk. XIV began replacing the First World War-era Course Setting Bomb Sight in 1942. The Mk. XIV was essentially an automated version of the Course Setting sight, using a mechanical computer to update the sights in real-time as conditions changed. The Mk. XIV required only 10 seconds of straight flight before the drop and automatically accounted for shallow climbs and dives. More importantly, the Mk. XIV sighting unit was much smaller than the Course Setting sight, which allowed it to contain a gyro stabilization platform. This kept the sight pointed at the target even as the bomber manoeuvred, dramatically increasing its accuracy and ease of sighting.

The Mk. XIV was theoretically less accurate than the contemporary Norden bombsight but was smaller, easier to use, faster-acting and better suited to night bombing. In practice, it demonstrated accuracy roughly equal to the Norden's. It equipped the majority of the RAF bomber fleet during the second half of the war; small numbers of the Stabilized Automatic Bomb Sight and Low Level Bombsight, Mark III were used in specialist roles. The Low Level Bombsight was built using parts of the Mark XIV, stabilized in pitch rather than roll.

A post-war upgrade, the T-4, also known by its rainbow code Blue Devil, connected directly to the Navigation and Bombing System computers to automate the setting of wind speed and direction. This eliminated the one potential inaccuracy in the system, further increased accuracy, and simplified operation. These equipped the V Bomber force as well as other aircraft until their retirement from service in the 1960s.

Mary Babnik Brown

Mary Babnik Brown (November 22, 1907 – April 14, 1991) was an American who became known for having donated her hair to the United States military during World War II. Thirty-four inches long (86 cm), her blonde hair had never been chemically treated or heated with curling irons, and therefore proved resilient enough to use as crosshairs in Norden bombsights for bomber aircraft, which had to withstand a wide range of temperatures and humidity.Brown declined compensation for her donation, believing what she had done was her patriotic duty. She was told at the time only that the hair was needed for meteorological instruments, and had no idea how it had been used until 1987. President Ronald Reagan wrote to her that year on her 80th birthday to thank her, and in 1991 she received a special achievement award from the Colorado Aviation Historical Society during a ceremony at the Air Force Academy in Colorado Springs.

Norden bombsight

The Norden Mk. XV, known as the Norden M series in US Army service, was a bombsight used by the United States Army Air Forces (USAAF) and the United States Navy during World War II, and the United States Air Force in the Korean and the Vietnam Wars. It was the canonical tachometric design, a system that allowed it to directly measure the aircraft's ground speed and direction, which older bombsights could only estimate with lengthy in-flight procedures. The Norden further improved on older designs by using an analog computer that constantly calculated the bomb's impact point based on current flight conditions, and an autopilot that let it react quickly and accurately to changes in the wind or other effects.

Together, these features seemed to promise unprecedented accuracy in day bombing from high altitudes; in peacetime testing the Norden demonstrated a circular error probable (CEP) of 75 feet (23 m), an astonishing performance for the era. This accuracy would allow direct attacks on ships, factories, and other point targets. Both the Navy and the USAAF saw this as a means to achieve war aims through high-altitude bombing; for instance, destroying an invasion fleet by air long before it could reach US shores. To achieve these aims, the Norden was granted the utmost secrecy well into the war, and was part of a then-unprecedented production effort on the same scale as the Manhattan Project. Carl L. Norden, Inc. ranked 46th among United States corporations in the value of World War II military production contracts.In practice it was not possible to achieve the expected accuracy in combat conditions, with the average CEP in 1943 of 370 metres (1,200 ft) being similar to Allied and German results. Both the Navy and Air Forces had to give up on the idea of pinpoint attacks during the war. The Navy turned to dive bombing and skip bombing to attack ships, while the Air Forces developed the lead bomber concept to improve accuracy, while adopting area bombing techniques by ever larger groups of aircraft. Nevertheless, the Norden's reputation as a pin-point device lived on, due in no small part to Norden's own advertising of the device after secrecy was reduced late in the war.

The Norden saw some use in the post-World War II era, especially during the Korean War. Post-war use was greatly reduced due to the introduction of radar-based systems, but the need for accurate daytime attacks kept it in service for some time. The last combat use of the Norden was in the US Navy's VO-67 squadron, which used them to drop sensors onto the Ho Chi Minh Trail as late as 1967. The Norden remains one of the best-known bombsights of all time.

SABS

SABS may refer to:

St. Andrews Biological Station, a Fisheries and Oceans Canada research centre

Sultan Abu Bakar School (SABS), Kuantan

Stabilizing Automatic Bomb Sight, a World War II bombsight used by the RAF Bomber Command

South African Bureau of Standards

Southern Appalachian Botanical Society

Skip bombing

Skip bombing was a low-level bombing technique independently developed by several of the combatant nations in World War II, notably Australia, Britain, and the United States. It allows an aircraft to successfully attack shipping by skipping the bomb across the water like a stone. Dropped at very low altitudes, the bomb never rises more than about 5 metres (16 ft) above the surface of the water, ensuring that it will hit the side of the ship as long as it is aimed correctly in direction.

As the technique required the aircraft to fly at very low altitudes directly at the ship, it made the task of shooting down the aircraft easier as well. In the immediate pre-war era, there was considerable effort to develop new bombsights that would allow the aircraft to remain at higher altitudes. The most notable was the US Navy's Norden bombsight, which was fit to most Navy aircraft. In practice, these proved largely useless, and the skip-bombing technique was soon introduced operationally.

After Pearl Harbor (December 1941), it was used prominently against Imperial Japanese Navy warships and transports by Major William Benn of the 63rd Squadron, 43rd Bomb Group (Heavy), Fifth Air Force, United States Army Air Forces in the Southwest Pacific area theater during World War II. General George Kenney has been credited with being the first to use skip bombing with the U.S. Army Air Forces.

Stabilized Automatic Bomb Sight

The Stabilized Automatic Bomb Sight, or SABS, was a Royal Air Force bombsight used in small numbers during World War II. The system worked along similar tachometric principles as the more famous Norden bombsight, but was somewhat simpler, lacking the Norden's autopilot feature.

Development had begun before the War as the Automatic Bomb Sight, but early bomber operations proved that systems without stabilization of the bombsight crosshairs were extremely difficult to use under operational conditions. A stabilizer for the ABS began development, but to fill the immediate need for a new bombsight, the simpler Mark XIV bomb sight was introduced. By the time the SABS was available, the Mark XIV was in widespread use and proving good enough that there was no pressing need to replace it.

The SABS briefly saw use with the Pathfinder Force before being turned over to No. 617 Squadron RAF, the famed "Dambusters", starting in November 1943. This squadron's Avro Lancasters were undergoing conversion to dropping the 12,000 pounds (5,400 kg) Tallboy bomb as a precision weapon, and required the higher accuracy of the SABS for this mission. In this role the SABS demonstrated superb accuracy, routinely placing bombs within 100 yards (91 m) of their targets when dropped from about 15,000 feet (4,600 m) altitude.

The system remained hand built throughout its history and was produced in small numbers. In the end, the 617 would also be the only squadron to see operational use of the SABS, using it with the Tallboy and the larger 22,000 pounds (10,000 kg) Grand Slam. Some Avro Lincolns also featured the SABS, but saw no operational use.

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