Bombsight

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.

Then, in World War II, tachometric bombsights were often combined with radar systems to allow accurate bombing through clouds or at night.

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.

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.

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.

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.

This action measured the wind speed, and moved the sights to the proper angle to account for it, eliminating the need for separate calculations.

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.

[16] 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.

[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.

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.

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.

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.

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.

Some of the traditional inputs, like airspeed and altitude, could even be taken directly from the aircraft instruments, eliminating operational errors, and allowing constant recalculation of essential target-tracking and bomb release parameters.

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.

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.

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.

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.

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.

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.

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.

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.

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.

[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 nuclear weapons – a circular error probable (CEP) of about 3,000 feet (910 m) was considered adequate.

[38] As mission range extended to thousands of miles, bombers started incorporating inertial guidance and star trackers to allow accurate navigation when far from land.

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.

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.