The Mark XIV Bomb Sight was a bombsight developed by Royal Air Force (RAF) Bomber Command during the Second World War. It 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 the 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. However, it 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.
History Course-setting sights A problem with early bombsights was that they could only correct for the effects of the wind in a simple way and required the bomber to fly directly up- or down-wind from the target, to minimise the complexity of the required calculations. This made it difficult to attack moving targets and allowed anti-aircraft artillery to sight their weapons along the wind line.
In 1917 Harry Wimperis introduced the Course Setting Bomb Sight (CSBS), which replaced the tables and timings used in earlier sights with a simple mechanical calculator capable of solving the sideways drift due to the wind. As the bomb aimer turned a wind direction knob, the main portion of the sight was pushed to the left or right, indicating the required angle to fly to take the aircraft over the target. The CSBS was the first bombsight that allowed the bomber to approach the target from any direction, which offered greatly increased tactical freedom.
The downside to the CSBS was that the settings, made through four main input dials, were useful for one operational setup, a given altitude and heading. If the aircraft manoeuvred, the entire system had to be reset. Additionally, the system required the bomber's direction to be compared to objects on the ground, requiring a time-consuming process sighting through thin metal wires against a suitable object on the ground below. As the sight was not stabilized, any manoeuvres to correct for misalignment interfered with the ability to measure the heading, so these corrections further extended the bomb run. The CSBS generally required the bomber to fly straight and level for a lengthy time.
Although the need for an improved CSBS was known in the 1930s, little work on developing such a sight was carried out. That was because an entirely new class of tachometric bombsights were being developed, which offered dramatically improved accuracy and automated much of the setup. The RAF was working on such a design, the Automatic Bomb Sight, but development was slow and it had not been accepted for use when the war started. Learning of a similar design developed by the US Navy, the Air Ministry began extensive negotiations in an effort to gain a production licence for this Norden bombsight. The US Navy constantly refused these requests, deeming it too sensitive to risk losing over Germany, and their refusals ultimately led to significant political friction between the two nations. Ironically, the plans of the Norden bombsight had been passed to the German military by a US-based spy in 1938.
As the war started, revised versions of the CSBS, the Mk. VII and Mk. IX, remained universal. The Mk. X, a more extensive improvement, was in mass production and being readied for service entry.
A pressing need
On 28 March 1939, the head of RAF Bomber Command Air Chief Marshal Sir Edgar Ludlow-Hewitt hosted a conference on the state of Bomber Command. Among the many problems with operational readiness, he noted that RAF bombs were much too small and that bombsight technology was obsolete. Given the problems of obtaining a modern bombsight, he pressed for the creation of a high-speed bomber design that could safely attack at low levels.
On 18 December 1939, Vickers Wellington bombers carried out a raid on German shipping in what became known as the Air Battle of the Heligoland Bight. Detected on radar and engaged en route to their targets, over half the attacking force was destroyed or damaged beyond repair. Ludlow-Hewitt presented a report on the attack on 22 December 1939, noting that flying straight and level for the CSBS made the bombers easy targets for fighters and anti-aircraft gunners. He again pressed for a new bombsight that featured stabilization to allow the aircraft to manoeuvre while it approached the target.
The CSBS and the improved version, the Mk. X, were insufficient, as both were too large to easily stabilize. Because of the way it was built, the Automatic Bomb Sight could be equipped with a stabilizer but it was estimated that it would be some time before it could be modified and brought into production. The Norden did offer stabilization but it also required relatively long setup times and was still not available for purchase.
Another solution to the vulnerability of the RAF bombers was to fly at night, which was taken up as the primary tactic of Bomber Command. The Mk. X proved to be very difficult to read at night and bombers that carried it were quickly refitted with the earlier Mk. VII or Mk. IX sight. The Norden was unable to work at night at all; the bomb aimer had to locate the target long in advance of the drop point using a built-in telescope and targets were simply not visible at the required distances in low light.
What was needed was a new bombsight, one that could be very quickly set up, had useful illumination of the crosshairs for night use, and was stabilized so the bomb aimer could watch the approach as the bomber was manoeuvring. An early attempt was the Mk. XI, which mounted a cut-down CSBS on the front of a gyro unit taken from a Sperry Gyroscope artificial horizon, to provide stabilization in the horizontal plane, useful for aiding drift measurements and corrections. But it was no easy task to manually calculate the range angle on the separate Course and Speed Calculator. It was introduced in 1941, but only a small number were produced.
Blackett's solution
The request for a new bombsight was quickly passed on to the Royal Aircraft Establishment, where Patrick Blackett of the Aeronautical Research Committee volunteered to lead the effort. His solution to the problem was a thorough revision of the CSBS concept.
The advance in the Blackett design was the way the sighting head was aimed. Instead of dialling the parameters into the sight directly as in the CSBS, these inputs were dialled into a separate console. The console was equipped with repeaters for each of the aircraft instruments needed to operate the sight, like altitude and airspeed. The operator simply turned the dials on the console so their indicator arrows matched the readings on the instruments displayed in the same location, known as laying needle on needle. This reduced the possibility that the numbers would not be changed as the bomber manoeuvred, but required so much manual working that a new crew member was introduced to operate the console, the bomb-aimer's mate.
The inputs operated by the bomb-aimer's mate drove a mechanical calculator inside the console, or computor. The output of the calculator drove flexible shafts that rotated the sight head to the proper angles in azimuth and altitude, representing the wind drift and range angle. The sight head replaced the older wire crosshairs with a modern reflector sight that was easy to see at night. The sight could be rotated manually to view objects well in front of the aircraft, allowing the bomb aimer to select among a wider variety of objects to use for drift measurements.
With the CSBS, the sighting system and calculator were the same device, which required the bombsight to be fairly large. With this restriction removed, the sight head was much smaller and lighter than the older versions. The resulting sight head was easy to mount on a stabilizer system, adapted from the same Sperry gyroscope as earlier experiments. With the sight head stabilized, the bomb aimer could continue to measure drift even as he indicated turns to the pilot, eliminating the need to correct, re-measure and correct again. The remote console and second operator eliminated the need for the bomb aimer to look away from the sights, to make adjustments while on the bomb run. As a result of these changes, short aiming periods of a few seconds would be enough for an accurate drop.
The new Mk. XII bombsight was first tested in September and October 1940 and by the end of October, 20 examples had been built. A slightly improved version, the Mk. XIII was designed but not put into production.
Automation
The need for the second crewman was an obvious problem with the Mk. XII, especially as few bombers of the era had enough room for the operator. Working with Henry Braddick, Blackett developed a new version of the calculator that included aircraft instruments inside the computor, eliminating the need for the needle-on-needle matching and completely automating the calculations. After the initial design was complete, Blackett moved onto other matters with RAF Coastal Command, where he continued development of his theories of operational research.
The new design reduced the bomb aimer's setup workload to dialling in four settings. Two of these could be set before the mission: the altitude of the target above sea level and the terminal velocity of the bomb, a function of the particular bomb being used on that mission. The only settings that had to be adjusted in flight were the measured wind direction and speed. The altitude, airspeed and course were all measured by the internal instruments and presented to the user in windows on the side of the computor case. Once set, the computor would automatically update the calculations and display the resulting bombing angle in another window. The computor could even account for steady changes in altitude, allowing the bomb run to take place in a shallow climb of up to 5degrees or dive of up to 20 degrees.