Mark XIV bomb sight facts for kids

The Mark XIV Bomb Sight was a special aiming device used by the Royal Air Force (RAF) during World War II. It was also called the Blackett sight after its main inventor, P. M. S. Blackett. The United States also made a similar version called the Sperry T-1. This bombsight became the RAF's main tool for dropping bombs in the second half of the war.

Developed from 1939, the Mk. XIV started replacing older bombsights from World War I around 1942. It was like an automatic version of the older sights. It used a mechanical computer to constantly update the aiming point as conditions changed. This meant bombers only needed to fly straight for about 10 seconds before dropping bombs. The Mk. XIV could even handle slight climbs or dives.

A big improvement was that the Mk. XIV's aiming part was much smaller. This allowed it to have a special gyro stabilization platform. This platform kept the sight pointed at the target even when the bomber moved around. This made bombing much more accurate and easier to do.

Even though another bombsight, the Norden bombsight, was thought to be more accurate, the Mk. XIV was smaller, simpler to use, and worked faster. It was also much better for bombing at night. In real missions, it was just as accurate as the Norden. Most RAF bombers used the Mk. XIV during the war. After the war, an improved version called the T-4 (or Blue Devil) was made. It connected directly to the plane's navigation computers, making it even more accurate and easier to use. These sights were used until the 1960s.

How Bombsights Changed Over Time

Early Bombing Challenges

In the early days, bombsights had a big problem: they couldn't easily correct for wind. This meant bombers often had to fly directly into or away from the wind when attacking a target. This made it hard to hit moving targets and also made the bombers easy targets for anti-aircraft artillery.

In 1917, a person named Harry Wimperis created the Course Setting Bomb Sight (CSBS). This sight used a simple mechanical calculator to figure out how much the wind would push the bomb sideways. When the bomb aimer turned a knob for wind direction, the sight would move to show the correct angle the plane needed to fly. The CSBS was the first bombsight that let bombers approach a target from any direction, giving them much more freedom.

Problems with Older Sights

The CSBS had some downsides. Its settings, like altitude and direction, were only good for one specific setup. If the plane moved, everything had to be reset. Also, the bomb aimer had to compare the plane's direction to objects on the ground using thin metal wires. This took a lot of time. Since the sight wasn't stable, any small turns by the plane messed up the measurements, making the bombing run even longer. Bombers had to fly straight and level for a long time, which made them vulnerable.

Even though everyone knew a better CSBS was needed in the 1930s, not much work was done. This was because new, more advanced bombsights were being developed that promised much better accuracy and automation. The RAF was working on its own design, the Automatic Bomb Sight, but it was slow to develop. They also tried to get the Norden bombsight from the US Navy, but the US refused, fearing it might fall into enemy hands. Ironically, the Norden's plans had already been given to Germany by a spy in 1938.

So, when the war began, older versions of the CSBS were still widely used. A slightly improved version, the Mk. X, was just starting to be mass-produced.

Why a New Bombsight Was Needed Urgently

On March 28, 1939, the head of RAF Bomber Command, Sir Edgar Ludlow-Hewitt, held a meeting. He pointed out that RAF bombs were too small and their bombsights were old-fashioned. He pushed for a new, fast bomber that could attack safely at low levels.

Then, on December 18, 1939, Vickers Wellington bombers attacked German ships. They were spotted by radar and many were shot down or badly damaged. Ludlow-Hewitt reported that flying straight and level for the CSBS made the bombers easy targets. He again asked for a new bombsight that was stable, so planes could move while aiming.

The CSBS and Mk. X were too big to be easily stabilized. The Automatic Bomb Sight could be stabilized, but it would take too long to get ready. The Norden was stable but also needed long setup times and wasn't available.

Another solution was to fly at night. But the Mk. X was hard to read in the dark. The Norden couldn't work at night at all because its telescope needed to see targets far away, which was impossible in low light.

So, the RAF needed a new bombsight that could be set up quickly, had good lights for night use, and was stable. An early attempt, the Mk. XI, tried to add a gyro stabilizer to a CSBS, but it was still hard to use.

Blackett's Clever Idea

The request for a new bombsight went to the Royal Aircraft Establishment. Patrick Blackett offered to lead the project. His idea was to completely redesign the CSBS.

Blackett's big change was how the aiming part worked. Instead of putting all the settings directly into the sight, these numbers were entered into a separate control box, called a computor. This box had dials that showed the plane's altitude and airspeed. The operator just turned the dials to match the plane's instruments. This made sure the settings were always correct. This new system needed an extra crew member, called the bomb-aimer's mate, to operate the computor.

The computor had a mechanical calculator inside. This calculator used the numbers entered to move the aiming part of the sight to the correct angles. The sight itself no longer used old wires but a modern reflector sight that was easy to see at night. The sight could also be turned manually to look far ahead of the plane, allowing the bomb aimer to pick any object on the ground to measure wind drift.

Since the calculator was separate, the aiming part of the sight was much smaller and lighter. This made it easy to put on a stabilizer system, using the same type of gyroscope as earlier experiments. With the sight stable, the bomb aimer could keep measuring drift even while telling the pilot to turn. This meant no more stopping, re-measuring, and correcting. The separate control box and second operator meant the bomb aimer didn't have to look away from the sight to make adjustments. Because of these changes, accurate bombing could happen after just a few seconds of aiming.

The first version, the Mk. XII, was tested in late 1940. An improved version, the Mk. XIII, was designed but not mass-produced.

Making It Automatic

The need for a second crew member was a problem, as many bombers didn't have enough space. Blackett and Henry Braddick then developed a new computor that had the aircraft instruments built right in. This removed the need for matching dials and made the calculations fully automatic. After this, Blackett moved on to other projects.

The new design, the Mk. XIV, made the bomb aimer's job much simpler. They only had to set four things. Two could be set before the mission: the target's height above sea level and the bomb's terminal velocity (how fast it falls). The only things adjusted during flight were the wind direction and speed. The computor automatically updated all other calculations and showed the correct bombing angle. It could even handle steady changes in altitude, allowing the plane to climb or dive slightly during the bomb run.

The Mk. XIV was first tested in June 1941. It was the first modern bombsight that allowed accurate bombing right after the plane had been turning sharply. It only needed about 10 seconds to settle. This quick settling time was very useful for night bombing. It allowed bombers to fly in a "corkscrew" path (climbing and turning) and then level out just before dropping bombs. This made it harder for night fighters to track them and helped avoid anti-aircraft fire.

The Mk. XIV wasn't as accurate as the Norden at very high altitudes (over 20,000 feet). But for typical night bombing altitudes (12,000 to 16,000 feet), the differences were small. For missions needing extreme accuracy, like with the Tallboy bombs, a different sight called the Stabilized Automatic Bomb Sight (SABS) was used in small numbers.

Making and Using the Mk. XIV

We don't know exactly when the Mk. XIV started being mass-produced in the UK. Testing began in January 1942, and squadrons started getting them in March. Small workshops made them at first, but production was too slow. By mid-1943, 900 sights were being made each month. This was enough to equip new heavy bombers like the Handley Page Halifax.

To meet the demand for other planes, like the de Havilland Mosquito, the RAF looked to US companies. Sperry Gyroscope was interested and felt they could quickly adapt the Mk. XIV for US production. Sperry arranged for AC Spark Plug to manufacture them.

The US companies made some small changes to make it easier to produce. The final design, called the Sperry T-1, was ready in May 1942. It worked perfectly with UK-made versions. Full production started in November, and T-1s arrived in the UK from March 1943. These were sent to medium bombers like the Wellington, while UK-made versions went to heavy bombers.

Later Versions and Post-War Use

In May 1943, Sir Arthur Harris, the head of Bomber Command, asked for the maximum bombing altitude to be increased to 30,000 feet. The Mk. XIVA, which arrived in December 1944, allowed bombing up to 25,000 feet and had a more accurate angle system. It also made it easier to correct for small differences in instrument readings between different aircraft.

Older Mk. XIVs used air pressure to power their gyros. The new Mk. XIVB and T-1B versions used electric gyros instead, which was simpler.

The Mk. XV was a version for the Royal Navy and Coastal Command, used for attacking submarines. Since these attacks were at low altitudes, even small changes in air pressure could cause big errors. The Mk. XV could get altitude readings directly from a radar altimeter, making it much more accurate. The Mk. XVII was a Mk. XV modified for the very fast Naval Mosquito planes, which flew at over 400 miles per hour.

After the war, improved versions based on the T-1 were made in the UK. These T-2 and T-4 (Blue Devil) designs could handle much higher altitudes, speeds, and wind speeds, perfect for jet stream bombing. These were usually part of a larger Navigation and Bombing System that combined information from many sources, like radar and star navigation. This system automatically fed information to the T-4 sight, making it incredibly accurate.

Most wartime optical sights like the Mk. XIV became useless for jet aircraft. Jets flew twice as high and three times as fast, meaning bombs traveled much farther (up to 7 miles). This made it impossible to see the target with an optical sight before the plane had already passed the drop point. So, bombing switched to using radar, and the Mk. XIV was removed from RAF service in 1965.

How the Mark XIV Bombsight Worked

Main Parts

The Mk. XIV had two main parts: the sighting head and the computor. The sighting head was in the bomb aimer's window at the front of the plane. The computor cabinet was separate and usually placed on the left side of the plane. The two parts were connected by flexible cables.

The computor cabinet had only four main controls. On the left side, from top to bottom, were dials for setting wind direction, wind speed, target altitude, and the bomb's terminal velocity. Small windows next to the dials showed the plane's airspeed, course, and the bombing angle. The computor also connected to the plane's air pressure system for accurate airspeed measurements and to the compass for direction.

The sighting head was mounted on a special platform. A small screw allowed it to be leveled. Above the platform was the stabilization gyro. This gyro kept the sight pointed at the target even when the plane rolled. The sight used a reflector to project crosshairs that looked like they were floating in space, making it easy for the bomb aimer to focus on the target.

Using the Sight

The main idea behind the Mk. XIV was to give the bomb aimer more time to focus on getting the plane to the right spot. Since the computor did all the calculations automatically, the bomb aimer could concentrate only on the sight during the bomb run.

The sight had a vertical line, but it was too short to measure drift directly. To fix this, the collimator handle could be used to manually turn the sight forward. This allowed the bomb aimer to pick any object on the ground, even the target itself, to measure drift long before the plane reached it. By moving the handle, the bomb aimer could make sure the drift line stayed on the target. When the handle was released, the sight automatically went back to tracking the correct bombing angle. The handle also folded the sight away for storage.

Many of the numbers needed for the bomb's path were fixed and entered before the mission. For example, the bomb's terminal velocity depended on the type of bomb and didn't change. Other measurements were entered as the plane approached the target.

Measuring the Wind

The only major measurement that couldn't be done automatically or before the mission was figuring out the wind speed and direction. Wind changes with time, location, and altitude. So, it had to be measured accurately near the target. The Mk. XIV manual described one method:

Before the bomb run, the pilot would fly the plane in several different directions, usually about 120 degrees apart. On each leg, the bomb aimer used the sight to measure the drift angle. This was done by turning the wind direction dial on the computor until objects on the ground appeared to move along the line on the sight. The plane's direction and the drift angle were recorded. Then, using a special calculator (the Mk. III Navigation Computor), these angles were entered to find the wind speed and direction. This final wind value was then entered into the Mk. XIV computor.

Other Details

Since the Mk. XIV could calculate for slight climbs or dives, the computor had its own leveling system. This was added to the calculated bombing angle to move the sight. Leveling the system was done on the ground and then left alone.

The bombsight also came with an Emergency Computor, which was a simple circular slide rule. This was used if the main computor broke down. In that case, the bomb aimer would manually enter the basic numbers and read out the correct aiming angle. Wind had to be guessed and calculated by hand.

A separate switchbox controlled the brightness of the lights for the drift scale and the aiming crosshairs.

Accuracy of the Mark XIV

During tests, the Mk. XIV was accurate to about 130 yards from 10,000 feet up. In real missions, the average error was about 300 yards, plus a random error of 385 yards. In comparison, the more complex Stabilized Automatic Bomb Sight (SABS) had a smaller average error of 120 yards under the same conditions.

Reports from 1944 tried to explain these differences. They found that most of the reasons were related to how missions were carried out, not problems with the bombsight itself. For example, the target marker flares used were very large (400 by 500 yards), bombers dropped many bombs at once, and the lead bomber might change the target point during the raid. All these things made it hard to tell exactly how accurate the bombsight was.

The difference between test results and real mission results wasn't unique to the Mk. XIV. The US Norden bombsight, which showed a circular error probable (CEP) of 75 feet in tests, had an average CEP of 1,200 feet in missions in 1943. Like the Mk. XIV, most of this difference was due to things like crew training and how well they could see the target. By 1945, the Norden's CEP improved to 900 feet with changes in bombing techniques.

A later report comparing the Mk. XIV and SABS when dropping Tallboy bombs found that SABS was twice as accurate for bombs that landed near the target. However, the report also noted that the Mk. XIV's ability to allow the plane to maneuver freely often made up for any accuracy difference, especially when a long, straight bomb run wasn't possible.

Images for kids

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  The Mk. XIVA sighting head, which would be mounted in the front of the aircraft and connected to the computor by the cables coiled up on the left. This example is found in the RAF Museum's reserve collection.

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  The Mk. XIVA computor, normally mounted on the left side of the forward fuselage. The wind speed and direction are set on the blue dials, the bomb's terminal velocity and the target altitude on the green dials.

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  The CSBS required the aircraft to remain level while the bomb aimer watched the drift along the thin parallel wires (white).

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  Bomb aimer on an Avro Lancaster demonstrating use of the Mark XIV

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  The T-1A computer, a US-built version of the Mk. XIVA computor. This example retains the scales in the reading windows and a blank levelling card.

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  Mk. XIVA in a Handley Page Halifax in its stowed position with the collimator handle rotated forward and the metal plate over the glass sight