Stabilised Automatic Bomb Sight facts for kids

The Stabilised Automatic Bomb Sight (SABS) was a special device used by the Royal Air Force (RAF) during World War II. It was a type of bombsight that helped planes drop bombs very accurately. The SABS worked in a similar way to the famous American Norden bombsight, but it was a bit simpler because it didn't have an autopilot feature.

Work on this bombsight started before the war. It was first called the Automatic Bomb Sight. However, early bombing missions showed that bombsights needed to be stable. This meant they had to stay pointed at the ground even when the plane moved. So, a new version, the SABS, was developed with a special stabilizer.

Before the SABS was ready, the RAF used a simpler bombsight called the Mark XIV bomb sight. By the time the SABS was finished, the Mark XIV was already widely used and worked well enough for most missions.

The SABS was used for a short time by the Pathfinder Force. Then, in November 1943, it was given to No. 617 Squadron RAF. This squadron's Avro Lancaster planes were being set up to drop huge, precise bombs like the 12,000 pounds (5,400 kg) Tallboy and the even bigger 22,000 pounds (10,000 kg) Grand Slam. They needed the SABS for its high accuracy. With the SABS, bombs often landed within 100 yards (91 m) of their targets, even when dropped from about 15,000 feet (4,600 m) high.

Only a small number of SABS units were made, all by hand. No. 617 Squadron was the only one to use the SABS in real missions. Some Avro Lincoln planes also had the SABS, but they didn't use it in combat.

How Bombsights Developed

Early Bombing Challenges

When a bomb leaves an airplane, it doesn't just fall straight down. It follows a complicated path because of air drag (air pushing against it), wind, and gravity. A bomb dropped from 100 meters will follow a different path than one dropped from 5,000 meters.

Early bombsights couldn't calculate this complex path directly. Instead, they used a "range" measurement. This was the distance the bomb traveled forward as it fell. This distance could be turned into an angle for the bombsight. The bomb aimer would set their sights to this "range angle" and drop the bombs when the target lined up.

However, these basic systems didn't account for wind. If there was wind, the bombs would miss the target. For example, a headwind would slow the plane's speed over the ground, making bombs fall short.

Solving the Wind Problem

Some early bombsights could adjust for wind blowing directly from the front or back. But this made it hard to attack targets that were moving or if the wind was blowing from the side.

Later, bombsights like the Course Setting Bomb Sight used vector algebra to figure out how wind affected the plane. To use these "vector bombsights," the bomb aimer needed to know the wind's speed and direction. Once these numbers were put into the system, the bombsight would move its aiming points to account for the wind. It would also show the best angle for the plane to approach the target.

These systems were limited by how long it took to measure the wind and calculate the settings. If the wind changed, it was hard to correct. Bomb runs often had to be very long to make sure the plane was flying on the correct path.

Newer "Tachometric" Bombsights

In the 1930s, new mechanical computers changed how bombsights worked. These computers could continuously calculate things. For bombsights, the unknown factor was how the wind affected the bomber's movement.

To measure this, the bomb aimer would guess the wind speed and direction. The computer would then move the bombsights to keep them pointed at the target. If the guesses were wrong, the target would appear to "drift" in the sights. The bomb aimer would then adjust the wind estimates until the target stayed still in the sights. This way, the wind was measured while the plane was flying towards the target.

This new method had two big benefits:

• The wind was measured during the actual bomb run, so it was always up-to-date.
• The bomb aimer just had to keep the sight on the target using a small telescope or reflector sight. All the complex calculations were done by the machine, reducing human error.

These "tachometric" or "synchronous" bombsights were a big step forward.

The Norden Bombsight

The US Navy found that bombsights were often not level with the ground, which caused errors. They realized that a system that automatically kept the sight level, called "stabilization," could make bombing twice as accurate.

So, in the 1920s, the US Navy started working with Carl Norden on a bombsight that used gyroscopes to stay level. The Norden bombsight was very accurate, able to drop bombs within a few yards of targets from 4,000 to 5,000 feet (1,200 to 1,500 m) high. The US Navy wanted to use it to attack ships from high altitudes, out of reach of ship guns.

The US Army Air Corps also saw the Norden as a powerful weapon. They planned to use it for strategic bombing – attacking factories and other important targets far behind enemy lines.

The British Air Ministry heard about the Norden in 1938. They were already developing their own Automatic Bomb Sight (ABS), which was similar but lacked stabilization. The British tried to buy the Norden, but negotiations were difficult.

The Mark XIV Bombsight

In early World War II missions, the RAF Bomber Command realized their old bombsights were not good enough. Planes had little time to spot targets and often had to dodge enemy fire. If the plane turned, the bombsight couldn't be used.

In December 1939, the RAF asked for a new bombsight that would let bombers take evasive action during the bomb run. This meant the bombsight needed to be stabilized.

The ABS was still far from ready, and adding stabilization would delay it more. The Norden was a good option, but the US Navy still wouldn't sell it to the RAF. So, in 1939, the Royal Aircraft Establishment began working on a simpler solution.

This led to the Mark XIV bomb sight. The Mk. XIV had a separate computer box that automatically took in altitude, airspeed, and direction. The bomb aimer just had to guess the wind speed and direction, and select the bomb type. Everything else was automatic.

The Mk. XIV was complex to build, but it was produced in both the UK and US. It quickly became the main bombsight for Bomber Command by 1942. It was much better than older sights, but it wasn't a "precision" system; it was more for "area bombing."

The SABS is Born

Even with the Mk. XIV, the RAF still needed a more accurate bombsight, especially for the new "earthquake bombs" which needed to hit targets very precisely. By 1942, the Norden was still not available to the British, even though US bombers were using it in the UK to attack Germany.

So, the British went back to their earlier idea: combining the ABS with a new stabilizer. This created the SABS. Like the Norden, the SABS's stabilizer was separate from the bombsight. But the SABS stabilizer moved the whole bombsight, not just the aiming crosshairs. The SABS didn't have an autopilot because RAF bombers already had one. Instead, the bomb aimer's corrections were sent to a display in the cockpit for the pilot.

SABS in Action

First Missions

A small number of SABS units became available in early 1943. They first went to No. 8 Group RAF, the "Pathfinder Force," but they only used them briefly. Then, the SABS units were given to No. 617 Squadron RAF. This squadron needed the high accuracy of the SABS for their new "earthquake bombs."

No. 617 Squadron first used the SABS in a real mission on November 11/12, 1943. They attacked a railway bridge in southern France. However, none of their ten 12,000 lb (5,400 kg) bombs hit the bridge.

Improving Accuracy

The SABS was used for aiming in daylight and for aiming at special "target indicators" dropped by other planes at night. The accuracy of night drops depended on how well the targets were marked. For example, during attacks on a German V-weapon site in December 1943, the Tallboy bombs landed very close to each other (within 94 yd (86 m)). But the target markers themselves were 350 yd (320 m) off the target.

Results got better over time. On February 8/9, 1944, Wing Commander Leonard Cheshire visually marked a factory in France. Eleven Lancasters then dropped bombs directly on the factory, knocking it out of the war with very few civilian casualties.

As crews became better at using the SABS, accuracy improved a lot. Between June and August 1944, 617 Squadron achieved an average accuracy of 170 yd (160 m) from 16,000 ft (4,900 m). By February-March 1945, this improved to 125 yd (114 m).

Famous Missions

The SABS is most famous for its role in sinking the German battleship Tirpitz on November 12, 1944. Thirty Lancasters from 617 and No. 9 Squadron RAF attacked the Tirpitz. At least two bombs from 617 hit the ship, causing it to capsize.

Another successful attack was on June 14, 1944, against German E-boat pens in France. One bomb went through the heavily protected roof, putting the base out of action.

After the War

As the war in Europe ended, plans were made for a bombing campaign against Japan, called Tiger Force. This force would use new Avro Lincoln bombers.

Since fewer than 1,000 SABS units were made, there weren't enough for the new force. The war ended before Tiger Force was sent to Japan.

Some Lincolns, including those of 9 and 44 Squadron, continued to use the SABS after the war. However, the SABS was no longer used after the Lincolns were replaced by jet bombers like the English Electric Canberra. The Canberra used the Mk. XIV bombsight, which was connected to the plane's navigation computer for accurate wind information.

How the SABS Worked

The SABS had three main parts: the bombsight itself (called the "range unit"), the stabilizing system, and a "bombing directional indicator" for the pilot.

The Range Unit

The range unit was the core of the SABS. It was a mechanical computer that did three main things:

• It calculated how fast the plane was moving over the ground. This information was sent to a reflector sight for the bomb aimer to see. It used a special part called a ball-and-disk integrator to do this.
• It adjusted for how long it took bombs to fall to the ground. Different bombs fall at different speeds. The bomb aimer would select the "bomb class," which would adjust the height setting to account for this.
• It adjusted for "trail." This is the distance the bomb falls behind the plane as it drops. The SABS tilted the entire range unit backward to account for this. If the plane was flying sideways into the wind (crabbing), the sight could also rotate to account for "side trail."

The range unit also had the bomb release system. As the plane got closer to the target, electrical contacts would line up, turning on warning lights for the pilot and bomb aimer. Then, at the exact right moment, the bombs would be released automatically.

The bombsight ran on the plane's 24-volt electrical power.

The Stabilizer

The stabilizer unit had two parts: a box with two gyroscopes and a system that used air pressure to keep the range unit level with the ground. It was like an inertial platform.

One good thing about the SABS was its automatic "erection" system. Gyroscopes usually need time to be set perfectly upright. The SABS used a pendulum that would gently push the gyro until it was level, making it faster to set up.

The gyros were connected to air valves. If the plane moved, causing the gyros to tilt, the valves would change air pressure to move pistons. These pistons would then adjust the platform to keep the bombsight level. This system was smoothed by oil-filled shock absorbers.

The entire bombsight sat inside this stabilized frame, which could move up to 20-25 degrees to stay level.

The stabilizer used compressed air from the same unit that powered the plane's automatic pilot. It took about 15 minutes for the gyros to get up to full speed and stabilize.

Autopilot Feature (Planned)

Near the end of the war, there were plans to add an autopilot feature to the SABS, like the American Norden. There were also requests for the sighting system to have variable zoom. However, neither of these changes were put into service.

Using the SABS

Using the SABS involved several steps, but they were done in order, making the bomb aimer's job easier during the final approach.

Setting Up

Before the mission, or early in the flight, the bomb aimer would set two dials on the range unit: the "trail scale" and "bomb class letter." These estimated how much the bomb would slow down and how quickly it would fall. These settings stayed the same for the whole mission.

During the Approach

About 15 minutes before reaching the target, the pilot would turn on the air supply to the bombsight. The bomb aimer would then start the stabilizer, waiting for the gyros to get up to speed. Once ready, the bombsight was good to go.

As the plane flew level towards the target, the bomb aimer would dial in the plane's altitude and airspeed. They could also put in rough guesses for wind speed and direction, which helped simplify the bomb run.

During the Bomb Run

When the target became visible, the bomb aimer would use a control wheel to point the reflector sight at it. There were two wheels: a large one for small, precise movements and a small one for quick adjustments. Once the target was roughly centered, the bomb aimer would switch to tracking mode, and the official bomb run would begin.

As the plane got closer, if the wind estimate was wrong, the sight would drift away from the target. The bomb aimer would make small adjustments to the control wheel to keep the sight on the target. This also updated the estimated wind speed. Usually, only a few adjustments were needed to fix any drift.

If the plane was off to the side of the target, the bomb aimer used another wheel to rotate the sight and put the crosshair back on the target. To get the plane back on the correct path, the pilot had to turn the plane *past* the correct heading, then turn back. The SABS helped with this by multiplying the error angle by four before sending it to the pilot's display. This made the pilot automatically overcorrect, bringing the plane back onto the right approach.

Dropping the Bombs

At this point, the bombsight had an accurate measurement of the plane's true movement over the ground. As long as the sight stayed on target, the bombsight would work correctly.

Setting the bomb type and trail moved a special cam inside the unit. As the plane got closer to the target, a metal part attached to the sight's rotation shaft would hit the first electrical contact, turning on the bomb drop timing lights. As the plane continued, more movement would cause the bombs to release.

Measuring Wind

The SABS could also be used to measure wind for navigation. By simply tracking any object on the ground with the control wheels, the wind speed and direction would be shown on the bombsight's dials.

Similar Designs

• Lotfernrohr 7, a similar German bombsight from late in the war.