dive bomber

Method

A dive bomber dives at a steep angle, normally between 45 and 60 degrees or even up to a near vertical dive of 80 degrees with the Junkers Ju 87, and thus requires an abrupt pull-up after dropping its bombs. This puts great strains on both the pilot and aircraft. It demands an aircraft of strong construction, with some means to slow its dive. This limited the class to light bomber designs with ordnance loads in the range of 1,000 lb (450 kg) although there were larger examples.

The most famous examples are the Junkers Ju 87 Stuka (short for Sturzkampfflugzeug, dive fighter), which was widely used during the opening stages of World War II often accompanied with the screaming sound of its sirens, the Aichi D3A "Val" dive bomber, which sank more Allied warships during the war than any other Axis aircraft, and the Douglas SBD Dauntless, which sank more Japanese ships than any other allied aircraft type. The SBD Dauntless helped win the Battle of Midway, was instrumental in the victory at the Battle of the Coral Sea, and fought in every US battle involving carrier aircraft.

An alternative technique, glide-bombing (attacks made from an altitude of less than 8,000 feet and at an angle of less than 70°), allowed the use of heavier aircraft, which faced far greater difficulties in recovering from near-vertical approaches, though it required greater use of sophisticated bombsights and aiming techniques, by a specialised member of aircrews, namely a bombardier/bomb aimer. The crews of multi-engined dive-bombers, such as variants of the Junkers Ju 88 and Petlyakov Pe-2, frequently used this technique. The heaviest aircraft to have dive-bombing included in its design and development, the twin engine Heinkel He 177, also utilised a glide-bombing approach; the requirement that the He 177 be able to dive/glide-bomb delayed its development and impaired its overall performance.

Dive bombing was most widely used before and during World War II; its use declined during the war, when its vulnerability to enemy fighters became apparent. In the post-war era, this role was replaced with a combination of improved and automated bombsights, larger weapons and even nuclear warheads that greatly reduced the need for accuracy, and finally by precision guided weapons as they became available in the 1960s. Most tactical aircraft today allow bombing in shallow dives to keep the target visible, but true dive bombers have not been a part of military forces since the start of the jet age.

Accuracy

When released from an aircraft, a bomb carries with it the aircraft's trajectory. In the case of a bomber flying horizontally, the bomb will initially only be travelling forward. This forward motion is opposed by the drag of the air, so the forward motion decreases over time. Additionally, gravity causes the bomb to accelerate after it is dropped. The combination of these two forces, drag and gravity, results in a complex pseudo-parabolic trajectory.

The distance that the bomb moves forward while it falls is known as its range. If the range for a given set of conditions is calculated, simple trigonometry can be used to find the angle between the aircraft and the target. By setting the bombsight to this "range angle", the aircraft can time the drop of its bombs at the instant when the target is lined up in the sight. This was only effective for "area bombing", however, since the path of the bomb is only roughly estimated. Large formations could drop bombs on an area hoping to hit a specific target, but there was no guarantee of success, and huge areas around the target would also be hit. The advantage to this approach, however, was that it is easy to build such an aircraft and fly it at high altitude, keeping it out of range of ground-based defences.

The horizontal bomber was thus ill-suited for tactical bombing, particularly in close support. Attempts at using high-altitude bombing in near-proximity to troops often ended in tragedy, with bombs both hitting their targets and friendly troops indiscriminately. In attacking shipping, the problems of inaccuracy were amplified by the fact that the target could be moving, and could change its direction between the time that the bombs were released and the time that they arrived. Successful strikes on marine vessels by horizontal bombers were extremely rare. An example of this problem can be seen in the attempts to attack the Japanese carriers using B-17s at altitude in the Battle of Midway, with no hits scored. The German battleship Tirpitz was subjected to countless attacks, many while in dock and immobile, but was not sunk until the British brought in enormous 12,000 lb (5,400 kg) Tallboy bombs to ensure that even a near miss would be effective.

An aircraft diving vertically minimises its horizontal velocity component. When the bomb is dropped, the force of gravity simply increases its speed along its nearly vertical trajectory. The bomb travels a virtually straight line between release and impact, eliminating the need for complex calculations. The aircraft simply aims at the target and releases its bombs. The primary source of error is the effect of wind on the bomb's flight path after release. As bombs are streamlined and heavy, wind has only a slight effect on them and the bomb is likely to fall within its lethal radius of the target.

Origins

It is difficult to establish how dive bombing originated. During World War I, the Royal Flying Corps (RFC) found its biplane two-seat bombers insufficiently accurate in operations on the Western Front. Commanders urged pilots to dive from their cruising altitude to under 500 ft (150 m) to have a better chance of hitting small targets, such as gun emplacements and trenches. As this exposed the aircraft and crew to destructive ground fire in their unprotected open cockpits, few followed this order. Some recorded altitude at the top and bottom of their dive in log books and in squadron records, but not the steepness of the dive. It was certainly not near-vertical, as these early aircraft could not withstand the stresses of a sustained vertical dive.

The Royal Naval Air Service was bombing the Zeppelin sheds in Germany and in occupied Belgium and found it worthwhile to dive onto these sheds to ensure a hit, despite the increased casualties from ground fire. Again, the angle of dive in these attacks was not recorded.

Beginning on 18 June 1918, the Royal Air Force (RAF), successor to the RFC, ordered large numbers of the Sopwith TF.2 Salamander, a single-seat biplane. The "TF" stood for "Trench Fighter", and the aircraft was designed to attack enemy trenches both with Vickers .303 machine guns and with 25 lb (11 kg) bombs. Of the 37 Salamanders produced before the end of October 1918, only two were delivered to France, and the war ended before those saw action. Whether the Salamander counts in more modern parlance as a fighter-bomber or as a dive bomber depends on the definition of "dive". It had armoured protection for the pilot and a fuel system to attack at low level, but lacked dive brakes for a vertical dive.

Heavy casualties resulting from air-to-ground attack on trenches set the minds of senior officers in the newly formed RAF against dive bombing. So not until 1934 did the Air Ministry issue specifications for both land-based and aircraft carrier-based dive bombers. The RAF cancelled its requirement and relegated the Hawker Henley dive bomber to other roles, while the Fleet Air Arm's Blackburn Skua was expected to do double duty: as a fighter when out of reach of land-based fighter support, and as a dive bomber. It had dive brakes that doubled as flaps for carrier landings. The Hawker Henley had a top speed only 50 mph (80 km/h) slower than the Hawker Hurricane fighter from which it was derived. The American and Japanese navies and the Luftwaffe chose vertical dive bombers whose low speed had dire consequences when they encountered modern fighters.

World War I

The Royal Naval Air Service developed dive bombing as a tactic against Zeppelin hangars and formed and trained a squadron at Manchester for this task. On 8 October 1914, a Sopwith Tabloid with two 50 lb (23 kg) bombs attacked a hangar at Düsseldorf after a dive to 600 ft (180 m). On 14 November 1914, four Avro 504s attacked the Zeppelin factory at Friedrichshafen on Lake Constance, diving from 1,200 ft (370 m) to 500 ft (150 m) to ensure hits. As Zeppelins were tethered close to stores of hydrogen, results were often spectacular.

The first use of dive bombing by the RFC, which had been urging its pilots to drop bombs at heights below 500 ft (150 m) in order to hit within 150 ft (46 m) of the target since February 1915, was later that year. On 27 November 1915, Lieutenant Duncan Grinnell-Milne arrived in his Royal Aircraft Factory B.E.2c over railway marshalling yards near Lys in Northern France, to find the target already crowded by other bombers. He dived from 10,000 ft (3,000 m) to 2,000 ft (610 m) before releasing his 20 lb (9.1 kg) bombs. A few weeks later, Lieutenant Arthur Gould dived to just 100 ft (30 m) to hit buildings near Arras.

The Royal Flying Corps developed strafing with diving aircraft using both machine guns and small bombs as a deliberate tactic. At the Battle of Cambrai on 20 November 1917, 320 Mark IV tanks and 300 aircraft, mostly Sopwith Camels and Airco DH 5s with 20 lb (9.1 kg) bombs, were used to suppress artillery and machine guns. The cost in pilots was very high, with casualties on some days reaching 30 percent. The initial impact at Cambrai was highly successful. The staff officer to the Royal Tank Corps Lieutenant-Colonel J. F. C. Fuller published findings which were later taken up by Heinz Guderian to form the basis for the blitzkrieg tactics of using dive bombers with tanks employed by the Germans in 1939–40.

Second Lieutenant William Henry Brown, a Canadian from British Columbia serving with the RFC and flying a Royal Aircraft Factory S.E.5a, made the first attack on a vessel on 14 March 1918, destroying an ammunition barge on a canal at Bernot near St Quentin, diving to 500 ft (150 m) to release his bombs. He was awarded the Military Cross for this and other exploits. Brown's technique was emulated by other British squadrons. But the heavy casualties to unprotected pilots cast a pall over the results and influenced RAF thinking for 20 years.

Interwar era

The Royal Flying Corps was initially impressed with the potential of the dive bomber, but was aware of its suicidal nature. It ran a series of tests at the Armament Experimental station at Orfordness in Suffolk. Sopwith Camels and Royal Aircraft Factory S.E.5as were used in early 1918 to dive bomb targets from various heights, with different bombs and with and without the use of the Aldis gunsight, which had been invented in 1916 to aid pilots to calculate the deflection required to hit a traversing enemy aircraft. In principle, it obviated the need for a vertical dive. The results showed that a vertical dive into the wind sighting along the top of rather than through the sight was best. But they were not considered good enough to justify the expected casualties. The Royal Air Force, which took over both army and naval aviation in April 1918, retired its Sopwith Salamander dive bombers at the end of the war.

Colonel, later general, Billy Mitchell arrived in France with the first US Army and Air Force units soon after 6 April 1917 and began to organise the US Army Air Force flying French Salmson 2s, a spotter plane. The later Salmson 4 was to be a ground attack and dive bomber, but production was cancelled at the end of the war. Mitchell became a strong advocate of dive bombers after witnessing British and French aerial attacks. Mitchell, by now assistant chief of the Air Service United States Army, arranged tests with captured German and obsolete US ships in June and July 1921 and repeated over the next two years using Royal Aircraft Factory S.E.5as as dive bombers and Handley Page O/400s and Martin NBS-1s as level bombers carrying bombs of different weights up to 2,000 lb (910 kg). The SMS Ostfriesland was sunk and so later were the USS Alabama, USS Virginia and USS New Jersey.

Opposite conclusions were drawn by the RAF and USAS, from two very different tests regarding the usefulness of dive bombers, with the RAF concluding that the cost in pilots was too high to justify the results and the USAS considering it as a potent anti-ship weapon. Both naval staffs opposed the view taken by the respective airmen.

In 1919, United States Marine Corps (USMC) pilot Lt. L. H. Sanderson mounted a rifle in front of the windshield of his Curtiss JN-4 (a training aircraft) as an improvised bomb sight, loaded a bomb in a canvas bag attached to the aircraft's underside, and made a solo attack in support of USMC troops trapped by Haitians during the United States occupation of Haiti. Sanderson's bomb hit its target and the raids were repeated. During 1920, Sanderson familiarised aviators of USMC units on the Atlantic coast with dive bombing techniques. Dive bombing was also used during the United States occupation of Nicaragua.

World War II

European theatre

On 10 April 1940, 16 British Royal Navy Blackburn Skuas flying at extreme range from the naval air station at Hatston in Orkney led by Lieutenant Commander William Lucy sank the German cruiser Königsberg in Bergen harbour, whilst trying to prevent the German invasion of Norway. On the German side Stukas augmented or replaced artillery support for the Wehrmacht's lightly armed parachute and airborne troops.

The invasion of Poland (September to October 1939) and the Battle of France (May to June 1940) saw the Stuka used to devastating effect. German blitzkrieg tactics used dive bombers in place of artillery to support highly mobile ground troops. The British Expeditionary Force had set up strong defensive positions on the west bank of the Oise River to block rapidly advancing German armour. Stukas quickly broke the defences, and the Wehrmacht forced a crossing long before German artillery arrived.

On 12/13 May 1940, Stukas flew 300 sorties against strong French defensive positions at the Battle of Sedan. This enabled German forces to make a fast and unexpected breakthrough of the French lines, eventually leading to the German advance to the Channel and the cutting off of much of the Allied army.

The skies over Sedan also showed the Stuka's weakness when met with fighter opposition; six French Curtiss H-75s attacked a formation of unescorted Ju 87s and shot down 11 out of 12 without loss. The Stuka was even more vulnerable to the Hawker Hurricane with its 100 mph (160 km/h) speed edge and eight machine guns, which it first met over France and then in larger numbers in the Battle of Britain (July to October 1940). Losses were such that the Luftwaffe rapidly withdrew Stukas from operations over the United Kingdom. A similar fate befell unescorted RAF Fairey Battles over France.

The Stuka had 7.92mm machine guns or 20mm cannons mounted in the wings. Some were modified to destroy tanks with heavy calibre, 37mm Bordkanone BK 3,7 autocannons mounted in gun pods below the wings. They were very successful in this role in the early days (1941) of Operation Barbarossa before the Red Army Air Force countered with modern fighters, such as the Yakovlev Yak-1 and later the Yakovlev Yak-3.

The most successful dive-bomber pilot, Hans-Ulrich Rudel, made 2,530 sorties. He contributed to the sinking of the Soviet battleship Marat at Kronstadt on 23 September 1941 using 1,000 kg (2,200 lb) bombs. Later, flying a tank-buster Stuka with 20mm cannon, he claimed over 100 Soviet tanks destroyed, mostly at the Battle of Kursk in July 1943. The Ju 87G Kanonenvogel, equipped with two 37mm BK 3,7 anti-tank guns, as suggested by Rudel, proved to be a lethal weapon in skilled hands. In the Soviet counter-offensive, Operation Kutuzov (July to August 1943), which concluded Kursk, the Luftwaffe claimed 35 tanks destroyed in a single day. Rudel co-wrote a post-war book about his experiences and consulted with the US Air Force.

Pacific theatre

The Vultee Vengeance was developed in the US as a private venture dive bomber for export. It first flew in March 1941. It had a zero incidence wing, which was perfect for vertical dives as there was no lift from wing or tailplane in a dive. But it had to fly in a nose up attitude to maintain level flight, which made landings difficult. Initial orders were 300 for France, but France fell before they could be delivered. The RAF, with the cancellation of the Hawker Henley and having noted the success of Stukas in Poland, took delivery instead. It was considered too vulnerable to German fighters for use in Europe or North Africa, but large numbers flew in Burma from March 1943. It flew close support for General William Slim's Burma campaign bombing Japanese supply routes, bridges and artillery. It operated in the Royal Australian Air Force and Indian Air Force as well as the RAF. Some were held back for the United States Army Air Forces after the attack on Pearl Harbor, but did not see combat.

Both the Imperial Japanese Navy (IJN) and the United States Navy invested considerable effort on dive bombers. Japan started the war with a very good design, the carrier-borne Aichi D3A ("Val"). As the war progressed, the design became outdated due to its limited speed, due in part to the limited horsepower of its power plant and to the greater drag of its fixed main landing gear (a shortcoming shared by the Stuka).

The main American dive bomber, the Douglas SBD Dauntless, had similar performance to the D3A Val. From December 1942, the Dauntless was replaced with the faster, but more complex and trouble-prone Curtiss SB2C Helldiver. Both American aeroplanes were ubiquitous, with 6,000 Dauntlesses and over 7,000 Helldivers built. Both the SBD and D3A were used at Pearl Harbor on 7 December 1941. The Japanese sent 54 D3A Vals carrying 550 lb (250 kg) bombs to attack parked aircraft at Wheeler Field and Ford Island. A flight of 18 Dauntlesses from USS Enterprise arrived over Pearl Harbor just as the Japanese attacked. Seven were shot down and many others destroyed on the ground at Marine Corps Air Station Ewa At the Battle of the Coral Sea, Dauntlesses sank the light carrier Shoho and damaged the fleet carrier Shokaku together with Douglas TBD Devastator torpedo bombers.

On 5 April 1942, the heavy cruisers HMS Cornwall and HMS Dorsetshire were leaving Colombo, Ceylon to join the British Eastern Fleet, but had been spotted by Japanese reconnaissance aircraft. They were attacked by a large number of Aichi D3As and both were sunk. On 9 April 1942 the Royal Navy aircraft carrier HMS Hermes escorted by the destroyer HMS Vampire were attacked by more than 32 Aichi D3As and both were sunk shortly before eight defending RN FAA Fairey Fulmars, of 806 Squadron, could reach them. The Fulmars shot down four D3As and damaged two while losing two Fulmars to the more numerous D3As.

Decline

When the RAF were attempting to stop the Panzers of Erwin Rommel's Afrika Korps in early 1942, a lack of dive bombers proved to be an impediment. However, the British Government's Chief Scientist, Henry Tizard, formed a panel of experts, which recommended using rockets. A rocket has a much flatter trajectory than a bomb, allowing it to be launched with reasonable accuracy from a shallow dive, and could be fitted on existing aircraft. The RAF used them on Hurricanes in June 1942 against Rommel's tanks. The British Army had used rockets against low-flying bombers during the Battle of Britain by enlarging the tube from 2 inches (51 mm) to 3 inches (76 mm) and fitting high explosive warheads; it became an anti-tank weapon. The more powerful Hawker Typhoon, originally developed as a fighter, proved even more effective, carrying eight RP-3 60 lb (27 kg) rockets and producing a similar effect to a naval destroyer's broadside.

On 23 May 1943, a Fairey Swordfish destroyed U-752 in the Atlantic, and five days later, a Lockheed Hudson of RAF Coastal Command sank U-755 in the Mediterranean, using specialised rockets fitted with iron spikes which were fired at a shallow angle into the sea. Once under water, they curved upwards and punctured the pressure hull below the waterline, disabling or sinking the submarine.

Caltech developed the 5-inch (130 mm) High Velocity Aircraft Rocket (HVAR) with a 24-pound (11 kg) warhead for the US Navy. It was rushed to Europe for use on D-Day and later used by Navy aircraft in the Pacific. By January 1943, American pilots who had been flying in RAF Eagle Squadrons before the US entered the war, converted from Supermarine Spitfires to Republic P-47 Thunderbolts to form the USAAF 4th Air Fighter Group. At over 4 long tons (4.1 t) unladen, one of the biggest single engine fighter bombers of the war, it could carry ten 5-inch (130 mm) HVARs.

By late 1944, the RAF was able to hit stationary targets with greater accuracy from greater heights inflicting far more damage with less risk. On 12 November 1944, two 5-long-ton (5.1 t) Tallboy bombs were dropped by Avro Lancasters from 25,000 feet (7,600 m) and hit the German battleship Tirpitz at supersonic speeds, sinking it. The Tallboy was developed by Vickers designer Barnes Wallis who followed it up with the even larger 10-long-ton (10 t) Grand Slam earthquake bomb which was used to destroy railway viaducts and bridges, targets that could previously only be damaged in diving attacks. Wallis also designed a bomb that bounced across water to destroy the Eder and Moehne dams, which needed to be hit repeatedly at the same spot under water to be breached but had nets to protect against torpedoes.