SB Dive Bomber [SB Scout Bomber]
Multiengine bombers usually dropped their bombs from a level flight attitude or, in the case of medium bombers in the ground attack mode, from a shallow dive. In contrast, an entirely different technique known as dive bombing was pioneered by the U.S. Navy during the decade preceding World War II. In this method of operation, the aircraft was put into a vertical or near-vertical dive at an altitude 15 000 to 20 000 feet and aimed directly at the target. Bomb release usually took place at about 3000 feet, after which the aircraft made a high-g dive recovery to a level flight attitude. Dive bombing was found to be especially suited for use against small, slow-moving targets such as tanks and ships and was employed with devastating effectiveness against Japanese naval forces during World War II.
The class designation system for aircraft developed in the early 1920 continued to remain a functional system and is still used today. There have been many additions, deletions, and major changes to the system over the years but the concept has remained intact. In July 1928, two new designations were also instituted, VB for bombing and VH for ambulance. A major change was instituted to the Aircraft Designation System on 2 January 1934. The SB Scout Bomber designation was in use by the US Navy from 1934 through 1946. The 1 July 1939 Model Designation of Airplanes included shore-based Patrol-Bombing (PB) and carrier based Scouting-Bombing (SB). The SB designation was superceeded in 1946 by the A for Attack designation, which remained in use through 1962, and with modification, to this date.
A great deal was said before the Great War on the subject of bomb-dropping as a means of attack on armored ships, and it was soon recognised how comparatively impotent a bomb, even charged with high explosive, may be when used without "tamping" against armor-plate. It is quite true that a certain amount of mischief would be wrought by a bomb of large size if successfully dropped on to the deck of a battleship or cruiser, and the effect would certainly be more disagreeable still if the recipient were a destroyer or some still smaller craft. The effect, however, would in no degree be comparable to that of a torpedo, where the inertia of the surrounding water plays an important role. It would, in any case, take a vast number of hits to put a first-class battleship or cruiser out of action. Further, the difficulties of aim were considerable, and with the counter-aircraft armament with which warships were being fitted, it would not, generally speaking, be possible for an airplane to descend to low altitude with impunity.
A consideration of importance in naval warfare or in coast defense, was the mine effect of the demolition bomb on sea craft of various types. None of the bombs, smaller than the 600-pound size, had any appreciable effect on armored vessels, when dropped in the water alongside. The latter bomb was effective however to a distance underwater of 15 feet. The mining effect increases, of course, with the size of the bomb, and reaches to a distance of 75 feet with the 4,000-pound bomb. On unarmored vessels, the effect is felt at a much greater distance. Even the 100-pound bomb had an appreciable effect at a distance of 25 feet, with the 4,000-pound bomb is effective up to 200 feet. The largest bomb used in the Great War, 1,650lbs, was dropped by a Handley Page 0/400 aircraft of the Independent Air Force on 14 October 1918.
In late 1920 the Navy conducted confidential bombing tests against an obsolete battleship well before the more famous 1921 Mitchell tests. The New York Times obtained damage photos and called for "a free and thorough discussion as to the effect of new weapons upon naval warfare." Army and Navy aerial bombs of all types and weights in service were used in the 1921 Bombing Experiments - from the smallest to the biggest - from the Army's small 25 pound Cooper fragmentation bombs for use against exposed personnel, and the Navy's antisubmarine 163 pound bomb - to the heavy Navy armor-piercing bomb and the Army's 2,000 pound demolition bomb, the heaviest bomb that could be carried by current types of aircraft in service with fuel enough to make them of any use in naval warfare. The bombs differed in types of construction, types of fuses used, and purpose for which they were designed. Bombs with the greatest penetrating power had thicker heads and heavier bodies and carried less weight of explosive than bombs of the same weight designed with lighter casings in order to carry greater explosive charge. The light case bombs had greater mining effect with near misses than the armor piercing type, and less effect against gunshields, splinter bulkheads, superstructures and protective decks, but the ones designed for penetrating power had to be dropped from higher altitudes.
The 1921 Ostfriesland tests began with an attack by Navy planes dropping thirty-three 250-pound bombs, scoring eight hits, followed by eight 550-pound bombs, making four hits. The Army then dropped eleven 600-pound bombs registering one hit. An examination of the vessel after these attacks showed that she had sustained little damage from direct hits; but the mining effect of the bombs that dropped close to her had damaged her considerably under water, and several compartments were leaking. In the final attack, seven 2000-pound bombs were dropped, none of which hit the vessel. The most effective 2000 pound bomb detonated close under the port quarter, throwing water up under both sides of the hull. She immediately began to settle rapidly by the stern. She turned completely over and went down by the stern at twenty-two minutes after the first 2000-LB bomb was dropped.
One of the difficulties faced by Naval aviation in the 1920s was to get planes with slow landing speeds to fly on an aircraft carrier. Only the lightest type of planes were able at that time to land on carriers. This meant that the heavy bombing planes could not be used at that time from carriers. The light pursuit or combat planes could not carry torpedoes nor bombs heavy enough to inflict serious damage to armored vessels. Until the heavier bombing planes were capable of flying on and off carriers, surface craft operating more than a few hundred miles from shore would not seriously be menaced by aircraft.
The case against carrier aviation was built on the idea that slow heavier planes could not stand up in aerial combat against fast light combat planes. This meant that protection of surface craft from bombing attacks could be insured if they are simply provided with light combat planes. For this reason it seemed probable in the 1920s that naval aviation on the high seas would be restricted to light combat, spotting, and scout planes which would be flown from surface craft accompanying the fleet, and that in operations over the water the use of bombing would be restricted to their limited radius of action from the coast line.
The armor piercing bomb was designed to penetrate the protective deck of the vessel, and to explode below decks through the action of a properly timed fuse. When compared with the demolition bomb, the armor piercing bomb had the great advantage that its explosive effect is felt in the very vitals of the vessel. The demolition bomb, when a direct hit is secured, created havoc with the upper works of the ship, but the explosive effect occurs before penetration, and therefore is not applied at the most vulnerable point.
But the armor piercing bomb has several disadvantages. In order to obtain the necessary strength for penetrating armor, the case of the bomb must be much heavier. The proportion of high explosive must, therefore, be reduced to an amount comparable with the shell, and one of the advantages of the demolition bomb surrendered.
Moreover, the armor piercing bomb should be dropped in such a was as to obtain from gravity the necessary striking velocity. There is a maximum velocity for bombs dropped from aircraft that cannot be exceeded no matter from what height dropped. This maximum velocity, known as "terminal velocity," is reached when the retardation due to air resistance is just equal to the acceleration due to gravity and consequently all acceleration ceases. The form, weight, and cross-sectional area of the bombs, of course, affect this. With 1000-lb bombs, having good stream lines, we can expect a terminal velocity of about 1,100 feet per second, but to attain such velocity they would have to be dropped from an altitude of about thirty or forty thousand feet. From an altitude of 4,000 feet one of these bombs would attain a velocity of about 500 feet per second. Armor piercing bombs would, therefore, be useful only when the ceiling available was high enough to insure their attaining sufficient velocity to penetrate the armor. In some cases however, it might prove a serious defect; a low ceiling, for example, might render the armor piercing bomb useless, where attacks with the demolition bomb would be feasible.
A further disadvantage lies in the necessity for greater accuracy when using the armor piercing bomb. The demolition bomb is many times more powerful in mining effect, weight for weight, than the armor piercing bomb. The latter is therefore almost useless, unless it obtains a direct hit. The demolition bomb, on the other hand, while it does a great deal of damage on a direct hit, is probably even more destructive, when it explodes in the water alongside the ship, within the distances mentioned above, and from 20 to 40 feet under the surface of the water.
A board of officers consisting of representatives of the Ordnance Department and the Air Service met in early 1920 and considered steps which should be taken in the development of bombs recommended the development of three large size demolition bombs drop bombs weighing 1,000, 2,000, and 3,000 pounds. The 1,000-pound bomb had already been partially developed and experiments were being conducted with a few experimental bombs of this size to ascertain whether or not the initial steps taken in the development were correct. The bomb board recommended the development of armor-piercing bombs weighing approximately 1,000, 2,000, and 3,000 pounds. These bombs were desired for use in attacking the decks of warships and other armored targets, and the contention was to modify the existing types of armor piercing projectiles so that they may be utilized as drop bombs. The initial work on the development of the 1,000 pound armor-piercing bomb had been started by early 1920 but no material progress had as yet been made.
By the mid-1920s it was still doubtful whether aircraft could carry bombs large enough to penetrate the armor of modern battleships. Live-fire exercises conducted by the navy were yielding an amazing 75 percent hit rate for antiaircraft fire from battleships against towed aerial targets. In contrast, the hit rate for bombing attacks against radio-controlled target battleships was abysmal.
The Quest for Precision
A bombsight is a device that is used to drop bombs accurately from aircraft. The Norden bomb sight was developed by the US Navy because of its concern with the difficulty of hitting a maneuvering ship. Carl L. Norden was a Swiss immigrant who was truly a mechanical engineering genius. He began designing the bombsight for the Navy in 1921. In 1923, Norden went into partnership with another Navy consultant, Theodore H. Barth. Norden designed bombsights, and Barth built and tested them. In 1924, the first Norden bombsight was delivered to the Naval Proving Ground, Dahlgren for five years of testing. In 1928, Norden and Barth received their first order from the Navy for 40 bombsights. At that point the two incorporated as Carl L. Norden Inc.
In 1928 the Navy negotiated a contract with Norden for the purchase of his bombsights. By 1931 Norden and Frederick I. Entwhistle of the US Navy had perfected the invention. In spite of the Norden bombsight's imperfections, it performed so much better than any other sight available in the early 1930s that it was quickly adopted by the Navy for all its bombers. The Norden bombsight had been developed primarily for the medium altitudes and slow speeds of Navy flying boars, such as the PBY bombers.
Before the 1939 torpedo bomber design competition, BuAer investigated glide bombing as an alternative to horizontal bombing. Elimination of the Norden bombsight would have saved space and weight but that was more than offset, in the eyes of BuAer engineers, by the fact that a stronger and therefore heavier plane would have been required. Unfortunately, there was no data on the relative accuracy of glide bombing and high-altitude horizontal bombing.
Tactical exercises conducted with the Norden bombsight in the early 1930s suggested to some that high altitude bombing could hit moving ships as well as stationary targets. But the Navy concluded that level bombing, even with the Norden bombsight, was largely ineffective. The Navy soon decided that it would have to use dive bombing for carrier-based aviation. In 1947 the U.S. Patent Office finally issued the patent on the Norden bombsight, which Carl L. Norden had applied for 17 years earlier.
The United States Army Air Corps originally backed the Sperry bombsight but switched when the Norden proved more accurate. The Army Air Corps acquired its first Norden bombsight in 1932, to facilitate hitting stationary targets. The Navy's agreement stipulated that if the Army also wished to procure the sights they would have to do so through the Navy -- the Army airmen were not permitted to deal directly with Norden Incorporated. And the Navy effectively blocked the AAF from access to the bombsight. As late as 1942, by which time carrier air doctrine emphasized dive-bombing for which the Norden sight was totally unsuited, the Navy was still gobbling up 75 percent of the Nordens.
By World War II the Norden bombsight had evolved into the best in the world, though that was not saying much. It was claimed that using this device, bombardiers could drop their bombs within a 100-foot circle from an altitude of well over 20,000 feet. But in practice the famous Norden bombsight was never as accurate as legend had it [less than a third of all bombs dropped during World War II by American bombers using the Norden bombsight fell within one thousand feet of the intended target]. On the famous bombing raid against the ball-bearing factories at Schweinfurt in October 1943, the bombardiers used Norden bombsights, but only one of every 10 of their bombs landed within 500 feet of their target. The difficultis Force experienced in hitting stationary targets on land vindicated the Navy's judgement of a decade earlier that something else would be needed to hit moving targets at sea.
The inability of high altitude horizontal bombers to pin-point naval ships underway at sea was proved in World War II. It is reported that no US naval warship was hit by a Japanese high flying plane when the ships were underway at sea. The many that were sunk by Japanese planes were sunk by dive bombers and torpedo bombers and Kamikazes.
Apparently, dive (or glide) bombing evolved in a number of air services during World War I. Both Allied and German pilots are reported to have used it in combat, and US Army fliers at Ellington Field, Texas, practiced it during 1917-1918, dropping their bombs from wing racks controlled by wires leading to the pilot's cockpit.
The British pioneered dive-bombing while conducting close-support operations in 1918. It did so in part through strafing and bombing raids from the air, a strategy the Colonial Secretary, one W.H.S. Churchill, strongly advocated because of its low cost. The success of the RAF's involvement against the Mad Mullah in 1919 allowed ground forces to be reduced and air-policing introduced.
As the first organized attack group to form within the Army Air Service, the 3rd Attack Group was instrumental in developing close air support doctrine in the inter-war period. Dive bombing was first demonstrated at Aberdeen Proving Ground, MD on 01 September 1919. The group pioneered dive bombing, skip-bombing, and parafrag attacks in the 1920s.
During the operations in Haiti, Marines began practicing a tactic fundamental to the carrying out of their close support mission . That tactic was dive bombing. During the summer of 1919, Lieutenant Lawson H. M. Sanderson of 4th Squadron, then stationed in Haiti, decided that he and his fellow pilots needed a more accurate method of delivering bombs against the enemy "Cacos." In experimental exercises, Sanderson abandoned the hitherto standard procedure of allowing his observer to release the bomb from horizontal flight while aiming with a crude sight protruding from the rear cockpit. Instead, he entered what was then considered a steep dive of 45 degrees, pointed the nose of his aircraft at the target, and released the bomb from the pilot's position at an altitude of about 250 feet [by World War II, the dive angle had increased to as much as 70 degrees, and the release altitude to 1,000 to 2,000 feet]. He found that this method made his bombing much more accurate, and other members of his squadron soon adopted it. By late 1920, Marines at Quantico were using it also.
By modern standards, what Sanderson was doing would be called "glide bombing," as a true, steep, powered dive was impossible in the planes of that day. At the time, however, they called it dive bombing and with sturdier machines like the Curtiss F6C series began to approximate the modern tactic. Lieutenant Sanderson never claimed to be the inventor of dive bombing, although probably he was the first Marine to use the tactic.
Close air support (CAS) became more effective due to dive bombing techniques and weaponeering. By trial and error, Marine aviation worked out basic tactics for CAS. Dive-bombing was one of the tactics practiced by the US Marine Corps in the 1920s that greatly improved its ability to support troops on the ground. In 1923 the 3d Attack Group was experimenting with dive-bombing. The Army Air Service (AAS) had cooled to this type of attack because many aviators felt aircraft were too vulnerable to anti-aircraft artillery [AAA] while dive-bombing. In contrast, the Marines became convinced that dive-bombing attacks could be most useful in small guerilla wars were AAA would be less of a threat.
Marine aviators during the 1920s used any scout or observation plane for dive bombing, including Jennies and DH-4Bs and later Curtiss Hawks and Helldivers. Biplanes could dive bomb without wing flaps or diving brakes because their "built-in headwind" of struts, wires, fixed landing gear, etc. kept their speed under 400 miles per hour even in a wide-open vertical dive.
The question remained as to how bombing by carrier aircraft could be made effective. An answer came from LCDR Frank Wagner, then CO of Fighting Squadron Two [VF-2], when he demonstrated the concept of dive-bombing in March 1926 and later trained his squadron to master the technique. From mid-August 1926 aviators on board USS Langley were "devoted to intensive study by practical operations of aircraft tactics." One innovative tactic examined was dive-bombing, which seemed to address the problem of accurately hitting targets (though it was unclear that this was a purely Navy development). On 22 October 1926, VF-2 made a mock attack on Pacific Fleet ships under way after having warned them of its intentions to do so ahead of time. It is likely that the commander of the surface force expected a standard low-altitude level bombing attack which would be detected in time for an effective response. Instead, Wagner's F6C-2 Curtiss Hawks came in high and unseen, nosed over, and from 12,000 feet screamed down upon their unwary victims and made simulated drops before the ships' anti-aircraft guns could be manned.
In another exercise, Wagner's squadron demonstrated dive-bombing accuracy by scoring nineteen hits out of forty-five bombs dropped on a 100 by 45 foot target. By December 1926 the accuracy of steep dive-bombing, later used throughout World War II, was established (though it was not clear that the method would kill ships given the weak ordnance of the period). By the 1930s, the most effective technology supporting the presumption of Naval aviation ascendancy was the dive bomber.
Since a bomb sometimes struck the propeller or lodged in the landing gear of the releasing plane, the tactic was in jeopardy until 1931 when displacing gear, or bomb yoke, was developed which swung the bomb clear. For several years, there was much overlapping among fighters, dive-bombers, and scouts. Some clarification took place in mid-decade; in 1934, BuAer held design competitions for both heavy (1,OOO-lb.) and light (500-lb.) dive-bombers and, in 1935, for a single-place fighter. Two aircraft were selected for development from each, and five of the six resulting designs were used at least briefly during WW II. The most important were the SBD dive-bomber and the F4F fighter.
The first major use of the airplane as an instrument of war in Central America took place during the mid-1920s in Nicaragua as a result of internal political strife. Marine pilots led by Major Ross E. Rowell were the first American forces to use dive-bombing (a technique earlier developed by Lt L.H.M. Sanderson) against an organized enemy. On 16 July 1927, Sandinista forces attacked the Marine garrison at Ocotal, Nicaragua. Marine Corps planes peeled out of formations at 1500 feet and dived to 300 feet, where they dropped their bombs. The battle at Ocotal proved significant for air power by introducing several innovations to air warfare. As the historian Neill Macaulay observed, the Marine aviators conducted "the first organized dive-bombing attack in history -- long before the Nazi Luftwaffe was popularly credited with the 'innovation'."
The Junkers A48 was originally designed during 1927 to fullfill a request of the Turkish Government for a fast fighter aircraft. At least seven civil A48 were built and used for experimental flight tests. During 1930/31 one was used for the world's first dive bomber experiments. The Heinkel He 50 was a single-seat dive-bomber and two-seat reconnaissance biplane of 1931, production examples of which went into Luftwaffe service. Samples of the Heinkel He 50 were exported to China and Japan.
By early 1930 the Martin XT5M-1, first US dive bomber designed to deliver a 1,000-pound bomb, met strength and performance requirements in diving tests. And in 1934 the XBT-1, a two-seat Scout and 1,000 pound divebomber, was developed. This aircraft was the initial prototype in the sequence that led to the SBD Dauntless series of dive-bombers introduced to the fleet in 1938 and used throughout WW II.
Before World War II, dive-bombers would drop at an altitude of 800 meters, but found this did not provide the desired accuracy. Releasing at 600 meters gave some improvement, and finally the release altitude became 400 meters. When dashing in a steep dive, it was neccessary to use the dive brake underneath of the wings, otherwise the aircraft would exceed its top speed and crash. A dive brake is a hinged or pivoted airfoil mounted in the surface of a wing. It is designed, when extended, to limit the speed of an aircraft in a dive.
Aggressive dive-bombing techniques that were developed during the 1930s produced severe acceleration during the pull-up at the end of the bombing run. Sporadic reports began to appear of blackout during these maneuvers as well as concern about acceleration-induced loss of consciousness (G-LOC). These problems were correctly attributed to "cerebral anemia produced by centrifugal action," and the Royal Air Force determined that 4 G (acceleration 4-fold greater than the force of gravity) was the limit of human acceleration tolerance. In 1934 the US Navy developed a pneumatic "acceleration belt" consisting of an abdominal bladder, which the pilot inflated prior to the dive-bombing run, but this device probably had only marginal effect on acceleration tolerance.
Ernst Udet, a senior official of the Nazi Air Force, visited the United States and saw a demonstration of a Curtiss Hawk dive bomber. He saw that such planes could drop bombs with high accuracy by diving toward their targets. Returning home, he insisted that Germany must have a dive bomber as well. This took shape as the widely feared Ju 87 "Stuka." The Luftwaffe's first dive bomber unit was created in 1937, and a handful of Ju 87A-1 Stukas were sent to Spain. In the early days of World War II, the images and screaming sirens of the Sturzkampfflugzeug became forever associated with the blitzkrieg. A new technology or program often must be combined with a new concept of operation and/or organizational change to produce a "transformational" effect or capability. The most well known historical example of this is the fact at the beginning of World War Two, the French had more and better tanks. However, the Germans combined their tanks with a new concept and organization (blitzkrieg) and other new systems such as the dive-bomber to produce a transformational effect that revolutionized warfare at that time.
In 1937 the prototype Blackburn B.24 Skua two-seat fighter/dive-bomber makes its maiden flight, piloted by "Dasher" Blake at Brough, Yorkshire; it was Britain's first dive-bomber.
The Army Air Corps became intensely interested in dive bomber attack aircraft in the late 1930s and early 1940s because of the success of the German Air Force (Luftwaffe) dive bombers in early blitzkrieg campaigns against Poland and France. The tactical successes of the Luftwaffe in France, and the close collaboration between Germany's ground forces and its Stukas, led the US Air Corps to contact the Navy in June 1940; the airmen were "extremely anxious" to obtain information on dive bombers. Initially, Navy aircraft were adapted to Army requirements and rushed into production. The Douglas A-24, based on the SBD-3 "Dauntless," and the Curtiss A-25, based on the SB2C "Helldiver," are examples of this. By the late summer of 1940, the US Air Corps had created two groups of dive bombers equipped with an Air Corps version of the Navy's A-24 "Dauntless." These Army dive bombers were not particularly successful and most were either assigned to secondary non-combat roles or later transferred to the U.S. Navy, which successfully used the dive bomber in combat.
From the outset of World War II, bombing in the horizontal plane was proven demonstrably ineffective at sea. The dive bomber was 30 or 40 percent more accurate than level bombing. Experience everywhere at sea - in the North Sea, the Channel, the Pacific, the Indian Ocean, the Mediterranean - proved the value of the dive bombers.
The USS Arizona exploded after it was hit by a 1,760-pound armor-piercing bomb that slammed through its funnel and ignited its forward ammunition. During this attack a five hundred pound armor piercing bomb fell into the hospital patio but fortunately it buried itself to a depth of about 50 feet before it exploded. Every wall in the hospital was cracked but not a single person in the building was even injured.
By the 1950s one of the more common type of runs is a dive from an altitude of about 8,000 feet with the bomb release and pullout started at about 3,000 feet. Other dive-bombing runs were made from high altitude. Here the dives began at altitudes as high as 20,000 feet and the pullouts were started anywhere from 16,000 to 12,000 feet.
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