In aviation, a drop tank (external tank, wing tank or belly tank) is used to describe auxiliary fuel tanks externally carried by aircraft. A drop tank is expendable and often capable of being jettisoned. External tanks are commonplace on modern military aircraft and occasionally found in civilian ones, although the latter are less likely to be discarded except in an emergency.
The primary disadvantage with drop tanks is that they impose a drag penalty on the aircraft. External fuel tanks will also increase the moment of inertia, thereby reducing roll rates for air maneuvers. Some of the drop tank's fuel is used to overcome the added drag and weight of the tank. Drag in this sense varies with the square of the aircraft's speed. The use of drop tanks also reduces the number of external hardpoints available for weapons, reduces the weapon-carrying capacity and increases the aircraft's radar signature. Usually the fuel in the drop tanks is consumed first, and only when all the fuel in the drop tanks has been used, the fuel selector is switched to the airplane's internal tanks. Some modern combat aircraft use conformal fuel tanks (CFTs) instead of or in addition to conventional external fuel tanks. CFTs produce less drag and do not take up external hardpoints but some versions can only be removed on the ground.
The drop tank was used during the Spanish Civil War to allow fighter aircraft to carry additional fuel for long-range escort flights without requiring a dramatically larger, heavier, less maneuverable fuselage. Mitsubishi A5M, designed by Jiro Horikoshi, began carrying an external underside drop tank to provide fuel for extended range in 1937 and during World War II, the Luftwaffe began using external fuel tanks with the introduction of a 300 L (66 imp gal; 79 US gal) light alloy model for the Ju 87R, a long-range version of the Stuka dive bomber, in early 1940. The Messerschmitt Bf 109 fighter also used this type of drop tank, starting with the Bf 109E-7 variant introduced in August 1940. Fitted also to the Focke-Wulf Fw 190, the 300 L (66 imp gal; 79 US gal) tank, available in at least four construction formats — including at least one impregnated paper material, single-use version — and varying only slightly in appearance, became the standard volume for most drop tanks in Luftwaffe service, with a rarely used 900 L (200 imp gal; 240 US gal), fin-stabilized large capacity drop tank used with some marks of the Messerschmitt Bf 110 heavy fighter and other twin-engined Luftwaffe combat aircraft.
The first drop tanks were designed to be discarded when empty or in the event of combat or emergency to reduce drag, weight, and to increase maneuverability. Modern external tanks may be retained in combat, to be dropped in an emergency. The Allies commonly used them to allow fighters increased range and patrol time over continental Europe. The RAF used such external fuel tanks in 1942, during the transit of Supermarine Spitfires to Malta. The Imperial Japanese Navy design specification for what came to be the Japanese Mitsubishi A6M Zero fighter included endurance with drop tanks of two hours at full power, or six to eight hours at cruising speed. Drop tanks were commonly used with the Zero, even on combat air patrol (CAP).  The Zero entered service in 1940.
American bomber theorists (the so-called Bomber Mafia) wanted to create vast fleets of heavy bombers. They insisted formations of heavy bombers with multiple machine-gun positions would be self-defending, and that long-range escort fighters would be unnecessary, or even impossible to design. They were also concerned that long-range medium bombers might compete for resources. For all these reasons, official policy in the United States held that drop tanks should not be developed or encouraged.   In the face of such entrenched attitudes in 1941 airmen such as Benjamin S. Kelsey and Oliver P. Echols worked quietly to get drop tank technology added to American fighters, beginning with the Lockheed P-38 Lightning. It was only with drop tanks supplying 450 US gallons (1,700 l; 370 imp gal) of extra fuel per fighter that P-38s could carry out Operation Vengeance, the downing of Admiral Isoroku Yamamoto's aircraft. (For this mission, each fighter carried one drop tank of approximately 150 to 165 US gal (570 to 620 L), and a larger one of approximately 300 to 330 US gal (1,100 to 1,200 L).  
Even after such experience showed the necessity for drop tanks, inflexible thinkers such as 8th Air Force General Ira C. Eaker had to be transferred out of commanding positions (and replaced with Maj. Gen. Jimmy Doolittle) so that drop tanks and range extension plans could be widely implemented in 1944 for American escort fighters.  External drop tanks turned the Republic P-47 Thunderbolt from a short-range interceptor aircraft into a long-range escort and air superiority fighter, enabling it to accompany bombers from the British Isles into Germany and made it possible for heavy bomber formations to undertake daylight raids under escort by North American P-51 Mustangs. The P-38 could also carry two 300-to-330-US-gallon (1,100 to 1,200 L) drop tanks for its longest sorties. This teardrop-shaped tank design was 13 ft (4.0 m) long and 3 ft (0.91 m) in diameter at its widest point. 
Faced by wartime metal shortages and a need to extend the range of fighter craft, the British came up with drop tanks made of glue-impregnated kraft paper, which had excellent tolerance characteristics for extreme heat and cold necessary for operation on an aircraft as well as being waterproof.
Since the glue would slowly dissolve from the solvent effects of the fuel (sometimes developing leaks within a few hours of being loaded with fuel) these were strictly a single-use item, used in typically chilly Northern European conditions, filled immediately before take off, jettisoned in the event of an aborted mission and only being required for the outbound portion of a flight. Such papier-mâché tanks were assembled from three main components, the nose cone, tail cone and the body, each shaped over wooden forms, the center section created by wrapping layers of the impregnated paper around a cylinder, the end caps hand-laminated with petal-shaped pieces sometimes called gores.
Before final assembly wooden anti-slosh baffles were installed, pipes and fittings were attached and the interiors coated with fuel-resistant lacquer and the three pieces were bonded together in press. Once the tank had cured, it was pressure tested to 41 kPa (6 psi ) and passing tanks were given two coats of cellulose dope followed by two coats of aluminium paint. (British paper drop tanks can be distinguished from outwardly similar metal tanks by colour, paper tanks were silver in appearance, while metal tanks were grey.)  Some 13,000 papier-mâché tanks were made and used by the RAF, the vast majority during the war, conserving a considerable amount of metal. Very few examples survive due to their expendable nature, low intrinsic value and the fact that they are not inherently robust.  While probably a nuisance for those under the flight path when the empty tanks were released, they were lightweight and comparatively fragile. Though they were likely to cause nothing but anxiety, the German authorities went so far as to distribute leaflets explaining that drop tanks were not bombs. 
U.S. paper tanks were developed by Col. Bob Shafer and Col. Cass Hough, who spent many hours developing a 110-gallon (416 liter) paper tank, then getting them into series production at Bowater-Lloyd's of London, only to be told by experts at Wright Field "paper tanks are absolutely unfeasible and will not do the job for which they are intended".  Since by the time the experts made that pronouncement 8th Air Force fighters had already used more than 15,000 paper tanks without a failure, the criticism was not taken seriously. It may explain why the most often-used fuel tanks for single-engined American fighters operating in Northern Europe were the 75-gallon (284 liter) capacity all-metal tank (made from two halves of formed aluminium with a prominent horizontal seam running along the tank's midline). Another common metal drop tank was the 150-to-165-US-gallon (570 to 620 L) model used by P-51s, P-47s and P-38s. 
The Matra JL-100 is a special hybrid drop tank and rocket pack; it combines a rocket launcher in front with 19 SNEB 68 mm (2.7 in) rockets and 250 litres (66 US gal ) of fuel behind into one single aerodynamically-shaped pod for mounting on combat aircraft such as the Dassault Mirage IIIs and English Electric Lightnings. The Convair B-58 Hustler, the first operational bomber capable of Mach 2 flight, had no bomb bay: it carried a single nuclear weapon plus fuel in a combination bomb/fuel pod underneath the fuselage.
After World War II, hot rodders raced the dry lakes of California to set new land speed records. War surplus drop tanks (aka belly pods) were plentiful, aerodynamically neutral and it did not take long to convert one into a car, dubbed a lakester. According to GM historians, Bill Burke of the So-Cal Speed Shop first attempted to convert a 168-US-gallon (640 L) P-51 Mustang belly tank, before switching to the larger 305-US-gallon (1,150 L) P-38 Lightning tank. Even now, lakesters compete at the Bonneville Salt Flats. 
The de Havilland DH.98 Mosquito is a British twin-engined, shoulder-winged, multirole combat aircraft, introduced during the Second World War. Unusual in that its frame was constructed mostly of wood, it was nicknamed the "Wooden Wonder", or "Mossie". Lord Beaverbrook, Minister of Aircraft Production, nicknamed it "Freeman's Folly", alluding to Air Chief Marshal Sir Wilfrid Freeman, who defended Geoffrey de Havilland and his design concept against orders to scrap the project. In 1941, it was one of the fastest operational aircraft in the world.
The Lockheed P-38 Lightning is an American single-seat, twin piston-engined fighter aircraft that was used during World War II. Developed for the United States Army Air Corps by the Lockheed Corporation, the P-38 incorporated a distinctive twin-boom design with a central nacelle containing the cockpit and armament. Along with its use as a general fighter, the P-38 was used in various aerial combat roles, including as a highly effective fighter-bomber, a night fighter, and a long-range escort fighter when equipped with drop tanks. The P-38 was also used as a bomber-pathfinder, guiding streams of medium and heavy bombers, or even other P-38s equipped with bombs, to their targets. Used in the aerial reconnaissance role, the P-38 accounted for 90 percent of the aerial film captured over Europe.
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The Republic F-84 Thunderjet was an American turbojet fighter-bomber aircraft. Originating as a 1944 United States Army Air Forces (USAAF) proposal for a "day fighter", the F-84 first flew in 1946. Although it entered service in 1947, the Thunderjet was plagued by so many structural and engine problems that a 1948 U.S. Air Force review declared it unable to execute any aspect of its intended mission and considered canceling the program. The aircraft was not considered fully operational until the 1949 F-84D model and the design matured only with the definitive F-84G introduced in 1951. In 1954, the straight-wing Thunderjet was joined by the swept-wing F-84F Thunderstreak fighter and RF-84F Thunderflash photo reconnaissance aircraft.
The Republic F-105 Thunderchief is an American supersonic fighter-bomber that served with the United States Air Force from 1958 to 1984. Capable of Mach 2, it conducted the majority of strike bombing missions during the early years of the Vietnam War; it was the only American aircraft to have been removed from combat due to high loss rates. It was originally designed as a single-seat, nuclear-attack aircraft; a two-seat Wild Weasel version was later developed for the specialized Suppression of Enemy Air Defenses (SEAD) role against surface-to-air missile sites. The F-105 was commonly known as the "Thud" by its crews.
The Douglas D-558-2 Skyrocket is a rocket and jet-powered research supersonic aircraft built by the Douglas Aircraft Company for the United States Navy. On 20 November 1953, shortly before the 50th anniversary of powered flight, Scott Crossfield piloted the Skyrocket to Mach 2, or more than 1,290 mph (2076 km/h), the first time an aircraft had exceeded twice the speed of sound.
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The escort fighter was a concept for a fighter aircraft designed to escort bombers to and from their targets. An escort fighter needed range long enough to reach the target, loiter over it for the duration of the raid to defend the bombers, and return.
The Grumman XTSF was a proposed twin-engine torpedo scout aircraft, designed by Grumman for the United States Navy towards the end of World War II. Based on the design of the Grumman F7F Tigercat fighter, but enlarged and with the addition of a bomb bay, the XTSF was deemed too large for carrier operations, and the project was cancelled before any aircraft were built. Instead, the Navy chose to order the single-engine XTB3F, which became the successful AF Guardian.
The Fairey Spearfish was a British carrier-based, single-engined, torpedo bomber/dive bomber that was ordered from Fairey Aviation for the Fleet Air Arm during World War II. Designed during the war, the prototype did not fly until July 1945. Much larger than earlier naval bombers, it was designed for use aboard the large Malta-class aircraft carriers that were cancelled after the war and was itself cancelled thereafter. Seven prototypes were ordered, but only five were built, of which four actually flew. They were mostly used for experimental work until the last aircraft was scrapped in 1952.
The Blackburn Firebrand was a British single-engine strike fighter for the Fleet Air Arm of the Royal Navy designed during World War II by Blackburn Aircraft. Originally intended to serve as a pure fighter, its unimpressive performance and the allocation of its Napier Sabre piston engine by the Ministry of Aircraft Production for the Hawker Typhoon caused it to be redesigned as a strike fighter to take advantage of its load-carrying capability. Development was slow and the first production aircraft was not delivered until after the end of the war. Only a few hundred were built before it was withdrawn from front-line service in 1953.
The Reggiane Re.2005 Sagittario was an Italian monoplane fighter and fighter-bomber produced for the Regia Aeronautica during the later years of World War II. Along with the Macchi C.202/C.205 and Fiat G.55, the Reggiane Re.2005 was one of the three Serie 5 Italian fighters. The lines of the fuselage were aerodynamically efficient, and the design was intended to exploit the famous Daimler-Benz DB 605 engine. The only drawback was a certain structural weakness in the rear section of the fuselage. Only 48 examples had been delivered before the Armistice, and these fighters took part in the defence of Naples, Rome and Sicily, with the survivors battling above the crumbling ruins of Berlin, in German insignia. British ace and military observer, Group Captain Duncan Smith, DSO DFC, said that "The Re.2005 was altogether a superb, potent aeroplane".
The Savoia-Marchetti SM.91 was a two-seat, twin-engined, Italian heavy fighter prototype, designed to compete in a 1942 revision to a long-range fighter-bomber contract offered by the Regia Aeronautica to Italian aircraft companies in 1938. The original 1938 specification yielded the Savoia-Marchetti SM.88, which the SM.91 was largely based on.
The Curtiss XBTC was a prototype single-seat, single-engined torpedo/dive bomber developed during World War II for the United States Navy. Four aircraft were ordered, powered by two different engines, but the two aircraft to be fitted with the Wright R-3350 radial engine were cancelled in late 1942, leaving only the pair using the Pratt & Whitney R-4360 radial. By this time, Curtiss Aircraft was overwhelmed with work and the Navy gave the XBTC-2 prototypes a low priority which delayed progress so the first flight did not take place until the beginning of 1945. One aircraft crashed in early 1947 and the other was disposed of later that year.
The Curtiss XBT2C was a prototype two-seat, single-engined dive/torpedo bomber developed during World War II for the United States Navy. Derived from the Curtiss SB2C Helldiver dive bomber, it was an unsuccessful competitor to meet a 1945 Navy specification for an aircraft to combine the roles that previously required separate types. Unlike the other competitors, the XBT2C was designed to accommodate a radar operator.
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