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 jettisonable. 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 the event of emergency.
The drop tank was first 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. During World War II, the German Luftwaffe began using external fuel tanks with the introduction of a 300 liter (80 US gallon) 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 liter tank, available in at least four differing construction formats and varying only slightly in appearance, became the standard volume of most subsequent drop tanks in Luftwaffe service, with a rarely used 900 litre (238 U.S. gallon), fin-stabilized large capacity drop tank used with some marks of the Messerschmitt Bf 110 heavy fighter.
The first drop tanks were designed to be discarded when empty or in the event of combat or emergency in order to reduce drag, weight, and to increase maneuverability. Modern external tanks are often intended to be retained both in combat and in normal long-range flight, and only dropped in an emergency.
The Allies commonly used them to allow fighters increased range and patrol time over continental Europe. The first RAF use of such external fuel tanks occurred in 1942, during the transit of Supermarine Spitfires to Malta.
Drop tanks were prohibited on fighter aircraft and on medium bombers by the so-called Bomber Mafia in the United States Army Air Corps prior to direct U.S. involvement in World War II. The bomber theorists assumed that bombers would be self-defending; they saw long-range fighters as "a myth", as they could be easily forced to drop the tanks by minor harassment just at the beginning of the raid. Long-range medium bombers were viewed as direct competition to their goal of vast air fleets of heavy bombers. Starting in 1941, airmen such as Benjamin S. Kelsey and Oliver P. Echols worked quietly to get drop tank technology added to American fighters such as the Lockheed P-38 Lightning. It was only with drop tanks supplying at least 450 US gallons (1,700 l; 370 imp gal) of extra fuel per fighter that the P-38 could have carried out Operation Vengeance, the shootdown of Admiral Isoroku Yamamoto. In this case, 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 General Ira C. Eaker had to be transferred out of commanding positions so that drop tanks and range extension plans could be widely implemented in 1944 for American escort fighters. External drop tanks also turned the Republic P-47 Thunderbolt from a short-range interceptor into long-range escort and air superiority fighter, enabling it to accompany bombers all the way from British Isles to Germany, and back.
The most-often used fuel tanks for single-engined American fighters in Europe were the 75-gallon (284 litre) capacity all-metal tank, made from two halves of formed aluminum with a prominent seam running along the tank's midline, and an impregnated-paper 110-gallon (409 litre) one-use drop tank. Another common drop tank was the 150-to-165-gallon model used by P-51s, P-47s and P-38s. The P-38 could also carry two 300-to-330-gallon drop tanks for its longest sorties. This teardrop-shaped tank design was 13 feet (4.0 m) long and 3 feet (0.91 m) in diameter at its widest point.
The primary disadvantage with drop tanks is that they impose a drag penalty on the aircraft carrying them. External fuel tanks will also increase the moment of inertia, thereby reducing roll rates for air combat maneuvers. A rule of thumb is that only about half the capacity of a streamlined drop tank actually goes towards increasing the aircraft's overall range, the rest going to overcome the added drag and weight of the tank itself. 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, and increases the aircraft's radar signature, both problematic for modern tactical aircraft.
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 turned on 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; however, some versions can only be removed on the ground.
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.
Automotive use 
After World War II, hot rodders raced the dry lakes of California to set new land speed records. War surplus drop tanks were plentiful and aerodynamically neutral, and it didn't take long to make 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-gallon P51 Mustang belly tank, before switching to the larger 305-gallon P38 Lightning tank. Even now, lakesters compete at the Bonneville Salt Flats.
- Rosen, Stephen Peter (1994). Winning the next war : innovation and the modern military (5th printing. ed.). Ithaca, N.Y: Cornell University Press. pp. 173–177. ISBN 0-8014-8196-1.
- Kelsey, Benjamin S. (1982). The Dragon's Teeth?: The Creation of United States Air Power for World War II. Smithsonian Institution Press. ISBN 0-87474-574-8.
- Air Power History 50. Air Force Historical Foundation. 2003. pp. 33–34.
- Polmar, Norman (2008). Aircraft Carriers: A History of Carrier Aviation and Its Influence on World Events, Volume II: 1946-2006. Potomac Books. p. 308. ISBN 1574886657.
- Bodie, Warren M. (1991). The Lockheed P-38 Lightning. Widewing Publications. ISBN 0-9629359-5-6.
- Christensen, Mark; Thacker, Tony (2005). So-Cal Speed Shop: The Fast Tale of the California Racers Who Made Hot Rod History. MotorBooks International. p. 66. ISBN 1610591852.
- Wilkinson, Stephan (2005). Man And Machine: The Best of Stephan Wilkinson. Globe Pequot. p. 97. ISBN 1599216795.
- 2003 So-Cal Lakester
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