Drop tank
In aeronautics, a drop tank (external tank, wing tank, or belly tank) is used to describe auxiliary fuel tank
Fuel tank
A fuel tank is safe container for flammable fluids. Though any storage tank for fuel may be so called, the term is typically applied to part of an engine system in which the fuel is stored and propelled or released into an engine...

s externally carried by aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

. A drop tank is expendable and often jettisonable. External tanks are commonplace on modern military aircraft
Military aircraft
A military aircraft is any fixed-wing or rotary-wing aircraft that is operated by a legal or insurrectionary armed service of any type. Military aircraft can be either combat or non-combat:...

 and occasionally found in civilian
A civilian under international humanitarian law is a person who is not a member of his or her country's armed forces or other militia. Civilians are distinct from combatants. They are afforded a degree of legal protection from the effects of war and military occupation...

 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
Spanish Civil War
The Spanish Civil WarAlso known as The Crusade among Nationalists, the Fourth Carlist War among Carlists, and The Rebellion or Uprising among Republicans. was a major conflict fought in Spain from 17 July 1936 to 1 April 1939...

 to allow fighter aircraft
Fighter aircraft
A fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets...

 to carry additional fuel for long-range escort flights without requiring a dramatically larger, heavier, less maneuverable fuselage
The fuselage is an aircraft's main body section that holds crew and passengers or cargo. In single-engine aircraft it will usually contain an engine, although in some amphibious aircraft the single engine is mounted on a pylon attached to the fuselage which in turn is used as a floating hull...

. During World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, the German Luftwaffe
Luftwaffe is a generic German term for an air force. It is also the official name for two of the four historic German air forces, the Wehrmacht air arm founded in 1935 and disbanded in 1946; and the current Bundeswehr air arm founded in 1956....

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
Dive bomber
A dive bomber is a bomber aircraft that dives directly at its targets in order to provide greater accuracy for the bomb it drops. Diving towards the target reduces the distance the bomb has to fall, which is the primary factor in determining the accuracy of the drop...

, in early 1940. The Messerschmitt Bf 109
Messerschmitt Bf 109
The Messerschmitt Bf 109, often called Me 109, was a German World War II fighter aircraft designed by Willy Messerschmitt and Robert Lusser during the early to mid 1930s...

 fighter also used this type of drop tank, starting with the Bf 109E-7
Messerschmitt Bf 109
The Messerschmitt Bf 109, often called Me 109, was a German World War II fighter aircraft designed by Willy Messerschmitt and Robert Lusser during the early to mid 1930s...

 variant introduced in August 1940. Fitted also to the Focke-Wulf Fw 190
Focke-Wulf Fw 190
The Focke-Wulf Fw 190 Würger was a German Second World War single-seat, single-engine fighter aircraft designed by Kurt Tank in the late 1930s. Powered by a radial engine, the 190 had ample power and was able to lift larger loads than its well-known counterpart, the Messerschmitt Bf 109...

, 300 liter became the standard volume of subsequent drop tanks in Luftwaffe service.

The first drop tanks were designed to be discarded when empty or in the event of combat or emergency in order to reduce drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

, 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
Malta , officially known as the Republic of Malta , is a Southern European country consisting of an archipelago situated in the centre of the Mediterranean, south of Sicily, east of Tunisia and north of Libya, with Gibraltar to the west and Alexandria to the east.Malta covers just over in...


Drop tanks were prohibited on fighter aircraft and on medium bomber
Medium bomber
A medium bomber is a bomber aircraft designed to operate with medium bombloads over medium distances; the name serves to distinguish them from the larger heavy bombers and smaller light bombers...

s by the so-called Bomber Mafia
Bomber Mafia
The Bomber Mafia were a close-knit group of American military men who believed that long-range heavy bomber aircraft in large numbers were able to win a war...

 in the United States Army Air Corps
United States Army Air Corps
The United States Army Air Corps was a forerunner of the United States Air Force. Renamed from the Air Service on 2 July 1926, it was part of the United States Army and the predecessor of the United States Army Air Forces , established in 1941...

 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 bomber
Heavy bomber
A heavy bomber is a bomber aircraft of the largest size and load carrying capacity, and usually the longest range.In New START, the term "heavy bomber" is used for two types of bombers:*one with a range greater than 8,000 kilometers...

s. Starting in 1941, airmen such as Benjamin S. Kelsey
Benjamin S. Kelsey
Benjamin Scovill "Ben" Kelsey was an American aeronautical engineer and test pilot who brought success in World War II to the United States Army Air Forces by initiating the manufacture of innovative fighter aircraft designs, and by working to quickly increase American fighter production to meet...

 and Oliver P. Echols
Oliver P. Echols
Oliver Patton Echols was an American military officer who brought success in World War II to the United States Army Air Forces by expanding the inventory of America's air arm to meet the needs of the coming war. More than any other man under Chief of the Army Air Forces, General Henry H...

 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 gal of extra fuel per fighter that the P-38 could have carried out Operation Vengeance, the shootdown of Admiral Isoroku Yamamoto
Isoroku Yamamoto
was a Japanese Naval Marshal General and the commander-in-chief of the Combined Fleet during World War II, a graduate of the Imperial Japanese Naval Academy and a student of Harvard University ....

. 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 fighter
Escort fighter
The escort fighter was a World War II concept for a fighter aircraft designed to escort bombers to and from their targets.The perfect escort fighter had long range, a lengthy combat loiter time to protect the bombers, and enough internal fuel to return home...

s. External drop tanks also turned the Republic P-47 Thunderbolt from a short-range interceptor
-Vehicles:* Interceptor aircraft , a type of fighter aircraft designed specifically to intercept and destroy enemy aircraft* Ford Crown Victoria Police Interceptor, a police car...

 into long-range escort and air superiority fighter
Air superiority fighter
An air superiority fighter is a type of fighter aircraft intended to gain air superiority in a war, by entering and seizing control of enemy airspace. Air superiority fighters are designed to effectively engage enemy fighters, more than other types of aircraft...

, enabling it to accompany bombers all the way from British Isles to Germany, and back.


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
Moment of inertia
In classical mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass, is a measure of an object's resistance to changes to its rotation. It is the inertia of a rotating body with respect to its rotation...

, thereby reducing roll rate
Roll rate
Roll rate may refer to:* Roll Rate Analysis: involves comparing worst delinquency in a specified “previous x ” months with that in the “next x” months, and then calculating the percentage of accounts that maintain their worst delinquency, get better, or “roll forward” into the next delinquency...

s 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 hardpoint
A hardpoint, or weapon station, is any part of an airframe designed to carry an external load. This includes a point on the wing or fuselage of military aircraft where external ordnance, countermeasures, gun pods, targeting pods or drop tanks can be mounted.-Rail launchers:Large missiles and...

s available for weapon
A weapon, arm, or armament is a tool or instrument used with the aim of causing damage or harm to living beings or artificial structures or systems...

s, 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 tank
Conformal fuel tank
Conformal fuel tanks are additional fuel tanks fitted closely to the profile of an aircraft which extend either the range or "time on station" of the aircraft, with a reduced aerodynamic penalty compared to external drop tanks....

s (CFTs) instead of or in addition to conventional external fuel tanks. CFTs produce less drag and do not take up external hardpoints, however as a result 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
The SNEB rocket is an unguided air-to-ground rocket projectile manufactured by the French company TDA Armements, designed for launch by combat aircraft and helicopters. Two other rockets were developed in the and caliber...

 68 mm (2.7 in) rockets) and 250 litre of fuel behind into one single aerodynamically-shaped pod for mounting on combat aircraft such as the Dassault Mirage III
Dassault Mirage III
The Mirage III is a supersonic fighter aircraft designed by Dassault Aviation during the late 1950s, and manufactured both in France and a number of other countries. It was a successful fighter aircraft, being sold to many air forces around the world and remaining in production for over a decade...

s and English Electric Lightning
English Electric Lightning
The English Electric Lightning is a supersonic jet fighter aircraft of the Cold War era, noted for its great speed and unpainted natural metal exterior finish. It is the only all-British Mach 2 fighter aircraft. The aircraft was renowned for its capabilities as an interceptor; Royal Air Force ...


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
So-Cal Speed Shop
The So-Cal Speed Shop is a specialty parts shop for hot rods, specializing in cars built in the 1940s. It offers parts online, as well as a number of retail stores in the western United States and one in Canada. The headquarters is located in Pomona, California....

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.

External links

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