Drop tank
In aviation, a drop 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.
Overview
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 maneuvres. Some of the drop tank's fuel is used 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, 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 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.
History
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. During World War II, the German Luftwaffe began using external fuel tanks with the introduction of a 300-liter 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 — including at least one impregnated paper material, single-use version — and varying only slightly in appearance, became the standard volume for most subsequent drop tanks in Luftwaffe service, with a rarely used 900 litre, 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 in order 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. The Zero entered service in 1940.
Bomber theorists insisted formations of heavy bombers with elaborate defensive armaments would be self-defending, believing long-range escort fighters to be "a myth" as they could be easily forced to drop the tanks by minor harassment at the beginning of the raid being more concerned that long-range medium bombers might compete for resources and so compromise their goal of creating vast fleets of heavy bombers. 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 such as the Lockheed P-38 Lightning.
with a drop tank
It was only with drop tanks supplying of extra fuel per fighter that P-38s could carry out Operation Vengeance, the downing of Admiral Isoroku Yamamoto's airplane. 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 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-gallon drop tanks for its longest sorties. This teardrop-shaped tank design was long and in diameter at its widest point.
Paper-based drop tanks
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 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 any 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 centre section created by wrapping layers of the impregnated paper around a cylinder, the end caps hand-laminated with petal-shaped pieces sometimes named 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 and passing tanks were given two coats of cellulose dope followed by two coats of aluminium paint.
Some 13,000 papier-mâché tanks were made and used by the RAF, the vast majority used in the course of the war, conserving a considerable amount of metal. Very few examples survive due to their expendable nature and low intrinsic value at the time of their creation, 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, as they were lightweight and comparatively fragile it is unlikely to cause anything but anxiety, the Germans authorities going so far as to distribute leaflets, explaining that drop tanks are not bombs.
U.S. paper tanks were developed by Col. Bob Shafer and Col. Cass Hough, who spent many hours developing a 110-gallon 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.
However it may explain why the most often-used fuel tanks for single-engined American fighters operating in Northern Europe were the 75-gallon capacity all-metal tank. Another common metal drop tank was the 150-to-165-gallon model used by P-51s, P-47s and P-38s.