Look at almost any photo of a fighter on a real mission and you will see them: big, smooth, bullet-shaped pods hanging under the wings or the belly. They are not bombs or missiles. They are fuel tanks, and many of them are designed to be thrown away in mid-air.

Drop tanks are one of the oldest tricks in military aviation and one of the least glamorous. Yet without them, an F-16 could not cross an ocean, a WWII Mustang could not have escorted bombers deep into Germany, and a fighter pilot about to enter a dogfight would be carrying several thousand pounds of dead weight.

More fuel, more range

A fighter's internal fuel tanks are squeezed into whatever space is left between the engine, the cockpit, the landing gear and the weapons bays. There is never enough. Hanging extra tanks outside is the quickest way to add range or time on station, whether for a ferry flight, a long patrol or a deep strike.

The F-16, for example, typically carries 370-gallon tanks under its wings and can add a smaller tank under the fuselage. Combat aircraft from the F-4 Phantom to the MiG-21 were routinely photographed with tanks hanging under them, as the Vietnam-era image below shows.

Why pilots throw them away

Tanks cost performance. They add drag, which burns more fuel and lowers top speed. They add weight and, hanging out on the wings, increase the aircraft's resistance to rolling, which matters enormously in a turning fight. They also occupy hardpoints that could carry weapons and make the aircraft easier to see on radar.

So when a fight is coming, or when the tanks are empty and only drag is left, pilots can jettison them. In the Second World War, fighters escorting bombers would drop their tanks the moment enemy fighters appeared. Today, the same switch is used in emergencies: after a bird strike, an engine problem or a fire on take-off, dumping heavy external stores can be what keeps a jet flying.

In the clip above, an Indian Air Force Jaguar jettisons its external stores after a bird strike on take-off. Jettison is not always tidy. Tanks can tumble, strike the aircraft or each other, and test programmes spend a lot of effort proving that they separate cleanly.

The paper tanks of WWII

During the Second World War, metal was precious. According to Wikipedia, the British developed single-use drop tanks made of glue-impregnated kraft paper, which were light and cheap. The fuel slowly dissolved the glue, so they could leak within hours of being filled, but they only needed to last for one mission. About 13,000 were produced for the RAF.

Drop tanks transformed the air war. Long-range escort fighters such as the P-51 Mustang, flying with external tanks, could accompany US bombers all the way to Berlin and back. On the other side, the Luftwaffe had introduced 300-litre tanks in 1940 for the Ju 87R and the Bf 109E-7.

Conformal tanks and the future

The modern alternative is the conformal fuel tank, shaped to hug the fuselage, as on the F-15E and some F-16 variants. Conformal tanks create far less drag and do not use up weapon stations, but most can only be removed on the ground, so the aircraft carries their weight for the whole flight.

Stealth aircraft have a bigger problem. A drop tank wrecks a carefully shaped radar signature, so stealth fighters rely on internal fuel and air-to-air refuelling in combat. The F-22, for example, carries external tanks for ferry flights rather than for combat missions. The humble drop tank, in other words, still shapes how far a fighter can go and how well it can fight when it gets there.

Sources: Wikipedia (Drop tank; General Dynamics F-16 Fighting Falcon), US Air Force