Self-sealing fuel tank

In aviation, self-sealing fuel tank is a fuel tank or fuel bladder technology in wide use since World War II that prevents fuel tanks (primarily on aircraft) from leaking fuel and igniting after being damaged by enemy fire.

Self-sealing tanks have multiple layers of rubber and reinforcing fabric, one of vulcanized rubber and one of untreated natural rubber that can absorb oil and expand when wet. When a fuel tank is punctured, the fuel will seep into the layers, causing the swelling of the untreated layer, thus sealing the puncture.

World War I
George J. Murdock applied for the patent "War Aeroplane Fuel Tanks" on February 7, 1917 but was temporarily blocked by an order of the Federal Trade Commission, on February 6, 1918, to keep any discussion or publication of the invention secret. The order was rescinded by the United States Patent Office on September 26, 1918 and Murdock was eventually granted United States Patent 1,386,791 "Self-Puncture Sealing Covering for Fuel-Containers" on August 9, 1921. Military aircraft built by the Glenn L. Martin Company used this self-sealing fuel tank.

World War II
In the newer generations of pre-war and early-war aircraft, self-sealing tanks were tanks used to minimize the damage from leaking or burning fuel. A conventional fuel tank, when hit by gunfire, could leak fuel rapidly. This would not only reduce the aircraft's effective range, but was also a significant fire hazard. Damaged fuel tanks could also rupture, destroying the airframe or critically affecting flight characteristics.

It was realized that, because of weight limitations, it was not practical to simply add armor plate to aircraft fuel tanks; a method of stopping fuel leaking from damaged tanks was necessary.

Early attempts at protecting fuel tanks consisted of using metal tanks, covered inside or outside by a material that expanded after being pierced. Research revealed that the exit of the projectile, rather than the entry, was the greater problem, as it often tumbled, thus creating a large exit hole. Among the earliest versions of these types of tanks were those manufactured in the UK at Portsmouth airport by Fireproof Tanks Ltd. These tanks were first installed in the Fairey Battle light bomber with other versions installed in Supermarine Spitfire and Hawker Hurricane fighters and larger aircraft such as the Avro Lancaster heavy bomber. The Henderson Safety Tank company provided crash-proof self-sealing tanks for the Miles Master trainer.

The Germans were using layers of rubber laid over leather hide with a treated fibre inner surface for the self-sealing tanks on their Junkers Ju 88 bombers early in the war.

In the US, Ernst Eger of United States Rubber Company (later Uniroyal) patented a self-sealing fuel tank design in 1941; one of many companies involved in developing this technology during the war. Goodyear chemist James Merrill was awarded a patent in 1941 for refining and successfully testing his method for manufacturing self-sealing tanks using a two-layer system of rubber compounds encased in a metal outer shell or the wing lining of the aircraft. In 1942, he received a War Production Board citation from President Roosevelt and the Goodyear tanks were subsequently placed in service in Goodyear-produced Corsair fighters, as well as other aircraft. By 1942 Fireproof Tanks had developed the first flexible fuel bladders as range extender tanks for the MkIX Spitfire. These tanks were flexible containers, made of a laminated self-sealing material like vulcanized rubber and with as few seams as possible to minimize leak paths.

As early tests showed that impact could overpressurize a fuel tank, the self-sealing fuel cell is suspended, allowing it to absorb shocks without rupture. U.S. Navy fuel tanks during the war were able to withstand Caliber .50 in bullets and, on occasion, 20 mm cannon shells.

Not all fighters were fitted with the relatively new invention. Self-sealing tanks tended to be heavier with lower capacity than non-sealed tanks. Nonetheless, aircraft that were fitted with self-sealing tanks regularly took more punishment than those without, and were able to return to base. Combat experience in the Pacific war showed that the heavily protected American aircraft could sustain far more damage than the lightly armored Japanese designs without self-sealing fuel tanks (for instance, the Mitsubishi A6M Zero).

The same principles were applied to give self-sealing fuel lines in aircraft (MIL-PRF-7061C).

Modern use
Most jet fighters and all US military rotary wing aircraft have some type of self-sealing tanks. Military rotary wing fuel tanks have the additional feature of being crashworthy. High altitudes require the tanks to be pressurized, making self-sealing difficult. Newer technologies have brought advances like inert foam-filled tanks to prevent detonation. This foam is an open cell foam that effectively divides the gas space above the remaining fuel into thousands of small spaces, none of which contain sufficient vapour to support combustion. This foam also serves to reduce fuel slosh. Major manufacturers of this technology include Amfuel (Zodiac) (formerly Firestone), Meggitt (formerly Goodyear), GKN USA and FPT Industries. FPT is now part of GKN. For military use, tanks are qualified to MIL-DTL-27422 (includes crashworthiness requirements) or MIL-DTL-5578 (non-crashworthy). An aircraft fuel tank sometimes consists of several interconnected fuel cells. The interconnecting hoses are typically also self-sealing.

In addition to fighter aircraft some military patrol vehicles and armoured VIP limousines feature self-sealing fuel tanks.

Self-sealing fuel tanks using military technology are also required in some motorsport categories.