Self-sealing fuel tank
Typical self-sealing tanks have multiple layers of rubber and reinforcing fabric, one of vulcanized rubber, and one of untreated natural rubber, which can absorb fuel, swell, and expand when it comes into contact with the fuel. When a fuel tank is punctured, the fuel seeps into the layers, causing the untreated layer to swell and thus seal 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 The order was rescinded by the United States Patent Office on September 26, 1918 and Murdock was eventually granted United States Patent 1,386,791 "Self-Puncture Sealing Covering for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.
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 filed a patent in 1941 (published in 1947) 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 Mk IX 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 .50 in (12.7 mm) bullets and, on occasion, 20 mm (0.79 in) 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).
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 Hutchinson, Amfuel (Zodiac) (formerly Firestone), Meggitt (formerly Goodyear), Robertson Fuel Systems, 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.
- "fuel tanks - henderson system - airscrew spinners - 1940 - 2064 - Flight Archive". flightglobal.com.
- "the Ingenious Ju-88", Flight, 1940
- Puncture Sealing Gas Tank
- US patent 2424701, Merrill, "Fuel tank casing", issued 1947-07-29 USPTO text and images
- A Study of Helicopter Crash-Resistant Fuel Systems
- "About GKN (Company History - Portsmouth)". Retrieved December 5, 2011.
- "UH-60A Student Handbook" (PDF). United States Army Warfighting Center. 2008. pp. D–3. Archived from the original (PDF) on January 5, 2012. Retrieved December 5, 2011.
- James A Merrill receiving citation from President Roosevelt (1942)
- Installation of a Self-Sealing Materials Systems in a C-130 Integral Fuel Tank Wing - DTIC
- Popular Science, November 1941 Lining Will Seal Bullet Holes In Bombers Fuel System see bottom half of page
- Gunfire Qualification Test of Self-Sealing Fuel Cells
- Self-sealing Fuel Tank Demonstration