To reduce the likelihood of damage to the A-10's fuel system, all four fuel tanks are located near the aircraft's center and are separated from the fuselage; projectiles would need to penetrate the aircraft's skin before reaching a tank's outer skin. Compromised fuel transfer lines self-seal; if damage exceeds a tank's self-sealing capabilities, check valves prevent fuel flowing into a compromised tank. Most fuel system components are inside the tanks so that fuel will not be lost due to component failure. The refueling system is also purged after use. Reticulated polyurethane foam lines both the inner and outer sides of the fuel tanks, retaining debris and restricting fuel spillage in the event of damage. The engines are shielded from the rest of the airframe by firewalls and fire extinguishing equipment. In the event of all four main tanks being lost, two self-sealing sump tanks contain fuel for 230 miles (370 km) of flight.