Science Fair Project Encyclopedia
The GAU-8/A Avenger is a 30 mm, seven-barrel Gatling gun that is mounted on the United States Air Force's A-10 Thunderbolt II. It is the largest (it is the size and weight of a family saloon car (sedan)), heaviest and most powerful aircraft gun in the United States military. Specifically designed for the anti-tank role, the gun delivers a very powerful shell at a high rate of fire.
The GAU-8 was created as a parallel program with the A-X competition that produced the A-10. The specification for the cannon was laid out in 1970, with General Electric and Philco-Ford offering competing designs. The A-X prototypes (both the A-10 and the Northrop YA-9) were both designed to incorporate the massive weapon, although it was not available during the initial competition and the M61 Vulcan was used as an interim weapon. On the resultant A-10, the GAU-8 fills half of the aircraft fuselage and represents one third of its unloaded weight. The gun is placed in the centre of the plane with the front landing gear positioned to the side.
The GAU-8 itself weighs 281 kg (620 lb), but the complete weapon, with feed system and drum, weighs 4,029 lb (1,830 kg) with a maximum ammunition load. The entire system is 19 ft 10.5 in (5.05 m) long. The magazine can hold 1,350 rounds, although 1,174 is the more normal load-out. Muzzle velocity with armor-piercing incendiary (API) ammunition is 3,250 ft/s (988 m/s), almost the same as the substantially lighter M61 Vulcan.
The standard ammunition mixture for anti-armor use is a four-to-one mix of PGU-13/B High Explosive Incendiary (HEI) rounds, with a projecticle weight of about 12.7 oz (360 grams) and PGU-14/B Armor-Piercing Incendiary (API), with a projectile weight of about 15.0 oz (430 grams). The PGU-14/B round incorporates a depleted uranium penetrator. Just six rounds are sufficient to destroy most current Russian main battle tanks. The use of the depleted-uranium round is controversial, with some reports linking its use to health problems among both survivors of DUP attacks and servicemen involved in the loading and handling of the rounds.
The Avenger's maximum rate of fire is 4,200 rounds per minute. In practice, the cannon is limited to one and two-second bursts to avoid overheating and to conserve ammunition. It is also said that this is to deal with the substantial deceleration of the plane that results from firing. This is however a myth (see below).
The GAU-8/A is also used on naval ships in the Goalkeeper CIWS.
The Recoil vs. Forward Thrust Myth
A persistent claim is that the recoil force of the Avenger matches that of the A-10's engines and as such the plane would slow down, stall and subsequently crash if the gun was to be fired for long periods of time. Some even add the fanciful notion of the plane beginning to fly backwards. These claims hold little truth. While the cannon does slow the aircraft when flying at high speed, it cannot stop the plane in mid-air.
The recoil force can be calculated by multiplying the muzzle velocity with the weight of the projectiles over one second (force = impulse per time). This gives an approximate recoil force of 30kN. On the GAU-8/A product homepage the recoil force is stated as 10 000 Pounds, or about 45 kN. The maximum combined output of the A-10 engines is 80 kN. Hence the recoil force of the gun is only about half of the total thrust of the engines. While this is quite significant, it is clearly not sufficient to stop the aircraft. However, it can slow the aircraft rapidly and poses a challenge for pilots. When the airplane is moving quickly, it experiences a backward force of aerodynamic drag that is proportional to its speed. Aggressive maneuvering increases drag significantly. During combat flight, the engines must operate near full-throttle just to counteract drag and maintain speed. When the gun recoil force is combined with the drag at 400+ knots, the total backwards force is much more than the combined thrust of the engines, even at full throttle. The plane slows down, fast. Word from A-10 pilots is that firing the cannon while moving at high speed feels like "hitting the brakes", even though the engines are automatically pushed full-on when the gun is activated. Long bursts from the gun can reduce airspeed enough to cause stall danger when executing a hard pull-up and turn at the end of a strafe. Of course, even if the gun could fire forever the plane would never fall out of the sky -- it would simply slow down to a speed where the engine thrust is equal to the gun recoil plus the aerodynamic drag.
Some claims have been made that the A-10 engines are susceptible to flame-out when subjected to gun powder gases, i.e. when firing, the smoke from the gun would make the engines stop. Gun exhaust is essentially oxygen-free, and is certainly capable of causing flame-outs of gas turbines. However, this well-known problem is addressed by engineers when designing modern aicraft, and engines are located such that gun gas is routed far away from intakes. Watching an A-10 fire, it is clear that the gun exhaust passes underneath the fuselage, and never ventures anywhere near the high-mounted turbines, even during negative-G maneuvers.
It may also be noted that impressively powerful weapons have always been surrounded by myths and exaggerations regarding their capabilities. The claimed ability of the GAU-8 to stop a Warthog in mid-air is certainly one of those myths and can perhaps be seen as an urban legend.
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