GAU-8 Avenger

The GAU-8 Avenger

The General Electric 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, heaviest and most powerful purpose-built aircraft cannon in the United States military, exceeded only by the specialised artillery pieces carried on the AC-130 Gunship. The GAU-8 was specifically designed for the anti-tank role, and delivers a very powerful shell at a high rate of fire.

History

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. Both the A-X prototypes, being the A-10 and the Northrop YA-9, were designed to incorporate the weapon, although it was not available during the initial competition and the M61 Vulcan was used as a temporary replacement. 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 A-10 and its GAU-8/A gun entered service in 1977. The gun is no longer in production. It was produced by General Electric, though Martin Marietta is now responsible for support.

Design

The GAU-8 itself weighs 281 kg (620 lb), but the complete weapon, with feed system and drum, weighs 1,830 kg (4,029 lb) with a maximum ammunition load. It measures 19.05ft 10.5in (5.06m) from the muzzle to the rearmost point of the ammunition system, and the ammunition drum alone is 34.5in (87.6cm) in diameter and 71.5in (181.6cm) long. (Spick, 2000, p. 44). 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 projectile 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 or 6,630 grains). The PGU-14/B round incorporates a depleted uranium penetrator. The Avenger is extremely lethal against tanks and any other armored vehicle. 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.

A very important innovation in the design of the GAU-8/A shells is the use of aluminium alloy cases in place of the traditional steel or brass. This alone adds 30 per cent to ammunition capacity for a given weight. The shells also have plastic driving bands to improve barrel life. They are imposing to examine and handle, measuring 11.4in (290mm) in length and weighing 1.53lb (694g) or more. (Spick, 2000, p. 44).

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 conserve ammunition; barrel life is also a factor, since the USAF has specified a minimum 21,000-round life for each set of barrels. It is also said that this is to deal with the substantial deceleration of the plane that results from firing (see below for details).

Each barrel is a very simple non-automatic design having its own breech and bolt. Like the original Gatling gun, the entire firing cycle is actuated by cams and powered by the rotation of the barrels.Template:Inote The barrels themselves are driven by the aircraft's dual hydraulic system.Template:Inote

The GAU-8/A ammunition is linkless, reducing weight and avoiding a great deal of potential for jamming. The feed system is double-ended: the spent casings are not ejected from the aircraft (which takes a great deal of force if the possibility of severe airframe damage is to be eliminated) but are cycled back into the ammunition drum. The feed system is based on that developed for later M61 installations, but uses more advanced design techniques and materials throughout, to save weight. (Spick, 2000, p. 44).

Variants

Some of the GAU-8/A technology has been transfereed into the smaller 25mm GAU-12/U Equalizer developed for the AV-8B, which is about the same size as the M61 but is considerably more lethal. GE has also developed the GAU-13, a four-barreled weapon using GAU-8/A components, which has been tested in podded-form, and the Avenger forms the basis for the Dutch-developed Goalkeeper naval air-defence gun. No current or contemplated aircraft other than the A-10, however, carries the full-up Avenger system. (Spick, 2000, p. 44).

The Recoil vs. Forward Thrust Myth

File:GAU-8 avenger.jpg

The GAU-8/A "Avenger" gatling gun next to a VW Beetle

A persistent urban legend 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 mass of the projectiles over one second (force = impulse per time). This gives an approximate recoil force of 30 kN. On the GAU-8/A product homepage the recoil force is stated as 10,000 pounds-force, or about 45 kN. The maximum combined output of the A-10 engines is 80 kN. Hence the recoil force of the gun is slightly more than half of the total thrust of the engines. While this is quite significant, it is 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, causing the aircraft to slow down. A-10 pilots report that firing the cannon while moving at high speed feels like "hitting the brakes", even though the engines are automatically pushed to full throttle when the gun is activated. Long bursts from the gun can reduce airspeed enough to cause a stall danger when executing a hard pull-up and turn at the end of a strafe. 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 gasses, such that when the GAU-8 firing, the smoke from the gun can make the engines stop. Gun exhaust is essentially oxygen-free, and is certainly capable of causing flame-outs of gas turbines. However, the A-10 is designed so that the gun exhaust passes underneath the fuselage, and never ventures near the high-mounted turbines, even during negative-G maneuvers.

See also

• List of aircraft weapons

References

• Spick, M. The Great Book of Modern Warplanes, Salamander Books, 2000. ISBN 1840651563
• GAU-8 Avenger, www.mindfully.org (viewed 27 Apr 2005)
• 30mm cannon GAU-8 Avenger, by Jirka Wagner (viewed 27 Apr 2005)
• General Electric GAU-8/A "Avenger" 30mm Rotary Cannon

External links

• FAS.org: GAU-8 Avenger, Hill Aerospace Museum (viewed 27 Apr 2005)
• Nazarian`s Gun`s Recognition Guide