The General Electric GAU-8/A Avenger is a 30 mm, hydraulically-driven seven-barrel Gatling-type rotary cannon that is mounted on the United States Air Force's A-10 Thunderbolt II. It is the largest, heaviest and most powerful aircraft cannon in the United States military. The GAU-8 was specifically designed for the anti-tank role, and delivers a very powerful round at a high rate of fire.
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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, 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. Once completed, the GAU-8 represents some 16% of the A-10 aircraft's unladen weight. The gun is placed slightly off centre in the nose of the plane with the front landing gear positioned to the side. In a similar class is the Russian Gryazev-Shipunov GSh-6-30, which is lighter and has a higher fire rate but has a lower muzzle velocity and overheats faster.
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 General Dynamics Armament and Technical Products has been responsible for support since 1997 when the division was sold by Lockheed Martin to General Dynamics.
The gun is loaded using Syn-Tech's linked tube carrier GFU-7/E 30mm Ammunition Loading Assembly cart. This vehicle is unique to the A-10 and the GAU-8.
Design
The GAU-8 itself weighs 2760 N (620 lb), mass 281 kg, but the complete weapon, with feed system and drum, weighs 1,830 kg (4,029 lb) with a maximum ammunition load. It measures 19 ft ½ in (5.81 m) from the muzzle to the rearmost point of the ammunition system, and the ammunition drum alone is 34.5 in (86 cm) in diameter and 71.5 in (1.82 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 (990 m/s), almost the same as the substantially lighter M61 Vulcan's 20mm round.
The standard ammunition mixture for anti-armor use is a four-to-one mix of PGU-14/B Armor-Piercing Incendiary (API), with a projectile weight of about 15.0 oz (425 grams or 6,560 grains) and PGU-13/B High Explosive Incendiary (HEI) rounds, with a projectile weight of about 12.7 oz (360 grams). The PGU-14/B round incorporates a depleted uranium penetrator. The Avenger is lethal against tanks and any other armored vehicles.
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% 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 11.4 in (290 mm) in length and weighing 1.53 lb (694 g) or more.
Originally, the Avenger's rate of fire was switchable between 1,800 rounds per minute (RPM) and 4,200 RPM. In later modifications, the 4,200 RPM rate of fire was replaced with a slightly lower 3,900 RPM. 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). Technically, however, there is no tech order limitation on the duration the gun may be continuously fired; therefore the pilot could in theory hold the trigger down and expend all 1174 rounds in one burst, with no damage or ill effects (though such an action would almost certainly be wasteful and tactically unsound).
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. The barrels themselves are driven by the aircraft's dual hydraulic system. 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.
Variants
Some of the GAU-8/A technology has been transferred into the smaller 25 mm GAU-12/U Equalizer developed for the AV-8B Harrier II aircraft, which is about the same size as the M61 but is considerably more lethal. GE has also developed the GAU-13/A, a four-barreled weapon using GAU-8/A components, which has been tested in podded form as the GPU-5/A, 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.
Recoil myth
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 were to be fired for long periods of time (some even claim that the aircraft would begin to fly backwards). However, the GAU-8/A product homepage states the recoil force as 10,000 pounds-force, or about 45 kN, which is less than the maximum combined output of the A-10 engines (82.6 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 and can noticeably slow the aircraft, it is not sufficient to stop the aircraft. During test firing of the gun in the A-10 in the early 1970s the USAF experimented with putting a muzzle brake on the end of the gun and extending the nose of the plane out around this muzzle brake to vent the gun gases backwards. It was decided during this testing that the effect of the gun was not significant enough to warrant the added expense and complexity of adding this to every plane in the inventory.
Each barrel fires when it reaches roughly the 9 o'clock position, when looking at the nose of the plane, resulting in off-center recoil forces. It was discovered during development that if the recoil forces were not transmitted down the exact centerline of the plane, they would push the entire plane off target during firing. As a result, the entire gun assembly itself had to be mounted off-center in the other direction—towards the 3 o'clock position to compensate. It also lies below the aircraft's center of gravity, being boresighted along a line 2 degrees below the aircraft's line of flight. This arrangement accurately centers the recoil forces, preventing changes in pitch and/or yaw when firing. This configuration also provides space for the front landing gear, albeit mounted on the right side of the nose.
Specifications
- Type: Electric-Motor, Hydraulic-Driven
- Caliber: 30 x 173 mm
- Rate of fire: 3900 rpm (rounds per minute)
- Muzzle velocity: 3500 feet/second (1,067 m/s)
- Maximum range: over (1,250 m)
- Number of barrels: 7
- Feed: Linkless feed system
- Accuracy: 5 mile (8,000 m), 80% of rounds fired at 4,000 ft (1,200 m) hit within a 20 ft (6m) radius
- Cannon weight: 619.5 lb (281 kilograms)
- Cannon length: 252.0 in (6.40 m)
- Barrel length: 90.5 in (2.299 m)
- Ammunition:
- PGU-14/B API Armor Piercing Incendiary (DU)
- PGU-13/B HEI High Explosive Incendiary
- PGU-15/B TP Target Practice
- Armor penetration:
- 69 mm at 500 meters
- 38 mm at 1000 meters
See also
References
- Spick, M. The Great Book of Modern Warplanes, Salamander Books, 2000. ISBN 1-84065-156-3
- GAU-8 Avenger, www.mindfully.org (retrieved February 17 2009)
- 30 mm cannon GAU-8 Avenger, by Jirka Wagner (retrieved February 17 2009)
External links
- FAS.org: GAU-8 Avenger, Hill Aerospace Museum (retrieved February 17 2009)
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