GAU-8 Avenger

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The General Electric GAU-8/A Avenger is a 30 mm, 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.

1 History
2 Design
3 Variants
4 Recoil myth
5 Specifications
6 See also
7 References
8 External links

[edit] 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 unladen weight. The gun is placed in the center 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.

[edit] 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 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. (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 (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 vehicle. 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. Residual DU in soil (and thus water-tables) has also been implicated in a sharp spike in birth defects observed in certain areas of Iraq immediately after the Gulf War.

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. (Spick, 2000, p. 44).

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 deleted and replaced with a slightly lower 3900 RPM. Also the low rate of fire (1800 RPM) was deleted as being tactically inferior to a high rate of fire - it is better to ensure a first-time kill by using a high rate of fire, rather than having to make a risky second pass just to put a sufficient number of rounds on the target [1]. 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. 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. (Spick, 2000, p. 44).

[edit] 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 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. (Spick, 2000, p. 44).

[edit] 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 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.

The average recoil force, <math>F</math>, can be calculated using fundamental principles of mechanics. The operative principle is change in momentum equals impulse. <math>mv = Ft</math>, where <math>m</math> is the projectile mass, and <math>v</math> is the muzzle velocity, <math>t</math> is time.

For a burst of fire, <math>F = \frac{mv}{t} \times r \times t</math>

which reduces to

where

<math>m</math> is the mass of the projectile (0.425 kg)

<math>v</math> is the muzzle velocity (1067 m/s)

<math>r</math> is the rate of fire in rounds per second (70 rounds/s)

At 4200 rounds/minute, 70 rounds are fired each second. A 0.425 kg projectile is accelerated to 1067 m/s every one seventieth of a second. This gives a recoil force of approximately 30 kN based on the relation above. On the GAU-8/A product homepage the recoil force is stated as 10,000 pounds-force, or about 45 kN, probably due to additional recoil from exhaust gas from the muzzle. 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, or to even slow its airspeed enough for the pilot to feel any noticeable reduction when fired in bursts.

According to 355th Fighter Wing Weapons and Tactics Chief at Davis-Monthan AFB in Tucson, AZ, there is no recoil problem with the GAU-8/A. The GAU-8/A utilizes recoil adapters. They are the interface between the gun housing and the gun mount. By absorbing (in compression) the recoil forces, they reduce the magnitude of the felt recoil, spread the time of the recoil impulse, and counter recoil energy transmitted to the supporting structure when the gun is fired. There is no noticeable feel to the aircraft that any reduction in airspeed is occurring, nor does gun-firing affect aircraft speed.

Some claims have been made that the A-10 engines are susceptible to flame-out when subjected to gunpowder gases, such that when the GAU-8 firing, the smoke from the gun can make the engines stop, and this did occur during initial flight testing. Gun exhaust is essentially oxygen-free, and is certainly capable of causing flame-outs of gas turbines. However, the A-10 is now designed so that the gun exhaust passes underneath the fuselage, and never ventures near the high-mounted turbines, even during negative-G maneuvers.

[edit] Specifications

Calibre: 30mm x 173mm
Rate of fire: 3900 RPM
Muzzle velocity: 1067 meters/second
Maximum range: over 1,250 meters
Number of barrels: 7
Feed: Linkless feed system
Accuracy: 5mil, 80% of rounds fired at 4,000ft hit within a 20ft radius
Cannon weight: 281 kilograms
Cannon length: 6.40 meters
Ammo types
- PGU-14/B API Armor Piercing Incendiary [DU]
- PGU-13/B HEI High Explosive Incendiary
- PGU-15/B TP Target Practice
Armor penetration
- 69mm at 500 meters
- 38mm at 1000 meters