The General Electric GAU-8/A Avenger is a 30 mm hydraulically driven seven-barrel Gatling-type
cannon that is typically mounted to the United States Air Force's Fairchild Republic A-10
Thunderbolt II. Designed specifically for the anti-tank role, the Avenger delivers very
powerful rounds at a high rate of fire. The GAU-8/A is also mounted to the Goalkeeper
CIWS.
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
of the A-X prototypes, the YA-10 and the Northrop YA-9, were designed to incorporate the weapon,
although it was not available during the initial competition; the M61 Vulcan was used as a
temporary replacement. Once completed, the entire GAU-8 assembly represents about 16%
of the A-10 aircraft's unladen weight. Because the gun plays a significant role in maintaining
the A-10's balance and center of gravity, a jack must be installed beneath the tail
of the plane whenever the gun is removed for inspection in order to prevent the aircraft
from tipping rearwards.
The gun is placed slightly off center in the nose of the plane with the front landing gear
positioned to the right of the center line, so that the actively firing cannon barrel
is directly on the aircraft's center line. The Russian Gryazev-Shipunov GSh-6-30 is a
similar class of weapon, although it is lighter with 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. It was produced by General Electric, though General Dynamics Armament and Technical
Products has been responsible for production and 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-8/E 30 mm Ammunition Loading Assembly cart. This vehicle is unique to the
A-10 and the GAU-8. Design
The GAU-8 itself weighs 620 pounds, but the complete weapon, with feed system and drum,
weighs 4,029 pounds with a maximum ammunition load. It measures 19 ft 5 1⁄2 in from
the muzzle to the rearmost point of the ammunition system, and the ammunition drum alone is 34.5
inches in diameter and 71.5 inches long. Power for operating the gun is provided by twin
hydraulic motors pressurized from two independent hydraulic systems. The magazine can hold 1,174
rounds, although 1,150 is the typical load-out. Muzzle velocity when firing Armor-Piercing
Incendiary rounds is 3,250 feet per second, almost the same as the substantially lighter
M61 Vulcan's 20 mm round. The standard ammunition mixture for anti-armor
use is a five-to-one mix of PGU-14/B Armor Piercing Incendiary, with a projectile weight
of about 14.0 oz and PGU-13/B High Explosive Incendiary rounds, with a projectile weight
of about 13.3 oz. The PGU-14/B's projectile incorporates a lightweight aluminum body,
cast around a smaller caliber depleted uranium penetrating core. The Avenger is lethal against
tanks and all other armored vehicles.
A very important innovation in the design of the GAU-8/A ammunition is the use of aluminum
alloy cases in place of the traditional steel or brass. This alone adds 30% to ammunition
capacity for a given weight. The projectiles incorporate a plastic driving band to improve
barrel life. The shells are imposing to examine and handle, measuring 11.4 inches in length
and weighing 1.53 pounds or more. The Avenger's rate of fire was originally
selectable, 2,100 rounds per minute in the low setting, or 4,200 rpm in the high setting.
Later this was changed to a fixed rate of 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 life of at least 20,000 rounds for each set
of barrels. There is no technical limitation on the duration the gun may be continuously
fired, and a pilot could potentially expend the entire ammunition load in a single burst
with no damage or ill effects to the weapons system itself. However, this constant rate
of fire would shorten the barrel life considerably and require added barrel inspections and result
in shorter intervals between replacement. 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, allowing the spent casings to be recycled
back into the ammunition drum, instead of ejected from the aircraft, which would require
considerable force to eliminate potential airframe damage. The feed system is based
on that developed for later M61 installations, but uses more advanced design techniques and
materials throughout, to save weight. Firing system
Accuracy The GAU-8/A is extremely accurate and can
fire 4,200 rounds per minute without complications. The 30-mm shell has twice the range, half
the time to target, and three times the mass of projectiles carried by comparable Close
Air Support aircraft.
The muzzle velocity of the GAU-8/A is about the same as that of the M61 Vulcan cannon,
but uses heavier ammunition and has superior ballistics. Its time of flight to 4,000 feet
is 30 percent less than that of an M61 round, the projectile decelerates much less rapidly
after leaving the barrel, and it drops a negligible amount, about 10 feet over the distance. The
GAU-8/A accuracy when installed in the A-10 is rated at "5 mil, 80 percent", meaning that
80 percent of rounds fired will hit within a cone with an angle of five-milliradians.
This equates to a 40 feet diameter circle at the weapon's design range of 4,000 feet.
By comparison, the M61 has an 8-milliradian dispersion.
Recoil Each barrel fires when it reaches roughly
the 9 o'clock position, when viewed from the front of the plane. Because the gun's recoil
forces could push the entire plane off target during firing, the weapon itself is mounted
off-center in the other direction, toward the 3 o'clock position, so that the firing
barrel lies directly on the aircraft's center line. The firing barrel also lies just below
the aircraft's center of gravity, being bore sighted along a line 2 degrees below the aircraft's
line of flight. This arrangement accurately centers the recoil forces, preventing changes
in pitch or yaw when fired. This configuration also provides space for the front landing
gear, which is mounted slightly off-center on the right side of the nose.
The GAU-8/A utilizes recoil adapters. They are the interface between the gun housing
and the gun mount. By absorbing the recoil forces, they spread the time of the recoil
impulse and counter recoil energy transmitted to the supporting structure when the gun is
fired. The A-10 engines were initially susceptible
to flameout when subjected to gases generated in the firing of the gun. When the GAU-8 is
being fired, 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. The A-10 engines now have a self-sustaining combustion section.
When the gun is fired the igniters come on to reduce the possibility of a flame-out.
The average recoil force of the GAU-8/A is 10,000 pounds-force, which is slightly more
than the output of one of the A-10's two TF34 engines. While this recoil force is significant,
in practice a cannon fire burst only slows the aircraft a few miles per hour in level
flight. Variants
Some of the GAU-8/A technology has been transferred into the smaller 25 mm GAU-12/U Equalizer,
which was developed for the AV-8B Harrier II aircraft. The GAU-12 is about the same
size as the 20 mm M61. 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. The Avenger
also forms the basis for the Dutch-developed Goalkeeper CIWS naval air-defence gun. No
current or contemplated aircraft other than the A-10, however, carries the full-up Avenger
system. Specifications
Precision: 80% of rounds fired at 4,000 feet range hit within a 40 feet diameter circle
Ammo: PGU-14/B API Armor Piercing Incendiary
PGU-13/B HEI High explosive incendiary PGU-15/B TP Target Practice
Armor penetration: 69 mm at 500 meters
38 mm at 1,000 meters
See also T249 Vigilante
Gryazev-Shipunov GSh-6-30 General:
List of aircraft weapons List of multiple barrel firearms
References
Spick, Michael. The Great Book of Modern Warplanes, Salamander Books, 2000. ISBN 1-84065-156-3.
External links General Electric GAU-8/A Avenger on USAF National
Museum site GAU-8 Avenger, Hill Aerospace Museum, fas.org
General Dynamics Ordnance and Tactical Systems GAU-8/A page
Video of the GAU-8 Avenger test firing