M61 Vulcan

The M61 Vulcan is a hydraulically or pneumatically driven, six-barreled, air-cooled, electrically fired Gatling-style cannon, which fires 20 mm rounds at an extremely high rate. The M61 and its derivatives have been the principal cannon armament of United States military fixed-wing aircraft for fifty years. The M61 was originally produced by General Electric, and after several mergers and acquisitions is currently produced by General Dynamics.[1]

Development
At the end of World War II, the United States Army began to consider new directions for future military aircraft guns. The higher speeds of jet-engined fighter aircraft meant that achieving an effective number of hits would be extremely difficult without a much higher volume of fire. While captured German designs (principally the Mauser MG 213C) showed the potential of the single-barrel revolver cannon, the practical rate of fire of such a design was still limited by ammunition feed and barrel wear concerns. The Army wanted something better, combining extremely high rate of fire with exceptional reliability.

In response to this requirement, General Electric Armament Division resurrected an old idea: the multi-barrel Gatling gun. The original Gatling gun had fallen out of favor because of the need for an external power source to rotate the barrel assembly, but the new generation of turbojet-powered fighters offered sufficient electrical power to operate the gun, and electric operation offered reliability superior to a gas operated weapon. With multiple barrels the rate of fire per barrel could be lower than a single-barrel revolver cannon while still giving a superior total rate of fire.

In and of itself, the idea of powering a Gatling gun from an external electric power source was not a novel idea at the end of the World War II era, as Richard Jordan Gatling himself did just that in 1893, in a patent[2] he filed at that time.

The Army issued GE the contract in 1946 for "Project Vulcan", a six-barrel weapon capable of firing 7,200 rounds per minute (rpm).[3] Although European designers were moving towards heavier 30 mm weapons for better hitting power, the U.S. initially concentrated on a powerful .60 caliber (15 mm) cartridge designed for a pre-war anti-tank rifle, expecting that the cartridge's high muzzle velocity would be beneficial for improving hit ratios on high speed targets. The first GE prototypes of the .60 caliber T45 were ground-fired in 1949; it achieved 2,500 rpm, which was increased to 4,000 rpm by 1950. By the early 1950s, the USAF decided that high velocity alone might not be sufficient to ensure target destruction and tested 20 mm and 27 mm alternatives based on the .60 caliber cartridge's case. These variants of the T45 were known as the T171 and T150 respectively, and were first tested in 1952. Eventually, the 20 x 102 mm cartridge was determined to have the desired balance of projectile and explosive weight and muzzle velocity.

The development of the F-104 Starfighter revealed that the T171 Vulcan (later redesignated M61) suffered problems with its linked ammunition, being prone to misfeed and presenting a foreign object damage (FOD) hazard with discarded links. A linkless ammunition feed system was developed for the upgraded M61A1, which subsequently became the standard cannon armament of U.S. fighters.

General Electric later sold its aerospace division, including GE Armament Systems along with the design and production tooling for the M61 and GE's other rotary cannon, to Martin Marietta. After Martin's merger with Lockheed, the rotary cannon became the responsibility of Lockheed Martin Armament Systems. Lockheed Martin Armament Systems was later acquired by General Dynamics, who currently produce the M61 and its various offspring.[1]

Description
The Vulcan is a Gatling gun: each of the cannon's six barrels fires once in turn during each revolution of the barrel cluster. The multiple barrels provide both a very high rate of fire—around 100 rounds per second—and contribute to long weapon life by minimizing barrel erosion and heat generation. Mean time between jams or failures is in excess of 10,000 rounds, making it an extremely reliable weapon. The success of the Vulcan Project and its subsequent progeny, the very-high-speed Gatling gun, has led to guns of the same configuration being referred to as Vulcan Cannon, which can sometimes confuse nomenclature on the subject.

Most aircraft versions of the M61 are hydraulically driven and electrically primed. The gun rotor, barrel assembly and ammunition feed system are rotated by a hydraulic drive motor through a system of flexible drive shafts. The round is fired by an electric priming system where an electrical current from a firing lead passes through the firing pin to the primer as each round is rotated into the firing position. The self-powered version, the GAU-4 (called M130 in Army service), is gas-operated, tapping gun gas from three of the six barrels to operate the mechanism. The self-powered Vulcan weighs about 10 pounds (4.5 kg) more than its electric counterpart, but requires no external power source to operate.

The initial M61 used linked, belted ammunition, but the ejection of spent links created considerable (and ultimately insuperable) problems. The original weapon was soon replaced by the M61A1, with a linkless feed system. Depending on the application, the feed system can be either single-ended (ejecting spent cases and unfired rounds) or double-ended (returning casings back to the magazine). A disadvantage of the M61 is that the bulk of the weapon, its feed system, and ammunition drum makes it difficult to fit it into a densely packed airframe. The feed system must be custom-designed for each application, adding 300–400 lb (140–180 kg) to the complete weapon. Most aircraft installations are double-ended, because the ejection of empty cartridges can cause a foreign-object damage (FOD) hazard for jet engines and because the retention of spent cases assists in maintaining the center of gravity of the aircraft. The first aircraft to carry the M61A1 was the C model of the F-104, starting in 1959.

A lighter version of the Vulcan developed for use on the F-22 Raptor, the M61A2, is mechanically the same as the M61A1, but with thinner barrels to reduce overall weight to 202 pounds (92 kg). The rotor and housing have also been modified to remove any piece of metal not absolutely needed for operation and replaces some metal components with lighter weight materials. The F/A-18E/F also uses this version.[4]

The Vulcan's rate of fire is typically 6,000 rounds per minute, although some versions (such as that of the AMX and the F-106 Delta Dart) are limited to a lower rate, and others have a selectable rate of fire of either 4,000 or 6,000 rounds per minute. The M61A2's lighter barrels allow a somewhat higher rate of fire up to 6,600 rounds per minute.

Ammunition
Until the late 1980s the M61 primarily used the M50 series of ammunition in various types, typically firing a 3.5 oz (100 gram) projectile at a muzzle velocity of about 3,380 feet per second (1,030 m/s). A variety of Armor-Piercing Incendiary (API), High Explosive Incendiary (HEI), and training rounds are available. Around 1988 a new round was introduced, the PGU-28/B,[5] which is now standard for US Navy and USAF aircraft. The PGU-28/B is a "low-drag" round designed to reduce in-flight drag and deceleration, and has a slightly increased muzzle velocity of 3,450 feet per second (1,050 m/s). It is a semi-armor piercing high explosive incendiary (SAPHEI) round, providing substantial improvements in range, accuracy, and power over the preceding M56A3 HEI round. The PGU-28/B has not been without problems, however. A 2000 USAF safety report noted 24 premature detonation mishaps (causing serious damage in many cases) in 12 years with the SAPHEI round, compared to only two such mishaps in the entire recorded history of the M56 round. The report estimated that the current PGU-28/B had a potential failure rate 80 times higher than USAF standards permit.

The main types of combat rounds and their main characteristics are listed in the table below.

Applications
The Vulcan was first used in the F-104 Starfighter. The gun was first used in aerial combat in the skies over North Vietnam when North Vietnamese Air Force MiG-17s dove through escorting F-100 Super Sabres and shot down 2 F-105 Thunderchiefs on 04 April 1965.[9]. One escorting Super Sabre engaged the MiGs with an air to air missile, while two other F-100s engaged with 20mm (M39) cannon.[10] It was installed in the Air Force's A-7D version of the A-7 Corsair II where it replaced the Naval versions' standard dual cannon, and subsequently adopted by the Navy on the A-7E[11] and in future fighters. Significantly it was designed into the F-4E Phantom II; no previous F-4 had a cannon as it had been believed that missiles had made guns obsolete. Combat experience in Vietnam showed that a gun could be more effective than guided missiles in many combat situations, and that a gun pod was less satisfactory than an internal gun.

The Vulcan was later fitted into the weapons bay of some F-106 Delta Dart models and the F-111 Aardvark. It was also adopted as standard in the teen-series air superiority fighters, the F-14 Tomcat, the F-15 Eagle, F-16 Fighting Falcon and F/A-18 Hornet. Other aircraft include the Italian/Brazilian AMX International AMX (on Italian aircraft only), and the F-22 Raptor. It was fitted in a side-firing installation on the AC-119, some marks of the AC-130 gunships, and was used in the tail turrets of both the Convair B-58 Hustler and Boeing B-52H Stratofortress bombers. Japan's Mitsubishi F-1 carried one internally mounted JM61A1 Vulcan with 750 rounds.[11]

Two gun pod versions, the SUU-16/A (also designated M12 by the US Army) and improved SUU-23/A (US Army M25), were developed in the 1960s, often used on gunless versions of the F-4. The SUU-16/A uses the electric M61A1 with a ram-air turbine to power the motor. This proved to cause serious aerodynamic drag at higher speeds, while speeds under 400 miles per hour (640 km/h) did not provide enough air flow for maximum rate of fire. The subsequent SUU-23/A uses the GAU-4/A self-powered Vulcan, with an electric inertia starter to bring it up to speed. Both pods ejected empty casings and unfired rounds rather than retaining them. Both pods contained 1,200 rounds of ammunition, with a loaded weight of 1,615 pounds (733 kg) and 1,720 pounds (780 kg) respectively. During service in the Vietnam War the pods proved to be relatively inaccurate: the pylon mounting was not rigid enough to prevent deflection when firing, and repeated use would misalign the pod on its pylon, making matters worse.

A variant with much shorter barrels, designated the M195 was also developed for use on the M35 Armament Subsystem for use on the AH-1G Cobra helicopter. This variant fed from ammunition boxes fitted to the landing skid and was developed to provide the AH-1 helicopter with a longer-range suppressive fire system before the adoption of the M97 Universal Turret mounting the M197 cannon.

The M61 is also the basis of the US Navy Mk 15 Phalanx CIWS system and the M163 VADS Vulcan Air Defense System (the M168 variant is used).