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M61 Vulcan

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The 20 mm M61 Vulcan is a hydraulically or pneumatically driven, six-barreled, air-cooled, electrically fired Gatling-style cannon with an extremely high rate of fire. It has been the principal cannon armament of United States military aircraft for five decades.

1 Development
2 Description
3 Disadvantages
4 Applications
5 Specifications (M61A1)
6 Notes
7 See also
8 External links

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 MG213C) 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 superior reliability 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. The Army issued GE the contract in 1946 for "Project Vulcan," a six-barrel weapon capable of firing 6,600 rounds per minute. Although European designers were moving towards heavier 30 mm weapons for better hitting power, the U.S. initially concentrated on a powerful .60 caliber cartridge designed for a pre-war anti-tank rifle. The thought was 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. As result, 20 mm and 27 mm alternatives based on the .60 caliber cartridge's case were also tested. 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 desired balance of projectile/explosive weight with muzzle velocity. The development of the F-104 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 feed system was developed for the upgraded M61A1, which subsequently became the standard cannon armament of U.S. fighters. It is likely to remain in service for at least another decade. General Electric later sold its aerospace division to Martin Marietta. This deal included GE Armament Systems along with the design and production tooling for the M61 and GE's other rotary cannon. 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 lb (4.5 kg) more than its electric counterpart, but requires no external power source to operate.

M61 ammo belt

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 (140-190 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 D 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 mass to 202 lb (91.6 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.^[2] 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. Until the late 1980s the M61 primarily used the M50 series of ammunition in various types, typically firing a 100 gram (3.5 oz) projectile at a muzzle velocity of about 3,380 ft/s (1,035 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, which is now standard for US Navy and USAF aircraft. The PGU-28 is a "low-drag" round designed to increase muzzle velocity, which rises to 3,450 ft/s (1,050 m/s). It is a SAPHEI (semi armor-piercing high-explosive incendiary) round, providing substantial improvements in range, accuracy, and power over the preceding M-56A3 HEI round. The PGU-28 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, 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. [2]

Disadvantages

Despite its reliability and tremendous rate of fire, the Vulcan has been increasingly criticized in recent years for its limited performance. The ballistic characteristics of the 20 mm round are relatively poor, with the projectile losing energy quickly, and its destructive power and accuracy are lacking compared to the heavier 25-30 mm rounds favored by European and Russian air forces. Efforts to develop a larger-caliber replacement for the M61 have thus far had limited success. The USAF spent a great deal of money in 1970s on the 25 mm GAU-7 cannon for the F-15 Eagle, using caseless ammunition, but it proved to be a failure and was abandoned in favor of the Vulcan. The five-barrel GAU-12 Equalizer 25 mm gun used in the AV-8B Harrier II is a Vulcan derivative, but despite greater hitting power (since it fires a heavier round at virtually the same muzzle velocity), it had yet to find wide application. A four-barrel variant of the GAU-12, the GAU-22/A, is being developed for the F-35 Lightning-II.^[3] The GAU-8 Avenger outfitted for the A-10 Thunderbolt II (or the Warthog) has largely been a success in the anti-tank role and others. It fires a 30mm depleted uranium round and has seven barrels vs the six on the Vulcan. Another criticism is that despite its high rate of fire, the Gatling-type weapon is hampered by the time it takes for the weapon to spin up to its maximum rotation speed (about 0.5 seconds). As a result, a one-second burst only fires about 70-75 rounds, which some experts feel is not enough of an advantage over revolver cannons like the ADEN/DEFA 30 mm weapons to justify the additional weight and complexity. To overcome this shortfall, the M61A2, with its lower inertia can be powered by a hydraulic motor running on 5,000 P.S.I. (34 MPa) of hydraulic pressure instead of the 3,000 lbf/in² (21 MPa) previously used on the F-15 Eagle and F-16 Fighting Falcon.

Applications

The Vulcan was first used in the F-104 Starfighter. The gun was also used in the F-105 Thunderchief in combat against Soviet-designed MiG fighters over Vietnam. It was installed in 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 and in future fighters. Most significantly, it was designed into of the F-4E Phantom II, as all previous versions the F-4 lacked a cannon as it was 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-15 Eagle, the F-14 Tomcat, F-16 Fighting Falcon and F/A-18 Hornet. Other aircraft include the Italian/Brazilian AMX International AMX, and the F-22 Raptor. It was fitted in a side-firing installation on the AC-119 and some marks of the AC-130 gunships, and was used in the tail turrets of the Convair B-58 Hustler and Boeing B-52H Stratofortress bombers. Two gun pod versions, the SUU-16/A (Army M12) and improved SUU-23/A (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 mph (644 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 lb (733 kg) and 1,720 lb (780 kg) respectively. Unfortunately, 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.

Main article: U.S. Aircraft Gun Pods

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 ranging 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). Both are considered inadequate for current missile and aircraft threats, and are being replaced by surface-to-air missile (SAM) systems like the FIM-92 Stinger and RIM-116 Rolling Airframe Missile.

Specifications (M61A1)

Type: six-barrel rotary cannon
Caliber: 20 mm (0.787 in)
Operation: hydraulically operated, electrically fired
Length: 73.8 in (1.88 m)
Weight (excluding feed system): 248 lb (112 kg)
Rate of fire: 6,000 rounds per minute (lab terms)
Muzzle velocity: 3,450 ft/s (1,050 m/s) (with PGU-28/B round)
Projectile weight: (HEI) 3.5 oz (100 g)

Notes

^ http://www.fas.org/man/dod-101/sys/ac/equip/m61.htm
^ [1] "The F/A-18E/F new lightweight gun system is the General Dynamics M61A2, which has a switchable firing rate of 4,000 or 6,000 shots per minute and a fully integrated linkless ammunition feed system"
^ Maher, Dave. National Defense Industry Association. 30 March 2006 25mm Gun Systems for the F-35 Joint Strike Fighter. Access Date: 13 August 2007