Science Fair Project Encyclopedia
The 20mm M61 Vulcan is a hydraulically driven, six-barreled, air-cooled, electrically fired gatling gun with an extremely high rate of fire. It has been the principal cannon armament of United States military aircraft for five decades.
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, practical rate of fire 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 6,000 rounds per minute. Although European designers were moving towards heavier 30mm weapons for better hitting power, the U.S. chose 20mm ammunition, trading projectile weight for rate of fire and muzzle velocity. The first GE prototypes of the T-171 were ground-fired in 1949.
The development of the F-104 revealed that the 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.
The Vulcan is a Gatling gun: each of the cannon's six barrels fires only once 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.
Most aircraft versions of the M61 are electric: the barrel assembly is rotated by an electric motor. The self-powered version, the GAU-4 (called M130 in Army service), is gas-operated, tapping gun gas from four of the six barrels to operate the mechanism. The self-powered Vulcan weighs about 10 lb (4.54 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 (140-190 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.
A lighter version of the Vulcan, the M61A2, is mechanically the same as the M61A1, but with thinner barrels to reduce overall mass to 202 lb (91.6 kg).
The Vulcan's rate of fire is typically 6,000 rounds per minute, although some versions (such as that of the AMX International 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 (1,052 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.
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 killing power and accuracy are marginal compared to the heavier 25-30 mm rounds favored by European and Russian air forces. Efforts to develop a higher-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, 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 has yet to find wide application.
Some experts feel 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 second). 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 cannon like the ADEN/DEFA 30 mm weapons to justify the additional weight and complexity.
The Vulcan was used on the F-104, F-105 Thunderchief, some F-106A Delta Dart models, the McDonnell F-4E Phantom II, the General Dynamics F-111, the McDonnell-Douglas/Boeing F-15 Eagle, the Grumman F-14 Tomcat, Boeing/Lockheed/General Dynamics F-16 Fighting Falcon, F/A-18 Hornet and Super Hornet, Italian/Brazilian AMX International AMX, and the F/A-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 marks 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 self-powered Vulcan, with an electric inertia starter to bring it up to speed. Both pods eject 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 (732.6 kg) and 1,720 lb (780.2 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.
The M61 is also the basis of the US Navy Mk 15 Phalanx CIWS system and the M163 VADS Vulcan Air Defense System. 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.
- Type: six-barrel rotary cannon
- Caliber: 20 mm (0.79 in)
- Operation: hydraulically operated, electrically fired
- Length: 73.8 in (1.88 m)
- Weight (excluding feed system): 248 lb (112.5 kg)
- Rate of fire: 6,000 rounds per minute
- Muzzle velocity: 3,450 ft/s (1,052 m/s) (with PGU-28/B round)
- Projectile weight: (HEI) 3.5 oz (100 g)
- M197 Gatling gun
- GSh-6-23 (the closest Soviet/CIS equivalent of the M61, used on a variety of Russian fighters)
- M61 A1 Vulcan - 20mm gatling gun system (F-16.net)
- M61 database item from FAS' website (Federation of American Scientists)
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