The Soviet GSh-6-30 aircraft cannon

 

Denis Evstafyev, edited by Anthony G Williams

 

This intriguing article, about the somewhat fraught development and use of the powerful, six-barrel 30 mm GSh-6-30 cannon, was sourced from a book by V. Markovsky and I. Prikhodchenko about the MiG-27 fighter-bomber.

 

During the development of the MiG-27 fighter-bomber, the onboard GSh-23L gun of the earlier MiG-23BN was replaced with a more powerful weapon. The power and striking force of the long-established 23 mm shells had become insufficient to ensure the destruction of ground targets and especially armoured vehicles, since NATO countries were introducing new and better-protected vehicles. The problem became aggravated by the disturbing tendency of Soviet aviation gun systems to lag behind Western equivalents, which surpassed them both in firing rate and in the power of the cartridge.

The Soviet military became interested in the ability of aircraft guns to destroy not only the new APCs and light tanks of their potential opponents, but also possibly the American main battle tank M1 Abrams. For this purpose, guns of larger calibre firing more powerful ammunition were required. To address that need the development of a 45 mm smoothbore aircraft gun was ordered, which would have used "active-reactive" rounds (involving a HEAT element in the nose) with increased armour penetration. However the creation of the new gun and ammunition for it demanded time so an interim solution was accepted: the installation of a new multi-barrelled 30 mm gun, ensuring a high rate of fire and a considerable weight of fire per second. The 45mm TKB-700 was eventually successfully tested, but abandoned as a result of the collapse of the USSR.

As the initiator of the decision to adopt a 30 mm gun calibre for all three services the Deputy Minister of Defence on Arms, General V.Ya. Shabanov, ordered the standardisation of weapons and ammunition for air force, navy and land forces [Ed: this may have been the idea, but in practice the three services developed different - and mostly incompatible - ranges of ammunition based on the same cartridge case]. The change from 23 mm to 30 mm calibre doubled the shell weight (from 175-185 gram to 400 gram), and increased the explosive charge by almost three times. Improved ballistics provided not only good armour penetration but also greater effectiveness against other targets. Accuracy was considerably improved and ammunition of new and more effective types was developed.

A new cannon of multi-barrelled rotary design increased the firing rate by 3-4 times, important given the short time for an attack. As with the US M61 'Gatling', the barrel group rotated continuously with each barrel firing as it came into position. The prototype for this was the 30 mm AO-18 six-barrel gun designed by V.P.Gryazev and A.G.Shipunov. Its history goes back to June 15th 1963 when the Council of Ministers of the USSR approved the development of a rapid-fire gun with a rotating group of barrels for the AK-630 naval mounting. The gun was designed in conjunction with a new 30 mm round with improved ballistics through an increased propellant charge and new shells. Both electric and hydraulic motors were considered to power the rotation of the barrel group and gun mechanism, but they needed about 40-50 h.p.. The designers therefore selected a self-powered scheme using gas tapped from the barrels. This permitted a compact gun system, of particular value for aircraft use.

However the naval gun was heavy, bulky and excessively powerful for installation in a fighter-bomber, which would suffer problems in absorbing the recoil. Before installation on the MiG-23BM the gun underwent significant adaptation. The barrels were shortened and the water-cooling deleted. The result was that the size of the gun was reduced. The dimensions of the naval AO-18, which were 2,176 x 295 x 336 mm (length - width - height) were reduced to 1,877.5 x 252 x 285 mm in the aircraft version. The new cannon, designated GSh-6-30A, had a weight of 145 kg (AO-18 = 205 kg) and rate of fire of 5,500-6,100 rounds per minute. The muzzle velocity was 850 m/s. The ammunition consisted of 300 rounds of high-explosive-incendiary (OFZ), high-explosive-incendiary tracers (OFZT), incendiary (FZ) or armour-piercing-explosive (BZ) shells in weights of up to 400 g. The effective range for aimed fire against ground targets was 1,200-1,600 m, and against aircraft was 200-600 m. The gun life was 6,000 rounds, assuming that it was allowed to cool between bursts of 100-200 rounds. The charging and spin-up of the barrel group before firing was achieved with the use of a pneumatic system which included, among other things, a pair of compressed air storage tanks and a "pneumostarter".

The gun was installed in the same place on the MiG-23BN as the GSh-23L had used under the compartment of the №1A tank. Gun installation was in an under-fuselage niche which was not enclosed by a fairing. That provided easy servicing and good cooling while shooting. A removable ammunition box was not used; in its place there was a special ammunition compartment taking up practically all of the space between frames №13B and №14. It was therefore necessary to make changes in the construction of the fuselage. The compartments aft of the cockpit, from frame №12 up to №14, were structurally new. Between frames №12 and №13 an additional section 200 mm long was installed, and frame №1ЗB, previously (for the MiG-23BN) connected with frame №14, was shifted forward for the length of the new compartment. The ammunition belt of 300 rounds, weighing under 300 kg, was loaded through the top hatches. Spent cases were ejected through a chute in the ammunition compartment.

The GSh-6-30A gun system had impressive characteristics, clearly superior to the majority of western systems. American warplanes used 20 mm guns with 100 gram projectiles, and the ADEN and DEFA 552/553 30 mm guns used in other NATO aircraft fired 270 g rounds at a muzzle velocity of 600-650 m/s, which led the famous designer of air arms A.E. Nudelman to characterize them as "guns with reduced characteristics") [Ed: this applies only to the early, low-velocity versions of the ADEN and DEFA cannon. By the end of the 1950s these had been replaced by higher-velocity guns with muzzle velocities of 760-820 m/s]. Only later did more high-powered guns appear in the air forces of the western countries: the 27 mm Mauser BK 27 designed for the Tornado and the American 30-mm GAU-8A specially developed for the ground-attack A-10. [Ed: another western aircraft gun was the Oerlikon KCA fitted to fighter SAAB Viggens, which fired 30 x 173 ammunition appreciably more powerful than the Russian 30 x 165.]

The main design problems of the "Shestistvolka" (six barrel gun) were solved during the development of the shipborne version. However mounting it on an aircraft demanded some adaptations. The guns of the first series could not fire the full ammunition load in one continuous burst, as required by the technical specification; after the first 150 shots, the gun barrels overheated. Firing could be recommenced only after the barrel group had cooled down. A number of other serious defects occurred which affected the reliability of system as a whole, concerning the mechanical operation, feeding system and durability of some parts.

During series production at the Tula machine-building factory it proved possible to eliminate in due course most of the design defects to achieve acceptable reliability. The modified gun allowed firing bursts of up to 300 rounds. However, another serious problem appeared with the aircraft gun mounting: despite the reduced ballistics [Ed: from the use of shorter barrels] the recoil force of the GSh-6-30A was about 5,500 kg. The impact loads caused by firing were very high for the aircraft to absorb, particularly as its structure was a development of a light fighter. For ground firing tests the gun was initially mounted on a wooden testbed, but at the first trial firing of the "Shestistvolka" the testbed simply fell apart.

Further problems occurred during the first air firing tests. It was discovered that the impact and frequency characteristics generating by firing the GSh-6-30A on the ground did not correspond to those which took place in the air. The first 25-round burst made in flight was ended by the failure of all of the avionics in the cockpit. In further test flights there were cases of deformation and even tearing away of the nose undercarriage door, and because of the strong vibrations the ammunition feed fell apart. Electronic equipment in an aft-of-cockpit compartment also failed.

To reduce the influence of gun firing on the fuselage, the axis of a gun was inclined downwards 113 '. [Ed: this would probably have been to prevent 'pitch-down' on firing.] The operational development of the gun system was carried out by Tula TsceKB and the "Zenith" arms development group, with the aid of experts from scientific research institute of aircraft systems (TsceNIIAS), who were in charge of test firing at the range situated near the Moscow suburb of Faustovo. In 1975 the gun system was accepted into Soviet Air Force service.

The capabilities of the gun and the force of its fire left few people unmoved. Even during the ground tests of "Shestistvolka", all of the workers wanted to sit down and put their hands over their ears. Firing did not even sound similar to the usual gun bursts: the deafening, rolling impact, lasting for a couple of seconds as the gun threw 100 kg of shells at the target, was felt rather than heard.

In combination with its sighting system the GSh-6-30А was very accurate. In Soviet Air Force regiments gun firing was often exercised during training flights. Usually the ammunition belts alternated two OFZ rounds with one BZ, but on examination flights each fourth or fifth round was a tracer. Firing was possible in a manual mode, with "approximate" targeting using the fixed grid of a sight. In automatic aiming mode the PrNK-23 navigation system was used. It gave the necessary corrections and lead on an S-17VG sight together with information about the target and effective firing range. When detonating on impact with the ground, the shell fragments scattered up to 200 metres in the air and represented a serious danger to the aircraft. In short 40-round bursts, lasting a fraction of a second, the gun sent to the target about 16 kg of "effective load". In training flights, the gun was loaded with only 30-40 cartridges, sufficient for improving shooting skills.

Parts of the gun, especially the gas drive system and mechanicals, were exposed to intensive thermal and mechanical loadings, working practically on the limit. Corrosion was especially severe and developed rapidly, requiring the gun be cleaned immediately after shooting, after any flight and even each 15-20 days when parked. The constant recoil impacts, despite the brief firing bursts, caused structural damage and punished the plane's equipment. Acoustic loadings from muzzle gases and the associated high-frequency vibrations literally loosened the fuselage, adding fatigue cracks to the tank in compartment №2 and to a back wall of tank №1A to which the gun mount was fastened. There was the risk of the doors of the forward landing gear being jammed. The fuel pump failed because of breakages in the power supply circuit.

As one of the measures to tackle the problems, the firing rate was limited to 4,000 rpm. Despite various adaptations and structural reinforcements, the risks involved in firing the gun had to be taken into account during operations. The general view of the aviation regiments was that bursts of 30-40 rounds had no serious consequences, but a long burst of 2.5-3 seconds was accompanied by "crunches and crashes". Apart from other advantages, restricting shooting to short bursts extended the effective life of the gun.

For ground crews the gun required repair works of varying complexity, but for the pilots there could be serious problems. In August 1980 a Major Shvyrev, of one of squadrons of the 722nd bomber regiment, was returning from a gun-firing flight when he discovered that the doors of the forward landing gear were deformed and it was not possible to lower the gear. The pilot had to land his aircraft on the two main wheels, for the first time during the service of the MiG-27 in the VVS. Despite this, the aircraft suffered no serious damage and soon flew again. Another case occurred on August 8th 1988, when a MiG-27D of Lieutenant M.V.Poluektov, of the 19th bombing regiment based in East Germany, returned from firing practice with the GSh-6-30. He also found that the forward gear was not down because of jammed doors. As was observed later in the report, "the pilot had high moral-political preparation" and was able to land the aircraft on the main wheels with minimal damage. A similar case with more serious consequences occurred in the 599th bombing regiment on May, 15, 1990. During firing a muzzle attachment broke, and this became jammed against the nose-gear doors, so the nose wheel could not release. The MiG-27K ploughed the runway with its nose, and had to be written off [Ed: the pilot obviously lacked something in "moral-political preparation"]

The avionics systems often became turned off because the electrical commutating switches became disconnected. On April 18th 1988 a MiG-27 from the 24th Air Division arrived at an air base without its communications or navigation systems working: during firing, the navigation, radio and gyroscopes had switched off. In East Germany on 2nd September 1989 a MiG-27 suffered full radio communication failure; contacts in the radio had broken and printed circuit boards had cracked. In aircraft of the 23rd Air Army in January 1989, gun shooting resulted in two incidents. In the 58th bomber regiment a broken electrical safety fuse caused almost complete failure of the stabilizer, flaps, landing gear and navigation system control. One week earlier in the 266th bomber regiment, a MiG-27K returned from a flight without the cockpit canopy:  emergency locks had released the canopy while shooting.

Even given this background, the case which occurred in the 24th Division on 29th March 1989 appears to have been unique. On recovery from a dive after gun shooting, the instrument panel fell onto the pilot's legs: the panel fastenings had been sheared by the recoil force. The pilot reached an airbase holding the panel, which was hanging via electric cables, by one hand.

It frequently happened that the recoil force of firing broke the reflector sight. Landing headlights broke so frequently, that before flights involving gun firing, they were removed and replaced with caps. The introduction of protective deflectors did not completely rescue a situation which demanded changes to the "MiG-27 Pilots Instruction": after night shooting, landing took place only on runways illuminated by floodlights.

A continuous burst of fire was dangerous because of the risk of barrel overheating. That could lead to the explosion of primers and cartridges, and even of shells. Each such case was considered as extraordinary and was examined by the OKB and Tula TcseKB. In the 16th Air Army such an incident on 22nd January 1990 resulted in the loss of a MiG-27K. The detonation of a shell in the barrel of the GSh-6-30A broke the gun apart. Fragments of the gun damaged a fuel tank, electric cables and a hydraulic system. The explosion of an oxygen tank "instantly inflated" a fire. The pilot lost control and ejected from the burning airplane. A similar case took place three months later on a firing range in Lunitcsa with a MiG-27K from the 39th Division. Some shells exploded near the nose, but the aircraft managed to return to base despite the shell-holes, broken hatches and glass housing of the sighting system, and dents on the compressor blades.

This type of "self-inflicted wound" happened even on the ground, usually the result of an oversight by the ground crew. As a protective measure a "ground/air" electric circuit, activated by weight on the landing gear, prevented the gun from firing while the plane was on the ground. Sometimes during flight preparations this interlock was disconnected or overlooked, so that the gun was armed as soon as the nosewheel left the ground. When parallel tests of different systems to check weapon control took place, this could result in the gun firing. In 1983 at the 236th bomber regiment in Chertok, such an accident demolished the forward landing gear, and a similar case took place in the 88th bomber regiment in Suurukul. In Lipetsk, on September 2nd 1986, only one cartridge remained in a MiG-27D's gun as it was returning from a flight. After landing, the gun fired and hit the nose gear, igniting the flammable AMG-10 hydraulic fluid which was contained in nose gear under high pressure.

With the retirement of the MiG-27 from Russian Air Force service, the GSh-6-30 guns were also withdrawn. [Ed: doubtless to sighs of relief from the pilots!]

 

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