Sunday, March 15, 2015

Sukhoi Su 25

The Su 25 is successor to the famous Il 2 Shturmovik of World War II. Fast and heavily armed, it is reputedly the most difficult plane in the world to shoot down.

The air war in Vietnam highlighted the need for simple close-support aircraft able to operate from unpaved strips close to the front. Such warplanes would also have to deliver heavy ordnance against targets with great accuracy and be able to survive intense ground fire. The United States parlayed its experience into the Fairchild A-10 Thunderbolt II, a heavily armored twin-engine bomber. The Soviets also watched these developments closely before deciding that they, too, needed similar aircraft and capabilities. During World War II Russia had deployed the redoubtable Il 2 Shturmovik aircraft for identical reasons, so in 1968 the Sukhoi design bureau became tasked with developing an equivalent machine for the jet age. The bureau settled upon a design reminiscent of the Northrop YA-9, which had lost out to the A-10 in competition. The new Su 25 was an all-metal, shoulder-wing monoplane constructed around a heavily armored titanium “tub” that housed both pilot and avionics. Engines were placed in long, reinforced nacelles on either side of the fuselage, and the fuel tanks were filled with reticulated foam for protection against explosions. To assist slow-speed maneuvering, the wingtip pods split open at the ends to form air brakes. Its profile is rather pointed, but a blunt noseplate covers a laser range finder/target designator. The Su 25 is somewhat faster than the A- 10, trusting more in speed to ensure survival than a dependency on agility and heavy armor. It is nonetheless an effective tank destroyer. 

Technical description
The Su-25K's service life was given as 1,500 flying hours before a major overhaul, and the service interval as 700 hours. They obviously did not expect high utilisation, since the 700-hour interval was also given as a seven-to eight-year gap. The first production Su-25 hardly differed from the later prototypes, and a technical description of one would apply just as well to the other. In fact, all Su-25s up until the Su-25T/TM were structurally similar, with much the same systems. Only a handful of changes were made as a result of later combat experience in Afghanistan, and they were limited in scope, despite their impact and significance.

The Su-25 was of conventional configuration and construction, apart from the extensive use of armour plate. The aircraft was an all-metal monoplane with a high-set, high aspect-ratio wing which was modestly tapered and slightly swept on the leading edge, but not on the trailing edge. The wing incorporated 2°30' of anhedral. Engines were mounted to the fuselage sides in semi-conformal nacelles. Sixty per cent of the aircraft's structure was of conventional Duralumin construction, with 13.5 per cent titanium alloys, 19 per cent steel, 2 per cent magnesium alloys and 5.5 per cent fibre-glass and other materials. Virtually no use was made of carbon-fibre composites or advanced aluminium lithium alloys.

Electrical power was supplied by a single 28.5-volt DC circuit, and by three 36-volt/400-Hz and one 115-volt/ 400-Hz AC circuits. The DC circuit consisted of a transformer, voltage regulator, and circuit breakers. Power was generated by a pair of engine-driven GSR-ST-12/400 generators, with two 25 Aph NiCad batteries available as an emergency power source.

A series of preproduction aircraft was subsequently deployed to Afghanistan, where the planes performed useful service against guerilla forces. Western intelligence had previously identified a Soviet parallel to the Fairchild Republic A-10 Thunderbolt and dubbed it `Ram-J' until it became better known as the Sukhoi Su-25 `Frogfoot'. A trial deployment is said to have been made as far back as 1980, but the type was most certainly used in the spring offensive. Working in conjunction with the Mi-24, this jet attack aircraft experimented with methods of co-ordinating the two disparate types, and the results will doubtless be noted for potential use in the Western theatre. They flew some 60,000 sorties, losing 23 machines in the process, but the decision was made to enter production in 1980. Since then 330 Su 25s have been built; they have received the NATO designation FROGFOOT.

The effect of the Stinger
The advantage of having two engines was fully exploited in the Su-25, in which the powerplants are mounted so close together that damage to one engine could cause collateral damage to the other. This became abundantly clear following the 1984 introduction of the Redeye SAM by the Mujahideen, and by the October 1986 delivery of General Dynamics FIM-82A Stinger SAMs. The introduction of Redeye was followed by the loss of two Su-25s in very quick succession, these aircraft having proved unable to decoy the SAMs away using flares. Flare capacity was increased from 128 to 256, by the addition of four 32-round dispensers scabbed onto the top of the engine nacelles. When the Mujahideen started using Stinger, the effect was even more dramatic. Four Su-25s were destroyed in three days, with two pilots lost. The Stingers tended to detonate close to the engine exhaust nozzles, piercing the rear fuel tanks with shrapnel and causing fires which could burn through control runs, or causing damage to the far engine. In order to prevent damage to one engine from taking out the other, a 5-mm armour plate was added between the two engines (acting as a giant shield and firewall), about 1.5 m (5 ft) long.

A new inert gas (Freon) SSP-2I/UBSh-4-2 fire extinguisher system was provided. This consisted of six UTBG sensors in the engine nacelles, which were connected to cockpit displays. The pilot had four push-buttons to actuate the extinguisher's first and second stages for each section of the engine. The Freon was stored in spherical 4-litre (0.87-Imp gal) bottles, each containing 5.64 kg (12 lb) of gas pressurised at 6.9 to 14.2 MPa.

These modifications proved a great success, dramatically reducing the Su-25's loss rate. No Su-25 equipped with the inter-engine armour was lost to a Stinger, although many were hit. The modifications were quickly incorporated on the production line, and were retrofitted to existing Su-25s.

Additional improvements were added during the period in which Su-25s were fighting in Afghanistan. On aircraft from the 10th production series, for example, the aileron control rod was fully faired in and the aileron trim tab was deleted. Elevator pivots were more effectively faired. Tenth series Su-25s also gained a second external APU/GPU socket. Other features appeared gradually, and cannot yet be pinpointed to a particular production series. The nosewheel was changed, from one which accepted a tubeless KN-21-1 tyre to one which took a tubed K-2106 tyre. The single long fuel tank access panel on the top surface of each wing was replaced by three shorter access panels, side by side. Small fins were added to the inboard faces of the bottom of each wingtip fairing, acting as glare shields when the PRF-4M pop-down landing lights were deployed. At the trailing edge of these pods, the airbrakes themselves were modified. Previously simply splitting 50° up and 50° down to give a > shape with the point forwards, they gained auxiliary segments which hinged upwards through another 90° at their trailing edges to give a shape reminiscent of a W turned on its side, with the central point pointing forwards. During production of the ninth production series the cannon muzzle was redesigned, with the ends of the twin barrels covered by a single muzzle shield. Many late production Su-25s had their distinctive SRZ and SRO 'Odd Rod' antennas replaced by simple blade antennas, similar to the SRO antennas fitted to later MiG-29s (which retained the traditional tripole SRZ antennas above their noses). The revised antennas may have combined interrogator and responder functions.

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