Tuesday, March 17, 2015

IL-102 (OES-1) close support aircraft Part I

The work on evolving operational requirements for an attack aircraft went further on for quite some time; not until 1972 was the customer, at last, in a position to present their final version. In accordance with the new specifications issued by the WS it was necessary to develop a subsonic attack aircraft possessing a maximum attainable survivability, high manoeuvrability, and an ability to be committed to action at the shortest possible notice when required by ground troops.

Although the IL-42 project had been rejected, the Ilyushin OKB, at its own initiative, continued working on the concept of a subsonic armoured attack aircraft intended for supporting ground troops on the battlefield. As a result, the IL-42 project was considerably reworked.

An analysis conducted with due regard to the peculiarities of combat aviation activities in the Arab-Israeli conflicts of those years showed that the two-seat IL-40 and IL-42 aircraft would represent the closest possible response to novel elements of the new military doctrine which gave up the use of mass destruction weapons on the battlefield and marked a transition to the use of precision-guided munitions. This was accompanied by a further increase of the pilot's workload, especially at low altitudes; in the opinion of the OKB's specialists, this workload was already in excess of what was humanly possible. This was corroborated also by the assessment of the combat efficiency of foreign single-seat attack aircraft which proved to be some 25% lower than expected. On the other hand, having a second crew member enabled the attack aircraft to spend more time attacking the target. The pilot sought out the target and attacked it on a head-on course, the gunner/ operator continued to bring his gun to bear on the target while the aircraft was leaving the scene; at the same time he protected the aircraft from being hit by an artillery shell or rocket launched in pursuit of the aircraft. There was one more point to be taken into account: the progress of avionics could enable the gunner/operator, despite his aft-facing position, to conduct observation of the battlefield together with the pilot. Using information from his display, the gunner/ operator could aim and launch rocket weapons against ground targets relieving the pilot from this operation and enabling him to concentrate on piloting, especially at extremely low altitudes, and on firing the guns. The participation of the gunner/operator in the attack made it possible to hit not one, but several targets during one pass, which served still further to enhance the attack aircraft's combat efficiency.

The new version of the project for a close support aircraft which was being developed as a 'private venture' under the guidance of Ghenrikh V. Novozhilov (who became General Designer of the OKS upon retirement of S. V. Ilyushin in 1970) received the designation IL-102. In comparison with the IL-42 it featured the following changes:
- cockpit visibility was improved;
- engines with thrust-vectoring nozzles were installed;
- the airframe incorporated a system of autonomous maintenance means which ensured the attack aircraft's high mobility in conditions of front-line deployment at short unprepared airstrips with a bearing strength of 5 kg/cm2 (71 Ib/sq in).

In designing the IL-102, special attention was paid to precluding the possibility of a situation in which firing the onboard weapons would affect engine operation, the aircraft's structure and the view from the pilot's cockpit. The mutual location of the cannon installation, the air intakes and the cockpit, dictated by these considerations, determined the general layout.

The aircraft was configured as a twin-engined low-wing monoplane with thick swept wings and dihedral tail planes mounted on top of the rear fuselage. The powerplant comprised two Klimov (Izotov) RD-331 turbofans (I stands for izmeneniye vektora tyagi vniz - downward thrust vectoring) with a take-off rating of 5,200 kg (11,466Ib) each. The RD-331 was a non-afterburning version of the production RD-33 engine developed by the Klimov OKS and manufactured in series for the MiG-29 fighter. The engines flanked the centre fuselage, breathing through individual air intakes which protruded slightly ahead of the wing leading edge. The nozzles could be deflected downwards both for take-off and in level flight, thus improving the aircraft's manoeuvrability and field performance considerably.

The thick swept wings developed for the IL-102 under the guidance of G. G. Muravyov jointly with TsAGI featured a new aerodynamic configuration. The wings enabled the aircraft to attain a speed of 950-1,000 km/h (559-621 mph) at sea level while keeping the minimum control speed down to a mere 250 km/h (155 mph). Such a wide range of speed characteristics enabled the crew to arrive quickly at the battle area in response to any request from ground forces. It could fly at an altitude of 30-100 m (100-330 ft) under conditions of ground fire, low visibility and uneven terrain, make a stealthy approach to the target and reliably identify it. The wing parameters in combination with the thrust/weight ratio, coupled with the availability of airbrakes, made the aircraft capable of performing a banking turn with a radius of only 400 m (1,310 ft); this enabled the pilot to make a second pass without losing sight of the target.

In comparison with the first prototype IL-40, the IL-102's maximum Mach number was reduced from 0.9 to 0.82, while the operational g-Ioad was reduced from 5.45 to 5.0. At the same time more effective airbrakes were fitted. The first prototype IL-40 needed one minute to decelerate from maximum speed to half of this value, whereas the IL-102 used only 45 seconds to bring the speed down from maximum to minimum.

The moderate wing loading, efficient high-lift devices, high thrust/weight ratio and reworked landing gear featuring twin wheels on each main unit, coupled with the thrust-vectoring engines, enabled the IL-102 to operate from ad hoc dirt strips in the frontline area. The IL-102's design characteristics included a take-off run of a mere 250-300 m (820-980 ft), while the landing run with the use of reverse thrust was expected to be 300-350 m (980-1,150 ft).

As distinct from Sukhoi's T-8 project, the designers of the IL-102 decided to relinquish the concept of an armour shell protecting all the vital components of the aircraft. Instead, a layout was evolved in which the vital structural elements and units were located in such a way as to provide mutual shielding. Armour plating was used only for the separate crew cockpits fitted with zero-zero ejection seats developed by NPP Zvezda (a standard K-36L for the pilot and a K-36L-102 for the gunner). Also, the engines and the fuel system were provided with partial armour plating. The fuel tanks had no armour protection - from the front and from the rear they were protected by armoured crew cockpits, while from below and from the sides they were shielded by the cannon installation and the engines. In addition, to prevent a fire and explosion, should a tank be pierced by a bullet, the empty space in the tank was filled with explosion-suppression polyurethane foam (as was the case with the Sukhoi T-8, too).

The IL-102 possessed a very potent complement of offensive armament. There were no fewer than 17 hardpoints for the carriage of external stores and chaff/flare dispensers; this enabled the aircraft to use a wide range of weapons. The aircraft's special feature was the versatile armament bays in the fuselage and the wings. The fuselage bay housed the easily detachable NU-102-1 cannon mount which could be deflected to an angle of 150 downwards; it carried the GSh-30 twin 30-mm cannon which had a 1,500-rpm rate of fire.

Eventually the NU-102-1 could also be equipped with a single-barrelled 30-mm (1.18 calibre) or 45-mm (1.77 calibre) aircraft cannon firing shaped-charge rounds intended for destroying heavy tanks. Gun pods housing 12.7-mm, 23-mm and 30-mm weapons could be carried externally under the wings. Bombs weighing up to 250 kg (551 lb) apiece could be carried in six wing bomb bays. The use of internal stowage of the bombs considerably enhanced the aircraft's performance and reduced its radar signature. Six underwing and two under fuselage versatile weapons racks could be used for the carriage of air-to-air and air-to- surface missiles, unguided rocket pods, bombs, gun pods and other weapons.

Additionally, two attachment points for the carriage of defensive armament were placed at the downturned wingtips. Mounted here were chaff/flare dispensers and radar warning receiver antennas. All this equipment provided passive protection from ground-to-air missiles with IR and radar homing warheads. Active protection from the rear hemisphere was provided by the tail barbette with a twin-barrel 23-mm cannon remote-controlled by the gunner/operator.

The availability of versatile internal weapon bays incorporated into the fuselage and the wings, and of the external racks, made it possible to modernise the armament complement without introducing changes into the airframe. The weight of the IL-102's rocket and bomb armament totalled 7,200 kg (15,880 Ib).

The aircraft was fitted with special equipment enabling it to operate efficiently under all weather conditions in the daytime and at night, making use of the full complement of its armament.

One more special feature of the IL-102 was its ability to operate autonomously and at the shortest possible notice when called into action by the ground forces. Engine starting was catered for by jet fuel starters mounted on each of them; there was a drive from the jet fuel starter to the on board generator and the hydraulic pump which made it possible to check the units and systems without making use of ground sources of electric power supply. Inbuilt onboard mechanisms for lifting and taking down the weapons and the possibility to lower the fuselage cannon mount for maintenance made it possible to prepare the aircraft for the next sortie very quickly. The IL-102 and its systems were notable for their utmost simplicity in manufacture and maintenance, earning for the aircraft the nickname 'a soldier aircraft'.

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