X-20 cruise missile (K-20 complex)

March 11, 1954 was issued the Decree of the Council of Ministers of the USSR on the creation of an aviation system of missile weapons K-20, which Tupolev Design Bureau-156, appointed by the leading executor, was ordered to develop a carrier aircraft Tu-95K on the basis of Tu-95MA bomber (carrier of nuclear warheads). The same document appointed organizations that develop components of the complex. In their capacity were enterprises that already had experience working together on anti-ship missile "Comet". The K-20 guidance system was entrusted with the development of Design Bureau-1 of the Ministry of Medium Machine Building under the leadership of V.M.Shabanov, the X-20 projectile aircraft - Design Bureau-155 A.I.Mikoyan (Chief Designer M.I.Gurevich).

Requirements for the system stipulated the possibility of hitting large strategic targets at a range of about 600 km with the flight of the carrier at altitudes up to 12-13 km. The X-20 speed had to be supersonic, not less than 1700-2000 km/hour, which was ahead of the existing fighters. The size of the missile was determined by the use of the agreed BC - nuclear charge, whose mass with the necessary systems was about 4000 kg.

Acting with reasonable conservatism and based on the experience of "Kometa", the engineers of OKB-155 chose a reliable TRD. The use of the turbojet propulsion system simplified the issues of power supply to the rocket - the TRD drive box provided operation of the electric generator and hydraulic system pumps. Taking into account the requirements on the speed and mass dimensional characteristics of the rocket, the thrust of about 10-11 tons was required. Suitable and the only engine in terms of thrust and dimensions was AL-7F - the newest TRD, which has just passed the test on the fighter I-7U. However, this power unit weighed 1700 kg, and flying on supersonic required the use of afterburning mode with the corresponding fuel consumption, the stock of which had to be about 4000 kg to achieve the calculated range.

In the development of the X-20 used the experience of Mikoyan design and construction of supersonic fighters (as in the design of "Comet" reflected many solutions of the MiG-15). The airplane shell was built according to an aircraft scheme with a frontal air intake, constructively borrowing many features of the same I-7U.

Only the general layout, wing geometry with a 55° sweep and supersonic profile were borrowed from the fighter. At the same time, features of the X-20 as a single-mode aircraft, not intended for maneuvering and low speeds at takeoff and landing, allowed to reduce the area of the wing and its chord, giving up flaps and brake plates. In view of the lack of takeoff and landing modes, the specific load on the wing of the projectile was brought to 470 kg/m2 (one and a half times greater than that of MiG-19), but on demand for range this parameter was chosen significantly less than that of the "near" and high-speed K-10, where it was 640 kg/m2. To avoid reversal of ailerons encountered on supersonic aircraft, especially with low rigidity of the lightweight structure of the wing of the missile, ailerons were placed closer to the root of the wing (good this did not interfere with the missing flap), halved their deflection angles with a simultaneous increase in their area to maintain sufficient control on the roll.

Initially, in the size of the tail fins were observed normal aircraft proportions, but due to moderate requirements for maneuverability (in the main modes of flight the task was to maintain the stability of the rocket) in the final version of the plumage significantly reduced, and the relative area of horizontal plumage was only 16% of the wing area, and vertical - 11.2% (against the conventional aircraft 20-25%). The keel thus shifted unusually close to the center of gravity of the rocket - already for layout reasons, its suspension under the carrier, where its usual location prevented the fuselage tanks. For stabilization and control in transient modes were sufficient small angles of deflection of the rudder altitude and one-piece stabilizer (the need for its use was revealed during tests of the MiG-19).

The scheme with the frontal air intake did not suit the layout of the missile in the best way, complicating the construction with long air channels that "eaten" internal volumes and caused aerodynamic losses. However, it provided the smallest middles, in addition, for supersonic speeds, such an air intake was then sufficiently worked out and did not promise problems. In general, the scheme and design of the X-20 became a practical compromise required and available to the designers.

To the full extent it concerned the guidance system as well - the classical radio command control was not suitable due to the given over-the-horizon range - the signal at such a distance was weakened, the method was unreliable due to failure of guidance in case of failure of the command line and was vulnerable to interference. In addition, it was difficult to control the missile's flight to the target. Radar guidance using the missile's CNS, active or illuminated by the target carrier, at such ranges was also ineffective, mainly due to insufficient radar visibility of targets - many strategic objects, however large, did not differ in radio contrast in the background, not to mention that the given range was outside the radio horizon.

The main combat tasks of long-range aviation were the defeat of the enemy's nuclear missile groups, its military-industrial and energy facilities, violation of state and military administration, defeat of operational and strategic reserves and transportation. The respective targets - military bases, airfields, warehouses and control points - were not only low-contrast on the ground (except for transport hubs and seaports), but they were usually camouflaged, which made radar, thermal and other similar systems of guidance of means of destruction useless. However, such objects were large and georeferenced, and their location was a point of reference for a combat mission, allowing them to be described as "targets with predetermined coordinates" (which, in turn, was an intelligence mission).

Insufficient accuracy and reliability of the then domestic systems impeded the implementation of fully autonomous targeting of such targets with the help of the missile's onboard system. The solution was the use of a combined guidance system of the aircraft-contractor using onboard programmable autopilot and radiometric equipment of the carrier.

The conceptual design of the Tu-95K was prepared within six months and signed by A.N.Tupolev on October 26, 1954. In autumn 1955, after approval, the military adopted a model of the missile carrier. The aircraft has received significant changes from the original bomber: in the bow of the two-antenna radar with separate fairings of the equipment, the navigator's workplace moved to the cockpit, gruzootseke to accommodate the fifteen-meter rocket extended almost twice, equipped with a beam holder BD-206, which in a semi-sink position hung X-20. To reduce resistance during flight along the route, the holder with the missile was in a retracted position, while the air intake of the X-20 was covered with a semicircular fairing. Before the launch, the cowl was rotated to clean the fuselage, the holder was lowered by 950 mm, the rocket engine was started and the detachment was performed. In flight without the rocket, the cargo area was closed with casements.

The increase of the cargo compartment entailed reconfiguration of the fuel system, the capacity of which was slightly reduced; to start the rocket engine, its heating and putting into operation in the fuselage installed an additional tank for 500 kg of kerosene, connected to the fuel system of the X-20 mainline with cut-off valves, which were blocked before starting. Additional more powerful converters in the AC electric system were introduced to power the equipment.

The deployment of the heavy missile caused significant changes in the fuselage, which affected a number of power elements.  The construction of experimental missile carriers was carried out by Kuibyshev Aviation Plant № 18, which used two standing in the slipway Tu-95 № 401 and 404. Redesign began on March 1, 1955, just a year after the issue of the task. It ended on October 31. The leader plane went up in the air on January 1, 1956, in summer it was joined by a second car. They were carried out factory tests of the complex, which took place in three stages with the development of the carrier itself, preparing for the launch of its systems and adjusting the control equipment of the complex and homing of the rocket. Factory test Tu-95K completed January 24, 1957.

For the flight development of drone systems used two production of MiG-19 fighters, converted to the OKB-155 aircraft analogues of SM-20/I and CM-20/II. They were equipped with carrier suspension units and a set of airborne radio control equipment placed instead of one of the tanks and in the fairing under the fuselage. Analogues, close to the mass and flight characteristics of the X-20, in the first flights took off from the airfield, and then lifted into the air on the Tu-95K and after resetting performed the flight using the control system K-20, practicing the characteristic modes. In these flights, they were piloted by test pilots Amet-Khan Sultan and VG Pavlov. During 1957, the analogous aircraft made 27 flights with independent take-off and 32 - with dropping from the carrier. They also investigated the interference immunity of the control system, for which they performed 9 flights at the Vladimirovka test site in the summer of 1958 with an imitation of radio counteraction. In total, a pair of SM-20 performed more than 150 test flights under the program K-20.

In the summer of 1957, they began testing the X-20. They lasted from June 6, 1957 to July 29, 1958.   The tests of the X-20 were satisfactory, without significant defects in the system. However, there were claims to range and accuracy: in the first launch on March 17, 1958 the missile did not reach the marching trajectory due to a malfunction of the IAP and the failure of the statoscope - a device that can withstand a given altitude, and the established range was not reached. In April, the next launch was made, and in July - two more, followed by the decision to transfer the complex K-20 for state testing.

The joint state trials began on October 15, 1958 and ended a year later, on November 1, 1959. Within the framework of them performed 16 launches of the X-20, of which 11 were recognized as offsets, although with reservations on the accuracy, not reaching the specified. Shortcomings were also identified in the work of the power plant and on-board equipment. Time and again at an altitude did not want to start the engine, which froze to -40 ° C. Usual kerosene in hypothermic condition did not ignite, and to start the aircraft was equipped with a make-up system with a tank of starting fuel - gasoline B-70, and the engine itself was equipped with an ignition system. Increasing the stability margin, the air inlet and inlet cone were improved and the air channel to the engine was extended.

At the same time, it was decided to replace the BC with an improved one with a new thermonuclear charge. Nuclear BC and its systems were developed in 1957. The modernized missile received the designation X-20M. In terms of spending X-20 of the first version, which was not adopted for service, the tests were continued on new samples.

Development of BC systems and automatics required a number of other test launches. X-20 launches were carried out under the influence of radio interference, in the course of testing was also similar to the operational conditions. In spite of a number of remarks, reliability, ease of mastering and maintenance, performance characteristics were positively evaluated. The K-20 aircraft and missile system was adopted for service by the government decree of September 9, 1960. As it was mentioned above, in pursuance of the same PSM of March 11, 1954, parallel works were carried out to create a missile-bearing version of the Myasischev ZM-K-20. According to the IAP Order of December 31, 1954, both carriers were to have a single guidance system and the K-20 rocket launcher. A serious problem was the very placement of the X-20 on the menschivskiy aircraft, hampered by the extremely small clearance under the fuselage and the bicycle landing gear. Consideration was given to the possibility of suspending the rocket with a special pit in the parking lot, its installation on the fuselage from above with a crane and even mounting the rocket under the cockpit in the upside down position, the keel down. In the end, a variant of X-20 suspension under the wing and symmetrical placement of a large PTB to maintain track stability and balance was considered acceptable. The missile was mounted on a special pivoting beam tilted to the starting position to move away from the carrier.

In January 1956, the State Commission considered the layout model. However, there remained a number of "bottlenecks", and in general the decision was inferior to Tupolev, whose aircraft had already been put to test. In the end, the choice of Air Force and MAP was made in favor of the Tu-95K.

The first production Tu-95K was released in March 1958. Production of the aircraft at plant number 18 continued until 1962. At the same time there was a training version of the rocket carrier Tu-95KU, which served to retrain the crews and train them to work with the equipment. However, already during the testing of the missile carrier revealed its significant disadvantage - the increase in weight of the aircraft and cumbersome suspension reduced the flight range by almost 2000 km, so the range of the complex began to yield to the bomber. Even taking into account the long-range launch of missiles, targets in the U.S. were on the limit of reach of the Tu-95K, and then subject to their placement in the Far North and Far East.

Already during the tests, July 2, 1958, came out PSM, which obliged developers to take measures to improve the performance of the aircraft and the complex as a whole. OKB-156 and the Air Force proposed to improve the range by refueling, in implementation of which appeared PSM from May 20, 1960, according to which the modified aircraft should be submitted for state testing in the 1st quarter of 1961.

Tu-95KD was equipped with a universal refueling system with a telescopic rod fuel receiver and radio equipment "Potok" for communication with the refueler. Joint testing of the Tu-95KD was completed on January 30, 1962, after which the aircraft was put into production. According to this sample was re-equipped and some previously produced Tu-95K. Subsequently, all machines with a refueling system were upgraded on the type of Tu-95KM with more modern communication and navigation equipment. It includes a new flight and navigation system "Way-1B", automatic navigation device ANU-1A and the exchange rate system KS-6D with Doppler meter DISS-1, which simplified management and had a positive impact on accuracy characteristics.

The first X-20 aircraft were assembled by the pilot production of OKB-155 and Plant 256 in Dubna. The Dubna plant also provided a series of missiles for testing. In connection with the transfer of the company to the production of new missiles X-22, the serial production of X-20M in 1960 was transferred to the plant number 86 in Taganrog, engaged in seaplanes and by this time the low-loaded main products. In 1963, the team of K-20 complex creators was awarded the Lenin Prize.