The Spiral seems to be one of the most ambitious and secret projects in the history of Soviet aviation. Therefore, only 40 years later it became possible to disclose the whole truth of this aircraft...

     Under the five-year plan of the Air Force on orbital and hypersonic aircraft, go-ahead to actually proceed with development of a manned orbital vehicle in the USSR was given in 1965 to the Artem Mikoyan's OKB-155 design bureau, and 55-year-old Chief Designer Gleb Lozino-Lozinsky was selected as project manager. The new project dubbed Spiral.
     The aerospaceplane project dated 29 June 1966 provided for the development of a 115-t orbital system consisting of three winged VTOL reusable aircraft: the 52-t HLA reusable hypersonic air-breathing launch aircraft, designated '50-50'; the RB expendable two stage rocket booster; and the OS manned orbital aerospaceplane, designated '50'. The system would take off vertically from a launch cart at a takeoff speed of 380-400 km/h.
     The launch aircraft would accelerate the whole system to a hypersonic speed of M=6 (about 1,800 metres per second), releasing the components at an altitude of 28-30 km. The launch aircraft would return to its launch base after completing the mission, and the booster, burning fluorine+hydrogen fuel, would propel the aerospaceplane to the orbit.
     The orbital aircraft would bring a payload of up to 10.3 tonnes to the circumpolar orbit of 130-150 km at the carrier offset of up to 750 km, if boosted by a launch aircraft with liquid hydrogen power plant, or 5 tonnes if the launch aircraft burnt kerosene.
     The manned reusable single-seated orbital aircraft would be employed in daytime photographic reconnaissance, radar reconnaissance, space targets interception or ground attack, equipped with a space-surface missile for the latter function. The aircraft weighed 8.8 tonnes in all configurations, carrying 500 kg of combat payload for reconnaissance and interception, and 2,000 kg in attack configuration. Launched from the territory of the USSR, the aircraft would reach the 130-150-km orbits, inclined by 45-135 degrees. Manoeuvrability characteristics of the aircraft, propelled by the rocket booster power plant, burning fluorine and amidol provided for variable orbit inclination (to allow second target run) by 17 degrees for the reconnaissance and interception aircraft, and 7-8 degrees for an attack aerospaceplane armed with a missile. The interceptor version would perform a combined manoeuvre of simultaneously changing the inclination and ascending to 1,000 km, after which it would lose the weight to 4,900 kg.

     To complete the flight, the pilot would start braking engines and the orbital aircraft would dive into the atmosphere at a large angle of attack, its roll deflecting from 0 to 60 deg. at stable angle of attack to control the aircraft on descent. It would be able of performing an aerodynamic manoeuvre at gliding trajectory within the range of 4,000-6,000 km and side deviation of 1,500 km. The aircraft would be guided to the landing area controlling the speed vector along the runway, which was achieved through the use of the roll programme. The agility of the aircraft would allow it to land at a speed of less than 250 km/h at night and in adverse weather at one of reserve 2nd class airfields in the USSR from any of its three orbit circuits, for which the RD36-35 kerosene-fuel turbojet engine of the Rybinsk-based OKB-36 design bureau (Chief Designer Pyotr Kolesov) would be used.

     The hypersonic launch aircraft had a layout of a large arrow-shaped variable flying wing, with vertical stabilisers mounted at the wingtips. The wing is made of double-wedge profile section featuring variable thickness ratio.
     HLA control surfaces included rudders of the fins, elevons and landing flaps. To improve its yaw stability, the tail unit incorporated a ventral fin, folding at take-off. The aircraft featured a two ejection seat insulated cockpit, its nose declining 5 degrees forward at landing to allow better field of view.
     Liquid hydrogen was used as propellant for the launch aircraft. The power plant included a set of four AL-51 turbojet engines boasting 17.5 tonnes of thrust each designed by Arkhip Lyulka's OKB-165. They had a single air intake and a single supersonic divergent nozzle.
     With the empty weight of 36,000 kg, the launch aircraft could carry up to 16,000 kg of liquid hydrogen in tanks of 260 m³ each.
     The engines' peculiarity consisted in the use of hydrogen vapours to gear the turbine that rotated the turbojet compressor. Hydrogen evaporator was placed at the compressor inlet. This is the way the powerplant for the aircraft was elaborated without combining turbofan, hypersonic and turbojet engines.
     The adjustable hypersonic air intake was yet another advanced feature of the launch aircraft. It used almost entirely the forward bottom surface of the wing and the specially designed fuselage nose to compress the air.
     Dedicated construction materials and heat-resistant coatings were applied to withstand the heat barrier.
     The launch aircraft might well have been used as a long-range hypersonic strategic reconnaissance aircraft, too. Equipped with kerosene engines, it was expected to accelerate to M=4.0-4.5 and have a range of 6,000-7,000 km at cruising M=4, while the hydrogen engine installed would allow it to speed up to M=6 and cruise 12,000 km at M=5.
     The OS was implanted into the launch aircraft on top, together with the rocket booster in the shape of two-stage launch vehicle with the total length of 27.75 m (18.0 m for the first stage and the fairing and the remaining 9.75 m for the second stage with the orbital aerospaceplane proper). The oxygen-hydrogen booster would be a meter longer and a half-meter thicker.
     The aerospaceplane itself was a flat-bottomed lifting body with a large upturned nose. The nosecone was made as a 60-deg. segment with 1.5-m radius of the sphere. Its design was found to greatly reduce afterbody heating during re-entry, when it heated to 1,400 deg. Centigrade.

     A unique feature of the aerospaceplane was the variable dihedral wings. The outer skin was articulated to permit thermal expansion during re-entry. The wings were set at an angle and had a specific form, so that during re-entry at a 45-60 deg. angle of attack and the hypersonic quality of 0.8 the air stream would flow from the body down to the wings, rather than to the wing leading edges. Also, the wings were made separately variable to better controllability in case of yawing.
     To improve landing parameters, after becoming subsonic, dual electric actuators moved the wings to a horizontal position, where they served as wings, substantially increasing the lift of the aerospaceplane for airbreathing operations, the wingspan reaching 7.4 m with a sweep of 30 deg. This increased the aerodynamic quality to 4.5.
     The 'hot structure' principle was used in ensuring heat resistant capability of the orbital aerospaceplane. Therefore, the aircraft had a welded frame, with a heat-resistant screen underneath, made of plates of the VN5AP clad columbium alloy coated with molybdenum dicilicide. The plates were placed as fish scales. The screen was attached with the help of ceramic bearings playing the part of heat barriers, as they relieved temperature stress by means of the screen moving relative to the body without changing the shape of the aircraft.
     Landing gear normally consisted of four skids in landing gear bays mounted on the sides of the aerospaceplane above the heat shield, and deploying in the manner not to split the screen before landing.

     The power plant included:
     - a liquid propellant engine (LPE) for orbital propulsion, with the 1,500 kg/f of thrust (specific pulse of 320 sec, fuel consumption of 4.7 kg/sec). It was employed to change orbital inclination and produce a braking pulse to dive off the orbit. Later versions would feature a more powerful LPE with thrust in vacuum of 5,000 kg/f, boasting a variable thrust capability, adjusting it slightly to 1,500 kg/f to allow orbital manoeuvres;
     - two emergency situation braking 16 kg/f LPEs, fed from main LPE fuel system by the compressed helium displacer;
     - an LPE attitude control unit of six 16 kg/f engines for coarse adjustment and ten 1 kg/f engines for fine manoeuvres;
     - a turbojet kerosene 2,000 kg/f engine with specific fuel consumption of 1.38 kg/kg*h to allow subsonic propulsion and landing. It had an adjustable ram air intake at the bottom of the fin, opened only before starting the turbojet.
     The first examples of combat orbital aerospaceplanes were to be equipped with intermediate LPEs, burning fluorine-ammonia fuel.
     The pilot-cosmonaut sat in an insulated 'headlight' escape capsule, equipped with powder charges to eject the capsule at any stage of flight from taking-off to landing. The capsule also had control engines for re-entry phase, as well as a radio beacon, an accumulator and an emergency navigation unit. It parachuted at a speed of 8 m/s, kinetic energy absorption effected by means of residual deformation of the specially designed cellular structure of the capsule bottom.
     The escape capsule with equipment, life-support system, escape system and the pilot-cosmonaut weighed 930 kg, and 705 kg at landing.
     The navigation and automatic control system included a self-sustained astro-inertial navigation system, a digital airborne computer, LPE directional equipment, astro-corrector, optical viewing device and radio altimeter.
     Manual backup on direction signals was available to control the aerospaceplane at descent.
     Specialised equipment of the combat manned orbital aerospaceplanes in reconnaissance and interceptor configurations was placed behind the cockpit in a 2 cu.m compartment, which was increased at the expense of the fuel compartment to allow housing the space-surface missile in the ground-attack configuration.

     Daytime photographic reconnaissance aerospaceplane would provide detailed intelligence on small-size land and mobile sea-surface targets, selected in advance.

     The photographic equipment ensured 1.2-m resolution, if employed from 130 km orbit. The pilot-cosmonaut was to detect the target and observe the terrain with the help of the optical viewing device. The device had an adjustable reflector to track targets from 300 km. Picture-taking would start automatically as soon as the pilot manually coincided the optical line of sight of the photo camera and the viewing system with the target. The pilot should have time to make photos of 3-4 targets during one circuit.

     The reconnaissance aerospaceplane would have short-wave (HF) and ultra short-wave (UHF) radios to transmit the data to ground control centre. If additional target run required, an automatic orbit inclination changeover was performed at pilot's command.
     The reconnaissance version would have a distinguishing 12x1.5-m external expendable antenna, which was expected to be effective at 20-30 m, which is quite enough to detect aircraft carrier groups and large land installations.
     The attack orbital aerospaceplane would be used to kill sea targets. It was expected to launch its space-surface missile over the horizon with target designation data available from an orbital re