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The Antonow An-325 was a planned enlarged and improved version of the Antonow An-225, which was never built. The An-225 was the largest production aircraft in the world and is designed to carry heavy loads on its back. The An-225 was initially designed to transport the Soviet Buran space shuttle and would be capable of launching the estimated 275-ton orbiter.

The AN-325 aircraft would have two additional engines, which should be mounted on the respective engine mounts in a manner similar to the American B-52. There would be an eight-jet plane with six engine gondolas. The aircraft, which never went beyond the planning stage, was intended as a launch platform for Russian and foreign spacecraft. It would have become by far the largest and most powerful aircraft in the world.

In mid-1985 the British National Space Centre approved a 2-year (1985 to 1986) study to evaluate the feasibility of a concept proposed by British Aerospace and Rolls-Royce for the Horizontal Take Off and Landing Spacecraft (HOTOL) single-stage-to-orbit aerospace vehicle. At the beginning of 1986, the British National Space Center allocated three million pounds to cover the expenses of a feasibility study on HOTOL. In ten years, this could be the first unmanned space-going aircraft, capable of placing cargoes on the order of ten tons into low orbit. Hotol's goal was to place 14,000 to 16,000 pounds of payload into low Earth orbit. It was to have a launch weight of about 500,000 pounds and cost $4-5 million per flight.

HOTOL would be powered by Rolls-Royce’s RB-546 (Swallow) engine, a combined air-breathing and rocket propulsion engine. The engine, which was classified secret by the British Ministry of Defence, would use atmospheric oxygen in the same way as an airplane’s jet engine at low altitudes and switch to HOTOL'S on-board liquid oxygen supply at Mach 5 at about 85,000 feet in altitude to boost the spaceplane into orbit. It would glide back to earth and land horizontally on a conventional runway.

Although the British National Space Centre participated in an evaluation of the HOTOL aerospace vehicle concept proposed by British Aero- space and Rolls-Royce, the British government’s participation in HOTOL'S development ended in July 1988 when it elected not to fund fur ther HOTOL research and development. Rolls-Royce ended its work on HOTOL in 1989. However, British Aerospace continued its efforts.

British Aerospace and the Soviet Ministry of Aviation Industry agreed in July 1990 to study the feasibility of air-launching an interim version of HOTOL from the back of the Soviet Union’s Antonov An-225 heavy-lift transport aircraft, The 6-month Joint Study Program was conducted simultaneously in the United Kingdom and in the Soviet Union with each organization sharing the data and analyses.

British and Soviet officials agreed that a low-cost reusable space launch vehicle was needed to support long-term manned space stations. According to British Aerospace’s Deputy Chief Executive for Engineering, a version of HOTOL launched at an altitude of about 30,000 feet from the An-225 could also provide a low-cost satellite launch system. He estimated that the vehicle would have the capability of placing a 7.7-ton payload into low earth orbit from an equatorial launch site. After deploying its payload, this version of HOTOL would land horizontally on a conventional runway.

British Aerospace and Soviet Ministry of Aviation Industry officials said the RB-545 engine could be replaced on an interim basis by high-performance rocket propulsion, since HOTOL would be launched from an airplane instead of taking off from a runway. According to British Aerospace, Interim HOTOL would provide a useful near-term initial operating capability without requiring the simultaneous development of an advanced airframe structure and an advanced air-breathing engine. Experience gained from such a vehicle, according to British Aerospace, could then be expected to lead to a single-stage-to-orbit HOTOL.

The Deputy Minister of the Soviet Ministry of Aviation Industry said that the project was based on a sound concept and that the An-225 could be a moving launch pad for HOTOL. Under the agreement reached between British Aerospace and the Soviet Ministry of Aviation Industry, the Soviets studied Interim HOIDL'S separation from the An-225 and the possibility of developing a high-performance, oxygen-hydrogen rocket engine for the vehicle. The Soviet work, which included wind tunnel testing, also assessed heat protection improvements to the An-225. British Aerospace studied the design and performance requirements for the vehicle, its operation and support, and the economic viability of the air-launch concept.

According to the Director of the Soviet Central Aero-Hydrodynamics Institute, the initial study results, as of October 1990, appeared to be favorable. In April 1991 British and Soviet engineers reported that using the An-225 as the first stage for Interim HOTOL was technically sound. Concerns included Interim HOTOL'S weight and stability on top of the An-225, the space launch vehicle’s ability to ignite its rocket engines and take off while the An-225 was in flight, and the type of equipment that would be required during high-speed separation of Interim HAL from the An-225. This conclusion was reached after 7 months of study and extensive scale-model tests in a wind tunnel at the Soviet Central Aero-Hydrodynamics Institute in Zhukovski near Moscow.

According to British Aerospace’s Deputy Chief Executive for Engineering, the estimated development costs of Interim HUIOL, as of September 1990, totaled about $4.6 billion, and recurring costs totaled about $16 million per flight, about one-third of the payload cost per kilogram of Ariane 5.

The principal technical uncertainty for any launch vehicle piggybacked on a carrier aircraft is launch vehicle separation, including both separation mechanisms and separation aerodynamics. The separation of a top-mounted vehicle from a carrier aircraft had been demonstrated, for example, with the Short-Mayo Composite, the Junker/Focke-Wulf Composite, and the Space Shuttle and Shuttle Carrier Aircraft [SCA-905] combination at subsonic Mach numbers; and with the D-21 drone and the M-21 aircraft at supersonic Mach numbers.

A paradigm change in the transportation of medium-weight payloads to low-Earth orbit (LEO) with respect to affordability, reliability, safety, resiliency, and operational responsiveness can be made only if existing chemical rocket engines are partially replaced with air-breathing propulsion. This paradigm change could be either a single-stage-to-orbit (SSTO) spaceplane that uses air-breathing propulsion during atmospheric flight, or with a two-stage-to-orbit (TSTO) system that uses air-breathing propulsion on the carrier aircraft stage and rocket propulsion on the launch vehicle stage. Two of the principal technical challenges with the TSTO concept are placing the launch vehicle at appropriate flight conditions to begin its ascent after separation from the carrier aircraft, and safely separating the two large vehicles in midair.

In 1938, Sänger and Bredt designed an SSTO, sled-launched, rocket-powered, winged boostglider called the Rocket Spaceplane. This design evolved from the Silbervogel (Silver Bird) that Sänger developed in the early 1930s. The Rocket Spaceplane design had to give way to the constraints of technology, however, and was replaced with a TSTO system dubbed Sänger I in the early 1960s.

In 1959, the need for a recoverable booster system to provide routine access to space led to the recoverable orbital launch system (aerospace plane) program in the United States. The US Air Force emphasised TSTO concepts as first-generation options, based on guidance from the Air Force Scientific Advisory Board and other ad hoc committees. In November 1965, after intensive study, review, and evaluation, the TSTO concept with an air-breathing first stage and a conventional rocket launch vehicle second stage was selected as the preferred approach. In 1970, when the US Space Shuttle Phase B award began, NASA and contractors were generally unanimous in considering fully reusable TSTO concepts as the most viable launch approach.

Around 1987, Sänger II – consisting of a turboramjet-powered, Mach 6·7 carrier aircraft and a rocket-propelled launch vehicle–was proposed. Efforts to develop a reusable, horizontal take-off TSTO system were restarted for the NASA space access study in 1993. Since then, a few such TSTO system concepts have been conceptualised and studied. These efforts considered air-breathing propulsion on the carrier aircraft, which staged the piggybacked rocket-powered launch vehicle at a subsonic, a supersonic, or at a hypersonic Mach number.

In the 1980s, the Antonov An-225 Mriya was designed to carry the Buran orbiter piggyback. It was considered as a carrier aircraft for three different Multipurpose Aerospace System (MAKS) configurations for space access. The An-225 was also considered for air-launching of the interim HOTOL. Aircraft such as the SCA, B747-400F, B747-800F, and A380-800F were not designed to carry a launch vehicle. The An-225 Mriya is longer than the B747-800F by 25·43ft and can carry significantly heavier launch vehicles.

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