Antonov An-225 Mriya An-325

The An-325 was Antonov's answer to a question the Mriya itself had left open: if an aircraft could carry the Buran orbiter on its back, why not use it as the first stage of a space system and launch an orbital plane from it in mid-flight? The project started from the An-225, the largest cargo aircraft ever built, but enlarged it, added fuel and power and turned it into a flying launch pad for Soviet and foreign space vehicles. It was meant to place payloads into circular, elliptical and high orbits, including geostationary orbit. It was never built.

Air launch was an old idea in the Soviet Union. Ever since the An-124 Ruslan appeared, there had been talk of launching a rocket from eight to eleven kilometres up to put its payload into orbit, and similar concepts were emerging in Europe and North America. The USSR was working on the Multipurpose Aerospace System (MAKS), whose essence was exactly that: a carrier aircraft based on the Mriya, the An-325, which at about ten thousand metres would enter a shallow dive to separate from the shuttle riding on top. After separation, the orbital plane would climb to orbit on its own engines while the carrier returned to land. The advantages claimed for MAKS included freedom from a fixed spaceport, the ability to rescue crews from objects in orbit and high-altitude reconnaissance. Sources disagree on dates: some place the An-325 within the MAKS work of the 1980s, while others describe studies in the early 2000s into producing this eight-engined derivative for use with Russia's then-developing MAKS space plane.

The best documented chapter of the An-325 is British. In the United Kingdom, British Aerospace and Rolls-Royce had proposed HOTOL, a single-stage spaceplane that would take off and land on a runway. The British government stopped funding it in July 1988 and Rolls-Royce left the work in 1989, but British Aerospace carried on. In July 1990 it agreed with the Soviet Ministry of Aviation Industry to study the feasibility of air-launching an interim version, Interim HOTOL, from the back of the An-225. The six-month Joint Study Program ran simultaneously in both countries, with data and analyses shared, and it led to the design of the An-325.

The division of labour was clear. The Soviets studied the separation of Interim HOTOL from the aircraft, the possibility of developing a high-performance oxygen-hydrogen rocket engine for the vehicle and the heat-protection improvements the An-225 would need; their work included wind-tunnel testing. British Aerospace handled the vehicle's design and performance requirements, its operation and support and the economics of the air-launch concept. According to the director of the Central Aerohydrodynamic Institute (TsAGI), the first results, as of October 1990, looked favourable. In April 1991, after seven months of study and extensive scale-model tests in the TsAGI wind tunnel at Zhukovsky near Moscow, British and Soviet engineers reported that using the An-225 as the first stage was technically sound. The concerns were Interim HOTOL's weight and stability on top of the aircraft, the vehicle's ability to ignite its engines and leave the carrier in flight, and the equipment needed for a high-speed separation.

In June 1991 the An-225 and Interim HOTOL, a 250-ton reusable vehicle, were presented at the European Space Agency's headquarters in Paris as a joint international aerospace system for near-Earth space exploration. Compared with vertical launch, the project promised a fourfold cut in the cost of placing payload in orbit. As of September 1990, British Aerospace estimated development costs at about 4.6 billion dollars and recurring costs at about 16 million dollars per flight, roughly a third of the cost per kilogram of Ariane 5; launched at around nine kilometres, the vehicle could place about 7.7 tons into low Earth orbit from an equatorial site.

On paper, the An-325 would have two more engines than the Mriya, mounted on the inner engine pylons, giving eight jets in six nacelles. Its dimensions would grow and it would gain an extra fuel compartment; because the weight of fuel it could lift exceeded the volume available inside, an external tank on the fuselage mounts was planned. On an Interim HOTOL mission the aircraft would fly for about an hour to around 9 km, the vehicle having been loaded with liquid hydrogen and liquid oxygen on the ground. From about 9.4 km the An-325 would begin a shallow dive, steepen it briefly for about 15 seconds to release the orbiter, and keep diving to clear the exhaust plume. The vehicle's four engines would light in pairs just before and just after release, with the sequence complete four to six seconds later. Free of its load, the An-325 would return to the ground.

The project ran into technical complexity and money. Neither the British government nor ESA, busy with Hermes and Ariane 5, agreed to fund it; funding ceased, development stopped, and the dissolution of the Soviet Union and the economic crisis that followed sealed its cancellation. It stayed frozen for more than twenty years: interest returned in 2012, and in April 2013 the Russian government announced plans to revive Soviet-era air-launch projects using a purpose-built modification of the An-225. The An-325 remains one of the great aircraft that never flew.