Spaceplanes may one day lift off from the backs of huge seaplanes that skim the ocean's surface at half the speed of sound, say space scientists in Russia and Japan. The developers believe that the technology could be highly competitive with traditional vertical take-off systems such as the space shuttle. The plan is to mount a spaceplane on the back of a 1500-tonne, rocket-propelled seaplane, known in Russia as an "ekranoplan".
Flying just a few metres above the water, the ekranoplan rides along on the cushion of air that forms under large flat objects near a surface, the "wing in ground" (WIG) effect.
Powered by jet engines, the ekranoplan reaches speeds of over 600 kilometres per hour (Mach 0·5) at which point the spaceplane's rockets fire and the two vehicles separate.
The spaceplane continues to climb until it reaches its escape velocity of around 966 kilometres per hour.
The chief advantage of the system, says Alexander Nebylov, director of the International Institute for Advanced Aerospace Technology in St Petersburg, is the high initial launch speed.
With vertical take- off, most of the fuel in the space shuttles' boosters is spent lifting more fuel. But with a horizontal launch, the spaceplane gains low-cost aerodynamic lift from the forward velocity of the ekranoplan.
The spaceplane has to carry slightly more fuel than if it was taking off vertically, but the ekranoplan will use less fuel than a rocket engine.
Because the launch will effectively be a single-stage affair, no massive external fuel tank or pair of solid rocket boosters as used on NASA's space shuttle is required. Furthermore, WIG craft have high fuel efficiencies due to low drag.
But if take-off sounds bizarre, consider the landing. Nebylov says the spaceplane will dock with a moving ekranoplan when it returns to Earth. The lack of landing gear will help balance out the extra fuel, he says.
Nebylov, together with Nobuyuki Tomita of the Musashi Institute of Technology in Tokyo, plans to undertake some initial sea trials next year with a scaled-down ekranoplan, weighing a mere 400 tonnes.
"We can launch from any point in the ocean, which is very important in achieving a necessary orbit," Nebylov says. Rocket scientists like to launch as near as they can to the equator, as the Earth's extra rotational velocity there helps spacecraft into orbit.
Jay Jayawant, who is working on a horizontal spacecraft take-off system using magnetic levitation at the University of Sussex, has his reservations. "Nothing travels at these speeds without having serious problems with stability," he warns.
But Nebylov says its size is the key to its stability. To work, the ekranoplan has to be three times larger than the Caspian Sea Monster -an enormous military ekranoplan built during the Cold War.
"It must also be rather big to have good seafaring abilities," says Nebylov. A larger craft will make a more stable platform that is capable of reaching take-off speeds even with waves 5 metres high, he says.
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Updated: Thursday 15th, August, 2013