Was it a boat? Was it a plane? A bit of both in fact. The Ekranoplan was one of the more obscure products of the fight for technological supremacy in the Cold War. Nigel Paterson, who joined Top Gear presenter James May for a test "flight", recounts its secret history.
In September 1966 an American spy satellite flew over a Soviet naval base on the Caspian Sea and took a series of photographs. This being the height of the Cold War, the results created quite a stir among the American intelligence community, because they showed an object, more than 100m long with inexplicably stubby, square wings, quite unlike anything they had seen before.
Their first guess was that this was a conventional aeroplane, possibly a seaplane, but one that was incomplete and much bigger than any aircraft the US had.
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But when the pictures were examined more closely, intelligence analysts calculated that, even if completed, it would actually fly really badly. This, coupled with the position of the engines, located well forward of the wing, made them realise what they were looking at was something entirely different. They had stumbled on one of the most top secret military projects of the Soviet era. The object was soon dubbed the Caspian Sea Monster.
What they were looking at was, in fact, an Ekranoplan; a wing in ground effect or WIG craft designed to fly at very high speed a few metres over the top of the sea. It sounds not unlike a hovercraft. But where a hovercraft floats on a skirt of air, the Ekranoplan sits clean above the surface and relies on a well known, if little understood aerodynamic phenomenon called "ground-effect".
In very simple terms the wing produces a dynamic cushion of air when it's close to the ground and the Ekranoplan effectively rides upon this. It's the same effect that pelicans use when flying low over the sea and it's a remarkably efficient way of flying, actually increasing lift by as much as 40%. All of which means the Ekranoplan was far more efficient than conventional aeroplanes.
But even more crucially, its ability to fly just a few feet above sea level lent it one huge military advantage - the fast and efficient Ekranoplan was stealthy, capable of carrying troops and armoured vehicles rapidly under the gaze of enemy radar.
But all this was still a mystery to the West in the 1960s. It would be a quarter of a century later, in 1991, before the first photographs of these "sea monsters" were finally published and their existence confirmed.
Behind the wheel - James May takes the Ekranoplan for a spin
Back in the 60s, the Ekranoplan project was so secret even to use the "E" word was forbidden in public. Not that nosey foreigners were likely to stumble on one during their travels. Developed on the Volga River near Nizhny Novgorod - formerly Gorky - this was a city closed to foreigners during the Soviet era.
The project owes much to the development of hydrofoils - fast boats that lift out of the water as they pick up speed. Today, hydrofoils are a staple of many ferry operators around the world, but it was the Soviet designer and inventor Rostilav Alekseev who is considered the father of the modern hydrofoil.
The Ekranoplan was handled by the Central Hydrofoil Design Bureau, with Alekseev at the helm. With the personal support of Soviet premier Kruschev, the project was given what amounted to an open chequebook.
The original Caspian Sea Monster spied by the Americans was a colossal 540-tonne research craft. At 100m long, it was bigger than a Jumbo Jet and twice as heavy as any contemporary aircraft, but much more efficient and capable of flying at up to 400km/h.
But such a craft was clearly considered unwieldy and after years of research the Soviet military scaled down their ambitions, developing and producing a smaller 125-tonne Ekranoplan, which entered service with the Soviet navy as rapid transports. Even then, the Ekranoplan was a machine with mighty potential.
The Caspian Sea Monster
Each could to whizz up to 300 troops on its split-level decks, or two armoured vehicles, swiftly and efficiently across open water.
A CIA report from the end of 1988 - just a year before the fall of the Berlin Wall - considered that initial deployment of the Ekranoplans to the Soviet Baltic, and Black Sea Fleets was due to begin in the early 1990s. But geopolitical events intervened.
Of the 120 craft planned less than a handful were ever built or saw service and after the end of the Cold War the entire Soviet Ekranoplan project was abandoned - the surviving monsters were mothballed at the same Caspian naval base where they were first discovered by the American spy satellites.
Yet the technology behind the Ekranoplan continued to grip military minds in the West. In 1993 the US even sent its own team of analysts to assess the technology of the advanced Soviet Ekranoplan project with a view to developing their own for heavy sealift transports.
Today, a private company, ATTK, (Arctic Trade and Transport Company), is once again producing Ekranoplans in the very same shipyards where the first prototypes were designed, built and tested. But these civilian heirs to the Caspian Sea Monsters are altogether smaller and more pocket-sized affairs, designed to be used as personal craft or water taxis.
The Aquaglide is a compact 5 seat, 10m craft with a cruising speed of 170km/h. Unlike its jet-powered forebears, it's powered by a Mercedes car engine, but the engines still sit in front of its short stubby wings. Two variable pitch propellers can push air under the wings and help develop the cushion that allows the craft to become airborne. And like their heftier siblings they are capable of flying over water, ice, or land.
Part of the problem with the development of Ekranoplans, has been their classification for matters of legislation and licensing. Were these aircraft or ships? The matter was only resolved as late as 2005 by the International Maritime Organisation who deemed that Ekranoplans were, in fact, "high flying ships".
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