May 7, 2009 In 1974 Evel Knievel launched himself across the Snake River Canyon in Idaho aboard the rocket powered Sky-Cycle in what was to be the biggest stunt and biggest flop of his daredevil career. The Skycycle X-1 and X-2 were more rocket than they were motorcycle and both were powered by overheated water rockets. This incredibly powerful means of propulsion also saw the team at Swissrocketman achieve a blistering 214.7 mph in 2.5 seconds back in 1992 - a water-rocket record that has remained unbeaten ever since.

Heated water is extremely powerful and plays a larger part in our lives than you might expect. In fact, up to 80 percent of the world's electricity is generated using the Rankine cycle where water is heated to become high pressure steam, which drives a turbine and, in turn, an electrical generator. The same principle is at work in heated water rockets where water is held in a pressure vessel then heated to a high temperature.

The temperature at which water boils and turns into steam is 212ºF (100ºC) at atmospheric pressure, approx 15 psi (1 Bar). However, when water is heated inside a sealed container such as a pressure vessel, the pressure within the container rises above atmospheric as the heat makes the water expand, raising the temperature at which it boils.

The water-rocket world record holder since 1992 has been the team at Swissrocketman (not to be confused with Fusionman Yves Rossi, who also hails from Switzerland) which built a six wheeled rocket car called "Waterthunder" that looks like a six wheeled fully enclosed go-kart with a water tank in the back.

Heated water rockets take advantage of this basic science to generate up to 15,000 hp by heating just over 100 liters of water using a 100kw electrical heater wrapped around he tank. It takes about 40 minutes to get the water to 500ºF (260ºC) which produces over 900 psi (60 Bar) of pressure. With the water at saturation temperature, pressure is reduced as water escapes into the atmosphere and it flashes into steam. If this steam and water is expanded through a suitable nozzle velocity can reach 1500 feet per second. This steam sustains the internal pressure and provides the pressure necessary to eject the liquid hot water from the container through the nozzle to produce thrust.

The reaction of this jet of steam has driven 'Waterthunder' to a world record 343.52 km/h (214.7 mph) in 2.5 seconds. Watch the video below to see the world record run.

Paul Evans

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