Agencies such as Nasa have also explored supersonic airline travel (Credit: Science Photo Library)

The Hytex’s’s turbojet engine has been successfully tested in a flight experiment which simulates speeds up to Mach 1.8. Hytex uses liquid hydrogen both as a fuel and coolant for air travelling at hypersonic speeds. “We have finished the conceptual design and aerodynamic wind tunnel tests of Hytex. The fuel consumption is one-fifth that of rocket engines,” says Hideyuki Taguchi, leader of Jaxa’s hypersonic airplane research. Hunger for hydrogen But, deriving hydrogen efficiently is one main factor for high operating costs. If the hydrogen can be sourced from natural gas, instead of from the electrolysis of water, the airfare tickets of a hypersonic trip could drop to about half the price of a business-class ticket. Based on current projections the ticket price will be about three times more expensive on average than current business-class subsonic tickets. One estimate puts the cost at €5,000 (£3,700) per seat for a Brussels to Sydney one-way trip. An alternative fuel could be liquefied natural gas such as super-cold liquid methane The big question now is how to create all of that hydrogen. “Wind turbines could actually store their energy by producing hydrogen,” says Steelant. “This has already been established by a Belgian supermarket chain where their forklifts are driven by hydrogen produced from an on-site wind-turbine park.” Even though hydrogen-fuelled airliners would not emit greenhouse-increasing gases such as carbon dioxide, sulphur oxides or soot like today’s subsonic airplanes, there is another issue. Water vapour produced by hydrogen combustion stays in the stratosphere for a long time, and could be a contributing factor to global warming.