Northrop Grumman's concept for a flying wing fighter has similarities to the Hortens' innovative design (Credit: Northrop Grumman) It looks about as futuristic as fighter aircraft can get, but its genesis goes far further back than you think – to a truly groundbreaking jet fighter design built and flown in Nazi Germany in the dying days of World War Two. That aircraft – the Horten Ho 229 – might be a footnote in aviation history, but it was so far ahead of its time that its aerodynamic secrets are still not completely understood. In fact, there’s a chief scientist at Nasa still working to discover just how its creators managed to overcome the considerable aerodynamic challenges that should have made it unflyable.

By Adrian Giordani 15 September 2015 “It is a magic aircraft… the pleasure of flying in it is almost a carnal one.” So said Joelle Cornet-Templet, a chief stewardess for Air France, about Concorde: one of the world’s first supersonic civilian airplanes, which flew from 1976 to 2003 and became a watchword for travelling in style. This thoroughbred airliner could fly from London to Sydney in 17 hours, three minutes and 45 seconds; compared to around 22 hours on a Boeing 747. Concorde was the best-known member of an exclusive club of two; the only other civilian airliner able to break the speed of sound was the Soviet Union’s Tupolev Tu-144, which flew until 1999. A variant of the Tupolev was used in experiments by Nasa, and American and Russian aerospace industries, in a joint research program after the end of the Cold War. The end of the Concorde and Tupolev airliners left the supersonic market empty. But now, 12 years after the Concorde fleet last flew, even faster airliners are taking shape in research facilities.

When Concorde bowed out in 2003, supersonic air travel went with it (Credit: Science Photo Library) One of those designs is by Lapcat-II, a European-designed aeroplane capable of cruising speeds up to eight times faster than sound (8,500 km/h or 5,280 mph) taking passengers from Brussels to Sydney in 2 hours and 55 minutes. At the AIAA Hypersonic Space Plane conference in Glasgow in Scotland in July, a paper submitted by Lapcat-II researchers said their early airliner tests suggested such a design would be greener than current aircraft, just as safe, and would not cost much more than today’s long-haul flights. Fuel factor Johan Steelant, a senior research engineer at the European Space Agency (Esa) and coordinator of Lapcat-II, with his colleagues, has been testing two prototypes. One is a Mach 5 plane – the Lapcat-A2 powered by a pre-cooled air-turbo ramjet; and a promising – Esa-designed – Mach 8 plane, also powered by a ramjet engine. A ramjet is an air-breathing jet, with no major moving parts. The engine's forward motion compresses incoming air travelling at high speed, ramming it into a combustion chamber. A similar concept powers the new missiles used by the Eurofighter Typhoon fighter plane, for instance. Ramjets can move a plane very fast. But how do you power them? Fuel choice is important, especially as one consideration for any future hypersonic fleet will be to try to keep its emissions as low as possible. This is why hydrogen was chosen, rather than a fuel based on hydrocarbons. Although hydrogen can be ignited, the risks of an explosion or fire are lower compared to conventional airline kerosene fuel What’s more, liquid hydrogen fuel is not highly combustible mid-flight. Although hydrogen can be ignited, the risks of an explosion or fire are lower compared to conventional airline kerosene fuel. Nasa used the same stuff to power the Space Shuttle. “If there is leak, the hydrogen is so light that it goes straight up; hence there won't be a pool of hydrogen on the ground as is the case for kerosene. Hydrogen, like kerosene, needs an igniter or a heat source to initiate combustion so it doesn't ignite spontaneously,” says Steelant. The Lapcat-II team is not alone in this field. They are sharing concepts and ideas with researchers across the Pacific Ocean. In Asia, Japan’s Aerospace Exploration Agency (Jaxa) is also working on a hypersonic airliner called Hytex intended to cross the Pacific Ocean in two hours at speeds of Mach 5. Both Lapcat-II and Jaxa are part of a hypersonic knowledge-transfer project between Europe and Japan called Hikari.