Aiming high

It Is five years since Concorde — the most beautiful plane ever built, many people would say — was retired. Unfortunately, it was also one of the least practical. Noisy, thirsty, and shunned by all but two airlines, the delta dart had great cachet, but ultimately had to go. As one commentator said, it looked like a window cut into the future — but it turned out not to be our future.

Now aerospace engineers across Europe plan new, larger and even faster aircraft. And although still at an early stage, a European-funded project to design hypersonic passenger aircraft, known as Long-term Advanced Propulsion Concepts and Technologies(Lapcat), is about to pass its first milestone with the completion of a feasibility study.

The aim is to determine if it is technically feasible to design an aircraft that can carry 300 passengers from Brussels to Sydney, more than 10,000 miles, non-stop in two to four hours.

The first fruits of that study look like being another window cut into the future — a sleek needle-nosed craft that would dwarf even an Airbus A380, and which, fuelled by liquid hydrogen, would produce no direct carbon emissions in operation.

The target is to design two aircraft — one that can do the trip in four hours, which would have to fly at four to five times the speed of sound (Mach 4-5, about 3,700mph or 6,000kmh), and one that would do it in two hours, at Mach 8. They are very different vehicles, propelled by different engines — neither bearing much resemblance to the turbofans used on current aircraft.

‘Since the 1960s, especially in the US, there have been studies to see whether hypersonic passenger flight is technically feasible, but in Europe there was nothing,’ said Lapcat co-ordinator Johan Steelant of ESTEC, the engineering branch of ESA. ‘But I want to have an independent view, seeing as we have the capabilities in Europe, to judge whether this was possible or not. And if it is possible from a technical point of view, is it worthwhile, or would it be so expensive that there’s no market for it?’

When Steelant’s team began work on the three-year project in 2005, it quickly determined that two technologies were necessary. For Mach 4-5, the system could use engines based, at least in part, on conventional jet engines, but these would switch to a different mode of operation at supersonic to hypersonic speeds. For this, they turned to a group of veteran UK propulsion experts who formed a company called Reaction Engines to develop their high-speed concepts after the collapse of a spaceplane project.

For the Mach 8 craft, no conventional engine would do. The only technology that could propel a plane in the atmosphere at those speeds was a supersonic combustion ramjet (scramjet), a deceptively simple device that compresses air by forcing it through a constricted tube, injecting a fuel, burning it, then ejecting the exhaust through a shaped nozzle to produce thrust. But to date, only two scramjets have been tested in flight. For this, Steelant looked to the German Aerospace Centre (DLR) in Berlin to lead the studies.

Reaction Engines has one of the most interesting stories in UK engineering. Formed in 1989, its founders had worked for Rolls-Royce on the Horizontal Takeoff and Landing vehicle (Hotol), a British space shuttle project, and designed an engine that could burn hydrogen in atmospheric oxygen as it climbed through the atmosphere, switching to a liquid oxygen-fuelled rocket phase once it entered space.

Hotol was shelved because of technical problems, but the engineers — Alan Bond, Richard Varvill, and John Scott-Scott — formed their own company, based at Culham near Oxford, to develop their concepts as a private concern.

Reaction’s main project is Skylon, a spaceplane powered by an engine developed from the Hotol system, called the Sabre. ‘Alan, who’s our thermodynamicist, took the Sabre engine as a starting point for Lapcat, and redesigned it to reduce the fuel consumption,’ said Varvill, Reaction’s chief designer. ‘And, of course, if you’re staying in the atmosphere, you don’t need the rocket mode at all.’

The new engine, called the Scimitar, is the cornerstone of Reaction’s prospective design for Lapcat, known as the A2.

The system needs liquid hydrogen fuel. Hydrogen contains almost three times as much energy per litre as kerosene, so the aircraft can carry less fuel and so is lighter. Just as important, it is also very cold, -250°C. As air enters a turbojet, it is compressed, which heats it up. The faster the jet is travelling, the hotter it gets. Therefore engines must be made from materials that can withstand high temperatures, and these tend to be dense, adding to the weight of the craft.

The Scimitar cools the air before it reaches the compressing turbines by passing it through a heat exchanger, which uses liquid hydrogen as a heat sink, increasing engine efficiency (unlike Concorde, it is efficient even at subsonic speeds) and allowing its weight to be minimised.

These heat exchangers are the main focus of Reaction’s research and development, both for Lapcat and for Skylon, and the company is about to build a full-size prototype. At higher speeds, the Scimitar switches into ramjet mode; it is travelling so fast that extra compression is not needed, and the velocity itself forces air into the combustion chamber.

This means that the craft can take off normally, fly subsonic over landmasses, then climb to its cruising height of 25km and accelerate to Mach 5 over the North Pole, making the bulk of its journey at hypersonic speeds over the Pacific.