Aiming high

A team of UK engineers plans to help prove the sceptics wrong and show that hypersonic flight could revolutionise aviation. Stuart Nathan reports.

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.

Scimitar engines allow the A2 to take off like a traditional airliner and fly subsonic over land while maintaining fuel economy. Below, the A2 is twice the length of an Airbus A380, and most of the fuselage is taken up by the fuel tanks

However, high-speed aircraft are not like conventional airliners, where a standard shape body can be attached to standard engines. The two have to be designed together, because the way air flows around the aircraft is crucial to the engines’ operation. Because of this, Reaction also designed a prospective configuration for the A2. Almost 140m long and 7.5m across, the fuselage is lifted by delta wings, each carrying two engines, one at the tip and one slung from a pylon at the centre of the wing. The windowless passenger compartment is 32m long, with the rest of the fuselage occupied by hydrogen tanks.

‘Our engine location is quite controversial,’ said Varvill. ‘It’s designed to avoid the thick boundary air layer which forms at hypersonic speeds — that’s the slow-moving layer of air due to friction. If that gets into the engines, it can destabilise the airflow into the intake and stop the engine working. So we think we’ve got to get the engine as far away from the aircraft as possible.’

The A2 needs four engines, so two have to be on pylons below the wings, away from the boundary layer on the wing underside. ‘People say we’re going to incur a drag penalty from that, which is true, but so what? The other possibilities won’t work. That’s real life for you.’

For the higher-speed part of Lapcat, design issues are even more pressing. Scramjet engines, like ramjets, rely on the velocity of the aircraft to force air into the engine, where the geometry of the engine duct compresses it and mixes it with the hydrogen fuel. ‘To get good performance at high speed, you have to maximise the air mass flow, so intakes become larger and larger — so large that they take up the whole front of the vehicle,’ said Steelant. The aerodynamics of the vehicle are so important to the engine’s performance that you could almost say the vehicle is the engine.

Scramjets, however, are unproven technology. Only two have ever flown — NASA’s X43 craft, which became the fastest ever free-flying air-breathing craft when it reached Mach 9.8 in 2004, and the Australian Hyshot, which first flew on the nose of a rocket in 2002.

The director of the Mach 8 part of Lapcat, Klaus Hannemann of DLR, says the two-pronged attack of Lapcat is deliberate. ‘When we decided to look at hypersonic flight, we thought of what could be done in the near future — which is 15 to 20 years off — and what can be done in 25 to 30 years and beyond.’

The focus of Hanneman’s work is therefore much more basic than Reaction’s, targeted at computer modelling and wind tunnel work on the compression, mixing and combustion sections of the scramjet, which is similar to the University of Queensland’s Hyshot engine. ‘There are still major questions to be answered, such as how do we get the hydrogen fuel into a supersonic flow such that it mixes efficiently with the air and burns quickly. That means that we can build the engine as small as possible, which reduces friction losses,’ he said.

The engine model Hannemann is working with ‘isn’t strictly an engine, because it isn’t designed to generate thrust,’ he said. ‘But it is a complete scramjet configuration, with an intake, combuster and exhaust.’

In the next phase of the project, Lapcat 2, which will start in October and last four years, the DLR team plans to convert the engine into a thrust model, designing a 2m-long model they can test in a wind tunnel and, probably after the end of Lapcat 2, build into a small-scale vehicle for test flights.

Almost 90 per cent of the project resources are directed towards propulsion research, but the remaining 10 per cent devoted to vehicle design is still important. The team has devised three possible configurations for the Mach 8 vehicle but as The Engineer went to press it had not released any details.

The researchers are unsure whether the scramjets will produce suffficient thrust for the final Mach 8 concept to carry 300 passengers — Hannemann says it might be downgraded to 200. The number of engines is also undetermined, although this would refer to the number of engine ducts within the single housing on the fuselage. ‘The engine area will be split in certain sections, but the propulsion system and the aircraft can’t be distinguished.’

Another issue yet to be solved is how to launch the craft, because scramjets don’t work below Mach 6. ‘We need to accelerate somehow, which could be by some sort of turbojet, or a rocket. Do we have a two-stage system, with a carrier aircraft? We’ve looked at integrating the two propulsion systems in one aircraft, with rocket-based and turbine-based combined cycles.’

One of the first tasks of Lapcat 2 will be to decide which combined cycle to concentrate on, and design a configuration around that, he added.

While Lapcat 2 continues, a parallel project, ATLLAS, is studying materials for hypersonic flight. ‘The leading edges can reach 600K, so we’re looking at materials which can withstand that sort of heat load, and also the aerodynamic load. We’re also looking at cooling systems,’ said Steelant, who is also directing this project.

But however fast, technically impressive or beautiful the fruits of Lapcat are, a question remains: is this the future of air travel, or a potential white elephant? Do the realities of civil aviation, the economics, and increasing concerns about the environmental costs of flying mean super-fast, super-high passenger aircraft are not needed?

Reaction Engines has calculated the cost of a ticket for a Brussels-Sydney flight. The result, said Varvill, was almost the same as a standard business class fare, but the cost breakdown was different. ‘Existing airline costs are only 20 per cent fuel,’ he said, ‘but for us, it would be over 80 per cent.’ Varvill calculated that a single A2 would cost about €46m (£35m) to buy and €21m a year to maintain, with indirect airline costs of €27m — but the hydrogen fuel, if it were produced by water electrolysis, would cost €460m per year. ‘It’s very dominated by energy prices, and it could stand or fall on what happens to the energy market,’ he said.

Fuel costs could be a major concern, with Concorde once again a worrying precedent. The oil shocks of the 1970s pushed up the cost of kerosene, so most airlines decided it was uneconomic to fuel the craft for its transatlantic flights. But aircraft manufacturers are under increasing pressure to move from hydrocarbon fuel, said Steve McGuire, a lecturer in international business at Bath University and an observer of civil aviation economics. ‘If hydrogen fuelling is the future, then airline economics will change considerably, and that could act in favour of this type of aircraft.’

The environmental impact of the craft is also uncertain. Running on hydrogen, it produces no carbon emissions in flight — but generating the hydrogen could produce emissions. If it were made by steam-reforming hydrocarbons the operating cost of the aircraft would halve, but the CO2 would need to be captured on the ground. Using renewable or nuclear electricity would also reduce the carbon footprint, and Varvill has said the use of hydrogen as aviation fuel might be an argument for expanding nuclear power.

The impact of the plane itself is not clear. ‘We’re flying in the ozone layer, and burning hydrogen produces water vapour,’ said Varvill. ‘We don’t know whether that will catalyse the decomposition of ozone. We think probably not, but we’re going to look at that [in Lapcat 2].’

The DLR team has tested a scramjet with an integrated intake and combustor in its wind tunnel

Steelant is optimistic about demand for hypersonic intercontinental travel. ‘From Schipol, there are two daily flights to Beijing, probably more from Heathrow, Paris and Frankfurt. These are big jets, with many business class seats. And they are all full. I guess if you propose to stop people sitting in a business seat, but say that for the same price I can get you there in less than four hours, you would soon get them on your side.’

Also, he noted, several aerospace companies are now working on small supersonic business jets, with Dassault and Sukhoi among the firms running projects for 6-12 seater aircraft. ‘You can imagine if the plane was bigger, the price would come down.’

McGuire, however, is pessimistic. ‘It’s such a big aircraft, and it’s going to be so expensive, that it would take a major change in the civil aviation paradigm for the airlines to be interested. I don’t think there would be that many takers for that sort of flight, so you’re looking at a small number of aircraft sold. That could easily scupper the whole thing. I love this kind of thing; it would be incredible to fly in it, and I really, really hope they succeed. But my God, talk about an economic challenge.’

This does not deter Steelant and his researchers. The project is still a feasibility study, Steelant stressed. ‘We aren’t seeking aircraft we can put into production, and we can’t come up with a sensible economic evaluation if the technical evaluation isn’t in place.’

There are already possible spin-offs into traditional aviation, such as Reaction’s pre-cooling technology, which Steelant thinks could be implemented on normal turbojets within a decade, and the new materials from ATLLAS.

‘Sometimes it’s very important to look at projects which have a driving goal that is 25 years ahead,’ he said. ‘You have to have revolutions in technology every so often, and these goals encourage revolutionary thinking. But by doing this revolution, you automatically have technology which is directly applicable to current technologies.’