High stakes

Hypersonic craft powered by scramjet promise low-cost satellite launches. Helen Knight reports on the technology.

As anyone who has travelled to Australia will know, long-haul flights are about as much fun as having your teeth pulled out. Crammed into an uncomfortable seat for hours, with nowhere to go but a bathroom suitable only for contortionists, and with just a bad film and the fear of deep vein thrombosis to keep you occupied, many people would be forgiven for wishing they had stayed at home.

Until now the only hope for reducing long-haul flight times has been Boeing’s not-quite-super-Sonic Cruiser, which the company claims will be 15-20 per cent faster than existing aircraft. But recently Boeing has publicly cast doubt on whether the plane will ever be built.Meanwhile, an international race is developing to build hypersonic air and spacecraft capable of flying at speeds many times faster than the world’s only supersonic passenger aircraft, Concorde.

Researchers in the US, UK, Australia, Japan, Russia, France, China and Germany are hoping to develop hypersonic aircraft and space vehicles, or at the very least to improve their understanding of the scramjet (supersonic combustion ramjet) engine technology which will be used to power them. There are already some big claims being made for scramjets – not least the prospect of two-hour flights between London and Sydney.

The air-breathing engines are also expected to dramatically reduce the cost of launching small payloads such as communications satellites into space, with the added benefit of not having to carry most of their propellant, as they use oxygen from the atmosphere. But how soon can we expect to see these vehicles, and is the technology all it is claimed to be? We are still a long way away from scramjet engines being used in aircraft, says Dr Terry Cain, Qinetiq project leader for the technology. ‘The basics of flying fast — such as the aerodynamics, what the propulsion system should look like, and what the materials should be – we know. But until we put them together we have not even started to uncover the problems.’

The Qinetiq hypersonics team – actually two specialists working in what looks like a bunker on the aptly named Rocket Road, with support from other departments within the company – is aiming to advance the understanding and development of scramjet engine technology. The company is a partner in Queensland University’s HyShot programme, led by Prof Allan Paull, which this summer successfully launched a Terrier Orion Mk 70 rocket containing a scramjet payload at the Australian Department of Defence’s Woomera range, 500km north of Adelaide. The 10-minute test flight, only the last few seconds of which contained the actual experiment, was designed to take the scramjet engine to Mach 7.6.

Qinetiq joined the project to test its own engine in flight, and collect some valuable data. ‘Our HyShot engine is a very simple scramjet combustor, with a simple intake that allows us to test supersonic conditions in our wind tunnel and in flight, and compare the differences,’ says Cain.

Unfortunately, Qinetiq was unable to launch this engine during the experiment, as the company had struck a deal with Queensland University’s hypersonics team that if a first rocket carrying the university’s engine was launched successfully the second launch would be available to test the Qinetiq engine. As it turned out the first rocket crashed, so the second launch on 30 July was again used to test the university’s second engine.

But Qinetiq still hopes to launch its engine at Woomera as soon as an opportunity arises, either next year or in 2004. This launch may be part of a joint project with Japanese hypersonic researchers, as the cost of paying Queensland University to build the payload and US specialist Astrotech Space Operations to provide the boosters means it would simply be too expensive to launch their engine alone.

Keeping costs to a minimum is the name of the game as far as the Qinetiq and Queensland teams are concerned. To do this the researchers have decided not to start with what they believe will be the end product, such as a missile or hypersonic aircraft, but with the basics, and from there to try to understand what they will be capable of achieving with the technology, says Cain. ‘In this way we don’t have to start with the mass [of the aircraft] we have got to carry. We can be working very small and learn nearly all the same lessons, and do it for relatively peanuts compared to what you would have to pay if you went straight to the application.’

To build the simple scramjet combustor for the HyShot project the Qinetiq team had to make some hefty design compromises, such as the use of stainless steel to build the engine, which could never be used in practice as it melts after just four seconds of use. So alongside this ‘short-burst’ project the team is also trying to address the problem of flying for more than these few seconds at hypersonic speeds, as part of its Sustained Hypersonic Flight Experiment, or Shyfe. The team expects to produce a prototype in 18 months, and is already conducting wind tunnel tests on the individual subsystems, such as the intake and combustor. The engine will be built from C/SiC, a composite of carbon fibres within a matrix of the ceramic silicon carbide. Once completed, the vehicle will be launched in a similar way to the HyShot experiment, with a Terrier/Orion rocket. But rather than being sent into space before being turned and diving back towards earth at Mach 7.6, as with the earlier project, Shyfe will be transported to an altitude of 15km by Orion, climbing at 20º to the horizon, where it will then accelerate away and climb to 32km. By the time it reaches this altitude it will be travelling parallel to the ground at Mach 6, and will cruise at that speed for around 300km.

The Shyfe project should provide some much-needed answers on the future of the technology, and these will be needed if scramjet-powered aircraft are to become a reality, never mind commercial hypersonic flight, says Cain. ‘One of the problems is that if someone asked what the take-off mass and payload fractions of a hypersonic aircraft would be I couldn’t tell them, and I don’t believe anyone else could. So until you have answers to these questions you can’t even begin the economic arguments.’But even when these questions have been answered Cain is far from convinced that commercial hypersonic aircraft will ever be built. ‘I’m not confident that the answers will turn out positively for a commercial aircraft flying hypersonically. If you want to put a guess on it, I think it’s unlikely. But it is a guess… because we haven’t even done the basic work yet.’

So if the much-heralded two-hour commercial flights between Europe and Australia are unlikely, at least in the near future, what are the potential early applications of this technology?

Cain believes the military will be the first to exploit scramjets, in the shape of a relatively small, unmanned vehicle. Although he will not be drawn further on what form this could take, it seems likely that one of its first uses could be to power missiles. The Defence Advanced Research Projects Agency (Darpa) and the US Navy are planning to launch a prototype hypersonic missile in 2004, as part of the four-year HyFly project. The missile, which would travel at Mach 6, would be carried by surface ships, submarines and aircraft, initially to combat highly mobile, time-sensitive surface targets such as Scud launchers. Eventually, says the US Navy, it could also be used against buried and heavily defended targets.

As well as high-speed military and possibly commercial aircraft and satellite launchers, scramjet engines are also likely to be used to build air-breathing spaceplanes. NASA’s Langley Research Centre, another participant in the HyShot project, is hoping to gather data from the Queensland flight tests to use in its Hyper-X programme, in which it is designing a scramjet-propelled spaceplane, says Charles McClinton, technology manager of the project. ‘We have been working with the Australians for quite some time, and have been using their wind tunnel testing facilities since the early 1980s. It has allowed us to get some completely supersonic data.’

Like Qinetiq’s Shyfe project, NASA’s own wind tunnel testing has concentrated on speeds below Mach 7. NASA researchers are also developing an entire vehicle powered by a scramjet engine, rather than testing the engine alone as the HyShot team has done. NASA is planning a Mach 7 test flight next year, and a Mach 10 flight in 2004, says McClinton. ‘We have finished all the wind tunnel testing from Mach 4.5 all the way to Mach 15, completed the data analysis and predicted the flight performance for the two tests, and we are now just waiting for them to occur. We hope to have the technology demonstrated from 0 to Mach 7 in 10 years.’

Air-breathing spaceplanes will use a scramjet to reach an altitude of around 50km, when the atmosphere becomes too thin for an air-breathing engine, and a rocket would take over to boost the craft into orbit. Such spaceplanes offer greater safety than shuttles, as the small rockets or turbojet engines needed to initially launch the craft into the atmosphere can be made safer without affecting the overall system. In contrast, modifying the much larger rockets needed to propel a space shuttle into orbit to make them safer dramatically increases their weight. says McClinton: ‘They design a rocket to within a millimetre of its life: it just barely holds together, and if it fails it does so catastrophically. But an air-breather can be made much safer, and if we are going to carry passengers into space it is going to have to be with an air-breathing system.’

He adds that by 2015 NASA hopes to be ready to start developing its spaceplane, and in just 10 years work on a commercial high-speed aircraft could begin, although at present NASA’s only interest is in space flight. ‘NASA has quit focusing on atmospheric flying. Our commercial people don’t seem to be interested in high-speed flight at the moment, but it could be a spin-off.’

The one remaining question mark over the future of scramjet engines at NASA is over funding. The last really high-speed aircraft built, the SR-71, cost over $10bn (£6bn) to develop, and at present NASA is not spending that amount of money on air-breathing engines, but investing it in rockets.

So suddenly the softly-softly approach of the UK and Australian teams looks quite attractive. Going all-out from the starting pistol on the basis of trying to build a spaceplane, as NASA has done, might get it to the finish line a little earlier. But by putting thoughts of potential applications on hold, starting small and working their way up, the UK and Australian teams are hoping to get to the end of the race without spending billions en route.

Sidebar: the thrust of the theory

Hypersonic aircraft are expected to fly at much higher speeds than conventional jet aircraft, even Concorde, while scramjet-powered spaceplanes will be much safer, smaller and less expensive than rocket-powered space shuttles and launchers.

Scramjet engines consist of an inlet, combustor and nozzle. Unlike a conventional jet they have no compressors or turbines. Air is compressed by the inlet, which is similar to a funnel, and passes into the combustor where fuel is injected in from the wall, mixing with the air and burning within 0.002 seconds. This combustion mix expands out of the engine through the nozzle, to produce thrust.

Scramjet engines only work when the aircraft is moving at speeds high enough to produce an extremely fast flow of air through the system, from around Mach 3. So spaceplanes and hypersonic aircraft are also likely to require low-speed systems to get them off the ground. But Qinetiq is hoping its Shyfe programme will lead to an air-breathing aircraft that can take off and accelerate with very small rockets or jets, reducing its weight.