Scramjets promise space travel for all
* 22 July 2009 by Greg Klerkx
* Magazine issue 2718. Subscribe and get 4 free issues.
* For similar stories, visit the Galleries and Spaceflight Topic Guides
Gallery: Spaceplanes and scramjets: A 50-year history
ON A bright autumn morning five years ago, the space-flight community was turned on its head by a little teardrop-shaped spacecraft built in a small workshop in California's Mojave desert. The successful flight of SpaceShipOne on 29 September 2004, the first of two flights en route to winning the $10 million Ansari X prize, seemed to usher in a new era of space travel - one in which space flight would be affordable, frequent and, perhaps most importantly, accessible to all.
SpaceShipOne was the first crewed spacecraft to be developed privately. Designed, built and flown on a budget of roughly $25 million, it was much cheaper than the multibillion-dollar US government-backed space shuttle. In its climb to just over 111 kilometres above the Earth, SpaceShipOne broke the world altitude record for a winged vehicle, set more than 40 years earlier by NASA's
X-15 rocket plane. It was also fully reusable, a feature long seen as an essential milestone on the path to a more accessible spacefaring future.
And yet, five years on, it is easy to regard SpaceShipOne as more anomaly than herald. After making two sub-orbital flights in two weeks, it never flew again: the craft now hangs in the Smithsonian National Air and Space Museum in Washington DC. The Spaceship Company, a partnership between SpaceShipOne creator Burt Rutan and airline tycoon Richard Branson has yet to unveil the larger, passenger-ready SpaceShipTwo, although the company has revealed the carrier aircraft needed to launch it on its way to space. Most other commercial space-flight projects remain on the ground.
"I think Burt Rutan did a great thing with SpaceShipOne," says Elon Musk, CEO and chief designer at commercial space company SpaceX.
"However, it is important to appreciate that it is only a Mach 3 [three times the speed of sound] terminal velocity vehicle. You need Mach 25 to reach low Earth orbit, and the energy required scales with the square of the velocity."
Whatever its limitations as a spacecraft, SpaceShipOne has galvanised attempts to break the "space access" problem. There are arguably more spacecraft development efforts under way now than at any point in the brief history of space flight. So which idea, or set of ideas, will produce the breakthrough vehicle? "To achieve a true revolution in cost and reliability, we have to make a truly reusable system," Musk says. "That's one of the biggest technical challenges known to man."
This challenge is gradually yielding to human ingenuity. SpaceX has successfully flown its Falcon rocket, after several aborted attempts, and other companies are doing ever more advanced tests on new engines, systems and designs. And one long-awaited test flight later this year may herald a major technological breakthrough in air-breathing engines that could power a winged vehicle from runway to orbit, ultimately fulfilling the dream that SpaceShipOne has rekindled.
Space vehicles can be broadly divided into two categories: those inspired by winged aircraft and those inspired by ballistic rockets.
In the early days of the space race, winged and ballistic craft were both considered to be viable options for reaching orbit. Yet they represent vastly different ideas about space travel, in terms of both engineering and economics. Ballistic spacecraft simply pile in the fuel and use brute force to push their way into space, shedding engines and fuel tanks on their way up to lighten the load.
Winged spacecraft are the more elegant option. Launching from the ground or from the back or belly of another aircraft, they use the Earth's atmosphere for lift as long as possible. On the way back, they glide down to Earth to be used again and again. Their potential reusability has led to the tantalising idea that winged spacecraft could, in time, be much cheaper to operate than ballistic throwaways.
They could even use the same facilities as commercial airliners, opening up space travel to commerce and even tourism.
In reality, winged spacecraft like SpaceShipOne and NASA's X-15 - which reached an altitude of 107 kilometres - have never made it past the lower reaches of space. Their on-board rocket engines lacked the oomph to propel them the extra 60 kilometres or so needed to reach orbit. The conspicuous exception to this rule is the space shuttle, a vehicle that was part winged spacecraft and part ballistic vehicle.
The shuttle isn't completely reusable, however: on each flight the expensive external fuel tank is dumped in the ocean. Yet it showcases the most important technologies for low-cost, reliable access to space, says Daniel Rasky, a NASA scientist who has developed flight hardware for several of the agency's spacecraft. "The key is reusability, combined with flying often," he says. "Where the shuttle falls down is the 'flying often' part."
Originally designed to fly hundreds of times annually and therefore reduce the cost of each flight, the shuttle fleet has never managed more than nine flights in a year. Lessons from the shuttle are being incorporated into NASA's Constellation Program, the project to design the shuttle's replacement, with the ultimate aim of sending people back to the moon. But while some elements of Constellation's spacecraft are designed to be reusable, Rasky believes that the design is a step back towards the Apollo era. "It can probably be made reliable enough, but it will never be low-cost because it will never be flown enough."
This is where commercial space companies come in. A new NASA initiative led by Rasky, called Space Portal, is forging partnerships between the agency and new space companies, with a focus on breaking the cheap-space-flight barrier. In the past year, for example, Space Portal has helped SpaceX develop high-performance heat shielding by, amongst other things, providing the company with the means to manufacture the material in-house.
Advanced technology is also breathing fresh life into winged spacecraft. The buzz centres on a hypersonic engine known as a supersonic combustion ramjet, or scramjet. Despite the name, scramjets are very different to the turbojet engines that power commercial aircraft, not least because scramjet-powered vehicles must first be accelerated to Mach 4 or so using jet engines or rockets before their scramjets can work. This is because, unlike turbojets, scramjets do not use spinning blades to compress the air entering the engine.
Instead, the high speed of the vehicle compresses the incoming air, which is then fed into a combustion chamber where the burning fuel creates an exhaust jetthat exits the engine faster than the air that entered (see diagram).
Advanced technology is beginning to breathe fresh life into winged spacecraft
Scramjets only work when travelling at high speeds - and getting them there requires complicated technology and lots of fuel. This has led some critics to liken scramjets to nuclear fusion reactors: a great idea but technically impractical. The first tests, in the 1990s, weren't promising - the scramjets required far more energy than they produced. Overcoming these problems is expensive, which until recently ruled out the scramjet for anything other than military applications.
Yet the first successful scramjet flight was delivered on a relatively modest budget of $1.1 million. It took place in 2002, when a joint British and Australian team flew their HyShot vehicle at the Woomera range in South Australia. To keep costs down, the scramjet was mounted on the nose of a conventional rocket. HyShot reached Mach 7.6, about 9000 kilometres per hour, for several seconds before breaking up.
Since then, scramjet development and testing has accelerated markedly, culminating in 2004 with NASA's X-43A scramjet aircraft, which flew at Mach 9.68 - a new world record for jet-powered travel. In the last few years, news of other scramjet research has trickled out, including programmes in Japan, Russia and China, as well as an Indian plan to develop a hypersonic spaceplane called Avatar.
Later this year, a consortium that includes NASA, the US air force and the Defense Advanced Research Projects Agency (DARPA) plans to fly a more advanced scramjet vehicle, the X-51A, over the Pacific Ocean. The programme's aim is to demonstrate that the small prototype engine can be scaled up. Designed to operate between Mach 4 and 6, the X-51A will also test a range of new materials that could, if successful, allow scramjets to operate for longer than the mere seconds that previous test vehicles lasted.
Critically, scramjet programmes like the X-51A are working towards reusability. This has proven elusive so far: all scramjet test vehicles have been intentionally destroyed after their brief period of use, largely because they burn out. The obstacles to greater scramjet robustness are many. Top of the list is keeping air moving through the engine at sufficient speeds for combustion, as well as preventing the engine from melting. Even with these problems, NASA is sufficiently encouraged by recent progress to have established several national science centres earlier this year. These will explore, and eventually exploit, scramjet technology.
Results from the X-51A programme may help another style of engine being designed by DARPA. Called Vulcan, it will be capable of launching a craft from a runway like a commercial airliner before accelerating it to hypersonic speeds. To do this, Vulcan will use a modified turbojet engine to accelerate to around Mach 2, at which point its hypersonic engine - possibly based on a pulse detonation design first used in Germany's V-1 flying bomb during the second world war - would kick in to push it beyond Mach 4. Both units would be integrated into an airframe similar to the X-51A. DARPA hopes to produce a test vehicle by 2012. Though it is initially aimed at military applications, the organisation suggests that future versions of the vehicle could allow for runway-based access to space.
So will these scramjets reach orbit unaided? Estimates for their top speed range from Mach 12 to Mach 20 - still short of the Mach 25 or so needed to propel them into orbit. To get round this problem, rockets will have to play a role on a scramjet-powered spaceplane, either as initial accelerants or final-stage boosters. This shouldn't affect their reusability too much, though, as most of the space vehicles'
power will come from the scramjet engine, making them far more reusable than the space shuttle. And because they are designed to draw oxidiser from very thin air, they would be much lighter than conventional ballistic vehicles. On the space shuttle, for instance, oxidiser accounts for about 85 per cent of the contents of the external fuel tanks.
Yet the debate over hypersonic-powered spacecraft is as contentious as that over winged versus ballistic vehicles. Critics of the hypersonic approach say that even if the technology does become viable, it is likely to remain in the military-industrial realm for some time to come, mainly due to expense. "Obviously, there are military applications for fast transport of small teams to a critical area,"
says Derek Webber of Spaceport Associates, which has conducted a market analysis for future space activities. The civilian equivalent, a hypersonic airliner, could take super-rich travellers right round the globe in just 4 hours.
Some engineers believe there is a third way forward: a relatively inexpensive, reusable winged spacecraft that does not rely on experimental technology such as scramjets or throwaway ballistic systems. British company Reaction Engines falls into this category:
its proposed Skylon vehicle would use an advanced air-breathing engine called Sabre to reach orbit.
At high speed, air entering an engine is compressed so fast that it heats up rapidly - that is why scramjets must be built using special, hefty heat-resistant alloys. To combat this, Skylon's jet engine would use a heat exchanger to cool incoming air from around 1000°C to less than -100 °C. The cooled air is then mixed with liquid hydrogen and burned. As a result, the Sabre will be lighter and, unlike a scramjet, it should work from launch up to a speed of Mach 5.5. Then, at an altitude of 26 kilometres, the engine would switch to normal rocket power and use on-board oxygen and hydrogen to propel the plane into space.
The Skylon is a return to the classic spaceplane dream. Launching under its own power, the unpiloted Skylon would fly into orbit, deliver its payload, and land on a runway. "Making launchers operate like civil aircraft is the route to unblocking the metaphoric dam,"
said Mark Hempsell, future programmes director for Reaction Engines.
The Skylon is a return to the classic spaceplane dream: launching under its own power, flying into orbit and landing on a runway
Skylon and its Sabre engine are new versions of work done by British Aerospace and Rolls-Royce in the 1980s, thereby continuing the trend in spacecraft development of building upon previously developed systems, rather than starting from scratch. Crucially, Reaction Engines does not propose to manufacture the Skylon nor the Sabre, but rather to contract those activities to aerospace firms with the facilities and experience to do so.
As the strategy of Reaction Engines suggests, smaller companies are recognising that they need the big players - and the results of their heavily funded research - to have any chance of achieving a true breakthrough. Likewise, Rasky and others at NASA now view space entrepreneurs as partners, not competitors. Another Space Portal programme, for example, would send science experiments on sub-orbital vehicles produced by companies such as Virgin Galactic, Mojave-based XCOR Aerospace, and Blue Origin in Kent, Washington. NASA would pay for the flights like any commercial customer, giving these companies a market for their services. And NASA is already funding a number of entrepreneurs, including SpaceX, through its Commercial Orbital Transport Services programme, which seeks to find cheaper, more reliable orbital space transport.
"Today's emerging commercial space offerings stand a much better chance of success because they're attacking the real issues," Rasky says, including purely commercial challenges such as focusing on expanding a customer base. "New and advanced technology will play an important role, but it will be evolutionary, similar to how technology impacts other forms of transportation, like cars and airplanes."
In this light, the success of SpaceShipOne created one undeniable result. As Hempsell points out: "It took away the giggle factor." In the short and often fractious history of space flight, that's one giant step towards a different, and perhaps more exciting, spacefaring future.