Scramjets promise space travel for all
* 22 July 2009 by Greg Klerkx
* Magazine issue 2718. Subscribe and get 4 free issues.
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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. Continue reading