杏吧原创

THE skies above Oshkosh, Wisconsin, are usually as sleepy as the town below, but in August they come alive with the howl of jet engines and whirr of propellers. Oshkosh is home to the world鈥檚 largest annual air show, properly called the EAA AirVenture Oshkosh but affectionately known as 鈥淥shkosh鈥� by the nearly 1 million fans who flock to it every year.

Over the years Oshkosh has seen it all, from elegant second-world-war Mustangs and Spitfires to cutting-edge 21st-century planes such as the stealthy F-22 Raptors. In 2005, it hosted the first and only public appearance of Burt Rutan鈥檚 SpaceShipOne after it had won the X Prize for being the first privately funded spacecraft to make it into space twice within two weeks.

This year鈥檚 show will feature another first: the throaty roar of rocket-powered planes going head-to-head in a race of speed and agility. After nearly three years of rumours, false starts and setbacks, the Rocket Racing League (RRL) will take to the air at . RRL has exhibition races scheduled for 1 and 2 August, and while these will be a mere taster for a full-blown event involving at least six teams (see table), the RRL鈥檚 founders hope the exhibition will light a fire under an idea that marries the other-worldly fascination of space tourism with the visceral excitement of big-time motor sport. The organisers also hope the RRL will boost the private space industry, both in terms of popular appeal and technological progress.

Rocket-powered planes have been around for more than 60 years, though as a commercial proposition they have never really got off the ground. In the second world war, the Germans and the British added rockets to propeller or jet aircraft to give them extra oomph on short runways or to lift heavy payloads. In the early days of the space race, rocket-powered planes clocked up some important aviation milestones.

In 1947, a US air force plane called the Bell X-1 became the first plane to break the sound barrier 鈥� around 1225 kilometres per hour, depending on air temperature and pressure 鈥� while in the 1960s the mighty North American X-15 set a speed record that still stands, 7274 kilometres per hour (Mach 6.7). The X-15 was also the first plane to reach the edge of space and held the altitude record until 2004 when it was beaten by SpaceShipOne 鈥� also rocket-powered.

The rocket planes lined up for Oshkosh can鈥檛 climb as high as the X-15 and with a top speed of around 480 kilometres per hour they won鈥檛 break the sound barrier. But the crowd should still be treated to a thrilling spectacle.

At the start of the race two planes will blast along the runway and take off steeply, engines aflame and roaring, then climb sharply to a height of about 400 metres where they will navigate several laps of an 8-kilometre virtual circuit, roughly 3.2 kilometres long and 800 metres wide. The route will keep the planes within view of the crowd at all times, with several close passes so spectators can see the flames and hear the howl of the engines. The winner will be the pilot who successfully completes the course in the fastest time. (The number of laps hasn鈥檛 been specified yet, though the RRL says it expects the real races to be four laps.)

Each plane will be powered by a single rocket engine capable of 6700 newtons of thrust, giving it similar acceleration to a state-of-the-art fighter plane. The engines will carry enough fuel to burn for about 4 minutes, meaning that the pilots will have to pursue a 鈥渂urn-and-glide鈥� strategy to stay in the air for the 12 minutes or so it will take to complete the course. The initial take-off burn will last about 45 to 60 seconds; after that the duration of the burns will depend on the course and the pilot鈥檚 strategy.

In a real RRL race, pilots will see the course as a series of fly-through hoops on displays inside their helmets. The RRL hasn鈥檛 released the details of the exhibition races, but they won鈥檛 be quite as sophisticated as that. Each plane will have its own track to reduce the risks of a catastrophic collision 鈥� though of course no one can guarantee the contest will be completely safe.

More lift, less drag

The planes themselves are an unusual design. Both employ a 鈥渃anard鈥� configuration in which the horizontal tailplane is mounted at the front of the fuselage rather than at the back. Because it offers greater lift and less drag, this design is particularly well suited to rocket planes and other 鈥減usher鈥� aircraft, where the propulsion system is at the rear.

The planes will be built by Velocity Aircraft of Sebastian, Florida, which was recently bought by RRL for this purpose. Both are off-the-shelf models from Velocity鈥檚 range of propeller-powered planes, one a Velocity SE and the other a Velocity XL, with the propellers replaced by a rocket engine. RRL will eventually choose one of the planes as the prototype for a standard RRL plane, which will be called the Rocket Racer.

For the engine, the RRL initially had a contract with just one company, XCOR Aerospace of Mojave, California, which has been experimenting with rocket planes since the late 1990s. In May, however, the RRL announced a second deal with Armadillo Aerospace of Rockwall, Texas, to design and build a competing engine. The idea is to give the RRL teams a choice of engine much as Formula 1 teams used to be able to choose between Bridgestone and Michelin tyres.

XCOR鈥檚 engine will use a mixture of liquid oxygen (LOX) and kerosene, a departure from the company鈥檚 traditional LOX-alcohol mix. The decision, says XCOR spokesman Douglas Graham, is largely aesthetic: alcohol gives a colourless burn, but the RRL wants to wow spectators with bright, colourful flames. Armadillo is likely to stick with a LOX-alcohol mix, but may seed the flame with sodium or another chemical to produce colour.

A key difference between XCOR and Armadillo鈥檚 engines is control. Most rocket engines have no throttle: they鈥檙e either on or off. XCOR鈥檚 engine will stick with this design, but Armadillo鈥檚 will have a throttle, allowing pilots greater flexibility with their burn-and-glide strategy.

Flying a rocket plane isn鈥檛 for novices. Veteran test pilot Dick Rutan, brother of SpaceShipOne鈥檚 Burt Rutan, is one of only a handful of experienced rocket-plane pilots, whose exploits include a stint in an XCOR powered experimental EZ-Rocket plane at Oshkosh in 2002. He told New 杏吧原创 that the feel of a rocket plane in flight is very different from propeller or jet-powered aircraft. 鈥淪ince the rocket motors consume a copious amount of fuel, the take-off and initial climb gross weight is very heavy,鈥� he says. 鈥淥nce the take-off, initial climb angle and airspeed are obtained the ride gets better and better, steeper and faster. The thrust on your back lets you know you are getting with the programme.鈥�

Another veteran military and test pilot with rocket-plane experience is Rick Searfoss, who has also flown three shuttle missions. He compares rocket-plane performance to that of the T-38, a popular training jet used by the US military. 鈥淚t鈥檚 definitely more challenging than a light aircraft,鈥� he says. 鈥淏ut it鈥檚 less challenging than performing a full-up mission in a fighter-type aircraft.鈥� However, he adds that because the RRL vehicles are still very much prototypes, 鈥渨e haven鈥檛 yet seen anything close to best performance鈥�.

Searfoss has long-standing links with XCOR and has been hired by RRL to fly in the exhibition race, where he will be piloting the Velocity SE with an XCOR engine. The other pilot, Len Fox, will fly the Velocity XL, which is slightly wider and longer than the SE, powered by an Armadillo engine. 鈥淭here鈥檚 not much difference between the aircraft,鈥� says RRL chief executive Granger Whitelaw. 鈥淲e鈥檙e just testing some safety and performance parameters.鈥�

Whitelaw, an entrepreneur who was part of the Indianapolis 500 winning teams of 1996 and 1998, co-founded the RRL in 2005 with X Prize impresario Peter Diamandis. The idea was to combine the pioneering spirit of the X Prize with the more down-to-earth world of motor racing, with its blend of high technology and crowd-pleasing thrills.

One important parallel between the RRL and motor racing, says Whitelaw, is that the competition will stimulate technological advancement. Formula 1, IndyCar and NASCAR all serve as test beds for a variety of technologies, from rubber compounds to valves, brakes and safety harnesses. 鈥淪o many things that go into a race vehicle end up in your car,鈥� says Whitelaw. 鈥淭hat鈥檚 why Indy and other races are heavily supported by manufacturers.鈥�

Much as motor racing teams generate intellectual property in the form of engines, materials and parts, so the RRL will end up owning new aerospace technologies. Velocity Aircraft will design a standard airframe and avionics for all teams in the league, although the teams will be allowed to make 鈥渟mall adjustments鈥�; XCOR and Armadillo will do the same for the engines. In this way the RRL and its affiliates will create intellectual property they can sell or license to the aviation and private space industries.

Big money

Whitelaw also hopes that the RRL will stimulate competition and cooperation in private space-flight, much as the X Prize did. 鈥淚f you get Virgin and Bigelow and Blue Origin [the space company created by Amazon.com founder Jeff Bezos] and others to work together, you鈥檒l have more success,鈥� he says. 鈥淭he league is a place they can do that.鈥�

Potentially, there is big money to be made from ticket sales and merchandise. Whitelaw points out that the two existing plane-racing events, the Reno and Red Bull air races, attract millions of people. 鈥淚t鈥檚 huge,鈥� he says.

Spectators at a real RRL race will see something quite different from the Oshkosh exhibitions. The real league has six teams so far, and races will feature multiple aircraft 鈥� possibly as many as 10 鈥� with several in the air at any one time. Since the races are not head-to-head contests but a set of individual time trials, there is no equivalent of pole position and no obvious race leader. Each race will have several heats and require at least one pit stop for refuelling, says Whitelaw.

He says that one of the more nerve-racking aspects of an RRL race will be the process of landing, refuelling and getting airborne again as quickly as possible. 鈥淭he big events in rocketry are start and shut down,鈥� says Rutan. 鈥淓ach has its own unique set of sequences that must go just right. If not, real bad things can happen.鈥�

It鈥檚 not hard to work out what those real bad things might be: since rocket engines rely on the controlled combustion of highly volatile chemicals, fires and explosions are not infrequent. Last year three people were killed at Burt Rutan鈥檚 Scaled Composites in Mojave, California, when an experimental propellent system for SpaceShipTwo exploded. Dick Rutan recalls one test flight where he had to 鈥済et out and run鈥� from a rocket plane while it was still on the ground 鈥� not an option when you鈥檙e shutting the engine down in flight during a burn-and-glide race. 鈥淵ou are miles high and all alone,鈥� he says. 鈥淭he words 鈥榶ou got a clean shutdown鈥� are most agreeable.鈥�

鈥淵ou are miles high and alone. The words 鈥榶ou got a clean shutdown鈥� are most agreeable鈥�

As an added complication, the rocket planes will land only after they have burned through all their fuel, meaning they鈥檙e essentially gliders performing a 鈥渄eadstick鈥� landing, at the mercy of gravity and what little aerodynamic performance the pilot can coax out of them. 鈥淣o second chance with that,鈥� Rutan says dryly.

On top of the flying challenges, the RRL teams will need smart strategy and effective teamwork. 鈥淚t鈥檚 definitely a thinking man鈥檚 game,鈥� says Searfoss. As in Formula 1 or NASCAR, successful RRL teams will need to employ detailed planning, clever engineering and meticulous operation before and during the race. 鈥淚t ain鈥檛 gonna be just turn-and-burn,鈥� says Searfoss.

Even if all goes well at Oshkosh, Searfoss isn鈥檛 yet convinced that the RRL will be ready to take off in earnest. 鈥淐onceptualising this rocket-plane racing operation is one thing and it鈥檚 a very exciting prospect,鈥� he says. 鈥淏ut actually designing, building, testing and flying these systems is orders of magnitude more difficult than I believe virtually anyone recognises.鈥� The league also has to decide how many races to hold in a season, and where.

Right now, RRL鈥檚 leadership are happy just to be getting the exhibition races off the ground. After the league formed in October 2005, several teams joined up only to drop out or fold. The first exhibition race was lined up for the 2006 Wirefly X Prize Cup, but didn鈥檛 happen. Along with Oshkosh though, the RRL is scheduled to fly three more exhibition races before the end of this year, and the first full RRL race could happen as early as next year.

鈥淚 really want to get through one or two exhibition races, to see how the tech looks and feels, how fans react,鈥� says Whitelaw. 鈥淭here鈥檚 a lot to consider. We have to produce multiple planes, train multiple teams, make sure the Federal Aviation Authority is comfortable with everything we鈥檙e doing. I just think you鈥檝e got to do things right.鈥� Get it right, though, and the NASCAR of the skies can start dreaming of soaraway success.

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