Last October, SpaceShipOne, developed by Burt Rutan’s company Scaled Composites, secured the coveted $10m X-prize (offered to the first privately funded craft to make repeated trips into space) and, with a subsequent tie-up with Richard Branson’s Virgin, sowed the first seeds of the space tourism industry.
Next month, Scaled Composites and Virgin are looking to break new ground again, when the Global Flyer aircraft takes off from Salina Municipal airport, Kansas, in a solo attempt to circumnavigate the globe without refuelling.
Its developers claim that if the flight is a success, the technology used on the aircraft could find its way onto unmanned aerial vehicles used to carry out atmospheric research and monitoring, terrain mapping, telecommunications and military reconnaissance work.
A circumnavigation of the globe has been achieved before â€” in 1986, by Burt Rutan’s brother Dick and co-pilot Jeana Yeager, in another Scaled Composites creation called Voyager. But the Global Flyer is powered by a jet engine, rather than the piston engine used last time.
The Global Flyer â€” which has a fully laden weight of 10,000kg, a 114ft wingspan and is powered by a single Williams FJ44-3 ATW jet engine â€” will climb to 45,000ft, where it will circle the globe at 285mph.
Following the jet stream across the Atlantic from Kansas to the UK, the aircraft will fly south over London and Paris towards the Mediterranean, cross Egypt, India, China and Japan, then the Pacific, before making the final stretch back to Kansas. The journey is estimated to take 80 hours.
So how did Scaled Composites manage to design an aircraft strong enough to carry four times its own weight in fuel, yet light enough for rapid flight?
The key challenge, according to Jon Karkow, Global Flyer’s chief test pilot and project engineer, was to get the fuel fraction (the fuel weight divided by aircraft take-off weight) as high as possible, to give the craft the best possible range.
With their previous attempt, Voyager’s more efficient piston engine produced a fuel fraction of 72 per cent; the Global Flyer’s jet engine produces a significantly higher fraction of 82 per cent. But, as Karkow explained, ‘the jet will allow us to complete the world flight in less than a third of the time Voyager took’.
The company has drawn on its composite materials expertise to reduce the fuel fraction and produce an aircraft that retains its strength, while being light enough to carry fuel for the 80-hour flight.
‘Structural strength to weight was the most important factor to success,’ said Karkow. ‘Aircraft need to be a certain strength regardless of what they are made of. So the only real way to reduce structural weight is to use higher performance materials.’ The Global Flyer is composed of sandwiches consisting of skins of carbon fibre and epoxy and a core of honeycomb.
However, strength and weight are not the only criteria for success; an efficient aerodynamic design is also imperative to ensure Global Flyer has the best chance of completing the challenge.
‘The main structural components were analysed using computer finite element modelling (FEM) methods,’ said Karkow. ‘FEM allowed changes to be made quickly, analysed and then rerun to get optimum structural weight.’ Once completed, the aerodynamics were analysed and optimised using computational fluid dynamics.
Karkow said the result was a body shape so aerodynamically efficient that two drag parachutes must be deployed to give the aircraft any chance of landing.
In one respect, said Karkow, designing the aircraft was easy, as it has just one purpose â€” to fly around the world without stopping. Its designers have not had to make the compromises which lie at the heart of most commercial aircraft projects.
But it would be foolish to suggest that the project has not been challenging. ‘There were still a zillion different areas of optimisation,’ said Karkow, ‘categorising these and determining which were the most beneficial was one of the most important parts of the process.’
The parts of the aircraft that needed the most development in the test phase were the fuel and environmental control systems. ‘We have a very simple fuel pressurisation system that is light, but also needed a lot of fiddling to get it to function properly,’ Karkow said. ‘And keeping the cabin at a comfortable temperature has been problematic.’
With 13 tanks on the aircraft, getting the fuel to reliably feed the engine was also a challenge. This required extra development work, during which many of the solenoid fuel valves were replaced with motor ball valves, a fuel-air separator was added to the vent system, and the pressure relief provisions in the header tank were reworked.
To ensure that the craft’s centre of gravity remains the same throughout the flight, fuel has to be burnt evenly from all the tanks. This is accomplished by an automatic system that cycles through the tanks at even weight increments. Should this system fail, the fuel can be manually selected so the tanks deplete evenly.
Another challenge, with the cabin so close to the engine, was reducing the noise. Here, as in every other area of the aircraft’s design, Karkow was particularly mindful of the weight issue, claiming that ‘nothing goes aboard without intense scrutiny’.
Use of vinyl and foam sound barriers and absorbers reduced the noise level to 95dBA, which is typical for a light aircraft, if noisy by commercial aircraft standards.
Virgin’s PR people set the original launch date as April 2004, so why the delay? Karkow said the April date was over optimistic, and failed to take into account the fact that the most favourable tailwinds exist between January and March. However, he added that the project has proceeded ‘surprisingly smoothly for such a unique vehicle’.
Should Global Flyer’s global circumnavigation succeed next month, Karkow hopes to move on to focus on Scaled Composites’ other big venture â€” commercial space travel. ‘We are looking forward to working with Virgin Galactic so that we can make space accessible to more people. I hope to be a part of that,’ he said.