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Skylon - revolutionary successor to the Shuttle?

Paul Sutherland, Feature writer
Jul 17, 2011, 23:00 UTC

Sen—With the retirement of the Space Shuttle after 30 years service, private companies and NASA are exploring new ways to carry man into orbit. For the forseable future astronauts will be hitching rides to the final frontier on Russian Soyuz spacecraft.

Most concepts for a Shuttle replacement appear to be reinventions of previous rocket technologies. Indeed, NASA’s own latest capsule closely resembles designs from the Apollo programme that first flew more than 40 years ago. They are tried and tested techniques but they are also enormously expensive.

However, British scientists at Reaction Engines Limited (REL) are developing an exciting alternative that will offer a brand new way to reach space. Called Skylon, it has the potential truly to revolutionise spaceflight and make it almost routine.

Skylon spaceplanes will operate more like aircraft than traditional rockets, taking off from and landing on familiar runways and being turned round rapidly after each mission to be ready for the next. They promise to slash the cost of getting cargo - or astronauts - into space.

Skylon resembles a sleek, futuristic jet with 82-meter long fuselage made from carbon-fiber reinforced plastic with a black ceramic skin to protect against the heat of re-entry. It needs no external rocket tanks such as the Shuttle has. After an automated take-off, it will fly directly into that region of space called low Earth orbit (LEO) where it will be able to dock with the International Space Station, launch satellites or even help construct interplanetary spacecraft.

Different cargo modules are envisaged, including one that could carry up to 24 passengers, offering another option for space tourists to experience space or stay in orbiting hotels, as well as for taking working astronauts to space stations.

Mission complete, the pilotless craft will then return into the atmosphere to land like the Shuttle does on a runway where it may be prepared for its next flight. Amazingly, its designers say this turnaround can be achieved in just hours because Skylon will be 100 per cent reusable.

Contrast that to the Shuttle which was only partly reusable and which, once landed, had to return to its Vehicle Assembly Building in Florida for a cumbersome process where various bits had to be bolted back on for its next launch months ahead.

Skylon will be able to carry 15 tonnes of cargo at a time into orbit in one of its various payload modules. That is less than the 24-tonne cargo capacity of the Shuttle into low Earth orbit. However, the average cost of a shuttle launch was $450 million, according to NASA. Skylon is expected to cost just $10 million per launch.

NASA's more costly approach has been because the conventional view has been that you needed to carry vast quantities of fuel with you in order to escape Earth's gravity and reach orbit and that entailed using multiple rocket stages. So how is it that Skylon promises to do the same without all this extra baggage?

The answer lies in its two completely new Sabre air-breathing engines, a remarkable hybrid breakthrough in design that will allow it to draw part of its fuel from the atmosphere as it flies towards space before switching to rocket mode. Sabre—it stands for Synergetic Air-Breathing Rocket Engine—is the brainchild of REL's chief engineer and managing director Alan Bond and this single-stage to orbit concept is no pie in the sky. The European Space Agency spent months conducting a thorough study of the principle by which Sabre will work and concluded in May, in a report for the UK Space Agency, that it has the potential to revolutionise rocket propulsion.

Bond and his team worked previously for Rolls-Royce and British Aerospace and Skylon is the direct descendant of a hypersonic aircraft called Hotol that they were designing with £2 million of UK government backing but which was cancelled in 1988. Wounded by the affair, Bond set up Reaction Engines and has kept the company private to avoid government from dealing Skylon a similar fate. In parallel to their work on Skylon, the team are also developing a revolutionary new aircraft called Lapcat that could whisk 300 passengers from London to Sydney in under four hours.

Most of Skylon's fuselage will contain large tanks of liquid hydrogen plus a smaller amount of liquid oxygen. On take-off, Skylon's Sabre engines will be powered by its on-board hydrogen mixed with oxygen from 400 kg per second of air drawn in from the atmosphere. This fuel mix will boost the craft to Mach 5.5, more than five times the speed of sound, and a height of 30km where the air becomes too thin to provide enough fuel. The engines will then switch to feed oxygen from Skylon's tanks to accelerate to orbital speed to take the ship out into space.

Drawing in oxygen will mean it has to carry 250 tonnes less fuel and so need to be just a little more than half the weight otherwise required at 275 tonnes when it takes off fully laden. But REL had to overcome major challenges to develop technology for their unique new thrusters.

As Skylon accelerated to speeds above 5,000 kph, air flowing in will reach a searing 1,000 C, far too hot for the engines to handle. Bond's team have developed a system that, in a flash, cools the air to around -120C by sending it flowing through thousands of tubes chilled indirectly by the liquid hydrogen on board, but at the same time uses so-called frost control techniques to avoid the engine icing up.

To fly fully laden with fuel and cargo into orbit it will need a new 3.5-mile long, strengthened runway. But with fuel used up, it could land at any major airport and take-off from them too for shorter hops which require less fuel aboard in the first place.

REL based at Culham Science Centre in Oxfordshire, have already received a million-euro grant from the European Space Agency towards work on their concept craft. It is estimated that it will cost $12 billion to complete development - about the same as for an Airbus jet or European Ariane rocket - which means companies with deep pockets need to come forward to support the project. But the $10 million cost of a Skylon launch into space will save a fortune on the current $160 million for a typical conventional rocket launch.

And once the production lie is rolling, REL estimate that there will be worldwide demand for up to 90 of their pilotless Skylons, at a sale price of around $1,200 million each.

Critics say that Skylon will not be able to reach the higher orbits where geostationary satellites for communications and TV need to be placed. To counter this, REL are proposing a separate transfer vehicle called Fluyt that could ferry satellites from low Earth orbit to greater heights.

REL expect it will take another ten years of development before their Skylon spaceplanes are flying in 2020. In an exclusive interview for Sen, Alan Bond told us: "What ESA have said is that assuming we get successful demonstration of the engine they don't see any impediment for going forward.

"That's quite important because that's actually saying that single stage to orbit, which is our ambition, is feasible. The world view at the moment is that single stage to orbit isn't feasible."

"We're already making the heat-exchanger modules on a regular basis. What we are about to do is put a pre-cooler made with 21 of these modules, in front of a Viper test engine. That will allow us to drag air through to prove the frost control works properly, look at the air distribution, show that it's stable, things like that. We should have completed all that by the end of this year.

"Next year we'll start the next phase, including demonstration of the engine technology. We will build a small engine built to Sabre's thermodynamic principles that will run completely autonomously as a demonstration that there's no black magic involved. It will be a working test of the principle.

"We're also looking at building a flight-test vehicle - we'll be flying about ten of those - to demonstrate the aerodynamics in operation of the engines' housings, known as nacelles. People think these are just to hold the engine to the end of a wing but the reality is that those nacelles carry out an amazing array of functions during the ascent and reentry and so they have to work."

Bond added: "A full engine will probably be built towards the end of 2016. The pre-production prototypes of Skylon will fly late 2016 or early 2017, and production Skylons in 2019 and vehicles in the shops for Christmas in 2020. That's the sort of timescale we're working to."

Breakout: Project Troy

The REL team have devised a concept which they call Project Troy for a manned mission to Mars to demonstrate how useful Skylon can be as a launch vehicle.

The spaceplane would deliver parts into orbit for the construction of two fleets of spacecraft to fly to the red planet. The first precursor mission would send three ships to deliver rovers, habitats, power units and other equipment to three sites on the martian surface.

Two years later, three crewed ships, each with six astronauts, would depart Earth orbit to land on Mars at the pre-prepared landing sites, staying to explore the planet for more than a year.

All the components needed to construct the ships would be carried into orbit using Skylon. Their crew would also be transferred to and from the Troy ships in Skylon's passenger module.

The landing sites would be selected to allow the astronauts to explore up to 90 per cent of Mars in their rover vehicles over a 14-month stay.