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Skylon is a serious and plausible design for a new class of aeroplane- an aeroplane that would be able to fly into orbit, and return, completely intact. Skylon is a clever near-term design for doing this.

Historically, getting into space has required something called 'staging'. Staging is when empty parts of a rocket are dropped off during the flight to save weight- so that the rocket and its payload can go on to make orbit- otherwise the rocket would be too heavy.

This staging causes a number of problems, it turns out that it is difficult and expensive to recover and reuse and reassemble the parts. Some relatively practical looking designs that stage exist, but at the end of the day, it pushes up costs and makes spaceflight much more expensive (see costs of launching to orbit to see why it isn't inherently expensive).

Clearly if it was possible to make orbit without staging then it would be much better.

The Skylon design promises to takeoff, from a specially strengthened runway, fly into space, and then reenter, and land back on the runway like a conventional aeroplane, without dropping off anything except possibly passengers; and burning some fuel.

The main secret to the design is the engine design, called SABRE. The engines are designed by the top British rocket scientist Alan Bond; and are capable of sucking air in and running like a jet engine at up to mach 6, and then closing off the air inlet and running as highly efficient rockets up to orbital speed.

This is actually an incredibly difficult trick- previous engines proposed by other designers have been good jet engines but lousy rockets; this engine is a fantastic rocket engine, as well as being a good jet engine up to mach 5.5. Mach 5.5 is faster than previous jet engines have managed, (for example the SR-71 maxed out at mach 3.2.) The problem has been that the air coming into the engine heats up as it is compressed into the engine, and the engine ends up melting.

The SABRE engine avoids this by using liquid hydrogen as fuel, and as the air enters the engine it cools the air using some of the hydrogen, the air is then burnt much like a conventional jet. (At high speed, beyond mach 6, the air would still end up too hot- so the inlet closes and the engine instead turns to burning the hydrogen with liquid oxygen like a rocket.)

Because the engine uses the atmosphere as reaction mass at low altitude, it doesn't need nearly as much propellent at takeoff; it burns about 5x less fuel there; and so takes off with much less. This, in turn, means that it doesn't need as much thrust, and it allows using conventional wings at takeoff. Using wings also reduces the fuel use, as the wings efficiently hold the vehicle up- conventional rockets use pure rocket power for that until they are near orbital speed.

And all this makes all the difference- a big, huge, enormous difference in that the design can make orbit- and back- in a single stage.

However, the vehicle is physically big- 82m long and 6.3m in diameter- mainly because it uses the low density fuel liquid hydrogen, so the tanks are relatively large, but are kept very light by storing propellent in them at low pressure. This large size is actually an advantage as it means that the vehicle has a much easier time during reentry, than say, the Space Shuttle. The vehicle ends up slowing down at higher altitude where the air is thinner- it turns out that this means that the vehicle doesn't get nearly as hot- the skin of the vehicle only goes up to 1500C or so, the extremely fragile tiles that the Shuttle uses are not required. This makes it safer and more practical (the Shuttles tiles get shredded even flying through rain- the Skylons skin would be reinforced ceramic and should be much more durable).

Indeed the fraction of the takeoff weight that is payload is more than twice the fraction that rockets can normally achieve and it should be fully reusable- that means if you think about it that each flight makes twice as much money; and the vehicle is cheaper to run because the vehicle doesn't get thrown away or reassembled after each flight further increasing the profit margin.

The projected ticket price is around $250,000 to get to orbit and back. Plenty of people can afford that. Space Tourism is going to be very popular activity if this vehicle actually works out as designed.

It's interesting to compare with the the previous project, HOTOL that Skylon was based upon, that essentially failed when the funding was cut by the UK government.

During the research for HOTOL it was discovered that the stability of the HOTOL vehicle was extremely poor- with HOTOL the heavy engines were located at the rear of the vehicle. This means that the center of mass of the vehicle was near the rear. However the center of drag was more central; this means the vehicle wants to turn around and fly backwards. Attempts to fix this problem ended up costing most of the payload the vehicle could supply, and contributed to the failure of the project. Skylon solves this by putting the engines on the end of the wings nearer the center of the vehicle, this moves the center of mass forward and allows the vehicle to remain pointed in the right direction- orbit.

Skylon Statistics:

  • Length: 82m
  • Fuselage Diameter: 6.25m
  • Wingspan: 25m
  • Unladen Mass: 41,000kg
  • Fuel Mass: 220,000kg
  • Maximum Payload Mass: 12,000kg
The project is currently moving relatively slowly. From full funding to flight we are probably looking at a R&D cost of $10 billion and a program perhaps 7-10 years long.

The cost of doing this is perhaps not terribly dissimilar to the real-term cost of Concorde. It might be assumed that this project could fail in similar ways; however Concorde failed mainly because of competition from subsonic aircraft, and Concorde was inherently more expensive than subsonic aircraft due to the higher drag at mach 2.2. However, with Skylon, in this case it seems unlikely that there would be any cheaper competition and the vehicle is in fact likely to be inherently cheaper.(Even other technologies like Space Elevators are initially much more expensive and further out technologically; early Space Elevators may be incapable of carrying people due to potentially lethal radiation issues with the Van Allen belts).

This design, or something very like it, may well be the future of transportation into space.


  • http://www.reactionengines.co.uk/
  • http://www.reactionengines.co.uk/main.php?content=downloads

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