Skylon

Helping orphans the way you would do it

Skylon is a plausible design by top British rocket scientist Alan Bond for an aeroplane that would be able to fly into low earth orbit, and return, completely intact.

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 because 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.

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 staging.

The main secret to the design are the engines, called SABRE. The engines are designed to run much like a jet engine at up to mach 5, and then closing off the air inlet and running as highly efficient rockets up to orbital speed.

Running at mach 5.0 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 great jet engine. The problem with running at mach 5.0 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 some of the liquid hydrogen fuel to cool the air, the air is then burnt much like a conventional jet. (At high speed, beyond mach 5, the air would still end up too hot- so the air inlet closes and the engine instead turns to burning the hydrogen with onboard 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 propellent; 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.

And this makes all the difference the design can make orbit- and back- in a single stage.

The vehicle design 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 running them at low pressure. This 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 in turn 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 [[Space Shuttle uses are not required. This makes it safer and more practical (the Shuttle's tiles get shredded even flying through rain- the proposed Skylons skin material is 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 $100,000 to get to orbit and back. Space Tourism may be a 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 loosely based upon, that essentially failed when the funding was cut by the UK government.

One difference is the undercarriage. HOTOL was to use sled launch. Skylon uses relatively conventional-looking retractable undercarriage. This is achieved by using high pressure tires on a specially strengthened runway, and using water cooled brakes. Upon successful takeoff, the water is jettisoned. This design reduces the weight of the undercarriage by many tonnes- weight that directly contributes to payload.

Skylon also uses a different engine design- HOTOL condensed the air and separated out the oxygen, whereas Skylon merely precools it. This is partly due to patent issues, but it turns out that the SABRE engine has higher performance anyway.

During the research for HOTOL it was discovered that the aerodynamic stability of the HOTOL vehicle was extremely poor. This means that the center of mass of the rear-engined vehicle was near the rear. However the center of drag was more central; this tends to make the vehicle 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 giving much more payload.

Skylon Statistics:

   * Length: 82m

   * Fuselage Diameter: 6.25m

   * Wingspan: 25m

   * Unladen Mass: 41,000kg

   * Fuel Mass: 220,000kg

   * Maximum Payload Mass: 12,000kg

   * ISP: 2000-2800 seconds (atmospheric) 450 seconds (exoatmospheric)

   * SABRE thrust:weight ratio >10

The project has a projected R&D cost of $10 billion and a program perhaps 7-10 years long.