Skylon
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Skylon.jpg
- For other uses of the word Skylon, see Skylon (disambiguation)
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.
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Single stage to orbit
Historically, getting into space has required something called 'staging'. Staging is when parts of a launch vehicle are dropped during the flight to reduce weight—otherwise the rocket and payload would be too heavy to make orbit.
This staging causes a number of problems, it is difficult, expensive and often even impossible to recover and reuse and reassemble the parts.
The Skylon design promises to take off from a specially strengthened runway, fly into space, reenter the atmosphere, and land back on the runway like a conventional aeroplane, without staging. This is known as single stage to orbit (SSTO).
The engines
The main features of the design are the engines, called SABRE. The engines are designed to operate like a jet engine at up to around Mach 5, and then close the air inlet and operate as a highly efficient rocket to orbital speed.
Operating a jet engine at up to Mach 5.0 is difficult. Previous engines proposed by other designers have been good jet engines but poor rockets; this engine is a good rocket engine, as well as being an excellent jet engine at all speeds. The problem with operating at Mach 5.0 has been that the air coming into the engine heats up as it is compressed into the engine, which causes the engine to overheat and eventually melt.
The SABRE engine avoids this by using some of the liquid hydrogen fuel to cool the air. The air is then burnt much like in a conventional jet. At high speed, beyond Mach 5, the air would still end up unusably hot, so the air inlet closes and the engine instead turns to burning the hydrogen with onboard liquid oxygen as in a normal rocket.
Because the engine uses the atmosphere as reaction mass at low altitude it burns about 1/5 as much propellent there. Therefore, it can take off with much less total propellant. This, in turn, means that it doesn't need as much lift or thrust, which permits smaller engines, and it allows using conventional wings at takeoff. The wings, in turn, hold much of the weight of the vehicle, and this also reduces the fuel needed--conventional rockets burn about a fifth of their propellent just holding the rocket up on a column of fire until reaching orbital velocity ("gravity losses").
Differences from HOTOL
It is worthwhile to consider how this design came to be. Skylon was based upon a previous project, HOTOL, that ended when the funding was cut by the government of the United Kingdom.
One difference is the undercarriage. HOTOL was to use sled launch. Skylon uses a 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 reduces the weight of the undercarriage by many tons--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 and Official Secrets Act issues, but it turns out that the SABRE engine gives higher performance anyway.
Another issue that Skylon has circumvented is the intrinsically poor stability of the HOTOL. The weight of the rear-mounted engine tended to make the HOTOL vehicle fly backwards. Attempts to fix this problem ended up sacrificing much of the payload the HOTOL vehicle could propel, 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 and thus moving the center of mass forward, ahead of the center of drag.
These differences make all the difference. The Skylon design can reach orbit and back in a single stage with a useful payload size.
Vehicle dimensions
The vehicle design is physically big—82 m long and 6.3 m in diameter—mainly because it uses low-density liquid hydrogen as fuel. The relatively large tanks required are kept very light by running them at low pressure. In some ways, this size is actually an advantage as it means that the vehicle has a much easier time during reentry than other vehicles, such as the Space Shuttle. The vehicle ends up slowing down at higher altitudes where the air is thinner. This means that the vehicle doesn't get nearly as hot during reentry: the skin of the vehicle only reaches 1500°C or so, and the extremely fragile tiles that the Space Shuttle employs are not required. This makes it safer and more practical. The Shuttle's tiles are damaged even flying through rain, whereas the Skylon's proposed skin material, reinforced ceramic, is much more durable.
Indeed, the proportion of takeoff weight to payload is more than twice that of normal rockets and it should be fully reusable. That means 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.
Skylon Statistics:
- Length: 82 m
- Fuselage diameter: 6.25 m
- Wingspan: 25 m
- Unladen mass: 41,000 kg
- Fuel mass: 220,000 kg
- Maximum payload mass: 12,000 kg
- ISP: 2000 to 2800 s (20 to 27 kN·s/kg) atmospheric, 450 s (4.4 kN·s/kg) exoatmospheric
- SABRE engine thrust/weight ratio: > 10
Economics
When carrying human payload the projected ticket price is around $250,000 to get to orbit and back. Space tourism may be a very popular activity if this vehicle works as designed.
The project has a projected R&D cost of under $10 billion and an estimated program length of 7-10 years. Skylon research, mainly associated with the SABRE engine heat exchangers, has been carried out by Alan Bond's company Reaction Engines Limited (http://www.reactionengines.co.uk/) in conjunction with Bristol university.
External Links
- Reaction Engines Limited (http://www.reactionengines.co.uk/) homepage
- Reaction Engines Limited download (http://www.reactionengines.co.uk/main.php?content=downloads) papers on Skylon/SABRE
- World Tiles (http://www.smallspace.demon.co.uk/Model_Services/Skylon/skylon.htm) Skylon model