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Aviation History
1988
1988 - 0536.PDF
New materials for Hotol growth, he says. A local increase in weight is multiplied by a factor of 20 when the vehicle is resized to lift the same 7-tonne payload into low Earth orbit. The British aerospaceplane is a very different vehicle from the US Space Shut tle, with its 24-tonne payload. To begin with Hotol is fully reusable: only the fuel, and hopefully the payload, are "thrown away". The vehicle is designed for a launch cost, per kilogramme transported into low Earth orbit, one-fifth that of Shuttle. Reusability and rapid turnaround are factors in the cost equation, but minimum size and weight are crucial. Many Hotol design features contribute directly to keeping size and weight down. The vehicle is unmanned, flying auto matically from take-off, into orbit, through re-entry, to the unpowered land ing. Weighing five times as much on take off as on landing, Hotol lifts off from a trolley, avoiding the 100-tonne penalty to be paid for an undercarriage able to support the fully laden vehicle, says Burns. The ability to burn atmospheric air contributes the major weight saving, although onboard liquid oxygen still accounts for 55 per cent of take-off weight. Further weight is saved by using liquid hydrogen and liquid oxygen for orbital manoeuvring and gaseous hydrogen for reaction control. The choice of airframe materials is also crucial to saving weight. Here BAe has been encouraged by recent developments in advanced high-temperature materials such as carbon-ceramic, metal-matrix, Keeping it down to size is perhaps the biggest challenge British Aero space faces in designing the Hotol aerospaceplane. Every kilogramme saved is £0-5 million off the development bill and £0 • 4 million off the unit cost of the unmanned satellite launcher, says project and technical manager, B. R. A. Burns. Hotol is best known for its "unique", and still classified, airbreathing propulsion system. From horizontal take off to Mach 5 and 85,000ft, the dual-mode rocket engine burns onboard liquid hydro gen with atmospheric air. Above 85,000ft, and on into orbit, the engine switches to onboard liquid oxygen. While the engine undoubtedly presents a major challenge to its developer, Rolls- Royce, the aerodynamic and structural design of Hotol will be equally demanding. Ascent and re-entry traverse a range of Mach numbers and flow regimes not encountered by conventional aircraft, notes BAe, while the single-stage-to-orbit launch concept imposes severe weight constraints. With onboard liquid oxygen still accounting for more than half Hotol's 250-tonne take-off weight, and liquid hydrogen another quarter, the vehicle's payload fraction of just 3 per cent—while twice that of expendable boosters—does not leave much room to manoeuvre, admits Burns. Hotol is particularly sensitive to weight Hotol's airframe will be protected from the heat of re-entry by large carbon-ceramic panels British Aerospace is asking UK materials suppliers to contribute towards Hotol aerospaceplane research, emphasising the spinoff appli cations of the advanced high- temperature materials required. Graham Warwick reports. FLIGHT INTERNATIONAL, 5 March 1988
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