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Aviation History
1988
1988 - 0537.PDF
and thermoplastic composites. The vehicle has a re-entry wing loading only 40 per cent that of Shuttle. This allows a slower, cooler re-entry, says Burns. Surface temperatures are roughly 300K cooler than on Shuttle, staying below 1,200K over most of the vehicle. The hottest areas are the intake lips, at around 2,000K, followed by the nosecone at 1,750K and the forward fin leading-edge at 1,700K, with the rest of the vehicle at or below 1.200K. For the hottest areas BAe proposes using carbon-carbon or carbon-ceramic composites. Carbon-carbon has certain drawbacks, principally its susceptibility to voids and oxidation, so the preferred high- temperature material is a carbon-ceramic composite comprising carbonfibres embedded in a silicon-carbide matrix, says Burns. Shielded by a carbon-ceramic thermal protection system, most of the Hotol airframe will be at or below 700K. While titanium is acceptably strong at these temperatures, BAe prefers a metal-matrix composite for the wing and centre and rear fuselage. The leading candidate is titan ium reinforced with 35 per cent by volume of boron carbide particles, says Burns. BAe had planned to use titanium for the liquid oxygen and liquid hydrogen tanks, but the thin walls required raised concerns about cracking. Instead, the tanks will be constructed from 3mm-thick filament- wound carbonfibre-reinforced thermo plastic. Thermoplastics are damage tolerant and easy to repair, says Burns, and the latest carbonfibre-reinforced polyethyl- ethyl-ketone (Peek) composites have increased upper temperature limits, so that less tank insulation is required, he notes. | The tanks are shielded from the fierce heat of re-entry by the thermal protection system. This comprises large carbon- ceramic panels forming an "aeroshell" covering the fuselage and lower wing surface. These panels do not carry any major loads, and are attached by pivoting pillars to allow for thermal expansion. Top BAe has already identified the most promis ing materials for use in Hotol. Above Through a series of subsonic and hypersonic tunnel tests, Hotol has evolved from "long and thin" through "short and fat" to its present shape, bottom There is just 150mm between the tank wall at 20K and the thermal protection panel at 1,200K, Burns notes. Between the two is a sandwich of foam-type cryogenic insulation adjacent to the tank wall and ceramic-fibre high-temperature insulation adjacent to the thermal protection panel. The liquid oxygen and liquid hydrogen tanks form the structural backbone of Hotol. Engine thrust is transmitted via links to the smaller but heavier oxygen tank, while the frame carrying the forward fin and nose gear is attached via a trellis to the hydrogen tank. The payload bay is located between the tanks, on the e.g. BAe completed a two-year, £2 million Hotol proof-of-concept (PoC) study in September last year, and has put forward to the UK Government a proposal for a Hotol "enabling technology" programme, leading to a development start in 1994 and a first flight around the year 2000. The "ET" programme would have two three- year phases, during the first of which BAe would select the materials to be used. Its design would be confirmed in the second phase. The ET programme will require consid erably more money than the just- completed PoC phase, however, and BAe proposes that industry provide half the funds. To that end the company has approached UK materials suppliers, seek ing their financial support. To encourage their involvement, BAe is stressing the application of new high- temperature materials beyond Hotol. While the fleet of five vehicles planned would need only 200 tonnes of advanced materials, future projects such as an advanced short take-off/vertical landing fighter and supersonic transport could require far greater qualities, "flotol leads the way," says Burns. BAe foresees a fleet of five vehicles, each with a 120-mission design life, based at the European equatorial launch site in Kourou. Between them the five vehicles, one equipped for manned missions with a pressurised capsule in the payload bay, would perform 28 missions a year, with a minimum turnaround between flights of just 48 working hours. With a recurring launch cost of just $5 million, each Hotol would probably only carry one satellite at a time, says BAe. Hotol will have the inherent capability to abort a launch in the event of payload problems, return the satellite to Earth, then offer a relaunch opportunity within days. "The satellite business would be vastly different today if we had such a reliable low-cost launch system," savs BAe. E FLIGHT INTERNATIONAL, 5 March 1988 •jr,
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