Crucial separationCrucial to a two-stage system is stage separation. МВБ has now added a pull-up manoeuvre to the original Mach 6.8/30km separation point, which takes the separation point to Mach 6.6/37km, increasing the final payload capability of HORUS by 1,100kg. Stage separation is initiated by igniting the two 40kN-thrust HORUS orbital control engines, and lifting the vehicle mechanically to an 8° angle of attack. The additional aerodynamic drag is counterbalanced by the thrust of the orbital engines, while at the same time a 5,OOOkN underbody pressure lifts HORUS away from the main vehicle. Four seconds after separation, the main engine propels HORUS into transfer orbit. Another crucial factor in realising Sänger is the availability of high-temperature materials that must at the same time be lightweight and extremely stiff. While a Boeing 747 weighs about 180 tonnes empty, the Sänger first stage, which could become a hypersonic airliner, will weigh just 142 tonnes. HORUS, which is similar in length to a Boeing 737 (empty weight 30 tonnes), has a target empty weight of just 20 tonnes. The lion's share of weight for both stages is accounted for by fuel—100 tonnes for the lower stage and 65 tonnes for the second. The ratio of empty weight to total weight of Sänger's first stage is about 42 per cent, compared with Concorde's 45 per cent. The ratio for HORUS is 22 per cent, significantly greater than HOTOL's 17-5 per cent. At critical locations, such as the nose, wing leading edges, and flaps, HORUS is subjected to temperatures far in excess of 1,000°C, while the fuselage surfaces heat up to 1,OQO°C. These temperature loads can only be withstood by fibre-reinforced carbon and ceramic structures. Thermal loads on the lower stage are not so extreme; at a cruise speed of Mach 4-4, temperatures between 300° and 700°C will be reached, depending on exposure within the flow field.
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