Engine integration
For the combined first-stage engine, a variety of solutions are being considered. They differ primarily in their degree of integration, and, therefore, in their thrust/weight ratio. The simplest and most reliable option is a design in which the turbojet and ramjet share a joint intake and airflow is directed by flaps to one engine or the other, or to both at the same time, depending upon flight speed.
Pic.4. Increasing the degree of turbojet and ramjet integration will improve thrusts/weight ratio The integrated engines include designs with a turbo core engine and low bypass ratio, or turbo core engine and high bypass ratio – windmilling the fan during ramjet operation. The combustion chambers for afterburner and ramjet operation are largely identical, and must be aerodynamically optimised for the individual modes of operation. The third version is an energy-optimised turbo-expander ramjet, also of integrated construction. The turbo core engine is bypassed during ramjet operation. The combustion-chamber wall is cooled by liquid hydrogen, thus serving as a heat exchanger, and the hydrogen gas produced powers the turbine for the compressor—during turbo mode—and the fuel pumps for turbo and ramjet modes of operation.
Pic.5. MBB has begun testing a hydrogen-fuelled ramjet combustor
Initial numerical analysis indicates that the integrated engines, including intake and nozzle, will exceed 20m in length and have a cross-section of several square metres. A one-tonne thrust hydrogen-fuelled ramjet has already been tested by МВB at Ottobrann. The major challenge in developing the engines is in optimising thrust and fuel consumption for various operating modes according to the phase of the ascent. For example, says Hogenauer, a high ramjet thrust can shorten the critical launch phase before stage separation at Mach 6.8, to the extent that thermal loading can be absorbed and active cooling of the first-stage airframe can be avoided.
Pic.6. MBВ has "rolled out" a 1:8.5-scale model of Sänger with HORUS atop
Hogenauer believes that past experience has proved that manned and unmanned spaceflight missions should be carefully separated and this requirement is met with two different upper stages: a 90-tonne four-crew re-usable hypersonic orbital upper stage (HORUS), capable of carrying 4 tonnes into orbit; and an expendable cargo upper stage (CARGUS) for unmanned payloads up to 14 tonnes into low Earth orbit or, 2.5 tonnes to geostationary orbit. The Sänger first stage is about 84m long, with a wingspan of 41m and a take-off weight of 330 tonnes when carrying HORUS, which is 32m long and has a wing span of 17m. The first stage could carry a small number of passengers on long-distance sub-orbital routes over 16,000km, for example from Frankfurt to Sydney in less than an hour.
Pic.7. The turbo-expander-ramjet is one option to power Sänger's hypersonic first stage HORUS would have a two-day lifetime in orbit, compared with Hermes' proposed maximum 33-day mission duration, and is seen as a transportation vehicle, not a laboratory. The HORUS manned orbiter, with l,200kN-thrust cryogenic rocket engine, "will require more advanced technology than the US Shuttle or Hermes," says Koelle. CARGUS will be powered by an Ariane 5 HM60 Vulcain rocket engine. The two АТС 700 engines planned for HORUS are powerful high-pressure topping cycle engines. "Thanks to support of the Federal Ministry of Research and Technology, this technology is basically known in West Germany," says Hogenauer, "but must be raised to the level of the current state of the art in the USA and Japan." Engine size is comparable with HM60. Used in the upper stage of a two-stage system, its nozzle length allows for a high expansion ratio, resulting in a high specific impulse of 472s. Sänger's flight profile would begin with the first-stage turbojet engines taking the vehicle to 10km altitude before passing Mach 1 under full afterburning thrust. At about Mach 3.5/19-5km, the ramjet propulsion mode begins, accelerating the vehicle to Mach 4.4/28km. A cruise phase over inhabited areas will extend 3,500km to the south and then due east until the vehicle is accelerated to Mach 6.6/37km for staging. The cruise phase "represents the focal point of the operation", Koelle says. "The cruise speed of Mach 4.4 seems to be the best compromise from the performance standpoint, but also for the economics of a hypersonic passenger 'plane."
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