Spacebus  

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About Spacebus

Spacebus is an enlarged and mature development of Spacecab, itself a development of Ascender. Its main use would be for transporting tourists and supplies to and from space hotels. It would also be used for launching medium satellites, and as a general purpose launch vehicle.

Spacebus Facts

Spacebus Design

The Spacebus booster comprises four turbo-ramjet engines to provide the lift upto 24 km and accelerate the booster to Mach 4, where rocket engines take over and power the booster to Mach 6 - separation speed. The orbiter has a capacity of 50 seats in a narrow body airliner-like fuselage. Alternatively some of the seats could be replaced by a viewing room for passengers and a zero-gravity gym for longer leisure trips of up to 10 hours.

Spacebus has much in common with the DASA Sänger project except for design features intended to minimise the use of new technology, and thereby to reach maturity sooner. These features include the use of jet plus rocket engines on the carrier aeroplane stage, thus avoiding the need for new hypersonic air-breathing engines. The use of rocket motors on the lower stage also permits a ski-jump separation which in turn allows separation with low air loads and the upper stage to reach orbit without aerodynamic lift. This in turn permits a blunt bubble tank structure designed for very low weight.

Mind-Set

We have become so used to the high risk and cost of manned spaceflight that the concept of a mature spaceplane takes some getting used to. A useful comparison can be made with a new supersonic or hypersonic airliner. The cost per flight will be of the order of $125,000 (250 passengers at $500 each). Such an airliner would be of comparable size and shape to the lower stage of Spacebus. Thus in broad-brush terms the two vehicles should have a comparable cost per flight, when Spacebus has matured to airliner standards. The upper stage is smaller but more advanced, and in broad-brush terms these two factors should tend to cancel out. Thus the Spacebus upper stage also has an estimated cost per flight of the same order.

When thinking about a new airliner, the maturity standard is taken for granted. When thinking about new launchers, whether reusable or not, the tendency is to assume the maturity of an expendable vehicle like a ballistic missile or of an experimental high performance aeroplane like the X-15. The new factor is a potential market - space tourism - large enough to require a large fleet size and to provide the operating experience and the commercial incentive for the continuous product improvement needed to reach maturity.

Cost and Programme

After some years of in-service experience and continuous product improvement, Spacebus would approach airliner standards of maturity, with a cost per flight of nearly four times that of the 747, at some $220,000. Spacebus carries some eight times fewer passengers than the 747 (50 compared with 400). The direct cost per seat is therefore some 30 times higher, at around $4500. This compares well with estimates of the profitability of space tourism that predict that "Spaceplanes using mature developments of existing technology and operating like airliners will have a direct operating cost per seat to orbit of around $5000".

Feasibility

As with any aeroplane development for commercial use it will take several years of in-service experience and continuous product improvement. The least mature systems, in terms of life and maintenance requirements, are the thermal protection system, rocket motors, hydrogen fuel system, and transparencies.

However, this is a lot less ambitious than many spaceplane proposals, and there is no reason why the maturation of these systems should involve more than straightforward product improvement over a decade or two of operational experience. An important objective of Ascender is to advance the maturation of these technologies and to demonstrate the feasibility of routine everyday manned spaceflight, albeit sub-orbital.

The remaining systems of Spacebus, such as structure, jet engines, hydraulics, power, landing gear, environmental control and avionics are straightforward adaptations of present airliner practice. This greatly reduces Spacebus's development cost, and greatly improves its reliability and certification from the start. In this way the development of Spacebus will be achieved both less expensively and with a more rapid return on investment.