Spacebus, BSP’s second generation orbital spaceplane, is a larger version of Spacecab, itself an evolution of Ascender. The design of Spacebus is such that a prototype could be built a few years after Spacecab without requiring a significant programme of enabling technology. Spacebus weighs about twice as much as Concorde, which is probably close to the practicable upper limit for a spaceplane using existing runways. Both stages are piloted and take-off and land horizontally.
Spacebus is designed to carry fifty people or equivalent cargo. Spacebus would be used for launching medium satellites and as a general-purpose launch vehicle, but its main use would be for transporting tourists and supplies to and from space hotels.
The basic design features of Spacebus were derived to provide the best return to investor or taxpayer, rather than for technical elegance or maximum efficiency. This means giving priority to early revenues, to minimising development cost and risk, and to accessing large new markets.
Spacebus Leading Data
|Max speed with jet engines||Mach 4||N/A|
|Separation Speed||Mach 6||N/A|
|Take-off Weight, tonnes||400||90|
|Empty Weight, tonnes||113||16|
|Payload, tonnes||90 (Upper Stage)||5|
The carrier aeroplane accelerates to Mach 4 using turbo-ramjets of new design but with existing technology. Rocket engines are then used to accelerate to Mach 6 and to climb to the edge of space where air and thermal loads are low. The orbiter then separates and accelerates to orbit.
The orbiter has a 50-seat capacity in a narrow, 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 design features intended to minimise the use of new technology, and thereby can reach maturity sooner than other proposed orbital space vehicles like the DASA Sänger project. 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 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.
When people think about the airline industry, they take for granted that the aircraft are mature, highly reliable flight-tested vehicles with a ready market. The space industry, however, conjures images of expendable rockets with limited demand, or experimental high-performance spaceplanes like the X-15. To understand the potential of Spacebus we need to change this mind-set and project the business of air travel into space. 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.
A useful comparison can be made with a new supersonic or hypersonic airliner. The cost per flight would be approximately $125,000 (250 passengers at $500 each). Such an airliner would be of comparable size and shape to the lower stage of Spacebus.
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 more advanced but is also smaller; as such, the smaller size counters the increased complexity. We can roughly estimate that its cost will be approximate to the lower stage. Thus the Spacebus upper stage also has an estimated cost per flight of the same order.
Cost and Programme
The least mature systems of Spacebus, 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 straight-forward product improvement over a decade or two of operational experience.
As with any aeroplane development for commercial use it will take several years of in-service experience and continuous product improvement.
The remaining systems of Spacebus, such as structure, jet engines, hydraulics, power, landing gear, environmental control, and avionics are 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.