Once the suborbital flights of Ascender are successful, Bristol Spaceplanes and others have much more ambitious plans to take passengers all the way to orbit. This is where the piggyback ride comes in (Fig. 7.7) . In Spacecab, a large winged booster vehicle with the six-passenger Spacecab on its back takes off from Bristol spaceport. The Concorde-like booster is unable to enter space, but does accelerate the Spacecab to Mach 2, using four turbojets, and then to Mach 4, with two rocket engines. Here, the Spacecab orbiter separates from its booster and ignites its own rocket engines while the booster returns to the spaceport. The orbiter carries a crew of two and has a cabin with a capacity for six passengers or space station crew or a payload of up to 750 kg of cargo. Its blunt swept-back shape reflects the fact that streamlining is not required for flight in space but reduces heating during reentry into the atmosphere. Spacecab's job of getting into orbit is now much easier, launching from the upper atmosphere and a speed of about 4,000 ft/s. Spacecab takes its passengers to an orbiting hotel, probably of an inflatable design currently being developed by Bigelow Aerospace.
Spacebus is a further enlargement of Spacecab. It will use turbo-ramjets to accelerate the 88-m booster to Mach 4 and two rockets to take the piggyback duo to Mach 6, where separation occurs. The orbiter is a 50-passenger 34-m spaceplane with a 21-m wingspan and a payload capacity of 5.4 tons or 50 passengers.3
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