The key to a successful spaceplane design concept is to identify whatever airplanes do better than rockets, as well as whatever rockets do better than airplanes, and incorporate these features into a single self-contained vehicle. That, in a nutshell, is the challenge of the spaceplane. For example, airplanes use the atmosphere in no less than three distinct ways: (1) aerodynamic lift, (2) oxidizer to sustain combustion of fuel, and (3) propellant or "working mass." Rockets, by contrast, use the atmosphere for none of these, instead carrying their own onboard propellant-oxidizers, and replacing aerodynamic lift by brute rocket thrust. These factors do allow the rocket to operate in the vacuum of space, however, where ordinary aircraft would be completely helpless and could never reach in any case.
The Space Shuttle solves only half the spaceplane equation, using its wings to land on Earth, but resorts to ballistic methods to enter space. This means that the Shuttle's wings and vertical stabilizer are dead weight during launch. It is still a step in the right direction, but advanced spaceplanes will go a step further. Utilizing the horizontal takeoff and horizontal landing approach, spaceplanes will cater much more readily to their passengers, while avoiding the pitfalls that have plagued the Shuttle program. They will operate routinely from aerospaceports around the globe. Horizontal takeoff means that spaceplanes will be bothered neither by the kind of falling debris that eventually doomed the Columbia, nor by hot gases escaping from strap-on solid rocket boosters, which resulted in the Challenger explosion. We will report on each of these tragedies later in the book, with a view to applying the lessons they teach us to future spaceplane design.
The main argument against the development of single-stage-to-orbit spaceplanes is the belief that they would not be able to carry enough propellants to reach orbit. Indeed, the Shuttle's space transportation system carries the bulk of its propellants in two huge solid rocket boosters and an enormous liquid propellant tank, both external to the delta-winged orbiter itself. And yet, the main problem with the Shuttle is that it is launched vertically. The spaceplanes of the future will all be launched horizontally, and will therefore not require nearly the propellant capacity of the Space Shuttle, because of greatly improved atmospheric utilization. In addition, spaceplanes will be far more versatile than either aircraft or present-day spacecraft have been. Most spaceplanes, at least in the short term, will be specifically designed to carry passengers and crew rather than freight. The Space Shuttle, by contrast, was designed as a heavy-lift "space truck" able to lift up to 65,000 lb of cargo into low Earth orbit.
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