Spaceplanes are an important part of our future in space. It is the aim of this book to show how spaceplanes of the near future will be developed, how they compare to conventional aircraft and launch vehicles, and how advanced spaceplanes will eventually become the dominant form of space access. In this book, you will find material on airplanes and rockets, in addition to rocketplanes and spaceplanes. The idea here is to allow true comparisons to be made, because only by comparing these modes of transport can one develop a good understanding of the subject.

The idea of winging one's way through airless space might seem strange. But a thoughtful analysis of every stage of flight, including the return trip, reveals that spaceplanes have a future not just in near-Earth space but also farther afield.

The idea of lunar and interplanetary spaceflight using spaceplanes begins to make more sense when several important factors are recognized. These factors include (1) the spaceplane's large propellant tanks, which would make it the ideal space tanker, (2) inflatable space habitats that could easily be carried on and deployed from spaceplanes, (3) the low G entries spaceplanes can make into planetary atmospheres, and (4) the fact that they can fly efficiently through any atmosphere, including the atmospheres on Mars, Venus, and Titan. Of course, the main factor is the reusability built into every spaceplane, which will make spaceplanes the space workhorses of the future. But let us start at the beginning, and get a handle on a few basic definitions.

What is involved in spaceflight? To understand this complex question, it behooves the wise reader to understand what precisely is meant by space and what precisely is meant by flight. Entering space does not necessarily mean entering orbit. In fact, the energy requirements for entering into low-Earth orbit are some 30 times those required for reaching space height, defined as 100 km above sea level. It is possible to orbit Earth at an altitude of 160 km, yet a stationary object - relative to Earth - placed 1.6 million km away will still fall to the ground as assuredly as any dropped object. The only exception is that it may fall onto the Moon first, if that celestial body happens to get in the way. The reason the X-15 and SpaceShipOne could not stay in space was not because they had not gone far enough, but because they had not gone fast enough. But back to the subject at hand, space itself.

The word space conveys the idea of room. The American Heritage Dictionary of the English Language defines space as room, and tells us the Indo-European root, rewe, also means "to open." Space certainly opens up new possibilities, broadens the mind, and expands the sphere of human activity. It gives us room to grow. It promises new technologies with improved capabilities. It unlocks the door to the future.

The idea of flight has three separate meanings: (1) to take wing above Earth, (2) to flow or run away rapidly, and (3) to float. These meanings are exemplified in the following kindred words: fly, flew, flown, fowl, flutter, flee, fleet, flow, flood, and float. The last term involves buoyancy in a fluid medium, such as water or air. Boats and balloons each rely on the same principle to stay afloat. And of course, spaceflight involves floating in space. Each of these terms can trace their origins back to the same Indo-European roots.1 The word for river, a flowing stream of water, also stems from the same root in several Germanic languages - the Swedish flod and German Fluss are two examples. And water is a compound of hydrogen and oxygen, the best rocket propellants known. Based on this simple etymology, the term spaceflight conveys the images of taking wing above Earth, an airflow over a lifting body, a rapid run into a roomy realm, a fleeing from the bonds of gravity, a flood of propellants into and a flow of high-speed exhaust from an engine, and finally, floating in space. Spaceplanes seem, etymologically at least, particularly suited for spaceflight.

There are a few other terms you should be familiar with before we dive into the meatier sections of the book. There is also a glossary of terms in the Appendix.

An aircraft is any craft built to operate in air, including balloons, blimps, gliders, airplanes, helicopters, and rocketplanes.

An airplane is a fixed-wing aircraft that flies by aerodynamic principles.

A ballistic missile is typically a rocket-powered aerodynamically shaped cylinder used to lob a projectile. It may have any range, and it may or may not enter space.

A rocket is a ballistic missile intended for spaceflight. It can also refer to a rocket engine, as in "rocket-powered launch vehicle."

A rocket engine is the device that produces the thrust for the rocket, launch vehicle, or spacecraft. Solid propellant rockets are called "rocket motors."

A rocketplane is an airplane powered by a rocket engine. It may or may not have the capability of entering space.

A rocketship is a spaceship powered by a rocket. The term is often used to describe spaceships that do not require separate launch vehicles. They are nonmodular, completely self-contained, and sport huge fins that serve to balance them when they land on other planets.

A spacecraft is any craft built to operate in space, including artificial satellites, planetary probes, space stations, spaceships, and spaceplanes. Spacecrafts have historically relied on launch vehicles to enter space.

A space launch vehicle is the rocket used to launch a spacecraft. It usually consists of two or more stages that drop off when their propellants are expended.

A spaceplane is an airplane that flies in space, or a fixed-wing spacecraft.

A spaceship is any manned space vessel, especially one intended for shipping crews or cargo.

A space shuttle is a spaceship that shuttles back and forth, typically between a planet and a space station, or between a planet and a moon. It may or may not be winged.

A space station is a large manned spacecraft, usually stationed near a planet.

Claims have been made that the Space Shuttle fleet operated by NASA are not true spaceplanes. In fact, the Space Shuttles are not fully developed spaceplanes, but they are true spaceplanes nonetheless. Likewise, both the X-15 and SpaceShipOne are true spaceplanes. They represent the first baby steps on the road to truly usable spaceplanes.

This book is about winged space vehicles. However, the reader should not take the term "winged" too literally, because the ultimate shape of spaceplanes has not yet been determined. It may very well turn out to be a "wingless" lifting body, or a blended wing-body of some kind. Also, the term "fuel" may sometimes refer to both the actual rocket fuel and the oxidizer necessary for operating rockets in space. The context should make it clear that sometimes "fuel" means "propellant," but we have used poetic license at times to make for a more enjoyable reading experience.

Rock River, WY Matthew A. Bentley

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