Although rockets are widely viewed as ultra-modern inventions, the principles behind them have been at least generally understood for thousands of years. The first mechanical devices that might be thought of as rockets were developed as steam-driven curiosities in Greece around the fourth century B.C. By the first century A.D. the Chinese had formulated an early type of gunpowder and were rolling it into tight paper tubes that could propel an arrow or other missile. The rocket was born.

In theory, the rocket is a fairly simple proposition. It is a vehicle that carries some type of fuel internally. The fuel is caused to expand (usually by way of some chemical reaction, such as ignition) and the exhaust from this expansion is forced out through a small opening at the rear. Forward momentum is simply the equal-but-opposite reaction to this exhaust. The mighty Saturn V rocket...