You don't have to be a rocket scientist to understand how things get into space. The energy begins in chemical reactions. Just as two people can live more cheaply than one, some compounds exist at a lower energy condition than their individual constituent atoms had when they were separate. As these compounds are created -- for example, when fuel molecules react with an oxidizer that violently snatches at their electrons, forming new compounds -- energy is released in the form of heat.
Heat increases the pressure and velocity of surrounding molecules, which escape as exhaust gas. Many people mistakenly believe that rockets move because the gas shoves against Earth or the air. In fact, gas pushes against the rocket. Simultaneously, the rocket pushes against the gas in a spectacular demonstration of Isaac Newton's Third Law of Motion: For every action, there is an equal and opposite reaction.
The same principles that powered the Saturn V and launched Apollo 11 toward the moon are still at work in the liquid- and solid-fuel components of today's space shuttles.
SPACE SHUTTLE PROPULSION
The shuttle's three main engines burn 400,000 gallons of liquid hydrogen (the fuel) and 142,560 gallons of oxygen (the oxidizer). Each engine generates 375,000 pounds of thrust at sea level. Each of the two solid-fuel boosters generates 3.3 million pounds of thrust using dry propellants. Thus a modern shuttle launch creates about the same amount of total thrust as the Saturn V first stage, moving the 4.5 million-pound combination of shuttle, boosters and tank off the pad at a higher initial acceleration than the 6.5 million-pound Saturn/Apollo.
The rocket nozzle is designed to maximize the velocity of exhaust gases. It first tapers into a bottleneck, which raises the pressure on the gas. But then it expands into a bell shape. As exhaust moves into this part of the nozzle, the pressure drops. Potential energy from high pressure turns to kinetic energy in the form of faster-moving gas.
To understand how fast the rocket moves, you need to consider another fundamental principle: conservation of momentum. Momentum is the product of mass times velocity. So to get more momentum, you can increase your mass or your velocity. But whatever you do, in any energy exchange, the total amount of momentum always remains the same.
A rocket and its load of fuel sitting on a launch pad have a total combined momentum of zero. Amazingly, it will remain zero throughout the launch. When ignition occurs, a mass of exhaust gas starts moving downward at a certain velocity, producing thrust. It's a fairly small mass, but it's really moving. Gas from liquid-fuel hydrogen-oxygen engines typically travels around 12,000 feet per second.
(Of course, you could get the same amount of thrust by using a larger mass of fuel at a lower speed, but that would add to the launch weight of your spacecraft. Better to make a smaller mass move faster, whether it's solid or liquid fuel.)
As the exhaust produces momentum in one direction, the rocket does so in the opposite direction. Conservation demands that the rocket's mass times velocity exactly equal the mass times velocity of the escaping gas. Its mass is huge compared with the exhaust, so its velocity is very small at first.