The amount of engineering required to launch a rocket into space is out of this world. There are several obstacles a spacecraft has to overcome, such as the effects of cosmic and solar radiation, orbital mechanics, the vacuum of space, and intense heat. But before rocket technology can jump any of those hurdles, it must first get into space – perhaps the hardest feat of them all.

The idea of the rocket sprung up nearly 3000 years ago in China. The Chinese knew that the secret to launching objects was a proper balance of chemical components in the fuel. Good rocket fuel burns very quickly, but also evenly over a longer period of time. Too much sulfur and not enough carbon in the fuel will create gunpowder, a substance that explodes when ignited.

The rocket engine also relies on a few Newtonian laws of motion. At their theoretical base, rocket engines release a lot of mass in one direction and, in reaction, are propelled in the opposite direction with equal force. This is different from a car or other mode of transportation because it does not rely on a rotary engine.

This reaction works something like a shotgun. When a shot is fired, the shooter feels a "kick" of the distributed force of the bullet propelled forward at 700 mph. Likewise, the force at which water sprays out of a firefighting hose supplies so much "kick back" to the hose that two or three firefighters are required to hold it steady.

Instead of bullets or water, the rocket engine releases gases at high pressure. When the mass of fuel inside the rocket is burned, it turns to gas and can no longer fit the space inside the shell. So, the gas pushes out through the bottom nozzle. The mass of the fuel does not change, but burning it causes the molecules to move faster, adding acceleration to the whole process.

Rocket power is measured in thrust. One pound of trust is the force it takes to stabilize a one-pound object against the Earth's gravitational pull. The amount of thrust required to put a space shuttle into space is relative to its payload. The fuel weight usually outnumbers the payload 36:1. That's why a space shuttle appears so tiny next to its rocket engines before liftoff.

An empty shuttle, or orbiter, weighs about 165,000 pounds, the external tank is about 78,100 pounds, and the two boosters weigh 185,000 each. When these are filled with liquid hydrogen and oxygen, the entire package weighs 4.4 million pounds. All of that fuel is necessary to lift 165,000 pounds of the shuttle into orbit.

The exhaust velocity of the rocket fuel falls between 5,000 and 10,000 mph, and the rockets only burn for two minutes. The main engines – the three that use the fuel in the external tank – burn for about eight minutes. The rockets and engines combined generate 4.5 million pounds of thrust.

Inside the cylindrical rocket, the fuel is solid and burns from the middle to the outside walls. The tube in the center is sometimes in the shape of the star to increases the surface area. This shape burns more at the beginning, but then more evenly as the shape rounds off and the shuttle is already in flight.

Solid fuel rockets are great because they are safe considering their lack of complexity and expense. However, once lit, thrust cannot be controlled, or more importantly, aborted.

Robert Goddard, a rocket engineer, solved this problem in 1926 with the liquid propellant rocket engine, which he filled with gasoline and oxygen. These engines are a little more complicated. The fuels are pumped into a combustion chamber, and their gases steam out the nozzle at high pressure. These methods do have a downside: millions of pounds of pollutants are released whenever they are used.

There are still other ways to get a rocket into space. Pressurized nitrogen thrusters or any substance that emits only gas exhaust can work just fine especially for smaller objects like satellites. NASA has been working on an ion engine that releases mass as atomic particles and ions, and the Deep Space-1 probe, launched in 1998, was the first to demonstrate the ion engine's efficiency.

Source:

How Rocket Engines Work
Marshall Brian
How Stuff Works
http://science.howstuffworks.com/rocket.htm