Space travel is an amazing and complicated thing. It is complicated because there are so many problems and obstacles to overcome. You have things like the vacuum of space, heat management problems, the difficulty of re-entry, meteorites and other space debris, cosmic and solar radiation, and going to the bathroom in space.
But the biggest problem of all is harnessing enough energy simply to get a spaceship off the ground. That is where rocket engines come in.
Rocket engines are reaction engines. Remember the famous Isaac Newton principle that "to every action there is an equal and opposite reaction."
When you see a fire hose spraying water, you will notice that it takes a lot of strength to hold the hose. The hose is like a rocket engine. It is throwing water in one direction, and the firemen are using their strength and weight to counteract the reaction. If they were to let go of the hose, it would thrash around with tremendous force.
A rocket engine is generally throwing mass in the form of a high-pressure gas. The engine throws the mass of gas out in one direction in order to get a reaction in the opposite direction. The mass comes from the fuel that the rocket engine burns.
The power of a rocket engine is called its thrust. Thrust is measured in "pounds of thrust". A pound of thrust is the amount of power it would take to keep a 1-pound object stationary against the force of gravity on Earth. So on Earth, the acceleration of gravity is 32 feet per second per second (21 mph per second).
If you have ever seen the Space Shuttle launch, you know that there are three parts, the Orbiter, the big external tank and the two solid rocket boosters. The whole vehicle, shuttle, external tank, solid rocket booster casings and all the fuel has a total weight of 4.4 million pounds at launch.
All of that fuel is being thrown out the back of the Space Shuttle at a speed of about 6,000 mph. The solid rocket booster burn for about two minutes and generate about 3.3 million pounds of thrust each at launch. The three main engines burn for about eight minutes, generating 375,000 pounds of thrust each during the burn.
Solid-fuel rocket engines were the first rockets created by man. They were invented hundreds of years ago in China and have been used widely since then.
The idea behind a simple solid-fuel rocket is simple. What you want to do is create something that burns very quickly but does not explode. As you are probably aware, gunpowder explodes. In a rocket engine you don't want an explosion. You want the power released more evenly over a period of time. When you light the fuel, it burns along the wall of the tube. As it burns, it burns outward toward the casing until all the fuel has burned. In a small model rocket engine the burn might last a second or less. In a Space Shuttle solid rocket booster containing over a million pounds of fuel, the burn lasts about two minutes.
Solid-fuel rocket engines have important advantages: simplicity, low cost and safety. They also have disadvantages: the thrust cannot be controlled and once ignited, the engine cannot be stopped or restarted. When you need to be able to control the engine, you must use a liquid propellant system. Robert Goddard created the first liquid propelled rocket engine in 1926.
In most liquid-propellant rocket engines, gasoline and liquid oxygen are pumped into a combustion chamber. There they burn to create a high-pressure and high-velocity stream of hot gases. These gases flow through a nozzle that accelerates them further and then they leave the engine.
NASA's Deep Space probes will begin using ion engines for propulsion. The ion propulsion engine is the first non-chemical propulsion to be used as the primary means of propelling a spacecraft and may change the way rockets work forever.