Invented by the Chinese about a thousand years ago, the rocket is the only kind of combustion engine that can work in space. This is because it carries not only fuel, but also the oxygen to burn the fuel. Other kinds of combustion engines, such as jet engines, rely on the oxygen in the air to burn their fuel. The jet and the rocket do have one thing in common, however. They both work on the same principle, the principle of jet propulsion. They burn fuel to produce hot gases, then these gases shoot out of the rear of the jet engine or rocket as a high-speed jet. The force of the escaping jet of gases acting backwards gives rise to an equal force acting forwards, which provides the means of propulsion. This follows from Isaac Newton's third law of motion: to every action there is an equal and opposite reaction. Rockets are often called reaction engines. Historically, the rocket has been used as a weapon since its invention until the present day. At the end of the last century every major army had a rocket squadron, using the ordinary firework rocket, which burned gunpowder. One person saw another potential use for rockets - as a means of propelling objects into space. He was the Russian Konstantin Tsiolkovsky. He foresaw that more powerful liquid-fuel rockets would be needed for space flight, but he didn't build any. It was not until 1926 that a liquid-propellant rocket was flown - by an American engineer, Robert H Goddard. Ten years later German rocket engineers began working on a rocket that was to revolutionize warfare and provide the stepping stone to outer space. This rocket was the A-4, later to become notorious as the V-2, Hitler's second revenge weapon of World War 2. The V-2 team was led by one Wernher von Braun, who after the war went to the USA and eventually directed their space-rocket programme. He designed a series of heavy-lift rockets, including the famous Saturn V Moon rocket. Other V-2 rocket experts went to the USSR after the war and provided the foundation of that country's space-rocket programme. The rockets that launched the first spacecraft were not specially designed for that purpose, but were modifications of the powerful ICBMs (intercontinental ballistic missiles) the USSR and USA had developed for warfare.
