We have seen that planes fly because air provides “lift” under their wings (post 17.16); any engines that moves the plane forward will enable air to lift the plane.
But there is no air in space (because air molecules are attracted to earth by its gravitational field – see post 16.16) so space-craft need to be propelled in another way – by a rocket. A rocket relies on burning its fuel very rapidly to cause an explosion. Since burning is another way of saying a chemical reaction with oxygen (post 16.33), a space rocket must carry oxygen, or a source of oxygen (called a propellant), as well as fuel. The fuel and oxygen mix in a combustion chamber. The mixture is then ignited, or ignites spontaneously (depending on the fuel and propellant used).
The explosion creates an exhaust gas at high temperature that expands (see post 16.35) and creates a pressure that acts equally in all directions – like the pressure in any other fluid (see post 17.5). The combustion chamber contains the high-pressure gas in much the same way as a wall supports someone who leans on it – the person exerts a force on the wall and the wall exerts an equal but opposite force on the person; this is Newton’s third law of motion (post 16.2).
However, there is a valve at the back of the combustion chamber that allows the high-pressure gas to escape. In its simplest form, this valve could be simply a trap door that is closed by a spring. If the area of the door is A and the pressure of the gas is p, the gas exerts a force of pA on the valve (post 17.5); the door will open if the force is sufficiently high to create an appreciable deformation in the spring (post 16.49).
Now the wall is only pushing back against the high-pressure gas at the front of the combustion chamber. According to Newton’s third law, the gas then pushes against the front of the combustion chamber – but not against the back.
So overall the gas exerts a force, called the thrust, that pushes the rocket forward.