Rocket propulsion is different from other forms of accelerating an object because the entire acceleration system is contained within the object. A runner makes use of the ground to accelerate; by pushing off of it with her foot. A car uses the road to accelerate; by the friction of wheels against the asphalt. A jet accelerates by utilizing the chemical energy from the burning fuel to heat and accelerate ambient air ejected backwards through a turbine. A rocket makes itself move entirely by expelling its own mass in the opposite direction. A rocket does not need anything to push against or pull on. A sailor throwing cannonballs off the back of a ship is using the same principle; his craft will accelerate forward with each throw.
There are several types of rocket propulsion. The most common and well-known is called chemical or conventional rocket propulsion. A chemical rocket relies on a highly explosive chemical reaction inside a tube to force fuel mass downward and the rocket up. NASA's Space Shuttle and the Saturn rockets used for the Apollo missions are all entirely chemical systems. The liftoff of Apollo 15 is shown above.
Besides chemical rockets, there are also nuclear thermal and electric rockets. A nuclear thermal rocket uses the heat from nuclear fission reactions to accelerate the fuel and create thrust. An electric rocket uses electrical power to accelerate propellant out of the back of the engine. An important parameter for all rockets is the speed at which the propellant is expelled from the rocket. The exhaust speed of propellant is far greater in electric rockets, making them much more efficient, but the level of thrust is much lower and restricts their operation for use only in space. Our mission is to develop the VASIMR® engine, a versatile form of electric propulsion, for the use in many in-space applications.