The Different Types of Rocket Engines

     A rocket engine is required to achieve space flight. A rocket works because of Newton's Third Law of Motion, "For every action, there is an equal and opposite reaction". 

Chemical

     There are two types of chemically powered rockets, solid-propellant engines and liquid-propellant engines. Both engines generally work alike: combusting the propellant and oxidizer, having the burning gases escape through the nozzle, and therefore, propelling the rocket forward. However, there are distinct differences and advantages to each type.

     Solid-propellant engines consists of a solid propellant, which is a compressed, solid mixture of propellant and oxidizer. Solid propellants are usually mixed in liquid form, and poured into a mold or the rocket casing, where it solidifies.
     The Chinese invented the earliest form of solid propellant. It was in the form of gunpowder and it used a mixture of sulfur, charcoal, and saltpeter. Today's solid propellant are much more powerful, consisting of ammonium perchlorate, powdered aluminum, oxidizer chemicals, and other additives.
     The advantages of a solid-propellant engine lie in its ease of use, reliability, and costs. They are easy to store and can be readied for launch very quickly. They can withstand changes in temperature very well and can produce powerful thrusts rapidly. And last but not least, solid propellants are much more inexpensive than liquid propellants. 
     The disadvantage of a solid propellant rocket is that once it has been ignited, combustion cannot be stopped. It will continue burn until the propellant is gone. 

     Liquid-propellant rockets use a mixture of liquid fuel and liquid oxidizer as propellants. Stored in separate tanks, the fuel and oxidizer are pumped into the combustion chamber where they mix, ignite, and burn to produce thrust. Some mixtures must be ignited to start the combustion process. While others, called hypergolic propellants, spontaneously ignite on contact.
     Some common liquid fuels include alcohol, purified kerosene, liquid hydrogen, and hydrazine. Some common oxidizers include nitrogen tetroxide and liquid oxygen.
     Advantages of the liquid-propellant rockets make it the most common kind of rockets used in spaceflight. Liquid-propellant rockets produce a lot of thrust in comparison to their weight. In addition, thrust can be controlled when using liquid-propellant. It can be shut off and started at any time. 
     However, many liquid-propellants are difficult or dangerous to handle, transport, or store. 

Electric

     Electric or ion rockets has not been brought into actual use yet, but has been under development since the 1950's. They work by placing a positive charge on the molecules of a single fuel by removing some of its negative electrons. Such fuel can be hydrogen, mercury, or even water. The charged atoms (ions) are then propelled out of the rocket by a strong magnetic field, producing thrust. There would be no oxidizer required, so more fuel can be carried. However, the thrust would be very small and insufficient in achieving escape velocity. But ion rockets are very efficient and would be able to function for a very long time. Future uses could include interplanetary or even interstellar probes. 

Nuclear

     Nuclear rockets work by using the incredible heat generated by the nuclear reactor to heat a gas and force it through the nozzle to develop thrust. Nuclear rockets have been tested and proven to be very efficient. However, safety and environmental concerns has prevented them from being used just yet.