Introduction to Rocket Engines

This page contains a very simple overview of rocket engines and terminology.

First, let's look at a jet engine. Basically, a jet engine is a tube. Air comes in the front and gets mixed with gaseous fuel and the mixture is ignited, causing the mixture to expand. The engine is designed that the expanded exhaust goes out the back end of the tube, which produces thrust. Compared to a rocket engine, one advantage of a jet engine is fuel efficiency. It carries fuel to heat but it doesn't carry air. Of course, getting enough incoming gas in outer space is a problem.

On the other hand, a rocket engine carries all necessary chemicals on board. Like a jet engine, it pushes gas out as exhaust to produce thrust. A disadvantage is that it has to carry more fuel than a jet engine because it is self-contained. An advantage is that it can work in places where there is not enough gas for a jet engine to work, e.g., outer space.

Thrust is "pushing force" and can be measured in pounds, kilograms, etc.

Blumrich discusses Specific Impulse (I_sp) extensively. In a way, Specific Impulse measures fuel efficiency. Specific Impulse says, "For one pound of fuel we can get X pounds of thrust." Some fuels are more efficient than others. For instance, one pound of nuclear fuel can produce more power than one pound of water.

Another way to think of Specific Impulse is that it measures how much energy is stored in a given chemical.

Example of different Specific Impulses: An ordinary flashlight battery and an alkaline battery.

Considering that we're talking about rocket fuels, it may seem odd to bring up flashlight batteries. But the point is the same: both batteries weigh the same, but the alkaline battery contains more power.

Do not confuse Specific Impulse with thrust. Just because a fuel is efficient does not mean it can produce a lot of thrust. Lay people have a tendency to think, "High Specific Impulse means a lot of thrust. Low Specific Impulse means only a little thrust." That is not true. The amount of thrust has to do with the design of the engine, not the type of fuel. Nuclear fusion of hydrogen and oxygen atoms releases a lot more power than burning oxygen and hydrogen!

Types of Propulsion Systems

Specific Impulse for Various Fuels