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Union University Department of Physics

The Science Guys

Science Guys > November 2002

November 2002

What is the difference in a jet airplane engine and a rocket?

In our September column, we explained how a rocket works in outer space where there is no air. People generally believe that a rocket must push on air in order to propel the rocket forward, but that is not the case. Briefly stated, a rocket works because of Newton’s third law, which says for every action there is an equal, and opposite reaction. The burning of fuel creates gases at high pressure, which exit from the exhaust nozzle and push the rocket forward. As gases exit the rocket, a reaction force (thrust) pushes on the rocket making it go forward. The faster the gases are expelled from the rocket, the greater the thrust. Think of how a garden hose creates a force pushing back on the hose as water squirts from it.

In fact, jet engines and rockets operate on the same general physics principle. Both eject fuel out the back. The momentum imparted to this exhaust is equal to the momentum gained by the vehicle, thus making the vehicle go forward. One difference between rockets and jets is found in the type of fuel they burn. Jet engines are air breathers. They take in air (which contains oxygen needed for combustion), mix it with fuel, burn it to increase the pressure, and exhaust the spent gases out the back at a high rate of speed. This high-speed ejection of mass propels the plane forward. Rockets do almost the same thing with two exceptions. Unlike jets, they carry their own oxygen along with them and a rocket does not have wings that add lift.

On the space shuttle, you notice an orange tank, which actually contains separate tanks of hydrogen and oxygen. These two ingredients are mixed in the liquid-fuel rocket engine, burned, and expelled out the nozzle. The white, solid-fuel rocket on each side contains a chemical mixture in which the oxidizing agent is part of the fuel. Rocket fuel can burn without external oxygen being present. As a side note, once a solid fuel rocket is ignited, it cannot be turned off. Jet engines must have outside oxygen from the air.

Another difference is that jet planes have wings for lift and rockets do not. The density of air and the speed of the plane affect the lift on the wings. For rockets the lift (thrust) is provided solely by the expelled gases.

Therefore, a rocket can travel in the vacuum of space void of air, but a jet engine could not. A jet plane has a ceiling limit above which it cannot fly because there is not enough air. The jet engine must be able to ’breathe’ in order to function. Rocket fuel is considerably more efficient than jet fuel and rockets usually are more powerful. However, the rocket generally is heavier because it must carry all of its oxidizer with it.