QUESTION #529

What are the differences between jet airplanes and rockets?

Both a jet engine and a rocket engine function by expelling hot gases opposite to the direction of desired acceleration. Newton's third law of motion says that the action of the hot gases produces a reaction in the opposite direction on the jet or rocket propelled vehicle; the momentum imparted to the gases is exactly opposite to the momentum imparted to the vehicle.

The main difference between the two types of engine is that a rocket carries its own supply of oxygen for combustion. A jet engine requires oxygen from the atmosphere for combustion, and so cannot operate in the vacuum of space.
Answered by: Paul Walorski, B.A., Part Time Physics/Astronomy Instructor
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Well, one is an airplane, and the other is a rocket! That's quite a difference in my opinion!

Joking aside, there are serious differences between the working principles of a jet airplane and a rocket. (The differences in detail are much greater I guess, but I am just a physicist.)

First, let me point out the similarities between the two: Both burn fuel and both eject this fuel mass to gain momentum. Ah, and of course, both 'fly'.

A jet engine works like this: It sucks in air from the front of the engine. This air is burned with the fuel within the engine. The resulting large mass of gas is ejected towards the rear at high velocity, which both propels the airplane forward, and gets more air sucked into the engine. In normal flight, the engines are used to propel the airplane _forward_. The actual 'uplift' is gained through the wings using the strong flow of the wind. (How actually the wings lift the airplane is a bit controversial, there is the 'Bernoulli' side, and the 'Newton' side... But that's another story.)

A rocket, in contrast, carries both fuel (which may be solid or liquid) and oxygen. Therefore it does not suck in air from the front. All it does is burn the fuel with the oxygen, and eject it at very high velocities backward. This momentum is used to both lift and propel the rocket. There are no wings for uplift. Any wings are for steering purposes.

A rocket is generally much more powerful and wasteful than a jet engine. Most airplanes (don't know of any exceptions) can not possibly climb vertically, while rockets are built just to do that. A jet airplane is pretty much useless outside the atmosphere, but a rocket will work just fine since it carries its own fuel and oxygen.
Answered by: Yasar Safkan, Ph.D., Software Engineer, Noktalar A.S., Istanbul, Turkey