Expert: Mark Janus - 10/11/2007

Question
QUESTION: I was once told that a rocket engine is actually an inertial propulsion device (totally unlike a jet engine). In other words this source was telling me that provided the rocket engine never ran out of fuel it would keep on accelerating at its inherent acceleration rate. Very similar to a falling object within a gravity field where the gravity field is the "rocket engine". Is this true?

ANSWER: A rocket engine and a jet engine operate on the same basic principle yet the jet must capture its own oxidizer (the atmosphere) whereas a rocket brings its own.  Your statement is a bit flawed in that the rockets supply of oxidizer (and fuel) has to be accelerated along with the rocket, thus if it is an inexhaustible supply then the rocket would have an infinite mass).  Setting that aside and assuming the rocket mass does not change, since the rockets thrust force does not depend on the rockets velocity and F=ma, then a=F/m which is constant.  For a real rocket the mass does change (it decreases) so for a constant force the acceleration actually increases over time.

Hope this helps,
mj

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QUESTION: Well, thanks. But more to the point is - all propulsion systems used on air, land or sea vehicles (including jet engines) have a speed limit. They all reach a point where adding more energy to the system produces no further results in forward movement.

My real question asks - does a rocket engine also have this inherent speed limit, or, as my source suggested, is a rocket engine a true inertial propulsion system that inherently has no speed limit. (I suppose we're assuming a constant supply and weight of fuel for discussion purposes)
Would a rocket engine thus supplied ever say, "I cannot go any faster, I can no longer maintain my acceleration"?

ANSWER: I would have to say that the "speed limit" you refer to for air, land and sea vehicles is imposed by the external forces on the vehicle which are proportional to V^2 and not due to an inevitable "limit" of the propulsion system.  So to compare apples to apples.  All things equal, a jet-powered missile and a rocket-powered missile will both experience an upper limit on velocity.  
On the other hand, in space, where only a rocket-powered vehicle can operate, a rocket has no upper limit on velocity (except that of the speed of light) and thus would continue to accelerate at a constant rate (given your constant mass premise)....this is due to the fact that in space there would be no external forces (to overcome)....assuming of course you were away from the gravitational effects of other bodies....

Thanx,
mj


---------- FOLLOW-UP ----------

QUESTION: Dialogue appreciated!  I'm still not sure if we're on the same page. Any propulsion system has to "push" against something - tires/pavement, prop/water, etc. Is this not true? What is it that sends a jet engine forward? Does it need to "thrust" against the air behind the engine? If there were no air behind the engine would any forward motion be achieved? (again assuming the jet engine had its oxygen supply) If the jet engine had its oxygen and air supply could it provide thrust in space where there is no atmosphere behind the engine?

Or, is a case where the combusting gases are "pushing" against the jet engine to provide the thrust? In that case, since the jet engine is a flow-through engine it could never travel or accelerate faster than the speed of its combustion and that would be its speed limit? Not so?

Does the rocket engine provide its forward thrust by exactly the same principle as the jet engine? It has NO flow-through so the big difference is that as far as the relationship between the combustion and the engine is concerned the engine is standing still. Therefore, it "never" reaches the speed of the combustion no matter how fast it is caused to travel. If this is true, it could keep accelerating forever, quite unlike any other propulsion system. Is this not the case? If it is, the rocket is in a class by itself and is truly an inertial propulsion system. Penny for your thots.

Answer
No it is not true that you must have something (surrounding you) to "push" on for propulsion...   A jet engine does not push on the air behind it for propulsion, it simply ejects air at a high speed out the back end.  In effect the other components of the engine (the solid parts) are "pushing" the air out the back....  action/reaction ....   the exhaust air goes one way, the solid goes the other (with equal momentum).  So yes a jet engine could do exactly what a rocket engine does in space provided it had some fluid (fuel and air) it could "throw overboard" at a very high speed.

In effect a jet engine scoops up air (accelerates it very quickly to a velocity near the speed of the vehicle) then reacts fuel with the air to release energy which is then used to accelerate the air in the opposite direction (to that of the direction of travel of the vehicle) at an even higher velocity.  The drag and aerodynamic heating issues occur (for the most part) before issues surrounding combustion speed and such....   There could be some issues associated with flame speeds, energy release and such that you would have with a jet engine and not a rocket (since a rocket has both working fluids (fuel and oxidizer) traveling at the same speed).

But, fundamentally rockets and jets work off the same principle, action/reaction, yet the jet accelerates it's oxidizer (very rapidly to a velocity close to the engine speed) and a rocket does not (its oxidizer is continuously being accelerated along with the vehicle).  If the jet didn't/couldn't accelerate the oxidizer then yes there would be limitations associated with flame speed and such....

mj