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Re: Rocket action



At 19:28 12/1/99 -0600, Greg Kifer wrote:
>I remember, but don't know from where, hearing that a rocket can go no
>faster forward than the exhaust gases leave the exhaust nozzle. Is this
>correct? If so, can someone explain the whys and wherefores to a simple
>high school teacher like me. Thanks in advance.
>
>Greg Kifer
>Olathe North High School
>Olathe, KS 66061

At 20.22 01/12/99 -0600, brian whatcott wrote:
A propellor driven plane needs to accelerate the mass flowing through its
blades to provide thrust. If the air was initially stationary to a ground
observer, it will be moved backwards. From the plane's perspective,
the efflux is moving backwards at a higher velocity than the plane is
moving forwards.

A rocket provides its own gas, and any rear directed acceleration of mass
in a vacuum provides a thrust force.

So it seems to me that the propellor model is closer to Greg's description
than a rocket model.

**************************************************
Brian's answer is clear to me - I hadn't thought of bringing a propellor
driven plane into the discussion. Clearly the thrust tends to zero as the
speed of the aircraft relative to the air approaches the speed of airflow
away from a stationary airscrew. The original contention about a rocket can
be seen to be problematic if we ask: "Faster relative to what?"

But what about a jet engine? Here the thrust comes partly from flinging
back air encountered on the way and partly (admittedly a small part) from
ejecting burnt fuel which is carried along like a rocket's propellant. Is
there an inbuilt speed limit in this case? My intuition says there is not,
but I'm not convinced of this.

Mark



_____________________________________
Mark Sylvester
United World College of the Adriatic,
34013 Duino TS, Italy.
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