Re: Rocket action

[Mark Sylvester <msylvest@SPIN.IT>]

> 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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