Re: turbojet riddle

["Bernard G. Cleyet & Nancy Ann Seese" <georgeann@REDSHIFT.COM>]

Right BW!

In starting one can hear the turbine spinning before there is any
flame.  I believe they use compressed air not an electric motor.    The
V-1 used a flutter valve (like those on hearth bellows or in hearts),
and intermittent combustion.

bc

brian whatcott wrote:

> At 10:52 1/2/00 -0500, John D wrote:
> > Hi --
> > Here's another riddle that you may find entertaining and/or
> > pedagogically useful.
> >
> > Short version:  At the most basic physics level, what makes a
> > turbojet engine go the right way? ...
>
> > Now, to a first approximation the primary effect of the flame
> > is to increase the temperature and pressure of the gas in the
> > guts of the engine.
> >
> > So the big question is, why does this high-pressure gas decide
> > to go out the back rather than going out the front? ...
>
> The Brayton cycle is isobaric during the combustion, so that the
> flame does *not* increase the pressure, but rather increases the
> volume, and above all the temperature which must peak above 1500F
> for half way decent fuel efficiency.
>
> The seventeen compressor stages ( disks of blades) or more, supply
>  the air at a pressure of 100 psi+ at which the combustion occurs
> through a waisted annulus.
>
>  It follows that to start this type engine, you must spin it at an
> appreciable speed to provide some reaction for combustion gases.
>
>  A similar situation faces rocket designers, who (in this case) must
> provide fuel and oxidizer at combustion pressure.
> In the Shuttle main engines, this head pressure of around 600 psi
> consumes humongo power to provide the necessary mass rate.
>
> brian whatcott <inet@intellisys.net>
> Altus OK