Re: Are "jets" really jets?

From:         tim@me.rochester.edu (Tim Takahashi)
Organization: University of Rochester, School of Engineering
Date:         08 Sep 95 02:35:35 
References:   1 2
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Keith Barr <barr@netcom.com> wrote:

>Daniel P. B. Smith <dpbsmith@world.std.com> wrote:

>>Is a modern-day "jet" really a jet at all?

>I guess you could think of modern engines as cowled turboprops
>A turbofan has a large fan disk on the front of the engine
>most of the thrust is provided by the bypass air
>Turbofans are more efficient because they need less fuel

Reason being is fairly straightforwards :

A Brayton cycle motor (turbofan,prop,etc.) is like a diesel
engine (rather than a conventional gasoline powered engine)
in that it always burns fuel in the presence of excess
oxygen. The efficiency of such a cycle is tied up in the
effective compression ratio of the motor. There is no
throttle (and associated pumping losses).

Clearly the maximum compression ratio will occur at a fairly
high turbine speed. At the same time, the compression needs
to be maintained against a certain backpressure. Optimal
backpressure is greater than that provided by well
designed exit turbines used to run the compressor.

In other words, the power drained from the exhaust turbines
and used to drive the fan disk (or propellor) is necessary
for compression. You could design a restrictive exit, in lieu
of secondary turbines... but that would be inefficient in
a thermodynamic sense. From a power to weight standpoint,
a restrictive exhuast with additional fuel injection
(i.e. an afterburner) works well.

A question?

Is there a high-bypass ratio turbofan with afterburners?
(I think the answer may be YES - the A-10 "thunderbolt" motors?)

-tim