Re: Concorde

Date:         23 Nov 96 03:36:25 
From:         "P. Wezeman" <pwezeman@blue.weeg.uiowa.edu>
Organization: The University of Iowa
References:   1 2 3
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On 21 Nov 1996, shahid siddiqi wrote:

> Peter Coe wrote:
>
> > On a supplemental thread, I vaguely recall that at the time of its entry
> > into service, Concorde's engines were claimed to be the most effecient
> > engine ever built - I presume in terms of thrust per pound of fuel.  The
> > plane isn't effecient overall, because it operates in such an ineffecient
> > environment.
>
> Come come lets not get carried away - the laws of
> Thermodynamics make this claim impossible. A
> turbojet engine such as that of the Concorde can
> never beat the fuel efficiency of a high by-pass
> turbo fan such as the GE-90 or the Rolls Royce
> Trent.  The thrust specific fuel consumption in
> cruise for the Olympus/SNECMA engine is most
> probably in excess of 0.8 pounds per hour of fuel
> per pound thrust produced as compared to 0.53
> pounds/hour per pound thrust produced by the GE-90.
> (In afterburner it would be in excess of 1.4 and
> thats why the Concorde doesn't cruise supersonic
> with afterburners on it would drink all its fuel
> before crossing the Atlantic).

   In order to compare the energy efficiency of the two types  of engines
you need to know the amount of fuel consumed by each engine and the amount
of useful work done by each engine. The useful work done by a jet engine
is the product of the thrust times the distance traveled (work equals
force times distance).
   A pound of thrust at 600 miles per hour is producing useful work at the
rate of (for convenience converting the speed to feet per second, in this
case 880 fps) one pound of thrust times 880 fps equals 880 foot pounds
per second, or 1.6 horsepower. At 1400 miles per hour (2050 fps) a pound
of thrust produces 2050 foot pounds per second, or 3.7 horsepower.
   Using your figures, the energy efficiency of the GE-90 is 0.53 pounds
of fuel per hour per 1.6 horsepower ( power of one pound of thrust at
600 mph), which is .33 pounds  of fuel per horsepower hour ( quite a
respectable figure). The energy efficiency of the Concorde's Olympus
engines would be .8 pounds of fuel per hour per 3.7 horsepower (one pound
of thrust at 1400 mph), which gives .22 pounds of thrust per horsepower
hour, 50% better than the GE-90.
   The greater efficiency of the Concorde's engines comes from two factors.
First, the engine exhaust is used for thrust directly, instead of being
routed through turbine blades and then a fan, which, good as they are, are
not 100% efficient. Obviously, the Concorde's exhaust nozzles are not
100% efficient either, but they're better than a fan and its drive, not
surprising when you consider the extra moving parts. The GE-90 of course
has the advantage of 25 years of progress in the design and materials of
its compressors, combustors, turbine blades, controls, etc., but these
same improvements would work just as well at supersonic speeds, and in fact
the core of any modern turbofan could serve as the basis for a new SST engine
if we ever decide to build such a thing, just as these cores now power
planes, ships, pipelines and electrical plants.
   Second, and more important, the speed of the Concorde's jet efflux is a
smaller multiple of its cruising speed than the corresponding figures
for a subsonic fan-jet, giving the Concorde greater propulsive efficiency,
the ratio of mechanical work produced by the engine to the useful work
delivered as thrust horsepower to the vehicle. Propulsive efficiency equals
2/(1+R), where R is the ratio of jet efflux speed to aircraft speed. Think
of an aircraft running up its engines with the brakes locked in a pre-flight
check. Plenty of fuel burned, plenty of air set into motion, zero work
done on the aircraft (you hope). Propulsive efficiency of a subsonic airliner
is the one really obvious weak point of the system, hence the periodic
interest in prop-fans, ultra-fans, Unducted Fans (TM), and other propulsors
optimised for subsonic speeds.
   The figure I have is 40% thermal efficiency for the Concorde in converting
chemical energy into thrust horsepower. A modern diesel ship may be better,
but not by much.

                        Peter Wezeman, anti-social Darwinist

                             "Carpe Cyprinidae"