Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
suitable for use in compressors, to a level of about 850 to 900 K. Ambient temperature is nearly constant at about 220 K (although the temperature varies considerably
even at the altitudes flown by supersonic transports). Thus, the temperature ratio is about 4, resulting in an ideal thermal efficiency of about 3/4. The actual efficiency is
lower because of various losses but remains quite high.
Propulsion efficiency, which is the ratio of power pushing the airplane to the hot gas power produced by the gas generator, ideally depends only on the ratio of jet
velocity to flight velocity. It can be increased toward the limit of unity by increasing the bypass ratio. The bypass ratio that is best for any given application is determined
by a balance between the increased drag and weight associated with the larger engine frontal area needed for increased airflow and the increased propulsive
efficiency associated with the larger airflow. There is an additional factor in fixing the bypass ratio-the fan pressure ratio. Engines for supersonic applications require a
higher fan pressure ratio than subsonics for similar improvements in efficiency. When these compromises are struck for the subsonic and supersonic propulsive
systems, the propulsive efficiencies of the two systems at cruise conditions are not very different. Over the past 2 decades, the bypass ratios of subsonic engines have
increased from 2 toward 10 as lighter weight and aerodynamically more sophisticated designs have evolved. For supersonic propulsion systems, the cruise bypass
ratio of choice is now in the range of 0.5-1.0, whereas for the Concorde and for the U.S. supersonic transport of the 1970s it was zero.
7.10.3. Supersonic Propulsion System Characteristics
In contrast to subsonic engines-in which the needs for low cruise fuel consumption and low take-off noise are synergistic in that they both favor high bypass ratiosupersonic
engines face a severe conflict between the need for low bypass ratio in transsonic acceleration and cruise, and the need for higher bypass at take-off to
limit jet noise. This conflicts leads to supersonic propulsion systems in which the effective bypass ratio can be small at cruise and relatively large at take-off. Such
variation can be achieved by several different approaches, characterized at the extremes as variable turbomachinery systems and ejector nozzle systems. Common to
all approaches is that the engine operates at a relatively high exhaust velocity at cruise and a low exhaust velocity at take-off.
The engine concept favored by the U.S. program is a low bypass turbofan (0.6) with a mixer-ejector nozzle for noise suppression at take-off. It is projected that a
supersonic transport with this propulsion system could meet Federal Aviation Regulations 36 Stage 3 noise requirements. An alternative concept proposed by the
European engine consortium has a fan at the midsection of the engine, fed by auxiliary inlets at take-off, to produce a bypass ratio of about 2 at take-off. The auxiliary
inlets are closed and the fan airflow is decreased via a geometry change to give a lower bypass in cruise. This engine concept is estimated to meet the Stage 3 noise
requirement with a conventional variable area nozzle such as that used on the Concorde.
For designs under current consideration, the compression pressure ratio is about 22 at take-off conditions, and the corresponding temperature rise is about 500 K,
producing a compressor outlet temperature of about 800 K. By comparison, a subsonic engine at take-off would have a pressure ratio of 35 with a compressor exit
temperature of 900 K. At supersonic cruise, the compressor outlet temperature is limited by the materials to about 950 K. For a subsonic cruise engine with a pressure
ratio of 40, the corresponding compressor outlet temperature is about 850 K or lower, limited by practical aerodynamics. The consequence is that the supersonic
engine runs hot at cruise and cool at take-off-the opposite of the subsonic engine.
7.10.4. SST Propulsion NO x Output
At cruise, subsonic engines have an overall pressure ratio, including inlet ram pressure rise, in the range of 35-50. The overall pressure ratio of a supersonic
propulsion system, including the intake pressure rise and the remaining turbomachinery pressure ratio, is in the range of 130-140, calculated as a ratio of the
compressor exit total pressure divided by the inlet static pressure. Whereas the conventional subsonic system might have EI(NOx ) of 10-15 g kg-1 fuel using a
conventional combustor, even with the double annular system the advanced supersonic engine would barely achieve EI(NOx ) of 30 (Lowrie, 1993). Combined with the
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