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12 Gas Turbine Handbook: Principles and Practices
The turbojet is the simplest form of gas turbine in that the hot
gases generated in the combustion process escape through an exhaust
nozzle to produce thrust. While jet propulsion is the most common
usage for the turbojet, it has been adapted to direct drying applications,
to power a supersonic wind tunnel, and as the energy source in
a gas laser. The turbofan (Figure 2-1) combines the thrust provided
by expanding the hot gases through a nozzle (as in the turbojet) with
the thrust provided by the fan. In this application the fan acts as a
ducted propeller. In recent turbofan designs the turbofan approaches
the turboprop in that all the gas energy is converted to shaft power to
drive the ducted fan (Figure 2-2). Turboprops (Figure 2-3) utilize the
gas turbine to generate the shaft power to drive the propeller (there
is virtually no thrust from the exhaust). Therefore, the turboprop is
not, strictly speaking, a jet engine.
Figure 2-1. Courtesy of United Technologies Corporation, Pratt &
Whitney Aircraft. The JT8D turbofan engine was one of the early
“bypass” engines (BPR 1.7:1). The JT8D-200 series produces over
20,000 pounds “take-off” thrust and powers the McDonnell Douglas
MD-80. The FT8 is the industrial, aero-derivative, version of this
engine.