Specific thrust
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Specific thrust is a term used in gas turbine engineering to show the relative bulk of a jet engine (e.g. turbojet, turbofan, etc.) and is defined as the ratio: net thrust/total intake airflow.
Contents
Low specific thrust engines
A modern civil turbofan has a low specific thrust (~30 lbf/(lb/s)) to keep the jet noise at an acceptable level, and to achieve low fuel consumption, because a low specific thrust helps to improve specific fuel consumption (SFC). This low specific thrust is usually achieved with a high bypass ratio. Additionally a low specific thrust implies that the engine is relatively large in diameter, for the net thrust it produces. Consequently such aircraft engines are normally located externally, in a separate nacelle or pod, attached to the wing, or the rear fuselage.
High specific thrust engines
On the other hand, a modern military turbofan features a fairly high specific thrust (45-110 lbf/(lb/s)), which keeps the cross-sectional area of the engine low, allowing (normally) to easily accommodate the engine within a narrow fuselage, thus minimizing aircraft drag. A high specific thrust implies a high jet noise level, which is not an important consideration for most military applications.
Maximum airspeed
Specific thrust has significant bearing on thrust lapse rate: the low jet velocity associated with a low specific thrust engine implies that there are large reductions in net thrust with increasing flight velocity, which can only be partially offset by throttle changes at rated conditions (e.g. maximum recommended climb rating).
Supersonic aircraft
Supersonic aircraft require a high specific thrust, since that gives a high exhaust speed.
Afterburners
Dry specific thrust has impact upon the performance of afterburning turbofans.
A low (dry) specific thrust engine has a low tailpipe temperature, which means that the temperature rise across the afterburner can be high, giving a good thrust boost in afterburning. Nevertheless, the afterburning specific thrust is still relatively low. The total fuel flow (main combustor plus afterburner) is fixed by the temperature rise from air intake to nozzle and, for a given airflow, changes little with dry specific thrust. Consequently, the low afterburning thrust implies a high afterburning SFC. However, the dry SFC is low.
The situation is completely reversed for a high (dry) specific thrust.
Consequently engine designers must select a level of dry specific thrust that is suitable for the engine application. A compromise may be required.
References
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