9. The Jet Engine.
A Jet Engine performs a unique physical function in its own right. The maximum power which it is capable of delivering, also, the thermal and mechanical efficiency of it's systems have nothing to do directly with the Aircraft into which it may be installed. As things are, and according to convention, the maximum Gross Thrust which a jet engine is capable of delivering is achieved at maximum throttle when the vehicle containing the engine is in fact stationary and operating at Sea Level. In this case, the Gross Thrust Force is that directly attributable to the fact the Air Mass caused to accelerate rearwards through the engine leaves its propelling nozzle at a final jet velocity much higher than zero. It is this Gross Thrust force that is delivered to the Aircraft/engine's mounting bolts in the "brakes on" position just prior to take off. At this point, it is still, essentially, a jet engine. By this it is meant that its thrust power is invested entirely in its own jet stream as it leaves the engine's final propelling nozzle.
The primary function and action of any form of Jet Engine is to convert chemical fuel energy into Air Mass Motion Energy as economically as possible. The size of any particular design of engine determines in the main its maximum air mass flow capacity and therefore, it's thrust per unit of engine weight. The thermal and mechanical efficiency by which this maximum state is achieved determines the cost in fuel consumption per unit of thrust delivered in time, per unit weight of fuel. And so, the physical efficiency of the engine as a Jet Engine, is determined by the efficiency in converting a unit weight of chemical fuel energy into Air Mass Motion Energy. This is principally the realm and jurisdiction of the Jet Engine manufacturers.
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But, consider, the physical operational efficiency of a Jet Engine is in no way directly dependant upon its being installed in an Aircraft. Consider also, the potential of any jet engine's use is established some time before it is actually tried as a possible suitable propulsion unit. And, this is only the beginning, because, a Jet Engine has no Propulsive Efficiency relative to itself when its thrust is produced whilst stationary. Its Propulsive Effectiveness cannot be truly determined in fact until the combination of engine and airframe forming the Aircraft has actually been extensively flown. Consequently, an installed Jet Engine in flight has no Propulsive Efficiency in its own right simply because it does not move relative to the Aircraft to which it is bolted. Only the Aircraft as a Motion Vehicle that is in this way caused to fly has, in fact, a determinable Propulsive Efficiency.
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