In the study of propulsion systems, we
often encounter the term "specific", as in specific
fuel consumption, specific thrust, or
specific impulse.
Specific is a scientific term meaning "divided by
weight or mass", You have probably encountered other "specific" terms
in physics and chemistry. The specific
volume of a gas is the volume of the gas divided by the mass of
the gas contained in the volume. Specific gravity is the weight of an
object divided by the weight of the same volume of water. Specific
heat is the calories per weight of substance.
Why are we interested in "specific" properties? First, it
simplifies our mathematical analysis of some problems by removing the
dependence on mass. For example, the equations for the thermodynamic
work of a gas contains the mass flow rate as a multiplier. We could
carry it along throughout the analysis, but since the mass flow rate
is a constant, the math is easier if we just divide it out at the
beginning and put it back in at the end. Second, specific properties
are an indication of efficiency. For example, two different engines
will have different values of specific thrust. The engine with the
higher value of specific thrust is more efficient because it produces
more thrust for the same amount of airflow. Third, specific
properties give us an easy way to "size" a preliminary design. For
example, the result of a thermodynamic analysis is a certain value of
specific thrust. The aircraft drag will define the required value of
thrust. Dividing the thrust required by the specific thrust will tell
us how much airflow our engine must produce and this determines the
physical size of the engine.
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