Bypass ratio the velocity of a very large volume and consequently mass of air produces a very large change in momentum and thrust: thrust is the engine's mass flow (the amount of air flowing through the engine) multiplied by the difference between the inlet and exhaust velocities in—a linear relationship—but the kinetic energy of the exhaust is the mass flow multiplied by one-half the square of the difference in velocities. A low disc loading (thrust per disc area) increases the aircraft's energy efficiency, and this reduces the fuel use. The Rolls–Royce Conway turbofan engine, developed in the early 1950s, was an early
Mass ratio from Tsiolkovsky's rocket equation: where This equation can be rewritten in the following equivalent form: The fraction on the left-hand side of this equation is the rocket's mass ratio by definition. This equation indicates that a Δv of formula_4 times the exhaust velocity requires a mass ratio of formula_5. For instance, for a vehicle to achieve a formula_6 of 2.5 times its exhaust velocity would require a mass ratio of formula_7 (approximately 12.2). One could say that a "velocity ratio" of formula_4 requires a mass ratio of formula_5. Sutton defines the mass ratio inversely as: In this case, the values