Thrust

Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that system. The force applied on a surface in a direction perpendicular or normal to the surface is also called thrust. Force, and thus thrust, is measured using the International System of Units (SI) in newtons (symbol: N), and represents the amount needed to accelerate 1 kilogram of mass at the rate of 1 metre per second per second. In mechanical engineering, force orthogonal to the main load (such as in parallel helical gears) is referred to as static thrust.

Examples A fixed-wing aircraft propulsion system generates forward thrust when air is pushed in the direction opposite to flight. This can be done by different means such as the spinning blades of a propeller, the propelling jet of a jet engine, or by ejecting hot gases from a rocket engine. Reverse thrust can be generated to aid braking after landing by reversing the pitch of variable-pitch propeller blades, or using a thrust reverser on a jet engine. Rotary wing aircraft use rotors and thrust vectoring V/STOL aircraft use propellers or engine thrust to support the weight of the aircraft and to provide forward propulsion. A motorboat propeller generates thrust when it rotates and forces water backwards. A rocket is propelled forward by a thrust equal in magnitude, but opposite in direction, to the time-rate of momentum change of the exhaust gas accelerated from the combustion chamber through the rocket engine nozzle. This is the exhaust velocity with respect to the rocket, times the time-rate at which the mass is expelled, or in mathematical terms:

T = v d m d t {\displaystyle \mathbf {T} =\mathbf {v} {\frac {\mathrm {d} m}{\mathrm {d} t}}}

Where T is the thrust generated (force), d m d t {\displaystyle {\frac {\mathrm {d} m}{\mathrm {d} t}}} is the rate of change of mass with respect to time (mass flow rate of exhaust), and v is the velocity of the exhaust gases measured relative to the rocket. For vertical launch of a rocket the initial thrust at liftoff must be more than the weight. Each of the three Space Shuttle Main Engines could produce a thrust of 1.8 meganewton, and each of the Space Shuttle's two Solid Rocket Boosters 14.7 MN (3,300,000 lbf), together 29.4 MN. By contrast, the Simplified Aid for EVA Rescue (SAFER) has 24 thrusters of 3.56 N (0.80 lbf) each. In the air-breathing category, the AMT-USA AT-180 jet engine developed for radio-controlled aircraft produce 90 N (20 lbf) of thrust. The GE90-115B engine fitted on the Boeing 777-300ER has a thrust of 569 kN (127,900 lbf). It was recognized by Guinness World Records as the "World's Most Powerful Commercial Jet Engine" until it was surpassed by the GE9X (fitted on the upcoming Boeing 777X), with 609 kN (134,300 lbf).

Concepts

Thrust to power The power needed to generate thrust and the force of the thrust can be related in a non-linear way. In general, P 2 ∝ T 3 {\displaystyle \mathbf {P} ^{2}\propto \mathbf {T} ^{3}} . The proportionality constant varies, and can be solved for a uniform flow, where v ∞ {\displaystyle v_{\infty }} is the incoming air velocity, v d {\displaystyle v_{d}} is the velocity at the actuator disc, and v f {\displaystyle v_{f}} is the final exit velocity:

d m d t = ρ A v {\displaystyle {\frac {\mathrm {d} m}{\mathrm {d} t}}=\rho A{v}}

T = d m d t ( v f − v ∞ ) , d m d t = ρ A v d {\displaystyle \mathbf {T} ={\frac {\mathrm {d} m}{\mathrm {d} t}}\left(v_{f}-v_{\infty }\right),{\frac {\mathrm {d} m}{\mathrm {d} t}}=\rho Av_{d}}

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