thrust

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Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a proportional but opposite force on that system.

Examples

Forces on an aerofoil

A fixed-wing aircraft generates forward thrust when air is pushed in the direction opposite to flight. This can be done in several ways including by the spinning blades of a propeller, or a rotating turbine pushing air from the back of a jet engine, or by ejecting hot gases from a rocket engine. The forward thrust is proportional to the mass of the airstream multiplied by the velocity of the airstream. 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 and thrust vectoring V/STOL aircraft use engine thrust to support the weight of the aircraft, and vector some of this thrust fore and aft to control forward speed.

Birds normally achieve thrust during flight by flapping their wings.

A motorboat generates thrust (or reverse thrust) when the propellers are turned to accelerate water backwards (or forwards). The resulting thrust pushes the boat in the opposite direction to the sum of the momentum change in the water flowing through the propeller.

A rocket is propelled forward by a thrust force 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:

where:

• T is the thrust generated (force)
• is the rate of change of mass with respect to time (mass flow rate of exhaust);
• v is the speed of the exhaust gases measured relative to the rocket.

For vertical launch of a rocket the initial thrust must be more than the weight.

Each of the three Space shuttle main engines can produce a thrust of 1.8 MN, and each of its two Solid Rocket Boosters 14.7 MN, together 29.4 MN. Compare with the mass at lift-off of 2,040,000 kg, hence a weight of 20 MN.

By contrast, the simplified Aid for EVA Rescue (SAFER) has 24 thrusters of 3.56 N 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.^[1] The GE90-115B engine fitted on the Boeing 777-300ER, recognized by the Guinness Book of World Records as the "World's Most Powerful Commercial Jet Engine," has a thrust of 569 kN (127,900 lbf).

Thrust to power

Thrust at zero speed is zero power. Power requires work to be done, so zero velocity indicates zero work and zero power. Therefore the power of a rocket or aircraft engine is thrust times forward speed.

power (watts) = thrust (newtons) x speed (metres/second)

power (horsepower) = thrust (lbf) x speed (feet/second) / 550

power (horsepower) = thrust (lbf) x speed (feet/minute) / 33000.

For example: the Messerschmitt Me 262 with 3,960 pounds of thrust at 559 mph equates to 5,903 horsepower.

Thrust to power

A very common question is how to compare the thrust rating of a jet engine with the power rating of a piston engine. Such comparison is difficult, as these quantities are not equivalent. A piston engine does not move the aircraft by itself (the propeller does that), so piston engines are usually rated by how much power they deliver to the propeller. Except for changes in temperature and air pressure, this quantity depends basically on the throttle setting.

Now, a jet engine has no propeller - it pushes the aircraft by moving hot air behind it. One could imagine that a jet engine could be rated by how much power it transmits to the hot air on the exhaust (this depends basically on the throttle setting), but that quantity is not useful for anything (other than finding out how hot and fast the air is). The useful measurement is how much power the jet engine transmits to the aircraft through it Thrust force. This is the propulsive power of the jet engine (do not confuse that with all the other power transfers the engine has -- to create sound, to vibrate, to push hot air, etc...).

So let's find out the propulsive power of a jet engine from its Thrust. Power is the Force it takes to move something over some Distance divided by the Time it takes to move that distance:

In case of a rocket or a jet aircraft, the Force is exactly the Thrust produced by the engine. If the rocket or aircraft is moving at about a constant speed, then Distance divided by Time is just Speed, so Power is Thrust times Speed:

This formula looks very surprising, but it is correct: the propulsive power (or power available) of a jet engine increases with its speed. If the speed is zero, then the propulsive power is zero. If a jet aircraft is at full throttle but is tied to a very strong chain to a tree, then the jet engine produces no propulsive power. It certainly transfers a lot of power around, but all that is wasted. Compare that to a piston engine. The combination piston engine/propeller also has a propulsive power with exactly the same formula, and it will also be zero at zero speed -- but that's for the engine/propeller set. The engine alone will continue to produce its rated power at a constant rate, whether the aircraft is moving or not.

Two aircraft tied to a tree

Now, imagine the strong chain is broken, and the jet and the piston aircraft start to move. At low speeds:

The piston engine will have constant 100% power, and the propeller's thrust will vary with speed
The jet engine will have constant 100% thrust, and the engine's power will vary with speed

This shows why you can't compare the rated power of a piston engine with the propulsive power of a jet engine -- these are different quantities (even if the name "power" is the same). There isn't any useful power measurement in a jet engine that compares directly to a piston engine rated power.

One could imagine "okay, so we just multiply the engine thrust by the maximum speed, and we have the equivalent power". That would be true if the actual thrust at full speed was the maximum thrust. But, the thrust changes with speed, altitude and external temperature. In order to provide a single number for publication, engine builders publish the Maximum Static Thrust, that is, the force measured when you put the engine in a test stand in the ground and submit it to full throttle. The actual thrust at speed will be quite different. Additionally, it is unusual to operate jet engines at full throttle continuously. For example, the SR-71 Blackbird requires full throttle to break the sound barrier, but once it reaches Mach 3 the throttle is cut down to 30%.

As a curiosity, a 747-200 at 40,000 feet requires a calculated thrust of 55,145 lb (25% of its maximum static thrust) to cruise at Mach 0.9. With the equation above we find that is equivalent to power of about 87,000 hp (64880 kW). However, the same aircraft flying at lower altitude will have a lower true airspeed at the same throttle, so it will be rated at a much lower power. By the way, the figure of 87,000 hp doesn't mean that if you put a piston engine developing 87,000 hp it will achieve the same speed on the 747-200 -- the quantities are, once again, different.^[2]

References

See also

• Aerodynamic force
• Gimballed thrust, the most common thrust system in modern rockets
• Thrust-to-weight ratio
• Thrust vectoring
• Tractive effort