Thrust-to-weight ratio


Thrust-to-weight ratio is a dimensionless ratio of thrust to weight of a rocket, jet engine, propeller engine, or a vehicle propelled by such an engine that is an indicator of the performance of the engine or vehicle.
The instantaneous thrust-to-weight ratio of a vehicle varies continually during operation due to progressive consumption of fuel or propellant and in some cases a gravity gradient. The thrust-to-weight ratio based on initial thrust and weight is often published and used as a figure of merit for quantitative comparison of a vehicle's initial performance.

Calculation

The thrust-to-weight ratio can be calculated by dividing the thrust by the weight of the engine or vehicle and is a dimensionless quantity. Note that the thrust can also be measured in pound-force provided the weight is measured in pounds ; the division of these two values still gives the numerically correct thrust-to-weight ratio. For valid comparison of the initial thrust-to-weight ratio of two or more engines or vehicles, thrust must be measured under controlled conditions.

Aircraft

The thrust-to-weight ratio and wing loading are the two most important parameters in determining the performance of an aircraft. For example, the thrust-to-weight ratio of a combat aircraft is a good indicator of the maneuverability of the aircraft.
The thrust-to-weight ratio varies continually during a flight. Thrust varies with throttle setting, airspeed, altitude and air temperature. Weight varies with fuel burn and payload changes. For aircraft, the quoted thrust-to-weight ratio is often the maximum static thrust at sea level divided by the maximum takeoff weight. Aircraft with thrust-to-weight ratio greater than 1:1 can pitch straight up and maintain airspeed until performance decreases at higher altitude.
In cruising flight, the thrust-to-weight ratio of an aircraft is the inverse of the lift-to-drag ratio because thrust is the opposite of drag, and weight is the opposite of lift. A plane can take off even if the thrust is less than its weight: if the lift to drag ratio is greater than 1, the thrust to weight ratio can be less than 1, i.e. less thrust is needed to lift the plane off the ground than the weight of the plane.

Propeller-driven aircraft

For propeller-driven aircraft, the thrust-to-weight ratio can be calculated as follows:

where is propulsive efficiency, is the engine's shaft horsepower, and is true airspeed in feet per second.

Rockets

The thrust-to-weight ratio of a rocket, or rocket-propelled vehicle, is an indicator of its acceleration expressed in multiples of gravitational acceleration g.
Rockets and rocket-propelled vehicles operate in a wide range of gravitational environments, including the weightless environment. The thrust-to-weight ratio is usually calculated from initial gross weight at sea level on earth and is sometimes called Thrust-to-Earth-weight ratio. The thrust-to-Earth-weight ratio of a rocket or rocket-propelled vehicle is an indicator of its acceleration expressed in multiples of earth's gravitational acceleration, g0.
The thrust-to-weight ratio for a rocket varies as the propellant is burned. If the thrust is constant, then the maximum ratio is achieved just before the propellant is fully consumed. Each rocket has a characteristic thrust-to-weight curve or acceleration curve, not just a scalar quantity.
The thrust-to-weight ratio of an engine exceeds that of the whole launch vehicle but is nonetheless useful because it determines the maximum acceleration that any vehicle using that engine could theoretically achieve with minimum propellant and structure attached.
For a takeoff from the surface of the earth using thrust and no aerodynamic lift, the thrust-to-weight ratio for the whole vehicle must be more than one. In general, the thrust-to-weight ratio is numerically equal to the g-force that the vehicle can generate. Take-off can occur when the vehicle's g-force exceeds local gravity.
The thrust to weight ratio of rockets typically greatly exceeds that of airbreathing jet engines because the comparatively far greater density of rocket fuel eliminates the need for much engineering materials to pressurize it.
Many factors affect a thrust-to-weight ratio. The instantaneous value typically varies over the flight with the variations of thrust due to speed and altitude along with the weight due to the remaining propellant and payload mass. The main factors include freestream air temperature, pressure, density, and composition. Depending on the engine or vehicle under consideration, the actual performance will often be affected by buoyancy and local gravitational field strength.

Examples

The Russian-made RD-180 rocket engine produces 3,820 kN of sea-level thrust and has a dry mass of 5,307 kg. Using the Earth surface gravitational field strength of 9.807 m/s², the sea-level thrust-to-weight ratio is computed as follows:

Aircraft

VehicleT/WScenario
Northrop Grumman B-2 Spirit0.205Max take-off weight, full power
Airbus A3800.227Max take-off weight, full power
Boeing 737 MAX 80.310Max take-off weight, full power
Airbus A320neo0.311Max take-off weight, full power
Tupolev Tu-1600.363Max take-off weight, full afterburners
Concorde0.372Max take-off weight, full afterburners
Rockwell International B-1 Lancer0.38Max take-off weight, full afterburners
BAE Hawk0.65
Lockheed Martin F-350.87 with full fuel
HAL Tejas Mk 10.935With full fuel
Dassault Rafale0.988Version M, 100% fuel, 2 EM A2A missile, 2 IR A2A missiles
Sukhoi Su-30MKM1.00Loaded weight with 56% internal fuel
McDonnell Douglas F-151.04Nominally loaded
Mikoyan MiG-291.09Full internal fuel, 4 AAMs
Lockheed Martin F-22> 1.09 Combat load?
General Dynamics F-161.096
Hawker Siddeley Harrier1.1VTOL
Eurofighter Typhoon1.15Interceptor configuration
Space Shuttle1.5Take-off
Space Shuttle3Peak

Jet and rocket engines

Fighter aircraft

In International SystemF-15KF-15CMiG-29KMiG-29BJF-17J-10F-35AF-35BF-35CF-22LCA Mk-1
Engine thrust maximum 259,420 208,622 176,514 162,805 81,402 122,580 177,484 177,484 177,484 311,376 89,800
Aircraft mass, empty 17,01014,37912,72310,90006,58609,25013,29014,51515,78519,6736,560
Aircraft mass, full fuel 23,14320,67117,96314,40508,88613,04421,67220,86724,40327,8369,500
Aircraft mass, max take-off load 36,74130,84522,40018,50012,70019,27731,75227,21631,75237,86913,300
Total fuel mass 06,13306,29205,24003,50502,30003,79408,38206,35208,61808,16302,458
T/W ratio 1.141.031.001.150.930.960.840.870.741.140.96
T/W ratio 0.720.690.800.890.650.650.570.670.570.840.69

Specifications / FightersF-15KF-15CMiG-29KMiG-29BJF-17J-10F-35AF-35BF-35CF-22LCA Mk-1
Engine thrust maximum 58,320 46,900 39,682 36,600 18,300 27,557 39,900 39,900 39,900 70,000 20,200
Aircraft weight empty 37,50031,70028,05024,03014,52020,39429,30032,00034,80043,34014,300
Aircraft weight, full fuel 51,02345,57439,60231,75719,65028,76047,78046,00353,80061,34020,944
Aircraft weight, max take-off load 81,00068,00049,38340,78528,00042,50070,00060,00070,00083,50029,100
Total fuel weight 13,52313,87411,55207,72705,13008,36618,48014,00319,00018,00005,419
T/W ratio 1.141.031.001.150.930.960.840.870.741.140.96
T/W ratio 0.720.690.800.890.650.650.570.670.570.840.69