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.
The thrust-to-weight ratio is calculated by dividing the thrust (in SI units - in newtons) by the weight (in newtons) of the engine or vehicle. The weight (N) is calculated by multiplying the mass in kilograms (kg) by the acceleration due to gravity (m/s). The thrust can also be measured in pound-force (lbf), provided the weight is measured in pounds (lb). Division using these two values still gives the numerically correct (dimensionless) 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.
Because an aircraft's weight can vary considerably, depending on factors such as munition load, fuel load, cargo weight, or even the weight of the pilot, the thrust-to-weight ratio is also variable and even changes during flight operations. There are several standards for determining the weight of an aircraft used to calculate the thrust-to-weight ratio range.
The thrust-to-weight ratio and lift-to-drag ratio are the two most important parameters in determining the performance of an aircraft.
The thrust-to-weight ratio varies continually during a flight. Thrust varies with throttle setting, airspeed, altitude, air temperature, etc. 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.[2] Aircraft with thrust-to-weight ratio greater than 1:1 can pitch straight up and maintain airspeed until performance decreases at higher altitude.[3]
A plane can take off even if the thrust is less than its weight as, unlike a rocket, the lifting force is produced by lift from the wings, not directly by thrust from the engine. As long as the aircraft can produce enough thrust to travel at a horizontal speed above its stall speed, the wings will produce enough lift to counter the weight of the aircraft.
| \left( | T |
| W |
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| D | |
| L |
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.
For propeller-driven aircraft, the thrust-to-weight ratio can be calculated as follows in imperial units:[4]
| T | |
| W |
=
| 550ηp | |
| V |
| hp | |
| W |
,
ηp
V
The metric formula is:
| T | =\left( | |
| W |
| ηp | \right)\left( | |
| V |
| P | |
| W |
\right).
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.[5]
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[6] and is sometimes called thrust-to-Earth-weight ratio.[7] 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, g.[5]
The thrust-to-weight ratio of a rocket improves as the propellant is burned. With constant thrust, the maximum ratio (maximum acceleration of the vehicle) 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 is greater than that of the complete 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 greater than one. In general, the thrust-to-weight ratio is numerically equal to the g-force that the vehicle can generate.[5] Take-off can occur when the vehicle's g-force exceeds local gravity (expressed as a multiple of g).
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 thrust-to-weight ratio. The instantaneous value typically varies over the duration of flight with the variations in thrust due to speed and altitude, together with changes in weight due to the amount of remaining propellant, and payload mass. Factors with the greatest effect 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.
| Vehicle | thrust-weight ratio | Notes | |
|---|---|---|---|
| Northrop Grumman B-2 Spirit | 0.205[8] | Max take-off weight, full power | |
| Airbus A340 | 0.2229 | Max take-off weight, full power (A340-300 Enhanced) | |
| Airbus A380 | 0.227 | Max take-off weight, full power | |
| Boeing 747-8 | 0.269 | Max take-off weight, full power | |
| Boeing 777 | 0.285 | Max take-off weight, full power (777-200ER) | |
| Boeing 737 MAX 8 | 0.311 | Max take-off weight, full power | |
| Airbus A320neo | 0.310 | Max take-off weight, full power | |
| Boeing 757-200 | 0.341 | Max take-off weight, full power (w/Rolls-Royce RB211) | |
| Tupolev 154B | 0.360 | Max take-off weight, full power (w/Kuznecov NK-82) | |
| Tupolev Tu-160 | 0.363 | Max take-off weight, full afterburners | |
| Concorde | 0.372 | Max take-off weight, full afterburners | |
| Rockwell International B-1 Lancer | 0.38 | Max take-off weight, full afterburners | |
| HESA Kowsar | 0.61 | With full fuel, afterburners. | |
| BAE Hawk | 0.65[9] | ||
| Lockheed Martin F-35 A | 0.87 | With full fuel (1.07 with 50% fuel, 1.19 with 25% fuel) | |
| HAL Tejas Mk 1 | 1.07 | With full fuel | |
| CAC/PAC JF-17 Thunder | 1.07 | With full fuel | |
| Dassault Rafale | 0.988[10] | Version M, 100% fuel, 2 EM A2A missile, 2 IR A2A missiles | |
| Sukhoi Su-30MKM | 1.00[11] | Loaded weight with 56% internal fuel | |
| McDonnell Douglas F-15 | 1.04[12] | Nominally loaded | |
| Mikoyan MiG-29 | 1.09[13] | Full internal fuel, 4 AAMs | |
| Lockheed Martin F-22 | |||
| General Dynamics F-16 | 1.096 (1.24 with loaded weight & 50% fuel) | ||
| Hawker Siddeley Harrier | 1.1 | VTOL | |
| Eurofighter Typhoon | 1.15[14] | Interceptor configuration | |
| Sukhoi Su-35 | 1.30 | ||
| Space Shuttle | 1.5 | Take-off | |
| Simorgh (rocket) | 1.83 | ||
| Space Shuttle | 3 | Peak |
| Engine | Mass | Thrust, vacuum | Thrust-to- weight ratio | ||
|---|---|---|---|---|---|
| (kN) | (lbf) | ||||
| RD-0410 nuclear rocket engine[15] [16] | 2000kg (4,000lb) | 35.2 | 35.2kN | ||
| Pratt & Whitney J58 jet engine (Lockheed SR-71 Blackbird)[17] [18] | 2722kg (6,001lb) | 150 | 150kN | ||
| Rolls-Royce/Snecma Olympus 593 turbojet with reheat (Concorde)[19] | 3175kg (7,000lb) | 169.2 | 169.2kN | align=right | |
| Pratt & Whitney F119[20] | 20500lbf | ||||
| PBS TJ40-G1NS jet engine[21] | 425N | ||||
| RD-0750 rocket engine three-propellant mode[22] | 4621kg (10,188lb) | 1413kN | |||
| RD-0146 rocket engine[23] | 260kg (570lb) | 98kN | |||
| Rocketdyne RS-25 rocket engine (Space Shuttle Main Engine)[24] | 3177kg (7,004lb) | 2278kN | |||
| RD-180 rocket engine[25] | 5393kg (11,890lb) | 4,152 | |||
| RD-170 rocket engine | 9750kg (21,500lb) | 7887kN | |||
| F-1 (Saturn V first stage)[26] | 8391kg (18,499lb) | 7740.5kN | |||
| NK-33 rocket engine[27] | 1222kg (2,694lb) | 1638kN | |||
| SpaceX Raptor 2 rocket engine[28] | 1600kg (3,500lb) | 2,256 | 2256kN | ||
| Merlin 1D rocket engine, full-thrust version[29] [30] | 914 | 205,500 | 199.5 | ||
| Specifications | F-15K | F-15C | MiG-29K | MiG-29B | JF-17 | J-10 | F-35A | F-35B | F-35C | F-22 | LCA Mk-1 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Engines thrust, maximum (N) | 259,420 (2) | 208,622 (2) | 176,514 (2) | 162,805 (2) | 84,400 (1) | 122,580 (1) | 177,484 (1) | 177,484 (1) | 177,484 (1) | 311,376 (2) | 84,516 (1) | |
| Aircraft mass, empty (kg) | 17,010 | 14,379 | 12,723 | 10,900 | 7,965 | 09,250 | 13,290 | 14,515 | 15,785 | 19,673 | 6,560 | |
| Aircraft mass, full fuel (kg) | 23,143 | 20,671 | 17,963 | 14,405 | 11,365 | 13,044 | 21,672 | 20,867 | 24,403 | 27,836 | 9,500 | |
| Aircraft mass, max. take-off load (kg) | 36,741 | 30,845 | 22,400 | 18,500 | 13,500 | 19,277 | 31,752 | 27,216 | 31,752 | 37,869 | 13,500 | |
| Total fuel mass (kg) | 06,133 | 06,292 | 05,240 | 03,505 | 02,300 | 03,794 | 08,382 | 06,352 | 08,618 | 08,163 | 02,458 | |
| T/W ratio, full fuel | 1.14 | 1.03 | 1.00 | 1.15 | 1.07 | 1.05 | 0.84 | 0.87 | 0.74 | 1.14 | 1.07 | |
| T/W ratio, max. take-off load | 0.72 | 0.69 | 0.80 | 0.89 | 0.70 | 0.80 | 0.57 | 0.67 | 0.57 | 0.84 | 0.80 |