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 indicates 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 the initial performance of vehicles.
Calculation The thrust-to-weight ratio can be calculated
by dividing the thrust by the weight of the engine or vehicle. It 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 manoeuvrability
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 changes of payload. For aircraft, the quoted thrust-to-weight
ratio is often the maximum static thrust at sea-level divided by the maximum takeoff weight.
In cruising flight, the thrust-to-weight ratio of an aircraft is the inverse of the lift-to-drag
ratio because thrust is equal to drag, and weight is equal to lift.
Propeller-driven aircraft For propeller-driven aircraft, the thrust-to-weight
ratio can be calculated as follows:
where is propulsive efficiency at true airspeed is engine power
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 Jet and rocket engines
Rocket thrusts are vacuum thrusts unless otherwise noted
Fighter aircraft Fuel density used in calculations: 0.803 kg/l
The number inside brackets is the number of engines.
Engines powering F-15K are the Pratt & Whitney engines.
MiG-29K's empty weight is an estimate. JF-17's engine rating is of RD-93.
JF-17 if mated with its engine WS-13, and if that engine gets its promised 18,969 lb
then the T/W ratio becomes 0.97 J-10's empty weight and fuelled weight are
estimates. J-10's engine rating is of AL-31FN.
J-10 if mated with its engine WS-10A, and if that engine gets its promised 132 KN(29,674 lbf)
then the T/W ratio becomes 1.03 See also
Power-to-weight ratio Factor of safety
References John P. Fielding. Introduction to Aircraft
Design, Cambridge University Press, ISBN 978-0-521-65722-8 Daniel P. Raymer. Aircraft Design: A Conceptual
Approach, American Institute of Aeronautics and Astronautics, Inc., Washington, DC. ISBN
0-930403-51-7 George P. Sutton & Oscar Biblarz. Rocket Propulsion
Elements, Wiley, ISBN 978-0-471-32642-7 Notes
External links NASA webpage with overview and explanatory
diagram of aircraft thrust to weight ratio