Editing Thrust-to-weight ratio (section)

==Rockets==
[[File:Thrust to weight ratio vs Isp.png|thumb|upright=1.6|Rocket vehicle thrust-to-weight ratio vs [[specific impulse]] for different propellant technologies]]

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''.<ref name="sutton">George P. Sutton & Oscar Biblarz, ''Rocket Propulsion Elements'' (p. 442, 7th edition) "thrust-to-weight ratio ''F''/''W<sub>g</sub>'' is a dimensionless parameter that is identical to the acceleration of the rocket propulsion system (expressed in multiples of ''g''{{sub|0}}) if it could fly by itself in a gravity-free vacuum"</ref>

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<ref>George P. Sutton & Oscar Biblarz, ''Rocket Propulsion Elements'' (p. 442, 7th edition) "The loaded weight ''W<sub>g</sub>'' is the sea-level initial gross weight of propellant and rocket propulsion system hardware."</ref> and is sometimes called ''thrust-to-Earth-weight ratio''.<ref>{{cite web
 |publisher   = The Internet Encyclopedia of Science
 |title       = Thrust-to-Earth-weight ratio
 |url         = http://www.daviddarling.info/encyclopedia/T/thrust-to-Earth-weight_ratio.html
 |access-date  = 2009-02-22
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20080320040846/http://www.daviddarling.info/encyclopedia/T/thrust-to-Earth-weight_ratio.html
 |archive-date = 2008-03-20
}}</ref> 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''{{sub|0}}.<ref name="sutton"/>

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.<ref name="sutton"/> Take-off can occur when the vehicle's ''g-force'' exceeds local gravity (expressed as a multiple of ''g''{{sub|0}}).

The thrust-to-weight ratio of rockets typically greatly exceeds that of [[airbreathing jet engine]]s 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 [[Field strength#Gravitational field strength|gravitational field strength]].