Thrust-to-weight ratio
The thrust-to-weight ratio is defined by the quotient of the thrust F s of a missile and its weight . The weight is the product of its mass m and the gravitational acceleration g .
A thrust-to-weight ratio of over 1 therefore enables flight vertically upwards, i.e. without aerodynamic lift . Missiles , VTOL aircraft and helicopters therefore require a thrust-to-weight ratio of over 1. In combat aircraft , the thrust-to-weight ratio is one of the two most important parameters in addition to the wing loading on the wings.
Examples
When the engines are fired with full tanks, a vertically launched manned rocket sensibly has a thrust-to-weight ratio of just over 1 and is held by the launch pad until the computer reports that the engines are working correctly. Due to the enormous fuel consumption, the weight decreases very quickly. The thrust-to-weight ratio rises to values of over 4 towards the end of the fire in the first stage, so that the power of manned rockets is throttled in order to limit the acceleration for the astronauts. An example of this is the Saturn V with a take-off thrust-to-weight ratio of 1.18, which switches off the inner engine after 139 s ( Apollo take-off profile ). By dropping the empty first stage, the mass of the remaining rocket is reduced. The next stage does not necessarily have to have an initial thrust-to-weight ratio of more than 1, as this mainly increases the horizontal speed until the rocket reaches orbit. For example, the Atlas V upper stage has a thrust of 99 kN and an (initial) mass of 23 t , which corresponds to a thrust-to-weight ratio of 0.44.
Normal aircraft, including many combat aircraft, have a thrust-to-weight ratio of well below 1 when taking off with a typical load and initially accelerate horizontally on the runway. Then they go into the inclined climb. It is true that aircraft can start a steep parabolic flight , vertical flight or looping for a few seconds, starting with a sufficiently high horizontal speed , but in the long run (i.e. longer than approx. 10 seconds) the engines or the climbing power are not strong enough. The plane would lose momentum, slow down, and crash unless steered back to a flatter angle.
Jet fighters and interceptors since the 1970s have a thrust-to-weight ratio of just over 1, even when loaded, and can accelerate vertically into the sky after take-off, like a MiG-29 . With an unarmed, relieved and suitably fueled F-15 , several records were set in 1975, for example reaching the altitude of 12,000 m in less than a minute. The fighter was held at take-off until maximum thrust with afterburner was reached, after which the F-15 took off after a taxiway of about 140 m or 7 aircraft lengths and broke the sound barrier after 23 seconds.
| model | First flight / origin | Thrust-to-weight ratio | Mass (normal takeoff weight) |
|---|---|---|---|
| Eurofighter Typhoon | 1994
|
1.18 | 15,500 kg |
| Boeing C-17 (empty) | 1991
|
0.62 | 122,016 kg |
| McDonnell Douglas C-17 (maximum load) | 1991
|
0.29 | 263,083 kg |
| Lockheed Martin F-35 A | 2006
|
0.88 | 22,280 kg |
| McDonnell Douglas F-15E Strike Eagle | 1986
|
0.93 | 28,440 kg |
| McDonnell Douglas F-15C Eagle | 1972
|
1.07 | 20,185 kg |
| MiG-29 | 1977.svg){kind=small}
|
0.99 | 16,800 kg |
| MiG-29K | 1989
|
0.95 | 18,500 kg |
| MiG-29M | 1986
|
1.03 | 17,500 kg |
| Su-27P | 1977
|
1.07 | 23,430 kg |
| Sukhoi Su-57 | 2010
|
1.15 | 26,000 kg |
| General Dynamics F-16 | 1974
|
1,096 | 12,003 kg |
| Space shuttle | 1981
|
0.5-3 | 2,046,000 kg (with boosters and tank) |
| Boeing 757-200 (empty) | 1982
|
0.68 | 57,840 kg |