# Rocket Engines

( Newtons 2nd and 3rd law )
A rocket is propelled forward by a thrust force equal in magnitude, but opposite in direction, to the time-rate of momentum change of the exhaust gas accelerated from the combustion chamber through the rocket engine nozzle. This is the exhaust velocity with respect to the rocket, times the time-rate at which the mass is expelled, or in mathematical terms:

T={dm}/{dt} * {v}

where:

* T is the thrust generated (force)
* {dm}/ {dt} is the rate of change of mass with respect to time (mass flow rate of exhaust);
* v is the speed of the exhaust gases measured relative to the rocket.

For vertical launch of a rocket the initial thrust must be more than the weight.

Each of the three Space shuttle main engines can produce a thrust of 1.8 MN, and each of its two Solid Rocket Boosters 14.7 MN, together 29.4 MN. Compare with the mass at lift-off of 2,040,000 kg, hence a weight of 20 MN.

By contrast, the simplified Aid for EVA Rescue (SAFER) has 24 thrusters of 3.56 N each.

In the air-breathing category, the AMT-USA AT-180 jet engine developed for radio-controlled aircraft produce 90 N (20 lbf) of thrust.[1] The GE90-115B engine fitted on the Boeing 777-300ER, recognized by the Guinness Book of World Records as the “World’s Most Powerful Commercial Jet Engine,” has a thrust of 569 kN (127,900 lbf).

MN – MegaNewton
lbf – pound force

Source