behaves as an ideal gas. As an example calculation using the above equation, assume that the propellant combustion gases are: at an absolute pressure entering the nozzle of p = 7.0MPa and exit the rocket exhaust at an absolute pressure of pe = 0.1MPa; at an absolute temperature of T = 3500K; with an isentropic expansion factor of γ = 1.22 and a molar mass of M = 22 kg/kmol. Using those values in the above equation yields an exhaust velocity ve = 2802 m/s or 2.80 km/s which is consistent with above typical values. The technical literature can be very confusing because many authors fail to explain whether they are using the universal gas law constant R which applies to any ideal gas or whether they are using the gas law constant Rs which only applies to a specific individual gas. The relationship between the two constants is Rs = R/M, where R is the universal gas constant, and M is the molar mass of the gas. Specific impulse Thrust is the force that moves a rocket through the air or space. Thrust is generated by the propulsion system of the rocket through the application of Newton's third law of motion: "For every action there is an equal and opposite reaction". A gas or working fluid is accelerated out the rear of the rocket engine nozzle, and the rocket is accelerated in the opposite direction. The thrust of a rocket engine nozzle can be defined as: the term in parenthesis is known as equivalent velocity, The specific impulse is the ratio of the thrust produced to the weight flow of the propellants. It is a measure of the fuel efficiency of a rocket engine. In English Engineering units it can be obtained as where: {| border="0" cellpadding="2" |- !align=right| , |align=left | gross thrust of rocket engine (N) |- !align=right| , |align=left | mass flow rate of gas (kg/s) |- !align=right| , |align=left | velocity of gas at nozzle exhaust (m/s) |- !align=right| , |align=left | pressure of gas at nozzle exhaust (Pa) |- !align=right| , |align=left | external ambient, or free stream, pressure (Pa) |- !align=right| , |align=left | cross-sectional area of nozzle exhaust (m²) |- !align=right| , |align=left | equivalent (or effective) velocity of gas at nozzle exhaust (m/s) |- !align=right| , |align=left | specific impulse (s) |- !align=right| , |align=left | standard gravity (at sea level