A Comparison of Adiabatic and Isothermal Expansion Processes in Rocket Nozzles

An analysis has been made of the expansion of gases through a de Laval nozzle under isothermal condit ions. It is shown that for ideal gases, the increase in kinetic energy of the gases is equal to the heat transferred into the gas to mainta in constant temperature. The initial energy, or temperature, enters only in the design of the nozzle and determination of the pressure ratio required for the transference of the fixed quanti ty of heat . Unlike the adiabatic case, the jet velocity is not l imited by the initial energy content of the gas but only by the pressure ratio or energy content of the reservoir transferring the heat . Formulas are derived for isothermal exhaust velocity, mass flow, and thrust coefficient which are analogous to those for adiabatic flow.