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The main objective of utilizing nozzles is to convert the chemical energy to kinetic energy producing thrust. Wide variety of parameters make significant impact on nozzle performance; one of which produces significant effect is back pressure or ambient pressure. Basically, a nozzle designed for a specific back pressure does not work properly when the engine is ascending. Consequently, designing of nozzles needs knowledge of full effect of back pressure on engine performance. In this study, numerical simulation of three solid propellant nozzles have been conducted in several flight conditions. In other words, simulations have done in some ambient pressures which represents specific flight altitudes. Numerical modeling has been conducted aiding commercial code FLUENT. k-ϵ RNG turbulence model has been used for calculating turbulence interactions with the flow. Mass flow rate, chemical species, and chamber temperature have been used as the inlet boundary conditions based on engine specifications. Numerical results show a reasonable accuracy in comparison with experimental measurements. Estimating nozzle thrust level as a function of altitude increment is the primary goal of this study. Furthermore, with the aid of this relation and a MATLAB code for computing average specific impulse, optimum expansion ratio can be achieved based on a specified mission.

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In the present paper a DSMC solver is utilized to study the effects of wall heating/heater plates on performance parameters of microthruster systems. The solver uses local Knudsen number based on the gradient of flow properties to distinct the molecular and continuum region. This solver uses theory of characterisitcs for determination of inlet and outlet boundary conditions. Proper cell dimensions, number of particles per cell, and grid study are performed to guarantee the accuracy of simulations. Three typical micropropulsion systems are studied. All three systems have a microchannel and a converging-diverging micronozzle. First type is cold gas micropropulsion system, second type is a microthruster with wall heated channel, third type is microthruster with heater plates inside. The first type is considered as reference case and two other systems are compared with type1. It is obsereved that heating the walls in microthruster type2 accelerates the flow and increase the specific impulse of the system. In micropropulsion device type3, heater plates increase downstream temperature of convergent-divergent nozzle and also elevate the specific impulse. Due to considerable mass flow rate decrease of system type3, its thrust is decreased whereas mass flow rate of system type2 is not decreased as much as type3 and therefore the thrust of microthruster type2 is more than type1 and type3. Hence the second microprolusion system configuration has higher performance paratmeters in comparison with two other systems. It is also observed that increasing of wall temperature in microthruster type2 decrease the thrust and specific impulse sensitivity to temperature increase.