A Comprehensive Cold Gas Performance Study of the Pocket Rocket Radiofrequency Electrothermal Microthruster

Abstract

This paper presents computational fluid dynamics simulations of the cold gas operation of Pocket Rocket and Mini Pocket Rocket radiofrequency electrothermal microthrusters, replicating experiments performed in both sub-Torr and vacuum environments. This work takes advantage of flow velocity choking to circumvent the invalidity of modeling vacuum regions within a CFD simulation, while still preserving the accuracy of the desired results in the internal regions of the microthrusters. Simulated results of the plenum stagnation pressure is in precise agreement with experimental measurements when slip boundary conditions with the correct tangential momentum accommodation coefficients for each gas are used. Thrust and specific impulse is calculated by integrating the flow profiles at the exit of the microthrusters, and are in good agreement with experimental pendulum thrust balance measurements and theoretical expectations. For low thrust conditions where experimental instruments are not sufficiently sensitive, these cold gas simulations provide additional data points against which experimental results can be verified and extrapolated. The cold gas simulations presented in this paper will be used as a benchmark to compare with future plasma simulations of the Pocket Rocket microthruster.

Microplasma thruster powered by X-band microwaves

“…A comparison of representative operating parameters/conditions and thrust performance is given in Table II, for several microplasma thrusters of electrothermal type (with power inputs on the order of 1-10 W) that have so far been reported in the literature. [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49]…”

“…The electromagnetic thrusters rely on the plasma acceleration via interaction of the current and magnetic fields: micro pulsed plasma thrusters. 31,32 The electrothermal thrusters rely on the plasma/gas heating via plasma discharge followed by supersonic plasma/ gas expansion through de Laval nozzles: micro arcjet thrusters, 33,34 hollow cathode thrusters, 35 and plasma thrusters using dc microdischarges, 36,37 ac/rf microcavity discharges, 38,39 capacitively coupled rf discharges in a dielectric tube, [40][41][42][43][44][45][46][47] and microwave discharges. 48,49 We have developed a mm-scale microplasma thruster of the electrothermal type with azimuthally symmetric surface wave-excited plasmas (SWPs), [50][51][52][53][54][55][56][57][58] consisting of a microplasma source followed by a converging-diverging micronozzle as schematically shown in Fig.…”

An Inductively-Coupled Plasma Electrothermal Radiofrequency Thruster

“…Fruchtman [24] also discussed the complexity of thrust imparted by low pressure and high pressure expanding plasma sources. For capacitive PR, the basic understanding of the plasma-generated thrust increase at 13.56 MHz from the cold gas thrust has been validated using CFD Ace+ [25] and HEMP fluid-plasma transient simulations [21]. In the present study, we demonstrate direct thrust measurement of a mm size inductively coupled plasma source using a vacuum compatible miniaturized impedance matching system at 40.68 MHz.…”

Vacuum Testing of a Miniaturized Switch Mode Amplifier Powering an Electrothermal Plasma Micro-Thruster

“…For both pumping conditions, the collision mean free paths are orders of magnitude longer than the typical dimensions of the thruster where ionizing collisions take place (choked flow regime [29]) and the plasma ON/OFF frequencies are essentially independent of the pumping equipment. The Converter frequency can be affected by the argon mass flow rate injected through a small channel where it is ionized.…”

“…Although it is beyond the scope of this study to fully quantify the thrust gain from such a pressure increase of Figure 6, a quick estimate similar to the calculations and computer simulations of Charles and Boswell [8], Fridman et al [12], and Ho et al = 322 m.s −1 (where T is the gas temperature, k B = 1.38 × 10 −23 J.K −1 is the Boltzmann constant, γ Ar = 1.667 is the specific heat capacity for argon and m Ar = 6.64x10 −26 kg is the atomic mass of argon), the corresponding thrust from the momentum term (neglecting the neutral gas pressure term [12,29]) would be F cold gas = c s dm dt ∼ 0.19 mN at T ∼ 300 K. Recent computer simulations [24] show that approximately one tenth of the power injected into the plasma is converted into gas heating: hence with 10 Watts into MiniPR, 1 Watt or 1 J/s of kinetic energy ǫ is effectively transferred into heating the gas: ǫ = . The thrust gain from the plasma would be F plasma ∼ 0.17 mN yielding a total thrust of F total ∼ 0.36 mN.…”