The need for low propellant weight, high efficiency propulsion systems is a glaring need for various space missions. This thesis presents the thrust modeling of the Dense Plasma Focus plasma motion phases. It also contrasts some of the engineering tradeoffs between the existing coaxial plasma thrusters and the Dense Plasma Focus. Modeling the thrust generated by the DPF started with seeing how far the working models for the MPD would take the DPF. The effect of pulsed compared to quasi-steady state operation is computed. There is no known experimental data regarding thrust measurements for any DPF, so the thrust is analytically calculated using experimental data for the TAMU DPF. The calculated thrust is slightly higher than the thrust predicted by the models. The developed model shows that the force generated by the DPF will produce a thrust roughly three times the thrust for the MPD for similar input currents and electrode geometry. For the TAMUDPF to compete with the MPD as a thruster, it will need to be able to fire roughly 75 more times a second than the MPD.

The need for low propellant weight, high efficiency propulsion systems is a glaring need for various space missions. This thesis presents the thrust modeling of the Dense Plasma Focus plasma motion phases. It also contrasts some of the engineering tradeoffs between the existing coaxial plasma thrusters and the Dense Plasma Focus. Modeling the thrust generated by the DPF started with seeing how far the working models for the MPD would take the DPF. The effect of pulsed compared to quasi-steady state operation is computed. There is no known experimental data regarding thrust measurements for any DPF, so the thrust is analytically calculated using experimental data for the TAMU DPF. The calculated thrust is slightly higher than the thrust predicted by the models. The developed model shows that the force generated by the DPF will produce a thrust roughly three times the thrust for the MPD for similar input currents and electrode geometry. For the TAMUDPF to compete with the MPD as a thruster, it will need to be able to fire roughly 75 more times a second than the MPD.

This study presents the overall design of a 3U CubeSat equipped with commercial-off-the shelf hardware, Teflon-fueled micro-Pulsed Plasma Thrusters (µPPT) and an attitude determination and control system. The µPPT is sized by the impulse bit and pulse frequency required for continuous compensation of expected maximum disturbance torques at altitudes between 400 and 1000 km, and to perform stabilization of up to 20 deg/s and slew maneuvers of up to 180 degrees. The study involves realistic power constraints anticipated on the 3U CubeSat. Attitude estimation is implemented using the q-method for static attitude determination of the quaternion using pairs of the spacecraft-sun and magnetic field vectors. The quaternion estimate and the gyroscope measurements are used with an extended Kalman filter to obtain the attitude estimates. Proportional and derivative control algorithms use the static attitude estimation in order to calculate the angular momentum required to compensate for the disturbance torques and to achieve specified stabilization and slewing maneuvers or combinations. Two control methods are developed: paired firing method, and separate control algorithm and thruster allocation methods which determines the optimal utilization of the available thrusters and introduces redundancy. Simulations results are presented for a 3U CubeSat under stabilization, pointing, and pointing and spinning scenarios.

"One of the general processes of interest in Pulsed Plasma Thrusters is the ablation of the solid fuel. In general, ablation occurs when a short pulse of applied energy removes a portion of the fuel surface. Although this ablation process is relatively straight-forward in simple materials that sublimate, ablation in Pulsed Plasma Thrusters is significantly more complicated. This is caused by the transient conditions and the complex behavior of Teflon that does not sublimate but rather undergoes both physical and chemical changes prior to leaving the surface. These two effects combine to make Teflon ablation a highly nonlinear function of heat flux, material property variations, changing molecular weight, and phase transformation behavior. To gain greater insight into the ablation process, a one-dimensional ablation model is developed that addresses the more detailed thermal and thermodynamic behavior of Teflon during simulated operation of a Pulsed Plasma Thruster. The mathematical model is based on the work of Clark (1971), which focused on two-phase, one-dimensional Teflon ablation in the context of thermal protection systems. The model is modified for use in simulated PPT operations and implemented numerically using an adaptive non-uniform grid, explicit finite-difference techniques, and a volume fraction method to capture the interface between the crystalline and amorphous Teflon phases. The ablation model is validated against analytical heat transfer and ablation solutions and compared with previous experimental results. The Teflon ablation model is used to analyze several general ablation scenarios in addition to specific PPT conditions to gain greater insight into long-duration thruster firing, post-pulse material ablation, variable heat flux effects, variable material property effects, and the impact of surface re-crystallization on particulate emission. These simulations are considered in the context of prior experimental investigations of Pulsed Plasma Thrusters. The results of these simulations demonstrate the success of the numerical ablation model in predicting experimental trend and suggest potential paths of moderately improving thruster efficiency and operational repeatability in the future. "

La modélisation des propulseurs ioniques de type SPT pose de nombreux <br />problèmes dans le domaine du transport des particules chargées. Nous nous <br />intéressons à deux de ces problèmes, à savoir le transport des électrons et <br />le calcul du potentiel électrique.<br /><br />Le transport des électrons résulte de l'influence conjuguée des champs <br />(électrique et magnétique) établis dans la cavité du propulseur et des <br />collisions des électrons (dans la cavité et avec la paroi limitant celle-ci). <br />Nous avons participé au développement d'un modèle SHE (Spherical Harmonics <br />Expansion) qui résulte d'une analyse asymptotique de l'équation de Boltzmann <br />munie de conditions de réflexion aux bords. Ce modèle permet d'approcher la <br />fonction de distribution en énergie des électrons en résolvant une <br />équation de diffusion dans un espace \{position, énergie\}. Plus précisément, <br />nous avons étendu une démarche existante au cas où les collisions en volume <br />(excitation, ionisation) et les collisions inélastiques à la paroi <br />(attachement et émission secondaire) sont prises en compte. Enfin, nous <br />avons écrit un code de résolution du modèle SHE, dont les résultats ont <br />été comparés avec ceux d'une méthode de Monte Carlo. <br /><br />\vspace*{1mm}<br />Dans un deuxième temps, nous avons étudié le calcul du potentiel électrique. <br />La présence du champ magnétique impose d'écrire le courant d'électrons sous <br />la forme ${\cal J}=\sigma \nabla W$<br /> où W est le potentiel électrique et le tenseur de conductivité $\sigma$<br />est fortement anisotrope compte tenu des grandeurs physiques en jeu dans <br />le SPT. Pour résoudre $\mbox{div }{\cal J}(x,y)=S(x,y)$, <br />nous avons implémenté une méthode de volumes finis <br />sur maillage cartésien permettant de résoudre ce problème elliptique <br />anisotrope, et nous avons vérifié qu'elle échouait lorsque le rapport <br />d'anisotropie devenait grand. Aussi nous avons développé une méthode de <br />paramétrisation, qui consiste à extrapoler la solution d'un problème <br />anisotrope à l'aide d'une suite de problèmes isotropes. Cette méthode a <br />donné des résultats encourageants pour de forts rapports d'anisotropie, <br />et devrait nous permettre d'atteindre des cas réels.