Numerical Simulation And Experimental Investigation On Operating Process For Laser Thruster With Gas Propellant

Using laser instead of chemical energy in conventional rockets, a laser propulsion vehicle can get into low earth orbit with higher payload ratio, less cost and more safety. Compared with molecular and particulate absorption mechanisms, higher temperature and specific impulse could be acquired by inverse bremsstrahlung absorption, with plasma generated by gas propellant. For two modes of gas propellant laser thruster, including air-breathing pulse detonation and continuous-wave(CW) thermal rocket, the operating mechanism and performance were investigated in this dissertation, by means of theoretical analysis, numerical simulation and experiments.The governing equations of flow-field under high power laser were established. For different propulsion mode, the characteristics were discussed and the corresponding equation forms were obtained. The physical models, including composition calculation, nonequilibrium and equilibrium thermodynamics properties, transport properties, and energy sources because of laser absorption, relaxation between internal energy modes, chemical reactions and radiation, were given to make the equations closed.Some key issues for the two propulsion modes were modeled and analyzed firstly. For one-dimension detonation thrusters, analytic expression of impulse coupling coefficient was obtained by characteristic method, considering effects of laser, thruster configuration and atmosphere condition. For parabolic detonation thrusters, a performance model was developed by using point explosion theory and an effective conical space. Correlative calculations coincide well with experimental data. And for stable plasma maintenance in CW thrusters, an energy equilibrium model was established by some simplification. Calculations show that the trend of stable mass rate for different pressures and laser powers is consistent with the experiment. And the existence of stable mass rate zone can be primarily explained.Then the operating processes of two thrusters were simulated numerically. For the unsteady flow of pulse detonation thruster, an implicit dual-time method was adopted to simulate the absorptive wave under different laser intensities, with thermo-chemical nonequilibrium model. As laser intensity increases, the critical electron number density would be attained, resulting in saturation of absorbed energy and descent of absorption efficiency. This result could properly explain the phenomena in Mori's experiment, that impulse coupling coefficient of short pulse duration is lower than the long one under high pulse energy. And the phenomena in Schall's experiment, that impulse coupling coefficient is opposite to the beam quality as pulse energy increasing, could also be explained. Subsequently, the operating process for parabolic thrusters was computed. Results imply the pressure recovery history has an important influence on total impulse received. And due to higher thrust peak and longer positive thrust time, the flat top and longer configuration would significantly enhance the performance. For CW thruster, the effects of propellant, laser power and thruster size were calculated by pressure-based SIMPLEC algorithm. The results indicate that given chamber pressure and nozzle throat, effective absorption efficiency would decrease with higher laser powers, and for laser power of 10kW level, thrusters with hydrogen has a better performance than argon due to its intrinsic properties. Furthermore, properly reducing internal space of thruster would increase the specific enthalpy and heat uniformity of the propellant, thus the specific impulse.Thruster prototypes were designed and manufactured, and corresponding experimental systems were set up. In the 10kW level and 1s ignition experiment for CW thruster, laser breakdown and heating led chamber pressure and thrust to continuously rise during irradiating. As a result, peak thrust of 1.075N and net thrust of 0.095N were obtained. The detonation thruster experiments between 340J and 480J show that both impulse and impulse coupling coefficient increases with pulse energy, and the simulation results are consistent well with experimental data. For point-focusing thruster, impulse saturation effect emerges above 440J, and this phenomenon supports the former absorptive wave calculation. Then multi-pulse vertical flight experiments were carried out with air-breathing pulse detonation and ablation detonation thrusters. Results show that for air-breathing thruster the experimental trace approximately accords with the calculation, and for ablation thruster the impulse characteristics are quite different from that of single pulse, possibly because of the stagnation of absorptive product.Combined the two propulsion modes, micro-satellite launch conception to low earth orbit was studied, and the trajectories with a single laser site were calculated by discretizing the control law. Results show that better capacity can be gained by increasing the height of mode switch, total energy can be reduced at higher launch altitude, and laser power should be selected to match a certain laser site altitude.