【作者】 李小康；

【导师】 张育林；

【作者基本信息】 国防科学技术大学 ， 航空宇航科学与技术， 2011， 博士

【摘要】 采用激光代替传统火箭的化学能,激光推进飞行器能够以高载荷比、低成本、安全的方式将有效载荷送入近地轨道。相对于激光的分子吸收和粒子吸收机制,在气态工质环境中产生等离子体进行逆轫致吸收的方式工质温度更高,可望达到更高的比冲。本文针对吸气式脉冲爆震和火箭式连续激光加热两种气体工质激光推力器的工作模式,采用理论分析、数值模拟和实验验证的方法对其工作机理与性能进行了研究。建立了强激光作用下气体流动的控制方程组,针对不同推进模式的特点进行分析并给出了方程组相应的形式。详细给出了化学反应组元计算、高温非平衡与平衡热力学性质、输运性质、以及由于激光吸收、各内能模式弛豫、化学反应、热辐射产生的各种能量源项模型。针对吸气式脉冲爆震和连续激光加热这两种推进模式着重关注的问题进行了建模和分析。对一维激光爆震推力器,考虑激光束、推力器构型以及大气环境三方面参数的影响,采用特征线方法推导了冲量耦合系数的解析表达形式。针对抛物形爆震推力器,采用点爆炸理论和等效锥形约束空间获得了推力器工作性能的直接计算方法,计算结果与实验数据一致性较好。针对连续激光推力器中的等离子体稳定维持问题,通过简化分析给出了解析计算模型,该模型能够正确反映实验中稳定质量流量随压力、激光功率的变化趋势,并初步解释了推力器中存在稳定质量流量区间的原因。对推力器内部工作过程进行了数值仿真。对脉冲爆震推力器的非定常流场采用热化学非平衡模型,以隐式双时间步进格式计算了不同光强下的激光支持吸收波。计算结果表明,随入射光强增加,等离子体电子数密度达到临界值,吸收能量趋于饱和,有效吸收效率随之下降。这一结果合理解释了Mori实验中高脉冲能量条件下短脉宽对应冲量耦合系数低于长脉宽的现象,以及Schall实验中冲量耦合系数与光束质量提高相悖的现象。进一步研究了抛物形爆震激光推力器的工作过程,结果表明推力主峰后的压力振荡恢复过程对冲量有明显影响;由于更高的推力峰值和较长作用时间,平顶和较长推力室的结构可以明显提高获得的冲量。针对连续激光推力器重点研究了推进剂物性、激光功率和构型相对尺寸对推力器性能的影响。计算表明,给定喉部面积和室压时增大入射激光功率将使有效吸收效率降低;在10kW级激光作用下氢等离子体相对于氩具有更小的辐射损失,更适合应用于连续激光推力器;确定适当的内部流道尺寸可以提高工质比焓,显著提高推力器比冲。设计和加工了推力器原型并建立了相应实验系统。连续推力器的10kW级1s点火特性实验表明,在激光入射时间内,由于其击穿并加热气体,造成推力器室压和推力均持续上升,获得的推力峰值为1.075N,净推力为0.095N。空气爆震推力器实验表明,在340～480J强激光脉冲作用下,随脉冲能量上升冲量和冲量耦合系数均有所上升;同数值结果对比表明,二者在数值及发展趋势上均表现一致;440J以上点聚焦推力器出现冲量饱和效应,支持激光吸收波的计算结果。进行了空气爆震和烧蚀爆震两种飞行器多脉冲垂直飞行实验,前者运动轨迹与利用理论计算的结果基本吻合,验证了机理研究的结果;后者在多脉冲作用下由于吸收性烧蚀产物滞留,其冲量特性显著有别于单脉冲性能。初步探讨了组合吸气式脉冲爆震和连续激光加热这两种推进模式发射近地轨道微小卫星的问题,通过将控制律离散化,计算了利用单站地基激光器发射的弹道。结果表明,提高推进模式的切换高度,可以明显提高飞行器的入轨能力;采用高海拔发射点有利于降低发射过程中的总能量损耗;对于某一初始发射点海拔,激光器功率的选择应与之相匹配。更多还原

【Abstract】 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.更多还原