中野 正勝

The authors have developed laser ignition micro solid rocket which is a propulsion system suitable for 10-kg-class microspacecrafts. Although the specific impulse is expected to increase by reducing the throat area, higher combustion chamber pressure with the smaller throat causes a couple of serious problems. One is throat erosion and the other is break of combustion chamber. In order to prevent these difficulties, we made three improvements in design of thruster. First, we reinforced the throat, and throat erosion did not occur. Secondly, we changed the ignition surface of the propellant. As a result, peak pressure reduced, but specific impulse also reduced. Thirdly, we made the propellant to encapsulate to constrain the combustion surface, and peak pressure reduced considerably. We designed thrusters which had smaller throat than conventional, and established the increase in specific impulse by 17%. We visualized the combustion chamber, and it was found that capsule could control the combustion surface of propellant.

A series of successful ion-propelled missions opened a new era of space exploration and development using high-Isp ion engines. Modifications and improvements of the current ion engines and the development of new ion engines are underway to enhance mission performance using ion engines. However, the life qualification tests for ion engines are difficult, time-consuming, and costly processes, which is a serious problem for the future use of ion engines. Therefore, the numerical evaluation of ion engine life has become very important. In 2006, JAXA started the JAXA Ion Engine Development Initiative (JIEDI) project to develop a numerical tool for the life qualification of ion engine optics. The developed code (JIEDI-1) can analyze the wear of the grid system caused by ion and neutral sputtering with a reasonable computational time. In this paper, the JIEDI-1 code is explained in detail and several examples of its use are presented.

Grid erosion analysis of a μ20 ion engine was carried out. The design principle of the grid optics for the μ20 ion engine is fairly unique, in that the acceleration grid is ion-beam machined to produce an optimum grid shape by the direct impingement of ion beams. By adopting a new grid surface model and strategy for updating the grid surfaces, the highly eroded grid surfaces of the μ20 ion engine were successfully analyzed. Parametric studies were also conducted to assess the grid lifetimes for different acceleration grid voltages.

A sensitivity analysis was carried out using a three-dimensional grid erosion analysis code to understand the effect of doubly charged ions on the erosion of an ion acceleration grid. In the preliminary analysis, it was found that the neutral flux value estimated by assuming all the ions to be singly charged has significant errors when the doubly charged ion fraction is non-negligible. Calculations were conducted for the μ10 EM1 ion acceleration grid system, varying the doubly charged ion fraction (0, 0.1, and 0.2), and the experimental and simulation results of the acceleration grid current and acceleration grid mass loss were compared. In the case of doubly charged ion fractions of 0.1 and 0.2, which are similar to the experimental condition, the simulation results were in good quantitative agreement with the experimental data. Calculation results showed that when the doubly charged ion fraction was varied from 0.1 to 0 (0.2), the acceleration grid current and acceleration grid mass loss changed by -14% (+9.5%) and -18% (+32%), respectively. Electron backstreaming was also found to vary with the doubly charged ion fraction; it occurred 14% slower (12% faster) when the doubly charged ion fraction was varied from 0.1 to 0 (0.2). The structural failure of the deceleration grid was less sensitive than electron backstreaming; the deceleration grid eroded 8.3% slower (8.6% faster) when the doubly charged ion fraction was varied from 0.1 to 0 (0.2).

On January 23, 2009, the first micro-laser propulsion onboard the KKS-1 microsatellite was deployed in

Activity on numerical plasma simulations by JAXA’s Engineering Digital Innovation (JEDI) Center is overviewed. Currently, R&D of two major numerical tools is conducted. First one is spacecraft charging analysis tool that can compute charging status of a spacecraft solving charged particle motions precisely. Using this information, we can evaluate onboard measurement of electrostatic probes or consider better configuration of onboard equipment of a spacecraft. Computation example of the interactions between solar wind plasma and a solar sail is shown in this paper. Second one is a numerical tool called JIEDI (JAXA’s Ion Engine Development Initiative), which aims to reduce the cost and the time required for an ion thruster life test. The JIEDI tool can numerically estimate ion bombardment to an ion thruster’s grid material to predict the erosion rate of the grid material, and preliminary analysis by the JIEDI tool showed good agreement with the real-time life test of a microwave ion thruster.

Research and development of small spacecraft have advanced extensively throughout the world and propulsion devices suitable for the small spacecraft, microthruster, is eagerly anticipated. The authors proposed a microthruster using 1—10-mm-size solid propellant. Small pellets of solid propellant are installed in small combustion chambers and ignited by the irradiation of diode laser beam. This thruster is referred as to a laser ignition microthruster. Solid propellant enables large thrust capability and compact propulsion system. To date theories of a solid-propellant rocket have been well established. However, those theories are for a large-size solid propellant and there are a few theories and experiments for a micro-solid rocket of 1—10mm class. This causes the difficulty of the optimum design of a micro-solid rocket. In this study, we have experimentally investigated the effect of thruster configurations on a laser ignition microthruster. The examined parameters are aperture ratio of the nozzle, length of the combustion chamber, area of the nozzle throat, and divergence angle of the nozzle. Specific impulse dependences on those parameters were evaluated. It was found that large fraction of the uncombusted propellant was the main cause of the degrading performance. Decreasing the orifice diameter in the nozzle with a constant open aperture ratio was an effective method to improve this degradation.

Improvements have been made to the grid erosion evaluation code (JIEDI-1) to achieve robust convergence and to enhance the accuracies of grid life estimations. The major improved features of the code are 1) the use of Newton-Raphson method and the ICCG solver, 2) modifications of the inlet boundary conditions, 3) incorporation of the differential sputtering yield and redeposition effect of the grid material, and 4) solution-adaptive mesh generation. The last feature is realized by the commercial mesh generation software "pro-STAR". Numerical code prediction of grid mass loss showed good agreement with that of μ10 ion engine life test, with robust convergence and improved simulation time by a factor of 3〜4 in comparison with the previous code.

In order to directly observe the ion production and extraction phenomena within an ion engine, a two-dimensional visualized ion thruster (VIT) have been designed and fabricated. Since the experimental visualized results on ion extraction behavior of the VIT grid system was in good agreement with the numerical results derived from an ion grid erosion analysis code, the VIT may be useful for the inspection and modification of the numerical model of the codes.

A laser ignition microthruster has been developed for microsatellites. Thruster performances such as impulse and ignition probability were measured, using boron potassium nitrate (B/KNO₃) solid propellant ignited by a 1 W CW laser diode. The measured impulses were 60 mNs ± 15 mNs with almost 100 % ignition probability. The effect of the mixture ratios of B/KNO₃ on thruster performance was also investigated, and it was shown that mixture ratios between B/KNO₃/binder = 28/70/2 and 38/60/2 exhibited both high ignition probability and high impulse. Laser ignition thrusters designed and fabricated based on these data became the first non-conventional microthrusters on the Kouku Kousen Satellite No. 1 (KKS-1) microsatellite that was launched by a H2A rocket as one of six piggyback satellites in January 2009.

For validation of an ion beam optics code, the behavior of ion beam optics was experimentally observed and evaluated with a two-dimensional visualized ion thruster (VIT). Since the observed beam focus positions, sheath positions and measured ion beam currents were in good agreement with the numerical results, it was confirmed that the numerical model of this code was appropriated. In addition, it was also confirmed that the beam focus position was moved on center axis of grid hole according to the applied grid potentials, which differs from conventional understanding/assumption. The VIT operations may be useful not only for the validation of ion beam optics codes but also for the fundamental and intuitive understanding of the Child Law Sheath theory.