中野 正勝

A laser ignition microthruster was developed for microspacecraft propulsion. Small solid-propellant pellets were ignited by a diode laser beam to produce a high impulse from a very small power source. Basic experiments showed that the microthruster produced an impulse of 60 mNs, using 60 mg B/KNO₃ pellets ignited by a 1 W laser beam. A space-based demonstration of the microthruster system is planned onboard a microspacecraft named KKS-1, which has been chosen by the Japan Aerospace Exploration Agency as one of the six small satellites to be launched with the Greenhouse Gases Observing SATellite, which is scheduled to be launched on a H2A rocket in early 2009.

Numerical analyses were performed to clarify the power conversion mechanisms in a 1kW class Continuous-Wave (CW) laser thruster. Laser plasma heating and radiation emission from the plasma, which dominate in the power conversion balance, were highlighted in the analyses. Calculation results showed that low laser power absorption and large radiation loss from the plasma cause low thruster performance. These results also showed that optimization of the nozzle shape and use of regenerative-cooling can substantially improve the thruster performance.

Impulse generation experiments of a liquid propellant laser thruster were conducted using glycerin propellants in the energy range of 60mJ–60J. Momentum coupling coefficients and specific impulses were obtained from impulse and propellant mass measurements. The maximum specific impulse was 18s at the laser beam energy of 55J. Experimental data were scaled in terms of energy and the radius of the glycerin droplet to extrapolate laser thruster performance. The results indicate that the radius of the glycerin droplet must be less than 0.24 mm in these experiments to achieve specific impulse more than 1,000s that will be required to compete with other space propulsion systems.

The characteristics of a laser ablation igniter were investigated systematically. This kind of igniter is expected to realize a lightweight rocket engine system that consists of many combustion chambers, such as the Reaction Control System (RCS), without having heavy spark igniters. In this igniter, ignition is accomplished utilizing a high-temperature metal vapor produced by focusing a high-intensity laser pulse transmitted through an optical fiber. The laser ablation igniter was tested in a GH₂/GO_x thruster (dia. 1 cm). As a result, the minimum ignition energy was found to be lower than 2 mJ, and the required ignition energy decreased with increasing pressure in the combustion chamber.

Irradiating a nano-second laser pulse on a solid surface, a GH<SUB>2</SUB>/GO<SUB>2</SUB> combustion chamber was operated to measure the minimum ignition energy for three parameters: the spot diameter, the thruster pressure, and the target material. It was confirmed that this method is useful since the minimum ignition energy can be much less than that of the laser spark ignition. It turned out that the minimum ignition energy increases as the spot diameter increases, and decreases monotonously as the thruster pressure increases. In addition, it was suggested that the ignition is possible even after the irradiation of a million laser pulses by applying the metallic targets (tungsten and stainless). On the other hand, the carbon target provided poor ignition probability.

A grid lifetime model is proposed for the lifetime estimation of a 3-grid ion engine system. Grid life is assumed to be determined by electron backstreaming caused by the decreased negative voltage due to the enlargement of accel grid hole radius by charge exchange ion erosion. The model is based on the following grid life scenario: 1) the charge exchange ion erosion enlarges the accel grid hole radius, 2) the enlarged accel grid hole radius decreases the extended effective acceleration length between the screen and accel grids, and 3) the decreased extended effective acceleration length decreases the negative voltage to satisfy the space-charge limited current density law. The characteristic of the model is that lifetime exhibits very strong dependence on beam current and accel grid voltage. A computer program was used to validate the model and it was shown that the model agrees fairly well with the simulations in high NP/H conditions.