As manned transportation to the moon and other planets and interstellar flight become more and more realistic, the potential of laser propulsion with the high directivity and focusing capability of laser is once again attracting attention. This special issue introduces representative examples of the wide variety of research on laser-based propulsion systems in Japan.

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Numerical simulations and related works are reviewed by focusing on gas-driven-type laser propulsion which enables low-cost space launch system due to its propellant-less principle. Process of forming a blast wave by laser absorption in the atmospheric environment was reproduced using thermochemical non-equilibrium flow simulation with detailed chemistry. The trajectory of a laser-launched vehicle obtained by a 6-degrees-of-freedom simulation agreed with a past fight experiment, and the simulation results suggest that the beam riding performance should be improved for actual use. Therefore, a new attempt to improve the beam riding performance by using a multi-parabola body is examined. Moreover, it was found that the dominant chemical process at discharge front driven by millimeter-wave beam changes depending on the beam intensity.

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Current status of research on the Laser Launch System are reviewed. The aim of the LLS development is to drastically reduce the launch cost from ground to the orbit. After looking back the history of the LLS research, two different kinds of propulsion, the RP laser propulsion and the CW laser propulsion are introduced. Recent research activities on each kind of the propulsion are reviewed. For the RP laser propulsion, the beam-riding performance is pursued. For the CW laser propulsion, the launch demonstration using a 100 kW-class-fiber laser is planned in Japan.

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This paper deals with a combustion-controllable solid propellant microthruster using laser heating. Conventional solid propellant thrusters, which require neither tank nor valve, have relatively high reliability due to simpler structures. However, the solid propellant thrusters were not applied to attitude or station keeping because of the difficulty in interrupt and restart of thrust production. Hence, the authors proposed a new throttleable solid propellant microthruster using laser heating. The proposed thruster has combustion-controllable solid propellant, combustion of which was sustained only while burning surface is heated with a semiconductor laser; and then, combustion is started and interrupted by switching laser heating. A 0.1 N class thruster was prototyped to evaluate performance, and heat flux balance under combustion was evaluated.

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This paper focuses on the energy conversion process of the laser rockets, which has not been covered in the previous special issues, the principle and basic physics of how thrust is generated from the laser beam via the shock wave. The propagation mode and threshold of optical detonation as a fundamental physical phenomenon are explained, and the author’s recent research on the fast-gas ionization wave (FIW) is introduced. It is obvious that the propulsion design cannot be achieved without understanding the physical phenomena as well as the combustion detonation as the basic research.

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We developed a new mechanism of laser propulsion whose driving force is a laser-induced bubble that inhales water. We experimentally demonstrated its propulsion and explained its phenomenon by numerical simulation and successfully launched a target up by horizontally irradiating a laser and spouting water up. We clarified that the bubble generated by laser irradiation follows three steps. First, the laser-induced bubble expands and pushes the surrounding water that spouts from the target’s nozzle. Second, the collapsing bubble inhales the surrounding water downward, and, the target gets an upward force as a reaction to the water’s downward flow. We expect this mechanism will be applicable for implementing a completely new engine that requires no chemical reaction but only the physical reaction of water.

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The laser fusion rocket is a large rocket that generates high-energy plasma through nuclear fusion and obtains high thrust by expelling it from the spacecraft. Although it is a promising technology for human planetary exploration, including a manned mission to Mars, it is difficult to experimentally evaluate its propulsion performance, and it is currently being evaluated through numerical simulations. However, the reproducibility of the simulations is not sufficiently evaluated. Therefore, we are developing simulations that quantitatively simulate experiments while evaluating propulsion performance through experiments. In this paper, we describe the principle of the laser fusion rocket, the methods for evaluating its performance, and the current state of the research.

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