The technology of beaming microwave propulsion energy, using a fusion application high power gyrotron operating in the 100-GHz band, has been developed in cooperation with the University of Tokyo. A high power, 1-MW class, microwave beam which was radiated from a so-called gyrotron microwave generation device developed for plasma heating and current drive of JT-60 and ITER, was focused on a parabolic reflector and produced plasma and a shock wave on it. The reflector received impulse thrust. A vertical flight test was performed using a rocket type thruster demonstration model. The model vehicle successfully achieved a few meters of altitude during the flight, as shown in Fig. 2-12.
In space development, reduction of launching costs is an important issue. The payload/launch vehicle ratio for launch-to-orbit systems for chemical rockets, which now are primarily the vehicles used, is only several percent. Beamed energy propulsion technology with a high power laser or microwave beam provided from a ground-based station as shown in Fig. 2-13 is expected to realize an extremely low launching cost system. Beamed energy propulsion vehicles do not need to carry an energy source, including fuel tanks and pump systems, because atmospheric air can be utilized as propellant in the atmosphere. Therefore, beamed energy propulsion is a very attractive method to reduce rocket-launch costs, since only the construction of ground-based laser or microwave system station is needed.
The concept of launching objects using beamed energy supplied from ground-based devices has been proposed since the early of 1970s. Studies of laser beamed energy propulsion with a high power laser oscillator have been carried out because of the desirable directional characteristics of the beam for a transmission range of ca. 100 km, which is the expected distance between the rocket and its ground-based station. We have estimated that a microwave with a frequency of ~100 GHz also has enough directivity for beam propagation. An experimental basic study on microwave beamed propulsion with shock wave and atmospheric plasma generation using a high power gyrotron developed for fusion applications has started. Microwave beamed propulsion has some advantages in comparison with laser beamed propulsion, i.e., the energy conversion efficiency from electrical to microwave energy can be higher than 50% in the high power gyrotron with an energy recovery system. Accordingly, it is expected that the construction cost GW-level ground- based stations will be reduced. Furthermore, the momentum-coupling coefficient, which is a parameter of thrust performance and a fraction of measured thrust and input beam energy, reached 400 newton (N) /MW at a 0.1 ms pulse length, as shown in Fig. 2-14. The thrust performance of microwave beamed propulsion is comparable to laser beamed propulsion. Payload-launch by high power microwave beamed propulsion has the potential of achieving a breakthrough in future launch-to-orbit systems. |
