Seeing rockets launch into space are spectacular images: tons of steel and advanced composites vibrate into motion, lifted into the air by a controlled explosion. For example, the European Ariane 5 burns 476 tons of solid fuel for a launch within 130 seconds and an additional 158 tons of liquid fuel in the first five minutes of a flight. That is enough to bring up to 16 tons of payload into the earth's atmosphere.
But the payload, such as a satellite, must also be able to maneuver in zero gravity or correct its own position. For a good 20 years now, a technology has prevailed that is more elegant and – what is important in space travel – more efficient: ion engines. prof Dr. Chris Volkmar from the Department of Electrical Engineering and Information Technology at the THM and a member of the Space Electronics working group there works with the German Aerospace Center, the rocket manufacturer ArianeGroup and Prof. Dr. Peter Klar from the First Physics Institute at Justus Liebig University is working on the “Camerado” project to design a variant of this engine, the radio frequency ion engine (RIT), even more “suitable for everyday use”.
The principle of action of chemical fuels as in rocket launch and ion engines is the same: fuel is pushed out of the engine at high speed, thrust is created by the recoil principle. The sole reason why rockets do not start with a gentle glow, but in a jet of fire, is that the thrust generated by electrically charged ions is not sufficient to free the enormous weight of a rocket from the shackles of gravity. It takes brute force to do that.
It's different in weightlessness: that's where the ion engine can play to its strengths. This consists above all in the efficiency of fuel use: it is around ten times higher than with chemical drives. So less fuel has to be heaved up – a price advantage when taking off. In the changing satellite market, away from special technology weighing tons and toward handy satellites that may be deployed in dozens or even hundreds, lightweight technology is gaining in importance. It is made for orbit and course corrections and, more recently, for continuous orbit elevations, as well as for long missions into the depths of the solar system.
In order to generate thrust, the RIT ionizes the gaseous fuel by coupling in high-frequency electrical alternating fields. This is generated by a so-called radio frequency generator (RFG), which is the focus of the "Camerado" project. "The state of the art is half-bridge inverters, which require expensive analog or digital circuit components," explains Volkmar. Because these components are sensitive to interference, complicated shielding mechanisms are needed, such as a radiation-resistant housing. "But the trend is moving away from special technology towards parts on an industrial standard," says the researcher. Such an approach could be worthwhile, adds his research assistant Christian Rößler: For example, if a mission is not designed to last. Then the industrial goods used could significantly reduce the final price of a mission. And the radio frequency generator is considered a crucial cost factor in radio frequency ion engines. So as an ideal adjusting screw.
The RFG converts the DC voltage supplied by the satellite's solar cells into AC voltage, which is used to power the actual engine. A high level of electrical efficiency is crucial in order to minimize conversion losses and thus make every captured ray of sunshine usable. For this purpose, the use of a class E amplifier is to be examined in the project, set up to be interference-free and compared with the half-bridge technology currently used.
The amplifier, whose switching principle is also used in high-efficiency radio and radio amplifiers, is operated unregulated and therefore does not require any complex control electronics. The component should be less susceptible to faults. Furthermore, the number of components is significantly reduced compared to the half-bridge, which should have a positive effect on the complexity and thus the costs of the system.
The project is funded by the state of Hesse's "Research for Practice" programme for one year with almost 40,000 euros.