The geophysical survey of the asteroids requires object-dependent and thus demanding maneuvers of the exploration satellite. The choice of thruster (e.g., ion thruster) and the method of control (e.g., maneuvering thrusters) must therefore be carefully considered.
There are currently several systems on the market and they are used for commercial and non-commercial purposes. Ion thrusters are state of the art and an indispensable part of modern space technology. Due to an insufficient mass of the ions, which can be balanced by the high outflow velocities, they are only used in space as continuous thrusters. They keep satellites on their orbits or act as thrusters in deep space missions. But to be able to control these satellites around all axes a high number of additional thrusters or nozzles is needed.
We would like to present here a main engine alternative to the often used thrusters. This ion thruster, invented and patented by our team member Pascal Koch (Pat. No. DE 10 2013 217 059), combines both thrust by the ion beam and the control of the same. Of course, we can only give a brief overview of the thruster's technology on this website.
(1) Connection flange for the plasma source
(2) Separation chamber with sensor technology and additional gas valves
(3) Ion acceleration unit (IBK)
(4) Electron acceleration and storage channels (4x EBK's)
A low-pressure plasma source is mounted on the flange (1). This generates a plasma by injecting a process gas (e.g. xenon) and supplying energy. The plasma enters the separation chamber of the engine without first discharging any electrons. The electron acceleration channels (4) are arranged around the separation chamber and their extraction grids point into the separation chamber. At the exit of the chamber is the ion acceleration unit (3) (Pic. 1).
By accelerating the massive, positive ions by means of the electric field of the ion acceleration unit, the engine receives its thrust. This ion acceleration unit consists of three grids arranged in parallel. The focusing and acceleration grid (outermost grid) is slightly curved inward to achieve bundling of the beam.
A special feature of the engine is the deflection of the ion beam by the electrons. The four EBK's can thereby receive four independent electronic settings of the components.
We start with the assignment of the EBK's, if only thrust is to be given and the ion beam is not to be deflected. The electrons must be continuously removed from the engine. Because after a long time, such a large number of electrons remain in the engine that it becomes negatively charged to such an extent that not all the ions can leave the engine.
If we want to redirect the ion beam, we need a large (but electrostatically possible) number of electrons. The EBK's thus receive a new assignment in order to be able to store electrons for a certain time. This process runs simultaneously in all four channels. Before each steering phase, electrons must be 'collected' in the channels. If enough electrons have been collected, the redirection of the ion beam can be initiated.
For this purpose, the ion acceleration process is continued as usual. The corresponding EBK, which have to deflect, gets the standard assignment of the acceleration of electrons put on again. The stored electrons from this channel are now strongly accelerated and leave the engine bundled to an electron beam. This electron beam deflects the ion beam if the velocity, beam density and exit angle are correctly adjusted (Pic. 2).
The channels that do not deflect become a new electrical assignment so that the electrons are shot back into the separation chamber.
The channel that performs the steering process thus provides its own electrons for the redirection and also extracts, in addition to the standard extraction, all electrons from the other channels out of the separation chamber. For the deflection process, an excess number of electrons is now available over a longer period of time.
Reversing the direction of thrust allows maneuvers such as course corrections. These maneuvers take place over longer periods of time (the thruster was designed for this purpose) and therefore this technique is especially useful for the flight to the asteroids or between the asteroids as a supporting method to the mostly used gas nozzles. To ensure full maneuverability, two thrusters of this type are needed, mounted opposite each other on the satellite.
However, for the geophysical survey of the bodies we depend on fast and flexible adjustment of the survey orbits. Therefore, we cannot store electrons for long periods of time or perform time-consuming maneuvers near the asteroid. Especially here we have to use the technique of gas nozzles.