RIT15S - A radio frequency ion engine for high specific impulse operation

Abstract

The significant increase of available electric power for the propulsion unit onboard satellites makes high and ultra-high specific impulse thrusters attractive for different applications and missions. Especially thrusters of the radio frequency type show a great potentiality for high Isp applications. Two different rf-thrusters are described: The RIT15LP engine optimized for a moderate input power consumption and the RIT15S designed for a high specific impulse. The main, but only difference between these engines is the extraction grid system. During the development of RIT15LP, principle improvements of rfthrusters were invented. The given description of RIT15LP is focused on these performance improvements whereas the advantages of the rf-principle for (ultra-) high specific impulse applications were pointed out on basis of the RIT15S thruster. The function principle of gridded ion thrusters can be described as a two step process. In the first step the propellant is ionized and after that, in the second step it is accelerated in the electrostatic fields of an ion extraction and acceleration grid system. Both tasks a nearly independent from each other. The acceleration has no effect on the ionisation process and therefore the beam velocity can simply be controlled by the selection of the voltages applied to the grid system. Amoung different types of electric thrusters this is a unique feature of all gridded ion engines. In contrast, the use of other electric propulsion concepts requires often deep changes in the thruster design for an increased beam velocity. For example, it was necessary to invent a two stage concept for SPTs to reach a beam velocity comparable with that of gridded thrusters. Such efforts are not required in case of the later ones. Presently, the beam velocity of gridded ion thrusthers is merely limited by their electric input power consumption and not in a conceptional way. It is important to be aware that the specific input power consumption (the total consumption of electric power Ptot over the thrust F, usually given in>luWatt per Millinewton') of any kind of thruster is a simple function of the beam velocity Vbeam and the electric efficiency of the thrusters rjel: