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Development of the Radio Frequency Ion Thruster RIT XT

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Development of the Radio Frequency Ion Thruster RIT XT Development of the Radio Frequeny Ion Thruster RIT XT – A Status Report Hans.J.Leiter, Rainer Killinger, Helmut Bassner, Johann Müller; Ralf Kukies Astrium GmbH Post Box 80 11 69 D-816633 München Germany Tel: +49 89 607 23682 IEPC-01-104 North south station keeping is no longer the only application for ion propulsion onboard satellites. A competitive thruster system should also cover the requirements of advanced orbital manoeuvers, especially orbit rising and de- orbiting. Astrium’s new RIT XT thruster is designed to fulfill these demands. The potential free radio frequency ionization principle allows easily to adapt the ion propulsion system on high thrust demands of orbiting manouvers as well as high specific impulse operation during north south station keeping. Initially, a brief explanation of the thruster’s function principle is given and the components of RIT XT are described. The test setup is explained and the most recent test results are presented. I.INTRODUCTION be the last, but decisive missing step to an “all electric satellite”. North South Station Keeping (NSSK) of heavy geo satellites is regarded as the commercial Commonly, ion propulsion is associated with application for ion propulsion (IP). Only few Kaufman-type ion engines. Although this kind of years ago this was seen solely as future thruster is indeed often considered, the radio- technology but nowadays IP has become “state frequency ion engines, named “RIT” (Radio of the art” for NSSK. frequency Ion Thruster) developed by Astrium GmbH and the 1st Institute of Physics at Giessen Also in the field of scientific missions electric University are interesting alternatives. They propulsion is successful: Since “Deep Space combine the principal advantages of any gridded One” is operated in space electric propulsion has ion engine with the special features of ion become quite popular not only in the community generation by high frequency electromagnetic of rocket engineers and scientists. fields. Due to significant improvements of solar cells Up to now, RIT 10 is the solely commercially and photo-voltaic electric power onboard available radio frequency thruster. Over the spacecrafts increases continuously. Especially years the nominal thrust of this engine (“10” orbit and de-orbiting of satellites by ion engines characterizes the diameter of the thruster) could will become possible in the near future. This will be enlarged from formerly 15mN (RIT 10 “ARTEMIS” configuration”) up to a peak performance of 40mN (RIT 10 EVO), but it is * Copyright 2001 by astrium GmbH obvious that this not sufficient to fulfill the Published by the Electric Rocket Propulsion requirements of the new generation of heavy Society with permission geostationary communication satellites. Figure 1 illustrates that the daily operational 1 time would exceed the upper reasonable value of III. RIT OPERATION PRINCIPLE 3hours of thrust-on time, if the satellite mass Radio Frequency Ion thrusters (“RF” – (Begin of Life, BOL) is significantly higher than Thrusters”) are operated without any hot cathode 2000kg). Considering only NSSK a thrust of (“main cathode”) inside the thruster’s ionization 100mN would be absolutely sufficient to operate unit [5,6 e.g]. Instead, the propellant is ionized even a Satellite of 6.000kg BOL. But in respect by electromagnetic fields. For that, the ionizer to offer a solution for more sophisticated orbit- chamber, a vessel made of an isolating material, control manaeuers Astrium Gmbh decided to is surrounded by an rf-coil. The coil induces an develop an electric propulsion system of the axial magnetic field. Finally, the primary 100-200mN class [1]. The system bases on a magnetic field induces by Maxwells Law a new thruster called RIT XT. secondary circular electric field in which free electrons gain the energy for impact ionization. 3.0 After any impact ionization a xenon ion and at 2.5 least on more free electron is gained. Once the 2.0 ionization process is triggered, a self-sustaining Thrust during one Node [h] 30mN plasma-discharge is formed. The employed 1.5 40mN frequency is typically in the range of one Time 60mN 80mN 1.0 megacycle. 100mN North-South StationKeeping 120mN Operation Operation 0.5 160mN It is important to point out that this type of 200mN discharge and the physics behind (thruster 0.0 0 2000 4000 6000 8000 10000 respectively) are totally different from ECR- Satellite Mass BOL [kg] thrusters operated with some giga-cycles [9]. The later ones require external static magnetic Fig 1 Operational Time for North South Staion Keepin in fields to establish an electron cyclotron dependence of satellite mass for different thrust levels. It is resonance. Therefore the propellant flow assumed that the NSSK manoeover is performed only at through an ECR-type thruster has to be has to be one node once a day. matched exactly to the resonance conditions given by the frequency of electromagnetic waves as well as on the strength of the static magnetic field. Such limitations do not apply for RIT-Engines: The mass flow can be varied over an extremely wide range. This makes the rf-ion thruster superior, if very fast changes of thrust level are necessary, because the desired thrust is reachable faster than milliseconds by simply changing the applied rf-power. Beam current and with that the thrust follows rf-power immediately. It is absolutely sufficient to adapt the mass flow within the given speed of the xenon flow control unit. Merely the specific impulse varies until the mass flow reaches its optimal value again. Although the way the propellant is ionized is totally different from Kaufman- or more generally spoken from “bombardment thruster” Io=Ionizer Chamber, RF=Radiofrequency Generator, and ECR systems, there is no difference in beam Neut=Neutralizer, S=screen grid A=Accelerator Grid, forming and acceleration between these different D=Decelerator Grid (optional) , Ins=Gas inlet types. Sets of at least two grids were used to Fig. 2 Function principle of RIT type radio-frequency ion extract the ions from the plasma and after that to thrusters. accelerate them. 2 Usually, a positive voltage in respect to satellites acceleator grids. The last inspection of the potential U+ is applied at the plasma sided grid thruster performed at 13.000h of operation and a negative voltage at the following one. The indicates a total grid lifetime in the range of negative voltage U- at the second “accelerator” 25.000-30.000h . grid prevents a backstreaming of electrons from In the further description, that RIT 10 thruster the downstream surroundings of the thruster into will be named “RIT 10 ARTEMIS” to avoid the discharge area and allows a higher voltage confusion with the further developed for ion extraction (U + | U |) than for the beam + - RIT 10 EVO (EVOlution). acceleration (U+)only. Meanwhile ARTEMIS was launched. Due to a Sometimes a third grid on nearly satellite ground failure at upper stage of the ARIANE launcher is used to prevent backstreaming charge the satellite could not reach the geostationary exchange ions generated in the downstream orbit in the foreseen way by chemical region of the ion beam hitting the acceleration propulsion. Thus the European Space Agency grid. Also this third grid might have an influence ESA worked out a strategy to orbit ARTEMIS of the beam’s shape. The specific advantages by use of the NSSK electric propulsion system. and disadvantages of triple and double grid Preliminary tests in space were already finished systems often discussed for bombardment-type and begin of the orbiting manoeuver is engines apply for radio-frequency ion thrusters scheduled for middle of November 2001. as well. Like all type of electrostatic ion engines, RIT thrusters need a device for neutralization of the 2.2 Sub-scale Tests (RIT 10 EVO) generated ion beam too. For that, commercially The evolution of RIT 10 is described because it available hollow cathodes are used as electron could be proved that RIT engines of different emitters. size behave in the same manner (Development of RIT 10, RIT 15, RIT 35 at Giessen III. RIT XT University). This opened the possibility to optimize the thruster’s grid system on the 2.1. Heritage smaller RIT 10 engine. RIT XT is Astriums first ion engine in the 100- In a first step the grid system of RIT10 200mN class. Nevertheless the development ARTEMIS was modified. The open area fraction bases on a long term experience in ion thruster was increased and the ion optics were modified. design and operation. Since the early sixties the All the other parts of the thruster remained NSSK engine RIT 10 has been continuously unaltered. This enabled to demonstrate the developed and improved. The first successful influence of the grid system on the performance operation in space was performed onboard the data directly. European technology satellite EURECA. The result of the comparison between the Presently, Astrium GmbH offers an worldwide RIT 10 ARTEMIS and the RIT 10 EVO is unique commercially available rf electric straight forward: propulsion system called RITA (Radio Frequency Ion Thruster Assembly). RITA bases • Thrust regulation bandwidth on the space qualified thruster system for ESAs The ARTEMIS thruster, qualified for 15 new communication satellite ARTEMIS. mN, was tested up to 18 mN. With the modified grid RIT 10 EVO could be The corresponding lifetime test performed at operated between 1 to 41 mN. ESA’s test facilities at Nordwijk/NL of this engine is extraordinary successful. Up to now, • Increased specific impulse Isp (thruster): the required 15.000h of operation have been nearly reached. When this paper releases, the Thrust RIT 10 ARTEMIS RIT 10 EVO lifetime test will be completed. 15 mN 3400 s 3700 s One of the most remarkable results of that test might be lifetime of thruster’s graphite 35 mN n/a 3400 – 3700 3 • Reduced acceleration grid current Iacc Moreover RIT 10 EVO shows an excellent The important lifetime limiting acceldrain behavior even at lowest thrust levels A special current is reduced from 1,5% test program dedicated to applications like air (RIT 10 ARTEMIS) to approx.
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