Resistojet Thrusters for Auxiliary Propulsion of Full Electric Platforms

Transcription

1 Resistojet Thrusters for Auxiliary Propulsion of Full Electric Platforms IEPC Presented at the 35th International Electric Propulsion Conference Georgia Institute of Technology Atlanta, Georgia USA G. Cifali 1, S. Gregucci 2, T. Andreussi 3, M. Andrenucci 4 Sitael S.p.A., Ospedaletto (Pisa), State, 56121, Italy Abstract: Today, the use of electric propulsion system is a trend even for small, medium and large platforms. However, the use of low thrust devices, as HET and GIE, does not exclude the need of high thrust devices as auxiliary propulsion system (APS) for all off-nominal operations, e.g. collision avoidance, reaction wheel de-saturation, de-orbiting and repositioning. In this respect, the baseline for medium-large platforms is represented by cold gas thrusters, which provide very low performance and operate with the propellant already available on board (usually Xe). In this frame, the use of electrothermal thrusters could mitigate the performance loss. In particular, resistojets represent the natural evolution of the cold gas thruster, because they provide twice the performance (in terms of specific impulse and, consequently, in terms of mass saving), with a limited increase of system complexity. In order to comply with the main requirements of the space system integrators, SITAEL developed the XR resistojet family with the aim to reduce both the component complexity and the cost, making the device more flexible in terms of power consumption and performance. The main component of the thruster, i.e. the heater, was selected as a COTS, industrial grade part, which is extensively used in industrial and aeronautics sector, and ensures extreme reliability over a wide range of environmental conditions. The thruster was designed in order to guarantee an easy coupling with different heaters and, consequently, to tailor the resistojets according to customer requirements in terms of power budget. In this paper, it is described a typical architecture for integrated electric propulsion system including resistojets for APS. In addition, the experimental results for two resistojet thrusters developed by SITAEL the 50W and 100W models of the XR150 family operated with Xe and Kr, both in cold gas and in hot gas modes are shown. Finally, the new XR1000, an evolution of the XR150 family, is presented. APS EP GIE GSE HET EPS RCS RJ Nomenclature = Auxiliary Propulsion System = Electric Propulsion = Grid Ion Engine = Ground Support Equipment = Hall Effect Thruster = Electric Propulsion System (main system) = Reaction Control System = Resistojet 1 Test Engineer, Propulsion Division, gianluca.cifali@sitael.com 2 System Engineer, Propulsion Division, stefan.gregucci@sitael.com 3 Technical Manager, Propulsion Division, tommaso.andrenussi@sitael.com 4 Division Head, Propulsion Division, mariano.andrenucci@sitael.com 1

2 T I.Introduction HE use of electric propulsion for small, medium and large platforms is firmly established. The need for integrated systems, more attractive in terms of simplicity and costs, led Sitael to the development of new EPS able to cover different in-orbit operations. In this frame, the company focused its efforts not only on the development of main propulsion systems, but also on auxiliary systems for off-nominal operations, which require high thrust level even at lower specific impulse. These systems are based on the electrothermal thrusters (i.e. the arcjets AT series and the resistojets XR series). In particular, integrated EPSs equipped with resistojets represent the direct upgrade of propulsion systems based on electric thrusters (HET or GIE) and cold gas thrusters. With respect to the latter, resistojets provide about twice the specific impulse for impulsive maneuvers. In addition, they also allow a completely new approach for off-nominal operations (e.g. de-orbiting, reaction wheels desaturation, etc.), thanks to the possibility to increase the duration of the shoots. The XR150 series was developed with the aim to integrate an auxiliary system with minimum modification of the general architecture. The design was focused on optimization of the thruster using xenon or krypton as main propellants. Nevertheless, the particular design allows the use of the thruster with any propellant available on board, including mixtures. II.System Architecture The resistojets developed in Sitael are designed to be easily coupled with main propulsion systems based on GIE and HET. The thruster geometry was optimized to operate in a range of pressures compliant with a standard lowpressure stage of common EP feeding systems, i.e bar. This allows the system to be directly installed downstream the main pressure regulation stage (bang-bang, proportional, mechanical), without the need for an additional pressure regulator. Figure 1. Coupled APS Architecture. Schematic of fully integrated propulsion system for full electric platforms. Figure 2. Decoupled APS Architecture. Schematic of partially integrated propulsion system for full electric platforms. A schematic of such kind of integrated propulsion system is shown in Figure 1. Moreover, the heaters can be tailored to the on-board power distribution system in order to operate in direct drive mode (i.e. without a dedicated Power Processing Unit). In case the EPS pressure regulator is unable to manage the high mass flow rate required for the APS, a different layout has to be considered: in this case the feeding line is split before the EPS regulator, and an additional, APS dedicated, single stage, single set point, mechanical pressure regulator is installed (Figure 2). 2

3 In general, each resistojet assembly comprises a thruster, an interface structure, and a thruster box containing the electrical connectors and the enabling (thruster) valve. An example of integrated thruster pack is shown in Figure 3. Nevertheless, different thruster pack configurations can be implemented decoupling the box from the interface (e.g. considering a general box for all the RCS systems). A picture of the XR150 thruster is shown in Figure 4. Figure 3. Example of RJ thruster pack. The thruster is installed on a dedicated interface providing mechanical support and thermal insulation, while the enabling valve and the electrical interface are located inside the thruster box. Figure 4. Sitael XR150 thruster. The picture refers to the thruster EM before and during the acceptance test (pictures on bottom-right corner). III.Experimental Test Campaign In this section, the experimental campaign performed on Sitael XR150 resistojet thrusters is presented. A. Test Articles Two XR150 models were built and tested with Xe and Kr. The XR and XR allow direct coupling with spacecrafts with a 28V unregulated power bus voltage. The main requirement specifications for the two thrusters with Xe and Kr are listed in Table 1. Thruster XR XR Weight [g] 220 (without piping) Size [mm] Ø 27 x 80 Propellant Xe Kr Xe Kr Nominal Thrust range [mn] Power bus [Vdc] 28 * Max 28Vdc [W] Specific impulse [s] Thruster inlet pressure 5.5 bar 4.5 bar 7.5 bar 6.5 bar Lifetime ** [hrs] 250 Total Impulse ** [Ns] 135k 110k 200k 160k (*) other bus voltages compatibility available on request (**) the lifetime and the total impulse are limited only by propellant shortage concerns. Table 1. Sitael XR150 family main performances. The XR150 resistojet thruster can be operated either in hot and cold gas mode. When the heater is switched-off, the thruster behavior is the same as a standard cold gas thruster, with typical performances that depend on the inlet pressure and the propellant characteristics. As all resistojet thrusters, the XR150 is designed to be started both in hot and cold gas conditions: in the hot start mode, the heater is switched-on before the propellant flow (pre-heating), and it allows to reach the desired specific impulse just at the beginning of the thruster operation. The pre-heating time varies according to the initial temperature of the thruster. 3

4 in the cold start mode, the heater and the propellant flow are switched-on at the same time and the thermal steady state condition is achieved after few minutes from the beginning of the operation (depending on the initial temperature of the thruster unit). Accordingly, the specific impulse increases from the value corresponding to the cold gas mode up to the one corresponding to the hot gas mode (for Xe, from 30 to s). This case leads to an increase of the amount of propellant required for the envisaged maneuver, but can be used in case of unexpected events that require quick thrust impulses. B. Test Setup The facility used for the XR150 test campaign was the Sitael IV3 Facility (Figure 5 and Figure 7). The vacuum facility has an internal volume of about 4.6 cubic meters and it is equipped and optimized for high enthalpy aerothermodynamic testing and testing of electro-thermal thrusters. Considering the high mass flow rate involved in such kind of tests, the facility features a double stage primary pumping system based on rotary and roots pumps; the ultimate back pressure is about 1e-3 mbar. Figure 6 shows the thrust balance used during the test. The main performance characteristics of the balance are listed in Table 3. Figure 5. Sitael IV3-ATD vacuum facility for aerothermodynamic and electrothermal thruster testing. Figure 6. Sitael XR150 installed on the Thrust Balance for Electro-thermal thruster. Test item weight Thrust balance characteristics Measurement mode Measurement sensor Thrust range Resolution & accuracy Drift compensation Bandwidth < 3 kg Time-resolved Load cell with electrical strain gauges mn 1 mn not installed (optional) 0.5 Hz Figure 7. Sitael XR150 installed inside IV3 hatch. Table 2. Sitael Thrust balance for Electrothermal thruster: main characteristics. Figure 8 shows the layout of the GSE used during the XR150 test campaign. During the last test of the XR with Kr the laboratory manual pressure regulator installed on the propellant bottle was replaced by a mechanical reducer with 30 psig outlet pressure. 4

5 C. Test Description & Results The test campaign was divided into four parts, the first two parts dedicated to the test of the XR (100W thruster) with Xe and Kr, and the second two parts for the test of the XR (50W thruster) with Xe and Kr. Each part was composed by two phases: a preliminary operation test, to assess the main characteristics of the thruster in cold gas mode, and a hot operation test. Table 3 lists the performances of the thrusters with Xe in cold and hot gas modes for different pressure setpoints, while Table 4 lists the performances with Kr. Cells in pale blue represent the experimentally validated data. Figure 9 shows the main performance parameters for the XR using Xe and Kr as propellants. During the test, a feeding pressure of 2.5 bar was set, with a pre-heating time of 5 min for both propellants. Figure 10 shows the main performance parameters for the XR using Xe as propellant at 2.55 bar feeding pressure. In this case, a pre-heating time of 10 min was selected for the shoot. Finally, Figure 11 shows the main performance parameters for the XR using Kr as propellant. During the test, a feeding pressure of about 3 bar (30 psig) was set using the fixed point mechanical pressure reducer. In this test, an evaluation of the effect of the pre-heating time on the stabilization of the performance was made, and three different pre-heating time, i.e. 10, 15, and 20 min were investigated. Figure 8. GSE layout for XR150 test. The thruster is installed on the balance by a dedicated mechanical interface. Two type-k thermocouples are mounted on the thruster to monitor the temperature close to the nozzle throat (TC1) and at the interface flange (TC 2). The DAQ allows to control the electrical output of the power supply to the heater, and to monitor both the mass flow rate (MFC) and the feeding pressure. This monitoring is performed using the mass flow controller and the pressure sensor installed on the feeding system flange (upstream the thrust balance). An auxiliary line before the main filter allows the draining and the purging of all the branches of the feeding system. Feeding Pressure Thrust Cold operation Hot 50W Hot 100W Mass Flow Rate (Xe) Specific Impulse Mass Flow Rate (Xe) Specific Impulse Mass Flow Rate (Xe) Specific Impulse [bar] [mn] [mg/s] [s] [mg/s] [s] [mg/s] [s] Experimentally validated Calculated by numerical model Table 3. XR150 series experimental performance characterization and numerical extrapolation for Xe operation. 5

6 Cold operation Hot 50W Hot 100W Feeding Thrust Mass Flow Specific Mass Flow Specific Mass Flow Rate Specific Pressure Rate (Kr) Impulse Rate (Kr) Impulse (Kr) Impulse [bar] [mn] [mg/s] [s] [mg/s] [s] [mg/s] [s] Experimentally validated Calculated by numerical model Table 4. XR150 series experimental performance characterization and numerical extrapolation for Kr operation. feeding pressure feeding pressure feeding pressure feeding pressure Figure 9. Main performances measured during the test of the XR with Xe and Kr at feeding pressure of 2.5 bar. 6

7 feeding pressure (Xe) feeding pressure (Xe) feeding pressure (Xe) feeding pressure (Xe) Figure bar. Main performances measured during the test of the XR with Xe at feeding pressure of feeding pressure (Kr) feeding pressure (Kr) feeding pressure (Kr) feeding pressure (Kr) Figure psig. Main performances measured during the test of the XR with Kr at feeding pressure of 7

8 During the test campaign, a dedicated test was performed to investigate the time delay from the thruster valve opening and the stabilization of the thrust signal. Figure 12 shows the experimental data for two consecutive shoots performed with the XR fed with Xe. Note that, in order to perform precise trigger of the flow, the mass flow controller was used as enabling valve. The distance of the mass flow controller from the thruster inlet pipe was about 750 mm (due to the presence of the thrust balance). Note also that the pressure transducer used for the evaluation was close to the mass flow controller (Figure 8). The plots show that the thrust stabilization occurs within 0.5 sec from the opening of the mass flow controller. Consistent results were found also with krypton (Figure 13). Figure 12. XR switching-on transitory analysis with 2.5 bar. Figure 13. XR switching-on transitory analysis with 2.5 bar. IV.Test results The test results demonstrate the ability of the XR150 series to provide thrust in the range from 40 to 150 mn with specific impulses higher than the correspondent values of cold gas thrusters (which are about 30 s for Xe and 40 s for Kr). The 100 W model is able to provide a specific impulse which is about 1.86 times the one of the cold gas, while the 50 W is about 1.6 times greater than the values recorded in cold gas mode. However, while the 100W model is able to guarantee a higher specific impulse for all the range of thrust, the 50W model is more suitable for thrust up to 80 mn. This is mainly due to the lower heat exchange efficiency of the 50 W heater. Concerning the pre-heating time, the experimental characterization suggests to tailor its duration not only to the initial thermal condition of the thruster, but also to the particular operation required for the APS. If a stabilized performance is requested during the shoot, a longest pre-heating time can be considered (up to 20 min). In case of short or zero pre-heating time, the thruster can operate with derated performances, assuring in any case specific impulses always higher than those of a cold gas thruster. For both startup conditions and both propellants (Xe and Kr), 8

9 the thruster is able to reach the desired thrust level after less than 0.5 s from the enabling valve opening. In addition, in case of failure of the heater, the thruster can be used as cold gas thruster assuring the same thrust level reachable in hot gas mode. V.XR1000 series presentation Pushed by market, Sitael has started the preliminary study of a new XR resistojet able to assure a thrust level of 1N. The XR is a 500W-class thruster born for medium-large platforms with 50V power bus. The main characteristics of the new thruster are shown in Figure 14. XR150 XR1000 Figure 14. Sitael XR dimensions and main characteristics. The drawings on the top-left side of the figure shows a dimensional comparison between the new thruster and the XR150 series. The table on the top-right side of the figure lists the main performance requirements, while the table on the bottom lists the operational parameters for one operational point at 1 N thrust. VI.Conclusion With the aim to identify a suitable architecture for an integrated propulsion system for full electric platforms, Sitael performed dedicated analysis using its XR150 resistojet thruster. The company developed and manufactured two XR150 engineering models with power consumption of 50W and 100 W (the XR and the XR ). A specific experimental test campaign was performed with Xe and Kr to investigate its ability to operate with such kind of propellants as a valid alternative to traditional cold gas thrusters. The test campaign confirmed the results of the computational analyses and demonstrate the effectiveness of these thrusters as auxiliary propulsion system. Moreover, to extend the company product catalogue, Sitael started the development of a new 1N, 500W thruster, named XR , for medium and large platforms. Acknowledgments The authors wish to express their gratitude to Giovanni Cesaretti, Luca Pieri, and Carlo Tellini for their valuable assistance in preparing and performing the experimental campaign. The work described in this paper has been funded by Sitael as internal R&D program. References 1 Passaro, A., Bulit, A., Development and Test of XR-150, a New High-Thrust 100 W Resistojet, 33rd International Electric Propulsion Conference, The George Washington University Washington, D.C. USA October 6 10, 2013, IEPC