The Xenon Feed System is an essential part of any electric propulsion system. The life cycle of the feed system is largely determined by lifetime of the valves and the amount of working substance in tank. Whereas the amount of working substance is selected based on the requirements for each specific mission valve parameters must be satisfying for various missions. Accordingly, the valves must have low power consumption, high reliability (more than 106 cycles), low weight and dimensions.
The paper presents a long lifetime solenoid valve which is appropriate for feed systems with different control techniques. The paper addresses the main components of the proposed valve, the associated features, the preliminary calculations, and overviews the performances obtained from the accomplished laboratory tests.
Typical Electric propulsion system (EPS) consists of following main subsystems: the electric propulsion thruster; the storage and feed system; the power-processing and control unit.
The storage and feed system in EPS structure must ensure three main tasks: storage the working substance in the high pressure (up to 180 bar) tank, decreasing the high pressure to the working level (1 bar) and feed the referenced values of the mass flow rates into electrical propulsion thruster. If the structure of EPS includes one Hall thruster the storage and feed system must has two output lines – into anode and hollow cathode units.
Tree main structures of the storage and feed systems for pressure decreasing are used: a) system with mechanical pressure regulator; b) system with accumulator tank (bang-bang controlled valves/ feedback control from pressure transducers); c) system with proportional valve [1 – 3].
SETS Company have designed Xenon storage and feed system (XFS) for application in EPS structures with the ST-25 and ST-40 Hall thrusters . The operation principle of this system is based on the accumulation tank (receiver) and controlled valves application.
2. XENON STORAGE AND FEED SYSTEM (XFS) STRUCTURE
The storage and feed system structure for electric propulsion systems with ST-25 and ST-40 application is shown in Figure 1. The structure of this system has one line to feeding the working substance into anode and two lines to hollow cathodes (one of them – for an operating cathode, the second – for reserved cathode).
Figure 1. Xenon storage and feed system block diagram
In Fig. 1 following designations are used:
Tank – High pressure tank for storage the
working substance during all mission;
T1 – Temperature transducer;
FDV1 – Fill and drain valve.
High pressure unit:
P1 – P3 – Pressure transducers;
F1 – F2 – Filters;
SVH1 – SVH2 – High pressure solenoid valves;
OR1 – Flow restrictor.
Low pressure unit:
AT – Accumulator tank (receiver);
T2 – Temperature transducer;
P4 – P6 – Pressure transducers;
F3 – F5 – Filters;
SVL1 – SVL3 – Low pressure solenoid valves;
OR2 – OR4 – Flow restrictors.
The working substance (Xenon), which is stored in high pressure tank, at SVH1, SVH2 valves activation fills the AT through the restrictor OR1 and filter F2. The pressure in the AT is increasing up to the set maximum operating pressure, after that the valves SVH1, SVH2 are closed according to the values of P4 – P6. The valves are controlled by the control unit in structure of the powerprocessing unit of the EPS. The pressure value in the AT is stabilized in a such way with referenced accuracy. The values of mass flow rates into the anode unit and the cathode unit are determined by OR2 – OR4 restrictors. General view of the Xenon storage and feed system engineering prototype is presented in Fig. 2 (without the high-pressure tank).
Figure 2. General view of the XFS engineering prototype for ST-25 thruster (without the highpressure tank)
As can be seen from the block diagram (Fig. 1), high pressure (up to 180 bar) and low pressure (1 bar) valves are required to build working substance storage and feed system intended for operation as part of the electric propulsion system. Thus, highand low-pressure valves are critical elements that determine the reliability and durability of a storage and feed system.
3. SOLENOID VALVE DESIGNED BY SETS
The solenoid valve SV-1 was designed, manufactured and tested by SETS Company. This valve has been proven to operate at both high and low pressures. The general view of the valve is shown in Fig. 3, and its technical parameters are presented in Table 1.
SV-1 is a direct current solenoid valve offers fast and safe switching, long service life, good medium compatibility of the materials used, and extremely small size package. SV-1 operates with high repeatability and is constructed of non-corroding stainless steel.
Figure 3. Engineering prototype of SV-1 valve
Table 1. SV-1 valve parameters
|Valve type||1-Way Normally|
|Operating working substance||Xe, Ar, Kr|
|Maximum operating pressure, bar||250|
|Temperature range, ºС||-30 to +60|
|Response time, sec||0,005|
|Orifice inlet size, mm||0,5|
|Actuation voltage, V||12+1|
|Operating voltage, V||7+1|
|Resistance of solenoid at 20ºС,Ω||62±0,5|
|Minimal insulation resistance, MΩ||20|
|External leakage, sccm of He||5×10-6|
|Internal leakage, sccm of He, max||2×10-5|
|Cycle life||> 1000000|
|Dimensions (without connectors), mm||Ø 15,5 x 22|
|Development stage||Engineering model|
4. CALCULATION AND MODELLING
Features of the valve design are shown in Fig. 4, where the following designations are introduced:
1 – moving element;
2 – membranes;
3 – spring;
4 – coil.
Figure 4. Solenoid valve structure (simplified view)
As shown in Fig. 4, the SV-1 valve design uses two diaphragms, which perform the function of centering the moving element, and also provide the required closing speed of the valve. If the diaphragm deforms, the moving element will shift, resulting in valve failure.
For this reason, diaphragms are one of the key elements of the valve and require preliminary analysis and calculations. The choice of material and design of the membrane was made based on the analysis of existing valve designs [5, 6]. The membrane was calculated using the Ansys package. The calculation results for the membrane are shown in Fig. 5.
Figure 5. Results of the membrane calculation
In the course of calculations, stainless steel was chosen as the membrane material. The maximum mechanical stresses in the membrane arise when the valve is opened (moving element displacement 0.2 mm) and reach 577 MPa. The results of the calculations made it possible to choose the material for the manufacture of the membrane – cold-worked stainless steel, the tensile strength of which is 980 MPa.
To clarify the parameters of the developed valve after its manufacture, laboratory tests were carried out, the results of which are shown in Fig. 6 – 9.
5. EXPERIMENTAL TESTS
Qualification tests were carried out on laboratory prototypes of the SV-1 valve to verify that the valve confirms specifications.
At the first stage of testing, the dependence of the valve opening time on the value of the working pressure was studied (Fig. 6) during the lifetime test at 106 actuations. Valve testing showed that when the working pressure changes in the range of 5 – 265 bar, the valve opening time changes in the range of 1.5 – 4.8 msec.
At the same time, the valve closing time is practically independent of the working pressure and is in the range of 0.64 – 1.1 msec.
At an operating pressure of 295 bar, the valve did not open, while the valve did not collapse, no visible signs of deformation were found.
The valve was tested when the operating voltage was applied to the electromagnet of 20 V. During the tests, the leakage of the solenoid valve did not exceed 0.0004 cm3 / sec.
Figure 6. Dependence of the valve response speed on the change in inlet pressure
The results of experimental tests of the developed valve shown in Fig. 6, show that the valve opening time does not exceed 5 ms, which fully meets the design requirements.
At the second stage of testing, the dependence of the valve opening time and actuation
Figure 7. Dependence of the valve response speed for different operating voltages at fixed operating pressures
Analysis of the data presented in Fig. 7 and Fig. 8 shows that at increasing of the actuation voltage above 12 V, the valve actuation speed practically does not change, and the actuation current value does not exceed 150 mA. Therefore, the valve’s operating voltage of 12 V provides the required response speed.
Figure 8. Dependence of the valve actuation current on the operating voltage at fixed operating pressures
Figure 9. Valve’s temperature regimes
To reduce power consumption and prevent possible overheating, the SV-1 valve goes into the holding mode after actuation. In Fig. 9 the dependences of the valve temperature change in the holding mode at various holding voltage values when the valve is operating in a vacuum of 10-4 Pa are showed.
The valve holding voltage of 7 V was selected based on tests performed.
At the third stage of testing, to simulate the operation of the SV-1 valve as part of the working substance storage and feed system of the electric propulsion system, two engineering model valves were tested for 106 actuations when the working pressure was reduced from 155 bar to 15 bar.
The valve connection diagram during testing is shown in Fig. 10. Compressed air was used as a working substance. The actuation voltage of 12 V was supplied from a stabilized power supply with a maximum power of up to 200 W, the output voltage instability was 0.05 V.
The measurements of the leakage level during testing were carried out after 1, 2, 4, 7, 10, 20, 40, 60, 80 and 100% of the valve actuation cycles at operating pressure 155, 150, 140, 130, 120, 100, 80, 40 and 15 bar respectively.
During the entire third test cycle (106 actuation cycles), the leakage of the both solenoid valves did not exceed 0.0004 cm3 / s.
Figure 10. Valve connection diagram during testing
1 – Manometer;
2 – Filter (5 µm);
3 – Tested Valve;
4 – Flow restrictor;
A – Tested Valve inlet;
С – Tested Valve outlet.
6. APPLICATION OF THE SV-1 VALVES IN THE WORKING SUBSTANCE STORAGE AND FEED SYSTEM FOR SPS-25 and SPS-40 ELECTRIC PROPULSION SYSTEMS.
The developed SV-1 valves were used in Xenon Storage and Feed systems (XFS) for electric propulsion systems SPS-25 and SPS-40 (Fig. 11).
The XFS structure contains the tank made of polymer composite materials, which provides storage of Xenon at pressures of up to 300 bar (not shown in the figure, selected depending on the spacecraft mission); a high-pressure unit, which provides a decrease of the pressure in the tank to 1±3% bar in the AT and the low-pressure unit, which supplies the working substance from the AT to the anode unit and the hollow cathode unit of the Hall thruster with predetermined mass flow rates.
For pressure measurement at the control points of the storage and feed system (Fig. 1), the pressure sensors with a measurement accuracy of ± 1% are used. To ensure the specified working substance mass flow rates into the anode unit and the hollow cathode unit, SETS has developed appropriate flow limiters with bores up to 80 microns and calibrated capillary tubes with an inner diameter of 40 and 60 microns. The accuracy of setting the mass flow rates of the working substance is set by stabilizing the pressure in the AT and is ± 3%.
Figure 11. XFS for application in SPS-40 electric propulsion system
The company Space Electric Thruster Systems (SETS) has developed, manufactured and successfully tested laboratory and engineering prototypes of lightweight electromagnet valves. Lifetime tests confirmed the valve’s performance over more than 1,000,000 actuations. Burst tests have confirmed that the valve can be used in feed systems with different control techniques (e.g. “bang-bang”) for high (up to 250 bar) and lowpressure propulsion systems. Also, actuation tests showed a quick response time (1-5 ms for working pressure 1-250 bar) of the valve.
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