uFCU - Results of a Prequalification Test Campaign

I. Introduction

Electric propulsion systems require controlled and steady flows of Xenon gas, and typical flow control units (FCUs) have a mass of 400 g to 1 kg per thruster – significant for spacecraft with up to 24 thrusters like LISA and increasingly a driver of system cost pressure even for new EP technologies. AST Advanced Space Technologies GmbH, a new player in FCUs for EP systems, led a joint team of five partners in a 22-month project funded by the European Commission’s 7th Framework Program that designed, manufactured and tested a new miniaturized flow control unit, the µFCU, with particular focus on manufacturing processes and dedicated measurement/ground support equipment.

After validating components and mastering manufacturing processes, two engineering and qualification models were built and tested, covering a full qualification program except a full lifetime test (whose lifetime capability was investigated separately on component/valve level). EQM tests were split into two groups – thermal stress and pressure tests, and vibration stress tests – applied in parallel to one EQM each, with EQM02 additionally undergoing proof pressure and thermal vacuum testing, so that both EQMs together were exposed to all stress types, demonstrating design maturity for a formal qualification despite the short development time.

II. Valve Component Test (Accelerated Wear Test)

At the valve manufacturer, 30 valves with three different seal materials were set into an accelerated wear test under worst-case conditions, with thermal cycles from -40°C to +110°C every 6 hours and flows exceeding 1000 sccm at 2 bar nominal pressure using Argon and Xenon to maximize gas-dynamic erosion. The finally selected Viton elastomer seal showed first degradation signs after more than 300 million switching cycles, exceeding the leakage requirement of 10⁻⁶ sccs GHe after 350 million cycles; testing continued to over 700 million cycles without mechanical failure, from which a lifetime cycle capability of at least 300 million cycles under worst-case conditions was estimated.

III. Pre-Qualification Test Campaigns

Two engineering and qualification models (EQM01, EQM02) of identical design were manufactured, each with a high-flow port up to 8 sccm Xenon and a small port up to 0.5 sccm nominal full scale at 2 bar, self-limited to about 145% of nominal flow, and protected by 5µm particle filters. EQM01 additionally carried temperature sensors at the valve “hot spot”. Both units were cleaned with de-ionized water and isopropyl alcohol prior to test.

The thermal vacuum test, performed at the 1st Institute of Physics at Giessen University, stepped temperature over an operational range of -35°C to +80°C (EQM02 planned -40°C to +100°C) under vacuum below 0.1 mbar, characterizing the FCU before and after thermal cycling and pressure proof tests. Key findings: the transfer function between PWM/FM and flow is acceptably linear; maximum flow is choked at about 145% above full-scale flow at nominal conditions, independent of temperature (less than 2% variation); nominal flow varies about ±25% over the full temperature range due to gas viscosity changes; internal leakage stayed below 10⁻⁷ sccs GHe at room temperature and nominal pressure, increasing with temperature due to elastomer diffusion; set-on/set-off voltages showed only minor thermal sensitivity; and settling time and flow stability measurements showed no detectable outlet flow ripple even at increased scaling, with noise level about 0.5% and no harmonics.

Valve temperature measurements showed the potting material at the valve tip remaining below +110°C at full +24V power and +80°C baseplate temperature, suggesting the operational range could be extended to +90°C or beyond with lowered driving voltage or pull-in/hold operation for the isolation valve. After slow dependency-test cycles between -35°C and +80°C, fast non-operational cycling between -50°C and +110°C was performed (14 cycles total, eight non-op), after which a further dependency test revealed no changes in transfer function or internal leakage, and external leakage was also verified as unaffected.

Proof Pressure Test (EQM01)

After thermal vacuum testing, EQM01 was placed in a de-ionized water bath for proof pressure tests. At MEOP (8 bar) the µFCU opened its valves successfully to release gas (venting capability); at proof pressure (12 bar, three cycles of 5 minutes dwell each, with closed and open valves) no leakage or degradation occurred, and valves could still be opened even at 12 bar. Post-test characterization confirmed all parameters remained within specification and measurement uncertainty, and microscopic inspection of weld seams showed no irregularities.

Vibration Test (EQM02)

The vibration test campaign on EQM02 followed an equivalent scheme: characterization, vibration stress exposure with in-situ leakage monitoring via flexible PTFE tubes connected to a leakage tester, then re-characterization. Out-of-plane vibration was tested up to 21.5 gRMS and in-plane up to 17.25 gRMS; no increase in leakage tester signal was observed during any test, confirming the valves remained closed, and post-test internal/external leakage measurements reconfirmed ultra leak-tightness of valves and welds. Two further high-flow models (up to 50 sccm Xenon) were under manufacturing for coupling tests with EP system manufacturers.

IV. Conclusion

A new miniaturized xenon flow control unit has been developed and pre-qualified by the AST-led project consortium, with all relevant tests successfully performed to qualification levels on two engineering and qualification models. The new ITAR-free design allows significant reduction in mass and size to 62 grams for a two-flow-line design, with excellent open-loop stability, simple interface requirements and relaxed driving-electronics requirements giving the µFCU great potential for system cost reduction, ready for a formal qualification program after reaching TRL 5.