Miniaturized Flow Control Unit

Project Information

µFCU

Grant agreement ID: 284100

Closed project

Start date 1 December 2011

End date 30 September 2013

Funded under

• Specific Programme "Cooperation": Space

Total cost € 2 059 936,50

EU contribution € 1 428 212,00

Coordinated by

AST ADVANCED SPACE TECHNOLOGIES GMBH
Germany

Final Report Summary - µFCU (Miniaturized Flow Control Unit)

Executive Summary:
Electric propulsion (EP) is a key technology for future space missions and satellites. Most of the used EP systems require controlled and steady flows of Xenon gas to supply the thrusters and neutralizers. Today, flow control units (FCUs) which are capable to drive one single thruster have a typical mass of about one kilogram. Only FCUs from US suppliers are available with masses of 400g per thruster. Assuming spacecrafts (S/C) with up to 24 thrusters for fine pointing capabilities like LISA, the FCUs are significantly contributing to the mass and power budget. Such future missions are restricted to low spacecraft masses. Consuming a large portion of the total system mass, the use of an EP system with actual FCUs is impractical. Even for larger satellites the mass of the electric propellant system is driving the overall system mass budget.
The new development of the "µFCU" miniaturize the flow control unit and lower the mass and footprint required on the satellite panels. The large potential of the used miniaturization technology has been impressively demonstrated. µFCU controls two independent gas flow lines. Although made of stainless steel the total weight remains below 60 grams (without harness).
Further driving requirements for FCUs are a low leakage and high lifetime capability. The millisecond fast switching valves converted for space applications within µFCU achieved initial internal leakages below 1E-9 scc/s GHe and still impressive 1E-6 scc/s GHe after 300 million switching cycles over an extended temperature range from -40°C to +110°C. The cycle lifetime capability is unique in the market and already exceeds the requirements for a typical satellite control application by a factor of three.
The µFCU has been developed to EQM level. The assembly and integration processes have been mastered in an extensive development program. The µFCU is a fully welded device with a stainless steel body. Media wetted surfaces are completely made of 316L steel except for the seal elastomere. Inlet and outlet flow lines are protected against particle contamination by 5µm filters.
Several units have been manufactured and tested. The prequalification test campaigns covered performance tests, thermal vacuum test, vibration test and pressure tests. After these successful tests the µFCU has reached technology readiness level TRL 5.
µFCU has been developed in close cooperation with future customers that supported the project with advice and specification review to ensure an adequate design for integration of µFCU into existing electric propulsion systems.
With µFCU and the developed miniaturization technology, Europe takes over the technological leadership in spaceborne gas fluidic control and distribution systems for electric propulsion.

More details on µFCU may be found under http://www.microfcu.com
Project Context and Objectives:
Electric propulsion (EP) is a key technology for future space missions and satellites. Most of the used EP systems require controlled and steady flows of Xenon gas to supply the thrusters and neutralizers. Today, flow control units (FCUs) which are capable to drive one single thruster have a typical mass of about one kilogram. Only FCUs from US suppliers are available with masses of 400g per thruster. Assuming spacecrafts (S/C) with up to 24 thrusters for fine pointing capabilities like LISA, the FCUs are significantly contributing to the mass and power budget. Such future missions are restricted to low spacecraft masses. Consuming a large portion of the total system mass, the use of an EP system with actual FCUs is impractical. Driven by these requirements and restrictions objective 1 can be derived as:
→Objective 1: Reduce the FCU system mass to less than 100g per thruster (thruster + neutralizer flow line).

The flow control units are a key component of an electric propulsion system. In consequence of market concentrations the availability of state-of-the-art flow control components is dependent on foreign countries, leaving Europe's satellite market vulnerable. Therefore a flow control unit for EP systems has been rated as critical space technology.
→Objective 2: Use only components of European origin.

For missions like telecom satellites the system operational time may exceed 15 years. As result, the propellant loss due to leakage must be limited to a minimum. At the same time it would be an advantage for system integration, if the operational temperature range is enlarge up to +90°C.
→Objective 3: Achieve internal leakage rates below 1E-6 scc/s GHe over lifetime and extend temperature range beyond +90°C.

The cost effective development and production of a flow control unit is only possible, if the full market potential is addressed. Therefore the basic design of the FCU shall be able to supply micropropulsion systems (<1mN thrust) as well as thrusters and neutralizers for satellite station keeping (typ. 50-100 mN thrust).
→Objective 4: Demonstrate the operation with at least two flow ranges a) 1.5 sccm F.S. Xe, b) 50 sccm F.S. Xe

The emerging ESA missions with low thrust and fine control requirements like LISA need a capable micropropulsion system. A potential candidate propulsion system requires a technology readiness level of at least TRL 5 for all components. If a technology is able to gain flight heritage on a demonstration mission this would be a significant advantage.
→Objective 5: Reach TRL 5 and prepare for a first mission towards TRL 7.

The performance of an unit has to be rated in the context of the embedding system. Development and system cost grow with system complexity and the number of interface requirements. Even if hard to measure, the simplicity of a design is one of the keys to acceptance in space business. For µFCU the simplicity has been achieved by a modular design with simple interfaces that can be satisfied by state-of-the-art equipment. In the competitive market segment of telecommunication satellites a robust flow control unit with low system requirements and low impact on the power supply design provides the chance of cost reduction.
→Objective 6: Use a modular concept and keep the design simple.



Project Results:
The µFCU project bases on a "spin in" development approach of ITAR-free European components. Five partners provide technologies and components that have been developed for high performance and high reliability ground applications. An advantage of the use of existing technology is a cost and time reduction for the development. A second advantage is the experience from development, production and application gained by the manufacturer during decades. Nevertheless some new aspects that should not be underestimated are added to a design if components are integrated into a system and if they are prepared for a space application. Within the scope of the µFCU project these technologies have been converted to space application. This conversion covers the exchange of materials to equivalent space proven types, the application of processes like cleaning and cleanliness control and an intensive test and verification campaign. This campaign was carried out on component and unit level.

A. The Baseline Design
The term "µFCU" covers on the one hand AST's technology to design and manufacture a miniaturized flow control unit based on conventional component technologies like solenoid valves. With this technology different types of flow control and management systems can be set-up. On the other hand, it stands for the baseline design of a Xenon flow control unit with one inlet flow line and two outlet flow lines in the context of this publication.
The µFCU has two independently controlled flow lines with commandable flow rates. The number of lines is sufficient to supply either two independent thrusters or one thruster/ neutralizer pair (SPT, HEMPT, RIT). The µFCU has no flow sensor element to keep the system complexity low (objective 6) . The control loop can be closed using a signal from the thruster like the anode current (SPT, HEMPT) or the deviation from a optimal operation point (RIT). Such a control is commonly used in state-of-the-art electric propulsion systems.
The FCU is supplied with Xenon gas (or other noble gases) through an inlet port. The inlet is protected by a 5µm stainless steel mesh filter. The flow into the µFCU is established or stopped by the inlet isolation valve (IV). Behind the isolation valve the flow splits into two branches. In each branch a chopping valve (CV) is operated in a pulse width modulation (PWM) or frequency modulation (FM) mode to control the average flow. Each valve can be controlled individually so that compared to some other designs the flow ratio between both outlet lines can be varied. The pulsed flow from the valve enters a microchannel structure. The channels, acting as flow restrictors. In conjunction with cavities the channels form a fluidic low pass filter to eliminate the flow ripple at the outlet. The flow channels and the cavities are embedded in a planar structure called "flow path board (FPB)". This FPB can be compared to a PCB in electronics. It interconnects the (surface mounted) components like filters and valves and provides the resistive and capacitive elements.
Both output paths are equipped with 5µm particle filters. The particle filters in inlet and outlet lines isolate the interior of the µFCU from contaminations during integration handling e.g. line welding. Particles from outside that have been trapped on the filter mesh can be flushed with isopropyl alcohol.
The valve manufacturer's heritage in building valves for leakage testers contributes to ultra low internal leakage. All valves act as isolation valves, providing a double serial redundancy against propellant loss. The internal leakage of the µFCU is specified to be better than 1E-6 sccs GHe. Typically values achieved during test campaigns are even one order of magnitude lower over full life.
The PWM/FM operation shows a number of system advantages compared to proportional valves. The valves of µFCU are completely switched open or closed. The switching is robust and has no drifting working point compared to the proportional valve. The drift of the working point would be critical for the control of small flows in environments with large temperature changes.
Also the driving electronics benefits of the reduced requirements if a simplex switched voltage (20V...24V) is required compared to a precisely controlled analogue current.

The major drawback of state-of-the-art PWM/FM controls is the large flow ripple introduced by the low frequency on/off cycles. The typical sound of the operation also triggered the nickname "bang/bang". The µFCU development was able to overcome this drawback by introducing a higher chopping frequency typically in the range between 1 Hz and 5 Hz. Combined with a fluidic low pass filter element embedded into the flow path board, the flow ripple at the outlet vanishes.

B. Component Development

1. Valves
A higher chopping frequency drives the number of lifetime switching cycles. As example, a µFCU running with 3 Hz average frequency for 20 000 hours activates the chopping valves for 216 million cycles. The valves used for µFCU utilize a very advanced plate anchor technology without bearing friction compared to a standard plunger. Only the small bending of a precision spring, the magnetic forces in the coil assembly and the hit of the armature onto its rest introduce a mechanical stress.
The major wear mechanism is linked to the abrasion of the seal elastomer. The mechanical life of the valves is very high. Individual valves have already been operated in ground application for some billion cycles. During the µFCU development, a set of 30 valves with three different seal elastomer has been tested in an accelerated wear test. To increase the wear, the temperature was also cycled in a climate chamber in 6 hours from -40°C (below glas transition temperature for the used Viton) to +110°C. Additionally a flow of more than 1000 sccm has been established with a differential pressure of approx. 2 bars to maximize gas dynamic abrasion.
The test was performed with Argon and Xenon as test gases in a closed loop pumping system. After some ten million cycles the test was interrupted to measure the leakage. After 300 million cycles the valves showed first wear effects. After 350 million cycles the µFCU internal leakage requirement of 1E-6 sccs GHe was exceeded by most of the valves. The test continued to investigate the mechanical life especially a potential fracture of the spring. After additional 350 million cycles (700 million cycles in total) without valve failure it has been decided to stop the test.
Using the lifetime limitation due to seal material wear under worst case conditions, the minimum cycle operational lifetime capability of µFCU can be estimated.

2. Flow Path Board
The function of the flow path board (FPB) is very similar to a PCB in electronics. They shall interconnect components, provide flow resistors and cavities and are the mechanical interface to the satellite panel.
FPBs are made of a stainless steel plate material. Microchannels are engraved into the surface. The channels interconnect holes and borings. Later the borings are the ports for surface mounted components like valves or particle filters. The individual plates are attached together to a stack. The stack is then bonded vacuum tight by a special process. The final board carries a three-dimensional network of channels and connecting ports for components.

3. Particle Filters
Particle filters have been designed for a surface mount interface required by the integration to the FPB. The filters are completely made of 316L stainless steel. The filter element is a woven mesh fabric with a maximum pore diameter of 5µm. The filtration grade of each filter is verified by a bubble test.

4. Flow Sensor
Although not included in the baseline design of the µFCU, a sensor for low Xenon gas flows has been developed. The sensor can measure flows in the low sccm range with a resolution of milli-sccm. A sensor housing and a prototype electronics have been developed to show a potential integration into a subsystem. The accurate measurement of flows of less than one sccm requires a new calibration technique that also has been developed within the project. This device is an advancement of a standard calibration method developed by the NIST. It is able to produce known flows with a pressure controlled piston displacement while the flow is re-measured using the pressure drop across the 50 µm orifice.

C. Prequalification Test Campaign
During the development phase the production and assembly processes have been mastered and transferred to the unit level. After the development phase, two EQMs have been manufactured in mid of 2013. EQM 01 executed performance tests and thermal vacuum tests. EQM 02 was subject to performance tests, vibration tests and a proof pressure test. Both units have been tested to qualification levels typical for electric propulsion system. With the end of the tests the µFCU technology has demonstrated its capabilities in "a relevant environment" and has reached technology readiness level TRL 5.

Potential Impact:
The new µFCU with a total mass of 62 grams and low complexity interfaces shows clear system advantages compared to competitor's solutions. The technology of integrating small components into a flow path board instead of having large tubes allows a size reductions and an improved production process. Especially for complex flow control and distribution systems, µFCU will provide a clear cost reduction potential.
The European sourced components guarantee the non-dependence from other countries. With its low complexity, ultra low leakage and enhanced operational range and lifetime µFCU outperforms all existing flow controls. It is expected that the technology can also be adapted to other flow control and distribution task as required e.g. by chemical or cold gas propulsion systems.
If flight heritage can be gained in a future mission for µFCU and its technologies, Europe will gain technical leadership in fluidic systems for space propulsion.

List of Websites:

http://www.microfcu.com