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Film-Evaporation MEMS Tunable Water Microthruster Array (FEMTA) By Tony Cofer FISO Telecon 7/20/16. Outline. Introduction Microfabrication Testing and results Current Work . INTRODUCTION. Motivation Thrust Requirement Options Thermal Valve Concept. Motivation.

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  1. Film-Evaporation MEMS Tunable Water Microthruster Array(FEMTA)ByTony CoferFISO Telecon 7/20/16 Tony Cofer

  2. Outline • Introduction • Microfabrication • Testing and results • Current Work Tony Cofer

  3. INTRODUCTION • Motivation • Thrust Requirement • Options • Thermal Valve Concept Tony Cofer

  4. Motivation • Miniaturization of satellite systems provides potential for reduced launch and operations costs • Small sats i.e. micro, pico, nanosats are emerging technologies. • Cubesats(4 inches cube) are low cost alternative for some applications. • Propulsion systems resist miniaturization • Viscous losses at small scale • Low mass fraction due to power processing mass • Excessive power demands for small sat applications Tony Cofer

  5. Cubesat Minimum Thrust Requirement • 1.33 kg cubesat • 0.1 meter cube • Need 180 degree slew in 1 minute ω = 0.1047 rad/s • = 0.0022 kg·m2 = moment of inertia • = 0.000230 N·m·s = = angular momentum • F = necessary thrust • d = distance from center of mass = 0.0707m • t = firing time = 60 s • = 54 µN Tony Cofer

  6. Electrothermal FEEP PPT Power vs Thrust for smallsat propulsion VAT Hall/Ion Electro-Spray Cold Gas FEMTA Max power–min thrust for 1U cubesat* * Assuming 180 degree/ 1 minute slew requirement – see slide 54 Tony Cofer

  7. Comparison of Cubesat Thrusters Cold Gas[1] FEMTA µCat Arc[3] Electrospray[2] • Minimum Power 0.01 W • Water propellant • Dry mass 1.5grams – propellant 1 grams • Max thrust ~230 µN • Min impulse bit 3 µN∙s • Total impulse 1 N∙sec/g • Nominal Isp = 95 s • Effective Isp = 38 s – 80s • Minimum Power 0.1 W • Metal propellant • Dry mass 200 grams – propellant 40 grams • Max thrust ~35 µN • Min impulse bit 0.1 µN∙s • Total impulse 1370 N∙sec • Nominal Isp = 3500 s • Effective Isp = 583 s • Minimum Power 1.5 W • Ionic Liquid propellant • Dry mass 100 grams – propellant 12 grams • Max thrust 100 µN • Min impulse bit NA 2hr startup • Total impulse – 140 N ∙s • Nominal Isp = 1200 s • Effective Isp = 137 s • Minimum Power ~5 W • Liquid butane propellant • Dry mass 456 grams – propellant 55 grams • Max thrust 53 mN • Min impulse bit 0.53 mN·s • Total impulse – 23 N ∙s • Nominal Isp = 43 s • Effective Isp = 4.6 s [1]http://www.vacco.com/images/uploads/pdfs/MicroPropulsionSystems_0714.pdf [2] Krejci, David, Fernando Mier-Hicks, Corey Fucetola, Paulo Lozano, Andrea Hsu Schouten, and Francois Martel. "Design and Characterization of a Scalable ion Electrospray Propulsion System." (2015). [3]Keidar, Michael, SamudraHaque, Taisen Zhuang, Alexey Shashurin, Dereck Chiu, George Teel, Elwood Agasid, Oriol Tintore, and Eddie Uribe. "Micro-cathode arc thruster for PhoneSat propulsion." (2013). Tony Cofer

  8. FEMTA ConceptPressure vs Surface Tension Under proper conditions pressure can be offset by surface tension, micron size screens have been used to filter vapor from liquid in resistojet systems. Tony Cofer

  9. Critical Capillary Size Young-Laplace Equation γ = Surface tension Pvap = Vapor Pressure θ = Contact angle d = gap width Tony Cofer

  10. FEMTA Concept ~10 microns ~40 microns Heater Meniscus Liquid is contained by hydrophobic surfaces, evaporation is temperature controlled at the meniscus Tony Cofer

  11. Proposed Device and Application Single FEMTA nozzle featuring substrate heater if waste heat is not available 10 by 10 element FEMTA array propellant reservoir is integrated Cubesat with 3 axis rotational control using 12 FEMTAs Tony Cofer

  12. Proposed Device and Application Yaw +/- Pitch +/- Roll +/- Cubesat with 3 axis rotational control using 12 FEMTAs Tony Cofer

  13. MICROFABRICATION • 1st Generation • 2nd Generation • 3rd Generation Tony Cofer

  14. Configuration Gold conductor 3750 micron long x 200 micron wide x 1 micron thick Silicon Slot Nozzle 2500 microns long 200 microns deep 60 microns wide (entrance) 10 microns wide (exit) Nichrome heater 2400 micron long x 6.6 micron wide x 0.5 micron thick Tony Cofer

  15. 1st Generation: AR~2 Lateral Cross Section 35 microns 140 microns 9 microns 250 microns Looking In Tony Cofer

  16. 1st Generation: AR~2 Type A Sample A1-5 – G Longitudinal Cross Section Heater Element • Broken by first design hold down washer • Split along longitudinal axis of nozzle Looking In Gold Overlap Roughness in throat Tony Cofer

  17. 1st Generation AR~8 48 µm 28 µm Cross Section Nichrome Heaters 88 µm 7.9µm 275 µm Looking In Tony Cofer

  18. 1st Generation Failure Modes and Reliability Considerations • Galvanic corrosion due to reactive heater metals Tony Cofer

  19. 2nd Generation FEMTA Units Built with Throat Aspect Ratios (AR) of 2-8 Nichrome 1.4 µm thick AR~2 AR~4 Oxide 1.9 µm thick AR~8 AR~6 Tony Cofer

  20. 3rd Generation FEMTA Units Built with Throat Aspect Ratios (AR) of 2-8 AR=6 AR=2 Nozzle Inlet and Throat AR=8 Tony Cofer

  21. FEMTA Configurations Tony Cofer

  22. TESTING • Evaporative Loss Measurement • Thrust Measurement • Mass Flow Measurement • Isp Calculation Tony Cofer

  23. 1st Generation Test Vessel • Miniature pressure vessel • Water leak rate 7 mg/hr • Power available to FEMTA for mass flow determination • Thermocouple (not shown) monitors fluid temperature Mounting Washer Retaining Washer FEMTA 50 mm Tony Cofer

  24. Test Set Up 100 TorrBaratron Power Valve Test vessel Vacuum Chamber Tony Cofer

  25. Evaporative Loss @ 22 C Free molecular rate = 0.206 g/hr Numerically calculated rate 0.030 ± 0.015 g/hr Tony Cofer

  26. Torsion Springs Linear Variable Differential Transformer (LVDT) Electrostatic Fins MicroNewton Thrust Stand at Purdue +++++ -- - - - - Base on design by Dr. Andrew Ketsdever* and constructed by Dr. IvanaHrbud *Development of a Thrust Stand Micro-Balance to Assess Micropropulsion; Pancotti, Lilly, Ketsdever et.al AIAA 2005-4415 Performance Tony Cofer

  27. Diffusion Pump Speed Curve • Pumping speed is constant below 1 milliTorr • Maximum test pressure is 0.1 milliTorr • Mass flow out of the chamber is proportional to Pump speed curve for the Varian HS-20 diffusion pump. Tony Cofer

  28. Diffusion Pump Conductance • Transconductance is treated as parallel resistances • Conductance 1 is conductance of duct • Conductance 2 sonic limit of duct aperture Tony Cofer

  29. Mass Flow Measurement Pressure change integrated from start of one pulse to start of next • 10 known masses of nitrogen injected into vacuum chamber on pressure history recorded. • 2 tests each of increasing, decreasing, and random mass flows – 60 data points. • Pressure change and mass flows integrated from beginning of a pulse to the beginning of the next Pressure history for 30 second mass flow pulses with 60 second delays from 5 – 50 sccm nitrogen in 5 sccm increments, integration interval for third pulse in green. Tony Cofer

  30. Mass Flow Measurement Mass vs ΔP∙t for water vapor@50 °C Conversion Factor = 5342 mg/Torr∙s Mass vs ΔP∙t for nitrogen @ 22°C Tony Cofer

  31. Measurement Uncertainty • Calculated using Taylor Series Method Tony Cofer

  32. 2nd Generation Thrust Test Setup • Test vessel from Gen 1 experiments mounted horizontally on a microNewton thrust stand • Internal pressure controlled via 0.5 psi relief valve • Power provided by Labview square wave generator passed through a unity gain power amplifier • Only AR~8 heaters provided any data Tony Cofer

  33. Maximum Expected Performance For 10 x 2500 micron throat = 540 µg/s mass flow = 67.7 mW input power = 1.15 W cooling power = 74 seconds • = 392 µN Thrust Tony Cofer

  34. Thrust Histories for Gen 2 AR~8 Nozzles – Test 1 Powered Interval • 211 mW powered tests on Gen 2 AR~8 nozzle • 7 Volt 100 Hz square wave applied for 60 seconds • Miniscule – barely measureable thrust Tony Cofer

  35. Thrust Histories for Gen 2 AR~8 Nozzles – Test 2 Powered Interval • 349 mW powered tests on Gen 2 AR~8 nozzle • 9 Volt 100 Hz square wave applied for 60 seconds • Miniscule – barely measureable thrust Tony Cofer

  36. Gen 3 Test Vessel • Teflon construction to reduce galvanic corrosion • Can hold up to 8 grams of water Tony Cofer

  37. Gen 3 Thrust Test Setup FEMTA Test Vessel • Thrust stand mounted in a 4.2 cubic meter vacuum chamber • Base pressure ~ 10 microTorr • Thrust stand calibrated from 8 – 768 µN MicroNewton Thrust Stand Tony Cofer

  38. Gen 3 AR~2 Thrust History for 30 Second 65 mW Pulse Powered Interval • Thrust levels higher than expected • Delay before firing • Erratic behavior shuts off prematurely • Unwanted impulse bits in other tests Tony Cofer

  39. Gen 3 AR~2 Isp Summary • Seems to indicate peak performance at around 50 mW • Due to excessive mass flow at higher power levels Tony Cofer

  40. Gen 3 AR~2 Thrust and Bulk Fluid Temperature Histories for 30 second 50 mW pulse • Thermocouple measurement of bulk fluid shows temperature drop due to vaporization energy drawn from water reservoir Tony Cofer

  41. Entropy Generation • Where is the specific entropy change for liquid water. The entropy change for the gas has two parts; the change of the liquid to FEMTA firing temperature and the change from liquid to gas, and is given by • Assuming the phase change occurred at T = 323 °K which is , and is the specific entropy change of vaporization, then Tony Cofer

  42. Gen 3 AR~4 Thrust and Power Histories for 10 Pulses @ 75 mW Tests performed at 25, 50, 75, 125, 150, 200, and 300 mW Tony Cofer

  43. 75 mW Test Tony Cofer

  44. 150 mW Thrust Test Tony Cofer

  45. Gen 3 AR~4 Thrust vs Power • Thrust to power ratio nearly constant at around 230 µN/W Tony Cofer

  46. Gen 3 AR~4 Mass Flow vs Power Tony Cofer

  47. Gen 3 Specific Impulse vs Power • AR~2 nozzles provide comparable Isps to AR~4 at low power • AR~4 nozzles provide consistent Isp ~90 s at higher power levels Tony Cofer

  48. Gen 3 Thrust/Power Ratio vs Power • AR~2 nozzles provide higher thrust/power – up to 6000 µN/W – but with unstable performance • AR~4 nozzles provide consistent 230 µN/W Tony Cofer

  49. Gen 3 Coefficient of Performance vs Power • AR~4 nearly constant ~0.4 twice as much heat in as out • AR~2 variable – high ~11 –theoretical limit is 18 Tony Cofer

  50. Flow Characteristics • Based on firing temperature 50 C or 323 K • Assume sonic flow M = 1 • Vapor pressure = 12 kPa – sonic pressure = 6.4 kPa • Assume ideal gas - density at throat = = 0.043 kg/m3 • Characteristic length = gap size = L = 8 microns • Reynolds number = = 13.6 • Knudsen number = = 0.1 • Mean Free Path = ~0.8 microns Tony Cofer

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