1 / 18

Electrodynamic Tether System Analysis Comparing Various Mission Scenarios

Electrodynamic Tether System Analysis Comparing Various Mission Scenarios. Keith R Fuhrhop and Brian E. Gilchrist University of Michigan. Introduction. Electron Emission Theory & Space Charge Limits Thermionic Cathodes Field Emitters Hollow Cathodes ED Tethers System Integration

rosalba-frank
Download Presentation

Electrodynamic Tether System Analysis Comparing Various Mission Scenarios

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Electrodynamic Tether System Analysis Comparing Various Mission Scenarios Keith R Fuhrhop and Brian E. Gilchrist University of Michigan

  2. Introduction • Electron Emission Theory & Space Charge Limits • Thermionic Cathodes • Field Emitters • Hollow Cathodes • ED Tethers System Integration • System Simulations

  3. Thermionic Emission • 2 Step Process – Thermionic Emitter then Electron Gun • TC production • Overcome Fermi Energy • Boils off Low E Electron • EG emission • To Overcome SCL • EG needs more E • Thermionic Emitter • Electron Gun • Richardson Eq.

  4. Field Emission • Quantum Mechanical Tunneling Effect • 108 - 109 V/m E-field • 107 tips / cm2 • No Heaters or Gas Required • Many Emitter Types • Spindt type, carbon nanotubes, BN nanostructure (UM) • Fowler-Nordheim Eq.

  5. Hollow Cathodes • Setup • TC Emission • Xe Ionized Gas • Stats • Fuel Flow Rate (4–14 sccm) • Potentials (10-40 V) • Diameter of Keeper (1-12 cm) • Double Sheath Possible • 2 Potential Drops • Across the Xe gas then Sheath

  6. Space Charge Limit 1-d C-L Law (vacuum gap) • Only so many Electrons can emitted at a time • Plasma Parameters Determine • Sheath • Emission Area • Emission e- Energy 3-d C-L Law (vacuum gap)

  7. De-boost Tether Example • Tether electromotive force (EMF) drives current through tether • Vemf = (vxBNorth)• l • Geomagnetic field, BNorth (0.18 - 0.32 Gauss) • Orbital velocity, v (~7500 m/s @ 300 km alt) • Vemf (35 – 250 V/km) along tether of length l • Electrons collected from ionosphere along positively biased upper bare tether and returned to ionosphere at lower end • Current I produces magnetic force (drag thrust) dF on each tether section of lengthdl:dF = dlIxBNorth. • Current magnitude varies along tether • Current magnitude determined by • Available EMF and tether resistance • Bare-tether electron collection efficiency • Electron ejection efficiency at lower end

  8. Grounded Cathode (HC’s) S/C Surface is negative by HC bias Grounded Gate (TC & FEA) S/C at floating potential Vemf powers emitter If can’t emit current then emmiter cathode or spacecraft pulled very negative Series - Bias Grounded Gate (TC & FEA) S/C at floating potential Emitter not Powered by Vemf Easily Control Emitter Potential Requires non-tether power source If can’t emit current then emitter spacecraft pulled very neg. Configurations Series – Bias Grounded Gate Grounded Tip Grounded Gate

  9. Differences in Mission Objective Tether length: 5005 m Geometry: Single Line Bare vs. Insulated: 50% Bare Boost vs. De-boost: Both cases Orbital Parameters: 0o Latitude, 35o Inclination HVPS: 2000 V S/C Surface area: Next Slide Emission device: TC, FEA, or HC Models: IRI-2001, MSIS-86, IGRF-91 Test Dates: 1-1-06 (Min) & 7-15-01 (Max) EDT Simulation System Setup

  10. EDT Simulation System Setup 2 Total Mass = 1055 kg Total SA = 15.622 m2 Ballistic Coeff. = 30.697

  11. Simulation Analysis 1 • Max boost [N]: • High Density: ~0.56 HC, ~0.51 FEA, ~0.076 TC • Low Density: ~0.048 HC, ~0.046 FEA, ~0.033 TC • Fewer TC Emitters: Potential near Max • System correspondingly reacts

  12. Simulation Analysis 2 • Total Power ( = PHVPS) [W]: • High Density: ~7800 HC, ~7300 FEA, ~1700 TC • Low Density: ~418 HC, ~ 421 FEA, ~462 TC • TC has weakest boost & uses most power (Min) • Same issue with Fewer Emitters in Max Case

  13. Simulation Analysis 3 • Efficiency = Orbit Power / Supplied Power • Identical (Min) • Dip being investigated (Max)

  14. Simulation Analysis 4 • Boost / Power [N/W] • HC most Efficient • Within 25% of max value through 2000 km • Investigating max density case

  15. Simulation Analysis 5 • Max boost [N]: • High Density: -0.57 HC, -0.52 FEA, -0.11 TC • Low Density: -0.038 HC, -0.036 FEA, -0.024 TC • Fewer TC Emitters: Potential near Max • System correspondingly reacts

  16. Simulation Analysis 6 • Total Power ( = PEMF) [W]: • High Density: ~7390 HC, ~6760 FEA, ~1580 TC • Low Density: ~330 HC, ~320 FEA, ~280 TC • Near equivalent power (Min) • Same issue with Fewer Emitters in Max Case

  17. Simulation Analysis 7 • Boost / Power [N/W] • TC most efficient until 400 km in high density case • Efficiency reaches min at 500 km then goes up • FEA highest after ~1300 km in low density case

  18. Conclusion • TC’s • Not Very Effective • FEA’s • Nearly identical to HC performance • HC’s • Proven, Most Powerful & Efficient • Requires use of a consumable gas! • Future Work: • Analysis on Other EDT Mission Objectives • Further analysis on current work

More Related