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MAGIC Tether Trade Study

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  1. MAGIC Tether Trade Study Anthony Lowrey Ryan Olds Andrew Mohler November 10, 2003

  2. Background • Purpose of trade study • To assess the feasibility of the MAGIC Tether system • Concern about design was raised at the PDR • Thought of as high risk for DINO • To investigate possible alternatives to the tether • Requirements from DINO • Spacecraft must be nadir pointing Colorado Space Grant Consortium

  3. Introduction to Tethers in Space • Gravity Gradient Stabilization • Lower mass has more gravitational than centrifugal force • Upper mass has more centrifugal than gravitational force • Lower mass slower • Upper mass faster Colorado Space Grant Consortium

  4. Introduction to Tethers in Space • Important issues • Tether length and tension • The longer the tether length, the more tension • Tether material properties • Coefficient of Thermal Expansion (CTE) • Shape Memory • Debris/Micrometeorite resistance • Tether deployment • Recoil • Tip-off rate Colorado Space Grant Consortium

  5. Brief History of Tethers • Tethered Satellite System 1 (TSS-1) • 1992 NASA shuttle tether • 550 kg satellite, 20 km electrically conductive tether • Deployment failed after 256 m from mechanical failure • Small Expendable Deployment System (SEDS) • 1993 NASA project • 25 kg satellite, 20 km tether deployed from a Delta 2nd stage • Successful mission: longest structure ever deployed to that time Colorado Space Grant Consortium

  6. Brief History of Tethers (Cont.) • SEDS-II • Launched in 1994 by NASA • Successful deployment • Tether was cut after only 3.7 days • TSS-1R • 1996 NASA reflight of TSS-1 • Spark severed tether just before deployment end • Tether Physics and Survivability Experiment (TiPS) • Built by Naval Research Lab. Launched in 1997 • 4 km tether survived about 3 years • Success lead to the ATEx project Colorado Space Grant Consortium

  7. Advanced Tether Experiment (ATEx) • Purpose • Demonstrate tether stability and control • Fly a long term, survivable tether • 6 km tether experiment was to last 61 days • Deployment • Deployed at steady 2 cm/s using a stepper motor • Deployment was to take 3.5 days • Sensors • Local angle sensor – 16 LED/detector pairs in a plane • Turns counter to measure length of deployed tether Colorado Space Grant Consortium

  8. ATEx Deployment Colorado Space Grant Consortium

  9. ATEx Failure • Launched atop STEX on 8/3/98 • Experiment began in 1/99 • Deployed 22 meters before being jettisoned by STEX • Tether blocked out-of-bounds LAS due to “excessive slack tether” • Determined reason for failure • Tether thermal expansion • From eclipse to sun, tether expanded 6 inches Colorado Space Grant Consortium

  10. ATEx Lessons Learned • Tethers can’t be fully tested on Earth • Good math models required in design • Provide large margins for error in design • Deployability of tether needed more consideration • Shape memory and CTE proved downfall • Experiment should be focus of mission Colorado Space Grant Consortium

  11. Post-DeploymentTether Dynamics

  12. Deployed Tether Geometry Tip Mass (5kg) Velocity 20m Libration Angle Nadir Zenith • Oscillating Frequencies: • Roll Oscillating Frequency = 0.000368 Hz • Pitch Oscillating Frequency = 0.000316 Hz • Yaw Oscillating Frequency = 0.000177 Hz Main Structure (25kg) Colorado Space Grant Consortium

  13. Current Issues • Tension and Libration • Pendulum Motion Requires Accurate Deployment • Tether Tape Material Properties Colorado Space Grant Consortium

  14. Tension Analysis • For a 20m tether, Tension will be approximately 0.3mN. • Tension this low could fail to provide adequate control in the pitch and roll axes of DINO. • At low tension, tip mass and main structure would rotate freely until tension builds up. Colorado Space Grant Consortium

  15. Pendulum Motion • Pendulum motion of DINO in the pitch and roll axes might not damp out over time. • Accuracy of the deployment would define the pointing accuracy of DINO. • ±10º off of nadir would be possible. Colorado Space Grant Consortium

  16. Material Properties • Thermal Expansion (20x10-6mm/mm/K) 13.7cm expansion in sun • Thermal Snap-Contraction (100x10-6/mm/mm/K) 68.6cm contraction in shade • Stress vs. Strain of Tether • Effective Modulus could differ from specs. Colorado Space Grant Consortium

  17. Conclusion • Issues/Risks • Lack of Tension • Pendulum Motion will not damp out • Tether expands and contracts in and out of sunlight • Possible solutions • A boom would be more rigid and could provide more predictable control. • Build a emergency release mechanism for the tether if it is used and provide a backup such as a momentum wheel. Colorado Space Grant Consortium

  18. Tether Deployment

  19. Brake Tether Tip Mass Wheel (turning) Tether Guides Velocity Brake shoe (fixed) Lightband Braking System Tether Z-fold Design at PDR • Open-Loop Deployment • Lightband will provide kickoff velocity of 2 ft/s • Deployment will take approximately 40 sec • Tether will be “left-behind” by tip mass • Braking system will slow tip-mass near end of travel • Simple compared to a complex motor system Colorado Space Grant Consortium

  20. Deployment Suggested Changes • Spoke with Jeff Slostad of Tethers Unlimited Inc • Longer tether • Having extra tether on board • Liked fast deployment • Liked “leave-behind” method • Feedback control system for braking Colorado Space Grant Consortium

  21. Booms

  22. Introduction to Booms • Provides gravity gradient stabilization on small spacecraft • Accurate to within 5 deg of nadir • Used for “short” deployments (< 6m) • High stiffness compared to tethers • Bigger and heavier than a tether Colorado Space Grant Consortium

  23. Boom Types • There are 5 main boom types to consider: • STEM Boom • Elastic Memory Composite (EMC) Boom • STACER Boom (SSTL) • Coilable Booms • Inflatable Boom Colorado Space Grant Consortium

  24. STEM Boom • STEM: Storable Tubular Extendable Member • One of the oldest and most successful deployable booms • Current stems use either Beryllium Copper or Stainless Steal • Limited in size due to stored energy strains and high density • Reel-stored Extendable Boom • Analysis shows: • Significant reduction of mass • Improved specific stiffness • Reduced stored strain energy Colorado Space Grant Consortium

  25. Elastic Memory Composite (EMC) Boom • CTD’s STEM boom • A coilable Longeron Deployable Boom • Deployment force provided by stain energy • Made of unidirectional S-glass/epoxy • Prototype EMC longerons exhibited • Highly predictable • Repeatable structural response • Packaging performance • Significant reduction in system mass • Reduced stored strain energy Colorado Space Grant Consortium

  26. STACER Boom • SSTL-Weitzmann 6m Deployable boom is • A rigid structure • Contains a prefabricated 1-13kg tip mass and deploying mechanism • Deploys at a rate of 0.3 m/s • Has a mass of 2.2kg (without tip mass) • Requires 5 A for >10 msec. • A history of 25 years, with over 600 Units used Cons: *Has a storage size of 102x115x264 mm *Deploys using Pyro-Cutter actuation Colorado Space Grant Consortium

  27. Coilable Booms • ABLE Coilable Booms • 100% Successful Flight Heritage • Two types • Lanyard Deployed • Most common • Compact mass stowage (2% of deployed length) • Extremely light weight capability (<50g/m) • Stowed strain energy gives positive deployment force • Least expensive • Canister Deployed • Motor driven • Retractable/deployable • Larger stowage volume Colorado Space Grant Consortium

  28. Inflatable Boom • Inflatable boom from ILC Dover • Thermoset composites • Thermally cured • Power requirement of 0.01W/in^2 • Heater performance(survivability) validated • Outgassing negligible outside of MLI • Deployment Component if desired (as shown above) BUT: -Expanded in a inflation gas reaction (gas tank required) -Less stiff of a structure than other boom types Colorado Space Grant Consortium

  29. Student-Designed Boom • Citizen Explorer • 4 m boom, 2 kg tip mass • Uses three roles of stanley tape measure • Deployed using Starsys’ HOP Colorado Space Grant Consortium

  30. Student-Designed Boom (Cont.) • Starsys • Designs many booms for customers • Jeff Harvey and Carlton Devillier offered to help • Both worked on booms at AEC Able for years • Suggested using 1 inch Stanley tape • Poor torsional stiffness, but more than tether • Deployment and damping mechanism still needed • Once deployed, it is sure to work • Said we should design ourselves • They will review our designs • Can provide flight qualified tape • Lightband could still be used Colorado Space Grant Consortium

  31. Conclusions and Recommendations

  32. Tether • Pros • Low mass • Already procured • Design started • Cons • Hard to predict dynamics • Very low tension at current length • Difficult to deploy • Tether material is not ideal Colorado Space Grant Consortium

  33. Ways Tether Could Work • Lengthen tether • Longer tether would mean more tension • Tether Spool • More predictable control of tether • Controlled braking • Prevents recoil • Treat as an “experiment” and provide backup • Focus more attention on subsystem Colorado Space Grant Consortium

  34. Boom • Pros • Structurally rigid • Easier to deploy • More predictable dynamics • A lot of flight experience • Cons • Greater mass and volume than tether • 6 meter (20 ft) maximum length • New design Colorado Space Grant Consortium

  35. Trade Study Conclusion • Tether could work • Boom is better decision for DINO • Less risk than tether • Easier to win flight competition • Direct help from industry • Still a lot of student involvment Colorado Space Grant Consortium

  36. Appendix A Colorado Space Grant Consortium

  37. Appendix B Colorado Space Grant Consortium

  38. Appendix C Colorado Space Grant Consortium

  39. Appendix D Colorado Space Grant Consortium