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

Colorado Space Grant Consortium 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 Affects Science, Power, Comm, ADCS Colorado Space Grant Consortium

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

Introduction to Tethers in Space Important issues Tether length and tension The longer the tether length, the more tension Tether material properties Thermal expansion Shape memory Debris/micrometeorite resistance Tether deployment Recoil Tip-off rate Colorado Space Grant Consortium

Colorado Space Grant Consortium Past Tether Missions Org Satellite/ Tip Mass Tether Length Success/ Failure TSS-1 NASA Shuttle/550kg 20 km SEDS Delta 2nd Stage/25 kg Success SEDS II Success (cut tether) TSS-R1 TiPS NRL 4 km ATEx STEX/50 kg 6 km Colorado Space Grant Consortium

Post-Deployment Tether Dynamics

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

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

Colorado Space Grant Consortium 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

Colorado Space Grant Consortium 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

Colorado Space Grant Consortium 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

Tether Deployment

Colorado Space Grant Consortium 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 Tether Wheel (turning) Brake shoe (fixed) Brake Braking System Tether Z-fold Tip Mass Lightband Tether Guides Velocity Colorado Space Grant Consortium

Deployment Suggested Changes Longer tether Have extra tether on board Put tether on spool Add a feedback control system for braking Keep fast deployment Keep “leave-behind” method Colorado Space Grant Consortium

Booms

Colorado Space Grant Consortium 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

Colorado Space Grant Consortium Boom Types There are 6 main boom types to consider: STEM Boom Elastic Memory Composite (EMC) Boom STACER Boom (SSTL) Coilable Booms Inflatable Boom Student-designed Colorado Space Grant Consortium

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

Conclusions and Recommendations

Colorado Space Grant Consortium 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

Colorado Space Grant Consortium 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

Colorado Space Grant Consortium 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

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

Colorado Space Grant Consortium Appendix A Colorado Space Grant Consortium

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

Colorado Space Grant Consortium ATEx Deployment Colorado Space Grant Consortium

Colorado Space Grant Consortium 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

Colorado Space Grant Consortium 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

Colorado Space Grant Consortium 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

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

Colorado Space Grant Consortium 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

Colorado Space Grant Consortium 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

Colorado Space Grant Consortium 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

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