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

2. 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

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
Thermal expansion
Shape memory
Debris/micrometeorite resistance
Tether deployment
Recoil
Tip-off rate
Colorado Space Grant Consortium

5. 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

6. Post-Deployment Tether Dynamics

7. Deployed Tether Geometry
Tip Mass (5kg)
Velocity
20m
Libration Angle
Nadir
Zenith
Oscillating Frequencies:
Roll Oscillating Frequency = Hz
Pitch Oscillating Frequency = Hz
Yaw Oscillating Frequency = Hz
Main Structure (25kg)
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8. Colorado Space Grant Consortium
Current Issues
Tension and Libration
Pendulum Motion Requires Accurate Deployment
Tether Tape Material Properties
Colorado Space Grant Consortium

9. 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

10. 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

11. 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

12. Tether Deployment

13. 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

14. 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
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15. Booms

16. 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

17. 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

18. 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

19. Conclusions and Recommendations

20. 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

21. 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

22. 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

23. 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

24. Colorado Space Grant Consortium
Appendix A
Colorado Space Grant Consortium

25. 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

26. Colorado Space Grant Consortium
ATEx Deployment
Colorado Space Grant Consortium

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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