MAGIC Tether Trade Study

DINO: MAGIC Tether April 20, 2023

MAGIC Tether Trade Study

Anthony LowreyRyan 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



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



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

lu

lu

mm

mmLT

23

lu

lu

mm

mmLT

23

lu

lu

mm

mmLT

23

lu

lu

mm

mmLT

23

Lmm

mmT

lu

lu

23



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



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



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



ATEx Deployment



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



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


Post-DeploymentTether Dynamics



Deployed Tether Geometry

Tip Mass (5kg)

Main Structure (25kg)

ZenithNadir

Libration Angle

20m

Velocity

Oscillating Frequencies:•Roll Oscillating Frequency = 0.000368 Hz•Pitch Oscillating Frequency = 0.000316 Hz•Yaw Oscillating Frequency = 0.000177 Hz



Current Issues

• Tension and Libration• Pendulum Motion Requires Accurate

Deployment• Tether Tape Material Properties



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.

0 100 200 300 400 500 600 700 800 900 10000

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

Tether length [m]

Ten

sion

[N

]

Tether Tension vs. Length



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.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-1

0

1

Roll

(deg

)

Euler Angles

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-2

0

2

Pitc

h (d

eg)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-2

0

2

time (hrs)

Yaw

(deg

)



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.



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.


Tether Deployment



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

Braking System

Tether Z-fold

Tip Mass

Lightband

Tether Guides

Velocity

Tether

Wheel (turning)

Brake shoe (fixed)

Brake



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


Booms



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



Boom Types

• There are 5 main boom types to consider:– STEM Boom– Elastic Memory Composite (EMC) Boom– STACER Boom (SSTL)– Coilable Booms– Inflatable Boom



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



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



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



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



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



Student-Designed Boom

• Citizen Explorer– 4 m boom, 2 kg tip mass– Uses three roles of stanley tape measure– Deployed using Starsys’ HOP



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


Conclusions and Recommendations



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



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



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



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



Appendix A



Appendix B

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-5

0

5x 10

-3

Roll R

ate(

deg/

2)

Euler Rates

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-5

0

5x 10

-3

Pitc

h Ra

te(d

eg/2

)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-2

0

2x 10

-3

time (hrs)

Yaw

Rate

(deg

/2)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-1

0

1x 10

-5

Roll A

ccel(

deg/

s2

Euler Accelerations

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-5

0

5x 10

-6

Pitc

h Ac

cel(d

eg/s

2 )

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-2

0

2x 10

-6

time (hrs)

Yaw

Rate

(deg

/s2 )



Appendix C

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-1

0

1x 10

-3

Rol

l tor

que

(N*m

)

Spacecraft Torque

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-1

0

1x 10

-3

Pitc

h to

rque

(N

*m)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-5

0

5x 10

-7

Time (hrs)

Yaw

tor

que

(N*m

)



Appendix D

0 100 200 300 400 500 600 700 800 900 10000

0.2

0.4

0.6

0.8

1

1.2x 10

-3

Tether length [m]

Elo

ngat

ion

[m]

Tether Elongation due to Stretching

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MAGIC Tether Trade Study

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