Few, Alex

NASA Technical Reports Server

Testing and Maturing a Mass Translating 
Mechanism for a Deep Space CubeSat
Alex Few
NASA Marshall Space Flight Center
44rd Aerospace Mechanisms Symposium
NASA Glenn Research Center
May 16-18, 2018
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https://ntrs.nasa.gov/search.jsp?R=20180005382 2019-08-31T14:50:37+00:00Z
Agenda
2
 Near Earth Asteroid (NEA) Scout Overview
• Active Mass Translator (AMT) Overview
• Current Design State
 Thermal Design Challenges
• Discoveries during thermal vacuum testing
• Design and motor contributions
• Poor assumptions
 Lessons Learned
• Observed mini motor primary failure modes in vacuum
• How to remove heat from mini motors
• Successful thermal interface design
• How to monitor mini motor health during testing
NEA Scout Overview
3
Goal:  
Characterize a NEA during flyby while 
demonstrating low cost reconnaissance 
capability (Solar Sail)
Vehicle and Mission Details:
• 6U CubeSat manifested on SLS 
Exploration Mission 1
• 86m2 solar sail propulsion
• 2.5 year mission
• 1.5 x108 km (1 AU) distance from Earth 
AMT
AMT Overview
4
CAPABILITIES
• Translation:  160 X 68 mm (X & Y respectively)
• Speed:  0.3 and 0.1 mm/s (X & Y respectively)
• Precision:  0.1 mm
• Volume:  220 x 99 x 14 mm (300 cc)
• Mass:  250 grams
AMT Overview
5
Purpose:  Alter the inertial 
properties of the vehicle 
while controlling the center 
of pressure (CP) and center 
of mass (CM) offsets
How:  The AMT will move 
one portion of the NEA 
Scout relative to the other.  
Why:  A conventional control 
system (Reaction wheels or 
sail vanes) cannot 
accommodate the possible 
CP/CM offsets within the 6U 
volume
Translation
Thermal Design Challenges
6
 The mechanism is completely 
exposed to deep space 
environments, requires -70 to 
70 C operational range
 Very low mass and surface 
area to conduct heat (this will 
cause problems later)
 Only viable motors are rated 
for -35 to 70 C but are “space 
rated”
 Motors are very low mass (1g) 
and draw 0.5 W
 Will spend vast majority of life 
shaded from sun
 Small area makes 
temperature monitoring very 
difficult during test
6mm 
200 mm
100 mm
 Moments into the first 
AMT thermal vacuum 
test, the mechanism 
failed.
 Context:  AMT required 
operational life is about 
1000 hours
 3 month investigation 
determined that the 
motors were overheating
 How to design 
motor/chassis interface 
capable of conducting 
heat?
• How to remove heat 
from such small areas?
• Clamps design? 
• Epoxy, grease and 
heat spring material 
traded
Discoveries During Thermal Vacuum Testing
7
Discoveries During Thermal Vacuum Testing
8
 After failure, thermal 
models were updated 
with more accurate 
motor internal and 
external conduction 
properties
 The first two columns 
reflect the old clamp 
design, showing clear 
design flaws
 Adding any material to 
the motor surface 
showed improvement
 Indium was chosen for 
ease of design and 
robustness
Max Coil Temp, 130 C
Motors analyzed at full and half power for old design, thermal grease and indium heat spring
 Design space required smallest motors on the 
market
 Motors did not have surface mounting or threaded 
interfaces
• Vendor gave suggested clamping 
configurations, but this configuration was 
crippled thermally 
• Thin wall cannot take high clamp loads
• Clamp location was far from heat source
 Insufficient internal and external conductive and 
radiative areas 
 Only 1 gram on thermal mass in motors   time to 
thermal-induced failure ≈ seconds
Design and Motor Contributions
9
Matured to…
Motor
Heat Source
Transmission
Thin walls and planetary gears
Shaft Bearing
Clamp
Clamp
Negligible conduction b/w 
motor and transmission (Xray
image to follow)
0.5W
 We thought…
• “Space-rated” 
• Internal conductive path 
was substantial between 
motor and transmission
• 0.5 W could be dissipated 
through radiation and 
conduction
 In fact…
• Space-rated only in 
theory.  Larger models 
have flown, but none this 
small
• Negligible internal 
conductive path
• Convection dominated 
bench top testing
Poor Assumptions
10
 Observed mini motor primary failure mode for low-load, low speed application in vacuum 
environment
• Motor overheats causing the windings to short
• Very little warning. We suggest to constantly measure motor coil resistance throughout 
testing.
• Coils’ resistance would decrease (Example: 120Ω to 100 Ω to 60 Ω to 0 Ω) over consecutive 
operations
• Motor windings may not degrade at same rate, perhaps caused by variation in windings 
and/or insulating layer
• Motor torque output decreases as coil resistance decreases until motion stops
• Larger currents increase rate of degradation
Lessons Learned
11Vs.
Healthy Coils Overheated Coils
 How to remove heat from mini-motors
• Clamshell design with indium heat spring
• Because heat doesn’t conduct into transmission, clamp is required over all motor surfaces
Lessons Learned
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 Mechanical-thermal interface design for small DC motors in vacuum
Lessons Learned
13
1. Cover base clamshell surfaces with indium
2. Completely wrap motor surfaces (black) with indium
3. Cover motor and top clamshell with indium
4. Fasten top clamshell to bottom, pre and post 
tightened heights to verify indium compression
5. Trim excess indium
6. Verify indium compression when deconstructed
1 2 543
6
 How to monitor mini motor health during testing
• If you cannot access motor surfaces or internals with thermocouples
• And cannot view motor surfaces with thermal imaging
• You can measure motor coil resistance immediately after operation to determine coil temp
• Increased resistance = temperature increase from T0
• Decreased resistance = temperature decrease from T0
• Use system known characteristics to create correlation b/w resistance and coil temperature
Lessons Learned
14
ܴ ܶ ൌ ܴ଴ሺ1 ൅ ߙ ௖ܶ െ ଴ܶ ൯
ܴ ܶ ൌ ܿ݋݈݅	݅݉݌݁݀ܽ݊ܿ݁	ܽݐ	ݐ݁ݏݐ	݁݊ݒ݅ݎ݋݊݉݁݊ݐ	ݐ݁݉݌݁ݎܽݐݑݎ݁
ܴ଴ ൌ ܿ݋݈݅	݅݉݌݁݀ܽ݊ܿ݁	ܽݐ	ݎ݋݋݉	ݐ݁݉݌݁ݎܽݐݑݎ݁	ሺ120	ߗሻ
ߙ ൌ ܿ݋݂݂݁݅ܿ݅݁݊ݐ	݋݂	ݐ݄݁ݎ݈݉ܽ	ݎ݁ݏ݅ݏݐܽ݊ܿ݁	݂݋ݎ	ܿ݋݈݅	݉ܽݐ݁ݎ݈݅ܽ	ሺܿ݋݌݌݁ݎ ൌ 0.0039	 ߗ °ܥൗ ሻ
௖ܶ ൌ ܿ݋݈݅	ݐ݁݉݌݁ݎܽݐݑݎ݁	ܽݐ	ݐ݁ݏݐ	݁݊ݒ݅ݎ݋݊݉݁݊ݐ	ݐ݁݉݌݁ݎܽݐݑݎ݁	
଴ܶ ൌ ݎ݋݋݉	ݐ݁݉݌݁ݎܽݐݑݎ݁	ሺ20	°ܥሻ
ܶܿ ൌ ૛. ૚૝ ∗ ࡾ ࢀ െ ૛૜૟. ૝
This equation with these knowns… produced our system’s simple equation…
Lessons Learned
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… and determined the motors’ operational range in vacuum environment
In Conclusion
 Indium is suitable for thermal interfaces with DC minimotors
• Coil temps decreased from 230 C to 90 C at comparable temperatures
• Inexpensive, predictable method produced significant results
• Simple disassembly, avoids epoxies, greases or oils
 Use motor coils as thermistor if internals are not understood
• Produce a simple method to characterize motor health over temperature range
• Recognize limitations: data delays after operation lessen actual maxima
 Use test data to determine operational ranges if vendor data is incomplete 
or unavailable
• Able to accommodate COTS motor for space environments with limited procurement 
costs (< $1k per motor)
• CubeSat form factor and budget did not allow more qualified vendor
 CubeSat philosophies that need to be adopted or amended
• Allocate schedule and resources to procure engineering development units (EDUs) 
• Perform as many low-fidelity tests as possible (simple vacuum tests, vibe tests)
• Arduinos accelerated development but were used for too long, transition to flight board 
EDUs as soon as possible 
Questions
17
TVAC Test Instrumentation, Limitations
18
AMT Design with Simulated Interfaces (aluminum beams)
19
Completed Clamp Design, X and Y Axes
20
Used Indium
21
Pre/Post Design Change, Thermal Data Comparison
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Testing and Maturing a Mass Translating Mechanism for a Deep Space CubeSat Mechanism for a Deep Space CubeSat

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