Low Temperature Mobility and Mechanisms

Preliminary - For Discussion Purposes Only Page 1

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Low Temperature Mobility and MechanismsLow Temperature Mobility and Mechanisms

By

Dr. Lutz RichterLutz RichterInstitute of Space Systems, German Aerospace Center

(DLR)

Bremen, Germany


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OverviewOverview

• Introduction• Example mechanisms: rover mobility• Example mechanisms: sampling systems• Tribological options & materials choices• Summary & Conclusions


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IntroductionIntroduction

• Low temperature mechanisms:– No universally accepted definition, but: environmental

temperature < Earth ambient (which can be as low as -50 C)– Typical cases:

• Mars surface missions (operation down to -90 C environmental temperature typically required)

• Outer planet moons surface missions (operation down to -200 C environmental temperature typically required)

• Special, cryogenic applications: mechanisms in front ends of cryogenically cooled space telescopes (operation down to -250 C environmental temperature typically required)

• Mechanisms in free space at large solar distances: e.g. pointing mechanisms

• Issues: proper choice of tribological elements & treatments; temperature gradients (temporal & spatial); use of local heaters to condition elements


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Example low temperature mechanisms – Mars surface mobilityExample low temperature mechanisms – Mars surface mobility

MER-A Sol 44

PHX

MER-B Sol 180


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Mars rovers – past and presentMars rovers – past and present

• Flown:– PROP-M (1971/73)– Mars Pathfinder/Sojourner

(1997)– MER Spirit & Opportunity

(2004-present)

• In development:– MSL (2009 launch)– ExoMars (2013 launch)

• Technology studies (over the years/decades):– Various: Marsokhod,…


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Mars rovers – where are the mechanisms?Mars rovers – where are the mechanisms?

• JPL rocker-bogie vehicles (Sojourner, MER, MSL) & ExoMars: thermally exposed to ambient conditions (e.g. in wheel hubs) but have heaters

• Concept studies: in rover warm compartment, with gears transmitting torque through suspension

SojournerSojourner MERMER MSLMSL ExoMarsExoMars

MIDD (study for ESA)MIDD (study for ESA)

MIDD (study for ESA)MIDD (study for ESA)SR-2 (study for NASA)SR-2 (study for NASA)

drives inside warm box



drives inside warm boxdrives inside warm box


drives outside warm box







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MER mobility mechanismsMER mobility mechanisms

Steering Actuator

Rocker Deployment Actuator

Rocker

Bogie Pivot (+/- 30 deg)

Bogie

Differential pivot (+/-20 deg)


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Engineering Model assemblyEngineering Model assembly


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• Wheel with internal traction and steering mechanisms


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MSLMSL

‚‚Scarecrow‘ outdoor testScarecrow‘ outdoor test

Wheel traction drive elementsWheel traction drive elements


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ExoMarsExoMars

Phase B1 chassis BreadboardPhase B1 chassis Breadboard


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Concept study: MIDD 4 kg, 4-wheeled rover for MarsConcept study: MIDD 4 kg, 4-wheeled rover for Mars

- actuators inside - actuators inside warm compartment!warm compartment!

- gear train going to - gear train going to wheelwheel


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Example low temperature mechanisms – Mars drills & other Example low temperature mechanisms – Mars drills & other toolstools

• Flown:– Viking lander telescopic sampling arm/scoop (1976)

– MPL & PHX jointed sampling arm/scoop (1999 & 2007)

– MER IDD arm (2003)

– MER RAT (2003)

– Beagle arm (2003)

– Beagle sampling mole (2003)

• In development:– MSL arm/scoop (2009 launch)

– MSL rock sampler (2009 launch)

– ExoMars rover drill (2013 launch)

• Other drills:– Rosetta lander sampling drill to a comet (2004 launch, 2014

operstions)

• Technology studies:– Various


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• Arms:

VL arm w/ VL arm w/ scoopscoop

MER IDD w/ MER IDD w/ contact contact instrumentsinstruments

Beagle arm w/ Beagle arm w/ contact contact instruments & instruments & samplerssamplers

PHX arm w/ PHX arm w/ scoopscoop


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• Drills & other subsurface samplers:

Rosetta Rosetta lander drilllander drill

Beagle Beagle sampling sampling molemole

ExoMars ExoMars sampling sampling drilldrill


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Tribology at low temperatureTribology at low temperature

• Moving parts: need to control friction and contain wear

• Method: use lubrication• Two basic choices for space:

– Solid lubricants: coatings– Liquid lubricants: greases, oils

• Low temperature missions (definition: see introduction & previous speaker): greases & oils may become solid-> general limit for space greases: ~ -75 C lubricant

temperature (pour point) -> foresee local heaters or choose solid lubricant instead


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Pro‘s & con‘s of lubricant optionsPro‘s & con‘s of lubricant options

• Solid lubrication (example: MoS2 coating by sputtering or bonding):– Good for very low (and also high) temperature– No outgassing/condensation issues– Can do accelerated mechanism life testing (saves time &

cost of test program)– Requires careful application & handling (degrades in

presence of humidity)

• Liquid lubrication (example: PFPE greases):– Robust method, easy to apply (however: care in parts

materials selection), insensitive to humidity (ground handling & testing)

– Temperature limits (previous slide)– Cannot do accelerated testing (would change flow regime)– Good for use in atmospheres (Mars, but not Titan as too

cold unless have heaters…)


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Mars surface mechanisms: how has it been done?Mars surface mechanisms: how has it been done?

• Sojourner & MER mobility mechanisms:– PFPE grease as lubricant– Local electrical heaters

• MER deployment mechanisms (short required life):– Dry lubrication

• Beagle mechanisms (deployment functions, arm, mole sampler, rock corer):– PFPE grease as lubricant

– Some elements: MoS2 bonded coating

• MSL mobility mechanisms:– Planned to use MoS2 coating on Ti gears (for lower allowed

operating temperature at high latitude landing sites) -> ran into problems -> final solution: ?


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Mars surface mechanisms: how has it been done?Mars surface mechanisms: how has it been done?

Beagle mole internal hammering Beagle mole internal hammering mechanism: bonded coating of MoSmechanism: bonded coating of MoS22

with Formaldehyde on slider bearingswith Formaldehyde on slider bearings


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Environmental testing: key to successEnvironmental testing: key to success

Beagle mole actuator tests with applied Beagle mole actuator tests with applied load (brakes) in Mars simulation load (brakes) in Mars simulation chamberchamber


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0.022 Nm0.018 Nm

0.044 Nm

0.036 Nm

0.081 Nm

0.098 Nm

0.113 Nm

0.113 Nm

0

50

100

150

200

250

300

-75-70-65-60-55-50-45-40-35-30

Motor temperature [°C]

Av

era

ge

op

era

tin

g c

urr

en

t [m

A]

Minimum loadMaximum load

Beagle mole actuator tests with applied load (brakes) Beagle mole actuator tests with applied load (brakes) in Mars simulation chamberin Mars simulation chamber

Sta

rt-u

p cu

rren

t [m

A]


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Beagle PAW Flight Model acceptance Beagle PAW Flight Model acceptance tests: thermal cycling with functional tests: thermal cycling with functional tests in Mars simulation chambertests in Mars simulation chamber


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Summary & conclusionsSummary & conclusions

• Large number of successful low temperature mechanisms for planetary surface missions

• Generally feasible:– liquid lubrication+local heaters -> but: energy expensive;

still: OK for Mars

• For operations below ~ -70 C ambient: solid lubrication preferred– But: has issues in atmospheres, requires careful selection of

substrate materials & processes

• There are design guidelines for mechanisms: e.g. Space Tribology Handbook (developed and used in Europe) -> use them!


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The EndThe End

Documents

Low Temperature Mobility and Mechanisms