NANOSAT THERMAL MANAGEMENT WORKSHOP Sept 2005

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NANOSAT THERMAL MANAGEMENT WORKSHOPSept 2005

• Charlotte Gerhart
• Mechanical Engineer
• Charlotte.Gerhart_at_kirtland.af.mil
• 505-846-2438

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Spacecraft Thermal Management

• Objective To describe what, why, when, and how
  spacecraft thermal management is accomplished.
• What is Thermal Management?
• Why is Thermal Management Important for
  Spacecraft?
• Temperature Effects
• Orbital Environment
• How are Thermal Systems Designed?
• Quick Check Calculations
• Numerical Modeling
• Accepted Orbital Thermal Modeling Standards
• Examples
• Thermal Design/Model Inputs
• Other Thermal Design Considerations
• AFRLs Role
• Thermal Engineers Toolbox

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What is Thermal Management?

• Assuring that a system operates within its
  temperature range
• Automobiles
• Computers
• Refrigerator
• Aircraft Fuselage
• Alaska Oil Pipeline
• Spacecraft
• OR determine temperature limits and make sure the
  system can handle the induced loads
• Sidewalks
• Railroads
• Power Lines
• Bridges

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Why is Thermal Management Important for
Spacecraft?

• Common Misconceptions
• Space is cold, getting rid of heat is no problem!
• It runs just fine in the lab so Ill have no
  problems
• If it gets too hot just put a fan on it
• Just put a heater on it if its cold
• Reality
• Heat transfer and rejection is via radiation in
  space, which is very different than terrestrial
  systems which are convection dominated
• The sun is VERY hot and produces a lot of heat
  that can be absorbed (1350W/m2)
• The earth is a cool (250K) body that strongly
  effects earth orbiting missions AND reflects some
  solar energy back out to space
• There is no air in space for a fan to blow around
  unless you are willing to build a pressure vessel
  (very heavy, prone to leaks), and you still have
  to eventually dump the heat by radiation.
• Heaters require power, usually mostly during
  eclipse bigger battery more mass/volume

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Temperature Effects

• Failure
• Expansion of deployable panel to stick in frame
• Freezing of lubricant
• Solar cells overheating, decreased efficiency
• Device out of tolerance/calibration
• Thermal fatigue
• Cracking, breaking solder joints
• Thermal distortions
• Stresses, bending

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The Earth Orbital Environment
Reflected Solar
ECLIPSE
Incident Solar
Earth IR
Waste Heat
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Thermal System Design and Analysis

• Iterative process with increasing fidelity
• Steady state thermal balance
• Transient analysis with minimal number of nodes
  and surfaces
• More detailed analyses with better information,
  focus on critical components
• Analysis code MUST include radiation and
  conduction as well as orbit propagation
• view factors
• solar inputs
• earth IR (earthshine)
• reflected (albedo)
• NASA standard
• Sinda (finite difference solver)
• Trasys (view factors)

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Steady State Estimations

• Quick check average environmental sink
  temperatures for earth, 3-axis stabilized cube
  (Aerospace Corporation)

Assumptions Surface properties abs 0.2, emis
0.8. 1400W/m2 solar, 250W IR, 0.33 albedo
orbit average fluxes
Beta 90
Beta is the angle between the earths orbit
plane(normal to the page in this example) and
that of the satellite
Beta 0
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Quick Check Example

• 500W radiated from 1m2 earth facing panel in LEO,
  beta0, emissivity0.8 (assume back side is
  perfectly insulated, abs0 neglecting solar and
  earth inputs)
• General radiative heat transfer equation
• Q ??A(Ts4 - T?4) 1
• Where
• ? Stefan-Boltzmann Constant 5.67e-8 W/m2K4
• ? emissivity
• A area, m2
• Ts Surface Temperature, K
• T? Sink Temperature, K
• Q Heat Input, W
• Re-arranging Equation 1 and solving
• Ts (Q/(A ? ?) T? )1/4 324K 51ºC
• Can be used to get first order estimates of
  temperatures, radiator and heater sizes

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Thermal System Design and Analysis

• Other first order checks
• System thermal capacity for transient response
  time ROM
• 10kg Aluminum (875 J/kgK) 2kg Copper (380
  J/kgK) 9150 J/K
• 9150 J/K 9150 Ws/K 152 W for 1 minute for 1K
  temperature change
• Energy balance on inputs, internal generation,
  and output for rate of change
• Next step is building FEMs to do steady state
  and/or transient analysis
• Lumped nodes, surface properties,
  contact/mounting conductances
• Trade different properties to obtain desired
  temperatures
• Surface coatings, insulation, mounting, heaters,
  etc.
• AFRL uses SINDA and RADCAD
• Have I-Deas for Structural, not Thermal

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Example Analysis (UN1)
Orion Sat
Emerald Sats
MSDS
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Example Analysis Results (UN1)
MSDS with Black Paint
MSDS with White Paint
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Other Thermal Considerations

• Temperature gradients
• Plasma thrusters (hot) next to IR sensor (cold)
• Surface degradation
• Space environment
• Handling
• Outgassing
• Thrusters
• Cost and handling procedures
• Abrade surfaces, toxic materials, ground
  processing
• Thermal shorts
• Harnessing, compressed MLI
• Need thermal balance testing to verify model
• Requires vacuum - preferably thermal vacuum
  solar sources - and lots of temperature sensors
  (ground only)

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Thermal Toolbox

• Coatings
• Black Paint (emis.95, abs.85) -GEO, internal
  faces
• White Paint (emis.9, abs.2) -LEO
• Silver Teflon (emis.9, abs.1)
• Insulation
• MLI properties vary with of layers and
  specific application
• Beta cloth shuttle/ISS material (touch
  temperatures)

NASA EOS Terra S/C in assembly
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Thermal Toolbox

• Interface conduction
• G-10, ceramic paper low conductivity interface
  material
• Calgraph (carbon paper), thermal grease, metallic
  foils, etc. for high conductivity gap filler
• Heat transfer
• High conductivity metals - aluminum, copper
• An-isotropic composites - graphite's, carbon
  fibers
• Heat pipes, loop heat pipes, capillary pumped
  loops
• Pumped fluid loops or circulators

Cryogenic CPL Experiment
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Thermal Toolbox

• Thermal energy storage
• Thermal mass
• Phase change materials (paraffin's for room
  temperature)
• Heat rejection
• Louvers
• Variable emissivity coatings
• Heat pumps
• Cryogenic systems
• Thermal electric coolers, cryocoolers, cryogenic
  radiators

• Suggested component temperature ranges
• Operational -10C to 60C
• Non Operational -40C to 100C
• Deployment, heat sensitive actuation -50C to
  150C

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AFRLs Role

• Responsible for system in shuttle through post
  ejection deployment
• Have vested interest in the success of University
  missions
• Provide thermal models to NASA for Shuttle
  manifesting
• Flight rules
• Limited thermal control (heaters, thermostats)
• Need detailed information to build STS paylaod
  level models
• Can provide satellite level analysis and
  review/oversight as desired

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Summary 1 of 2

• Thermal design and analysis essential for
  spacecraft
• Keep all spacecraft components in their
  temperature ranges during all mission phases
• Ambient operation is generally not indicative of
  space
• Iterative process throughout design and mission
• Analysis codes must solve conductive and
  radiative heat transfer, and as be able to
  simulate the orbital environment

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Summary 2 of 2

• Models are only as good as the information used
• No simple one size fits all solutions
• AFRL is here to provide pre-deployment survival
  as well as help as needed to assure mission
  success
• Suggested component temperature ranges
• Operational -10C to 60C
• Non Operational -40C to 100C
• Deployment, heat sensitive actuation -50C to
  150C