Payload Thermal Issues

1
Payload Thermal Issues Calculations

• Ballooning Unit, Lecture 3

2
Thermal Requirements

• All payload components can function properly only
  within particular temperature ranges
• Operating temperature range (narrowest)
• In this temperature range the component will
  perform to within specified parameters
• Non-Operation temperature range (wider)
• Component will not perform within specs, but will
  do so when returned to operating temperature
  range
• Survival temperature range (widest)
• If this range is exceeded component will never
  return to proper operation
• Thermal requirements constitute specifying these
  ranges for all components

3
Thermal Control Plan

• Systems and procedures for satisfying the thermal
  requirements
• Show that thermal system (i.e. heaters,
  insulation, surface treatment) is sufficient to
  avoid excursions beyond survival temperature
  range
• Show critical components remain mostly in the
  operating temperature range
• Specify mitigation procedures if temperature
  moves to non-operating range (e.g. turn on
  heaters)

4
Determining Temperature Ranges

• Start with OEM (original equipment manufacturer)
  datasheet on product
• Datasheets usually specify only operating
  temperature range
• Definition of operating may vary from
  manufacturer to manufacturer for similar
  components
• Look for information on how operating parameters
  change as a function of temperature
• Your operating requirement may be more stringent
  than the manufacturer
• Find similar products and verify that temperature
  ranges are similar
• Search for papers reporting results from
  performance testing of product
• Call manufacturer and request specific information

5
Survival Temperature Range

• Survival temperature range will be the most
  difficult to quantify
• Range limits may be due to different effects
• Softening or loss of temper
• Differential coefficients of expansion can lead
  to excessive shear
• Contact manufacturer and ask for their
  measurements or opinion
• Estimate from ranges reports for similar products
• Measure using thermal chamber

6
Heat Transfer

• The payload will gain or lose heat energy through
  three fundamental heat transfer mechanisms
• Convection is the process by which heat is
  transferred by the mass movement of molecules
  (i.e. generally a fluid of some sort) from one
  place to another.
• Conduction is the process by which heat is
  transferred by the collision of hot fast moving
  molecules with cold slow moving molecules,
  speeding (heating) these slow molecules up.
• Radiation is the process by which heat is
  transferred by the emission and absorption of
  electromagnetic waves.

7
Convection

• Requires a temperature difference and a working
  fluid to transfer energy
• Qconv h A ( T1 T2 )
• The temperature of the surface is T1 and the
  temperature of the fluid is T2 in Ko
• The surface area exposed to convection is given
  by A in m2
• The coefficient h depends on the properties of
  the fluid.
• 5 to 6 W/(m2 Ko ) for normal pressure calm
  winds
• 0.4 W/(m2 Ko ) or so for low pressure
• In the space environment, where air pressure is
  at a minimum, convection heat transfer is not
  very important.

8
Conduction

• Requires a temperature gradient (dT/dx) and some
  kind of material to convey the energy
• Qcond k A ( dT / dx )
• The surface area exposed to conduction is given
  by A in m2
• The coefficient k is the thermal conductivity of
  the material.
• 0.01 W/(m2 Ko ) for styrofoam
• 0.04 W/(m2 Ko ) for rock wool, cork, fiberglass
• 205 W/(m2 Ko ) for aluminium
• Need to integrate the gradient over the geometry
  of the conductor.
• Q k A ((T1-T2)/L) for a rod of area A and
  length L
• Q 4? k r (r x) ((T1-T2)/x) for a spherical
  shell of radius r and thickness x

9
Radiation

• Requires a temperature difference between two
  bodies, but no matter is needed to transfer the
  heat
• Qrad ? ? A ( T14 T24 )
• The Stefan-Boltzmann constant, ?, value is 5.67 x
  10-8 W/m2 K4
• The surface area involved in radiative heat
  transfer is given by A in m2
• The coefficient ? is the emissivity of the
  material.
• Varies from 0 to 1
• Equal to the aborptivity ( ? )at the same
  wavelength
• A good emitter is also a good absorper
• A good reflector is a bad emitter
• In the space environment, radiation will be the
  dominant heat transfer mechanism between the
  payload environment

10
Emissivity Absorptivity 1

• Kirchoffs Law of Thermal Radiation At thermal
  equilibrium, the emissivity of a body (or
  surface) equals its absorptivity
• A material with high reflectivity (e.g. silver)
  would have a low absorptivity AND a low
  emissivity
• Vacuum bottles are silver coated to stop
  radiative emission
• Survival space blankets use the same principle
• Kirchoffs Law requires an integral over all
  wavelengths
• Thus, some materials are described as having
  different absorptivity and emissivity value.

11
Emissivity Absorptivity 2

• Manufacturers define absorption and emission
  parameters over specific (different) wavelength
  ranges
• Solar Absorptance ( ?s ) absorptivity for 0.3 to
  2.5 micron wavelengths
• Normal Emittance ( ?n ) emissivity for 5 to 50
  micron wavelengths
• The Sun, Earth and deep space are all at
  different temperatures and, therefore, emit power
  over different wavelengths
• A blackbody at the Suns temperature (6,000 Ko)
  would emit between about 0.3 and 3 microns and at
  the Earths temperature (290 Ko) would emit
  between about 3 and 50 microns

• For space we want to absorb little of the Suns
  power and transfer much of the payload heat to
  deep space.
• Want a material with low ?s and high ?n .
• Sherwin Williams white paint has ?s of 0.35 and
  ?n of 0.85

12
Steady State Solution

• In a steady state all heat flows are constant and
  nothing changes in the system
• Sum of all heat generators is equal to the sum of
  all heat losses ( Qin Qout )
• Example flow of heat through a payload box wall
• Assume vacuum so no convection
• Input heat ( Qin ) generated by electronics flows
  through wall by conduction and is then radiated
  to space.
• Qcond Qin or kA ( T1 T2 ) / L Qin (1)
• Qrad Qin or ??A (T24 Ts4 ) Qin (2)
• Use eq. 2 to determine T2 and then use eq. 1 to
  determine T1
• But real systems are never this simple

13
Balloon Environment is Complex

• Multiple heat sources
• Direct solar input (Qsun), Sun reflection
  (Qalbedo), IR from Earth (QIR), Experiment power
  (Qpower)
• Multiple heat sinks
• Radiation to space (Qr,space), Radiation to Earth
  (Qr,Earth), Convection to atmosphere (Qc)
• Equation must be solved by iteration to get the
  external temperature
• Then conduct heat through insulation to get
  internal temperature

QsunQalbedoQIRQpower Qr,spaceQr,EarthQc
14
Solar Input Is Very Important

• Nominal Solar Constant value is 1370 W / m2
• Varies 2 over year due to Earth orbit
  eccentricity
• Much larger variation due to solar inclination
  angle
• Depends upon latitude, time of year time of day
• Albedo is reflection of sun from Earth surface or
  clouds
• Fraction of solar input depending on surface
  conditions under payload

ATIC-02 data showing effects of daily variation
of sun input
15
Other Important Parameters

• IR radiation from the Earth is absorbed by the
  payload
• Flux in range 160 to 260 W/m2, over wavelength
  range 5 to 50 microns, depending on surface
  conditions
• Radiation is absorbed in proportion of Normal
  Emittance ( ?n )
• Heat is lost via radiation to Earth and deep
  space
• Earth temperature is 290 Ko and deep space is 4
  Ko
• There is also convective heat loss to the
  residual atmosphere
• Atmosphere temperature 260 Ko
• For a 8 cm radius, white painted sphere at
  100,000 feet above Palestine, TX on 5/21 at 7 am
  local time with 1 W interior power
• Qsun 9.5 W, Qalbedo 3.7 W, QIR 1.6 W
• Qr,Earth 0.1 W, Qr,Space 15.3 W, Qconv
  0.4 W

16
Application for BalloonSat

• Can probably neglect heat loss due to convection
  and radiation to Earth
• Simplifies the equation you need to solve
• Need to determine if the solar inclination angle
  will be important for your payload geometry
• e.g. a sphere will absorb about the same solar
  radiation regardless of time of day and latitude
• Spend some time convincing yourself that you know
  values for your payload surface ?s and ?n and
  your insulation k.
• Biggest problem will be to estimate albedo and
  Earth IR input
• Use extremes for albedo and IR to bracket your
  temperature range

17
References

• HyperPhysics web based physics concepts,
  calculators and examples by Carl R. Nave,
  Department of Physics Astronomy, Georgia State
  University
• Home page at http//hyperphysics.phy-astr.gsu.edu/
  hbase/hph.htmlhph
• Thermodynamics at http//hyperphysics.phy-astr.gsu
  .edu/hbase/heacon.htmlheacon
• Heat Transfer at http//hyperphysics.phy-astr.gsu.
  edu/hbase/thermo/heatra.htmlc1
• Vacuum Flask at http//hyperphysics.phy-astr.gsu.e
  du/hbase/thermo/vacfla.htmlc1
• Thermal Conductivity Table at http//hyperphysics.
  phy-astr.gsu.edu/hbase/tables/thrcn.htmlc1
• Table of Solar Absorptance and Normal Emmittance
  for various materials by Dr. Andrew Marsh and
  Caroline Raines of Square One research and the
  Welsh School of Architecture at Cardiff
  University.
• http//www.squ1.com/index.php?http//www.squ1.com/
  materials/abs-emmit.html
• Sun, Moon Altitude, Azimuth table generator from
  the U.S. Naval Observatory
• http//aa.usno.navy.mil/data/docs/AltAz.html