Thermal Control Systems

External Heat Transfer

In space it is only possible to transfer heat via radiation

Internal Heat Transfer

Conduction - thermal energy transfer through matter in the absence of fluid motion

Blackbody

Idealised material that absorbs all incident radiation.

Planks Radiation Law

E = (2 h c²) / λ⁵ . 1/(e^(hc/(λkT))-1)

Surface Properties

Incident radiation can be reflected, absorbed, or transmitted through a particular surface layer

• Mostly concerned about absorption and emission

Radiator Temperature

Include a waste heat term in the thermal equilibrium equation (Q_W/A_R):

Q_W = Heat dumped by radiator

A_R = Area of radiator

Surface Irradiation (I)

Total amount of radiation incident upon the surface per unit time and area.

Surface Radiosity (J)

Total amount of radiation leaving the surface per unit time.

The sum of the proportion of reflected incident radiation (ρI) and the energy emitted (εE)

View Factors

Net radiation exchange between two black body surfaces

Q_net1−2 = A₁F₁₋₂σ (T₁⁴ − T₂⁴)
Q_net2−1 = A₂F₂₋₁σ (T₂⁴ − T₁⁴)

For a particular surface A₁F₁₋₂ = A₂F₂₋₁

Oppenheim Radiation Networks

• Represent surfaces as nodes with potentials of σT⁴

• Connect to radiosity nodes J with surface conductance Aϵ/(1 − ϵ)

• All radiosity nodes are interconnected with view factors

• The sum of view factors out of a node is equal to 1

Absorptivity (α)

Percentage of incoming radiation that is absorbed by a material

Emissivity (ε)

Proportion of radiation emitted by a surface when compared to a ‘black body’ at the same temperature

Simplifications to Oppenheimer Networks

• Perfect insulation so no heat transfer (J = σT⁴)

• Surface has an emissivity of one (J = σT⁴)

• Deep space has a temperature of zero so J_S=σT_S⁴=0

Conduction Networks

Q = kA(T₁-T₂)/Δx

Now the potential of a node is T, and the conductor value is kA/Δx

Active Thermal Control Devices

Pumped loop, heater + thermostat, mechanical refrigeration

• Manned spacecraft

• Close control (a few degrees)

• Dissipate large quantities of energy

Passive Thermal Control Devices

Space radiators thermally coupled to heat sources

• Unmanned Spacecraft

• Lighter + less power

Semi Passive Thermal Control Devices

Variable conductance (e.g. thermal switches)

Heat Pipe (TCD)

• Heat applied at one end vapourises liquid in wick.

• Vapour flows to condenser section.

• Vapour condenses releasing heat.

• Liquid absorbed by wick.

• Capillary forces in wick draw liquid from condenser

Careful selection of materials/fluids prevents chemical reactions.

Performance measured in Wcm (5080Wcm = 508W over 10cm)

Louvre (TCD)

• Act like venetian blind

• Positioned between radiator and space

• Slats have low ϵ coating

• Radiator has high ϵ coating

• Heat rejection governed by angle of slats

• Controlled by bellows or bimetallic spring

Thermal Switches

contacts in the heat conduction path that can open and close

Space Radiator

Heat exchanger, dissipates heat to outer space

Electrical Heaters

Used to bring temperature up to desired level

Optical Solar Reflector

Reflective surface with low absorptivity
(high cost, fragile)

Silver-coated teflon

Similar to OSR, less expensive, more durable

Phase Change Devices

Useful for absorbing heat from sources
that use high power over a short period.

Thermostats

On/off switches that react at pre set temperatures

Thermal Coatings

Control absorptivity/emissivity of surfaces

Thermal Environment

• Direct Solar Energy (Solar radiation)

• Earth IR (Terrestrial radiation)

• Reflected Solar Energy (Albedo radiation)

If in the shadow there is no solar/albedo radiation

Cryogenic systems

Used to cool IR detectors.

Active Refrigeration Systems

long duration missions, similar to
domestic fridge, problems with vibration, reliability, operating life

Expendable Cooling Systems

Short duration missions, Low temp
fluid/solid absorbs excess heat and is then vented into spac