Thermal Design Pt 1

Question 1

Why can heat transfer not occur through convection in space and what does it mean for heat transfer to occur through radiation and conduction?

Answer 1

Fluids do not flow in a microgravity environment i.e. warm molecules will not move on top of colder molecules thus transferring heat. Conduction happens via the interfacing of two surfaces with a temperature differential; the molecules will collide and exchange energy leading to heat transfer.

Question 2

What is an example of passive heat control and active heat control?

Answer 2

Passive thermal control often is handled through selection of materials when designing the spacecraft.

Active thermal control is implementing some powered device to assist with cooling and heating of the spacecraft systems such as on-board electronics.

Question 3

What are 4 aspects of space that make it ‘hostile’ from a thermal management perspective?

Answer 3

1. Space is extremely cold
2. Rapidly changing levels of exposure to radiation i.e. sun-facing surfaces versus shadowed surfaces
3. Microgravity means there is negligible convection
4. planetary entry through atmospheric boundaries is an energy-intensive process

Question 4

What is the thermal advantage of using milspec electronics on a spacecraft?

Answer 4

MILSPEC (millitary specification) are more robust from a spec viewpoint meaning they have better tolarances. Thermally, milspec electronics have a wider range of operating temperatures (-55 to +125 C) than commercial and other electronics

Question 5

What are the typical operating temperatures of solar cells and bearing mechanisms?

Answer 5

solar cells: -100 to +85 C
bearing mechanisms: -45 to +65 C

Question 6

What are the typical operating temperatures of batteries and propellants?

Answer 6

Batteries: 0 to 45 C
Propellants: +10 to 40 C

Question 7

At what altitude should heating from atmospheric drag be considered in the thermal model?

Answer 7

altitude <= 140 km

Question 8

In general, a spacecraft’s thermals are understood through understanding the radiative heat transfer environment. What are some examples of radiative heating?

Answer 8

• Solar radiation
• Albedo radiation (solar radiation reflected from other bodies
• Planetary radiation
• Radiation from the spacecraft

Question 9

What is meant by the acronyms AM0 and AM1?

Answer 9

Air mass 0 is the solar intensity (1366 Wm^-2) just outside of the earth’s atmosphere. Remaining air mass numbers correspond to the amount of scattering and attenuation experienced by the sun’s rays leading to reduced solar intensities. The air mass is proportional to the solar zenith angle.

Question 10

Describe what is meant by the following terms:
- blackbody radiation
- Planck curve
- Wein’s law
- Spectral radiant exitance

Answer 10

A blackbody is a perfect radiator and emits radiation with a wavelength that is inversely proportional to its absolute temperature

The planck curve highlights the relationship between the radiance of the radiation emitted by the blackbody and the wavelength of the radiation which gets sharper at higher temperatures

Wein’s law describes the relationship between the perfect radiators temperature and the wavelength where the radiance is highest.

Radiance is the the power per area per wavelength [Wm-2um-1]

Question 11

What does the stefan-boltzmann law describe and what is the equation for it?

Answer 11

• describes the total radiated power per unit of surface area of a blackbody and is given by:

E_b (T) = σT⁴ [Wm-2]

Question 12

Knowing the solar intensity at AM0 how is the the blackbody temperature of the sun calculated?

Answer 12

Use the stefan boltzmann equation but with a factor for the ratio of the areas of the sphere with radius of the sun and another sphere with radius of 1 AU (distance to AM0).

Φ=(r<div>a</div>)²σT⁴

Question 13

How is emissivity defined?

Answer 13

It is the ratio of the radiance (radiated intensity) of a hemispherical body to the radiance of a blackbody AT THE SAME TEMPERATURE:

ε=q_λ / E_bλ

Question 14

How is emissivity defined?

Answer 14

It is the ratio of the radiance (radiated intensity) of a hemispherical body to the radiance of the the equivalent hemispherical body if it was instead a blackbody:

ε=q_λ / E_bλ

Question 15

In general, how does the emissivitty look of the following groups of materials:

• polished metals
• Oxidised metals
• plastics, polymers, paints, glass, organic materials

Answer 15

• very low 0.01 - 0.05
• Low 0.2
• high at 0.9 ish

The higher the emissivity the better the body radiates, the more it resembles a blackbody.

Question 16

How is absorptivity defined and what does having an absorptivity of 1 mean versus 0?

Answer 16

If some radiance is incident on a surface and some of it is absorbed, the ratio of absorbed to incident gives the absorptivity.

1 means it absorbs all incident radiation and is perfectly black

0 means it is perfectly reflective or transparent

α = q^a / qⁱ

Question 17

How is the absorptivity used to calculated power absorbed for some incident flux on a surface?

Answer 17

q = αA_projectedΦ

Question 18

In general what are the absorptivity values of the following groups of materials:
- paints
- metals
- coloured metals

Answer 18

Black paint 0.95
White paint 0.2
Polished metal like silver 0.1 aluminium 0.2

Coloured metals like gold 0.5

Question 19

We can define the reflectivity and transmittivity of opaque and semi transparent objects similarly to absorptivity with what equations?

Answer 19

ρ = q^ref / qⁱ

Opaque: ρ + α = 1
Semi-transparent: ρ + τ + α = 1

Question 20

What are the two wavelength ranges of interest for the sun and other planets?

Answer 20

Solar (visible): 0.3 - 3 μm
Heat (IR): 3 - 30 μm

Question 21

What is the greybody assumption?

Answer 21

We assume that the emissivity and absorptivity are flat across the spectrum of interest

Question 22

how is Kirchoff’s law stated?

Answer 22

The spectral emissivity for the emission of radiation at temperature T is equal to the spectral absorptivity for radiation coming from a blackbody at the same temperature

α (T) = ε (T)

Question 23

For a system/body what is the equation of state used to determine the total heat stored?

Answer 23

Q_stored=Q_absorbed+Q_internal-Q_radiated

Question 24

What is the composition of the atmospheres of the following planets:

Earth
Venus

Answer 24

Earth - trace gases (water, CO₂ and 0₃). 78 % Nitrogen, 21 % Oxygen.

Venus - 96.5% CO₂, rest is C0₂

Question 25

How is the view factor defined?

Answer 25

The view factor represents the fraction of radiative energy leaving one surface that strikes another surface directly.

Question 26

State the reciprocity rule for viewing factors

Answer 26

For a body 1 with area A₁ interacting with body 2 with area A₂, the reciprocity rule is A₁F_1-2 = A₂F_2-1, where F_1-2 is the viewing factor from body 1 to body 2

Question 27

Why do spacecraft in near-earth orbit not attain thermal equilibrium with the atmosphere?

Answer 27

The mean free path of particles in the atmosphere is much larger than the dimensions of the spacecraft.

Question 28

How can we use the plank curve to explain the colour of warm objects such as the sun and an iridescent lamp?

Answer 28

the spectral radiant exitance peaks for the sun in the visible EM band. For an incandescent lamp, the peak is slightly higher wavelength (because it’s cooler) and so the emitted colour is slightly red-shifted

Question 29

What is the difference between a specular and diffuse reflector?

Answer 29

Specular reflect at the angle of incidence, whereas diffuse reflects through a range of angles

Question 30

Under what condition can most real bodies be treated as greybodies?

Answer 30

When we restrict the spectrum of interest i.e. to the solar spectrum OR the IR spectrum

Question 31

What are 4 examples of passive thermal control?

Answer 31

• Surface finishes
• MLI
• Heat pipes
• Radiators

Question 32

What are the advantages of surface finishes for PTC

Answer 32

• simple
• Cost effective
• many types available with different a/e values

Question 33

What are the disadvantages of surface finishes

Answer 33

• performance degradation during handling on the ground
• atomic oxygen, solar flux etc cause degradation
• degradation leads to increase in a which needs to be considered throughout the spacecraft operation

Question 34

What are 5 major causes of degradation of the surface finishes in the space environment

Answer 34

• solar UV radiation
• ionising radiation
• atomic oxygen attack (LEO)
• outgassing in a hard vacuum
• temperature extremes

Question 35

Why are optical solar reflectors used? How thick are the plastic/glas materials used?

Answer 35

To make spacecraft less sensitive to solar degradation. The thickness is between 12 to 125 microns and another back layer metal is used to determine absorptivity

Question 36

What is the radiator panel usually coated with?

Answer 36

High emissivity coating like glass or paint

Question 37

How might radiator panels be organised on a spacecraft?

Answer 37

Multiple might be used to radiate heat from each subsystem

Or a single central panel might have Heat routed to it using thermal pipes or fluid loops which it can then dissipate