thermal design Flashcards

1
Q

why is thermal design important?

A

without thermal control, components would overheat or overcool, making for inefficient operation

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2
Q

define conduction

A

molecular excitation

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3
Q

define convection

A

heat transport by fluid mixing

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4
Q

define radiation

A

EM wave emission

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5
Q

fill the missing word: in high vacuum, heat can only be transferred to or from the spacecraft via ________

A

EM radiation

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6
Q

define direct solar radiation

A

can be approximated to a parallel beam of light

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7
Q

define albedo radiation

A

direct solar radiation reflected by nearby planets

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8
Q

define earth shine

A

thermal radiation radiated by every planet

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9
Q

what must be true for a system in thermal equilibrium?

A

what is absorbed is equal to what is emitted

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10
Q

list the features of white, gold and grey paint

A
  • white: highly absorbent in IR and highly reflective in visible part
  • gold: not good absorber of visible and IR parts, good material to use for thermal insulation coatings
  • grey: totally absorbing, used in passive thermal control
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11
Q
A
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12
Q

define view factor

A

fraction of radiation leaving surface 1 that is intercepted by surface 2

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13
Q

what needs to be considered for circular non-co-planar orbits?

A

planetary position and angle the orbit plane makes with earth-sun vector

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14
Q

what is true in non-circular non-co-planar orbits?

A
  • view factors change around the orbit
  • projected areas change around the orbit
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15
Q

define thermal cycle

A

behaviour of spacecraft temperature over time as spacecraft passes between direct solar and eclipse conditions in the orbit

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16
Q

what does a larger thermal time constant mean?

A

reduced temperature variation

17
Q

define passive thermal control

A

thermal control methods requiring no additional power input

18
Q

true or false: in high earth orbit, the view factor is quite small therefore the albedo and earth shine terms can be considered negligible

A

true

19
Q

passive

for a fixed spacecraft geometry, how is the equilibrium temperature controlled?

A

ratio of absorptance to emittance

20
Q

list properties of solar and flat reflectors and absorbers

A
  • solar reflector: low absorptance in peak visible region, high E, a/E < 1
  • solar absorber: high absorptance in peak visible region, low E, a/E > 1
  • flat reflector: low absorptance in peak visible region, high E, a/E ~ 1
  • flat absorber: high absorptance in peak visible region, high E, a/E ~ 1
21
Q

define super insulation

A

insulation blanket which is used to screen hot or cold components

22
Q

define variable external radiation devices

A

bimetallic fins which passively vary the a/E ratio of the external surface

23
Q

define internal pressure thermal control

A

used to transport heat from one section to another

24
Q

where is internal pressure thermal control commonly used?

A

where onboard components generate heat that needs to be rejected to radiators that are facing deep space

25
Q

what do heat pipes consist of?

A

sealed tube with fluid that evaporates and condenses within the required temperature range

26
Q

list how a heat pipe works

A
  • fluid evaporates due to heat input from component that needs to be cooled
  • mass transport of evaporated fluid due to pressure gradient
  • fluid vapor condenses, introduces heat rejection
  • condensed fluid returns to evaporator due to capillary flow
27
Q

list the key characteristics of heat pipes

A
  • no moving parts
  • pipe insulation required to reduce heat loss
  • pipe routing can be complex
  • can be made active by incorporating thermostatic heater
28
Q

what is meant by the ‘viscous limit’ for a heat pipe, and which temperature range does it occur at?

A
  • low vapour density = small pressure gradient, flow dominated by viscosity
  • low temperature
29
Q

what is meant by the ‘sonic limit’ for a heat pipe, and which temperature range does it occur at?

A
  • due to lower speed of sound at lower temperatures, vapour velocity can often reach or exceed sonic velocity
  • low temperature with increasing flow rate
30
Q

what is meant by the ‘pumping limit’ for a heat pipe, and which temperature range does it occur at?

A
  • sum of pressure losses through the pipe can equal or exceed the capillary pressure and evaporator can dry out, stopping fluid flow around heat pipe
  • mid/high temperature
31
Q

what is meant by the ‘entrainment limit’ for a heat pipe, and which temperature range does it occur at?

A
  • significant liquid vapour interaction and liquid drops can be removed from flow before reaching the evaporator = evaporator dry out
  • mid temperature
32
Q

what is meant by the ‘boiling limit’ for a heat pipe, and which temperature range does it occur at?

A
  • boiling of fluid can occur resulting in evaporator dry out
  • high temperature
33
Q

define active thermal control

A

thermal control methods requiring additional power input

34
Q

describe louvre systems

A

sensors used to activate a motor which opens shutters (louvers) to reveal surfaces with different a/E ratios

35
Q

decribe fluid loops

A

similar to heat pipes but have a pumping system

36
Q

describe peltier coolers

A

use thermoelectric effect to transfer heat from a very localised area, direct conversion of voltage to temperature difference in p-n junction

37
Q
A
37
Q
A