thermal design

Question 1

why is thermal design important?

Answer 1

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

Question 2

define conduction

Answer 2

molecular excitation

Question 3

define convection

Answer 3

heat transport by fluid mixing

Question 4

define radiation

Answer 4

EM wave emission

Question 5

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

Answer 5

EM radiation

Question 6

define direct solar radiation

Answer 6

can be approximated to a parallel beam of light

Question 7

define albedo radiation

Answer 7

direct solar radiation reflected by nearby planets

Question 8

define earth shine

Answer 8

thermal radiation radiated by every planet

Question 9

what must be true for a system in thermal equilibrium?

Answer 9

what is absorbed is equal to what is emitted

Question 10

list the features of white, gold and grey paint

Answer 10

• 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

Question 12

define view factor

Answer 12

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

Question 13

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

Answer 13

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

Question 14

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

Answer 14

• view factors change around the orbit
• projected areas change around the orbit

Question 15

define thermal cycle

Answer 15

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

Question 16

what does a larger thermal time constant mean?

Answer 16

reduced temperature variation

Question 17

define passive thermal control

Answer 17

thermal control methods requiring no additional power input

Question 18

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

Answer 18

true

Question 19

passive

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

Answer 19

ratio of absorptance to emittance

Question 20

list properties of solar and flat reflectors and absorbers

Answer 20

• 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

Question 21

define super insulation

Answer 21

insulation blanket which is used to screen hot or cold components

Question 22

define variable external radiation devices

Answer 22

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

Question 23

define internal pressure thermal control

Answer 23

used to transport heat from one section to another

Question 24

where is internal pressure thermal control commonly used?

Answer 24

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

Question 25

what do heat pipes consist of?

Answer 25

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

Question 26

list how a heat pipe works

Answer 26

* 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

Question 27

list the key characteristics of heat pipes

Answer 27

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

Question 28

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

Answer 28

* low vapour density = small pressure gradient, flow dominated by viscosity * low temperature

Question 29

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

Answer 29

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

Question 30

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

Answer 30

* 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

Question 31

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

Answer 31

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

Question 32

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

Answer 32

* boiling of fluid can occur resulting in evaporator dry out * high temperature

Question 33

define active thermal control

Answer 33

thermal control methods requiring additional power input

Question 34

describe louvre systems

Answer 34

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

Question 35

decribe fluid loops

Answer 35

similar to heat pipes but have a pumping system

Question 36

describe peltier coolers

Answer 36

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