thermal design Flashcards
why is thermal design important?
without thermal control, components would overheat or overcool, making for inefficient operation
define conduction
molecular excitation
define convection
heat transport by fluid mixing
define radiation
EM wave emission
fill the missing word: in high vacuum, heat can only be transferred to or from the spacecraft via ________
EM radiation
define direct solar radiation
can be approximated to a parallel beam of light
define albedo radiation
direct solar radiation reflected by nearby planets
define earth shine
thermal radiation radiated by every planet
what must be true for a system in thermal equilibrium?
what is absorbed is equal to what is emitted
list the features of white, gold and grey paint
- 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
define view factor
fraction of radiation leaving surface 1 that is intercepted by surface 2
what needs to be considered for circular non-co-planar orbits?
planetary position and angle the orbit plane makes with earth-sun vector
what is true in non-circular non-co-planar orbits?
- view factors change around the orbit
- projected areas change around the orbit
define thermal cycle
behaviour of spacecraft temperature over time as spacecraft passes between direct solar and eclipse conditions in the orbit
what does a larger thermal time constant mean?
reduced temperature variation
define passive thermal control
thermal control methods requiring no additional power input
true or false: in high earth orbit, the view factor is quite small therefore the albedo and earth shine terms can be considered negligible
true
passive
for a fixed spacecraft geometry, how is the equilibrium temperature controlled?
ratio of absorptance to emittance
list properties of solar and flat reflectors and absorbers
- 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
define super insulation
insulation blanket which is used to screen hot or cold components
define variable external radiation devices
bimetallic fins which passively vary the a/E ratio of the external surface
define internal pressure thermal control
used to transport heat from one section to another
where is internal pressure thermal control commonly used?
where onboard components generate heat that needs to be rejected to radiators that are facing deep space
what do heat pipes consist of?
sealed tube with fluid that evaporates and condenses within the required temperature range
list how a heat pipe works
- 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
list the key characteristics of heat pipes
- no moving parts
- pipe insulation required to reduce heat loss
- pipe routing can be complex
- can be made active by incorporating thermostatic heater
what is meant by the ‘viscous limit’ for a heat pipe, and which temperature range does it occur at?
- low vapour density = small pressure gradient, flow dominated by viscosity
- low temperature
what is meant by the ‘sonic limit’ for a heat pipe, and which temperature range does it occur at?
- due to lower speed of sound at lower temperatures, vapour velocity can often reach or exceed sonic velocity
- low temperature with increasing flow rate
what is meant by the ‘pumping limit’ for a heat pipe, and which temperature range does it occur at?
- 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
what is meant by the ‘entrainment limit’ for a heat pipe, and which temperature range does it occur at?
- significant liquid vapour interaction and liquid drops can be removed from flow before reaching the evaporator = evaporator dry out
- mid temperature
what is meant by the ‘boiling limit’ for a heat pipe, and which temperature range does it occur at?
- boiling of fluid can occur resulting in evaporator dry out
- high temperature
define active thermal control
thermal control methods requiring additional power input
describe louvre systems
sensors used to activate a motor which opens shutters (louvers) to reveal surfaces with different a/E ratios
decribe fluid loops
similar to heat pipes but have a pumping system
describe peltier coolers
use thermoelectric effect to transfer heat from a very localised area, direct conversion of voltage to temperature difference in p-n junction