Thermal Design Part 2 Flashcards
Why does a greenhouse feel slightly warm. What properties of glass allow for this state?
The glass is opaque to IR whilst it is transparent to the UV radiation from the sun
This means the glass can absorb and re-emit the IR
What is the absorptivity of white paint?
0.2
What is the absorptivity of black paint?
0.9
What is the absorptivity of silver-coloured metals?
0.1 - 0.2
What is the absorptivity of coloured metals like gold?
0.5
What is the emissivity of paints?
0.9
What is the emissivity of polished metals?
0.05 Al and 0.01 for silver
what is the emissivity of oxidised metals?
0.2
For planets with no atmosphere and slow rotation, how is the equilibraium temperature different than for earth?
At what time is this modified equation true?
the factor for is removed:
T=4RT((a/e)(flux/sigma))
At local noon time when the sun is directly overhead
Fot a satellite in GEO during eclipse, what is the dominant source of heat?
EArth’s IR emissions
Why cant we predict the temperature of a spherical spacecraft in LEO?
there are internal sources of heat for the satellite (onboard electronics etc)
Albedo radiation from earth
When does viewing factor become relevant?
When the heat source can no longer be considered as a point source
Why does sputnik never actually achieve its calculated eclipse and irradiated equilibrium temperatures?
Because it does not spend enough time in the respective phases of its orbit, it therefore oscillates at some intermediate temperature point
For a LEO orbit, what is eclipse time and sunlight time?
60 minutes sun, 30 minutes eclipse
What are the four sources of heat for sputnik?
What is the earth’s emissivity in this calculation?
Albedo
insolation
Earth IR
Internal
The earth is treated as greybody where a_IR=e_IR
Describe how a TMM works and its purpose
It is used to model the spacecraft thermal behaviour which is complex as a result of the varying space environments
the spacecraft is modelled as a number of descrete regions called isothermal nodes
-each node is characterised for its thermal properties
-nodes that can ‘see’ space will include the corresponding radiative characteristics in the nodes
What is fourier’s law of conduction
Q_c=h_c.deltaT
How is the stored heat energy calcualted for a node
energy stored = energy from edxternal source + energy internal - heat radiated to space - heat conducted to neighbouring nodes - heat radiated to other nodes
what are the external heat sources in the TMM
solar, planetary and albedo radiation
What is the purpose of the TVAC test
the thermal vacuum test tests the accuracy of the TMM by checking the thermal performance of the spacecraft
The chamber has a shroud which can be electrically heated, and cooled using LN
A platten can also be used to stimulate the spacecraft thermal response
What state does the spacecraft need to be in?
It needs to be operated as normal to capture the full thermal behaviour of the spacecraft resulting from operation of on board systems
What is the purpose of the intial hot cycle?
The initial cycle is known as the bake out where volatiles are driven off that could later result in outgassing and redeposition impacting over spacecraft equipment
What can be done with outgassing products?
They can be analysed by capturing them on a ‘cold finger’ and then analysed on mass spectrometer
What must be done before the spacecraft can be returned back to room temperature and pressure?
It must be warmed up in the vacuum such that when repressurised, water does not condense on the spacecraft
