Thermal Analysis & Design
Basic hand calculations for steady state temperatures for cube in space:
- Calculate incident solar radiation
- Calculate infrared radiation of satellite into space
- Steady state means a thermal equilibrium between incident solar radiation and the satellite producing infrared radiation
- Accurate Thermal Properties are important, see UKube Thermal Analysis for baseline properties (TA_SSEDS_005.pdf)
- Construct an ESATAN model of the satellite, validate any results by referring to the hand calculations
- Models can be made with increasing levels of detail and accuracy
- Best practise would be to start with a simple model and build more complexity as knowledge of programme develops
- Refer to tutorials and manual provided with ESATAN software but treat tutorials with caution as some areas are not covered in detail
Geometry
- Importing CAD into ESATAN is not advisable as the meshing becomes complex and the process of importing is difficult
- Geometry is created by producing faces and joining them together.
- Box geometry options do not create a solid block but rather a 6 face sided box
- To create for example solar panels where the solar cells only cover a proportion of the surface, there are two main options:
- Use multiple faces placed side by side, as individual faces can have separate material properties defined for them
- Or, use cutting tool and input face to fit in hole left by cutting tool. However unlike the previous option the programme will not generate conductive interfaces, see , therefore the conductive links would have to be inputted manually into the model.d file for the analysis case.
- The node numbers for shells have to be chosen manually, make sure that different faces do not have the same node number assigned to them
- If faces are to be defined as inactive be careful and check the user manual
Defining Heat Transfer
- Heat transfer for conduction, radiation and if necessary convection have to be considered carefully as they are sometimes defined by the programme and in other situations require manual input from user
Conduction
- Conduction through faces are calculated by the programme as long as a material and thickness if defined for that particular face.
- Conduction links (conductive interfaces) between adjacent faces can be inputted with the aid of ESATAN. Once the geometry has been assigned to your model the option can be taken for Auto Generate Conductive Interfaces from a drop down heading at the top of the page.
- Each conductive interface will require definition as either not processed, fused or contact connection.
- For a dry contact connection a value of around 20 W/m^2 K is usually appropriate
- Not processing interfaces is sometimes necessary if the start and end point of an interface is almost at the same point, otherwise the model may produce errors
- Conduction between surfaces that are not adjacent requires the creation of a conductive link between either faces on a shell or a shell side to one face. It is not possible to link a shell side to a shell side.
- These links require the path length and cross sectional area as well as the conductivity of the material creating the link
- Defining these links in the model.d file once an analysis case is created is much quicker than using the Graphical User Interface (GUI)
Radiation
- Shells that have been defined fully will emit radiation as the view factors between that shell and others will be automatically calculated
- If heat loads or temperatures are applied to faces or the whole shell then these shall also automatically be incorporated into radiative calculations
- However if non-geometric thermal nodes are used then radiative exchanges will have to be defined manually
Convection
- Required for modelling heat pipes, usually there is no need for considering it for cubesats
Radiative Case
- At this stage you define the orbit you want to simulate for the cubesat
- Running a radiative case calculates the solar and infrared radiation incident on the cubesat throughout the orbit and is necessary for the analysis case to run
Analysis Case
- The analysis case incorporates the heat transfer around the cubesat, including any heat loads present and the data on incident radiation from the radiative case
- The analysis case is defined by choosing an appropriate radiative case, specifying what boundary conditions are present and what kind of analysis is to be run, transient or steady state
- Boundary conditions include initial temperatures, constant temperature and heat loads applied to faces or shells
- Different options available for analysis:
- Steady state can be done for a particular point in the orbit
- Transient will model the satellite completing the defined orbit
- It will start with the temperature at 0 degrees therefore more than one orbit will have to be run to gain accurate starting temperatures
- This can be done using the SOLCYC function when defining the transient analysis
- If using SOLCYC convergence has to be checked manually to ensure the model is working this is done by checking the ‘.out’ file in the result file folder for errors
- Or, multiple orbits can be run with coding similar to that shown below where the initial orbits are calculated quickly and the later orbits done in more detail
IF(SSEOL_PR__.GT.0.5)THEN
MAXORB= 3
ILP=-3
REPEAT
ILP=ILP+1
ORBNUM= ILP
IF(ILP.LE.1) DTPMAX= 1.0
IF(ILP.EQ.2) DTPMAX= 0.2
IF(ILP.EQ.3) DTPMAX= 0.2
TIMEO= 0.0
TIMEND= PERIO2 #24hrs
CALL SLFWBK
UNTIL (ILP.EQ.MAXORB)
ENDIF
Common Errors
- An error complaining about multiple faces is likely to be the result of different faces having the same node number assigned to them
- A conductive interface cannot be calculated if it starts and ends in the same place
- Updating the model requires the radiative case to be rerun for an analysis case using that updated model
- Convergence is not achieved, have to check ‘.out’ file to find this, try increasing NLOOP, MAXCYC or ensuring the orbit time is divisable by the timesteps
- NOTE: If the model is not converging limit the NLOOP to about 1000
- The importance of modelling the cubesat is to ensure that components will remain in operational temperatures during the orbit. The following are methods which enable the temperature characteristics to be altered:
- Black and white paint used for altering heat transfer
- Black is particularly good for enabling heat transfer around the cubesat
- Gold foil can be placed on the outside to transfer more heat the cubesat
- Multi-layered insulation can be used to maintain temperature
- Copper leads connecting to the outside can create conductive leads to enable heat to travel away from the component
- There should be no holes on the spacecraft from which sunlight could reach inner components
- Black and white paint used for altering heat transfer