Detailed Thermal Analysis with NX

LiBus uses NX's space systems thermal program for a detailed thermal analysis. Please see software techniques for more technical NX documentation.

Model Simplification

Begin with the full assembly CAD model, prepared by the structure lead, and perform the following model simplifications. 

1.  Inspect the model for assembly errors, this includes 

a. Mechanical interference: two parts that go through each other

Make extruded cuts on parts that are overlapping. Note that if a part is common to many subassemblies, changes to the part will cause changes in this part in all of its occurrences.

b. Undesired gap: a part not mechanically connected to the rest of the structure, or two parts that are supposed to be connected are not.

This is commonly a result of uncomplete structural task. For the purposes of thermal analyses, speak to the structure lead, and design simple stand-in components. 

2. Simplify PCBs

a. CDH, power, ADCS subsystems all have complex PCB designs. It is unnecessary to include all IC components for the purposes of thermal analysis. Speak to the leads of each subsystem, and learn which components consume the most power, and leave only these components in the CAD model.

3. Simplify bolts and screws

a. Helical coils on each bolt or screw is unnecessary for the purposes of thermal analysis. When possible go to the part, and turn on the configuration that has the helical coil removed. If this configuration is unavailable, manually measure and make a screw stand-in.

4. Remove all other unnecessary components 

a. Review the model and identify all other unnecessary components for the thermal analysis. These could include

i. nuts

ii. washers

iii. springs 

5. Use the defeature tool to simplify the model.

Mesh and Material Creation (.fem file)

Once you import the simplified CAD file into NX, open NX - Pre/Post, and begin a new .fem file. 

Mesh Generation

In analyses for this project, I used 3D TET4 mesh for the entire model, and assigned each component their respective material. Table 1 lists each component's material. In case of a component with complex internal geometry with unknown thermal properties, such as the reaction wheels, I modeled them as 2 mm thick aluminum.

The properties of all the materials assigned to each component in Table 1 have properties summarized in Table 2. 

Boundary Conditions (.sim file)

Boundary conditions in NX are separated into Load Type, Constraint Type, and Simulation Object Type.

Load Type

Load types in NX Space Systems Thermal are Thermal Loads, you can choose between heat load (W), heat flux (W/mm^2), or heat generation (W/mm^3). In this analysis, I used LiBus's power budget (orbit average consumption with margin), provided by the power lead for each subsystem. The boundary conditions I used in this project are summarized in Table 3. 

Constraint Type

Commonly used constraint Types in NX Space Systems Thermal  are temperature constraint and initial temperature conditions. In operational analyses (normal, low power, survival and critical hold modes), I used an initial temperature of 10C. This value was derived from LiBus's 2nd iteration detumbling numerical analysis. I used temperature constraint boundary conditions to check for proper mesh creation and conductive heat transfer. Initial temperature conditions are very helpful to ensure the transient solution's setup is as desired.

Simulation Object Type

Simulation Object contains many heat transfer properties. I used the following in my analyses. 

1. Orbital Heating

a. I chose to use Illuminate Selected Elements, which allows me to select which sides of the space craft receives orbital heating from the sun. I selected the deployed solar panel surfaces, and surfaces that are coplanar with the solar panels

b. Note that this select is under "Top Side Illuminated Region", which corresponds to the upward normal of each meshed element, be sure to check for mesh normal orientation before applying this step. 

c. I named the object with "Orbital heating (condition)" where (condition) describes the orbit used in this object. 

d. LiBus's orbital parameters are

Orbit Type Sun Synchronous Orbit

Minimum Altitude 575 km

LTAN 18:00:00

Max Eclipse Date June Solstice

e. For this orbit, the hot case has sun planet character "Max solar flux", while the cold case has sun planet character "June Solstice". 

2. Radiation

Internal and external enclosure radiation are applied all large surfaces in the model. Such as shell surfaces, and PCB surfaces; and crucial component surfaces, such as battery surfaces. 

Solution Creation

Environmental conditions are defined in when a solution is created. In this project, I first used a steady state solution to check that all coincident components have conformal mesh, and that proper conductive heat transfer occur in the model. I then used transient solutions to perform thermal analyses. 

I used the following transient solution parameters. 

Radiative Environment Temperature 4K

Initial Temperature Uniform 10 C

End Time 6000 s (orbital simulation)

End Time 1800 s (detumbling simulation)

Number of Time Steps 100

Number of Results 20

With orbital parameters defined as a simulation object, NX calculated the orbit period to be 5761.276 s (96.021 min). A 6000s time span will cover the entire orbit. With 100 time steps, temperature is calculated every 60s (minute) for 100 times. LiBus's detumbling time is 30 min (1800 s) long, therefore detumbling was run for 1800 s. 

Battery Saddle Design

The Iris heritage battery saddle was designed to house 4 batteries on the top side and 2 batteries on the bottom side, and be made of aluminum. However, upon further thermal simulation, an aluminum battery saddle that is directly connected to the shells transfers too much heater power to the shells, and does not retain much heat to keep the batteries warm. Figures 1 and 2 show LiBus's overall temperature under normal and survival modes. The batteries are around 11C under the normal mode, which is within its operating temperature. The batteries are around -2C under survival mode, which is colder than its operating temperature.

To solve this problem, the structure lead and I redesigned the battery saddle. The battery saddle now house 3 batteries on top, and 3 batteries on the bottom. This design shortens the battery saddle width, and allows us to put two Delrin insulations on either side. The saddle itself is still made of aluminum, ensuring proper heat distribution from the heaters to the batteries. Figure 3, 4 and 5 show the battery temperature under LiBus's normal mode. Numerical simulation suggests that the new battery saddle design for LiBus is sufficient in keeping the batteries at their operating temperature in both the hottest and coldest operating modes. 

Results

Table 4 shows the highest and lowest temperature and their location for each power mode of LiBus. Figures 6, 7, 8, and 9 show the entire Bus's temperature distribution.