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5. Geometry and materials
This chapter shows how to transform the geometric model of spacecraft architectures into a GMM by means of the available tools.
Figure 5.1 - Topic of the chapter (yellow box)
Generally, the geometric model is an input and is provided at a certain level of detail, for example, by a client. It is the task of the thermal analyst to modify the model such that it fits the thermal code without altering the real architecture. Same considerations stand for materials, which must be described in a way that the thermal code is able to handle.
5.1 From geometry to GMM
As introduced in Chapter 1, the tool that permits to generate FEM meshes from surface geometric models is the Mesher. Although it also has all the basic shapes to generate a geometry from scratch, this function has been exploited only to make local adjustments during test campaign.
91 Figure 5.2 - Mesher's main menu (courtesy of IDS)
The Mesher accepts as input files like IGES and STEP, which can be generated by the most common CAD software. These files should preferably contain surface skins, since any volumetric information is lost once they are imported within the Mesher.
To illustrate the Mesher environment and the main features of the thermal code, a generic architecture of spacecraft is considered: the Reference Satellite. Although other geometries have been successfully analyzed, this one better lends itself as a benchmark to describe in detail all the rationale followed in the rest of the work.
Figure 5.3 - Internal view of the Reference Satellite
92 The figure above shows the Reference Satellite at an intermediate point during the use of the Mesher. In fact, its skin has been generated with CATIA® V5 software (Generative Shape Design) and, when required, only the aluminum block has been added on with the geometry generation tools inside the Mesher. The use of a CAD is advantageous for at least two reasons: the creation of a geometry is more intuitive and fast and, above all, the computation of the volumes is done automatically (e.g.
“Element Measure” command in CATIA® V5).
Figure 5.4 - Overall dimensions (mm) of the Reference Satellite
Inside the Mesher, the following two geometric-mathematic entities are defined:
wall: any surface to which a material is assigned;
compartment: any closed empty volume delimited by walls.
This means that there is no distinction between solid and non-solid volumes; but this piece of information is available from the CAD and can be manually assigned outside the Mesher environment, together with information about the material of solid compartments.
Each wall is representative of a component and all the components made of the same material (as intended in the next paragraph) can be englobed in a single group, which is univocally identified by a positive index and a name, e.g. the name of the
93 component. Therefore, each group is characterized by a single material, but the same material can be used to characterize more groups. This turns out to be convenient when dealing with complex models where similar components are placed too far so that it is not intuitive to englobe them in the same group.
Once the skin has been adjusted, each group is meshed according to the accuracy required and the size of the components. As a rule, mesh size should be smaller on the external walls, while it can be larger for interiors, where a rough approximation of temperature field is acceptable. Nevertheless, most figures in the next chapters do not respect this trend and mesh schemes are rather uniform (and sometimes not fine enough). This choice is consistent with the scope of the work, which is to build and validate a prototype code. Furthermore, the large simulation times associated with finer meshes would be unaffordable for the available laptop. Below is a mesh example of the Reference Satellite.
Figure 5.5 - Reference Satellite mesh example
Each 2D-triangular mesh element is called facet and is described in the Mesher Frame (MF) by the coordinates of its vertices and the vector normal to its surface. A facet can be:
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single: if only one normal is considered; a facet is single if it is on a wall that is boundary of a solid compartment, so its normal points outward. Examples of single facets are those of battery, camera and momentum wheel walls;
double: if its normal is considered perpendicular to both sides; in this case the facet is duplicated and two different facets, the “real” and the “doubled”, are created. This is typical of walls that separate two internal empty volumes, outer space and internal empty volume and walls that are completely immersed in outer space. Examples of double facets are those of plate, main structure and solar panels.
The only exception is the interface between two solid compartments: intuitively, double facets would be required, but in practice, the same information is supplied by the total area of the surface in common between the compartments. This is the case of the interface between the aluminum block and the battery.
To sum up, at the end of the mesh process the GMM of the Reference Satellite is composed of facets and compartments. The latter are generated at the end of the process, before closing the Mesher, and can be of three different type:
Unknown temperature: imposed by default;
Constant temperature: by definition, setting a constant value of T, [K], should make the whole compartment at the same temperature. This is what happens inside VIRAF, where all the compartments are full of air and T represents an average value inside the compartment. Instead, this function is employed in a different manner in this work: T is assigned only at the geometric centroid of a solid compartment and conduction inside the solid creates a temperature difference between the centroid and the walls.
In the Reference Satellite, the camera (which is roughly schematized as a lens laterally covered with aluminum) is kept at constant T in most simulations. This is consistent with the fact that some parts of a spacecraft (e.g. sensors, such as IR detectors, [22]) can sustain very small operating ΔT and therefore require active control throughout the mission not to overcome this stringent limit. Therefore, the name “camera” in this context wants to
95 describe a larger class of spacecraft components having the requirement to be at fixed T;
Constant heat flow: the compartment generates an internal heat power Qint, [W], which is then dissipated through its walls. An example is the battery, which is on during eclipse periods, or on-board electronics that constantly dissipates electric power as Joule heating, [22].
Furthermore, there is the possibility to assign a fixed temperature also to facets. This could be of interest when describing external surfaces of antennas for which, as anticipated in Chapter 1, thermal requirements are very stringent.
Once the GMM is built and the model saved, a list of files is generated and three of them, with the following default names, contain useful information:
test.skin: meshed skin of the geometric model in input; useful to adjust the model, verify the correctness of pre-processing (how data are transmitted to the thermal code) and read information not available on the other files (e.g., the number of facets per each group);
test.msh: binary file containing information about the facets. It is the input file for the VFS in order to obtain view factors; then, it can be converted in ASCII file to read information word for word;
IR_PropertyAssign.js: directly extracted from the SKIN file, it contains information about the compartments. The following missing information have been added by simply opening and modifying it:
o full or empty compartment (solid or vacuum), o geometric centroid (read in MF),
o volume (computed with CATIA® V5), o material.
Up to now, all the Mesher’s outputs useful for next analyses have been described. It is worth remembering that the entity “compartment” is specifically designed to deal with gases like air or, at most, vacuum, but not with solids. This explains why the possibility to assign a constant T to the whole compartment, which is obviously not physical if it were full of solid material (unless infinite thermal conductivity be
96 assumed!). Despite this, in order to respect the fundamental choice to use as most information from the Mesher as possible, even solid compartments are for the moment set at constant T and then re-modelled inside the thermal code as explained before.
The Mesher is able to not only assign, but also define materials through an extensive library of materials. For the sake of simplicity, materials have been defined outside the Mesher, as explained in next paragraph.
5.2 Thermo-optical model for materials
The driving criterion to model materials directly derives from the fact that the Mesher handles surfaces and not volumes. The extension of each wall in 3D space is done by defining the wall thickness to the material it is made of. In doing so, an error is committed at the interface among walls whose normals have different direction (usually, at edges and corners), but such error is acceptable if thicknesses are much smaller if compared to the two dimensions of the wall.
Figure 5.6 - Corner and edge errors due to material modelling
According to this choice, the following important considerations hold when modelling 2D surface skins:
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components having only one dimension much smaller than the other two should be modelled as walls;
components having either one dimension much larger than the other two (e.g.
stiffeners) or all dimensions of the same order of magnitude should be modelled as compartments.
In any case, the material is homogeneous and composed of a certain number of layers, each of which having its thickness and thermal and optical properties defined in Chapter 2: density, thermal conductivity, specific heat, absorptivity and emissivity.
Obviously, optical properties are defined for the first layer only, except for walls having double facets, for which optical properties are defined also for the last layer of material.
Figure 5.7 - Thermo-optical model of materials
For any skin model, a material file called materials.txt is written “ad hoc” in order to be consistent with the geometry. Below is an excerpt of a TXT file to show how materials are defined for each group and how are displayed in the Mesher, where the function to name materials has been exploited.
98 Figure 5.8 - Excerpt and explanation of materials.txt
Although only layers of solid materials have been used, there is no limitation in inserting a convective layer made of gas or liquid. An example could be the electrolytic solution inside batteries.
While in the Mesher the material is assigned to walls only, there is the need to assign it also to solid compartments. This is done by assigning in the JS file the index of one the materials of the walls surrounding the compartments, but inheriting only the properties of the inner layer of material, which is “fictitious” since its thickness is put intuitively equal to zero. On the other hand, facets receive information about materials in the MSH file.
The following scheme summarizes how files are managed to pass from the geometric model of a spacecraft architecture to the thermal code.
99 Figure 5.9 - Schematics of file management
