Thermal modelling

The thermal design and modelling of SUNRISE III at system level is led by UPM which is also responsible for several subsystems at unit level as shown in Figure 4.51.

INTA, MPS and APL also participate in the thermal modelling process. Information exchange, model’s integration and result reporting play a key role in the development of the thermal model built by several institutions. In order to establish an efficient working methodology, some actions have been taken from the system level:

• Boundary conditions have been provided for the unit level model developers to perform their own analyses.

• A thermal modelling guideline document has been prepared in order to homogenize the structure of every thermal model easing the integration to the system model and defining the required analysis cases.

• A hierarchical file management system has been used allowing a direct integration of different subsystem models. Each subsystem has its own coordinate system which is then transformed into the main one.

• Once integrated, the required outputs from the model are provided for the unit level developer and the boundary conditions are updated. By doing so, the design can be iterated.

Figure 4.51: Hierarchy of the SUNRISE III thermal model.

The design of the gondola cannot be undertaken independently of the payload.

Also, its thermal model must consider the presence of the payload units. In addition, APL, which oversees the gondola design and its instruments, uses Thermal Desktop instead of ESATAN which complicates the information exchange. Keeping both models updated and performing the same analysis in both softwares has been a challenge during the design process.

The Geometrical Mathematica Model of SUNRISE III is shown in Figure 4.52.

Main parts of it can be identified such as the solar panels, the gondola, the PFI structure, the E-Rack structure and the telescope. Its design started with the thermal control concept used in SUNRISE I and SUNRISE II. The main parts of it are explained in the following subsection together with the followed strategy to analyze each unit apart from the system model.

4.4.1.1 Post Focus Instrumentation

The Post Focus Instrumentation consists of a honeycomb frame with different compartments for the scientific instruments. The front part of it, the one facing the Sun, is protected with a Sun shield and the whole structure is covered by styrofoam with aluminized mylar on the outer face to get high solar reflectivity (ρ= 0.8). The baseline for the thermal design of the instrumentation has been the use of radiators coated with Aeroglace A276 (α = 0.23; ε = 0.9) on the top to dissipate the internal

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Figure 4.52: SUNRISE III ESATAN thermal model.

power. The rest of the top surface has been also covered with styrofoam blocks.

Every instrument is insulated from the structure with Single Layer Insulation (SLI) providing them with more thermal stability. Finally, linear low-density polyethylene (LLDPE) film, 1.5-mil thick, has been used to cover the top part of the PFI in order to act as a wind shield to avoid freezing of the instruments during the ascent phase. This solution was also used in SUNRISE I and II successfully after the test flight performed from Fort Summer (New Mexico) in 2007 [23]. Each part can be identified in Figure 4.50 corresponding to the PFI thermal model.

Boundary conditions for the individual thermal analysis should be obtained from the corresponding analysis cases. To do so, not only the thermal environment has to be defined but also the influence of the nearby instruments, their dissipating power and their thermo-optical properties should be considered. The way the boundary conditions are provided depends on the configuration of the system. In this case, scientific instrumentation is attached to the PFI structure at several points and the thermal loads only reach the instruments through the upper face as shown in Figure 4.53. For that reason, not only albedo, OLR and direct solar fluxes have been extracted from the model but also the equivalent blackbody temperatures of each side have been calculated.

Figure 4.53: Boundary conditions for the scientific instrumentation in the PFI.

4.4.1.2 Electronic Racks

The instrument control electronics are placed on two racks. They consist of two honeycomb plates acting as radiators placed behind the Solar panels. These electronic boxes are painted in white to diminish albedo radiation input from Earth.

In addition, thermal fillers have been used to enhance heat transfer between the most dissipative boxes and the rack. As done for the PFI, a wind shield to protect the instrument during the ascent phase has been used.

Boundary conditions should be more detailed in this case as shown in Figure 4.54.

In contrast with the scientific instrumentation on the PFI, the electronic boxes are not insulated from the nearby instruments, which are going to affect its thermal behavior. Five radiative interfaces, one conductive interface and thermal loads over each face are provided to perform the analysis independently.

Figure 4.54: Boundary conditions for the electronic boxes in the racks.

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4.4.1.3 Telescope

One of the main advantages of the LDB missions is that the payload can be recovered and reused. This is the case of the telescope. It was designed and integrated by the German company Kayser-Threde under contract with MPS for SUNRISE I.

The strategy followed for the thermal control is explained in Ref. [19]. This concept, due to the success of previous missions, remains in SUNRISE III with slight modifications. Due to its complex geometry and the strong dependence on the nearby elements, the telescope cannot be decoupled from the system thermal model. During the operational phase, the telescope together to the PFI, attached to the gondola structure through a shaft, is continuously pointing towards the Sun.

4.4.1.4 Balloon

The balloon film can reach a diameter of around 120 meters at the floating altitude.

Depending on its thermo-optical properties [65], the thermal load reflected on to the scientific instruments cannot be ignored. However, some considerations have to be taken into account regarding its thermal modelling. Including the balloon film in the analysis would increase the computational weight in a huge way. In addition, uncertainties would appear unless an important increment in the number of rays (of the Monte Carlo ray tracing method used by ESATAN) is set.