Analysis cases

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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.

and minimum temperatures of the different instruments are obtained for every potential worst-case environment. The absolute maximum and minimum values for each case are chosen resulting in the Cold and Hot Operational Case environmental conditions. Operational heaters will be used when it is necessary and the required power for them is obtained with this analysis. When having movable parts, this study is even more important to account for reflections. In SUNRISE III, the telescope is always pointing the Sun during the float phase, going up and down making relative position between elements to change together with environmental conditions, which are a function of the SZA.

As the base line in SUNRISE III thermal design is to reduce the amount of direct solar flux and albedo heat load, parametric analyses for most of the scientific instruments and electronic units results in values shown in Table 4.5 for the Hot and Cold Operational Cases.

In addition, the solar constant varies during the year due to the variable distance between the Earth and the Sun. Values for the solar constant have been obtained from the SORCE data [52] being 1316 W/m² and 1324 W/m² the minimum and maximum values, respectively, during the considered period of time (from June 1^st to July 15^th).

Table 4.5: Worst extreme cases vales for SUNRISE III thermal analysis.

Hot Operational

Case

Cold Operational

Case

Albedo 0.64 0.77

OLR [W/m²] 237.5 176.5

SZA [^o] 45.5 87.7

Solar irradiance [W/m²] 1324 1316

In addition to these two operational cases, a Cold Non-Operational Case should be performed where the power dissipation of the equipment is not the same and the primary mirror of the telescope is covered with a deployable curtain that avoids a direct solar flux. Non-operational heaters at float are sized through this analysis.

4.4.2.2 Ascent Phase

Both convective and radiative parameters are obtained as a result of a local study for the location and epoch launch site (Esrange, Sweden), and they are defined as a function of the altitude and the SZA. The profiles of the radiative and convective parameters used to define the hot and cold cases during the ascent phase are

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obtained following the methodology presented in Section 4.3. The hot case is defined considering the launch time at 10 am Coordinated Universal Time (UTC), whereas the cold case is defined considering the launch time at 21 pm UTC.

The thermal environment to be found during the ascent makes the thermal study of this phase is essential to the survival of the equipments. Traditionally, this phase has been analytically studied due to the huge amount of variable environmental conditions as a function of the altitude. Not only the definition of the particular thermal environment of LDB launched from Esrange has been a challenge [35], but also the implementation in ESATAN-TMS has required a big effort to automatize the analysis execution.

The tool developed allows us to study the influence of different parameters for the ascent phase such as the launch time, the spin rate or the environmental parameters.

In the case of SUNRISE III, the analysis has been performed without considering the balloon film, thus reducing a huge amount the computational weight. To show the behaviour of the system and the influence of the launch time, two analyses have been carried out. The key element which makes both analyses different is the SZA profile. However, it is not the only parameter which has a dependency on the time of the day as it was shown in Section 4.3.1.

Here, three elements have been considered to show the temperature results:

• Solar panels: They are tilted at an angle of 23° with respect to the vertical plane. The gondola will start pointing to the Sun from 31 km of altitude making the solar panels normal vector to be contained in the solar plane.

• Electronic racks: Located in the lateral part of the gondola just behind the solar panels. When SUNRISE is at float, there is no direct solar flux to the electronics, but they are painted white to reduce the albedo heat load.

However, during the ascent phase, the gondola will be freely rotating provided a solar load to the E-Racks.

• SUSI-O radiator: SUSI-O is one of the scientific instruments on board the Post Focus Instrumentation [98]. It consists of an optical bench divided into two floors where the most dissipative components (two spectrograph cameras and slit jaw camera) are connected to two radiators through thermal straps.

During the ascent phase, this instrument will be switched off, therefore, the temperature will be controlled by non-operational heaters.

The only component of these three elements which is not directly exposed to external convection is the SUSI-O radiator. The PFI is covered by a polyethylene film, which is almost transparent to infrared and solar radiation, which prevents this equipment from the effect of convection. Nevertheless, the effect of the force convection on the external parts of the PFI should be considered.

Results are shown in Figure 4.55a and Figure 4.55b for launch times 21 h and 10 h, respectively. The different instrument behaviours can be appreciated depending on their location and characteristics. A temperature of 5 ^◦C has been used as initial condition for the whole system for launch time at 21 h and a temperature of 15 ^◦C for launch time at 10 h.

(a) (b)

Figure 4.55: Temperature result for a launch time at (a) 21 h and (b) 10 h.

For the launch time of 21h, it is shown that the solar panels reach their minimum temperature during the ascent phase due to the non-constant heat flux due to both the rotation and the convective heat transfer. From 31 km, once the system starts pointing the Sun, its temperature raises up to around 35 °C. SUSI-O radiator temperature has a direct view of the Sun during the ascent due to the horizontal orientation of the PFI reaching a lower temperature for the case at 21 h due to the high SZA. In contrast, E-Racks can reach temperatures below -25 °C. The slope of the curve becomes more critical when the gondola starts pointing. During the first part of the ascent phase, even the convective heat dissipation is huge, there is a higher solar heat load over the electronic racks due to the rotatory motion of the gondola. When it starts pointing, electronics equipment remains behind the solar panels, which blocks the direct solar radiation so they should switch on to meet the requirements.

When executing an analysis with a launch time at 10 h, results are quite different from the previous analysis. Solar panels keep an almost constant temperature until

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the pointing control starts. SUSI - O radiator and E-rack follow a similar behaviour in both analyses but here, its minimum temperature is higher due to the incoming radiative flux. Besides, the lower SZA makes SUSI - O radiator temperature increase once the pointing control starts because the PFI is kept in a horizontal position during the whole ascent phase.