Implementation in ESATAN-TMS

4. Planetary thermal environment for stratospheric balloon thermal design 99

Table 4.1: Input variables required to compute relative velocity.

Input variables SUNRISE I Air density, [kg/m³]

ECMWF Air temperature, [K]

Air pressure, [Pa]

Wind speed, [m/s]

Gross mass, [kg] 6098

Drag coefficient, [-] Conrad and Robbins model.

Balloon film thermo-optical properties

α= 0.024 ε= 0.134 τ = 0.916 τ_IR = 0.176

Payload mass, [kg] 1920

Ascent ballast mass, [kg] 81 Balloon film mass, [kg] 2330

Auxiliary mass, [kg] 1767

Volume at float altitude, [MCF]

34

Launch site and date 08/06/2009. Esrange.

Percentage Free Lift 10%

Figure 4.41: SUNRISE I GPS ascent profile compared to simulation outputs.

(a) (b)

Figure 4.42: SUNRISE I GPS (a) latitude and (b) longitude profile compared to simulation outputs.

Figure 4.43: SUNRISE I relative velocity profile obtained from simulation outputs.

density is low, the balloon shows a shape similar to a sphere with a huge volume.

Due to the large size of the balloon, the consideration of the balloon during the simulation increases notably the time of simulation. Thus, it is mandatory to do a previous study in order to decide if the balloon should be included or not as done in the previous Section. In case it should be taken into account, as ESATAN-TMS does not allow to change the geometry during the analysis, it is necessary to run N_s chain simulations, where the final state of the simulation n must be the initial state of simulationn+1. Hence, the execution process should be automatized and

4. Planetary thermal environment for stratospheric balloon thermal design 101

therefore the balloon GMM and TMM build-up need to be parametrized.

Figure 4.44: Scheme of the process followed to carry out the ascent phase analysis with B-TASEC.

As has been commented in Section 4.3.1, there is a huge quantity of parameters with a thermal influence during the ascent phase. They are variable during the ascent and extremely dependent on the epoch and site of launch. For that reason, it is necessary the use of an external tool in order to control the full simulation. With this purpose, a tool named B-TASEC, which allows to automatize the simulation,

has been developed in Python [24]. The flow diagram followed during the analysis and the inputs and outputs of the different subtools which B-TASEC is composed of are shown in Figure 4.44. B-TASEC flow and the tasks of each subtool are explained step by step.

4.3.5.1 Environment Characterization Tool

The main objective of this tool is to provide the Flight Characterization tool and the Pre-Process tool with the environmental parameters as a function of the altitude and the time of the day. To do so, different data sources as is explained by Ref. [31], are used. These sources are summarized in Table 4.2 together with the generated outputs.

Table 4.2: Input variables required to compute relative velocity.

Source Output To

CERES

Albedo, a(td, h)

Pre-Process Tool and Flight Char- acterization Tool Earth IR radiation, q_E(t_d, h)

Solar irradiance,G_S(t_d, h) Sky temperature, TSky(h) Andoya Space Centre

soundings / ECMWF / ISA

Air temperature,T_air(h) Air density,ρ(h) Air pressure, p(h)

ESRAD / ECMWF Wind speed, w(h) Flight Charac-

terization Tool As mentioned before, the geographical and seasonal variability of this data make it necessary to particularize the analysis to the launch window and the launch site area considered. A statistical study is required. Averaged values for the radiative and convective parameters as a function of the altitude are used in the Flight Characterization Tool together with the extreme wind velocities to derive the highest relative wind speed. However, extreme values as a function of the daytime are used by the Pre-Process Tool to study the behaviour of the system under the worst cases.

4.3.5.2 Flight Characterization Tool

Depending on the characteristics of the system analysed, the flight performance during the ascent phase could be different. As explained before, a dynamic model is used to compute the ascent velocity,v(h) , of the balloon-borne system and the horizontal wind relative speed, vw(h) . Moreover, the balloon geometry is also dependant on both the system characteristics and the environmental parameters coming from the Environment Characterization Tool. The Flight Characterization

4. Planetary thermal environment for stratospheric balloon thermal design 103

tool provides the Pre-Process Tool with the output parameters shown in Table 4.3, which are used to derive the ascent ephemeris, the convective effects and the balloon parametric geometry as a function of the altitude.

Table 4.3: Input variables required to compute relative velocity.

Process Output

Dynamic model

Ascent velocity, v(h) Wind relative speed, v_w(h)

Height profile, h(t) Balloon Geometrical

model

z_max(h) r_max(h) zrmax(h)

4.3.5.3 Pre-Process Tool

This tool is mainly used to translate all the data about the environment and the flight parameters into the ESATAN-TMS language. In addition, it is used to set the analysis parameters. Three user inputs are required:

• Launch time,t₀: Several parameters are defined as a function of the time of the day. This value is obtained by setting the launch time as t_d=t₀+t [s].

By doing so, all these parameters can be expressed now as a function of the time of the flight or elapsed time,t.

• Array of heights,h [N_s+ 1] : This parameter is used to divide the execution inN_s ranges. All the environmental and flight parameters, which are inputs of this tool, are defined for such heights resulting in several arrays. During the execution, steps between two values of heights interpolate in the defined arrays. Moreover, using the h(t) input array, the time of the day at each altitude can be obtained and the array of elapsed time can be defined.

• Spin rate, ˙φ(t): The spin rate is going to be defined as an array of Ns

dimension. Each constant value is applied to the corresponding model.

All the input parameters are interpolated in height and the time of the day. The input profiles (1D or 2D) are transformed in 1D arrays whose values corresponds to the array of heights.

Controlling the Solar Zenith Angle and the height as a function of the time with ESATAN-TMS is quite complicated. It allows defining the trajectory using orbit parameters which in this case it is not recommended. An alternative solution has

Figure 4.45: Definition of the orbit trajectory. It is considered the Sun in a constant position with respect to the Earth and the balloon borne system is going to move along a meridian.

been applied based on considering the Sun in a constant position with respect to the Earth as shown in Figure 4.45. The balloon borne system is going to move along a meridian to change the SZA with time. Ephemeris are defined by 7 parameters:

• Elapsed time, t.

• Three Cartesian position coordinates in the Inertial Coordinate System (ICS), x.

• Three velocity vector coordinates in the ICS, ˙r. They have been defined only considering the ascent velocity in a radial direction.

They have been obtained from h(t) and θ(t) as follows,

x={(h(t) +R_E)·cosθ(t), 0, (h(t) +R_E)·sinθ(t)} (4.39)

˙

r=ⁿh(t)˙ ·cosθ(t), 0, h(t)˙ ·sinθ(t)^o (4.40) All this information needs to be translated to ESATAN-TMS language and structured in a way it could be used to execute the analyses. For that reason, several files are automatically created. These files are shown in Table 4.4.

4. Planetary thermal environment for stratospheric balloon thermal design 105

Table 4.4: ESATAN-TMS files automatically created in Pre-Process tool.

Files Type Information

.ARRAYS Analysis Mission times and environmental parameters at each time.

.VARIABLES1 Analysis Modifies standard QS, QA, QE and sky and ambient nodes temperatures with corresponding values interpolating in

.ARRAYS.

.ERG Execution Creates an ephemeris array and modifies the parametric balloon geometry.

.ERK Execution Defines a radiative case per time range setting the spin or not.

.ERE Execution Defines an analysis case per time range.

4.3.5.4 Execution Tool

The Execution tool is simply a developed tool that sends commands to ESATAN- TMS automatically. As above mentioned, the analysis is done chaining different geometries (and for instance different models, N_s) for a defined time range in case the balloon geometry is considered. Otherwise, two radiative cases would be executed and then chained in the analysis case (N_s = 1). The analysis of this time range is going to use temperatures of the previous time range last step as initial condition. The execution sketch of the performed analysis is shown in Figure 4.46.

The only input needed by this tool is the template file, which stablishes the different files included in the analysis case of the model.

Figure 4.46: Schematic process developed by Execution tool.

4.3.5.5 Post-Process Tool

Finally, the Post-Process tool allows the user to visualize the ascent phase analysis results directly in ESATAN-TMS or using different libraries in Python. To do so, the TMD file is obtained and then read by Python in order to store the results in a DataFrame. The information in this file can be also visualize in ESATAN-TMS, as it is shown in Figure 4.47.

Figure 4.47: Visualization of the results in ESATAN-TMS.

4. Planetary thermal environment for stratospheric balloon thermal design 107