Coupled Environmental Flight System Simulation for Characterizing the Thermal and Radiation Balance of an Atmospheric Sounding Probe

Kurzfassung

The objective of the project Mars Atmospheric Boundary Layer Explorer (MABLE) is to investigate the Martian atmosphere using a balloon-borne sounding probe. The probe contains temperaturesensitive components, such as batteries and a radiometer, among others. In preparation, two test flights have already been carried out on Earth. One a tethered flight at an altitude of 32 m, one a free flight up to 12 km. Primarily, this work provides a thermal model to simulate the temperatures during both flight cases. The model represents the probe as several nodes with specific thermal capacities and radiative properties. The interaction between these nodes is modelled by applying convective, conductive and radiative heat exchange. External heat sources such as the Sun and the ground, are modelled as external heat fluxes and boundary nodes. The model is developed in several steps, expanding from a simple two-node model to a more detailed seven-node model with consideration of the soil and the atmosphere among others. The results of the model are evaluated by comparing them to the data obtained during the test flights. Following this, the model is used to estimate the temperatures and the temperature gradients to which sensitive parts are exposed. It shows that the temperatures closely follow the daily cycle during the tethered flight and the temperature of the surrounding air during the free flight. Both the spatial and the temporal gradient are greatest during the free flight. Although these conditions do not match the radiometer's design case, they are expected to have only a minor effect on its reading. Secondarily, a radiometer model is developed that estimates the fluxes that reach the radiometer detector. For this, the atmosphere is assumed to contain only CO2 and water vapour as relevant species in the infrared region and their absorption cross section spectra are used to calculate the radiative properties of the atmosphere. To account for the different conditions at different altitudes, the atmosphere is modelled as several layers. For comparability, the radiometer readings from the test flights are converted to fluxes. The calculation shows the greatest deviation of no more than 12% during the free flight. During the tethered flight the deviation is larger which is suggested to be due to uncertainties in the modelling of the ground surface temperature. For further test flights, the model is adjusted to accommodate a radiometer with different specifications. The objective is to generate a temperature gradient of the atmosphere based on the Ground temperature and the mean atmospheric temperature, both measured by the radiometer, as well as the air temperature measured by a sensor on the gondola. However, the calculation shows that, while the ground temperature can be determined from the modelled radiometer readings, this is not true for the atmosphere. Therefore, only a linear temperature gradient of the atmosphere, based on two grid points, can be estimated.