XPL stands for Aerothermodynamics Experiment Payload. These are the scientific payloads that fly on board of the satellite to investigate the thermal behavior of the Thermal Protection System (TPS) during the re-entry.
There are 6 sub-payloads with different aims in XPL:
TPS Efficiency Payload: The objectives of this payload are to measure the heating and the ablation of the front TPS, which is made of Cork P50. This can be done by temperature measurements. Thermocouples are inserted in plugs which plug in to the front TPS.
TPS & Environment Payload: This payload consists of pressure measurements in the front part of the vehicle to understand the aerothermodynamic environment together with the XPL01. It will allow us to later reconstruct the trajectory and will also act as input to understand the ablative material behavior and efficiency.
Stability Payload: This payload aims to determine the stability of the satellite during the re-entry phase of the mission by measuring the static pressure on the side panels of the spacecraft.
Shear Force & Transition Payload: Sharing the same pressure sensors with the former payload, the objective of this payload is to determine whether or not a laminar to turbulent transition will happen. The side pressure ports also contribute to CFD validation.
Off-stagnation Temperature Payload: Due to the nature of the boundary layer transition, an increase in heat is expected downstream of the transition location. Thus, temperature measurements are added to the side wall pressure ports to identify the transition region. Additionally, these temperature measurements will allow us to determine the behavior of the side ceramic TPS. In summary, this payload contributes to the transition determination and side TPS efficiency and also to CFD validation.
Emission Spectroscopy for Radiation Payload: an emission spectrometer is inserted in the TPS nose to monitor the species concentration and temperatures of the stagnation point of the vehicle.
The XPL PCB is placed in the front part of the satellite, right behind the TPS nose.