Optical fiber sensor for atmospheric reentry experiments

Reliable temperature measurements of hot structures of re-entry vehicles are one of the main challenges associated with atmospheric heating. During re-entry, several minutes at hypersonic velocities results in severe aerothermal loads on some components and leads temperature increase of more than 1000°C. In contrast to single-point measurements provided by thermocouples, optical fiber sensors allow temperature measurement at multiple positions along the fiber line. Key challenges of this technique include packaging, integration, and extraction of the temperature contribution from a signal that is also influenced by strain effects. MPBC developed optical fiber sensors for such temperatures with special packaging optimizing between protective capability and fast thermal conductivity. The fiber sensors were initially calibrated with thermocouples using a standard oven, then through a test in the DLR arc-heated wind tunnel L3K at 1000°C. To monitor accurately the fast heat fluxes in reentry two special ruggedized interrogation modules (Interrogator) were developed with data acquisition at 100 Hz and 3.5 kHz. Each Interrogator has a large memory capacity to save data during 1 hour, and a USB memory stick as back up. The Interrogators were validated for vacuum, thermal cycling, and vibrations, being completely functional during the tests. The vibration tests were performed successfully with the accurate sweeping tunable Fabry-Perot Interferometer functional during the vibrations in the X, Y, and Z-axis. DLR integrated both 100 Hz and 3.5 kHz into the hypersonic flight experiment ATEK for measuring temperature distribution of the motor case of the second stage motor and hybrid module structure, respectively. The 3.5 kHz Interrogator was integrated into the hybrid module, which was part of the launcher block equipped with a parachute. The 100 Hz Interrogator was integrated into the rocket second stage and performed the return flight without parachute and allowed testing the impact resistance of the new DLR’s data acquisition system and some measurement techniques. The ATEK flight experiment was successfully launched on the 13th of July 2019 from the launch site Esrange in Kiruna and provided valuable flight data [1,2,3]. The second stage and the payload reached an apogee of approx. 240 km and continued the descent without any thrust and landed about 500 seconds after the take-off at a distance of approx. 67 km from the launch site. The Health Monitoring System allowed the measurement of aerothermal and mechanical loads on the hybrid payload structure and the motor case, along with the complete flight. Part of the data has been transmitted during the flight to the ground via telemetry at a low sampling rate of several Hertz. In addition, several impact-resistant data acquisition units could acquire the data at a high sampling rate up to several Kilohertz and stored it onboard. The complete flight data of all data acquisition units is available. The housing of the 3.5 kHz interrogator and its memory stick box was completely undamaged. All four fiber optic connectors were still attached. MPB will recuperate and evaluate its functionality. The 100Hz Interrogator, without protection, impacted the ground with a velocity of about 95 m/s and was damaged due to the impact.