Characterization of Light Deflection on Hot Exhaust Gas for a LIDAR Feasibility Study

Using a 10 N thruster engine of a LunarLander application developed by Airbus Defence & Space® and a λ = 632.8 nm HeNe laser, an investigation was carried out regarding the deflection behavior of light for different pulse durations of the engine in order to evaluate interferences of hot exhausting gas on a nearby LIDAR system. The experiment took place under vacuum conditions with the laser beam traversing the hot exhausting gas and the collecting optics focusing on the point of intersection. An obvious correlation between engine pulse length and backscattered light intensity is revealed. The shortest pulses result in the most intense backscattering, indicating an incomplete combustion process between the hypergolic constituents MMH = monomethylhydrazine and NTO = (di)nitrogen tetroxide for very short pulse lengths. On the other hand, a prolonged pulse mode of 120 ms firing time causes only marginal deflection of the laser beam. Furthermore, the permanently firing engine leads to a negligible signal of backscattered photons, accompanied by increasing emission bands of combustion products such as CN, O2, and CO2. However, the disappearance of the OH* emission band, typical for this hypergolic combustion, shows a nearly complete reaction of the hydroxyl radical within the combustor for all pulse modes. In accordance with HITRAN calculations emission as well as absorption spectroscopy reveals a broad H2O background in the near infrared regime of the optical spectrum