Characterization of Light Deflection on Hot Exhaust Gas for a LIDAR Feasibility Study Robert Stützer 1 , Stephan Kraus 2 , Michael Oschwald 1 1 DLR – German Aerospace Center, Space Propulsion; D—74239 Hardthausen, Germany 2 Airbus Defence & Space, Space Transportation; D—74239 Hardthausen, Germany corresponding author: robert.stuetzer@dlr.de Abstract 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, O 2 , and CO 2 . 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 H 2 O background in the near infrared regime of the optical spectrum. Introduction In order to realize European research projects aiming at moon surface exploration, ESA commissioned Airbus Defence & Space with the development of a lunar landing space craft. All engine tests for the LunarLander application were carried out under space and ground conditions on the DLR test site Lampoldshausen, Germany. Due to their space storability, the hypergolic constituents MMH and NTO were used for propulsion, path and position correction as well as for landing maneuvers. A lidar system (light detection and ranging), however, ensures distance and velocity measurements in particular while approaching and landing on the celestial body. Several potential sources of interference must be identified and tested in order to ensure a sound operation under space conditions. Since the laser beam traverses the exhaust plume during landing maneuver an error-prone attenuation of the beam intensity can be possible for critical engine parameters such as a high pulse rate