Investigation of optical laser beam impairment on hypergolic lunarlander exhaust plumes for a lidar feasibility study

Abstract

Plumes of two hypergolic bipropellant thrusters of a LunarLander application, developed by ArianeGroup® Lampoldshausen, were optically examined regarding their potential to interfere with laser beams of a nearby LIDAR system. On one hand, hot exhaust gas of a 22 N Vernier thruster was used to investigate the scattering of a λ = 632.8 nm HeNe laser beam. A series of several engine pulse modes were conducted. An obvious correlation between engine pulse duration 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, steady operation 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. Mie scattering calculations show a correlation between the incident laser beam wavelength and the backscattered light intensity. On the other hand, near infrared spectroscopy on the exhaust plume of a 500 N apogee thruster revealed that interfering flame emission and absorption in the optical range around λ = 1064 nm can be neglected. The relatively intense flame emission measured for λ ≥ 1300 nm, on the other hand, is a potential risk for the application of a laser beam of a similar wavelength.