Hot-fire tests were performed with a single-injector research combustor featuring a large optical access (255 × 38 mm) for flame imaging. These tests were conducted with the propellant combination of liquid oxygen and compressed natural gas (LOX/CNG) at conditions relevant for main- and upper-stage engines. The large optical access enabled synchronized flame imaging using OH* and CH* radiation wavelengths covering an area of the combustion chamber from the injection plane to shortly before the contraction section of the nozzle for two sets of operating conditions. Combined with temperature, pressure and unsteady pressure measurements, these data provide a high-quality basis for validation of numerical modeling. Flame width and opening angle were extracted from the imaging in order to determine the flame topology. A two dimensional Rayleigh Index was calculated for an acoustically unexcited and excited interval. These Rayleigh Indices are in good agreement with the thermoacoustic state of the chamber.

Experiments at the European Research and Technology Test Bench P8, located at the DLR Institute of Space Propulsion, were carried out to validate the feasibility of laser ignition on the research rocket combustor “BKA”. Propellant injection in BKA was carried out via 15 coaxial injectors with liquid oxygen as oxidizer and gaseous hydrogen or gaseous methane as fuel. Crucial parameters such as the equivalence ratio between oxidizer and fuel (ROF), plasma temperature, ignition time, and breakdown thresholds can be determined using appropriated methods for plasma analyses. Two settings for LIBS measurements have been used for the comparison between cryogenic rocket combustion (ignition) and experiments on gaseous compositions. The research rocket combustion chamber “Brennkammer A” (BKA) was laserignited with the opportunity to evaluate the ignition spark, and on the other hand, well-defined gas mixtures inside a laboratory-scale cavity were analyzed using plasma spectroscopy. Moreover, taking the molar masses of the constituents into account, the local ROF inside the ignition spark is derived.

A Q-switched laser system was applied to a research rocket combustor in order to ignite cryogenic propellants by inducing several plasma breakdown events. The optical emission was analyzed using LIBS and other techniques.

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.