Towards Determining OH*, CH* and C2* Concentrations in LOX/Methane Rocket Engine Tests via Emission Spectroscopy as a Potential Means to Assess Climate Impact

With the number of rocket launches increasing almost exponentially in the last years, a trend that will presumably continue, the question of the environmental impact of rocket launches becomes more and more important. However, rocket plume investigations in the past were mostly focused on engine monitoring and not on environmental aspects, so the amount of experimental data related to ozone-destroying radicals, carbon oxides and soot is limited.

As a first step in tackling this problem, spectroscopic measurements of rocket exhaust plumes were taken during ground-based LOX/methane rocket engine tests at the test benches at DLR Lampoldshausen.

Emission spectroscopy in the UV-VIS range enables non-intrusive measuring of light emitted by chemically excited species within the plume as they fall back to their ground states. Each atom or molecule emits light at characteristic wavelengths, so it can be identified and analysed in the measured spectra. The focus was placed on OH* and CH*, well-known intermediate products of methane combustion, as well as C2* which could serve as an indicator for soot formation.

Since the shape of the exhaust plume, i.e. the location of the Mach disk, its diameter or its inner structure, can vary drastically during different operating conditions throughout a test run, time resolved comparison of measurement position and plume structure was made possible with complementary imaging of the plume.

Through careful intensity calibrations, post-processing and geometric analysis, the actual amount of the emitting excited state molecules in the plume can then be calculated from the measured spectra and the results will be presented at the conference. While these excited state species do not immediately provide information about the total species population without further analysis, they nonetheless serve as an indicator and solid first step towards a better understanding of near-field rocket exhaust plume chemistry and could potentially also be used to validate numerical models.