Premixed green propellants: DLR research and test activities on nitrous oxide/ hydrocarbon mixtures

Since the 1960s hydrazine (N2H4) is the commonly used monopropellant for various space propulsion systems. Besides chemical stability, Hydrazine offers superior long-term storability and good performance characteristics. Moreover, propulsion systems operating with hydrazine provide excellent cold start capabilities. However, hydrazine is highly toxic and carcinogenic, so a large number of safety precautions (e.g. SCAPE suits) are needed for the personal operating with the propellant. These precautions result in higher handling and transportation costs. Caused by the toxicity of N2H4 numerous research and test activities on so called “green propellants” were started across the globe. A green propellant should offer a comparable performance while maintaining the costs of a conventional hydrazine propulsion system. Additionally its toxicity should be significantly reduced compared to hydrazine. Among ADN based propellants (LMP-103S, FLP-106) and hydrogen peroxide (H2O2), the German Aerospace Center (DLR) in Lampoldshausen investigates a so called premixed monopropellant consisting of N2O and hydrocarbons (e.g. C2H4). DLR calls the mixture HyNOx (hydrocarbons mixed with nitrous oxide). The idea behind this propellant is to store oxidizer and fuel premixed in a single tank. Thus the high Isp (here >300 s) of a bipropellant can be combined with the simple propulsion system of a monopropellant. Beside those benefits several challenges concerning the propellant occur: When oxidizer and fuel are stored premixed, a flame flashback across the injection system of the propulsion system must be avoided. If a flashback during operation of a combustor would occur, the flame might propagate upstream the feeding line to the tank structure and destroy the spacecraft. A further challenge connected with the high specific impulse of the propellant is the combustion temperature. Due to temperatures around 3000K the combustion chamber must be equipped with an active cooling system (e.g. a regenerative cooling). To investigate the propellant mixture and to overcome the mentioned challenges, DLR set up an experimental rocket combustion chamber and an ignition and flashback test section: a) The model combustion chamber offers a versatile platform to test different injection or ignition systems, to analyze the propellant performance, to derive the characteristic combustion chamber length and to derive heat loads to the chamber walls. Since 2014 DLR conducted more than 400 successful combustion tests with the N2O/C2H4 propellant mixture. For most of the tests the gaseous, premixed propellant was used, while in 2017 first tests with liquefied gases were conducted. During various test campaigns, the characteristic exhaust velocity (c*) and the combustion efficiency (eta c*) depending on mixture ratio and combustion chamber length were derived. During the experiments a maximum c* of 1550 m/s and a combustion efficiency of 95% could be achieved. b) The ignition and flashback test setup is used to study the ignition and flame propagation behavior of the N2O/C2H4 propellant. Furthermore different flashback arresting elements (capillaries and porous materials) are tested and evaluated regarding their ability to prevent a flame flashback. The ignition is initiated by a spark plug and the flame propagation is recorded via high speed camera. The results obtained in the ignition setup are then used to improve flashback arresting and injection elements in the experimental combustion chamber