High Performance Propellant Development - Overview of Development Activities Regarding Premixed, Green N2O/C2H6 Monopropellants

Abstract

The global search for green alternatives to the highly toxic monopropellant hydrazine (N2H4) is still ongoing. Different alternatives for hydrazine with different degrees of maturity are currently under investigation. One of these alternatives are mixtures of nitrous oxide and fuels. These propellants offer a high performance (Isp approx. 300 s), low propellant costs and are non-toxic. In the frame of the ESA activity High Performance Propellant Development, DLR investigates a premixed N2O/C2H6 propellant and assesses the safety and performance properties of the mixture. To study the propellant properties DLR built a liquefaction and mixing setup which allows condensation, pressurization and mixing of liquid N2O and C2H6. The produced propellant mixture was used to feed a thermal stability, a material compatibility as well as a priming/water hammer test setup. Furthermore, the gaseous propellant was used to conduct ignition test under vacuum conditions. A last work package included hot runs with liquefied N2O/C2H6 propellant supplied to a monopropellant like setup from a single tank. Despite the premixed nature of the propellant, the thermal stability tests showed no decomposition or combustion of the propellant. The compatibility testing of the propellant with metal alloys Al 2219, Ti64, and stainless steels SS 316 and CRES-15-5 showed no degradation of the material or propellant. In addition, the polymers PTFE (Polytetrafluoroethylene), Kalrez and FEP (Fluorinated ethylene propylene) were tested, here only FEP was incompatible with the propellant. The priming/water hammer tests also showed no decomposition or combustion of the propellant, even for supply pressures of up to 60 bar. The ignition tests under vacuum conditions showed a reproducible, good ignitability of the propellant in a 22 N research thruster. The mixture was ignitable for mass mixture ratios of ROF=4 to 11 while the mass flow was changed in between 4 g/s and 10 g/s. The final task was to conducted a series of hot gas combustion tests with an experimental 22 N thruster. The thruster was fed from a tank with liquified, premixed propellant. During 40 test runs, the propellant showed a reliable ignition and combustion behaviour. The mixture ratio of oxidizer to fuel in the test runs was in between 5.4 and 9.9 and showed very low combustion roughness and high combustion efficiencies (up to 96 %). Due to the positive results of the activity, the next step is the development of higher TRL thrusters and propulsion systems for the N2O/C2H6 propellant.