Ignition Overpressure in Laser Ignited Reaction and Control Thrusters

Advanced ignition systems have been subject of great interest due to the need of developing a new generation of ignition systems for alternative propellant combinations for reaction and control system (RCS) and orbital and manoeuvring system (OMS) thrusters which move away from the classic but toxic hypergolic propellants monomethylhydrazine (MMH) and dinitrogen tetroxide (NTO). Laser ignition is one of a number of advanced ignition methods which are currently under investigation for use on reaction and control thrusters using green propellants. An investigation examining the laser ignition energies required to successfully ignited an experimental RCS chamber fed with liquid oxygen/gaseous hydrogen (LOx/GH2) and with liquid oxygen/gaseous methane (LOx/GCH4) revealed that all successfully induced ignitions using on a single-point energy deposition were hard igntions associated with a signficant ignition overpressure. The investigation of laser ignition of a single injector RCS thruster has shown that the ignition delay, defined as the time elapsed from valve opening to ignition of the combustion chamber, and which is frequently used to explain observed overpressures, is not the only parameter which impacts on the ignition overpressure. Patterns of flame growth have been identified and qualitative relationships have been established between flame growth behavious and the observed ignition overpressure.