Foam-Based Ignition System for Non-Toxic Propellants, Phase II

Hydroxylammonium nitrate (HAN)-based monopropellants promise many improvements over monopropellant hydrazine and bipropellant nitrogen tetroxide/monomethyl hydrazine (NTO/MMH). But before HAN-based monopropellants can be commercialized, a reliable ignition system must be implemented. In this Phase II project Ultramet, working with American Pacific Corporation (AMPAC) In-Space Propulsion, will develop an ignition system for HAN-based monopropellants based on a monolithic open-cell foam catalyst bed that will be a non-sintering drop-in replacement for granular catalyst beds. As demonstrated in Phase I, several benefits are afforded by resistively heating the foam bed. Less electrical energy is used to heat the catalyst bed because the heat is generated within the bed, so surrounding hardware such as the combustion chamber is heated less. The ramp rate to operating temperature is very fast: less than 2 seconds to reach over 1100°C. The high preheat temperature realized by heating the foam directly enables reliable ignition with fast lightoff time. And, operating the resistance heater during propellant flow offers a performance advantage. In a 1- to 2-lbf class thruster, an additional 7 seconds of specific impulse is possible. In Phase II, laboratory testing will demonstrate the ability of the foam-based ignition system to transfer thermal energy to a stream of flowing propellant under a range of conditions, such as bed volume and flow rate at various power levels. The temperature and heat capacity of the liquid used for this laboratory simulation will be approximately that of AF-315e monopropellant in a satellite on orbit. After selecting the foam based on material type, pore size, and density, attention will be turned to designing the foam resistojet by incorporating electrode attachments and provisions for electrical feedthrough into the chamber. The iridium or platinum-iridium alloy wire electrodes will be attached to the foam by plasma spraying or chemical vapor deposition bonding with iridium metal. Electrical feedthrough will be accomplished by incorporating a thin spacer between AMPAC’s existing injector and combustion chamber through which the insulated current-carrying wires will run axially. The overall goal of the project will be to further develop the foam igniter and, working with AMPAC, test it in several ways. Initial hot-fire testing will be performed on a monolithic piece of foam coated with iridium. A conventional bed heater will be used (no resistojet function) and testing performed in standard fashion as if granular catalyst were being used. Subsequent testing will be performed in the form of an electrothermal thruster, in which current is applied to foam occupying the full bed length. The final test will be a hybrid approach using a thin disk of foam just downstream of the injector and the remainder of the bed filled with granular catalyst. The specific impulse increase afforded by the foam-based electrothermal thruster will be characterized. In all tests, the goal will be to demonstrate restart capability after one hour of testing time. The project seeks to advance the state of the art in AF-315e ignition and may be leveraged in Spiral 2 efforts under AMPAC’s Multi-Mode Propulsion System (MMPS) program.