One of the limitations of ADN is the high combustion temperature. The combustion temperature of LMP-103S, the most mature propellant blend, is about 1600 °C; much higher than the one of hydrazine, which is about 900 °C. Cheaper and ITAR-free combustion chamber materials could be used, if the combustion temperature of the propellants is reduced. Theoretical calculations were performed in order to predict the influence of increased water content in the two baseline propellants (LMP-103S and FLP-106) on the combustion temperatures and on the performance.
Currently, the ignition of LMP-103S, the most mature ADN-based propellant, is achieved with a pre-heated catalyst. In the project ignition systems that require less pre-heating energy were investigated, with development activities both on catalytic thermal ignition.
The research on catalytic ignition showed that a vaporization of the water content is necessary before the decomposition can start. Therefore the development of a cold-start capable catalytic system was deemed not possible. Therefore the focus shifted towards developing catalysts that can survive the harsh conditions in the combustion chamber. To achieve this goal a significant effort was invested in developing hexaaluminates as catalyst supports. Hexaaluminates are attractive because they offer excellent resistance and stability to thermal shocks and high temperatures. Three different types of hexaaluminates were synthetized, namely barium-, lanthanum-, and modified barium-iridium-cobalt- hexaaluminates.
Further activities were conducted on the development of 3D printed catalyst supports. Three different designs were chosen for the printed monoliths: Straight channels (in order to have a direct comparison with an extruded part), cellular structure, and polyhedral structure. For the manufacturing of monoliths 4 different ceramic materials were evaluated, namely: cordierite, aluminium oxide, magnesium oxide and silicon nitride.
The decomposition and combustion of the baseline propellant LMP-103S creates conditions particularly harsh for the catalyst: the temperature reaches 1600°C and one of the main reaction products is water vapour, which enhances the sintering of the ceramic support, destroying the mesoporosity and thus dramatically reducing the specific surface area. In order to simulate these conditions, several catalyst samples were tested with a procedure called simulated firing testing.
Based on the results of the simulated firing the catalyst selected for the 20 N thruster demonstrator was Ba-Ir-Co-Al hexaaluminate in granulated form. Hot firing of the thruster was conducted. The thrust measured corresponded to that expected from a vaporization of the propellant, without ignition. The temperature measured in the heater dropped following the firing. It was concluded that the catalyst was not active enough to assure the decomposition of the propellant in the thruster tested.
In the third reporting period further activities on the thermal ignition of ADN-based propellants were conducted. Three different chamber designs were built and tested. These designs included a porous material to facilitate the vaporization of propellant and to increase the propellant residence time in the chamber. Such design allowed igniting of both baseline propellants. No sustained combustion was achieved.
Ways to improve the synthesis of this chemical were studied both in Rheform and in the H2020 project GRAIL. In Rheform the focus was on continuous nitration of ammonium sulfamate to guanylurea dinitramide (FOX-12), while in GRAIL a one-step method to convert FOX-12 to ADN was studied.
