Development of a catalytic bed for the 1N thruster HTP

The project is carried out in cooperation with the European Space Agency (ESA). Together with the Warsaw University of Technology and the Institute of Aviation – Łukasiewicz Research Network, the project covers the development of a 1N thruster structure powered by concentrated hydrogen peroxide (HTP).

Catalyst beds are key elements of every monopropellant chemical thruster. The catalyst bed is filled up with a suitable catalyst or its mixture. Usually it refers to the sum of the carrier (support) whashcoating (if utilised), and active surface (phase). In some cases the active surface (phase) and support are the same material. One of the most known catalyst that are utilised in monopropellant thruster systems are those which decompose hydrazine (N2H4). Catalysts for hydrazine space thrusters (e.g. Cnesro or Shell 405) are prepared from gamma alumina that is used as support (100-200 m2/g) with hexachloroiridic acid in aqueous solution as precursor, by wetness impregnation method. The catalyst is then activated by reduction in gaseous hydrogen flow. This step enables the formation of small iridium crystallites onto the surface of alumina pellets (support). The procedure is repeated many times – in order to obtain the expected amount of the active phase and to guarantee a sufficient life of a catalyst.

The technology (catalyst) described above is the keystone for the hydrazine thrusters that are in use for decades now (since 1960’s). On the other hand, traditional silver catalyst (metallic one, as silver gauzes, screens, etc.), has been used successfully with hydrogen peroxide of HTP (High Test Peroxide) since the 1950’s, since silver has the best decomposing performance for HTP. However, concentrations of hydrogen peroxide higher than 90-92% may not be used in practice with such catalyst due to the adiabatic decomposition temperature of peroxide which is close to the melting point of silver. This in turn causes, especially in long duration operations, that silver (or silver coated) catalyst loses its performance due to the formation of silver oxide and/or sintering effect. The effect is much stronger in the case of 98% hydrogen peroxide which decomposition is extremely exothermic, and thus the catalyst bed reaches the temperature in the range of 950 – 960°C within a few seconds, whereas the melting point of silver is 962°C.

The solution (to replace the traditional silver catalyst) may be to use other than silver materials (metals, alloys, compounds) as catalytically active for hydrogen peroxide decomposition. One of the current approach is to use manganese oxides – MnOx/Al2O3 (sometimes mixed with cobalt oxides) supported on a suitable ceramic support (e.g. alumina). Other types of this kind of catalyst are also possible, e.g. such as cordierite monolith honeycomb (MnOx/2MgO*2Al2O3*5SiO2). The impregnation procedure is very important for sufficient anchorage of the active phase onto the support for use manganese oxides, especially in the case of MnOx/Al2O3 pellet catalyst.

Therefore, the analysis of the open and current literature shows clearly that mixed oxides, containing cations of transition metals, may be considered as one of the potential options. Nonetheless, such catalysts exhibit many problems that need to be solved. Some of them is activity and survivability issues connected to the oxidation changes within the active phase (e.g. active MnO2 transformers into Mn2O3 that is lower active towards peroxide). This issue, together with low mechanical strength of some of the ceramic supports or/and relatively low active phase loadings, cause that operating life (cycles) duration is rather limited.

Hydrogen peroxide with concentration 98% by weight is the most desirable in monopropellant applications due to its propulsive performance. Therefore, the development of the catalyst for 98% HTP that would not suffer from activity or life durations problems, seems to be the most important issue in this field. The main objective of the project is to perform research that would answer the question of which technology enables us to obtain the most promising catalyst for the efficient decomposition of 98% hydrogen peroxide of rocket grade. Such catalyst would be an essential component for the catalyst bed of a near-future monopropellant thrusters that operates on 98% hydrogen peroxide as green propellant. Moreover, we think that our (but also in terms of European) technology development in the field green (based on hydrogen peroxide) space propulsion has already demonstrated the need for such catalyst.

The final goal of this project will be an operational (working in laboratory conditions) catalyst bed for 1 N 98% HTP monopropellant thruster. Provided that this solution has met currently defined requirements and is promising to meet higher level requirements, the development of a 1 N monopropellant thruster will be continued.

photo: ESA

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