Space has an ever increasing influence on our daily lives. When switching on the TV to watch events happening on the other side of the world, satellites in space are involved. If we go to an unknown place, we often rely on satellite navigation to get there. The chemical...
Space has an ever increasing influence on our daily lives. When switching on the TV to watch events happening on the other side of the world, satellites in space are involved. If we go to an unknown place, we often rely on satellite navigation to get there.
The chemical substances used in rocket propulsion systems to generate thrust are called propellants. One of the most used storable liquid propellants is hydrazine, often used as monopropellant, i.e. without a separate oxidizer. In a thruster, hydrazine is decomposed by a catalyst, providing the thermal energy needed to generate thrust. Hydrazine is highly toxic and carcinogenic. The associated health and safety concerns lead to a highly complex infrastructure and costs for testing, shipping and handling. In 2011, hydrazine was added to the candidate list of substances of very high concern by the European authority in charge of evaluating chemicals (REACH).
The substitution of hydrazine with less toxic green propellants will make space propulsion more sustainable and better suitable for the requirements of future missions. Particularly interesting for substituting hydrazine are propellants based on ammonium dinitramide (ADN). They have the following advantages compared to hydrazine:
â€¢ Lower overall life cycle cost due to simplified handling, especially at the launch site.
â€¢ Higher overall performance (higher specific impulse, i.e. more thrust per kg of mass of propellant burned)
â€¢ Higher density leading to smaller tanks, and therefore reduced structural weight.
The aim of the H2020 project Rheform is the replacement of hydrazine by ADN-based liquid propellants for orbital and launcher propulsion systems. More specifically, technological solutions are developed to overcome some of the difficulties with recently developed ionic liquids in general and ADN fuel blends in particular.
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 Â°C; much higher than the one of hydrazine, which is about 900 Â°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 1600Â°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.
Procedures to synthesize several hexaaluminates were developed. They are interesting for all applications where a large specific surface area and an excellent resistance to both higher temperatures and thermal shocks are required (for example in chemical industries and catalytic combustion).
The use of 3D printed ceramic monoliths as catalyst supports for ADN-based propellants is a novelty. 3D printing gives much more freedom to design the structure of the monoliths compared to conventional extrusion. The development of 3D printed catalyst supports can be applied in different areas, from automotive to chemical industries.
Hot firing testing with different catalysts was conducted with highly concentrated hydrogen peroxide (HTP). An in-house developed code was adapted to describe the propellant decomposition in a monolithic catalyst with a complex geometry. These results are the groundwork to develop improved catalyst for new thrusters.
The production process of ADN was improved, developing a continuous nitration to replace the currently used batch process. Such a procedure could be exploited to produce ADN on a larger scale and at lower costs.
A better understanding of the thermal ignition of ADN-based propellants was obtained. Ignition of the two baseline propellants was achieved when the propellant was pre-vaporized before coming into contact with the ignition source. These results are important to develop new thermal igniter for ADN-based propellants.
A 22 N catalytic thruster was designed and built. This thruster used as catalyst iridium-cobalt-doped barium hexaaluminate in form of pellets. The results of the testing showed that this catalyst is not active enough to ensure the ignition of the propellant in the thruster tested. This result is important because it shows the importance of the catalyst for the correct functioning of a thruster.
More info: http://rheform-h2020.eu/.