Development and Testing of ADN-Based Monopropellants in Small Rocket Engines

This paper presents the initial research, development and testing of a novel rocket monopropellant which has potentially higher performance, significantly less toxic and environmentally benign compared with hydrazine, which is the current state-of-the-art. The new monopropellant is based on the oxidizer Ammonium Dinitramide (ADN). Swedish Space Corporation (SSC) has since 1995 been working on new chemical propulsion systems for small spacecraft. In 1997, SSC in cooperation with the Swedish Defence Research Establishment (FOA) and Chalmers University of Technology began research and development of ADN-based liquid monopropellants for small rocket engines. The objectives of the work presented here, have been research of the basic principals, design and experimental tests of critical functions and characteristic. The outcome of this work was formulation of a new ADNbased monopropellant candidate, LMP-101, consisting of 61% ADN, 26 % water and 13% glycerol. LMP-101 has a theoretical vacuum specific impulse of 2420 Ns/kg (exp. ratio 50) and an adiabatic combustion temperature of 1970 K. LMP-101 has been tested in different experimental rocket engines and operated both pulsed in steady state. "Proof of concept" has been demonstrated, i.e. the propellant ignites rapidly and is capable of a sustained, complete combustion with clean exhaust gases (i.e. approximately 50% water, 30% nitrogen and 20% carbon dioxide). Restarts have been performed without observed degradation in ignition characteristics. A two-year development phase of a flight-like rocket engine is planned to start during 2000. The work was supported by the Swedish National Space Board and the European Space Agency (ESA). INTRODUCTION Hydrazine and hydrogen peroxide have been recognized as rocket propellants for more than 50 years. During the 1950's and 1960's the use of hydrogen peroxide decreased due to its inferior storage stability and hazards in production and handling. Simultaneously, the hydrazine propulsion system technology developed. With the development of a reliable and long lived catalyst, hydrazine-based monopropellant propulsion system became commonly used for space propulsion applications. Hydrazine emerged as the standard liquid monopropellant. Since then, hydrazine-based systems have performed a vast number of space flights demonstrating the capability of millions of pulses and mission duration of more than 20 years. Moreover, there are today a wide range of qualified commercial off the shelf components suitable for hydrazine based propulsion systems. Personnel safety and recent increased environmental awareness are, and will continue to be, important issues in the context of propulsion system handling and operation. There is also an increased awareness on how safety and handling contributes to the vehicles overall costs. This is of particular importance for small spacecraft handled and operated by organizations not having the infrastructure to handle a propellant such as hydrazine. In the past decades, the payload cost has decreased while the cost of fuelling a spacecraft has increased along with the repeated lowering of the limit value of hydrazine exposure. Over the past few years, hydroxylammonium nitrate (HAN)based monopropellants have emerged as "Green Propellant" candidates for space propulsion '''. Although other candidates such as hydrogenperoxide also could be envisaged as a "Green Propellant" candidate, SSC decided at an early stage to focus on ADN-based monopropellants as the "Green Propellant" candidate for the work described in this paper. For the successful development of a new propellant, it must lead to a more cost effective overall propulsion system. * Head of Dept. Propulsion R&D t Research Engineer, Dept. Propulsion R&D I Research Engineer, FOA Defense Research Establishment Copyright 2)2, is a solid oxidizer salt, mainly intended for high performance solid rocket propellants. It is synthesized from mixed acid (nitric and sulfuric acid), ammonia and salts based on sulfamicacid. All components are standard industrial chemicals. No solvents, except water, are needed to produce ADN. All waste chemicals are recycled. ADN is highly hygroscopic which makes it possible to formulate a liquid monopropellant by dissolving ADN in water and adding a suitable fuel. The amount of ADN that can be dissolved in a solvent depends on the temperature at which the blend is saturated. E.g. at room temperature measurements have shown that it is possible to dissolve 80% ADN in water, while at 0 °C, 70% ADN can be dissolved. Hence, low temperature requirements on the propellant will decrease the specific impulse and the density of the propellant. Figure 4 shows a Differential Scanning Calorimeter (DSC) curve of ADN. The DSC used was a Mettler DSC 30 with a ceramic sensor. The curve shows the endotherm melting peak at 91 °C, followed by an exothermal decomposition at 154 °C. Table 1 gives of the basic properties of ADN. Melting point Heat of formation Density Molecular weight Enthalpy of melting Oxygen balance 91 °C -148kJ/mol 1.81 g/cm 1 24.056 g/mol 140 J/g +25.79 % Table 1: Basic properties of ADN