Recent Advancements in Green Propulsion

Embarking on the space exploration demands propulsion systems that not only push the boundaries of efficiency but also align with the principles of environmental concerns, this chapter offers a comprehensive discussion of their merits and demerits. From the eco-friendly promises of hydroxylammonium nitrate (HAN) and ammonium dinitramide (ADN) to the potential drawbacks of inherent defects and costly raw materials, this chapter navigates the intricate landscape of propellant technologies. The chapter unfolds with a spotlight on the merits of green propellants, emphasizing their role in reducing pollution and enhancing operational safety. The advantages of non-toxic, safe, and environmentally friendly propellants are the focus of this chapter, promising a paradigm shift in propellant technologies. Yet, challenges such as inherent defects, expensive raw materials, and the quest for higher energy performance present challenges that demand innovative solutions. The contrast between green chemical propulsion and non-chemical propulsion systems becomes apparent as the discussion widens to the propulsion systems. Despite their maturity, solid propulsion systems grapple with limitations in control and safety. In contrast, the promise of electric, solar, and nuclear systems remains on the horizon, with technical issues remaining unsolved. The chapter concludes with a call to action for the future of space propulsion technology.

Hydroxylammonium nitrate, abbreviated as HAN, is an ionic solution comprising NH₃OH⁺ as the cation and NO₃⁻ as the anion. This study involved the preparation of binary HAN-water solutions, ternary HAN-Methanol (MeOH)-water solutions, and HAN-Ethanol (EtOH)-water solutions. The characterization of these solutions as monopropellants was conducted through analytical techniques such as Differential Scanning Calorimetry (DSC) and Raman spectroscopy. Subsequently, the thermal decomposition of both binary and ternary HAN-based monopropellants was investigated using various methods with solid synthesized catalysts. Diverse metals and various support shapes were employed to prepare a range of catalysts. Simultaneously, the reaction mechanism was developed, compared with existing literature, and thoroughly discussed. Lastly, firing tests of the HAN thruster were conducted using a 20N class thruster over monolithic shape catalysts.

In recent years, extensive research has been conducted globally to explore environmentally friendly alternatives for sustainable energy in space propulsion. Hydrogen peroxide (H₂O₂) has emerged as a promising green propellant system, particularly for satellite reaction control systems (RCS) and liquid propulsion. This study involved a comprehensive examination of the thermal decomposition of H₂O₂ monopropellant at a laboratory scale using a batch reactor, a differential pressure device, and differential thermal analysis–gravimetric analysis. The objective was to gain insights into the behavior of this monopropellant before its application in space missions. Concurrently, various catalysts with diverse support shapes were synthesized through different preparation methods. These catalysts were then subjected to characterization using several physico-chemical techniques and evaluated for their effectiveness in facilitating the chemical reactions involved in H₂O₂ decomposition. Notably, the synthesized catalysts proved to be more cost-effective than noble-metal catalysts, demonstrating excellent performance in enhancing the thermal processes.

Green monopropellants play a crucial role in advancing propulsion systems for micro and nanosatellites, offering enhanced performance and safety compared to hydrazine. The development of feed and pressurization systems, particularly for CubeSats, poses a contemporary challenge impacting propulsion system efficiency, onboard power consumption, and spacecraft size optimization. As small satellites transition from passive planetary orbiting to active orbital operations necessitating impulsive maneuvers, there is a growing need for innovative primary propulsion systems. This emphasizes the demand for high-thrust, small-size propulsion systems utilizing green monopropellants due to their advantageous physiochemical properties and simplified system designs compared to gaseous and bipropellant alternatives. The exploration of feed and pressurization systems for small satellite propulsion encompasses conventional pressurization techniques and introduces unconventional approaches such as autogenous pressurization and micro electric pump feed cycles. Miniaturized propulsion system architectures struggle with the optimization of limited onboard space in small-sized spacecraft. Recent trends indicate a significant interest in conceptualizing and employing multi-mode propulsion configurations, integrating various propulsion types such as cold-gas, monopropellant, bipropellant, and electric propulsion. This chapter delves into the challenges and discussions surrounding the main architectures utilizing multi-mode configurations of green monopropellants in conjunction with other propulsion methods.

The chapter gives an overview on the research and development activities regarding mono- and bipropellants based on nitrous oxide and fuels. Nitrous oxide-based propellants offer significant advantages compared to conventional toxic and carcinogenic propellants, e.g., easy handling, non-toxicity, low cost, and good availability. Initially, an overview of so-called premixed monopropellants is given. Here, nitrous oxide and different fuels are stored premixed in a single tank. Therefore, a monopropellant-like system with a bipropellant performance is obtained. During numerous test activities with this kind of propellants, the explosive characteristic and the safe use of such propellants remain a major focus of the research activities. In the second part of the chapter, an overview on nitrous oxide/fuel bipropellants is given. These propellants offer low cost, good availability, safe handling, and self-pressurized propulsion systems while avoiding the explosion hazards of the premixed propellant. Several companies across the globe investigate nitrous oxide fuel bipropellants, and the available technology has a different degree of maturity. The current development focus is on thruster improvement to achieve a long term as well as reliable operation. Furthermore, due to the propellant properties, propulsion system components have to be newly developed and qualified. Due to the advantages of nitrous oxide-based propulsion, it is perfectly suitable for the New Space market.

Space thrusters employ hydroxylammonium nitrate (HAN), a high-performing, newly developed energetic ionic liquid. The development of high temperature-tolerant catalysts for the breakdown of HAN-based compositions is necessary to achieve the high performance that results from the compositions’ higher adiabatic temperature. The chapter reviews the developments in this field and summarizes the results from the development of a novel robust catalyst based on cobalt-doped cerium oxide. The reactivity of the catalyst and catalyst endurance in decomposing HAN aqueous solutions and HAN-based propellant compositions are briefly discussed along with the catalyst characterization and possible reaction mechanism. The performance level of the ceria-based catalysts is compared with that of conventional alumina-supported iridium catalysts.

In the realm of rocket propulsion, hydrogen peroxide has been a subject of historical intrigue and modern fascination. This comprehensive study delves into the evolution of high-concentration hydrogen peroxide as a propellant, aiming to elucidate its potential in future rocketry and satellite propulsion. Beginning with the pioneers and challenges in The Early Days of Hydrogen Peroxide, it explores innovative European contributions and significant advancements in hydrogen peroxide propulsion in Russia. Subsequently, past endeavours with hydrogen peroxide propulsion in the United States are highlighted. Additionally, the authors dispel misconceptions about hydrogen peroxide. Satellite propulsion emerges as a crucial focus, detailing contemporary applications and exploring future perspectives. Delving into the essential parameters of hydrogen peroxide, the chapter provides a holistic understanding of its characteristics. Present uses showcase its relevance today, while the discussion of temporal postponement sheds light on setbacks faced. The chapter ends by drawing insightful conclusions, emphasizing the promising trajectory of hydrogen peroxide. This comprehensive exploration serves as a valuable resource, guiding future research and shaping the landscape of space propulsion.

The privatization of the space sector has led to a significant increase in the volume and affordability of space technologies. This development underlines the need for more cost-effective, safer, and environmentally sustainable space technology solutions. Propellant technologies play an essential role in shaping space systems, serving as the foundation for achieving greener and more cost-efficient space operations. The European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA) are actively engaged in the quest for cleaner, higher-performing propellants capable of displacing traditional alternatives. This chapter explores modern trends and innovations in both mono and bipropellants, offering insights into the latest developments in this field.

Green combustion catalysts have gained significant attention due to their ability to enhance combustion rates while reducing environmental impact. This chapter focuses on the advancements and potential applications of metal oxides, nitrotriazolone (NTO) metal, azoles, and graphene-modified azoles as green combustion catalysts. Metal oxides, such as copper oxide and titanium dioxide, exhibit unique catalytic properties that facilitate rapid fuel oxidation reactions. NTO metal catalysts, which are composite materials consisting of low-sensitivity explosives and metals, have shown remarkable catalytic activity and safety performance. Azole catalysts, characterized by their azole-ring structure, possess favorable combustion catalytic properties. These catalysts exhibit high thermal stability and combustion ability, effectively enhancing combustion rates and efficiency. In conclusion, the investigation of metal oxides, NTO metal, azoles, and graphene-modified azoles as green combustion catalysts offers great potential for advancements in combustion control and environmental sustainability. These catalysts could contribute significantly to the development of more efficient combustion technologies while minimizing environmental impact.

Ammonium perchlorate (AP) is the most commonly used oxidizers in solid rocket propellants. Although AP possesses high oxygen balance, high density, high thermal stability, and good compatibility, it also exhibits biological toxicity. What’s more, the combustion of AP releases hydrochloric acid, which not only causes ozone layer depletion and acid rain, but also generates smoke trail and results in tactical disadvantage. Plenty of work has been carried out to develop environmentally friendly substitutions for AP. However, applications of these candidates are still restricted due to some critical limitations. In this chapter, we mainly reviewed several green oxidizers which could potentially replace AP in solid rocket propellants. The preparation, modification, properties, and applications of these environmentally friendly oxidizers are discussed. Further research and development of new oxidizers are still needed in future to find green oxidizer products.

A detailed reaction mechanism model of ammonium perchlorate-nitrocellulose (AP/NC) micro-propellant microscale combustion was established to further study the mechanism of propellant combustion in micro-solid rocket engine, which includes 64 components and 408 steps. Through user-defined functions (UDF) secondary development, the numerical calculation of microscale combustion of AP/NC micro-sized propellant was carried out. By coupling the primitive reaction with the flow and considering the diffusion mixing process of species transport, the prediction accuracy of micro-propellant combustion behavior was improved. The numerical results show that the critical premixed flame height rises first and then falls when the combustion pressure is increased. When the mass ratio of NC to AP in the propellant formulation is 1.1, the critical pressure is 8 MPa. At the same time, with the increase of pressure, the gas phase has a stronger conduction feedback effect near the burning surface, resulting in an increase in temperature. However, with the increase of NC content in the propellant formulation, the steady flame height decreases gradually.