By: Vandana A.M, Associate Director- Propulsion & Founding Member, Stardour Aerospace Technologies
Green Revolution in Space Propulsion
For several decades, spacecraft have been utilising powerful yet often toxic, corrosive, hazardous, and expensive-to-handle chemical propellants. In recent times, space exploration has shifted towards a greener and cleaner propellant era to drive satellites.
A few green alternatives, such as HTP, ASCENT, and LMP-103S, are being employed instead of conventional chemical propellants, with a focus on safety, environmental impact, and cost concerns.
In terms of sustainability, deep space exploration, and interplanetary ambitions, an improbable fuel has surfaced – “WATER”!
Water is emerging as the future fuel as it amalgamates safety, in-space availability, versatility, compelling performance, and cost efficiency for future missions, aligning with upcoming space reforms & regulations and operational demands.
Water is environmentally friendly, non-toxic, non-corrosive, and highly stable across a wide range of operational conditions, unlike traditional hazardous propellants such as Hydrazine, MMH, UDMH, N2O4, MON – 3, and other Earth-storable fuels. The challenges of personnel risk during handling, fuelling / servicing, integration, and safety aspects associated with these conventional propellants are beyond doubt eliminated. With zero complexity in fuelling operations and simpler tank storage feasibility, water emanates to be the ultimate green propellant.
Moreover, water is abundantly available not only on Earth but also on celestial bodies like the Moon, Mars, and asteroids. This reduces dependency on Earth-based propellant launches, as in-situ resource utilization (ISRU) can be employed. This is one of the biggest advantages of using water as a propellant improving overall payload capability and cost-effectiveness of spacecraft.
ISRU seeds an innovative vision for sustainable space exploration, including propellant depots in space and lunar or Martian supply chains.
Water is already being utilised by crew aboard spacecraft for drinking, hygiene, cooling, and oxygen support. When stored water is also used as a propellant for orbital manoeuvring and station keeping, it helps reduce overall mass. This multi-functionality of water combining propulsion, life support, and power is vividly acknowledged by ESA.
Water, when transformed into an energetic form, can be employed across multiple propulsion modes, viz., bi-propellant thrusters, electrothermal thrusters, and plasma thrusters.
Demonstrations and Global Developments
NASA’s Pathfinder Technology Demonstrator (PTD-1) successfully proved water propulsion in orbit, attaining approximately 310 seconds of specific impulse using the HYDROS thruster.
This HYDROS hardware unit is a water-based propulsion system designed for CubeSats. Credits: Tethers Unlimited Inc. / Mason Freedman [1]
Additionally, one of NASA’s technical reports indicated that water propulsion systems exhibit a thrust-to-power ratio of ~0.2 N/kW and electrical conversion efficiencies exceeding 85%.
Similarly, ESA, along with its industry partners pursuing water propulsion technologies, had accomplished performance up to 14 times higher than electric propulsion systems at comparable input power. This showcases strong competition with conventional rocket fuels.
Pioneering Efforts in Asia
Tokyo-based deep-space technology organisation Pale Blue had trademarked multiple types of water-based propulsion systems. The company had successfully demonstrated the potential of water propulsion through various small satellite missions using their in-house developed resistojets and ion thrusters, proving the technology’s integrity, feasibility, and reliability.
The AQT-D (Aqua Thruster Demonstrator) was the first ISS-deployed satellite using a water-based propulsion system called AQUARIUS-1U (Aqua Resistojet Propulsion System -1U), developed by the University of Tokyo. It utilises water as propellant for trajectory control and reaction wheel desaturation of the 6U CubeSat EQUULEUS (EQUilibriUm Lunar-Earth point 6U Spacecraft).
The ESA’s AQUAMAG project further advances water propulsion technology by using magnesium and steam to generate a controlled hybrid engine capable of throttleable thrust, supporting de-orbiting or rendezvous missions.
Challenges and the Road Ahead
Although water-based propulsion systems offer numerous advantages, several challenges remain crucial for their effective implementation. Advanced propulsion technologies like these require robust power supplies, miniaturised fluidic and versatile mechanical systems, and efficient propellant management devices. And with very little precedence and experimental data, this is a niche in propulsion that needs fundamental R&D: something that Stardour has always focused on.
Further challenges include thermal management system preventing freezing in storage systems and fluid lines under harsh temperature gradients and vacuum conditions due to water’s low freezing point. Thus, operational complexities are foreseen with these systems.
Additionally, ISRU-based propulsion systems face difficulties related to resource extraction, processing, and on-site utilisation.
Nonetheless, collaborative efforts across space agencies, research institutions, and private industries continue to drive the development of advanced propulsion technologies like water-based systems. These efforts aim to enhance performance, simplify operational complexities and integration, and ultimately create cost-effective, reliable, and sustainable propulsion solutions for deep-space missions.
Conclusion
Water is a simple yet powerful, distinctive and unique fuel that integrates enhanced performance, safety, versatility, and multi-functional capabilities, offering long-term sustainability and cost-effectiveness for future deep space explorations. It outshines as an indispensable choice for future space propellants clean, green, abundantly available across the solar system, and supporting both life and propulsion activities.
With its ability to function in multi-mode propulsion techniques, and the potential of ISRU technology enabling in-space refuelling, water represents a transformative step in sustainable space travel.
NASA and ESA have already laid the foundation for water-based propulsion systems. This technology is poised to emerge as a promising propulsion eventually replacing toxic conventional propellants in deeper and more advanced missions.
Water is not only a precious resource for humanity; it may also be the key to unravelling the mysteries of the universe and this is exactly why Stardour is pioneering this technology.