A 2012 NASA study titled “In-Space Propellant Production Using Water” by William Notardonato et al. explored the groundbreaking concept of using water as the key propellant feedstock for in-space propellant depots. Published over a decade ago, this study laid out a compelling vision for establishing cryogenic propellant production and storage infrastructure in space by launching water and electrolyzing it on-orbit to produce liquid hydrogen and oxygen.
The authors argued that a new era of ambitious space exploration, including manned missions to the Moon and Mars, will require the ability to efficiently store and manage cryogenic propellants in space for long durations. In-space propellant depots emerged as a key enabling technology, offering benefits such as lower launch costs, smaller launch vehicles, and enhanced mission flexibility compared to launching all propellant from Earth’s surface.
The Water-Based Propellant Production Architecture
The most notable aspect of the proposed depot architecture was the concept of launching water to a propellant production and liquefaction spacecraft (PPLS), which would then electrolyze the water and liquefy the hydrogen and oxygen products for storage in customer spacecraft propellant tanks. This approach aimed to minimize the total energy needed for propellant production and liquefaction by leveraging direct solar power and eliminating intermediate energy conversion steps.
Water was chosen as the ideal feedstock fluid due to its storage simplicity compared to cryogenic hydrogen and oxygen. Water launches would not require vacuum jacketed tanks or complex insulation, making it easier to store during ground and launch phases. Supplying water also enables much higher payload mass fractions for the propellant resupply launches, helping minimize the repetitive costs that drive the overall depot cost structure.
The proposed PPLS spacecraft is a multifunctional vehicle that includes all necessary subsystems for command and control, propulsion, attitude control, docking, power supply, and thermal management. It receives water from simple water supply modules (WSMs), electrolyzes it at high pressure, and liquefies the oxygen and hydrogen output streams for transfer to customer vehicles.
Advantages of the Water-Based Depot Concept
Using water as the propellant feedstock offers several key advantages:
- Simplified launch operations: Launching water is much simpler and safer than launching cryogenic hydrogen and oxygen. Water does not require vacuum jacketed tanks or complex insulation systems.
- Improved payload mass fractions: Supplying water enables much higher payload mass fractions for propellant resupply launches compared to launching liquid hydrogen and oxygen. This helps minimize the repetitive launch costs that drive the overall depot cost.
- Reduced fluid types: Using water allows for a reduction in the number of different fluids required on the depot and customer spacecraft. The water can be electrolyzed to provide gaseous hydrogen and oxygen for propulsion, life support, and power reactants.
- Reusability: The active components for propellant production, liquefaction, and transfer reside on the PPLS depot, allowing customer vehicles to be simpler and more reusable. Refueling can be done with the customer’s tanks kept cold and empty.
- Scalability: The depot can be scaled up over time by adding more PPLS modules and increasing the water resupply rate as demand grows. The system is inherently flexible.
Development Path
The 2012 study acknowledged that the minimal cryogenic storage durations required for exploration missions beyond low Earth orbit would exceed current capabilities by orders of magnitude. Developing and validating the suite of technologies needed, from large scale electrolysis to zero-g liquefaction to active cooling, was identified as a critical path forward.
Outlook for Water-Based Propellant Depots
In the decade since the publication of this seminal NASA study, the concept of water-based propellant depots has only grown more appealing. Advancements in solar power, cryogenic refrigeration, and in-space operations have helped mature the key technologies.
While a NASA-led development path was outlined, the authors presciently noted that commercialization of propellant depots by private space companies may be a more realistic near-term scenario. Today, companies like SpaceX and Orbit Fab are actively pursuing reusability and in-space refueling, recognizing the revolutionary impact it could have on spaceflight.
Looking ahead, the use of water as the propellant feedstock, combined with solar-powered electrolysis and liquefaction, offers a compelling path to establishing an in-space propellant production infrastructure. With its inherent simplicity, reusability, and scalability, the water-based depot architecture laid out in the 2012 study will no doubt continue to inform and inspire the development of this game-changing capability.
The ability to refuel vehicles in space with propellants produced from water is the key to enabling sustainable exploration of the Moon, Mars, and beyond. By establishing propellant depots in low Earth orbit, at L1, and eventually on the lunar surface, we can dramatically reduce the cost and complexity of deep space missions while enabling new capabilities like reusable lunar landers and interplanetary transports.
The water-based propellant depot architecture presented by Notardonato and his NASA colleagues in 2012 represents a brilliant concept whose time has come. As we embark on a new era of space exploration and development, tapping the power of in-space resources like water will be essential to long-term success. With focused investment in the key technologies and operational concepts, the vision of water-based propellant depots can soon become a reality, forever changing the economics and possibilities of spaceflight.
