Ideas on the Future of Hydrolox Rocketry

For more than a year, I have been proposing rocket architectures. A rocket design starts with the propellant choice. Liquid fuels, due to their density, allow for more compact rockets (the tanks are smaller and even the engines are more compact). That's why for most of my designs I proposed methane and LPG as the ideal rocket fuel. However, recently I want to focus more on hydrogen. It has very potent advantages besides its obvious disadvantages. I see the challenges as room for improvement. Compared to other fuels which are already perfected considerably, perfecting a hydrolox system would close the gap and become really competitive in the rocket market. Due to technical difficulty, I reserve hydrogen rockets for developed nations with considerable technological know-how and infrastructure. For countries with limited resources and know-how, I recommend they use LPG instead.

The major attractive point of hydrogen for me is that it can be produced on demand from the sea using electricity only. Given that most launch sites are close to the shore, sourcing water and even cheap energy (offshore renewable energy farms) is highly feasible. The good thing about the electrolysis of water is that it produces the ideal amount of hydrogen and oxygen required for a rocket (though rockets use less oxygen than the ideal, which is not a problem). So there is no need to source a hydrocarbon fuel, transport it to the launch site, and try to produce oxygen from air. This flexibility also allows for mobile launch sites. Imagine a nuclear-powered space launcher. It can generate the propellant on demand, and if the rocket is recoverable including the upper stage, the platform can keep launching rockets without much need to be in contact with land. An ideal launch platform would be a repurposed aircraft carrier. I will write a short article on that later.

Finally, I would like to propose a propellant production process that lowers the cost of liquid hydrogen and oxygen. Electrolysis requires a lot of power, and liquefying propellants is also very energy-intensive—especially hydrogen. My proposition is a unified production system where the heat rejected during the liquefying process is used to vaporize seawater. The hot steam also lowers the electrolysis power consumption compared to ambient temperature. In this setup, low-temperature heat has more than 85% recoverability, whereas trying to generate electricity from such low heat would have at most 15% efficiency. Additionally, this setup negates the need for complex, energy-consuming, and maintenance-requiring reverse osmosis to obtain pure water.

The ability to develop such unified systems and perfect them is beneficial for the future of space exploration. The easiest way to generate propellant for a rocket is to generate liquefied hydrogen and oxygen from water or ice on another terrestrial body. This requires approaching the problems in unison, instead of trying to perfect electrolysis and liquefaction as two independent processes.