Spinoff is NASA’s annual publication featuring successfully commercialized NASA technology. This commercialization has contributed to the development of products and services in the fields of health and medicine, consumer goods, transportation, public safety, computer technology, and environmental resources.

NASA has been planning for a water-powered rocket engine since the Agency’s early years. The water-electrolysis rocket offers advantages such as high performance, safe materials, simple storage, long lifetime, and low power requirements. During decades of experimentation, though, no one actually produced a practical water-electrolysis engine. Until now.

“Managing hydrogen and oxygen and superheated steam, there are a lot of materials issues like corrosion,” said Robert Hoyt, co-founder and CEO of Tethers Unlimited Inc. (TUI) in Bothell, WA. With this in mind and with NASA’s help, TUI built the first viable water-electrolysis engines. Currently, NASA’s interest in the technology stems from its effort to advance the state of technology for CubeSats — tiny, low-cost satellites built with off-the-shelf parts.

NASA regularly carries little satellites built by universities, startups, and others into orbit as secondary payloads but NASA is also enhancing small satellites’ capabilities to use them more in future exploration and science missions. In 2010, NASA’s Small Business Innovation Research (SBIR) program released a solicitation for propulsion systems for the minisatellites and TUI was selected to receive funding through NASA Ames Research Center.

At the time, CubeSats didn’t have propulsion systems. Building small enough thrusters has been a challenge. And there are concerns about allowing pressurized propulsion systems onboard launches. As a result, CubeSats have largely been stuck with the orbit of the primary payload they hitched a ride with, unable to change their altitude or maintain their orbit long-term.

The HYDROS-M thruster is meant for small satellites weighing between 110 and 400 pounds. The tanks on either side hold water, while the central unit holds an electrolyzer and bladders for oxygen and hydrogen gas, along with avionics. The hole in the top is the thruster nozzle.

With the SBIR funding, TUI built two prototypes and started testing them — control avionics, software integration, and performance and lifetime improvements were still necessary. NASA found a corporate partner — Millennium Space Systems — to share the cost of maturing what TUI now calls its HYDROS thrusters, in exchange for three of the resulting propulsion systems for use on its Altair small satellites. Millenium wanted larger versions of the technology, known as the HYDROS-M, designed for satellites weighing between 110 and 400 pounds, which have already been delivered. Meanwhile, NASA selected the CubeSat version, the HYDROSC, to fly on its first Pathfinder Technology Demonstrator (PTD) project, which tries out new CubeSat technology.

Both HYDROS versions launch with tanks carrying enough water to power them for about three years. Once in orbit, an electrolyzer powered by solar panels splits the water into oxygen and hydrogen gases, stored in separate bladders, to be transferred to the combustion chamber as needed. The system is a hybrid of electric and chemical propulsion — one that’s powered by solar panels but can unleash the powerful thrust of a chemical engine.

The HYDROS approach avoids the risks of sending up a pressurized system, as it’s unpressurized until it starts filling its gas bladders in orbit. The system is also relatively inexpensive and easy to manufacture, scalable for different satellite sizes, and fuel-efficient. HYDROS’ ability to deliver powerful bursts of energy and its long operating life make it ideal for any satellite that needs to move from one orbital altitude to another and then stay there long-term.

The next generation of HYDROS thrusters will include refueling ports, extending their operational lifetimes indefinitely. And TUI looks forward to harvesting water for deep-space travel.

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Tech Briefs Magazine

This article first appeared in the January, 2021 issue of Tech Briefs Magazine.

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