The Case for Alternative Propellants
Traditional electric propulsion systems today rely on xenon and krypton. These gases are easily ionized and have heavy atomic masses — qualities that make them high-performing fuels in both power efficiency and thrust.
Xenon and krypton are also extremely rare on Earth. They’re trace gases in atmospheric air. Xenon is found in 0.08 parts per million (ppm), or roughly 1 in 10 million, and krypton, 1 ppm.
To separate atmospheric air into its component elements, chemical engineers use a process called cryogenic distillation. It’s not dissimilar from the process for distilling, say, gin or vodka, but it’s done at very, very cold temperatures.
Air is first cooled until it liquifies. Then, its various components are boiled off at their respective boiling temperatures. Oxygen, for instance, boils at -183°C (at 1 atm).
After one round of distillation, liquid oxygen typically includes very trace amounts of xenon and krypton. In order to extract pure xenon, oxygen goes through a second round of distillation before a xenon-krypton mixture can be separated. That xenon-krypton mix is then further distilled to extract out various grades of xenon and krypton based on purity. Aerospace grade xenon, used in Hall effect thrusters, is usually defined as 99.9995% xenon.
This purity matters for Hall effect thrusters. As plasma physicist Derek Thompson describes on the P4 podcast, propellant impurities can cause degradation in Hall thruster components. Because of the additional infrastructure required, complexity of processes, and overall scarcity, aerospace grade xenon is challenging to source and expensive to buy.
Supply constraints pressure pricing and national security
While many air separation units (ASUs) can separate out oxygen and nitrogen from air, there are only about 100 ASUs capable of producing crude krypton and xenon, and 20 worldwide that can produce research- and aerospace-grade xenon.
Of those 20 ASUs, four ASUs are based in allied nations, and only one in the United States. The remaining 15 are spread throughout former Soviet states and China. These procurement limitations can pose significant issues for national security missions in times of duress. They also mean that xenon and krypton are very supply inelastic, that is, their prices fluctuate wildly based on small changes in demand. For example, between 2007 and 2009, demand for some rare gases exceeded supply by 5%, spurring a 400% price increase.
This supply inelasticity casts a foreboding shadow over the future for new satellite constellations: over the next 10 years, demand for noble gases is forecasted to increase by 7–11%. Assuming similar future pricing dynamics in line with historical fluctuations, we’d expect prices to rise 4–5x, from $4,000 per kilogram to over $30,000. For new commercial megaconstellations and defense proliferated LEO constellations, this threatens to be an enormous cost driver and supply risk.
Enter the propellant-agnostic radio-frequency thruster
One hallmark of the Phase Four radio-frequency thruster (RFT) is its propellant agnosticism. This means that the RFT is capable of running on any neutral gas, as well as a range of alternative non-noble gas propellants, without significant impact to its lifetime.
Given the supply risks and challenges with rare gases like xenon and the RFT’s unique qualities, we asked ourselves: what other propellants might be worth exploring?
In the lab, we’ve been looking closely at alternative propellants like water vapor — propellants with features that are important to our customers: 1) they can be quickly and inexpensively procured, and 2) are densely stored and volume-efficient without a highly pressurized propellant tank. High-pressure storage is often complex and heavy, and contributes to range safety issues on launch. We also hope to provide a propulsion system with shelf stable propellant, allowing for storage of ready-to-fly satellites, negating the need for fueling operations on the launch pad.
At Phase Four, we get excited about solving big problems, problems that matter to our customers. The supply chain and cost challenges these promising propellants can solve fit directly with our mission: to unlock the full potential of space missions. If you’re interested in joining our team, we’re hiring at phasefour.io/careers.
