A novel solution for propelling CubeSats in space
Everything is shrinking thanks to the miniaturization of electronics, and spacecraft are no exception. For example, take nanosatellites, part of a category called small spacecraft (SmallSats). These miniature satellites, the best-known of which are CubeSats, have a mass ranging from 1 to 10 kg and are the size of a loaf of bread. Developed in 1999 as a vehicle for education and space exploration, CubeSats provide a current and potential platform for scientific experiments, remote sensing, mapping, tracking assets like ships and trucks, meteorology, environmental protection, agricultural crop monitoring, and communications, among applications.
There has been tremendous growth in the use of nanosatellites in recent years, due to dramatic reductions in the size of electronics utilized onboard the spacecraft to power things like cameras and sensors. As a result of these advances, more than 1,500 CubeSats had been launched as of April 4, 2021, according to Nanosats Database.
But how do you propel and maneuver CubeSats, to adjust their orientation and position in space? The development of CubeSats has triggered R&D efforts to provide novel propulsion units because legacy solutions would not work due to size and power limitations. The methods for propelling CubeSats include such technologies as cold gas thrusters, monopropellant engines, electrosprays, pulsed-plasma thrusters, and vacuum arc thrusters.
While these methods have been developed specifically to fit the miniature requirements of CubeSats, they have a number of drawbacks — low specific impulse (meaning they do not use propellant efficiently), inability to operate for the necessary lifecycle of several years due to triggering systems failures, and contamination of the spacecraft and thruster by the exhaust plume.
We think liquid-fed pulsed-plasma thrusters (LF-PPTs) can help overcome the disadvantages of current propulsion systems, including low fuel efficiency, premature thruster failure, and contamination dangers. LF-PPTs use green liquid propellant based on hydroxylammonium nitrate, also known as AF-M315E, which is safe for the spacecraft. PPTs utilize a capacitor bank to store energy, which subsequently is converted into kinetic motion, heating, and propellant ionization upon initiation of the discharge. The capacitors can be charged by onboard or solar energy.
The propellant is accelerated via the Lorentz force, an electromagnetic force exerted on a charged particle that was identified by Dutch physicist Hendrik Lorentz in 1895. A classical demonstration of the Lorentz force is a railgun, which is a gun that uses electromagnetic force to launch high-velocity projectiles. The difference in the case of the LF-PPT is that the “projectile” is made of a conducting plasma gas cloud rather than solid material. The liquid propellant is converted to a plasma gas phase, using a high-current discharge pulse initiated by an initial spark.
In Purdue’s Electric Propulsion and Plasma Laboratory (EPPL), where I am principal investigator, we conduct experimental research inside vacuum chambers to model space conditions. We are developing the LF-PPT concept and, in particular, improving the efficiency of converting electrical energy into the fast exhaust velocity of plasma gas flow. Our research involves state-of-the-art experimental equipment like nanosecond high-voltage pulsers, vacuum hardware, and plasma diagnostics, as well as numerical simulations.
The work is supported by our partnership with General Atomics, an American energy and defense corporation with headquarters in San Diego and multiple locations in the U.S. and around the world. Currently, we are looking for opportunities to launch and test our LF-PPT propulsion system in space.
Our ultimate goal is to introduce a superior propulsion solution for CubeSats that is attractive for industry, and provides a large practical impact, furthering the exploration and use of space for scientific and commercial advancement.
Alexey Shashurin, PhD
Associate Professor, School of Aeronautics and Astronautics
College of Engineering, Purdue University
Senior Member, American Institute of Aeronautics and Astronautics (AIAA) and Institute of Electronics and Electrical Engineers (IEEE)