A Cubesat propulsion system that uses phase change material to store solar thermal energy for use when needed has been awarded a $225,000 National Science Foundation SBIR grant. The ThermaSat concept, developed by Howe Industries of Tempe, Ariz., is designed to provide propulsion for a typical 15kg cubcubesat for 10 years.
Cubesats are tiny satellites — weighing as little as 200 grams — that orbit close to Earth’s atmosphere. They are cheaper to develop and launch than larger satellites. Cubesats have a wide range of purposes, including the collection of mapping and weather data. More than 1,100 have been successfully deployed.
Troy Howe, owner of Howe Industries, answered questions about the ThermaSat propulsion system.
Q: How long has your company been working on the concept?
A: "We have been working on this topic for only about a year in preparation for our NSF proposal, but have experience with optical systems and phase change materials going back about five years."
Q: Can you briefly describe how the system works?
A: "The ThermaSat works by heating liquid water propellant to high temperature steam using incident sunlight. Normally, it is difficult to reach high enough temperatures to use water as propellant, but our optical filtration system is designed to reject long wavelengths of light and only transmit short wavelengths- similar to the greenhouse effect. The phase change materials in the thermal capacitor store the solar energy over a period of hours and then heat the propellant during a 'burn' phase.
"The PCM will be distributed throughout a graphite matrix in the form of small beads. Flow channels will run axially down the length of the cylinder for the propellant to pass through. The design is based loosely on the old NERVA fuel elements from the nuclear rocket program in the 1970s, with the UC kernels being replaced with our PCM. The drawing here shows a cutaway of the thermal capacitor surrounded by the optical system.
"The system is very conceptual at this point and has not been tested, although the propulsion characteristics are well understood. Our task at this point is to show that the optical system works as predicted and can reach the desired temperatures. Phase II will address the effects of a vacuum environment on a prototype."
Q: What type of PCM is used?
A: "We chose a salt (80LiOH+20LiF) as our PCM, it melts at 700K and has a latent heat of fusion of 1163 J/g. The material was selected based on a study performed by NASA in 1986 on space energy storage. The paper was called 'Technology for Brayton-Cycle Space Powerplants Using Solar and Nuclear Energy' by Robert English.""
Q: How much PCM would be used in a system powering a typical Cubesat?
A: "The standard design includes 0.62 kg of PCM. "
Q: Are you working with any Cubesat manufacturers at this point?
A: "We received letters of interest from Pumpkin Space Systems, Aster Labs, and Arizona State University. They all expressed interest in having a safe and reliable Cubesat propulsion system but we have not formally formed collaboration with any manufacturers at this point.”
Q: How will you use the NSF SBIR grant?
A: "Our goals for this topic include demonstrating the optical system in a lab bench test, fabricating photonic crystals, and performing computational analysis on the thermal, structural, and propulsion systems."
Q: What's the next major step in commercializing the system?
A: "Our commercialization strategy right now is to build a functioning prototype and demonstrate operation on earth. From that point we will aim to do a flight test which performs a set of orbital maneuvers and successfully de-orbits itself. From there we will work with Cubesat manufacturers to move forward."
Q: What excites you most about this project?
A: "We are excited about how near term and effective this technology will be for the upcoming Cubesat revolution. We hope to provide a safe, reliable, and effective propulsion solution that can be used with thousands of different satellites and drastically increase the performance of new technologies in space in the timeframe of just a few years.”