Small satellites are becoming more and more capable, taking over missions that used to require larger spacecraft. However, adding propulsion systems to these smaller platforms remains a challenge, which means many small sats are limited to applications that do not require increases in altitude, or changes in inclination.
Engineers at The Aerospace Corporation, working in conjunction with the University of Southern California, are developing a monopropellant vapor propulsion system that could help solve this problem.
“This type of system could enable the satellite to perform formation flying, auto rendezvous, extend mission durations, or change orbits while not substantially affecting power or volume budgets,” said Dr. Brandie Rhodes, the principal investigator on this project.
Small satellites face some unique challenges in terms of propulsion. To start with, they don’t have much space for a propulsion system. In addition, they often launch with other satellites, and any propellant they carry must not present a risk to the satellite sitting next to them in the launch vehicle.
Small sat propulsion systems do exist. Aerospace developed a xenon cold gas thruster that launched in 2006 and a steam thruster that flew in 2017.
The problem is that these and other small satellite propulsion systems require some compromise in pressure, power, or performance.
Rhodes and team are looking at a different way of tackling the problem. They patented a method of evaporating the vapor off a liquid propellant (such as hydrogen peroxide), and then delivering the vapor to a catalyst to react.
In the future, they envision using a process called thermal transpiration to help regulate the flow of the vapor and increase the pressure further.
They call their system the Hydrogen Peroxide Vapor Thruster, or HyPer. HyPer provides thrust with low pressure and minimal power requirements while still providing higher specific impulse than comparable systems. It only requires around ¼ unit (5 x 5 x 10 cm) of space.
“There are no other propulsion systems of this style or with these benefits,” Rhodes said.
The team has developed several iterative prototypes, completed performance and safety testing, and is working on refining their design, with the goal of having a flight-ready system later this year.
As part of the modular future of space, small satellite manufacturers could use this technology to add a simple, small, higher performance propulsion unit to their satellite.
As Rhodes puts it, “If we want to replace large, expensive satellites, we need to develop new propulsion systems that work on a small scale.”
We checked in with Rhodes to find out the latest on HyPer.
What is the latest with Hyper?
HyPer is currently in the environmental test phase with the other Slingshot payloads prior to being integrated into the cubesat bus. Launch of Slingshot is currently planned for December 2021.
How maneuverable are small satellites currently?
Most small satellites do not contain a propulsion system either because it is not required for their specific mission or it introduces too much difficulty to their design, build, and integration. Due to small satellite volume constraints and shared launch conditions, trade-offs between subsystem performance, footprint, and power must be made when considering propulsion. Additional complexities include subsystem reliability and propellant hazards. Without propulsion, systems are limited to drag-based maneuvering and torque rod and reaction wheel orientation control. This means that they cannot maintain their orbit, increase their altitude, or change their inclination.
What would Hyper allow satellites to do?
HyPer could enable small satellites to perform orbit maintenance, formation flying, auto rendezvous, extend mission durations, or change orbits while not substantially affecting their power or volume budgets. These functions are necessary to expand small satellite capabilities.
What are the advantages of a system like Hyper?
In HyPer, reactive hydrogen peroxide vapor is vacuum-evaporated from the surface of the stored liquid. The vapor flows to a catalyst bed where a chemical reaction occurs producing hot product gases, which then can be utilized to generate thrust. This propellant feed system represents the mirror image of a tradition monopropellant, which pressure-feeds liquid propellant to the catalyst. HyPer’s design provides millinewtons of thrust with low tank pressure (< 2 psia), no additional pressurant gas system, and minimal power requirements, all while using a green propellant. A vapor system utilizes all of the liquid propellant (i.e., none stays trapped in the system) and many of the hydrodynamic complications that prevent full decomposition in low thrust liquid systems, do not occur in a vapor-phase propulsion system.