The University of Michigan
Department of Aerospace Engineering
| Plasmadynamics & Electric Propulsion Laboratory |
PEPL Thrusters: CubeSat Ambipolar Thruster

Above: Our team's design of a 5 kg 3U CubeSat with CAT engine performing initial testing in Low Earth Orbit.

The CubeSat Ambipolar Thruster is a new design for a permanent magnet helicon generated plasma thruster. Its small plasma volume (~10 cm3) and low power requirements (<100 W) make it ideal for propelling nanosatellites (<10 kg). The source is powered by a novel DC to RF oscillator with air-core inductors suitable to be flown on small spacecraft. Specifically, the CAT is being made to fit the CubeSat form factor, a design of nanosatellites made up of 10x10x10-cm units (1U). Permanent magnets generate a converging-diverging magnetic nozzle with a magnetic field that decreased to the strength of earth's magnetic field within 50 cm allowing the entire exhaust plume to develop in the vacuum chamber. Low gas flow rates (~4 sccm) and high pumping speeds (~10,000 l/s) are used to more closely approximate the conditions of space.

Above: A computer design of the CAT engine with the power system and propellant tank.

In the CAT, the plasma is generated continuously using a Radio Frequency (RF) antenna to launch a helicon wave. This type of plasma wave is extremely efficient in ionizing the gaseous propellant. The wave also heats the electrons, which are insulated from the walls of the plasma liner by a magnetic field generated by rare earth magnets. The electrons stream out of the plasma liner through the magnetic nozzle, dragging the ions with them. The thermal energy of the electrons is converted to kinetic energy in the ions to generate thrust.

We are currently in the process of raising funds through grants and donations to build the CAT thruster and put it in orbit on an actual satellite. The Plasma Ambipolar Thruster for Rapid In-Orbit Transfers (PATRIOT) mission will involve flying multiple CubeSats loaded with CAT engines to demonstrate a variety of orbital maneuvers that the CAT will enable. If you are interested in this project and want to have a direct influence on humankind's exploration of the solar system, consider contributing to the CAT research project! For mission control sponsorship, spacecraft naming, or donations exceeding $10,000 please contact Daryl Weinert, Associate Vice President for Research.

The Future of Space Exploration

Our new thruster technology will be able to send low-cost satellites from the Earth to go observe and radio-tag asteroids for retrieval, search for life on the moons of Jupiter, and explore uncharted regions of the solar system. The future of space exploration is not limited to large, expensive spacecraft! Most satellites and interplanetary spacecraft launched by NASA and private industry today are the size of a car and can cost up to one billion dollars or more. Nanosatellites are changing all of that. CubeSats cost 1,000 to 10,000 times less to develop and launch than conventional satellites.

Currently, these CubeSats are deployed from larger rockets, and once in space, they drift around Earth, trapped in their original orbit. However, this will soon change as we are developing the CAT that once in space, will send these small spacecraft to the moon, asteroids, Mars, and beyond!

A Big Idea for Small Satellites

We are developing the CAT, a plasma propulsion system designed specifically to fit in 1U of a 3U (or larger) CubeSat. Plasma is an ionized gas that can be accelerated to produce thrust. Just like a normal rocket that produces thrust from the burning and expansion of hot gases, our thruster will produce thrust from the expansion of a super-heated 200,000 degrees Celsius plasma stream. The force generated by this thruster will be very low (milli-newtons) but very efficient. The engine will be turned on for weeks at a time, accelerating the spacecraft to much higher velocities than a typical chemical rocket. Initial testing will be performed in our lab on the ground and then in low Earth orbit (LEO) to validate the performance and physics models developed by our team. Once these tests are completed, every attempt will be made to perform a series of maneuvers to climb to higher and higher altitudes in order to escape the Earth, demonstrating an operational performance of the thruster.

Above: Orbital simulation for the CAT engine pushing a CubeSat with continuous-thrust. The initial orbit is a 500 km altitude circular orbit around the Earth, out to a rendezvous and fly-by of the Moon.

How Does the CAT Engine Work?

First, from a propellant tank, our fuel propellant will be injected from its storage tank into the plasma liner, a quartz chamber that distributes the gas and contains the plasma. The gas is turned into a plasma by a radio frequency antenna that surrounds the liner and launches a plasma wave known as a "helicon." Then the plasma is launched out of the liner with magnetic fields from extremely powerful permanent magnets, pushing the satellite in the opposite direction. Unlike conventional rockets, almost any gas can be used as propellant for the CAT - even water vapor!

Above: The quartz plasma liner supported by plastic stand. The gaseous propellant enters from the left, is ionized into a plasma in the bottle shaped region, and accelerated out of the nozzle to produce thrust.

Plasma thrusters (ion engines, Hall thrusters, resistojets, arcjets, etc) have been used on satellites for decades, but they have been large, bulky devices that weigh up to 20 lbs, suitable only for large satellites. The CAT design scales down previously demonstrated technology (see the VASIMR engine) to make it usable for CubeSats . Most of the thruster components have been tested by themselves, and now we will be assembling everything into one compact thruster unit for testing in the lab, then testing in space, then in operational use in space.

While the CAT thruster is firing, the satellite will be aimed with a space qualified control system consisting of small reaction control wheels (four small gyroscopes) and magnet torque rods. Flight qualified solar panels mounted on the outside of the CubeSat will power the CAT and other onboard systems like the radios and computers. All of these core satellite components have flown previously on CubeSats from the University of Michigan.

Above: Radio Auroral Explorer - 2 developed by the Michigan Exploration Laboratory at the University of Michigan.

    CAT Engine Specs
  • Up to 2 mN thrust for 10W (20mN for 100W pulsed)
  • Up to 20,000 m/s plasma exhaust velocity
  • Up to 10 Watts continuous (or higher power when pulsed)
  • >90% efficient solid-state DC to RF converter
  • Expected engine lifetime, >20,000 hrs of operation
  • Expected propellant: Iodine or Water
  • Expected propellant mass: <2.5kg (for a 3U CubeSat)
  • Permanent magnet converging-diverging nozzle
    Spacecraft Specs:
  • 3U CubeSat (30 cm x 10 cm x 10 cm)
  • 2.5 kg dry mass (5 kg total mass)
  • 20 W of power produced from deployable solar panels
  • Passive magnetic attitude stabilization from nozzle magnets interacting with Earth's magnetic field
  • Anticipated lifetime in LEO: 5 yrs (radiation limit for onboard chips)
  • Anticipated lifetime beyond Earth: 10 yrs (battery lifetime)
  • Anticipated lifetime before micrometeorite impacts degrade spacecraft beyond recognition: 100,000,000 years

Laboratories on the Ground and in the Sky

The CAT engine is being developed here at the Plasmadynamics and Electric Propulsion Laboratory (PEPL). We have the largest university-run vacuum chamber in the world, which will be used to test the thruster in conditions as close to space as we can achieve on the ground. Here the thruster will be fine-tuned and a fully operational satellite loaded with a CAT engine will be tested in the vacuum chamber.

Our team also includes the Michigan Exploration Laboratory (MXL), which has over six years of experience building and flying CubeSats (the design of the CubeSats itself is only 10 years old). In fact, our team launched the first CubeSat in the world and the first CubeSats for JPL and the NSF. We have designed, tested, and flown most major CubeSat components including radios, power systems, computers, structures, solar panels. This technology will provide a stable, proven platform for testing the CAT.

Above: A photo of our high-speed payload interface module from the RAX missions. This is the flight unit used on the RAX-2 satellite, which launched in 2011.

Once the satellite is ready for launch it will be deployed as a secondary payload on a large satellite launch. These space-based tests will be called the PATRIOT Mission.

The World is Not Enough

Above: A screen capture of orbit tracking RAX-2.

Ground testing can only go so far. To truly understand the CAT's performance the device must be flown and tested in space. The first PATRIOT mission will be in Low Earth Orbit to ensure that the thruster performs as expected. Once the CAT has been OK'd for operation through a series of rigorous tests, we will set our sights skyward and start to propel the CubeSat away from Earth.

A flight tested CubeSat propulsion system could be added to any future CubeSat mission, enabling a wide variety of exciting experiments. Future missions that we envision include:

  • An array of CubeSats flying in formation, gathering spatially resolved data in Earth's magnetosphere.
  • Loaded with scanning equipment and a radio beacon payload, a CubeSat can be sent to the asteroid belt to identify and mark an asteroid of interest for retrieval or mining.
  • Simple diagnostic equipment could be put into orbit around Europa (a moon of Jupiter that is thought to contain more water than in Earth's oceans) to determine its potential to support alien life in our own solar system.

This is just the beginning of how these CAT CubeSats could be used, but we need your help to get things off the ground - literally.

PATRIOT, this is Echo Base. Do you copy?

While the satellite is in orbit we will communicate with it via the Peach Mountain Observatory and Dish, a University of Michigan ground station located in Dexter, MI. A 20-minute drive from campus, this former radio astronomy lab is being upgraded with a sophisticated satellite tracking system to follow the satellite as it orbits the Earth.

Above: The Peach Mountain deep space ground communication station.

Above: The Michigan Space and Atmospheric Research Control Center (M-SPARCC).

Once the CAT propulsion system has been tested in low Earth orbit, the next phase of the mission is to propel the CubeSat away from Earth, into deep space. If enough funding is secured, two spacecraft will be launched and raced against each other to achieve the farthest distance from Earth. Each spacecraft will have slightly different and competing designs that will be tested and characterized on the ground and then put to the true test once in space! A large number of network nodes could be launched into deep space onboard our CubeSats and pushed out into position with the CAT engine, effectively forming a daisy chain of deep space satellites in order to form the first interplanetary internet.

Made in Michigan

Michigan has an incredible reservoir of talent and knowledge of precision manufacturing from nearly a century of high quality auto manufacturing. Whenever possible we will take advantage of local skill and resources when building our spacecraft.

Selected Relevant Publications

  1. Sheehan, J.P., Collard, T.A., Ebersohn, F.H., Longmier, B.W., "Initial Operation of the CubeSat Ambipolar Thruster", IEPC 2015-243 , 34th International Electric Propulsion Conference, Kobe, Japan, July 6-10, 2015.
  2. Collard, T.A., Longmier, B.W., Gallimore, A.D., Sheehan J.P, "A Pressurized Inert-Gas Propellant Feed System for the CubeSat Ambipolar Thruster", IEPC 2015-364 , 34th International Electric Propulsion Conference, Kobe, Japan, July 6-10, 2015.
  3. Sheehan, J. P., B. W. Longmier, E. A. Bering, C. S. Olsen, J. P. Squire, M. D. Carter, et al. (2013). Plasma Adiabaticity in a Diveriting Magnetic Nozzle. International Electric Propulsion Conference. Washington, D.C.: IEPC-2013-159. [pdf]
  4. S. C. Spangelo, B. W. Longmier, BravoSat: Optimizing the Delta-V Capability of a CubeSat Mission with Novel Plasma Propulsion Technology, Interplanetary Small satellite Conference, Pasadena, CA, June 20-21, 2013
  5. B. W. Longmier and J. P. Sheehan, "Initial Experiments of a New Permanent Magnet Helicon Thruster," International Conference on Plasma Science, San Francisco, CA, June 2013.
  6. B. W. Longmier, E. A. Bering, M. D. Carter, L. D. Cassady, W. J. Chancery, F. R. C. Diaz, et al., "Ambipolar ion acceleration in an expanding magnetic nozzle," Plasma Sources Science and Technology, vol. 20, p. 015007, Feb 2011. [pdf]
  7. B. W. Longmier, L. D. Cassady, M. G. Ballenger, M. D. Carter, F. R. Chang-Diaz, T. W. Glover, et al., "VX-200 Magnetoplasma Thruster Performance Results Exceeding Fifty-Percent Thruster Efficiency," Journal of Propulsion and Power, vol. 27, pp. 915-920, Jul-Aug 2011. [pdf]
  8. Rivas, J. M.; Han, Y.; Leitermann, O.; Sagneri, A. D.; Perreault, D. J. “A High-Frequency Resonant Inverter Topology With Low-Voltage Stress” IEEE Transactions on Power Electronics, Volume 23, Issue 4, July 2008, Pages: 1759-1771.
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