TEPCE (Tether Electrodynamics Propulsion CubeSat Experiment)
TEPCE is a tethered spacecraft being built by U.S. NRL (Navel Research Laboratory), Washington D.C. to demonstrate electrodynamic propulsion in space. Electrodynamic propulsion holds the promise of limitless propulsion for maneuvering of spacecraft without using expendable fuel. The spacecraft, in its orbital configuration, will consist of two CubeSat end masses attached to the end of 1 km of electrically conducting tether.
Electrodynamic propulsion works on electromagnetic principles similar to an electric motor. The magnetic field in an electric motor attracts an electric current that flows through the windings of the armature causing the armature to spin. In space, the Earth has a naturally occurring magnetic field and for TEPCE, the tether wire serves the purpose of the armature. By inducing an electric current to flow along the tether, a mutual attraction between the Earth's magnetic field and the tether will occur. This electromagnetic attraction can propel TEPCE to higher altitudes or to change the orientation of its orbit.
NRL researchers conducted the deployment tests in the Naval Center for Space Technology's high bay facilities at NRL. The tests exercised a spring deployment mechanism, called a "stacer", which pushes the two CubeSats apart at a relative velocity of 4 m/s. The tests were conducted in free fall that simulated the weightlessness of space. The CubeSats were instrumented with angular rate gyros and accelerometers that measured rotations and accelerations. 1)
The TEPCE deployment tests determined the effectiveness of the stacer mechanism to produce the required separation velocity while holding tip-off rotations to an acceptable level. The deployment experiment was a milestone in the development of the first tethered spacecraft to demonstrate electrodynamic thrusting for orbit maneuvers using energy derived from the sun instead of from expendable fuel.
TEPCE is a 3U CubeSat demonstration of emission, collection, and electrodynamic propulsion. Two nearly identical endmasses with a stacer spring between them are used in TEPCE, which separate the endmass and start deployment of a 1 km long braided-tape conducting tether. TEPCE will use a passive braking to reduce speed and hence recoil at the end of electrodynamic current in either direction. The main purpose of this mission is to raise or lower the orbit by several kilometers per day, to change libration state, to change orbit plane, and to actively maneuver
Figure 1: Photo of NRL's TEPCE 3U CubeSat (image credit: NRL)
TEPCE uses a stacer spring to energetically push the ends apart, to pay out a 1 km conductive tether stowed around the stacer. It can use either the tether or 5 m EDDE-like metal tapes at each end to collect electrons from the plasma, and EDDE-like hot wire emitters at each end to emit electrons into the plasma.
Each endmass has isolated high voltage supplies, magnetorquers, GPS, a camera, and plasma sensors. TEPCE has too little power to counteract drag near ISS altitude, and will reenter within a few days after tether deployment. But that is enough to test hardware operation and measure plasma interactions.
Once in space, it will divide into two objects connected by a 1km long tether. The system will collect electrons from the Earth's space environment and transmit the electrons from one object to the other. Its designers expect the Earth's magnetic field to exert a force on the electrons in the tether, producing a velocity change that will affect both the magnitude and direction of the spacecraft.
"What this means is a possible new propulsion capability for spacecraft," said Shannon Coffey, TEPCE's principal investigator. "Which may decrease the amount of propellant that we have to use."
Figure 2: U.S. Naval Research Laboratory's Tether Electrodynamic Propulsion CubeSat Experiment's CubeSat will split into two objects connected by a 1 km long tether (image credit: NRL) 2)
Launch: TEPCE is a secondary payload on the STP-2 rideshare mission of USAF, launched on 25 June 2019 (06:30 UTC) aboard a SpaceX Falcon Heavy launch vehicle from Launch Complex 39A at NASA's Kennedy Space Center. The STP-2 payload includes six FormoSat-7/COSMIC-2 satellites (primary payload, each with a mass of 280 kg), developed by NOAA and Taiwan's National Space Organization to collect GPS radio occultation data for weather forecasting. The mission also carries several NASA technology demonstrations. The STP-2 mission is led by the Air Force Space Command's Space and Missile Systems Center (SMC). The total IPS (Integrated Payload Stack) has a mass of 3700 kg. 3)
The secondary payloads on this flight are:
• DSX (Demonstration and Science Experiments) mission of AFRL
• GPIM (Green Propellant Infusion Mission), a demonstration minisatellite of NASA (~180 kg). 4)
• FalconSat-7, a 3U CubeSat mission developed by the Cadets of the U.S. Air Force Academy (USAFA) at Colorado Springs, CO.
• NPSat-1 (Naval Postgraduate School Satellite-1) of the Naval Postgraduate School, Monterey, CA. A microsatellite of 86 kg.
• OCULUS-ASR (OCULUS-Attitude and Shape Recognition), a microsatellite (70 kg) of MTU (Michigan Technological University), Houghton, MI, USA.
• Prox-1, a microsatellite (71 kg) of SSDL (Space Systems Design Laboratory) at Georgia Tech.
• LightSail-2 of the Planetary Society, a nanosatellite (3U CubeSat, 5 kg) will be deployed from the parent satellite Prox-1.
• ARMADILLO of UTA (University of Texas at Austin), a nanosatellite (3U CubeSat) of ~ 4 kg.
• E-TBEx (Enhanced Tandem Beacon Experiment), a tandem pair (3U CubeSats) of SRI International.
• TEPCE (Tether Electrodynamics Propulsion CubeSat Experiment), a 3U CubeSat (3 kg) of NPS (Naval Postgraduate School).
• CP-9 , a joint CP-9/StangSat experiment, which is a collaboration between PolySat at Cal Poly and the Merritt Island High School, and is sponsored by the NASA LSP (Launch Services Program). CP-9 is a 2U CubeSat while StangSat is a 1U CubeSat.
• PSat-2 (ParkinsonSAT), a student built 1.5U CubeSat of USNA (US Naval Academy) with a mass of 2 kg.
• BRICSAT-2, a student built 1.5U CubeSat of USNA (US Naval Academy) to demonstrate a µCAT electric propulsion system and carry a ham radio payload.
• OTB-1 (Orbital Test Bed-1) a minisatellite developed by SSTL (based on the SSTL-150 bus, 138 kg) and owned by General Atomics' Electromagnetic Systems Group (GA-EMS) of San Diego. One of the hosted payloads is NASA's DSAC (Deep Space Atomic Clock), a technology demonstration mission with the goal to validate a miniaturized, ultra-precise mercury-ion atomic clock that is 100 times more stable than today's best navigation clocks.
Figure 3: SpaceX's Falcon Heavy rocket, carrying LightSail-2 and 23 other spacecraft for the U.S. Air Force's STP-2 mission, lifts off from Kennedy Space Center on 25 June 2019 at 06:30 UTC (image credit: NASA)
The STP-2 mission will be among the most challenging launches in SpaceX history with four separate upper-stage engine burns, three separate deployment orbits, a final propulsive passivation maneuver and a total mission duration of over six hours. It will demonstrate the capabilities of the Falcon Heavy launch vehicle and provide critical data supporting certification for future National Security Space Launch (NSSL) missions. In addition, [the USAF] will use this mission as a pathfinder for the [military's systematic utilization of flight-proven] launch vehicle boosters.
The three orbits of the STP-2 mission for spacecraft deployment are:
1) The small secondary CubeSat satellites will be deployed into an elliptical orbit of ~300 x 860 km, inclination of ~28º. These are: OCULUS-ASR, TEPCE, E-TBEx, FalconSat-7, ARMADILLO, PSAT-2, BRICSAT, and CP-9/StangSat.
2) The second deployment batch of the STP-2 mission will occur at a circular altitude of 720 km and an inclination of 24º.
- Deployment of LightSail-2, Prox-1, and NPSat-1
- Deployment of OTB-1 with NASA's DSAC and GPIM
- The six FormoSat-7/COSMIC-2 satellites will be deployed into the initial circular parking orbit of 720 km. Eventually, they will be positioned in a low inclination orbit at a nominal altitude of ~520-550 km with an inclination of 24º (using their propulsion system). Through constellation deployment, they will be placed into 6 orbital planes with 60º separation.
3) The third and final deployment will be the Air Force Research Lab's DSX spacecraft as well as the ballast, which will be delivered to an elliptical MEO (Medium Earth Orbit) with a perigee of 6000 km and an apogee of 12000 km, inclination of 43º.
TEPCE mission status
• April 7, 2020: According to information of Jerome Pearson, President of Star Technology and Research, Inc. (Mount Pleasant, SC), TEPCE deployed on 17 November 2019 and re-entered on 1 February 2020. TEPCE actually did change its orbit slightly, but was limited by the power available (around 50 watts from solar cells on the body of the 3U CubeSat).
• November 4, 2019: In early November the Tether Electrodynamic Propulsion CubeSat Experiment (TEPCE), already in orbit, is set to make its move under the watchful gaze of telescopes on the Hawaiian island of Maui. The Earth-bound control team is waiting for an ideal 10-minute period at dawn or dusk, when the dim sunlight will offer the best possible view of the shoebox-size spacecraft involved. Once the crew triggers the process, TEPCE should separate into two identical minisatellites joined by a kilometer-long tether as thick as several strands of dental floss. If deployment goes smoothly, the mission can observe how the tether interacts with Earth's magnetic field in the ionosphere (where much of the space junk orbits) to change the satellites' velocity and orbit; the results could possibly enable future spacecraft to move around while orbiting Earth—without having to carry unwieldy chemical propellant. 5)
- "In other words, it is the sailing ship of space," says Enrico Lorenzini, a professor of energy management engineering at the University of Padova in Italy, who is not involved in the TEPCE mission. But instead of wind, the electrodynamic tether technology moves thanks to the physical laws that govern electric and magnetic fields. A tether in Earth's ionosphere—an upper atmospheric layer filled with charged particles such as free electrons and positive ions—can collect electrons at one end and emit them at the other, generating an electric current through itself. The electrified tether's interactions with Earth's magnetic field produce an impetus known as the Lorentz force, which pushes on the tether in a perpendicular direction.
- Several TEPCE components work together to create the necessary current. After separating, each of the two mini-satellites deploys a five-meter steel tape that can collect free electrons from the ionosphere. Each satellite also has a tungsten filament that uses power from onboard solar panels to heat up until it reaches a temperature at which it can emit those electrons. This produces a current that runs from the electron collector on one satellite through the tether to the electron emitter on the other.
- Because each tethered satellite contains both components, the current can flow in either direction—depending on which end of the tether is gathering electrons, and which is releasing them. When the current flows in one direction the Lorentz force pushes in a direction opposite to the spacecraft's motion, producing drag that eventually slows the satellite down and reduces its orbital altitude. Run the current in the opposite direction and the direction of the Lorenz force will reverse as well, creating propulsion instead of drag. The resulting increase in velocity could help maintain or increase orbital altitude, enabling more complex maneuvers without requiring any additional fuel.
- This mission will focus on testing the fundamental technology, rather than moving the satellites significant distances; the Lorentz force TEPCE generates will be small, due to the weak electric current involved and the reliance on solar panels—which barely supply enough power for an ordinary lightbulb. For this reason, mission planners do not expect ground-based radar and telescopes to have an easy time detecting small changes in the tether's velocity. So the team will work on measuring the tether's current flow and check positional data from GPS receivers on each satellite.
- "The actual maneuver to be able to demonstrate that the orbit actually changed requires a lot more current than we can flow," says Shannon Coffey, principal investigator for TEPCE at the Naval Research Laboratory in Washington, D.C. "So this is an experiment to show that we have all the parts working for an electrodynamic system."
- TEPCE's power (and goals) are limited by size—the team had to cram all the onboard electronics, sensors, tether deployment devices and other mechanisms into two identical satellites with a combined capacity of about 3,000 cubic centimeters, roughly half the volume of a soccer ball. By using such tiny satellites, often called CubeSats, mission planners kept launch costs low and gained more opportunities for TEPCE to ride into space as a secondary payload. TEPCE hitched a ride aboard a SpaceX Falcon Heavy rocket in June 2019.
- "TEPCE would demonstrate electrodynamic tether propulsion under extremely hard volume and weight constraints," says Gonzalo Sánchez-Arriaga, an aerospace engineer and astrophysicist at the University Carlos III of Madrid in Spain, who is not involved in the current mission. Sánchez-Arriaga notes that this technology is not new; the 1990s represented a "golden decade for tethers in terms of funding level and missions" conducted by space agencies, he says. Altogether, missions successfully deployed more than 65 kilometers of tethers, demonstrating they could produce both thrust and power. But funding and interest tapered off after NASA cancelled a particularly ambitious mission due to the tragic loss of the space shuttle Columbia in 2003.
- Now TEPCE is among a new wave of space-tether missions testing improved versions of the technology, and how it could help remove retired spacecraft. Sánchez-Arriaga is leading a separate project called Electrodynamic Tether technology for Passive Consumable-less deorbit Kit (E.T.PACK), funded by the European Commission. That mission aims to test how a tether, attached to future satellites, could deploy as a passive brake that brings down dead or decommissioned spacecraft at the end of their lives.
- A successful TEPCE demonstration could also pave the way for a larger tether spacecraft. The aerospace company Star Technology and Research, based in Mount Pleasant, South Carolina, is developing a craft called the ElectroDynamic Debris Eliminator (EDDE) that is designed to act like an orbital tugboat: it could grab dead satellites or space junk with nets and a mechanical device somewhat akin to a catcher's mitt. This company helped design the electron collector and emitter components for the TEPCE mission, and sees the current test as a precursor to EDDE. "This is basically a mini EDDE test because it's based on our EDDE spacecraft," says Jerome Pearson, an aerospace engineer and president of Star Technology and Research.
- Once the TEPCE tether deploys, the countdown toward the mission's end starts immediately. This is because the long tether will create additional drag that will lead the spacecraft to burn up in Earth's atmosphere within approximately 60 days. But if all goes well, TEPCE's sacrificial end could mark the beginning of a new era in decluttering Earth's orbit—and preserving humanity's access to space.
EDDE (ElectroDynamic Debris Eliminator)
An attractive option for active removal of space debris threatening low Earth orbit (LEO) assets is a concept referred to as the ElectroDynamic Debris Eliminator (EDDE). EDDE is a new type of conceptual space vehicle: it is a propellantless spacecraft that can operate in the Earth's ionosphere in LEO. EDDE is solar-powered, uses electric current in a long conductor to thrust against the Earth's magnetic field, and rotates slowly to stabilize itself. The rotation also lets EDDE push and pull against the Earth's magnetic field as desired, allowing controlled changes in all 6 orbital elements. The concept, illustrated below, uses a wide reinforced aluminum-foil tape as a 10-km-long electron collector and conductor. The solar arrays are distributed along its length to serve as "electron pumping stations" that limit peak voltages relative to the local plasma. The multi-amp current can be allowed to flow in either direction using hollow cathodes as electron emitters. 6) 7)
Figure 4: Electrodynamic Debris Eliminator Concept (image credit: NASA)
Figure 5: Propellantless electrodynamic propulsion (image credit: NASA)
An SBIR Phase III contract, entitled "Technology Maturation of the EDDE Vehicle," has been awarded to Star Technology and Research Inc. (STAR), to mature the EDDE hardware and operating concepts through detailed analysis and ground-based technology development. STAR will focus on reducing the main technical risks that affect the design, cost, mass, safety, control, performance, reliability, and survivability of the EDDE vehicle, and on maturing the technology associated with safe and reliable deployment, orbit transfers, collision avoidance, and rendezvous.
Plans include collaborating with NRL on a CubeSat (TEPCE) mission and evaluating secondary launch opportunities for a TetherSat. In parallel to the EDDE investigation, engineers at the Marshall Space Flight Center are also performing studies on using a electrodynamic tether for space debris removal; the title of this task is Propulsion Using Electrodynamics (PROPEL). As part of the Clean Space project potential debris capture devices and mechanisms will be identified and assessed through separate studies planned in the near future.
1) "NRL's TEPCE Spacecraft Undergoes Successful Deployment Test," NRL. May 19, 2010, URL: http://www.nrl.navy.mil/media/news-releases/2010
2) "Payloads deployed by SpaceX to study space weather and spacecraft propulsion," NRL, 25 June 2019, URL: https://phys.org/news/2019-06-payloads-deployed-spacex-space-weather.html
3) Stephen Clark, "Falcon Heavy launches on military-led rideshare mission, boat catches fairing," Spaceflight Now, 25 June 2019, URL: https://spaceflightnow.com/2019/06/25
4) "GPIM Spacecraft to Validate Use of 'Green' Propellant," NASA, Aug. 19, 2014, URL: http://www.nasa.gov/content/gpim-spacecraft-to-validate-use-of-green-propellant/
5) Jeremy Hsu, "Kilometer-Long Space Tether Tests Fuel-Free Propulsion," Scientific American, 4 November 2019, URL: https://www.scientificamerican.com
6) Drew Hope, "Clean Space," NASA Space Technology, 30 January 2020, URL: https://gameon.nasa.gov/archived-projects-2/clean-space/
7) Jerome Pearson, Eugene Levin, John Oldson, Joseph Carroll, "ElectroDynamic Debris Eliminator (EDDE): Design, Operation, and Ground Support," 2010, URL: https://www.amostech.com/TechnicalPapers/2010/Posters/Levin.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).