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TEPCE (Tether Electrodynamics Propulsion CubeSat Experiment)

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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.

1) "NRL's TEPCE Spacecraft Undergoes Successful Deployment Test," NRL. May 19, 2010, URL:

2) "Payloads deployed by SpaceX to study space weather and spacecraft propulsion," NRL, 25 June 2019, URL:

3) Stephen Clark, "Falcon Heavy launches on military-led rideshare mission, boat catches fairing," Spaceflight Now, 25 June 2019, URL:

4) "GPIM Spacecraft to Validate Use of 'Green' Propellant," NASA, Aug. 19, 2014, URL:

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 (

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