A Japanese led international team is developing a suborbital test of orbital-motion-limited (OML) bare wire anode current collection for application to electrodynamic tether (EDT) propulsion. The tether is a tape with a width of 25 mm, thickness of 0.05 mm, and is 300 m in length. This will be the first space test of OML theory. The mission will launch in the summer of 2009 using an S520 Sounding Rocket. During ascent, and above approx. 100 km in attitude, the tape tether will be deployed at a rate of approx. 8 m/s. Once deployed, the tape tether will serve as an anode, collecting ionospheric electrons. The electrons will be expelled into space by a hollow cathode device, thereby completing the circuit and allowing current to flow. The total amount of current collected will be used to assess the validity of OML theory. This paper will describe the objectives of the proposed mission, the technologies to be employed, and the application of the results to future space missions using EDTs for propulsion or power generation

Fujii, H. A.

Johnson, Les

Sanmartin, J. R.

NASA Technical Reports Server

FORTISSIMO: A JAPANESE SPACE TEST OF BARE WIRE ANODE TETHERS 
 
L. Johnson 
NASA George C. Marshall Space Flight Center 
Huntsville, Alabama 
c.les.johnson@nasa.gov 
 
H.A. Fujii 
Tokyo Metropolitan Institute of Technology 
Tokyo, Japan 
 
J.R. Sanmartin 
Universidad Politecnica de Madrid 
Madrid, Spain 
 
ABSTRACT 
 
A Japanese led international team is developing a suborbital test of orbital-motion-limited (OML) 
bare wire anode current collection for application to electrodynamic tether (EDT) propulsion. The tether is 
a tape with a width of 25 mm, thickness of 0.05 mm, and is 300 m in length. This will be the first space 
test of OML theory. 
The mission will launch in the summer of 2009 using an S520 Sounding Rocket. During ascent, 
and above ≈100 km in attitude, the tape tether will be deployed at a rate of ≈8 m/s. Once deployed, the 
tape tether will serve as an anode, collecting ionospheric electrons. The electrons will be expelled into 
space by a hollow cathode device, thereby completing the circuit and allowing current to flow. The total 
amount of current collected will be used to assess the validity of OML theory. 
This paper will describe the objectives of the proposed mission, the technologies to be employed, 
and the application of the results to future space missions using EDTs for propulsion or power generation. 
 
NOMENCLATURE 
 
Ie electron current 
Ii ion current 
Lb length of conductive beam 
mi ion mass 
N plasma density 
wb width of conductive beam 
 supply voltage 
 yield per unit bias (electrons per kV and impacting ion) 
 
INTRODUCTION 
 
Selected by the Japan Aerospace Education Agency (JAXA) in 2007 for flight in 2009, a team  
of researchers led by Professor Fujii at Tokyo Metropolitan Institute of Technology will develop and fly  
a sounding rocket payload that, for the first time, will demonstrate bare wire electron collection by an 
electrodynamic tether (EDT) in space.1 Data sufficient to validate the performance of the bare wire 
anode’s performance as predicted by orbital motion limited (OML) theory will be obtained. In addition,  
the overall current collection efficiency of the tether system will be used to update performance models  
of future EDT propulsion and power systems being considered for space application by NASA, JAXA,  
and others. 
 
https://ntrs.nasa.gov/search.jsp?R=20090014062 2019-08-30T06:29:19+00:00Z
RESULTS AND DISCUSSION 
 
 
EXPERIMENT OBJECTIVES 
 
 The objectives of the experiment are summarized in Table 1. 
 
Table 1.  The engineering and science objectives of the Fortissimo Space Tether Experiment. 
 
Objective Comment 
Fast deployment of a bare EDT Engineering: Deploy a bare, conductive tape tether in space
Fast ignition of a hollow cathode Engineering: Ignite a hollow cathode in space for 180 sec 
Dynamics of tethered space 
robot 
Engineering: Analyze a tethered robot system for a 
deployment/retrieval maneuver 
Verification of EDT operation in 
ionospheric plasma 
Science: Eject electrons from an ignited hollow cathode and 
collect electrons by using a bare EDT 
Verification of OML theory Science: Measure electron collection by a positively biased 
boom and a negatively biased bare EDT 
 
The rocket will first deploy a 300-m aluminum tape tether within minutes of launch. The bias 
voltage of the vertically deployed tether, which results just from its orbital motion through Earth’s magnetic 
field, is positive with respect to the ambient plasma at the top and negative at the bottom. The tether 
generates and forms part of a unique type of electrical circuit, which has been successfully demonstrated 
in space by flights of the plasma motor generator (PMG) in 19932 and the Tethered Satellite Systems 
(TSS-1 and TSS-1R) in 1992 and 1996.3 Both missions deployed long conducting tethers from orbiting 
spacecraft and successfully generated a current. The tethered system extracts electrons from the 
ionospheric plasma at one end (upper or lower, depending upon the deployment direction and intended 
thrust motion) and then carries them through the tether to the other end, where they are returned to the 
plasma. The circuit is completed by currents in the plasma.4 
The efficiency with which electrons are collected is predicted by a theory, which applies when 
their Debye length is much greater than the object’s size—as it can also be stated, when the object is 
small with respect to its sheath size. This theory is known as OML.5 The uninsulated tether will collect 
electrons along part of its length. The efficiency with which is collects will be used to determine the validity 
of OML predictions. 
  
EXPERIMENT IMPLEMENTATION 
 
After launch, the sounding rocket will achieve a maximum altitude of 300 km. During ascent, a 
round conductive boom with a diameter of 1 cm and length of 20 m will be deployed, followed by the  
300-m tape tether. The overall mission timeline is shown in Figure 1.  
 
 
 
 
 
 
Figure 1. Mission timeline from launch at T = 0. 
 
At the first stage of the experiment, the positive terminal of power supply is connected to the 
conductive boom with length, Lb, and the negative terminal of power supply the tape tether with width, wb, 
(Fig. 2). Electrons collected by the boom cross supply to tape, where they leak at the rate of ion impacts 
plus secondary yield. ( ) ( ) (1 )e iI boom I tape    where ionkVe /15.0~  and the following 
equation is obtained: 
 
 .)1(2
2)1(22 
  
i
t
t
e
bb m
ewLeN
m
eRLeN  (1) 
 
 
The plasma density will vary rapidly as the rocket ascends and descends, and the experiment  
will be conducted over six different voltages every 2 sec for a total of 12 sec: 0.5, 0.6, 0.7, 0.8, 0.9,  
and 1.0 kV. The maximum expected current will be 0.1 A.  
 
During the next stage of the experiment, the negative terminal of the power supply will be 
switched to being connected to the hollow cathode, and the tape tether will be connected to the positive 
terminal (Fig. 3). Electrons collected by the tape tether will cross the supply and be ejected at the 
cathode. OML predicts the following: 
 
 Ie (tape)  eNLt 2wt
2e
me
. (2) 
 
 
The supply voltage, , sweeps across a range of values from 100–1,500 V during this phase. 
Each sweep will be obtained by either varying the voltage supply or across embedded resistors and will 
take 20 sec.  
 
 
 
Figure 2. Configuration for testing OML theory without the use of an active plasma 
contactor. The boom and tether are biased positive and negative, respectively. 
 
 
 
 
 
Figure 3. Configuration for testing OML with the use of a plasma contactor. 
 
EXPERIMENT HARDWARE 
 
Instruments onboard the rocket will include Langmuir Probes (for plasma diagnostics), a 3-axis 
magnetometer (to measure the local magnetic field), accelerometers, and an ammeter (to measure the 
current at the onboard power supply). 
 
The first objective of the experiment is to fully deploy the tape tether using a “new” deployment 
scheme that is actually derived from a very common technique used by firefighters in paying out very long 
firehoses. The tape will be folded and stacked into a box with an opening on one end—resembling a 
tissue box (Fig. 4). A spring will eject an endmass attached to the rocket by the tether. The tether will be 
deployed to its full 300-m length within 120 sec. Three springs, with a total energy of 60 J, will kick off the 
subsatellite with an initial velocity of 4 m/sec. Braking is accomplished by applying a coating to the last 
several meters of the tether, increasing its deployment friction. 
 
                                                            
Figure 4. Concept drawing showing the 
Fortissimo tape tether stored and being 
deployed from an open-ended box. 
 
Colorado State University is contributing a fast-staring hollow cathode to the experiment. Hollow 
cathodes, which are devices that emit electron current into the plasma, have flown many times in space; 
but these heritage cathodes typically require both a lot of power and significant pre-ignition conditioning 
before operation can commence. The Colorado State team has examined the cold start of the cathode 
and has found a way to reduce the conditioning time significantly by paying special attention to its storage 
environment. 
 
ANTICIPATED RESULTS AND APPLICATIONS OF THE DATA 
 
EDT thrusters work by virtue of the force a magnetic field exerts on a wire carrying an electrical 
current. This phenomenon was first observed by Ampere, one of the pioneers in the study of electromag-
netic phenomena, around 180 years ago. The details of the force, which acts on any charged particle 
moving through a magnetic field, (including the electrons moving in a current-carrying wire) were 
concisely expressed by Lorentz in 1895 in an equation that now bears his name. The force acts in a 
direction perpendicular to both the direction of current flow and the magnetic field vector. Electric motors 
make use of this force: a wire loop in a magnetic field is made to rotate by the torque the Lorentz Force 
exerts on it due to an alternating current in the loop times so as to keep the torque acting in the same 
sense. The motion of the loop is transmitted to a shaft, thus providing work. Michael Faraday demon-
strated the first simple electric motor in 1821.  
Although the working principle of EDT thrusters is not new, its application to space transportation 
may be significant. In essence, an EDT thruster is just a clever way of getting an electrical current to flow 
in a long orbiting wire (the tether) so that the Earth’s magnetic field will accelerate the wire, and conse-
quently the payload attached to the wire. The direction of current flow in the tether, either toward or away 
from the Earth along the local vertical, determines whether the magnetic force will raise or lower the orbit.  
The bias voltage of a vertically deployed metal tether, which results just from its orbital motion 
(assumed eastward) through Earth’s magnetic field, is positive with respect to the ambient plasma at the 
top and negative at the bottom. This polarization is due to the action of the Lorentz force on the electrons 
in the tether. Thus, the “natural” current flow is the result of negative electrons being attracted to the 
upper end and then returned to the plasma at the lower end. The magnetic force in this case has a 
component opposite to the direction of motion, and thus leads to a lowering of the orbit and eventually to 
reentry. In this “generator” mode of operation the Lorentz force serves both to drive the current and then 
to act on the current to decelerate the system. This operational mode was thoroughly verified to work in 
space by the TSS and PMG missions described above, but no measurements were made to quantify the 
resulting small orbital changes.  
One of the most important features of tether thrusters is that they use renewable energy sources 
to drive the electrical current flow in either the orbit-raising or orbit-lowering modes. Sources inherent to 
Earth orbit are used. To raise the orbit, sunlight can be converted to the electrical energy required to drive 
the tether current. To lower the orbit, the orbital energy itself (supplied by the Earth-to-orbit launcher when 
it raises the system into orbit) is the energy source of the tether current via the action of the Lorentz force. 
EDTs can be directly applied to a wide spectrum of uses in space. As a propulsion system, they 
include satellite de-orbit, transfer of a satellite from one orbit to another, altitude maintenance for large 
spacecraft such as the International Space Station (ISS), and—since it works wherever there is a 
magnetic field and an ionosphere—planetary exploration missions.  
Space is becoming increasingly cluttered with debris, from old satellites and rocket stages to bits 
of trash thrown overboard by early space travelers, and near-Earth space is becoming filled with collision 
hazards to current and future satellites. To mitigate the growth of this problem, a requirement that 
spacecraft de-orbit themselves after completion of their mission is being considered by various govern-
ments. One method to do this would be to carry extra propellant so that the satellite can thrust to reentry. 
This requires a large mass of propellant, and every kilogram of propellant that must be carried reduces 
the weight available for payload. Moreover, it requires that the propulsion system must be functional after 
sitting in orbit for 10 years or more. EDTs provide a lower mass and potentially more reliable means of 
bringing old satellites out of orbit. A tether system could reside in a small package bolted to the satellite. 
When the end of the satellite's useful life is reached, a conducting tether several kilometers long would be 
deployed from the satellite, and the EDT propulsion orbit decay process would begin. The orbital decay 
process can be very rapid. Calculations indicate that a tether weighing as little as 2% of the satellite 
weight can bring a satellite out of some orbits in just a few weeks. (Compare that to centuries without any 
de-orbit system.) 
An EDT upper stage could be used as an orbit transfer vehicle (OTV) to move payloads within 
low-Earth orbit. The OTV would rendezvous with the payload and launch vehicle, grapple the payload, 
and maneuver it to a new orbital altitude or inclination without the use of boost propellant. The tug could 
then lower its orbit to rendezvous with the next payload and repeat the process. Conceivably, such a 
system could perform several orbital maneuvering assignments without resupply, making it relatively 
inexpensive to operate. However, due to the rapidly diminishing plasma density around the earth, the tug 
would be limited to operation at altitudes of less than 2,300 km.  
Outfitting the International Space Station (ISS) with an electrodynamic reboost tether might 
eliminate the most critical and constraining dependency on Earth—propellant resupply. The ISS can 
supply its own power but not its own propellant.  
A concept design for an EDT thruster capable of delivering 0.5–0.8 N of thrust to the ISS at a cost 
<10 kW of electrical power consists of a 10-km-long aluminum tether in the form of a thick ribbon (0.6 mm 
 10 mm).6 Despite its length, the tether would weigh only about 200 kg. The tether reboost system would 
operate continuously and virtually eliminate the need for expensive resupply missions to keep the station 
in orbit. Over its 10-yr life, considerable cost savings might result. However, many questions, particularly 
those associated with crew safety, must be resolved before such a system can be placed on the space 
station. For example, a method for preventing recoil of a tether accidentally severed by a micrometeoroid 
orbital debris particle must be developed to mitigate the possibility of the tether wrapping itself around the 
station or one of its critical components.  
Perhaps the most exotic use of the technology would be to provide propulsion and power to 
spacecraft exploring the outer planets. The environment of the Jovian system has properties that are par-
ticularly favorable for use of an EDT. Specifically, the planet has a strong magnetic field and the mass of 
the planet dictates high orbital velocities, which when combined with the planet’s rapid rotation rate, can 
produce very large relative velocities between the magnetic field and the spacecraft. In a circular orbit 
close to the planet, tether propulsive forces are found to be as high as 50 N, and power levels are as high 
as 1 megawatt.7 With current spacecraft being extremely limited in power consumption due to their dis-
tance from the Sun (typically using less than 100 W), this level of available power could make available a 
whole new suite of science instruments such as high-power radar. However, the power levels are so high 
that power conversion, energy dissipation, and tether temperature become issues that must be addressed. 
 
SUMMARY AND CONCLUSION 
  
The planned sounding rocket test of OML theory will be sufficient to validate bare wire tether 
anodes for future mission applications. In addition, a new type of tether deployment will be tested, 
possibly leading to a viable, survivable alternative to wire-type tethers used in past space missions.  
 
REFERENCES 
 
1.  Fujii, H.A.; Takegahara, H.; Oyama, K.; et al.: “A Proposed bare-tether experiment on board a 
sounding rocket,” AIAA 2004–5718, 2nd International Energy Conversion Engineering Conference, 
Providence, RI, August 16, 2004. 
 
2.  McCoy, J.E.; Grossi, M.D.; and Dobrowolny, M.: “Plasma Motor Generator (PMG) Flight Experiment 
Results,” Proceedings of the Fourth International Conference on Tethers in Space, Smithsonian 
Institution, Washington, D.C., 1995, pp. 57–82. 
 
3.  Strim, B.; Pasta, M.; and Allais, E.: “TSS–1 Vs. TSS–1R,” Proceedings of the Fourth International 
Conference on Tethers in Space, Smithsonian Institution, Washington, D.C., 1995, pp. 27–41. 
 
4.  Stone, N.H.; Wright, K.; Winningham, J.D.; et al.: “Identification of Charge Carriers in the Ionospheric 
Branch of the TSS-1 Tether Generated Current System,” Proceedings of the Fourth International 
Conference on Tethers in Space, Smithsonian Institution, Washington, D.C., 1995, pp. 359–371. 
 
5.  Sanmartin, J.R.; Lorenzini, E.C.; Estes, R.D.; et al.: “Analysis of ProSEDS Test of Bare-Tether 
Collection,” 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Huntsville, AL,  
July 20–23, 2003. 
 
6. Dankanich, J.; Bonometti, J.; Sorenson, K.; et al.: “Free Re-boost Electrodynamic Tether for the 
International Space Station,” AIAA–2005–4545, 41st AIAA/ASME/SAE/ASEE Joint Propulsion 
Conference and Exhibit, Tucson, AZ, July 10–13, 2005.  
 
7.  Gallagher, D.L.; Johnson L.; Moore, J.; et al.: “Electrodynamic Tether Propulsion and Power 
Generation at Jupiter,” NASA/TP—1998–208475, Marshall Space Flight Center, AL, 
<http://mtrs.msfc.nasa.gov/mtrs/98/tp208475.pdf>. 
 
 
GLOSSARY 
 
EDT electrodynamic tether 
ISS International Space Station 
JAXA Japan Aerospace Education Agency 
OML orbital-motion-limited 
OTV orbit transfer vehicle 
PMG plasma motor generator 
TSS Tethered Satellite System 
 


Fortissimo: A Japanese Space Test Of Bare Wire Anode Tethers

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