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AiAA A01-34592

AIAA-2001-3980

Electrodynamic Tether Optimization for theSTEP-AIRSEDS Mission

J. Van Noord and R. SturmfelsThe Michigan Technic CorporationHolland, Michigan, USA

37th AIAA/ASME/SAE/ASEE Joint Propulsion Conferenceand Exhibit

8-11 Jul 2001Salt Lake City, Utah

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AIAA-2001-3980

ELECTRODYNAMIC TETHER OPTIMIZATION FOR THE STEP-AIRSEDS MISSION

Jonathan Van Noord*and Rich SturmfelsttThe Michigan Technic Corp.

1121 Ottawa Beach Road, Suite 200, Holland, MI 49424

Abstract

In order to produce an optimal electrodynamic tetherseveral factors need to be considered while designingfor a specific mission. This paper presents thedesign process being used for the electrodynamictether used on the STEP-AirSEDS mission. Thedesign factors considered include tether survivabilityfrom micrometeoroid and orbital debris, atmosphericdrag, current collection, thrust produced, tetherresistance, tether strength, mass, manufacturability,tether thickness, and coatings. The coatingsinfluence the tethers ability to collect electrons,insulate from the ionosphere, and temperature. Thecoating also must withstand the atomic oxygenenvironment present in the ionosphere. The STEP-AirSEDS tether design has iterated on these designfactors and preliminary designs are undergoingtesting and application of potential coatings.

Electrodynamic Tether Background

Electrodynamic tethers produce thrust for satellitesusing the force produced when a current runsthrough a wire in a magnetic field. A deceleratingforce on the tether is produced when a current flowsup and in a direction away from earth. However, if acurrent is forced to flow down or towards the earthby means of solar panel, batteries or some othersource, an accelerating force is produced.

For deboost, a simple conducting tether withoutinsulation is used to collect electrons from theionosphere. The tether naturally biases positive andso current is induced from the wire passing throughthe earth's magnetic field.

For a boost, this induced voltage must be overcomeby the use of an external voltage source, such as a

battery or solar panels. The optimal boost tetherconfiguration consists of an insulated tether attachedto a collecting tether below it.1 This is due to thefact that regardless of the presence of the insulatedportion, about the same length of collecting tetherwill be used. This added insulated portion allows forthe maximum amount of current to flow through alonger length of tether.

The most efficient current collection results from acollecting tether with width, thickness or radius lessthan a debye length. These thin collectors have themaximum amount of current collection per surfacearea.1 However, there are other issues that drive atether design, including tether survivability. TheSTEP-AIRSEDS mission has a year operationalrequirement.2 The International Space Station couldbenefit greatly from a tether with a 10-year lifetime.3The survivability of a tether from micrometeoroidsand debris for these lifetimes requires a tether widthon the order of a few centimeters.4 The best tetherfor both collecting current and increasingsurvivability seems to be one with a rectangular crosssection that is thin and wide, also known as a tapetether. The design of this tether is presented here.

Tether Design Flow

The STEP-AirSEDS tether design flow wasinfluenced greatly by taking into account the largestperceived risks to completing a yearlongelectrodynamic tether mission. One of the greatestthreats to the mission is risk of being struck by amicrometeoroid or orbital debris (M/OD). Typicalelectrodynamic tethers to date are on the order ofmillimeters in width. One strike anywhere on thetether would likely be a mission-ending event. Thedesign flow starts from that issue and is shown inFigure 1.

Project Engineer, AIAA MemberProject Engineer, AIAA Member

Copyright © 2001 by Jon Van Noord. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission.

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The tether design factors are given in bold and in theboxes. The design implications that resulted fromeach factor are given in the arrows. Currently threedesigns are being considered and testing has begunto evaluate these designs. The following sectionswill describe each design factor and a summary ofthe results determined for each one.

• M/OD Survivability

Wider

r •Atmospheric Drag•CurrentCollection(thrust)

V* _____ . _____ J

—— 1\• Porous•Shorter• Raise InitialAltitudei —————— . i

• Manufacturing

3Candidates

Coating(Thermal, AO,

Insulate)

Figure 1 - STEP-AirSEDS Tether Design Flow

M/OD Survivabilitv

It is required that the STEP-AirSEDS tether have atleast a 95% chance of surviving M/OD impacts on aone-year mission. The STEP-AirSEDS mission isplanned to orbit in the 400-700 km altitude range. Arepresentative cumulative particle flux is shown inFigure 2. It is important to note that this figuredepicts the cumulative particle flux that includes theparticles of a certain diameter and larger.

•3 l.E+02| £^ 1 l.E+00§ t-1 I l.E-02o —

1 1 l.E-04.S I1 .3 l.E-06£ Q

Orbital Debris

Meteoroid

1 .E-08 ————————-——0.001 0.01 0.1 1 10 100

Particle Diameter, cm

Figure 2 - Meteoroid and Orbital DebrisParticle Flux for Year 2004, 570 km altitudeand 51.6 degree inclination

Based on this M/OD information analysis can beperformed to estimate the chance that a tether of aparticular width and length will be cut. Figure 3depicts the sever risk determined for tape tethers(rectangular cross section).

Initially, the STEP-AirSEDS tether was designed tobe 10 km long. It can be seen that tethers with awidth less than 1 cm involve significant risk thatthey will be severed in a 1 year time span. It canalso be seen that above 2 cm little improvement isgained. It should be noted that most of the objectsabove 2 cm in diameter are orbital debris as shownin Figure 2. It should also be noted that the largerorbital debris is tracked and the satellite couldpotentially be moved to avoid these larger objects.This would increase the probability of surviving

Based on the analysis done for the STEP-AirSEDSmission, the long life electrodynamic tether shouldbe at least 2 cm in width.

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Figure 3 - M/OD Sever Risk for a Tether 0.015cm thick based on flux depicted in Figure 2

Atmospheric Tether Drag

Since relatively wide tethers are essential for anymission of considerable lifetime, drag must beconsidered. A tape tether that is 2.5-cm wide and 10km long has a cross-sectional drag area of 250 m2.This is a significant area, especially at low altitudes.However, to be accurate in estimating the drag area,twisting must be accounted for. The twisting can beaccounted for by multiplying the drag area by afactor of 2/Ti to give approximately 159 m2. This isstill a considerable area, especially when comparedto typical satellites that are on the order of single totens of square meters.

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Using standard atmospheric drag equations, thelifetime of a satellite with no thrust for reboost canbe calculated.5 The results support the fact that dragis a significant factor. Figure 4 shows that, at solarmax for varying tether widths, a non-operatingdeployed tether at 400 km altitude has 1 to 10 daysbefore falling back to earth. The corresponding dragforces for different altitudes and tether widths areshown in Figure 5. The thrust provided by the tethermust be greater than the values in Figure 5. The dragforce can be minimized by reducing the area crosssection by shortening the tether, reducing the width,or increasing the porosity of the tether.

100000Altitude

0.005 0.01 0.015 0.02 0.025Tether Width (n*

0.03 0.035 0.04

Figure 4 - Satellite lifetime for various tetherwidths (10 km long) at various altitudes

tether will not operate. This can be accomplished in3 primary ways:

1. Use a porous tether instead of a solidtether.

2. Shorten the tether by only using the lengthof tether required.

3. Start at the highest initial altitude that theprimary rocket will allow and dip to lowerorbits after the system has been proven out.

Calculating Current Collection for Wide Tethers

Orbit motion limited (OML) theory was used tocalculate the current collection for the range of tetherwidths presented here. Even though the originalbounds of the theory were only a few debye lengths,(A,D's) OML theory has been shown to be reasonableout to 15 XD's for tape tethers.6 The XD is expectedto be from 2.5-9 mm/ This would make the tethersevaluated here with widths of 0.1-5.0 cm to be 0.1-20 XD. Further testing of current collection isplanned for larger A,D*s. Drag was incorporated inthese calculations by using the 2/n area adjustmentfor a twisting tether.

Boosting for Tethers of Various Widths

5 km Insulated plus 5 km Collecting Tether

A 10 km long tether that is 50% insulated has beenshown to be a good design for thin tethers (-1.8mm).7 However, drag becomes a considerable factoras the tether width increases. Figure 6 shows theorbit transfer time for a 1 kW, 10 km long, and 50%insulated tether to travel from 400 to 600 km.

§26

220

14

700 750 1.5 2

Tether Width (cr3 3.5 4

Figure 5 - Drag Force for a 10 km long tether atvarious widths and altitudes

It is therefore critical to minimize drag to allow forextra time to diagnose a problem in the event the

Figure 6- 400 to 600 km Orbit Transfer Time forTethers of Varying WidthsIt is important to note that the resistance of the tethervaried. The 2.5-cm wide tether had a resistance of15 Q/km. The resistance at other widths wasadjusted based on the change in area cross section.This is a significant reason why the orbit transfer

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time decreases as the tether width initially increases.However, for a tether width around 1.0 cm, dragbecomes the predominant factor such that the timeincreases and the orbit becomes more elliptical asdemonstrated in Figure 7. Wider tethers thenapproach an altitude where the drag cannot beovercome and their orbit decays back to earth.

0 2 4 6 8 10 12 14Mission Timeframe (days)

Figure 7 - 2.5 cm Wide 10 km Long TetherStarting at 400 km

Figure 9 shows the neutral point for a 2.5 cm widetether operating at 1000 W with 5 km of insulatedtether. The top of the y-axis is at 5-km from theupper unit where the collecting tether begins. Thebottom of that axis is 6-km from the upper unit andwhere the collecting tether ends. The x-axis is thetime from start of mission with the day/hour. Figure9 also shows that for the 2.5 cm case, less than 1 kmof bare tether is needed to collect the necessarycurrent since the neutral point is rarely locatedbeyond 1 km.

Neutral Voltage Position

In order to minimize the effect of drag, it isimportant to deploy the minimum amount of tetherrequired. In a boosting scenario for a fixed-powertether, only a portion of the bare tether is needed tocollect electrons. Figure 8 depicts the location of theneutral voltage point. This is the point at which thetether above it collects electrons and the tether belowit collects ions.1 The portion below the neutral pointreduces the amount of current sent through theinsulated tether, reducing thrust.

Col led inn electrons

Collectinn ions

Insulated

Neutral voltage point

Figure 9 - Position of Neutral Voltage along a 2.5cm wide ribbon tether over a period of 14 hoursduring a simulated STEP-AIRSEDS maneuver.The y-axis indicates location along the tether. Theinsulated tether is above the -5000 m point andthe collecting tether extends below the -5000 mpoint.

The variation of the location seen is due to thechanging magnetic field and ionosphere densitiesthroughout each orbit. This ability to compensate forvarying conditions and still provide the necessarycurrent is one of the reasons why an electrodynamictether is attractive. Yet it is important to not haveexcessive collecting tether to minimize drag and ioncollection.

The estimated length needed to collect the electronsa majority of the time for the 2.5-cm wide tethershown in Figure 9 is 700 meters. Figure 10 showshow this estimated length changes with width for atether with 5km of insulating tether and a systemrunning at 1 kW. The lengths required for widetethers are only a fraction of what is needed for thetethers previously considered with widths of only afew millimeters.

Figure 8 - Tether Neutral Voltage Point.

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2 3

Tether Width (cm)

Figure 10 - Estimated length of tether needed tocollect electrons with 5 km of insulated tether in a1 kW system for altitudes up to 700 km.

5 km Insulated plus Varying Collecting TetherLengths

It would be more realistic to compare how tethers ofvarying widths perform by only adding the collectingtether lengths that are necessary. Figure 11 showsthe orbit transfer time for tethered satellites to travelfrom 400 to 600 km. They all have 5 km ofinsulated tether and the length of the collectingtether for each width corresponds to the lengthscalculated and shown in Figure 10.

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Tether Width (cm)

Figure 11-400 to 600 km Orbit Transfer Timefor Tethers of Varying Widths and CollectingLengths

By using shorter length collecting tethers, the drag isreduced and the performance is increased for tetherslarger than 1 cm in width. The lower drag alsocreates less elliptical orbits. The time between thesatellite first entering the orbit at 600 km and thetime that all of the orbit is at 600 km is noticeablyless in Figure 11 than in Figure 6. An orbit is moreelliptical as the time increases between entering orbitat 600 km and fully arriving at 600 km.

It is also evident that tether widths larger than 2 cmfor this tether scenario do not significantly improvethe performance. To increase the performance ofthese wider tethers, it is essential to minimize thedrag on the tether even further.

Deboost

A short collecting tether is desirable for boostoperations since the current flows through theinsulated tether to produce a significant amount ofthe thrust. However, in the deboost case, theinsulated tether is only contributing drag and nocurrent is flowing through is. Therefore, the thrustand the amount of current drawn by the collectingtether during deboost is highly dependent upon thecollecting tether length. Three deboost cases will beexamined here. These cases are derived at solar maxand have a tether width of 2.5 cm and a 5 km lengthof insulated tether. The three cases have differentcollecting lengths: 1 km, 3 km, and 5 km.

Table 1 gives the deboost time for the variouslengths. From this table it is evident that the lengthof the collecting portion greatly affects the deboosttime. It is important to note that this is only usingthe collecting portion of the tether, so the insulatedportion remains unutilized. One way to increase thedeboost performance would be to collect electrons onthe upper unit using a collecting body and run acurrent through the insulated portion. This iscurrently under investigation.

Table 1 - Deboost TimeBare Tether Length

(km)135

Deboost Time from 700to 300 km (days)

-50~7

Work is also under way to evaluate how well poroustape tethers can collect current8. A porous tapestructure offers the possibility of collecting similaramounts of electrons with a lower atmospheric dragcross section.

The research regarding the current collection andboost/'deboost transfer times indicates that for widetethers only a fraction of collecting length is needed.A collecting body might aid the shorter collectingtether length to decrease deboost transit times.Porous tape tethers are another possibility forreducing the drag and yet maintaining the currentcollection necessary.

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Material Properties

The key material properties of the tether areelectrical resistance, strength, and mass. The goalsare to have a tether that has a resistance of no morethan 15 Q/km. Decreasing the resistance beyondthis level does not significantly improve theperformance of the tether. The tether also mustwithstand any tension spike that might result duringthe mission. While the nominal peak loads expectedare around 10 N, it is possible that the tether couldexperience a 100 N load. It is desired for the tetherto not exceed 100 kg, so a material with minimalmass is desired. It will also be important that thetether have good ductility since it could be woundand unwound multiple times.

Table 2 lists several of the materials considered forthe construction of the tether. A resistance of 15Q/km was used as a baseline to compare all thematerials. Aluminum 1350-O can be seen to be themost lightweight material to achieve the resistancerequired.

Table 2 - Potential Tether Materials and Alloys

v ^ , Stress (MPa)

18 23 28j33 38 43 48 53 58 63^6815

Material

CopperSilverGold

Al 5056A15154A16061AlSxxx

Al 1350-0

Density(g/cc)

8.910.519.32.72.72.7

2.712.71

Resistance/length(ft/km)

1515151515151515

Mass/length(kg/km)

10.111.228

10.59.66.65.15.0

Aluminum 1350-O is not only a good lightweightlow electrically resistance material, it also has goodstrength characteristics. The curve in Figure 12indicates for a certain tether cross section using AL1350-O the level of stress (for a 100 N load), themass per length and the resistance per length. Theshaded area of the graph indicates the region wherethe tether is either too heavy or the resistance is toohigh.

Aluminum 1350-O has been identified as a materialthat meets the desired specifications for strength,electrical resistance and weight.

Manufacturing

The manufacturability of a tether is a criticalcomponent. This greatly affects the cost, time of

5 6 7 8 91011121314151617181920Resistance (Ohm/km)

Figure 12 - AL 1350-O Design Space Assuming a100 N Loadproduction, quality and feasibility of a tether design.For proprietary reasons, this category will not beextensively evaluated here.

Certain manufacturing problems that have occurredinclude not being able to manufacture some designsthin enough.. The thickness is a significant factorfor storing the tether on board before deployment.Fractions of a millimeter in thickness translate intosignificant diameter changes for 10 km of spooledtether.

Other issues that have arisen include some processesthat exceed the yield strength of the material. Thedesigns are also tied to the ability to coat the tetherdesigns.

Currently, candidate designs have been identifiedand manufactured by The Michigan TechnicCorporation.

The coatings on a tether have three primaryfunctions. These are to provide protection fromatomic oxygen (AO), electrical insulation (orconduction for the collecting portion), and thermalcontrol. More specifically, the properties desiredare:

• Withstand AO front fluence of 1.4 x 1022

atoms/cm2

• The insulating coating should have aminimal dielectric breakdown voltage of4,000 V after 1-year exposure at 450-kmaltitude.

• The conducting coating should have amaximum bulk resistance of 1x108 Q*cm.

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• Thermal optical properties with a solarabsorptivity over emissivity (o/s) of lessthan 1 with a minimum emissivity of 0.2.

• It must withstand a temperature range of-150°Cto+150°C.

• It must withstand multiple bending cycles(100) around a 10 cm diameter spool.

• For porous structures it must maintainopen ings/voids and coat uniformly.

To date there have been no coatings developed for amission of this level, but there have been coatingsdeveloped for previous missions. The ProSEDSmission used COR/PANi conductive polymer coatingand a TOR-BP coating for insulation.9

Several candidate coatings are currently underdevelopment to meet the needs of the STEP-AirSEDSmission.

A majority of the work in the STEP-AirSEDSprogram has been analysis. There have been severalassumptions used in this analysis based on previousmissions. However, since the technology is stilldeveloping, there are many tests that need to be doneto verify the assumption used.

General tether tests include hypervelocity testing tosimulate M/OD particles impacting the proposedtethers. These tests will reveal more into how thetethers withstand the impacts and the resulting holesize from impacts. There is also testing underway toexamine how well wide and porous tethers are atcurrent collection.8 There will also be testingevaluating the conditions of pinholes present fromM/OD strikes in the insulation under high voltageconditions in a simulated ionosphere. Mechanicaltesting will also verify the tethers strength.

Several tests will also be used to verify the coatings.These tests are essential since the coatings are key tothe performance of the tether and since they are notoff the shelf products. These tests include a coatingresistance and dielectric strength test. The coatingswill be tested in an atomic oxygen environment. Thecoatings will also be subjected to bending andadhesion tests. The coated tethers will also undergothermal testing.

These tests are either currently underway oranticipated in the next year on the STEP-AirSEDStether.

Conclusions

There are several factors that need to be consideredwhen designing an electrodynamic tether for aparticular mission. These include survivability fromM/OD strikes, atmospheric drag, currentcollection/thrust, electrical resistance, strength,mass, coatings and manufacturability.Manufacturability is probably one of the mostsignificant since an exceptional theoretical tethercould be impractical or intractable to build. AtTMTC several designs have been manufactured forthe STEP-AirSEDS mission based on the designprocess outlined here. These tether designs arecurrently undergoing testing to determine theoptimal tether design.

Acknowledgements

This research was sponsored by NASA/MSFC. Theauthors would like to acknowledge Randy Baggett,Andrew Santangelo, Jennifer Robinson, and BrianGilchrist for their contributions.

References

1 Estes, R. D., Sanmartin, J. R., and Martinez-Sanchez, M., "Technology of bare tether currentcollection," Proceedings of the Tether TechnologyInterchange Meeting, NASA CP-1998-206900,1998, pp. 379-398.

2 Santangelo, A., Sturmfels, R., Van Noord, J.,"Applications of Electrodynamic Tethers and theSTEP-AIRSEDS Mission," AIAA 39th AerospaceSciences Meeting & Exhibit, AIAA-2001-1144,RenoNV, Jan. 2001.

J Estes, R., Lorenzini, E., Sanmartin, J., Pelaez, J.,Martinez-Sanchez, M., Johnson, L., and Vas, I.,"Bare Tethers for Electrodynamic SpacecraftPropulsion," Journal of Spacecraft and Rockets,Vol. 37, No. 2, March-April, 2000, pp. 205-211.

4 Sturmfels, R., and Robinson, J., "Design of SpaceTethers for Meteoroid and Orbital Debris ImpactSurvivability," AIAA 39th Aerospace SciencesMeeting & Exhibit, AIAA-2001-1140, Reno NV,Jan. 2001.

5 Van Noord, J., West, B., Gilchrist, B.,"Electrodynamic Tape Tether Performance withVarying Tether Widths at Low Earth Altitudes,"

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AIAA 39th Aerospace Sciences Meeting & Exhibit,AIAA-2001-1141, RenoNV, Jan. 2001.

6 Gilchrist, B., Bilen, S., Patrick, T., Van Noord, J.,"Bare Electrodynamic Tether Ground Simulations ina Dense, High-Speed Plasma Flow," 36th

AIAA/ASME/SAE/ASEE Joint PropulsionConference and Exhibition, AIAA 2000-3869,Huntsville, AL, June 2000.

7 Estes, R.D., and Lorenzini, E. C., "Performanceand Dynamics of an Electrodynamic Tether," AIAA38th Aerospace Sciences Meeting & Exhibit, AIAA2000-0440, Reno NV, 2000.

8 Gilchrist, B., Bilen, S., Gallimore, A. D., "GroundSimulations of Electron Current Collection to WideProbes for Electrodynamic Tethers," 37th

AIAA/ASME/SAE/ASEE Joint PropulsionConference and Exhibition, AIAA 2001-3337, SaltLake City, UT, July 2001.

9 Vaughn, J., Finckenor, M, Kamenetzky, R.,Schuler, P., Sorenson, J., "Development of PolymerCoatings for the ProSEDS Tether," AIAA 38th

Aerospace Sciences Meeting & Exhibit, AIAA-2000-0244, RenoNV, Jan. 2000.

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