Development of the Flight Tether for ProSEDS

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Deployer System (ProSEDS) space experiment will demonstrate ... deploy a newly designed and developed tether which will provide tether generated dragĀ ...
Development Leslie 1Space

Transportation

of the Flight

Curtis

1, Jason

Vaughn

National

Aeronautics

Directorate,

AL 35812, 256-544-2486,

2 Tether Applications,

1, Ken

Gotham

256-544-9347,

ken. welzyn@msfc,

nasa.gov,

for ProSEDS

Welzyn

and Space

Inc., 1813

[email protected],

Tether

1, Joe

Carroll

Administration, St. Chula

jason,

Marshall

Space

Flight

Center,

Vista, CA 91913-2624

vaughn@msfc,

619-421-2100,

2

nasa.gov,

tether@home,

256-544-1731,

com

Abstract. The Propulsive Small Expendable Deployer System (ProSEDS) space experiment will demonstrate the use of an electrodynamic tether propulsion system to generate thrust in space by decreasing the orbital altitude of a Delta II Expendable Launch Vehicle second stage. ProSEDS will use the flight-proven Small Expendable Deployer System to deploy a newly designed and developed tether which will provide tether generated drag thrust of ~0.4 N. The development and production of very long tethers with specific properties for performance and survivability will be required to enable future tether missions. The ProSEDS tether design and the development process may provide some lessons learned for these future missions. The ProSEDS system requirements drove the design of the tether to have three different sections of tether each serving a specialized purpose. The tether is a total of 15 kilometers long: 10 kilometers of a non-conductive Dyneema lead tether; 5 km of CCOR conductive coated wire; and 220 meters of insulated wire with a protective Kevlar overbraid. Production and joining of long tether lengths involved many development efforts. Extensive testing of tether materials including ground deployment of the full-length ProSEDS tether was conducted to validate the tether design and performance before flight.

PROSEDS

ProSEDS

is an electrodynamic

payload

on

delivered, +/-1

a Delta

the ProSEDS

degree

tether

II Global mission

inclination.

propulsion

Positioning begins.

An endmass

from

the Delta

MISSION

system

System First,

weighing

at an initial

OVERVIEW

space

(GPS)

the Delta

experiment

scheduled

replacement

second

stage

mission. is placed

approximately

21 kg is then deployed The

ejection

of 3 m/s.

endmass, conductive

is a nonconductive material which provides portion of the tether. The deployer control

first

the

as a secondary

primary

in a 360 km circular

rate

by spring

to fly in 2002 After upward

10 km of tether,

the gravity gradient force system applies appropriate

(away

which

required braking

payload

is

orbit

with a 36

from

the earth)

is connected

to the

to deploy the remaining to bring the total tethered

system to a vertical stable orientation at the end of deployment. The end of the 5 km conductive tether remains attached to the Delta II. As the system moves through the earth's magnetic field, a motional-induced potential attracts flow

electrons

from

to the Delta

the surrounding

stage where

plasma.

a plasma

contactor

The

electrons

are collected

is used to emit them back

by the conductively into space

to complete

coated

tether

the circuit

and

through

the space plasma. A force is exerted on the current-carrying tether by the earth's magnetic field which causes the altitude of the Delta to decrease. The Delta stage, with ProSEDS attached, will continue it's orbital decay until it bums

up upon

capability plasma data

reentry

by lowering density

obtained

into the atmosphere.

The ProSEDS

the orbit of the stage

by at least

and other from

conditions

the ProSEDS

to determine experiment

experiment 5 km a day.

the current will

be used

will

demonstrate

electrodynamic

tether

Instrumentation

on ProSEDS

collection

capability

of the electrodynamic

to predict

the performance

of future

will tether

thrust

measure

the

tether.

The

missions.

!

TETHER The tether

deployer

deployer,

which

winding

system

for ProSEDS

has flown

on an aluminum

as counters

to monitor

DEPLOYER is based

successfully core.

three

length

on the flight

times.

The canister

the tether

SYSTEM

also houses

insulated Kevlar

assembly,

to prevent overbraid

for

combined comprise core. The Kevlar characteristics. tolerant

remains

wear

attached

and

handling

For the uninsulated to provide

to lower

by

conductive

good

temperatures

the ejection

connection

conductive

portion

The entire The brake

Tether

and infrared The

tether

protection.

from

Both

stage

the

light

insulated

and

the aluminum

wire

strands

are coated

conductivity

(for

electron

collection)

momentum

of the

to the endmass

of the tether,

which

to the conductive

to provide deployment endmass

emitting

the entire

contactor.

surface

Connected

section

mission.

uninsulated

that act is 15

until

sufficient

gravity

This

tether

provides

will be subjected

to horizontal

This section

conductive

is

with

a

sections

wire twisted around a Kevlar windability and deployability

with

a conductive

atomic

oxygen

improved

surface

optical

and

of tether

is not required.

forces

diodes experiment

It is also is covered

is a 10 km long tether

improved resistance to meteoroid properties are needed to allow also

(SEDS) the tether

section, an uninsulated conducting section is secured to the tether core in

section, on orbit.

System

that houses

for the ProSEDS

during

the plasma

Deployer

canister

gradient

impact. enough

forces

a stabilizing

much

like the wind

made

(This end of tether to be

develop. tension

Also, force

pushing

a

to the

on a sail.)

tether passes through the brake and ammeter, which are mounted above the canister, as it is deployed. consists of a post and tether guide that can be rotated under the action of a stepper motor (commanded by

the deployment at MSFC

Expendable

together: a non-conducting 1). The 220 meter insulated

to the tether

from Dyneema fiber braided into a flat geometry the tether is deployed first, as its low friction deployed

deployment.

to the Delta II second

reconnection

Small

has an aluminum

5 km and consist of 7 continuous strands of 28 AWG aluminum core provides ample tensile strength for the tether and improved

polymer

properties

which

any electron

proven

deployer

phototransistors

and rate during

km long with three distinct sections connected section, and an insulated conducting section (Fig. the deployer

The

DESCRIPTION

control

with

other

Applications

law) to control

hardware

Inc. of Chula

Non-Conductina Section

the tether

(Fig 2). Vista,

and hence

canister

the deployment

and brake

rate.

The deployer

were

designed

subsystem

is assembled

and fabricated

by

CA.

Tether:

A-B:

tension

The deployer

10 km. Section A-C

20 m Kevlar

Leader

Section B-C: 10 km Dvneema Flat Braid Conductino Tether: 5220 m. Section C-E Section C-D: 5 km CCOR Coated Aluminum Wire. Kevlar Core Section D-E: 220 m Insulated Aluminum Wire. Kevlar Core. Overbraid

FIGURE

Upon

command

from

Delta

the endmass

1. ProSEDS

is ejected

from

Tether Schematic.

the

second

stage

initiating deployment. The brake is applied at various times during deployment tether. The brake control law is a modification of what was used on previous finalized The

and tested

control

law

numerous

and brake

times settings

during

deployment

are preprogrammed

testing into

of flight the data

pulling

the

tether

from

the canister,

to control the deployment rate of the SEDS mission. The control law is

type tethers

in a vacuum

subsystem

electronics

chamber box

before

at MSFC. launch

i t

because

there

transferred

is no uplink

to ground

command

stations

during

capability. the mission

All of the data for post mission

TETHER The

design

of the

tether

for the

ProSEDS

on tums

tests

were

conducted

of various

and analysis on the tether performance, constraints to the tether design. These

experiment

of the canister

and support

material

was

based

on analysis

and tether

tether

to improve

An analysis

samples

and 5 km.

missions

of the ionospheric

It was

the tether to demonstration

space

environment thermal lengths

the design.

fitting

determined

In addition

is predicted

capability

of tethers.

to trades

had

2. ProSEDS

successfully

Data System

Deployer Hardware

deployed

Assembly.

non-conducting

tethers

of 20 km lengths

from

of the non-conducting plasma

conditions

tether

that would

in case of impact be present

the volume

constraints

that a length

of the SEDS

of 5 km would

with a micro-meteoroid

SEDS

in space

during

the mission

particle. was conducted

by various lengths and sizes of metallic 1.2 mm in order to provide the needed

canister.

be required

Lengths

to generate

that

a tether

were

considered

current

to be 1400 V, which

will provide

an ample

demonstration

of the electrodynamic

tether. current

were

of 3 A.

3, 4,

Shortening

save system mass would lower the current collection capability of the system and of the electrodynamic tether generated forces. The maximum value of tether generated

ProSEDS

for the

environments, and materials,

to use a 10 km non-conducting tether for initial deployment and for tether stabilization was that was made was to use a flat (1.2 mm x 0.16 mm) braided tether instead of a cylindrical

the survivability

area while

be

GPS Receive_r

to determine the electromotive force (EMF) that could be generated The outer diameter of the conductive tether needed to be around collecting

to finalize

ister

FIGURE

tether

of the

and orbital debris, natural were performed on tether

Tether

previous

will

structure.

HVCM

hardware, the decision made. The one change

and deployment

the decision to use the SEDS flight proven hardware provided several constraints included volume limitations of the canister and mass limitations

GPS Antenna

Since

tension,

DESIGN

experiment including the ionospheric plasma conditions, meteoroid radiation and solar conditions, and atomic oxygen. System trades and many

counts,

analysis.

limit the EMF for

tether

thrust

The5kmconducting tetheruses seven wirestwisted around ahighstrength braided core.Thisisaproven design forproduction oflonglength cables andprovides aflexible, strong (greater than250N tensile strength) tetherfor winding anddeployment. Trades wereconducted onthemetallic material tobeusedfortheconducting portion of thetether.Thetwomaterials thatwerestudied indepth werecopper andaluminum. Some oftheconsiderations included: current collection capability, system mass, material deployability, manufacture andhandling ofthetether. Comparisons weremade forthecurrent collection capability of copper andaluminum wire.If a slightlylarger diameter aluminum wireisused thecurrent collected iscomparable tocopper withamass savings of9.3kg. The largerdiameter (28AWG)1350-0 aluminum wirealsoallowsimproved manufacturability of acontinuous 5 km length tether.Toaddress concerns ontemperature cycling andtheeffectontheelectrical conductivity ofthewire, metallic coatings wereinvestigated. Thedesire wastofindacoating thatwouldimprove theabsorptivityemissivity (a/e)ratioofthebarealuminum, whilemaintaining theabilityofthetether tocollect electrons. Thecoating that wasselected isapolymer based coating developed byTritonSystems, Inc.called C-COR, whichprovides ana/e ratioof1.14. Thefinaltethersection is 220mlonganduses asimilarseven wireconstruction asthe5kmconducting section. TritonSystems, Inc.developed atwo-layer insulating coating toprevent anyelectron reconnection tothetether by theplasma contactor neartheDeltastage.Thelargeroutside diameter ofthiscoated wirerequired theuseof a largerdiameter braided Kevlar corethissection. Inaddition, thissection isalsoprotected byaKevlar overbraid to prevent anydamage tothetetherduring deployment orthroughout themission. Thetetheriswound onacorefor deployment (Fig.3).

FIGURE 3.ProSEDS Tether Wound onCore. Consideration ofallenvironments andconditions mustbemade before developing therequirements forsystem designs. Some oftheconditions mayprovide conflicting restrictions onthedesign, sosystem interactions mustbe understood andtrades conducted earlybefore hardware isdesigned andbuilt.Whiletherearesignificant benefits to development timeandcostbyusingexisting flightproven hardware, therearealsosubstantial constraints imposed

i

to the rest of the system

in order

to make

the entire

system

fit and work

together.

System

design

trades

constrained

by the use of existing designs may limit the use of some design solutions that would provide an overall improved system. When the focus of an experiment is the development and demonstration of a new technology, such as the bare electrodynamic systems

as much

tether

it would

be beneficial

to let the design

TETHER

The

ProSEDS

processing

tether of the

is manufactured

wires

and

and the use of carefully and supporting

The

the other

for the

steps.

Production

conducting

and

and

braiding

insulating

production

manufacturing

personnel

procedures.

was a key element

The cooperation

in developing

of NASA,

and producing

10 km non-conducting per

inch.

in sections

tether

is a flat braid

The Dyneema

that are spliced

to a much

Tether

successful

of the

requirements for close monitoring Applications

flight

Inc.,

tethers

heavier

of Dyneema

material together

20 m length

material

is an ultra high by Tether

of Kevlar

using

molecular

Applications,

braid

used

11 strands weight

Inc.

for heat

of 135 denier

PE which

fibers

for the

metallic

tether

is manufactured

in several

steps:

wire

and to prevent

the conducting

deployer

core.

section

The wire

of tether

can be spliced

is manufactured

and coating,

to the non-conducting

and coated

by Kanthol

Kevlar

in Palm

tether Coast,

section

FL.

core

during

Kanthol

tether

the tether uses

is

from several

fabrication

to insulating wires, overbraiding section. All of these steps must

at

at Western

One end of the non-conducting resistance

production

twisting, wire twisting over Kevlar core, cold welding conducting tether segment, and application of cross-straps to the conducting

braided

is produced

snagging on the endmass to which it is attached. The splice of the Kevlar braid to the Dyneema braid stages of taper to allow a smooth transition from the Kevlar leader to the 10 Km non-conducting tether.

before

section,

and joining

experiment.

Filament

The

supporting

of the non-conducting

section,

sections, and splicing of each section together. Due to precise of the ProSEDS tether, the manufacture of the tether requires

developed

contractor

7.5 to 8 picks spliced

drive

MANUFACTURING

in several

coatings

conducting and insulating tether final dimensions and performance

ProSEDS

of the new technology

as possible.

and

the insulated be completed

winding

specializes

onto the in coating

fine magnet wires and wires for surgical processes with the capability to coat aluminum wire down to 0.4 mil in diameter. The of 28 AWG 1350-0 aluminum wire is drawn at Kanthol from 0.125" wire to 0.0126" with strict tolerances

on wire

roundness

insulating,

TOR

polymer

polymer

TM

is a blend

of Triton's

necessary conductive, covered with TOR-BP the aluminum steps.

The

that is required coatings

wire,

to perform

are fabricated

Colorless

Oxygen

the coating

by Triton

Resistant

(COR)

emissive and AO resistant properties TM for atomic oxygen protection. The

to produce

polyimide

a final 0.35 mil coating

requires

two passes

through

process.

Systems,

and Polyanaline

thickness.

the coating

The

insulated

process

The wires

processing.

Cable

in Cortland,

(Pani)

NY for further

coating

to build

at Kanthol

to Cortland

the conducting, MA.

CCOR The

in a blend

is applied

to a 1 mil final and cut to 6,750

Cortland

Cable

TM

and

conductive

to provides

for the tether. The insulating coating CCOR TM coating is applied in 12 even

TOR-BP applied as the final step. The wires are wound onto spools conductive wires, and 300 m per spool for the insulated wires. are shipped

Both

Inc of Chelmsford,

the

is polyimide increments to

in two application thickness

with

m per spool

developed

the

for the

and uses

the "Hi-Wire" design to manufacture many of their cable products. The Hi-Wire process involves using strands of wire twisted around a polymeric core that is used as the strength member of the cable design. Two different diameter cores of Kevlar TM 49 are used for the ProSEDS tether because the coated wire diameters are different. The smaller larger strand

diameter

conductive

section

uses a smaller

diameter insulating wires use a core of 390 denier material. These Kevlar

so that the wire twisting process without any induced stresses. The wire

twisting

magnetic

torque

process control

removes

at Cortand devices

used

Cable

core made by braiding

6 strands

of 390 denier

material.

The

section made by braiding 8 strands cores are then twisted in a direction

of 390 denier material around one opposite the wire twisting direction,

the initial

core

allowing

spools

of wire

twist

is performed

to control

of the Kevlar

by using

the payoff

tension.

seven

The wires

the tether

to be produced

set onto a pay-off

are fed through

a twisting

rack with die that has

7 equally spaced holes spool also with tension

in a circle around a center hole. The Kevlar core is fed through the center hole from a supply control. The wires and core are threaded onto a take-up spool in a Cook twister machine and

are pulled

at a rate to apply

and twisted

approximately

3 twists

per inch.

In order

to make

a 5 km section

of tether

in

thismanner, theremustbeanunderstanding ofprocessing requirements andclose control oftension andmachine operating parameters mustbemaintained. It is mostdesirable toproduce theentirelengthof tetherin one production runwhichisatimeconsuming process thatrequires watchful careandcontrol.Aftertheconductive section oftether istwisted it mustbeconnected totheinsulating section oftether inamanner thatprovides smooth transition between segments andprovides adequate tensilestrength forthemission.Thisis accomplished by performing asplice withthetwodifferent sized Kevlar cores toeach otherandcoldwelding thetwotypes ofwires toeach other.Thetwisting process isstopped forthecoldwelding procedure thatisperformed byusingaHuestis coldwelder toweldeach pairofwirestogether (Fig.4). Thecoldweldsarestaggered inlength, byabout 3cmin ordertohaveamoreuniform finishtothetetheranda smooth transition between thetwodifferent coated wire sections. Thecoldwelded wireshavethesame orbettertensile strength thanvirginwire,andtheymaintain their electrical conductivity. Afterallseven wireshave been coldwelded together andtheKevlar coresplice ismade the twisting process iscontinued.

FIGURE

The next two

step s in tether

4. Huestis Cold Welder for Splicing Wires on Tether.

manufacturing

are done

to reduce

risks to tether

damage

and to provide

improvements

to mission success for the ProSEDS experiment. To provide'protection from damage or snagging to the insulated section of the tether (which will be closest to the Delta stage after deployment) an overbraid of Kevlar TM 49 is applied. to apply picks

This provides this overbraid

per inch.

and testing. 16 cross-straps

After

a thin but complete sleeve to cover the insulated tether section. to the tether. 16 strands of Kevlar _ 49 380 denier material completion

In case one or more are applied

of the overbraiding of the wires

to the tether

the tether

is packed

in the 5 km conductive

to allow

the current

to continue

and shipped

section

of tether

to flow

A 16 carrier braider is used are braided with about 13.5 to MSFC

for cross-strapping

is damaged

in the tether.

These

on orbit,

a set of

cross-straps

are

about 6 cm long and are made by wrapping a thin copper wire around the tether and then covering the copper wire section with an Aracon fiber overwrap. During the mission if up to 3 wires get broken on the tether the copper provides

a means

wires on the other

of carrying side.

current

from

the broken

This will allow the mission

wire

to continue

side of the cross-strap even if some

damage

and distributing occurs

it back

to the tether.

into all 7

Aftereachsection oftetherisproduced theymustbespliced together tomakea continuous 15kmlongtether. Except forthesplices between theinsulated andconductive sections, whicharedoneduringthewiretwisting process atCortland Cable, allofthetethersplices aredoneduringtetherwindingatTether Applications. The Kevlar leader tonon-conducting Dyneema splicehasbeen discussed previously. Themainsplicethatisperformed during winding isthesplice between theDyneema material andtheconducting wiretether.During thisprocess the wiresarepeeled backfromtheKevlarcoresothattheKevlarcorecanbespliced to theDyneema material. Painstaking caremustbetaken toprovide asmooth splice thatwill withstand theloadsduring deployment andthe mission. Afterthesplice iscompleted thewiresaretucked intothecoreatstaggered distances toprovide asmooth transition. Thedevelopment ofthenecessary materials, coatings andmanufacturing processes fortheProSEDS tether involved theefforts ofmanyindividuals. During thedevelopment phase oftheproject thereweremany testsconducted on thetethermaterials, andseveral production runsmadeof tetherprocessing. Untila complete tetherwas manufactured anddeployed alloftheinfluences ofthemanufacturing steps were notclearly understood. Seemingly minorchanges tothemanufacturing resulting inmajorchanges intheperformance andbehavior ofthefinished tether.Adequate timeto develop a newtechnology mustbeprovided forin projectscheduling anda clear understanding ofthemanufacturing processes should bedeveloped before production isinitiated. TETHER

It was

the philosophy

tethers.

An approach

of the ProSEDS

possible

to test under

of test what the exact

On individual properties. conducted

conditions,

samples,

this approach

tests

This section

were

performed

describes

wire samples

to verify

testing

on all of the hardware

test was

implemented.

was used

as much

allowable

voltage

some

in a plasma

The optical properties of the coatings were on the coatings to verify AO survivability.

insulated

extensive you

Although

as possible.

of the ProSEDS tether. In addition there were many process. Finally, deployment tests were performed

in flight configuration.

wire

to conduct

you fly, and fly what

flight

throughout the development phase as part of the tether manufacturing made and wound

Project

TESTING

standoff

of the significant

chamber

measured Finally

to verify

to verify dielectric

including

it was

Tests

were

conducted

tests that were incorporated after the tethers had been

tether

testing

the CCOR

emissivity. breakdown

that took place.

coating's

conductive

Atomic oxygen tests were tests were conducted on

capability.

At the completion of the wire manufacturing and coating process and after the twisting process a spark performed to detect pinholes or defects in the insulated section. Spark testing is used routinely in the wire for this purpose. voltage

detector

Spark testing

involves

head and controlling

high voltage

power

and very low current

supply.

The spark

In addition

carrying voltage

capability and handling ability was conducted testing of all electric field triple points (such

sections) A_er

post-joining

was performed

the tethers

were

reduce

the tendency

tested

to

demonstrate

samples

in a vacuum wound

were

plasma

gathered

structural

during

low speed

integrity.

for further

During off-line

as well

performed

after each deployment

during

the maturation test.

tester

consists

at 3000

of a high V, which

is

all of the splicing procedures and the to insure the breaking strength of the

testing.

Testing

prior to final selection as at the splice between

they were vacuum

deployment)

Finally,

deployment

baked

and drive

of the cross-straps

for current

of the cross-strap design. insulated and conductive

tests

of the

tether

design.

Spark

to relax

out moisture. were

temperatures to ensure that the final tether design would perform as predicted. to verify the performance of the overall tether and winding design. Extensive conducted

The spark

was conducted

test was industry

High tether

chamber.

in the flight configuration,

to unspring

(_tA).

test for ProSEDS

a factor of two higher than the highest expected EMF during flight. cold welding steps for wire joining, samples were made and .tested joints.

the

not always

tests

the wire residual

strain

They

vibration

conducted

were

then

in vacuum

(to

at various

Tension data was collected and used development deployment tests were of the

insulated

tether

sections

were

CONCLUSIONS The ProSEDS tether experiment is scheduled to fly in June of 2002. The new tether design that was developed for this experiment has been designed to perform as needed to collect the electrical current from the space plasma and demonstrate the electrodynamic thrust capability of tethers. In addition several risk mitigation designs were incorporated to improve the probability of mission success. Extensive testing was conducted during the development and production of the tethers for ProSEDS. Much was learned in the process that can be used for future systems that use electrodynamic tether propulsion technology. ACKNOWLEDGMENTS The authors would like to gratefully acknowledge the support and cooperation of personnel at MSFC, Kanthol, Triton Systems, Inc., and Cortland Cable Company. The close collaboration of these organizations was a key element in developing and producing successful flight tethers for the ProSEDS experiment.