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.