Nestor Voronka, Robert Hoyt,Nestor Voronka, Robert Hoyt,Brian Gilchrist, Keith Brian Gilchrist, Keith FuhrhopFuhrhop

TTETHERS ETHERS UUNLIMITED, NLIMITED, IINC.NC.11807 N. Creek Pkwy S., Suite B11807 N. Creek Pkwy S., Suite B--102102

Bothell, WA 98011Bothell, WA 98011(425) 486(425) 486--0100 Fax: (425) 4820100 Fax: (425) 482--9670 9670 voronka@tethers.comvoronka@tethers.com

Modular Spacecraft with Modular Spacecraft with Integrated Structural Integrated Structural

Electrodynamic PropulsionElectrodynamic Propulsion

NIAC Phase I Fellows MeetingNIAC Phase I Fellows MeetingAtlanta, GeorgiaAtlanta, GeorgiaMarch 7March 7--8, 20068, 2006

MotivationMotivation–– Traditional propulsion uses propellant as reaction Traditional propulsion uses propellant as reaction

massmass–– Advantages (of reaction mass propulsion)Advantages (of reaction mass propulsion)

•• Can move spacecraft center of mass, readily and relatively Can move spacecraft center of mass, readily and relatively quicklyquickly

•• Multiple thrusters offer independent and complete control of Multiple thrusters offer independent and complete control of spacecraft (6DOF)spacecraft (6DOF)

–– DisadvantagesDisadvantages•• Propellant is a finite and mission limiting resourcePropellant is a finite and mission limiting resource•• Propellant mass requirements increases exponentially with Propellant mass requirements increases exponentially with

mission mission ∆∆V requirementsV requirements•• Propellant may be a source of contamination for optics and Propellant may be a source of contamination for optics and

solar panelssolar panels–– Are there innovative alternatives?Are there innovative alternatives?

–– PresidentPresident’’s Vision Mandates NASA to s Vision Mandates NASA to ““implement a implement a sustainablesustainable and and affordableaffordable human and robotic program to human and robotic program to explore the solar system and beyondexplore the solar system and beyond””

–– Current architectures require very large total masses Current architectures require very large total masses to be launched from Earthto be launched from Earth

–– Propellant mass fractions for InPropellant mass fractions for In--situ resource situ resource utilization (ISRU) and mining based architectures are utilization (ISRU) and mining based architectures are significant and costly significant and costly

NASANASA’’s Vision of Explorations Vision of Exploration

There exists a critical need for highly efficient low-cost propulsion to assure access to space & in-space propulsion

Space Propulsion LandscapeSpace Propulsion Landscape10,000 sec

2,000 sec

Courtesy Gallimore, A., UMich

Isp

Electrodynamic Space Tether PropulsionElectrodynamic Space Tether Propulsion–– InIn--space propulsion systemspace propulsion system–– PROS:PROS:

•• Converts electrical energy into Converts electrical energy into thrust/orbital energythrust/orbital energy

•• Little or no consumables (propellant) are Little or no consumables (propellant) are requiredrequired

–– CONS:CONS:•• Long (1Long (1--100km) flexible structures 100km) flexible structures

exhibit complex dynamics, especially in exhibit complex dynamics, especially in higher current/thrust caseshigher current/thrust cases

•• Gravity gradient tethers have constrained Gravity gradient tethers have constrained thrust vectorthrust vector

•• Relies on ambient plasma to close Relies on ambient plasma to close current loop current loop

Proposed SolutionProposed Solution–– Multifunctional propulsionMultifunctional propulsion--andand--

structure system that utilizes structure system that utilizes LorentzLorentz forces generated by forces generated by current carrying booms to current carrying booms to generate thrust with little or generate thrust with little or no propellant expenditureno propellant expenditure•• Utilizes same principles as Utilizes same principles as

electrodynamic tether propulsionelectrodynamic tether propulsion

–– Utilize relatively short (Utilize relatively short (≈≈100 100 meter), rigid booms with meter), rigid booms with integrated conductors capable integrated conductors capable of carrying large currents, that of carrying large currents, that have plasma contactors at the have plasma contactors at the endsends

Performance of Proposed ApproachPerformance of Proposed Approach–– Current flowing in a moving wire through space Current flowing in a moving wire through space

interacts with the ambient magnetic fieldinteracts with the ambient magnetic field•• EarthEarth’’s Magnetic Field in LEO s Magnetic Field in LEO ≈≈ 30,000 30,000 nTnT•• Interplanetary Magnetic Field Interplanetary Magnetic Field ≈≈ 5 5 nTnT

–– LorentzLorentz Force: F = Force: F = iLiL x Bx B–– Space Tether Electrodynamic PropulsionSpace Tether Electrodynamic Propulsion

•• Example: 10km conductor, 1Ampere in LEOExample: 10km conductor, 1Ampere in LEO–– Thrust |Thrust |iLxBiLxB| | ≈≈ 0.3 Newtons0.3 Newtons

–– Proposed Integrated Structural PropulsionProposed Integrated Structural Propulsion•• Example: 100m conductor, 100 Ampere (!) in LEOExample: 100m conductor, 100 Ampere (!) in LEO

–– Thrust |Thrust |iLxBiLxB| | ≈≈ 0.3 Newtons0.3 Newtons–– Torque Torque ≈≈ 750 N750 N·· mm

‘‘StructuralStructural’’ ED PropulsionED Propulsion–– By connecting six booms to a spacecraft along By connecting six booms to a spacecraft along

orthogonal axes, full 6DOF of motion can be orthogonal axes, full 6DOF of motion can be controlled (translational and rotational)controlled (translational and rotational)

Modular SpacecraftModular Spacecraft–– By making booms and spacecraft modules By making booms and spacecraft modules

modular and modular and interconnectableinterconnectable, we create self, we create self--assembling assembling TinkertoyTinkertoy®® like components for like components for space structures and systemsspace structures and systems

Optimal Path PlanningOptimal Path Planning•• Chemical Systems nearChemical Systems near--impulsiveimpulsive

–– HohmannHohmann and Biand Bi--elliptical transferselliptical transfers

•• LowLow--thrust trajectory planning (e.g. thrust trajectory planning (e.g. electric propulsion)electric propulsion)

–– Near continuous low level thrustNear continuous low level thrust–– Additional constraints for optimization Additional constraints for optimization

problemproblem•• Available Power (eclipse periods)Available Power (eclipse periods)

•• Tethers and Structural Tethers and Structural Electrodynamic PropulsionElectrodynamic Propulsion

–– Additional constraints due to ambient Additional constraints due to ambient magnetic fieldmagnetic field

•• Thrust Vector direction limitedThrust Vector direction limited•• Thrust dependent on magnetic field Thrust dependent on magnetic field

strength!strength!

LowLow--Thrust Trajectory OptimizationThrust Trajectory Optimization–– EP Orbit Raising from GTO to GEOEP Orbit Raising from GTO to GEO

•• Optimizing both thrust magnitude & angleOptimizing both thrust magnitude & angle•• Variable thrust can increase payload mass Variable thrust can increase payload mass

fraction up to 3%, and be 5fraction up to 3%, and be 5--10% more fuel 10% more fuel efficientefficient

–– Secondary Effects to considerSecondary Effects to consider•• J2 effects, solar eclipsing, solar cell J2 effects, solar eclipsing, solar cell

degradation due to radiationdegradation due to radiation

Kimbrel, M.S., “Optimization of EP Orbit Raising”, MIT, 2002.

ESAESA’’ss SMARTSMART--1 Mission1 Mission–– Small Missions for Advanced Research in Small Missions for Advanced Research in

Technology Technology -- Launched on 27 Sept 2003Launched on 27 Sept 2003•• Arrived in lunar orbit 15 Nov 2004 Arrived in lunar orbit 15 Nov 2004 •• PPSPPS--13501350--G Hall Effect Ion Thruster (70 G Hall Effect Ion Thruster (70 mNewtonmNewton))

–– Propellant mass fraction = 82.5 kg / 370 kg = 22.3 %Propellant mass fraction = 82.5 kg / 370 kg = 22.3 %

•• 22ndnd time ion propulsion used for primary propulsiontime ion propulsion used for primary propulsion–– 11stst was NASA Deep Space 1 launched Oct 1998was NASA Deep Space 1 launched Oct 1998

•• Utilized nearUtilized near--constant thrust constant thrust •• Trajectory optimizationTrajectory optimization

–– Propellant consumptionPropellant consumption–– Radiation Belt Transit TimeRadiation Belt Transit Time–– Available power (limited thrust duration during eclipse)Available power (limited thrust duration during eclipse)

•• Thruster 1190W max out of available 1850W BOLThruster 1190W max out of available 1850W BOL

System ElementsSystem Elements–– NodesNodes

•• Energy StorageEnergy Storage•• System ControlSystem Control

–– BoomsBooms•• Structural Propulsion BoomsStructural Propulsion Booms•• Plasma ContactorsPlasma Contactors•• Docking Mechanisms and Docking Mechanisms and

SensorsSensors

–– Key ElementsKey Elements•• Energy Source (Solar)Energy Source (Solar)•• Energy StorageEnergy Storage•• Electron and Ion SourcesElectron and Ion Sources

Energy Storage TechnologiesEnergy Storage TechnologiesBattery SystemsBattery Systems–– NiH2NiH2

•• 35 35 –– 55 cell whr/kg55 cell whr/kg•• 20 20 –– 300 A300 A--hr ampacityhr ampacity•• 30% DOD for LEO30% DOD for LEO•• 5 5 –– 7 Year LEO life7 Year LEO life•• 5 5 –– 10 whr/kg system SE10 whr/kg system SE

–– Li ExpectationsLi Expectations•• 70 70 –– 150 Cell whr/kg150 Cell whr/kg•• 20 20 –– 60 A60 A--hr ampacityhr ampacity•• 10 10 –– 15% DOD for LEO15% DOD for LEO•• 5 5 –– 7 Year LEO life7 Year LEO life•• 1010–– 30 whr/kg system SE30 whr/kg system SE

Flywheel SystemsFlywheel Systems–– Near TermNear Term

•• 25 25 –– 40 whr/kg40 whr/kg•• >4 kW hrs capacity>4 kW hrs capacity•• 90% DOD for LEO90% DOD for LEO•• 15 Year LEO life15 Year LEO life•• 10 10 –– 20 whr/kg system SE20 whr/kg system SE

–– Far TermFar Term•• 50 50 –– 75 whr/kg75 whr/kg•• Unlimited thru parallelingUnlimited thru paralleling•• 90% DOD for LEO90% DOD for LEO•• > 15 Year LEO life> 15 Year LEO life•• 40 40 –– 75 whr/kg system SE75 whr/kg system SE

Courtesy NASA GRC P&PO

Flywheel Technology Challenges and GoalsFlywheel Technology Challenges and Goals

Auxiliary Bearings –touchdown and launch loads,

stability, caging

Magnetic Bearings – low losses, higher speeds,

sensors, dynamic control

Housing – system and component integration,

structural/dynamic response

Composite Rotor – long life, safety without containment,

light-weight hubs, design and cert. standards

Motor/Generator – low losses, higher speeds, drive controls

Far Term GoalsFar Term Goals–– Integrated Integrated

Power & Power & Attitude SystemsAttitude Systems•• 75 whr/kg75 whr/kg•• 92% efficiency92% efficiency•• 25 year LEO life25 year LEO life•• --5555--220220°°CC

–– Energy StorageEnergy Storage•• 100 whr/kg100 whr/kg•• 30 year life30 year life

–– Pulse PowerPulse Power•• 2,000 W/kg2,000 W/kg

Courtesy NASA GRC P&PO

–– High System Specific Energy, Specific Power, Long LifeHigh System Specific Energy, Specific Power, Long Life–– High Round (Charge/Discharge) Trip EfficiencyHigh Round (Charge/Discharge) Trip Efficiency–– Multiple Functionality (Power and Torque)Multiple Functionality (Power and Torque)–– Long Storage Life Without DegradationLong Storage Life Without Degradation

The Ultimate Spacecraft Battery

Flywheel BenefitsFlywheel Benefits–– Life is virtually independent of Depth of DischargeLife is virtually independent of Depth of Discharge–– Performs equally well with lowPerforms equally well with low-- and highand high--power loadspower loads–– State of charge easily determined by measuring flywheels State of charge easily determined by measuring flywheels

rotational velocityrotational velocity–– Demonstrated net (charge/discharge) efficiencies up to 93.7%Demonstrated net (charge/discharge) efficiencies up to 93.7%

•• EddyEddy--current and current and hysteresishysteresis losses in magnetic bearings and motorlosses in magnetic bearings and motor--generator generator

–– Two counterTwo counter--rotating flywheels produce no net torque (OR rotating flywheels produce no net torque (OR can be used for attitude control)can be used for attitude control)

!!

Integrated Structural ED BoomIntegrated Structural ED Boom–– RequirementsRequirements

•• Rigidity based on ApplicationRigidity based on Application•• Conductive Conductive Element(sElement(s))

–– Boom (Tether) OptimizationBoom (Tether) Optimization•• Goal: Maximize Efficiency of Power Goal: Maximize Efficiency of Power

to Orbital Energy Conversionto Orbital Energy Conversion–– There is no optimal tether length, nor There is no optimal tether length, nor

optimal current level for a desired optimal current level for a desired thrust forcethrust force

–– Resistive Losses in boom (tether) Resistive Losses in boom (tether) should be minimizedshould be minimized

Integrated Structural ED Boom ConstructionIntegrated Structural ED Boom Construction–– TensegrityTensegrity ((tenstensile integrityile integrity) )

StructuresStructures• “an assemblage of tension and compression components arranged in a

discontinuous compression system..” R.B. Fuller Patent, 1962.

– Tubular Booms (e.g. Stem)–– RigidizedRigidized InflatablesInflatables

•• Foam Foam RigidizedRigidized•• Mechanically Mechanically RigidizedRigidized•• UV Cured UV Cured ThermosetThermoset CompositesComposites•• Thermally Cured Thermally Cured ThermosetThermoset Composites Composites •• Work Hardened Aluminum LaminatesWork Hardened Aluminum Laminates

–– OnOn--orbit Constructionorbit Construction

UV dissolving

filmstrength

and conductive elements

Electron EmittersElectron Emitters–– Field Emissive CathodesField Emissive Cathodes

•• MicrofabricatedMicrofabricated Emitter tips rely on sharp emitter Emitter tips rely on sharp emitter tips, and close nontips, and close non--intercepting electrodes to intercepting electrodes to generate high field required to enable electrons to generate high field required to enable electrons to quantum tunnel out of the material into spacequantum tunnel out of the material into space

•• High current densities (5000A/cmHigh current densities (5000A/cm22) have been ) have been demonstrateddemonstrated

•• Development undergoing to increase total current Development undergoing to increase total current output and reduce environmental constraintsoutput and reduce environmental constraints

–– Hollow CathodesHollow Cathodes•• Electric discharge ionizes neutral gasElectric discharge ionizes neutral gas•• Technology well developed Technology well developed –– neutralizers for EPneutralizers for EP•• 100A 100A HCsHCs have been tested (9have been tested (9--40sccm 40sccm XeXe flow)flow)

–– Annual fuel requirement for 100A @ 20 Annual fuel requirement for 100A @ 20 sccmsccm•• Xenon Xenon –– 61.6 kg61.6 kg•• Hydrogen Hydrogen –– 0.47 kg 0.47 kg

•• High current High current --> High temperature > High temperature --> lifetime limit> lifetime limit

Electron Emitter SummaryElectron Emitter SummaryDeviceDevice Power RequiredPower Required DetailsDetails

ThermionicThermionicCathode+GunCathode+Gun

2.1 MW2.1 MW 18 emitters, 18 emitters, VVff<1.25V for <1.25V for SCLSCL

Field Emission Field Emission ArrayArray

5.9 kW5.9 kW 10 emitters, 10 emitters, VVff<0.4V for <0.4V for SCLSCL

Hollow CathodeHollow Cathode 1.25 1.25 –– 10 kW10 kW Consumable Required!Consumable Required!(9(9--40 40 sccmsccm XeXe))

1.8 cm

TO5 Header

A

BC

Electron CollectionElectron Collection–– Passive Electron CollectionPassive Electron Collection

•• Space Tethers typically utilize large Space Tethers typically utilize large collection areascollection areas

–– Solid or grid spheres, bare tethersSolid or grid spheres, bare tethers

•• To collect 100A, 46.6kV needed To collect 100A, 46.6kV needed (4.7 MW) for a 1 meter sphere (!)(4.7 MW) for a 1 meter sphere (!)

–– Hollow CathodeHollow Cathode•• 6.2 kW @ 280 6.2 kW @ 280 sccmsccm to collect to collect

100A of electrons100A of electrons–– 6.6 kg of Hydrogen for 1 year6.6 kg of Hydrogen for 1 year

Hollow Cathode Ion SourceHollow Cathode Ion Source–– Hollow cathode Ion EmissionHollow cathode Ion Emission

•• VERY inefficient as compared to electron emission VERY inefficient as compared to electron emission (ionization efficiency is 1:1)(ionization efficiency is 1:1)

•• Ion emission requires Ion emission requires ≈≈ 14 14 sccmsccm /Ampere of emission/Ampere of emission–– Annual fuel requirement for 100A @ 1440 Annual fuel requirement for 100A @ 1440 sccmsccm

•• Xenon Xenon –– 4400 kg (!)4400 kg (!)•• Hydrogen Hydrogen –– 33 kg 33 kg

•• 4.7kW @ 1440 4.7kW @ 1440 sccmsccm to emit 100A of ionsto emit 100A of ions–– OPTION: Combo plan OPTION: Combo plan –– ion thruster ion thruster

(without neutralizer) as contactor/thruster(without neutralizer) as contactor/thruster

Liquid Metal Ion SourceLiquid Metal Ion Source–– Micro Ion Source Technology Micro Ion Source Technology –– Liquid Metal IonLiquid Metal Ion

•• Scalable system, including a passive material supply (no valves)Scalable system, including a passive material supply (no valves)•• Goal: Wide range of ion currents from addressable large area arrGoal: Wide range of ion currents from addressable large area arrays ays •• Goal: Optimized Power (> 80%) and Mass (Goal: Optimized Power (> 80%) and Mass (≈≈100%) efficiencies100%) efficiencies•• Power efficiencies on the order of 300 Watts/Ampere expectedPower efficiencies on the order of 300 Watts/Ampere expected•• Controllable current over 7 orders of magnitudeControllable current over 7 orders of magnitude•• Development Objectives:Development Objectives:

–– 2006 2006 –– 100 mA/cm100 mA/cm22 density, with 1mAdensity, with 1mA--10mA total current10mA total current–– 2015 2015 –– 10A/cm10A/cm22 density, with >10A total currentdensity, with >10A total current

_

++Classical

FieldIon Emission

(a wetted needle)

MicrofabricatedCapillary

Architecture

No energy loss, only ionization energyLess contamination, can only produce ionsIncreased reliability from lower voltage

operation, reduced arcing

Simple physics of field ionization and Taylor cones

Accelerating Grid+ +

Liquid MetalReservoir

Extracting Electrode

Electric field and surface tension balance to form a “Taylor cone” at liquid surface

High Current Liquid Metal Ions (under development)

Low Current Gas Ions

ApplicationsApplications–– SelfSelf--Assembling Modular Spacecraft (SAMS)Assembling Modular Spacecraft (SAMS)–– SelfSelf--Assembling Structure for Refueling StationAssembling Structure for Refueling Station–– SelfSelf--Assembling Space TugAssembling Space Tug–– SelfSelf--Assembling Structure for Large Mirror Assembling Structure for Large Mirror

or Antenna Arraysor Antenna Arrays–– Formation Flying Space Systems Formation Flying Space Systems

•• Terrestrial Planet Finder (TPF)Terrestrial Planet Finder (TPF)

SummarySummary–– Proposed Concept IS feasibleProposed Concept IS feasible

•• Almost Almost propellantlesspropellantless –– required consumable for ion sourcerequired consumable for ion source•• Almost full 6DOF control Almost full 6DOF control –– no thrust in Bno thrust in B--field directionfield direction•• Competitive with tradition Electric Propulsion with added benefiCompetitive with tradition Electric Propulsion with added benefit of t of

structural elementsstructural elements–– Technology ChallengesTechnology Challenges

•• High Current Plasma ContactorsHigh Current Plasma Contactors–– Devices exist Devices exist –– robust units with higher efficiencies neededrobust units with higher efficiencies needed

•• Plasma Contactor Space Charge LimitingPlasma Contactor Space Charge Limiting–– High current densities may be environmentally limitedHigh current densities may be environmentally limited

•• Collision proof coordinated control laws for formation flight, aCollision proof coordinated control laws for formation flight, and nd selfself--assembly assembly

–– Additional constraints imposed on lowAdditional constraints imposed on low--thrust control lawsthrust control laws

–– Potential ApplicationsPotential Applications•• Space Tug and Commodity DepotSpace Tug and Commodity Depot•• Structure for Beamed Power Solar Array/Antenna FieldsStructure for Beamed Power Solar Array/Antenna Fields•• Structure for Space Habitats with Integral Drag MakeupStructure for Space Habitats with Integral Drag Makeup