The document describes a proposed Integrated Structural Electrodynamic Propulsion (ISEP) system that uses rigid booms with integrated conductors to generate thrust via Lorentz forces without consuming propellant. Short, 100m booms could carry high currents of 100A to provide comparable thrust to longer 10km tethers. The system could enable modular, self-assembling spacecraft. A technology demonstration on a picosatellite aims to generate measurable torque and thrust pulses by driving 1A currents through 10-20m booms. If successful, ISEP could provide propulsion and structure for applications like space tugs, refueling stations, and large structures.

1. NIAC 7th Annual Meeting
Tucson, Arizona
October 18th, 2006
An Architecture of Modular Spacecraft
with Integrated Structural
Electrodynamic Propulsion (ISEP)
Nestor Voronka, Robert Hoyt,
Brian Gilchrist, Keith Fuhrhop
TETHERS UNLIMITED, INC.
11711 N. Creek Pkwy S., Suite D-113
Bothell, WA 98011
(425) 486-0100 Fax: (425) 482-9670 voronka@tethers.com

2. Motivation
–– Traditional propulsion uses propellant as reaction mass
–– Advantages (of reaction mass propulsion)
•• Can move spacecraft center of mass, on-demand, and
relatively quickly
•• Multiple thrusters offer independent and complete control of
spacecraft (6DOF)
–– Disadvantages
•• Propellant is a finite and mission limiting resource
•• Propellant mass requirements increases exponentially with
mission ΔΔV requirements
•• Propellant may be a source of contamination for optics and
solar panels
–– Current Architectures of NASA’’s Vision of Exploration
require launching and transporting large masses
–– Are there innovative alternatives?

3. Space Propulsion Landscape
10,000 sec
2,000 sec
Isp
Courtesy Gallimore, A., UMich

4. Electrodynamic Space Tether Propulsion
–– In-space propulsion system
–– PROS:
•• Converts electrical energy into
thrust/orbital energy
•• Little or no consumables are required
–– CONS:
•• Long (1-100km) flexible structures
exhibit complex dynamics, especially in
higher current/thrust cases
•• Gravity gradient tethers have constrained
thrust vector
•• Relies on ambient plasma to close
current loop Given a fixed amount of power available
and fixed conductor mass, thrust
efficiency is independent of the length
of the conductor

5. Proposed Solution
–– Multifunctional propulsion-and-structure
system that utilizes Lorentz forces (F =
iL x B) generated by current carrying
booms to generate thrust with little or
no propellant expenditure
•• Utilizes same principles as electrodynamic
tether propulsion
–– Utilize relatively short (≈≈100 meter),
rigid booms with integrated conductors
capable of carrying large currents, that
have plasma contactors at the ends
•• Space Tether Electrodynamic Propulsion
–– Ex: 10km conductor, 1Ampere in LEO
•• Thrust |iLxB| ≈≈ 0.3 Newtons
–– Proposed Integrated Structural
Propulsion
•• Ex: 100m conductor, 100 Ampere (!) in
LEO
–– Thrust |iLxB| ≈≈ 0.3 Newtons
–– Torque ≈≈ 750 N· m

6. Single Boom Concept of Operation
– Apply potential to overcome motion-induced electric field and drive
current across magnetic field
– Current flowing down boom produces thrust force
– Plasma waves close the electrical circuit

7. ‘‘Structural’’ ED Propulsion
–– By connecting six booms to a spacecraft along
orthogonal axes, 4DOF of motion can be
controlled (translational and rotational)

8. ISEP Booms
–– Cold biased, low-resistance
element to maximize propulsive
performance
•• Copper Clad Aluminum (CCA)
offers low specific resistivity, yes
remains easy to work with
–– Ex. 0.01ΩΩ/50 meter boom has a 0.44
kg/m density
–– Stiffness dictated by application
–– Boom ends contain plasma
contactors & docking sensors
–– Hemaphroditic high-current
capacity docking mechanism

9. 100 Ampere Contactors for ISEP
Emission Device Power Notes
TC + Electron Gun 2.1 MW 18 emitters, < 1.25 V for SCL
Electron FEA 5860 W 10 emitters, < 0.4 V for SCL
Emission
HC 1250 W to 10 kW Flow Rates & Ion Type
9 sccm to 40 sccm Xe
Passive Sphere a 4.7 MW 1 m radius, 6.6E-3 N Drag
90% Porous – 6.6E-4 N
Passive Sphere b 1 MW 2.29 m radius, 3.46E-2 N Drag
90% Porous – 3.46E-3 N
Passive Plate a 61.3 MW 5 m2 – 5.26E-5 N Drag
Passive Plate b 1 MW 54.52 m2 – 5.73E-4 N Drag
Electron
Collection
HC 6150 W + 330 W
(20 A ion prod.)
280 sccm fuel
27.35 mg/s Xe+ or 0.21 mg/s H+
Ion Emission + Ion
Gun
1 MW + 1650 W
(100 A ion prod.) 83,334 emitters needed
Ion
Emission
HC 1000 W + 1650 W
(100 A ion prod.)
1400 sccm fuel
27.35 mg/s Xe+ or 0.21mg/s H+

10. ISEP Nodes
–– Node geometry
•• Simplest has 6 orthogonal
mating surfaces
•• Can also utilize polyhedrons
with mating surfaces spaced
45° along the circumference
–– Node components
•• Energy storage (flywheels @
75 W-hr/kg)
•• System & Navigation
Controllers

11. Modular Spacecraft
–– By making booms and spacecraft modules
modular and interconnectable, we create self-assembling
Tinkertoy® like components for
space structures and systems

12. ISEP Applications
–– Self-Assembling Modular Spacecraft (SAMS)
–– Self-Assembling Structure for Refueling Station
•• Integral rail gun for commodity delivery
–– Self-Assembling Space Tug
–– Self-Assembling Structure for Large Mirror
or Antenna Arrays
–– Formation Flying Space Systems
•• Terrestrial Planet Finder (TPF)

13. 40000
35000
30000
25000
20000
15000
10000
5000
0
200.0 400.0 600.0 800.0 1000.0 1200.0
Circular Orbit Altitude (km)
Delta-V (m/se
Inclination = 0.0º
Inclination = 28.5º
Inclination = 51.6º
Inclination = 90.0º
Inclination = 97.0º
ISEP Performance Analysis
–– Six 50m orthogonal booms in 300-1100km, 0-97°
inclination orbits
–– FEAs for electron emission, HCs for ion emission
–– Total system mass –– 1000kg with 20kg H consumable
(approx 3 years full time operation)
–– Current commanded to 100A continuous for 30 days
•• System Input power - 6500 Watts
–– Performance metrics tabulated & averaged
40000
35000
30000
25000
20000
15000
10000
5000
0
200.0 400.0 600.0 800.0 1000.0 1200.0
Circular Orbit Altitude (km)
Delta-V (m/se
Inclination = 0.0º
Inclination = 28.5º
Inclination = 51.6º
Inclination = 90.0º
Inclination = 97.0º
40000
35000
30000
25000
20000
15000
10000
5000
0
200.0 400.0 600.0 800.0 1000.0 1200.0
Circular Orbit Altitude (km)
Delta-V (m/se
Inclination = 0.0º
Inclination = 28.5º
Inclination = 51.6º
Inclination = 90.0º
Inclination = 97 0º
Along-track Total ΔV Cross-track Total ΔV Radial TotalΔV

14. ISEP Performance (cont.)
–– Thrust magnitude and efficiency
highly dependent on alignment
of boom(s) with magnetic field
–– Torques in the 1-10 N-m range
as compared to disturbances in
the 10-8 to 10-1 range
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
200.0 400.0 600.0 800.0 1000.0 1200.0
Circular Orbit Altitude (km)
Thrust st (
Inclination = 0.0º
Inclination = 28.5º
Inclination = 51.6º
Inclination = 90.0º
Inclination = 97.0º
Average Along-track Thrust (N)
200000
180000
160000
140000
120000
100000
80000
60000
40000
20000
0
200.0 400.0 600.0 800.0 1000.0 1200.0
Circular Orbit Altitude (km)
Specific Impulse (se
Inclination = 0.0º
Inclination = 28.5º
Inclination = 51.6º
Inclination = 90.0º
Inclination = 97.0º
10
9
8
7
6
5
4
3
2
1
0
Inclination = 0.0º
Inclination = 28.5º
Inclination = 51.6º
Inclination = 90.0º
Inclination = 97.0º
200 400 600 800 1000 1200
Along-track current torque (N-m)
200000
180000
160000
140000
120000
100000
80000
60000
40000
20000
0
Circular Orbit Altitude (km)
Inclination = 0.0º
Inclination = 28.5º
Inclination = 51.6º
Inclination = 90.0º
Inclination = 97.0º
0 5 10 15 20 25 30 35
Thrust to Power Ratio (μN/W)
ISEP boom Torque (N
Specific Impulse (s
Average Specific Impulse (Isp) Isp vs. Thrust to Power (uN/w)

15. ISEP Performance
–– ISEP is competitive with other EP technologies
–– In systems where collinear booms are assembled, performance
improves
–– For missions where structural elements are required, ISEP’’s dual use
(propulsive/structural) has significant advantages
1000000
100000
10000
1000
100
10
1
Resistojet
Arcjet
Pulsed Plasma Thruster
Hall Effect Thruster
Ion Thruster
Integrated Structural EP
0.001 0.01 0.1 1
Thrust-to Power (N/kW)
Specific Impulse (se
–– KEY TECHNOLOGY CHALLENGE: Power- and Mass- Efficient
collection and emission of electrons from/to the ambient plasma

16. Technology Demonstration Experiment
–– Primary Experiment Objectives
•• Generate directly detectable torque
•• Generate directly measurable thrust
–– Secondary Experiment Objectives
•• Validate performance of Field Emissive
Electron device(s)
•• Validate performance of lightweight
electron collectors
–– GOAL: Drive 1 Ampere of current
through lightweight deployable,
conductive 10-20 meter booms
•• 0.2-1.0 second impulses > 0.5 mN

17. Experimental Method
–– Picosatellite launched as a secondary
payload
–– Target platform –– CubeSat
•• 1kg –– 10x10x10 cm envelope
•• Standard initially designed for University class
experiments and educational purposes
•• Typically 1-2 launch opportunities a year
•• Launch costs $40-120K for a 1U CubeSat
•• Ideal for simple experiments and
technology demonstrations

18. Picosat Experiment Contactors
–– Field Emissive Cathodes
•• Microfabricated Emitter tips rely on sharp emitter
tips, and close non-intercepting electrodes to
generate high field required to enable electrons to
quantum tunnel out of the material into space
•• High current densities (5000A/cm2) have been
demonstrated
•• Development undergoing to increase total current
output and reduce environmental constraints
–– Passive Electron Collector (‘‘Hedgehog’’)
•• Bundle of conductive yarns
•• Yarns tied together at root, and when charged will
form approximately a ‘‘Koosh-ball’’ like spherical
structure due to electrostatic repulsive forces
•• 2 meter diameter structure with yarns every 40°,
and filament every 1° can weigh 10 grams(!)

19. Experiment Conops
–– Converted Solar Energy is stored onboard in capacitor bank
•• Allow for thrust pulse every 4-6 orbits
–– At desired B-field alignment, discharge capacitor to generate 1Ampere pulse
–– Measure Thrust with onboard accelerometers
–– Measure Torque with body attitude rate change

20. Summary
–– Proposed Concept IS feasible
•• Requires small amount of consumables for ion source
•• 4DOF propulsion–– no thrust in B-field direction
•• Competitive with tradition Electric Propulsion with added benefit of structural
elements
–– Technology Challenges
•• High Current Plasma Contactors
–– Devices exist –– robust units with higher efficiencies needed
•• Plasma Contactor Space Charge Limiting
–– High current densities may be environmentally limited
•• Collision proof coordinated control laws for formation flight, and self-assembly
–– Additional constraints imposed on low-thrust control laws
–– Experiment in Phase II will demonstrate system feasibility and validate
component technologies
–– Potential Applications
•• Space Tug and Commodity Depot (with integral rail gun?)
•• Structure for Beamed Power Solar Array/Antenna Fields
•• Structure for Space Habitats with Integral Drag Makeup