The Large Structures and Tethers Working Group sought to clarify the meaning of large structures and tethers as they related to space systems. Large was assumed to mean that the characteristic length of the structure was greater than one of such relevant plasma characteristics as ion gyroradius or debey length. Typically, anything greater than or equal to the Shuttle dimensions was considered large. It was agreed that most large space systems that the tether could be better categorized as extended length, area, or volume structures. The key environmental interactions were then identified in terms of these three categories. In the following Working Group summary, these categories and the related interactions are defined in detail. The emphasis is on how increases in each of the three spatial dimensions uniquely determine the interactions with the near-Earth space environment. Interactions with the environments around the other planets and the solar wind were assumed to be similar or capable of being extrapolated from the near-Earth results. It should be remembered in the following that the effects on large systems do not just affect specific technologies but will quite likely impact whole missions. Finally, the possible effects of large systems on the plasma environment, although only briefly discussed, were felt to be of potentially great concern

Barnett, A.

Garrett, H.

Katz, I.

Murphy, G.

Raitt, J.

Samir, U.

Sullivan, J.

English

NASA Technical Reports Server

N87- 20057
LARGE STRUCTURES AND TETHERS WORKING GROUP
G. Murphy, Chair, H. Garrett, U. Samir, A. Barnett, J. Raitt, J. Sullivan,and I. Katz
I. INTRODUCTION
The Large Structures and Tethers Working Group sought to clarify the meaning of "large
structures" and "tethers" as they related to space systems. "Large" was assumed to mean that the
characteristic length of the structure was greater than one of such relevant plasma characteristics as
ion gyroradius or debye length. Typically, anything greater than or equal to the Shuttle dimensions
was considered "large." It was agreed that most large space systems and the tether could be better
categorized as extended length, area, or volume structures. The key environmental interactions
were then identified in terms of these three categories. In the following Working Group summary,
these categories and the related interactions are defined in detail. The emphasis is on how increases
in each of the three spatial dimensions uniquely determine the interactions with the near-Earth space
environment. Interactions with the environments around the other planets and the solar wind were
assumed to be similar or capable of being extrapolated from the near-Earth results. It should be
remembered in the following that the effects on large systems do not just affect specific
technologies but will quite likely impact whole missions. Finally, the possible effects of large
systems on the plasma environment, although only briefly discussed, were felt to be of potentially
great concern.
II. EXTENDED LENGTH
Structures for which one dimension is large relative to the plasma are best represented by the
space tether although other systems could be envisioned. Examples include the Space Shuttle
oriented with the nose-tail axis simultaneously along the magnetic field and velocity vector, such as
sometimes occurs for polar orbits near the equator. In this case the wing axis is the large
dimension - a narrow plasma beam before it has spread. Although both conducting and
nonconducting structures were considered, the following discussion will focus on conducting
structures, perhaps with a nonconducting surface to insulate them from the space plasma. For
such structures, the primary interaction is the well known Lorentz force, which produces an
induced electric field in the reference frame of the structure:
E =vxB
or
V -E.L=L. (vxB)
where:
E = induced electric field
V = induced potential drop
L = characteristic length of structure
v -- velocity of vehicle
B = magnetic field
https://ntrs.nasa.gov/search.jsp?R=19870010624 2020-03-20T11:10:39+00:00Z
At ShuttleandSpaceStationaltitudes,v is7 to 8 km/sandB about.0.3 g sothattypicalpotential
dropsare0.3 V/re. As spacetethersareexpectedto reachtensto hundredsof km, kilovolt or
higherpotentialsareanticipated.
Theexistenceof kV or greater potentials will lead to several key interaction issues. Chief
among these for extended-length structures is the question of return currents where the plasma
"contact" area may be only a few meters of conducting length on the two ends (large area and
volume structures will have a similar problem but not, it is believed, nearly as severe as will the
extended-length structures). Any application of the high voltages created in a conducting tether or
similar extended-length structure will by necessity drive large return currents. It is anticipated that
the ambient environment may not be able to directly support these currents so that any space
systems wishing to draw power/current from the environment could be very inefficient. Plasma
contactors are devices that have been proposed to eliminate this problem for extended-length
systems by allowing efficient current collection from the ambient environment. Examples are
electron beams, ion beams, neutral plasma generators, or large conducting spheres that could create
artifically large current collection areas. The I-V curves of such devices at high potentials are not
well characterized and represent a critical problem for the development of electrodynamic tethers.
The consensus of the working group was that while in most areas of plasma interaction studies
laboratory experiments were still timely and valuable, they have been essentially exhausted in this
area and proper in-space experiments are critically needed.
The dynamics and stability of extended-length structures need to be carefully investigated. A
specific concern in terms of the plasma environment are the electrodynamic torques and drag
produced by the ambient plasma and fields. Whereas for most "normal" spacecraft, electro-
dynamic drag is small compared to neutral atmospheric drag, the large area-to-mass ratio of a tether
will make it potentially sensitive to normally weak electrodynamic forces - although the cross
section of a short segment is miniscule, the total cross section of a 100-kin tether will be very large
and is dependent on the details of the plasma interaction. (Note: in some applications, such as
around Jupiter, it may actually be possible to draw electrical propulsion power from the tether,
producing "antidmg"!) N. Stone has made a separate written contribution to the Working Group
on tethers, which is included as an appendix.
III. AREA
"Large" area means that each dimension of the object's two-dimensional cross section is
larger than the characteristic plasma dimensions. As an example, the cross sectional dimensions of
a 1-m-diameter spacecraft are typically large compared to the electron gyroradius and the debye
length - it is not compared to the ion gyroradius. In contrast, the Space Station cross section will
be large compared with most plasma characteristic dimensions. Large solar arrays are another
prime example of large-area structures.
The main concern with large-area interactions is the shadowing effect of the area. That is, the
area will create a complex wake behind a surface in the direction of the velocity vector and,
although primarily a three-dimensional structure, the wake will depend principally on the cross-
sectional shape of the area normal to the velocity vector. Models currently exist, but more
sophisticated models that incorporate magnetic field effects are lacking. In particular, in situ
experiments and comparisons between in situ experiments and the models are desperately needed.
Laboratory measurements have proven valuable but do not adequately address the relevant range of
parameters and are generally limited in the (Ro/Ld) 2 ratios that can be measured (Ro=characteristic
bodylength;Ld=debyelength). In situ measurementsareneededwhereRo>>Ld (i.e.,Ro>104
Ld). Measurementsfor both largeconductingandnonconductingsurfacesarenecessary.Both
laboratoryandin situexperimentsneedto concentratemoreonvarying the surfacepotential in
orderto determinewakevariationsdependenton thiscritical parameter.For truly largearea-to-
massstructures(suchastheSolarSail,theSpaceBasedRadar,andtheSolarPowerSatellite),the
electrostaticdragandthepondermotive forces may become as important as the neutral atmospheric
drag; few accurate models of these phenomena currently exist. Finally, the basic issue of how the
plasma wake and magnetic field couple to produce the observed EMI needs to be carefully studied
- such noise may set a lower limit on noise levels in the vicinity of large structures. The scaling
law that relates the EMI to size is not known since measurements aboard the Orbiter are
complicated by the introduction of large amounts of outgassing products.
IV. LARGE VOLUMES
Unlike extended-lengths and large areas, it is much more difficult to determine the scaling
laws for large volumes. Here we assume that "large volume" encompasses the concept of the
perturbed volume around a body. In particular, we include the interaction of the local gas cloud (as
from emitted gases) surrounding a body with the plasma and ambient neutrals and the resulting
modification of the local environment in the concept of large volume.
Key outstanding issues, in addition to the definition of scaling laws, are how plasma heating
can take place (as is observed in the Shuttle ram) and how the heating scales with size and
composition. Although studied as a function of area, the breakdown characteristics of high-voltage
solar arrays have yet to be determined in terms of the perturbed volume at the surface of the arrays
- in particular, the volumetric effects and differences between ac, dc, and pulsed power (as in the
SDIO beam weapons) systems need to be considered. The dynamics of emitted fluids (liquid or
gas) need to be investigated in terms of collective behavior. The so-called "critical Alfven
ionization velocity" effect whereby reflected, high-velocity neutral particles in the ram direction
self-ionize is another potential plasma interaction issue about which we know little. As locally
generated magnetic fields of very high amplitude are being considered for various experiments,
their effects on a large volume must be characterized. Models and in situ experiments to evaluate
these effects need to be carded out hand in hand.
A potentially dangerous problem was identified as regards one very complex system that
depends on volume -liquid droplet cooling. In these systems, millions of tiny droplets are
emitted in order to eliminate heat. Because of their large area-to-mass ratio, they would be much
more efficient at emitting radiation than current solid-surface emitters or cooling vanes. The
interactions of such systems of concentrated macroparticles with the environment (for instance,
gravity effects could be dominated by electromagnetic effects) over a large volume are virtually
unknown. In the early days of the space program, one attempt at creating such a population of
macroparticles using thousands of tiny metallic needles to create an artificial, radio-reflecting layer
was actually successfully carded out. It is not clear what the differences between conducting or
nonconducting droplets would be. Although the system is intended to be closed (the particles
would be captured and recycled), the environmental impact of such a cloud if the particles were to
escape is frightening considering the current state of affairs vis-a-vis space debris.
Although a concern for all three systems, the adequate measurement of the ambient and
perturbed environment around and within a large volume will be potentially difficult and
expensive. As local variations may be critical to an adequate understanding of the interaction of a
8
large volume with the environment, it will be necessaryto make many measurements
simultaneouslyin timethroughoutheperturbedvolume. To date,suchmeasurementshavebeen
virtually impossibledueto theexpenseof themanytypesof instrumentsrequired,themassive
amountsof datathatneedtobecorrelated,andthesheardifficultyof deployment.Noeasyanswer
currentlyexistsfor thisproblemalthoughproliferationof cheap,simple,probesystemsis currently
being investigated. It is, however, morethan likely that an entirely new technology,one
encompassingmeasurmentand analysistechniques,will be required before an adequate
understandingof theinteractionsof largevolumeswith theenvironmentwill bepossible.
V. SUMMARY
Table 1 summarizesthe findings of the Large Structures and Tethers Working Group.
Briefly, the key plasma technology issues have been defined in terms of large one-, two-, and
three-dimensional systems. The addition of each spatial dimension compounds the potential
plasma issues that need to be considered for successful missions in the year 2001 and beyond.
The most critical issues are: (1) how will large, extended structures be grounded to the plasma
environment? (2) what effects does the magnetic field have on wake shape and EMI for large
areas? (3) how do large volumetric structures/environments respond to the ambient plasmas and
fields? and, finally, (4) large structures may, by their very size, seriously impact the natural plasma
environment - a plasma issue little studied in the past.
Table1. Summaryof Key Plasma Interactions for Large Space Structures and Tethers
(see text for discussion of issues).
Spatial
character Key plasma issues
Extended length
Large area
Large volume
General
(V x B) • L
Plasma contact (grounding)
CmTent loop closure for conducting tethers
Wake structure
Magnetic field effects
Electromagnetic noise turbulence
Electrostatic drag
Variable surface potentials
Ponderrnotive forces and effects on system
dynamics
Effects of emitted fluids (gases and liquids)
Macroparticle assemblies
High-voltage breakdown
Plasma heating in the ram
Simultaneous measurements in three dimensions
Scaling laws with size, magnetic field, and
particle environment
l0
APPENDIXI.
Commentsby
N. Stone
MarshallSpaceflightCenter
Theuseof a tetheredsatellitesystemto placeaninstrumentpackageintoa low-altitudeorbit
to mapout the plasmacharacteristics,currents,winds, and in the lower thermosphere(i.e.,
altitudesin the rangeof 100 kin) will require severalextensionsof existing technological
capabilities.Thefollowingareexamples:
(1) The presentlydesignedNASATSSiscapableof deployingasatelliteto amaximumof
100km onanonconductingtether.TheSpaceStationwill orbit ataltitudesin therange
of 350 to 500 kin. Therefore,(i) a deploymentsystemmust bedevelopedthat can
handleupto 400kmof tether.(ii) Thedynamicsof tether,onceestablishedbytheTSS-
1andTSS-2missions,shouldbereassessedto includevery longtethersof therequired
length.A modificationtothecontrolsystemmayberequired.
(2) Tether degradation should be studied in more detail (this will be more important for
long-duration Space Station tethers than the 4- to 7- day TSS missions). This includes,
for example, micrometeorite strikes and atomic oxygen erosion.
(3) Tether and tethered satellite thermal control will be a critical issue for altitudes below
125 km. At 90 to 100 km (the most interesting range because of the turbopause),
heating will be a significant problem and active thermal control techniques (such as
phase change materials) will be required.
(4) Satellite aerodynamicsdplasmadynamics will also be important at 90 to 130 krn altitudes.
The shape of the satellite and the location of instruments will be critical.
(s) Tethered system dynamic noise and its possible interference with experiments as well
as other Space Station activities (in particular, microgravity experiments) will be
important. Avoidance and/or control of dynamic noise will require careful evaluation of
the theory and of data obtained from the TSS-1 and TSS-2 missions
11


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Large structures and tethers working group

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