Some Novel Space Propulsion Systems
Copyright © 1997, Forrest Bishop, All Rights
Reserved
This is a draft paper
for a talk at the
Fifth
Foresight Conference on Molecular Nanotechnology.
The final version has been submitted
for publication in the special Conference issue of Nanotechnology.
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Introduction
The rapidly improving ability to build atomically precise
structural materials of extreme strength-to-mass ratio will
permit a revolution in aerospace engineering. Graphenes, or
'Buckytubes' may become available in commercial quantities
[Smalley], allowing the realization of previously untenable
proposals, such as the 'Skyhook', or geosynchronous tether, for
example [Pearson, Moravec, 1977, Zubrin]. A number of novel
spacecraft propulsion systems and machines are presented, as well
as new applications for some very ancient devices. The crossbow
and other mechanical catapults are re-examined in light of this
materials development [Bishop, 1997d].
A Solar System-wide transportation system is proposed by the
author [Bishop, 1997b] for freight and for spacecraft propulsion.
A network of accelerator/decelerator stations (e. g. Mass Drivers
[Clarke, 1950, Chilton, 1977, Lemke, 1982], lasers [Kantrowitz,
1972, Forward, 1962], etc.) in various positions around the Solar
System pass 'Smart Pellets' and other forms of matter and energy
between each other, to planets and other bodies, and to
spacecraft in transit [Early]. The systems presented here are
amenable to inclusion in that proposal.
Between the proposals for particle beam [Nordley, 1994] and
pellet stream [Singer, 1980] spacecraft propulsion lies an
immense, largely unexplored spaceflight regime [Bishop, 1997c].
Pushing a spacecraft using a collimated beam of mesoscopic
particles, very roughly on the order of a nanogram mass each
(plus or minus several orders of magnitude), presents new
opportunities for high speed interplanetary manned
transportation. This kind of beam can be tailored in velocity,
mass flow, and beam profile parameters to fit the mission
requirements. The ballistic coefficient, or mass-to-cross-section
ratio of this type of particle is much greater than single atom
particle beams, allowing more precise control over pointing and
dispersion. With atom counts per particle reaching into the
millions, molecular nanotechnologies may permit the inclusion of
entire guidance systems [Drexler, 1992a]. The receiver onboard
the spacecraft may be as simple as a pusher plate, or may
incorporate particle ionization and magnetic mirroring [Singer,
1980].
The performance of the rotating tether, or sling, [web ref] is
considerably enhanced by constructing it of Graphene fiber. A
relatively short sling with a reasonable taper ratio can attain
tip velocities of several tens of km/sec, along with firing rates
of several hertz, making it an attractive substitute for the Mass
Driver. Scaling down further, the notion of a mesoparticle sling
is introduced. A planar array composed of many thousands of
centimeter-size slings and associated support systems forms a
type of mesoparticle beam projector.
Combining the concepts of solar sailing [Drexler, 1979,
Forward, 1984a, 1984b] and mesoparticles leads to the notion of
mesoscopic solar sails. A mesoparticle beam composed of thin film
sails with nanoscale electronics and actuators may be able to
accelerate, turn, and navigate itself to a target spacecraft. Its
accelerator may be the Sun, or a laser located on a deep space
'relay station' [Bishop, 1997b]. The magnetic sail [Andrews,
1990, Zubrin, 1991] analog , though suffering a scaling
disadvantage, is investigated briefly.
The concept of pushing a spacecraft with small, high velocity
lightsails may be feasible in the near future. A proposal to test
that notion is introduced [Appendix B].
Short sling substitute for the Mass Driver
[Full size Figure 1: 4K, 681 x 224 pixels]
Figure 1. For slings between ~1 cm and ~1 km
length. Maglev guideway might be replaced with atomically precise
sliding surfaces and electrostatic vernier velocity control in
the smaller sizes.
In the course of researching this article, it
was discovered that a proposal similar to this has already been
made, as is often the case for inventors [web ref]. An internal
pellet tube with electromagnetic guideway. Time the release, no
tip release mechanism needed. The maglev guideway should be
capable of vernier velocity control.
Crossbow, Ballista
Not finished. See http://www.speakeasy.org/~forrestb for update. A non-linear composite-structure analysis
is being performed on the Graphene bow. This member may be on the
order of 100 meters across.
Figure 2. Crossbow launcher with velocity
ëtunerí and electromagnetic regenerative braking.
Mesoparticle Beam Propulsion
Between the mighty powerplants of the laser sailors and the
gigantic accelerators of the pellet stream riders lies an
immense, largely unexplored, spaceflight regime [Bishop, 1997c].
By using very many dust or smoke sized particles a high speed
accelerator can be built that is quite a reasonable size, with a
moderately proportioned electric powerplant. This kind of beam
can be tailored in velocity, mass flow, and beam profile
parameters to fit the mission requirements. The problem of being
able to maintain the collimation of this kind of force beam over
large distances is a difficult one, but probably solvable,
particularly if molecular nanotechnology [Drexler,1992a] is
brought to bear. For high accelerations over short distances,
picoradian beam collimation may not even be desirable.
An earlier example (apparently the first) of a mesoparticle
accelerator is a proposal of G. Landis [Landis, 1989] to allow
accelerated mercury atoms to coalesce into droplets enroute.
Another example is the authorís "Starseed/Launcher"
accelerator [Bishop, 1996b]. By ganging many thousands of these
devices together, several grams per second can be fired in a
nearly continuous, collimated matter beam, at speeds from a few
meters per second, to some fraction of lightspeed. The receiver
onboard the spacecraft may be as simple as a pusher plate or may
incorporate particle ionization and magnetic mirroring [Singer,
1980].
The same baseline design parameters were used for each of the
propulsion methods outlined in this study, where applicable. A
spaceship massing 10000 kg is accelerated by various mesoparticle
force beams, with a relative velocity, or closing speed, of 10
km/sec, except where otherwise noted. An Earth-Mars transit is
used for illustration, with trajectories similar to those
explored in the authorís 1982 study (Appendix A.).
Constant closing speed with ship.  Collision coefficient (1
= elastic, .5 = inelastic)
Ship speed
Ship acceleration (~1/10 gravity)
Ship Mass
Reaction force on ship
 
Mass flow received by ship.
The underlying design philosophy is to keep the spacecraft
propulsion system as simple as possible by having a constant mass
flow and closing velocity. More technical demands are then placed
on the beam generators and power supplies, but these are
relatively stationary pieces of capital equipment.
For this case, the launcher power goes as the cube of the beam
launch velocity ( ).
Kinetic power processed by the shipís capture system is:  
Of this amount, one portion is reflected backward or sideways,
the remainder is converted to heat.
 
For the example, with  (a 20% rebound velocity), 96% of the kinetic energy
incident on the pusher plate is converted to heat (neglecting
sideways rebound) . This would approach zero for the magnetic
mirror [Singer,1982]. For a plate that is its own radiator, a
minimum area  can be established (discounting transient and higher
order effects) by a power balance, assuming the backside of the
plate is a black body and the temperature of space is 0oK.
 where 
For an aluminum (m.p. 933o K) plate, with Tmax =
800oK and emissivity  =.06,  The
aluminum plate would have to be no thicker than one millimeter
for the example, to stay within the mass budget.
A higher specific power might be attained by creating a
plasma/hot gas cloud behind the pusher plate (which might be bell
or cup-shaped for containment and increased performance). This
would then function as the decelerator for the mesoparticles.
Cooling would have to involve active elements, such as liquid
droplet radiators. A magnetic mirror would be geometrically
compatible with this system.
An effective instantaneous specific impulse for a force beam
propelled craft can be defined as
 or  which gives  for
this example, with no capture losses. This exceeds the best
chemical rocket performance by a factor of nearly three, in
addition to not incurring the exponential degradation of carrying
and accelerating its own propellant (propellant mass expended is
a linear function the desired change of velocity for the example
propellant.
Figure
3. Mass ratio vs. terminal speed.
n = ship terminal velocity (km/sec)
An equivalent mass ratio can be defined as
Where  = total
stream mass launched
(This figure is based on 10 km/sec closing
speed, .6 elastic coefficient, with no capture losses.)
Figure 4. "Smart" Mesoparticle
made of Active Cells (Bishop, 1995, 1996a, 1996 b).
Mesoparticle sling , mesoparticle beam projector.
Figure 5. Slings for mesoparticle beam.
Rotation rate
Sling length
Density of Graphene
Working tensile strength
Tip area
The 'working tensile strength' for Graphene
'rope' is a guess, based in part on communications with B. I.
Yakobson [Yakobson, 1997].
 
Taper function
Tip acceleration
Tip speed
Taper ratio, cutoff at 120:1 Tip speed
(km/sec)
Tip acceleration in megagravities.
Microscale lightsails for beam propulsion.
Combining the concepts of solar sailing and mesoparticles
[Bishop, 1997c] leads to the notion of mesoscopic solar sails. A
mesoparticle beam composed of thin film sails with nanoscale
electronics and actuators may be able to accelerate, turn, and
navigate itself to a target spacecraft. Its accelerator may be
the Sun, or a laser located on a deep space 'relay station'
[Bishop, 1997b]. The nominal length of the
"accelerator" is the distance between the deployment of
the sails and their impact against the spacecraftís pusher
plate. This distance can range between several thousands of
kilometers (using high performance sails with laser assist
[Landis, 1989, 1995) and several light years (for interstellar
travel) [Forward, 1984b].
In most of this study it is assumed the manufacturing
tolerances are held to those of a capable nanotechnology
[Drexler, 1992a], removing the need to analyze dispersion due to
ballistic coefficient (mass-to-area) variations.
A thin film lightsail needs support against the photon
pressure. For macroscopic sails, this support is provided by
spars, weights on the ends of rotating members, and so forth
[Drexler, 1979]. This is sometimes expressed as a fraction
of the ëbareí sail mass-per-area, one value is 1.3 (total
mass/area is 2.3 , load factor is 2.3/1.3) for a very high
performance interstellar laser lightsail [Landis, 1995]. Drexler
estimates a minimum structural mass-per-area of .03 gm/m^2 for
sails larger than 10 km diameter. The areal density of 16 nm Al
film is .043gm/m^2, a near-minimum for any lightsail.
As the width of the sail is scaled down from hundreds of
meters to hundreds of nanometers, it becomes self-supporting, and
the parasitic structural mass can be eliminated, doubling the
acceleration performance.
The maximum areal dimensions of the unsupported film can be
increased somewhat if need be by slight geometric departures from
planar, such a radial crimping, rolled edges, and other compound
surfaces (keeping the thickness in the incidence direction
constant). A cone or other surface of revolution can provide
passive stabilization.
Figure 6. Acceleration performance of Solar
sails at 1 AU (a1, in m/sec^2) vs. mass per square meter (ma, in
grams/m^2).
Figure 7. A minimalist, self-supporting
steering mesoparticle lightsail The inherent stiffness of the
film at small length scales substitutes for structural
components. The mass for sensors and nanocomputers is distributed
over the backside area (not shown). Control is effected by
tensioning the four linear members at the corners.
The nominal thickness of a lightsail optimizes
near the skin depth; some fraction of the incident light is allow
to transmit through as a loss.
Density of Aluminum
Bounce coefficient
Reflectance
Solar flux ëconstantí
Sail thickness
 
Absorptivity Transmissivity
(16 nm Al @ 500nm incident wavelength)
An equation for acceleration of a flat plate
sail perpendicular to the incidence vector is:
 , which yields .181 m/sec^2 for the
16nm film at one AU. A lightsail with these specs released from
LEO adds 10 km/sec to its velocity vector while still in
cis-Lunar space {Figure 14}.
Spherical
Lightsail
In the course of researching this article, it
was discovered that a proposal similar to this, for a macroscopic
spherical solar sail, has already been made, as is often the case
for inventors [web ref].
The sphere suffers a geometric disadvantage
over the flat plate, to wit the frontal area is one-fourth the
total surface area, and the useable surface is at an angle to the
incident radiation. Its inherent simplicity and no need for
attitude control, does make it attractive. A number of design
parameters can be adjusted to minimize this deficiency, such as
making the film semi-transparent to allow the use of the rear
disk area in addition to the frontal area {Figure 8}. Using a
material, or a surface microstructure, that has a reflectivity
and absorptance that varies with the angle of incidence (high
absorptivity at low angle of incidence) may also increase
performance. An internal pressure insufficient to maintain
sphericity against the light pressure may help increase the
frontal area, as well as rotate much of the surface normal
towards the incident light vector.
Minsky [Minsky, 1997] suggests a half silvered
sphere incorporating nematic crystals (liquid crystals) in
portions of the film for attitude and thrust vectoring. These
would either switch from absorbing to transparent, absorbing to
reflective, or transparent to reflective. In any case, the force
on the affected area would change, effecting the vectoring.
[Full size Figure 8: 10K, 757 x 496 pixels]
Figure 8. A spherical lightsail with film
thickness less than the skin depth (ëpartially silveredí)
receives some additional thrust from its back surface.
Heliogyro, pivoting vanes
A more efficient design is a microscopic analog
of the "Heliogyro", which has performance equal to the
flat plate. The vanes that make up the sail can pivot about the
long axis, allowing reaching. Vanes that are a few microns wide
by a few microns long do not need to be tensioned by rotation,
nor do they need the roller furling used in the original concept.
The pivoting mechanism is thus greatly simplified; no swash plate
(or equivalent) is required {Figure 9}.
Beam Divergence
A fictitious ribbon of material of the same
average density and thickness of the mesoparticle lightsails,
closing with the ship at  , defines an equivalent sail width:
Density of sail material.
Thickness of sail material.
 
Equivalent width of a ëribbon sailí,
which is well under one meter wide for the
study design. A "shadowing factor" can be defined in
terms of this equivalent width as:  which for a beam width
of  is 
A statistical analysis of beam divergence
caused by one sail shadowing another can be undertaken using a
more sophisticated, time-dependent version of this type of
factor. At  =0, when the particles are released, the shadowing
factor blows up. A controlled means of dispensing and dispersing
the little sails at the emitter is therefore needed. One method
is to employ a small laser, perhaps a semiconductor diode array,
to give an initial kick. Sweeping the laser(s) over the departing
beam may provide some ëherdingí capability as well.
Some other factors influencing the trajectory
include the Solar wind, the various magnetic fields in space,
electrostatic charging [DeForest, 1979] gravitational
perturbations, manufacturing tolerances and so forth. For the
ëdumbí sails, these dispersive factors severely limit their
acceleration range and utility. Putting more sophisticated
guidance systems on larger "shepherd" lightsails may
reduce the total system cost [Kim, 1997].
Mesoparticle Magsails
Not finished. See
http://www.speakeasy.org/~forrestb for updated version.
Interstellar work
The idea of lightsail or Magsail streams may be
extended to the remote support of a starship. Although a lens
system similar to the "O'Meara Paralens" [Forward,
1984] would be needed to maintain laser collimation over the
acceleration path, the distance (d) is much smaller than for a
loaded (structure and payload) lightsail. The size of the lens
needed is correspondingly reduced, as is the pointing accuracy
requirement. The receiver might be a Magsail, rather than a
pusher plate. In this case, the particle stream has to have a
closing velocity great enough to ionize the atoms making up the
lightsails. With a bounce coefficient approaching unity, the
momentum imparted by the colliding lightsails is doubled.
The lightsails would need to reach a little
(develop lift perpendicular to the incident radiation) to stay
out of the rebound corridor. Alternatively or additionally, the
ship is reaching slightly. Each lightsail launched in this manner
might have to break apart to form a stream (similar to the
Shoemaker-Levy comet) before arriving at the starship. The
cost-effectiveness [Andrews, 1994] and feasibility of this has
not been studied in depth.
A Near-Term Proposal
Figures: emitter array, ship w/ thin film
pusher plate. MEMS sail.
It may be currently feasible to build a version
of the solar sail particle beam system. In this proposal, a
supply ship (ëtankerí) in Earth (or Solar) orbit releases a
stream of small, thin-film particles (lightsails). These are
accelerated by incident solar radiation over the course of a few
hours or days. The pusher plate onboard the (Mars-bound) ship is
perhaps several hundred meters wide to partially compensate for
beam divergence, and constructed of a thin film as well. It may
have a dual function as a very low-performance solar sail,
depending on the amount of degradation caused by particle impact.
The film thickness of the pusher plate may be made variable, such
that the inner portions are more durable. This capture film needs
to be thick enough that a particle impact transfers its momentum
without completely blowing through.
The particles are small lightsails with no
payload; either on the order of a few square meters each with
MEMS or NiTi actuators and photolith microprocessors, or perhaps
a few square microns (mesoparticles) with little or no guidance
capacity {Figure 7}.
For current state-of-the-art thin films, such
as aluminized Kapton, the maximum acceleration is on the order of
.01 m/sec. In order to reach the velocities of interest
for beam propulsion, these would need an acceleration path of
several million km. This can be accomplished by launching their
ëtankerí Sunward, perhaps to the L1 Lagrange point between
Earth and Sun, 1.5 Gm from Earth.
Figure 10. Numerically integrated trajectory
(solid line) in the Earth-frame for ëdumbí Solar sails released
from L1 at 1.5 million km. Sail acceleration = .01 m/sec^2.
Velocity at perigee (300 km altitude) ~20 km/sec (dashed line).
Closing velocity with ship in circular orbit at perigee radius
~12 km/sec) Distances in kilometers.
'Dumb' mesoparticles (conical) might be
manufactured currently by vapor deposition on an organic
substrate. The substrate has to be removed, perhaps by solvation,
the micro sails cut apart (not necessarily in that order), and
stacked in holders. A possible alternative to metal film is the
dielectric film quarter-wave plate, with a thickness on the order
of 50 nm [Landis, 1989]. The reflectivity is about 50%.
If this idea pans out, a ratio of 1.5:1
for mass in LEO to mass on Mars transfer orbit might be
attainable {ratio figure}. The best current chemical rocket
proposals (e.g. "Mars Direct") [Zubrin, 1996] have an
equivalent ratio of 3.5:1. In other words, the mass
required to be launched from Earth for a manned Mars mission
would be cut in half. This proposal can be 'proof-of
concept' tested in the near future with a few kilograms
piggybacked into LEO. An SBIR grant proposal has been submitted
to NASA to study this further [Appendix B].
Figure 11. RE=
Radius of Earth; Vri= distance from
Earth center (km/10); Vvi= velocity
relative to Earth ('smart sail') m/sec; Vv2[=
velocity relative to Earth (ëdumb sailí) m/sec; i= integration
step.
Figure 12. Numerically integrated
trajectories in the Earth-frame for 'smart' (solid line) and
'dumb' (dashed line) Solar sails released at apogee of 30000 km.
('dumb' at 25000 km) Distances in meters.
Figure 13. Trajectories in the Sun-frame for
'smart' (solid line) and 'dumb' (dashed line) Solar sails
released at apogee of 30000 km (dumb at 25000 km). Distances in
meters.
[Full size Figure 14: 5K, 883 x 246 pixels]
Figure 14. Numerically integrated trajectory
(solid line) in the Earth-frame for 'dumb' Solar sails released
at apogee of 900000 km. Velocity at perigee ~16.5 km/sec (dashed
line). Distances in kilometers.
On Mars
It requires much less energy and technological
sophistication to decelerate a ship nearing its destination than
to accelerate it using force beams. A facility on high Mars
orbit, for example, can emit a matter stream (exploding pellets
or mesoparticles) at velocities of 1-3 km/sec retrograde to
intercept an inbound ship from Earth {Figures 10-14}. In the
authorís opinion, the first expedition to Mars should be made
with the intent of staying there and establishing support
facilities for future travelers [Appendix A]. The "Mars
Direct" proposal [Zubrin, 1996] encompasses means of
producing rocket propellants, air and water on Mars for life
support and for the return journey. It is not too much extra
trouble to use these capabilities to establish an orbital base
supplied from the Martian surface, on Deimos, for example
(launching and landing on Mars is much easier than on Earth).
Material from Deimos could be scooped up and processed to form
the decelerator particles.
A low-speed accelerator, such as a rotating
tether might orbit (or be tethered to) Deimos and launch Smart
Pellets that disintegrate as they close with the inbound ship,
perhaps one or two million km from Mars. Two or more such
accelerators would be employed for redundancy. The shipís
aeroshell would have an expendable cover, or pusher plate, over
it. After decelerating to a safe speed, the pusher plate is cast
off, the ship makes a course correction with rockets, and
performs a normal aerobraking maneuver. The transit time from
Earth to Mars would be reduced from 180 days (current thinking
for a safe aerobraking) [Park, 1990, Tauber, 1990] to some
fraction of that, perhaps 60 days. Something similar could be
rigged up to assist returning to Earth if some need for that came
up.
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Appendix A
InterPlanetary Mass Driver
(IPMD) (1982)
This earlier beam-propulsion proposal used a
Mass Driver in a highly elliptic Earth orbit (HEEO), preferably
supplied from extraterrestrial resources, to shoot supplies and
momentum to ships in low Earth orbit (LEO), similar to the
"Micro Lightsails for Beam Propulsion" proposal. At the
time it was thought that 'pellet stream propulsion' was original.
The project was abandoned when this was found not to be the case.
In the main study, a Mass Driver, rated at 8
km/sec, would begin a salvo after it passed through apogee, timed
to intercept a ship after it performed part or all of its
hyperbolic burn. Some work was done on using the thrust generated
by the reaction of the payloads on the Mass Driver to help adjust
its orbit for a follow-on launch after its next apogee passage.
The intent was to send a fleet of ships (e.g. 20) in sequence to
Mars on (one way) high energy transfers. Copies of some original
notes available on request.
Appendix B
NASA SBIR Phase 1 Proposal
#97-1 08.02-5268 Oct. 9, 1997
Micro Lightsails for Beam
Propulsion Institute of Atomic-Scale Engineering
Micro Lightsails for Beam
Propulsion
Topic 08.02 Advanced/Exotic
Space Propulsion Systems Technologies
Part 1:
Identification and Significance of the Innovation
A force beam composed of hundreds of thousands
of thin film Solar sails with micro or nanoscale electronics and
actuators should be able to accelerate, turn, and navigate itself
to a target spacecraft. The nominal length of the
"accelerator" is the distance between the deployment of
the sails and their impact against the spacecraftís pusher
plate. It may be currently feasible to build a version of this
Solar sail particle beam system. In this proposal, a supply ship
(ëtankerí) in Earth (or Solar) orbit dispenses a stream of
small, thin-film particles (lightsails). These are accelerated by
incident solar radiation over the course of a few hours or days.
Stream trajectories can be chosen that intercept a ship in LEO,
hitting its ëpusher plateí at several km/sec. The ëequivalent
specific impulseí can reach well over 1000 seconds, with no fuel
carried and accelerated by the ship itself. If this idea pans
out, a ratio of 1.5:1 for mass in LEO to mass on Mars
transfer orbit might be attainable. The best current chemical
rocket proposals (e.g. "Mars Direct") have an
equivalent ratio of 3.5:1. In other words, the mass
required to be launched from Earth for a manned Mars mission
would be cut in half. This proposal can be ëproof-of
conceptí tested in the near future with a few kilograms
piggybacked into LEO.
The pusher plate onboard the (Mars-bound) ship
is perhaps several hundred meters wide to partially compensate
for beam divergence, and constructed of a thin film as well. The
film thickness of the pusher plate may be made variable, such
that the inner portions are more durable. This capture film needs
to be thick enough that a particle impact transfers its momentum
without completely blowing through, and strong enough to resist
tearing.
The particles are small lightsails with no
payload; either on the order of a few square meters each with
MEMS or NiTi actuators and ëbare dieí silicon photolithography
microcontrollers, or perhaps a few square microns (mesoparticles)
with little or no guidance capacity.
For current state-of-the-art thin films, such
as aluminized Kapton, the maximum acceleration for sails without
payload is on the order of .01 m/sec. In order to reach the
velocities of interest for beam propulsion, these would need an
acceleration path of several million km. This can be
accomplished, with a launch velocity penalty, by launching their
ëtankerí Sunward, perhaps to a loiter orbit at the L1 Lagrange
point between Earth and Sun, 1.5 Gm from Earth.
The lightsails, ëtankerí and associated
hardware still have to be launched to high orbit by conventional
means- existing launchers to LEO, then chemical, ion or, Solar
sail to high orbit. This cost can be roughly compared to the cost
of launching the rocket and its fuel into LEO that the system is
replacing.
One interesting feature of this idea is the
wide latitude for abort. The ship in LEO has several days to
decide whether or not to place itself in the oncoming beam path.
A few m/sec burn will put it outside the stream corridor. The
lightsails that make it to Earthís vicinity have enough velocity
to end up in either Solar orbit, or to escape the Solar System
entirely (presuming they remain unfurled and operational).
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