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Mass beam propulsion, an overview

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  • Icarus Interstellar

Abstract and Figures

An alternative to rockets is to push spacecraft with a reflected beam. The advantage is that it leaves most of the propulsion system mass at rest. Use of mass beams, as opposed to photons, allows great efficiency by adjusting the beam velocity so the reflected mass is left near zero velocity relative to the source. There is no intrinsic limit to the proper frame map velocity that can be achieved. To make a propulsion system, subsystems need to be developed to acquire propulsive energy, accelerate the mass into a collimated beam, insure that the mass reaches the spacecraft and reflect the mass. A number of approaches to these requirements have been proposed and are summarized here. Generally no new scientific discoveries or breakthroughs are needed. These concepts are supported by ongoing progress in robotics, in nanometre scale technologies and in those technologies needed to use of space resources for the automated manufacture of space-based solar power facilities. For mass beams specifically, work in particle sizing, acceleration, delivery and momentum transfer is needed. For human interstellar flight, a notional schedule to provide a mass beam propulsion system within a century is provided.
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Mass Beam Propulsion, An OverviewJBIS, Vol. 68, pp.?-?, 2015
This paper was presented at the 100 Year StarshipTM Study Symposium,
30 September - 2 October 2011, Orlando, Florida, USA. It was
presented in the Time Distance Solutions technical track.
MASS BEAM PROPULSION, AN OVERVIEW
1. INTRODUCTION
Instead of reacting against its own exhaust, a beam-propelled
spacecraft is pushed by something else. This general concept
is sometimes called “momentum beam propulsion.” This
eliminates the exponential problems of mass ratio, exhaust
velocity and power-associated mass that lead an interstellar
rocket to be either very slow, or be many (perhaps thousands)
of times more massive at the start of its journey than at the end;
even with nuclear energies available. The agents of momentum
transfer can be photons, objects of ordinary mass, or perhaps
something like “dark matter” that physics has yet to describe
will enough for propulsive uses. The system consists of a
projector, the momentum transfer agents and the reector on
the spacecraft, as sketched in Fig. 1 for mass and photon beams.
If the beam is made of photons, the beam-propelled spacecraft
is a light sail of the sort about which Dr. R.L. Forward, among
others, has written extensively [1].
Granting the enormous potential of laser sails, there is a
signicant advantage to using something that transfers more
momentum per unit energy than a photon. A photon must travel
at the speed of light and until relativistic velocities are reached,
a reected photon carries away almost as much energy as it
started with. A massive particle’s velocity, however, can be
tuned so that the reected mass is left almost dead in space
relative to the beam generators, having surrendered almost all
of its kinetic energy to the starship. One can, of course, imagine
many options for reectors, mass particles, beam drivers and
space energy infrastructure for this concept.
Figure 1 compares the results of reecting a TJ laser beam
segment with the results of the reection of mass with one TJ
of kinetic energy in the sidereal frame of reference (0.67 TJ in
GERALD D. NORDLEY1 AND ADAM JAMES CROWL2
1. 1238 Prescott Avenue, Sunnyvale, CA 94089, USA.
2. 4 Ulmarra Crescent, Strathpine 4500, Queensland, Australia
Email: gdnordley@aol.com1
An alternative to rockets is to push spacecraft with a reected beam. The advantage is that it leaves most of the propulsion
system mass at rest. Use of mass beams, as opposed to photons, allows great efciency by adjusting the beam velocity so
the reected mass is left near zero velocity relative to the source. There is no intrinsic limit to the proper frame map velocity
that can be achieved. To make a propulsion system, subsystems need to be developed to acquire propulsive energy, accelerate
the mass into a collimated beam, insure that the mass reaches the spacecraft and reect the mass. A number of approaches to
these requirements have been proposed and are summarized here. Generally no new scientic discoveries or breakthroughs
are needed. These concepts are supported by ongoing progress in robotics, in nanometre scale technologies and in those
technologies needed to use of space resources for the automated manufacture of space-based solar power facilities. For mass
beams specically, work in particle sizing, acceleration, delivery and momentum transfer is needed. For human interstellar
ight, a notional schedule to provide a mass beam propulsion system within a century is provided.
Keywords: Mass beam, pellet beam, particle beam, interstellar, propulsion
the proper frame). The white area represents the sidereal frame
(s.f.) of the system where the beams are generated and the gray
area represents events in the spacecraft, or proper frame (p.f.)
of reference. The light pulse frequency, and thus its energy E,
is lower in the p.f. It delivers (approximately) a momentum
change, dp = E/c coming and going for a net momentum
change of 2 (E/c), [1] where M is the mass of the spacecraft and
reector, resulting small downshift in frequency and energy
in the p.f., neglected in this illustration. The reected light
pulse, no going the opposite direction, is further downshifted
with respect to the s.f. resulting in an approximate energy of
0.44 in the s.f. The physical particle’s initial velocity of 0.8 c
translates to an incoming velocity of -0.5 c in the p.f. In the
approximation of an elastic collision it delivers a momentum of
γ m vb to the reector coming and going, departing at about 0.5
c in the elastic approximation. At this relative velocity, about
40% of the laser energy is lost, while almost all of the particle
energy is delivered to the spacecraft.
Even a conjectural “space drive” that uses the rest of the
universe, somehow, as its reaction mass would not perform as
well as a mass beam propelled spacecraft if the mass of the
energy source to power the “space drive” must be carried on the
“space drive” starship. By E = mc2, that energy has an inertial
mass, m = E/c2 [2] and so the “space drive” has a mass ratio
similar to that of the rocket; it must lose mass to gain relative
velocity. No propulsion system that must carry its own energy
source can go faster than a system that can use the virtually
unlimited energy of its home star.
In 1980, in a JBIS paper C.E. Singer [3] proposed an
interstellar mass beam propulsion scheme that contains most
of the elements of mass beam propulsion discussed below.
Singer’s work was noted in The Staright Handbook [4] and
James Early’s [5] work on force beams. Landis included a short
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Gerald D. Nordley and Adam James Crowl
suggestion of using mercury atoms to push sails in a paper on
solar sails [6].
In the late 1980’s, Zubrin and Andrews [7, 8] and Vulpetti
[9] studied using the solar ion wind to push magnetic sails.
However, the rather tenuous and velocity-limited solar wind
has obvious limitations on acceleration and ultimate velocity.
Why not provide a much denser beam? Nordley [10] worked on
variable beam-velocity dynamics and suggested self-steering
pellets [11].
A conference on near-term robotic interstellar probes was
held in 1995 and featured several mass-beam-related papers
subsequently published in JBIS [12, 13].
Jordin Kare presented the idea of a two-stage propulsion
system using laser-propelled sails to push a larger spacecraft
in several papers following 2001 [14, 15, 16, 17]. At the
International Astronautical Federation (IAF) Conference in
Toulouse, France, in 2001, Nordley [18] outlined nano-pellet
guidance. Forest Bishop documented his studies on particle
size, guidance and acceleration in 2003 [19]. Andrews, in a
2003 paper [20] reiterated the point that mass beam propulsion
works with known physics.
The mass beam propulsion systems described in the
literature would be complex; decisions about some parts
will affect other parts. Understanding the general kinematics
leads to performance targets for reectors, which in turn
place constraints on the mass beams, their acceleration and
projection system requirements. Some choices will result in
greater technological challenges than others. However, at no
point is any physical process needed that has not already been
demonstrated and there are engineering models for much of it.
2. GENERAL KINEMATICS
The notional mission examples used in [10] assumed a constant
acceleration. Constant acceleration makes mission studies
easier, but it is probably what one wants to do anyway. For any
particular reector design, the more force it must withstand, the
heavier it will be. So it makes sense to operate the reector at
its maximum design acceleration (with a reasonable margin of
safety) for the entire acceleration period; one wants to get up to
speed as quickly as possible, make the acceleration path as short
as possible and make efcient use of any mass that isn’t payload.
It is immediately apparent that a spacecraft propelled by
reected mass cannot go faster than the velocity of the mass
Fig. 1 Photon and Mass
Momentum Beam Reection.
3
Mass Beam Propulsion, An Overview
propelling it. It is also apparent that starting out with a very high
beam velocity would waste a great deal of energy in the form of the
kinetic energy of the reected particles; this is the basic efciency
problem of photon sails. The solution to this is to increase the
beam velocity during the acceleration of the spacecraft.
For this velocity increase program, one needs to nd the beam
velocity in the originating frame of reference of the sun and other
(relatively) “xed” stars, (hence the “sidereal reference frame”
or s.f.) as a function of the spacecraft position and velocity.
If the spacecraft relative velocity is v and the mass beam
particles arrive with a negative velocity, -v, in the spacecraft
proper reference frame (hence “p.f.”), after they are reected
inelastically, they are left with a zero residual velocity, “vr,” in
the sidereal reference frame. This means that all their kinetic
energy and momentum has been transferred to the spacecraft.
Numerical experiments with the model developed in [10] by
Nordley and later replicated by Crowl initially indicated that
the greatest momentum delivery efciency was, as one would
expect, at the relativistic equivalent of vp = 2 vs, where vp is the
velocity of the particle and vs the velocity of the spacecraft.
An energy efciency factor, “e”, was added to account for
reected beam transverse velocities, reection inefciencies
and particle losses. For a perfect colinear inelastic reection
of all particles, e=1. The efciency maxima in specic cases
as a function of residual velocity were broad (Fig. 2) however,
and became broader as relative velocities got higher. For lower
values of e, minima moved to higher values of vr.
It was apparent that as spacecraft velocity approached a
gamma of two, where incoming particle relative velocities
approaches those of cosmic rays, there was not much to be
gained by further increases in relative beam velocity. Indeed,
signicant reduction in the nal, peak, power needed can be
had at little cost to efciency by tolerating fairly high residual
beam velocities. It was also apparent that in the very early
stages of acceleration, the formula, combined with a constant
acceleration, led to and extremely high mass ow rate at low
relative velocity.
Numerical experiments indicated that a good result overall
would be achieved with a beam velocity program that uses:
(a) a xed beam velocity of about .01 c until a spacecraft
velocity of 0.005 c is reached
(b) the vb 2vs law until signicant relativistic velocities
are reached and
(c) velocity increases as needed for a constant velocity
relative to the spacecraft in the proper frame.
The velocity at which the beam velocity program changes
from (b) to (c) above would be a trade that depends on specic
beam and vehicle engineering parameters. Figure 3 illustrates
the application of this program to accelerate a 1,000 ton starship
to 0.866 c.
Note the difference between the pellet launch time and the
beam power curves; high beam powers don’t occur until late
in the program. Because most of the energy consumed is in
the high-velocity regime, the (c) strategy of maintaining a
simple xed velocity relative to the sail, as in Kare [16], is a
reasonable compromise for rst order analysis that results in
high energy transfer efciencies over a broad range of nal
vehicle velocities.
3. REFLECTORS
Singer [3] proposed magnetic mirrors as reectors and Nordley
[11]] chose a magnetic mirror with two loops as a reector (Fig.
4). This was inspired partly by Andrews and Zubrin’s magsail
work [8], but also by work on magnetic nozzles for nuclear and
antimatter pulse spacecraft [16, 21, 22, 23] which work with similar
peak relative particle velocities and eld strength requirements.
To operate, a magnetic mirror requires the mass hitting it to
have an electric charge. Most of the incoming mass must thus
be converted into plasma as it approaches the starship; lasers,
particle beams, or particle explosions could all accomplish
this. If the impact plasma is dense enough once “ignited,” it
might even serve to ionize the incoming mass itself, as a plasma
contained in front of the spacecraft might serve to vaporize
interstellar dust particles, as proposed by Landis [24].
The outer loop, essentially, channels this incoming plasma to
the inner loop, where most of the force is felt. Particles escape
the eld as they recombine and become neutral or follow eld
lines that merge with the galactic magnetic eld. Some of the
plasma squirts forward along the eld lines running along the
axis of the current loops. This is not necessarily a bad thing; the
starship will thus be preceded in space by a “guard plume” of
hot gas which will tend to ionize and perhaps deect some of
the already tenuous interstellar medium in front of it, reducing
drag and making less work for shielding systems.
Fig. 2 Energy transfer efciency η and power as a function of the
s.f. frame velocity of residual beam mass. Three sets of three curves
are shown, for spacecraft relative velocities of 0.4 c, 0.6 c and 0.866
c respectively. Each set has curves for reection efciencies, e, of
1.0 (upper curve, for no losses), 0.95 (middle curve) and 0.9 (lower
curve).
4
Gerald D. Nordley and Adam James Crowl
For interstellar thrust levels, the loops would probably need
to be superconducting and probably will have to be cooled. For
larger pellet sizes, variations in plasma pressure will need to be
considered, [14, 16] but how much energy this will require is a
question for future science and technology. Higher temperature
superconductors would be desirable, but aren’t required. Kare
[16] estimates a mass of about a metric ton for a sailbeam
reector with a loop radius of 100 m. Nordley’s models use a
50 m radius loop.
For unguided beams, Landis [25] proposes that the size of
the reecting surface be increased by “inating” an articial
magnetosphere as described by Winglee et al. [26] created by
the superconducting loop with ions provided by the ionized
beam mass, resulting in an impressively large target. The drag of
this object on the interstellar medium would be signicant, but
the effective area of the sail would be reduced after acceleration.
In 1994, Nordley [27] proposed a non-superconducting
aluminium toroid propelled by a neutral sodium beam for a
Jupiter mission, as a rst step toward more capable interstellar
systems. The aluminium served as the hull of the spacecraft as
well as the current conducting element.
An auxiliary power source will be needed to maintain the
eld and ionize the mass beam, at least at the start. Ordinary
ssion systems would work, but by the time such spacecraft
are built, some kind of compact nuclear fusion reactor might be
available. Once underway, one might be able to bleed off some
Fig. 3 Acceleration prole. p.f. acceleration. = 1 LY/Y2 , e = 0.9
M = 1 × 106 kg.
Fig. 4 Dual loop magnetic mirror reector.
5
Mass Beam Propulsion, An Overview
of the propulsion beam energy for additional power during the
propulsion phase, if needed, with systems such as described by
Hyde [21] for pulsed nuclear propulsion.
Note that the plasma temperatures, densities and pressures
for higher accelerations will have much in common with such
pulsed nuclear systems, though probably with smaller, more
frequent “pulses.” See Andrews [20, 22] and Lenard [22] about
“Mini-MagOrion.”
After the spacecraft reaches cruising velocity; the magnetic
mirror will probably stay on. It is still needed to deect
relativistic “wind of passage” interstellar ions around or through
the empty centre of the spacecraft. Also, there’s signicant
energy in the currents of those loops; quenching them could
be a nontrivial exercise in heat rejection. Finally, the magnetic
mirror is needed to decelerate at the starship’s destination. As
discussed in Kare [16], the size of the mirror might be reduced
for the cruise phase, then be greatly expanded for deceleration.
For voyages to previously settled star systems, the deceleration
particle stream needs to be projected with only enough velocity
to get it deployed in time for the incoming starship to use it,
so its energetics would be trivial in comparison to launching a
starship. It would not necessarily need to be a “smart” stream;
since one is not pushing the deceleration stream to relativistic
velocities, one could use a brute mass approach and ood the
general area with nearly stationary mass at the needed density,
a bit like a cosmic runaway truck lane.
Of course, the rst spacecraft to reach an uninhabited star
system will not have a cooperating beam of particles coming out
to meet it to help it slow down. First missions may be entirely
automated, go more slowly than subsequent travellers and their
mass may consist mainly of systems to slow themselves down.
Deceleration systems include magnetic sails, Forward’s giant
retro mirrors that reect photon beams focused all the way from
the solar system and various forms of nuclear rockets. Two or
more of these systems might be combined; a magnetic sail
might be used in the initial stages, of deceleration, followed by
a nuclear pulse system like Mini-MagOrion. Dr. Gerald Smith
[28] has worked on an antiproton-ignited ssion/fusion system
that may be easier to do than many other pellet fusion systems.
However a rst mission decelerates, once in the target
planetary system, the vehicle would locate a suitable source of
mass and energy and its replicators would then start gathering
materials and replicating. They would gather information,
manufacture an infrastructure for subsequent exploration and
send out the slowdown path for the next set of visitors.
It is essential for the mass beam be turned into plasma as
it approaches the starship for two very important reasons:
magnetic elds only reect charged mass and the impact of
uncharged, undeected relativistic mass propulsion mass on
the spacecraft could be disastrous. Generally, one approaches
such a situation with several layers of contingency planning.
For atomic beams, lasers can readily ionize the incoming
mass. If not, interaction with the dense plasma trapped in the
reecting magnetic eld may do the job. If the spacecraft has a
toroidal geometry (and particle guidance is good), un-ionized
mass ux will go through the central hole. Finally, an extra
layer of water (such as a swimming pool) at the bottom end of
particularly vulnerable parts of the spacecraft could be a last
defence against wind-of-propulsion radiation.
For actively guided particles (see below) the particles
can be composed mainly of metastable, explosive material
designed to detonate and vaporize given the correct signal
and/or conditions. Unexploded pellets/particles with guidance
intact will pass through the central hole of a toroidal geometry.
Unexploded pellets/particles without functioning guidance
systems are likely to miss the spacecraft by a wide margin. For
the very few that survive all the above, sufcient aft-end mass
should serve to prevent disaster as long as impacts are few.
Reectors deliberately designed for impact would be simple
but would erode over the course of the mission and so be
massive and need to be replaced after every voyage (but this
could be a trivial chore, all considered). Simple sails designed
for use with low-relative-velocity neutral atomic beams have
also been proposed.
In a very advanced system, the reector could be a coaxial
frictionless electromagnetic launcher (EML). These could (in
principle) be run in reverse to catch incoming relativistic pellets,
bend them around and send them back again. Nordley [29]
described a much more modest system to, in sense, react against
the gravitational eld of a planet, albeit at much lower relative
velocities by sending mass in a retrograde orbit around it.
In an interstellar version, it has been noted that one starship’s
returned pellet stream could perhaps to serve as the deceleration
path for the next accelerator/starship, whose deceleration would
provided the energy and pellet stream to accelerate yet another
starship. If this could be done with perfect efciency, the
traveling accelerators could bounce back and forth between stars
trading energy with pellet streams, becoming, in effect, a “free”
interstellar transportation system. Some of the ideas mentioned by
Lebon [29] for “Magnetic Shepherding of Orbital Grain Streams”
might be applied to this. Of course, in practice, there would be
both energy and pellet losses to be made up, but there would a
great energy multiplier effect over non-recycled pellet streams.
“Flying Dutchman” starships could roam the galaxy,
deected by beams from star to star instead of being decelerated,
carrying data between civilizations in much larger chunks than
could be managed by lasers or masers. Passengers could be
picked up and delivered to such starships for a fraction of the
energy that would be needed to accelerate/decelerate complete
starships. The pellet guidance accuracy for this would be, of
course, much greater than that needed for simply hitting an
extended starship reector.
4. MASS BEAM DELIVERY
The major problem for a mass beam propulsion system is to
hit the starship with momentum particles at distances that
approach a light-year at the end of the acceleration period.
Even a hundred-meter-radius reector is a target of only about
2 × 10-14 radians as viewed from a beam driver half a light-year
away; think of a three-millimetre ball bearing at the distance of
the Sun from Earth.
There are two types of particle trajectory error to be
considered for correction: a systematic error due to insufciently
accurate pointing of the beam driver and random dispersion or
spreading of the beam due to cross-beam velocity differences
of individual particles, either initially, or caused by impacts
with interstellar gas.
The rst kind of error could be solved by the starship moving
6
Gerald D. Nordley and Adam James Crowl
toward the beam centre as the beam centre drifts. However,
the end of the beam might be hard to chase; toward the end of
the acceleration period the starship may be tenths of a light-
year from the projectors and if beam pointing changes by
microradians even on the scale of weeks, the beam centre might
move at several kilometres per second, too fast to chase with
a reasonable on board fuel supply. For an unregulated beam,
pointing stability, as opposed to absolute accuracy, would be
essential [6, 23, 26].
The second kind of error is beam spreading, sometimes called
the beam “temperature” since, in a frame of reference traveling
along with the beam, the random velocities of the particles look
like thermal expansion. Redirecting these velocities toward the
beam centre is called “beam cooling.” Singer [3] proposed to
use lasers stationed along the acceleration path to push errant
pellets back into line.
Andrews [p.7, 20] proposed using accelerations up to 200
gravities to keep the spacecraft close enough for an unguided
plasma beam that follows interplanetary eld lines. A crew in
such a starship would have to be totally immersed. Even a depth
of 0.20 m at such a g load would be the equivalent to scuba
diving at a depth of 40 m. Bracing, but perhaps survivable.
Landis’ neutral mercury beam is cooled to interstellar
effective temperatures. The beam is “stiff” relativistically and
may effectively reduce its dispersion velocity by condensing
into more massive mercury droplets in route. Andrews [20]
mentions hydrogen as beam material. Nordley [27] used
sodium atoms for a near-term interplanetary suggestion.
Laser cooling techniques such as a photon eld lens, described
by Minogin [31], could be used during acceleration, immediately
after acceleration and possible along the route of the beam to
improve collimation. A photon eld lens is basically a set of four
tuned lasers pointing at the atomic beam path. The lasers are
tuned just below a signicant absorption frequency of the atoms;
if the atoms stay on the beam, they don’t absorb a photon. If they
move toward the laser, the doppler shift of their motion brings
them into resonance with the laser and they get pushed back.
While the neutral beam cooling infrastructure needed to see
that the atoms/cluster hit the reector would be complex, this
might be a candidate for near term systems in that it doesn’t
require anything that hasn’t already been built, at least on a
laboratory scale.
Nordley [11], with a nod toward the milder forms of
nanotechnology, proposed that the beam particles steer
themselves to the spacecraft following a homing beacon. The
manoeuvre capability of the beam particles would be limited
- as Kare points out [16], one wants them to hit the starship
with some mass left! - but so would the “thermal” dispersion
velocities, particularly for more massive particles.
The circuitry would be much smaller and simpler than
today’s integrated circuits [18, 32], but a similar kind of thing.
The particles could all be identical and stamped out in the
billions and billions by automated facilities. This would tap into
the burgeoning interest in nanotechnology and its possibilities,
as described in Drexler [33, 34] or Vinge [35].
Fig. 5 Notional self-steering pellet
architecture.
7
Mass Beam Propulsion, An Overview
Figure 5, from [18, 32], shows an approach to a self steering
particle. The particle would spin with its spin axis nominally
pointed at the spacecraft, which would provide a homing
beacon. If all is well, the error sensors on the sides of the
particle don’t see the beacon and it doesn’t do anything. With
the right choice of manoeuvre size, a very simple sequence of
alternating attitude and velocity manoeuvres might eventually
bring the particle back on a path to the beacon. If it works,
the “computer” that alternates manoeuvre modes could be a
simple one-bit ip op, reset each time the propulsion capacitor
discharges. This means the particle homing system could be
extremely small.
In this particular example, the particle would have a
radius of about 500 nm and a mass of 1.66 × 10-16 kg. Even
at this small mass, most of the particle would be some form
of structural matrix. It would be useful if this bulk could be
made of something fairly easy to turn into plasma, or perhaps
it could be made of materials endothermic enough to facilitate
their own decomposition into a plasma or at least an ionizable
vapour as the enters the plasma behind the magnetic mirrors.
Bishop [19] and Kare [16], have proposed manoeuvrable
momentum transfer particles at opposite ends of the size
spectrum. Bishop’s “meso particles” might consist of only a
few million atoms while the microsails of Kare’s sailbeam are
large enough to tolerate repeated collisions with the interstellar
medium.
Kare [16] discusses terminal guidance options for his
sailbeam that also would apply to other self-guided particles
and points out that techniques that work at radio frequencies
for aircraft precision guidance could be downscaled to optical
dimensions for mass beam particles.
Because much less mass would be needed to simply provide
a directional reference for self-steering particles than to actually
push them back into line, the starship could carry and shed such
reference stations along the course of its progress, as shown in
Fig. 6.
How small to make momentum delivery particles is a future
design trade. The smaller the particles, the easier it will be to
accelerate them to relativistic velocities, the smaller the target they
make and the easier it will be to steer them back on course. The
larger they are, the less sensitive they will be to collisions with
atoms of interstellar gas. Beam pellet/particle interaction with
the interplanetary/interstellar medium was and continues to be a
signicant concern for Ruppe [36], Singer [2] and was a primary
consideration for the size of the Pellets described by Singer.
Atomic nuclei are very small compared to atoms, thus
impacting atoms would almost always pass completely through
the particle, transferring only the momentum of their stripped
electron shell. Whether this would be a survivable event in
terms of momentum transfer is not clear. If it is, the particles
should contain a passive nano-damping mechanism in case an
impact with an interstellar gas atom causes precession. There
are also very rare interstellar dust motes, but should a small
beam particle strike one of these, one can presume it lost from
the beam entirely. One simply adds sufcient beam particles to
make up for such losses.
While this paper is primarily concerned with the other end
of the space vehicle, there are an excellent and cautionary
discussion of interstellar wind of passage in Andrews [37] and in
Martin [38]. Landis has proposed a plasma shield and calculated
that it would actually become more effective at higher velocities
[25]. Note also Arthur Clarke’s ctional response to the problem;
i.e., to place what was essentially a large iceberg in front of his
spacecraft [39]. When all else fails, given a sufciently large
source of beam-driver energy, one can always throw more mass
at the problem. Matloff and Fenelly (as reported in the Staright
Handbook [p.115, 4]) investigated forward-pointed ultraviolet
lasers to increase interstellar media (IM) ionization for ram
scoop purposes. The same technology would be used to ionize
most of the neutral IM for deection purposes.
It is also important to note that the particle beam is not
passing through virgin interplanetary or interstellar space. It is
propagating through the wind-of-passage shadow of the starship,
its magnetic elds and associated efuvia, as qualitatively
illustrated in Fig. 7. The relatively high temperature reected
mass should quickly vacate the volume behind the starship.
Fig. 6 Spacecraft shedding guidance
beacons.
8
Gerald D. Nordley and Adam James Crowl
The charged particles deected by the ship’s magnetic
eld should do a fairly good job of sweeping away the ions
in the interstellar medium. Any matter ahead of the ship
that is not initially ionized should be ionized by onboard
lasers by the time it gets near the ship, though deection by
neutralized atoms in the forward -leaking propulsion boom
may be signicant as well. It is not clear how rapidly the “hot”
component of interstellar medium, mainly protons, will ll
in behind the starship because, for one, it is not clear how
much of a void will be initially created. This all needs to be
modelled.
Particle shape is also subject to trade analysis; Fig. 8 shows
a “hockey puck” but an open ring, or a “snowake” like
Bishop describes for his mesoparticles, or a miniature net like
Forward’s Starwisps [40] might work better. R. L. Forward
suggested that his starwisps could be used in this fashion this to
Nordley.
The Benford experiments [41] demonstrated levitation of
carbon lm sails at one gravity albeit with thrust mainly derived
from outgassing. Building sail beams of high temperature
materials that can radiate absorbed energy effectively may
prove an effective strategy for high acceleration microsails and
thus sailbeams.
5. MASS BEAM ACCELERATION
Many people are already studying how to throw very small
things very fast. Machines like the relativistic heavy ion
accelerator at Brookhaven National Laboratory already exist
and experiments with ultrahigh velocity cluster ion impacts
have been also been conducted. The Brookhaven machine
accelerates bunches of billions of gold atoms up to a gamma
of about 100 [41]. Beam luminosities of nearly a mole per
second have been achieved (though for intervals of much less
than a second). This in itself exceeds the velocity and mass
luminosity needs for simple particle beam propulsion by
orders of magnitude, though machines like the Brookhaven
Relativistic Heavy Ion Collider are not designed for continuous
operation nor necessarily for efciency at the levels desired by
propulsion engineers.
The process of accelerating a physical object will be
substantially different than that of a bunch of heavy ions;
however, some of the same techniques, such as stochastic
cooling [42], might be used to lower beam temperature
during the acceleration process. Particles large enough to
have homing systems might also be made with permanent or
accelerator energized magnetic dipoles, providing a way other
than electrostatic charge for accelerator elds to interact with
them.
Singer [4] proposed a linear accelerator for his pellets that
would be of planetary dimensions (say, 10,000 km, of 1/4 the
circumference of the Earth) in length. These would be anchored
to convenient asteroids. In the weightless vacuum of space and
given space resources to use, the construction effort required
for such accelerators would be much less than their size implies
and much, much less than required for, say, the superconducting
supercollider on Earth. Many would be needed and economies
of mass production would ensue.
For self-steering particles of less than a billion atomic mass
units, it may be reasonable to think in terms of hundreds rather
than tens of thousands of kilometres of beam line. Bishop
Fig. 7 Wind shadow and Bow
Shock waves.
9
Mass Beam Propulsion, An Overview
[15, 16] and Andrews [14, 20] have written extensively on this.
If the sails are substantially conductive, they may inductively
self-vaporize on encountering a Tesla-class magnetic eld at
relativistic velocities.
6. ENERGY FOR MASS BEAM PROPULSION
At a gamma of two, the kinetic energy of a starship equals its
rest mass energy, mc2. A thousand-ton starship moving at a
gamma of 2 would thus have a kinetic energy of 9×1022 joules,
about a hundred times as much as the current total annual
world nonfood energy consumption, or about two million one-
megaton nuclear weapons.
To get this amount of energy into the starship, considering
inevitable conversion and transportation inefciencies, several
times that much energy would need to be collected, let’s say
around 5×1023 Joules. While this is an awe-inspiring number,
it is not characteristic of mass beam propulsion; it would be
true of any starship of that mass moving at that velocity. Note
that a million-ton “generation” starship moving at a gamma of
1.001 (0.045 of light speed) is equivalent in this respect. Due
to the relative efciency of the processes involved, mass beam-
driven spacecraft are likely to need less total energy to get a
starship up to relativistic velocities at accelerations compatible
with short trip times.
Where will all the energy for interstellar travel come from?
A breakthrough in fusion power technology might bring the
moons and atmospheres of the giant into play, but solar energy
seems the most straightforward choice. This is generally the
choice of photon sail designers, such as Forward [1], who
envisioned vast arrays in solar orbit to gather energy for the
lasers that would power such systems.
Figure 9, from [18] shows the growth of installed solar power
capacity with time, assuming each factory system reproduces
itself and a 1-gigawatt (a U.S. billion watts) solar power array
each year. The mass of a few medium sized asteroids could
provide the matter needed to make the collection area.
This would be a huge, but repetitive, construction project
suited for robotic means. It could be done by a system of
devices that, collectively, have the following two properties:
First, it can make all necessary hardware out of raw solar
system materials (asteroids, lunar regolith, etc.). Second, it can
assemble said hardware to produce solar power stations, particle
beam drivers and copies of themselves. If a reproductive unit of
the system, call it a “factory,” can reproduce itself and one solar
power station each year, then the necessary energy collection
hardware can be in operation within a few decades. The cost
would be that of making the rst factory and supervising the
subsequent operation.
The engineering details of which solar energy conversion
systems are most appropriate, how big the robots should be and
so on, can be left to the future. Today’s solar power conversion
systems will get better, simpler and more efcient with time.
Machines that make parts of machines are an increasingly
relevant part of daily life in these times, as are (human scale)
robotic assemblers. Already solar-array farm manufacturing/
installation machines can operate largely autonomously.
Obviously, a real system won’t produce exactly one replica
and exactly one gigawatt-class power station in exactly one year.
Nor would it be fully autonomous - some human supervision
Fig. 8 Smart pellet beam.
[19] has proposed miniature linear accelerators of only a few
millimetres diameter for his Starseed Nanoprobes and would
use similar machines to launch mesoparticles to push mass
beam sails. Millions of accelerators would be needed, but
their total cross sectional area would be only a few square
meters.
Lasers are simpler to build and techniques for ganging
together solid state lasers offer the hope of relatively high
efciency as well. Thus, a two-stage process, ensues whereby
micro photon sails are pushed at very high acceleration and fairly
good efciency by photons at higher gamma values. The sails
then transfer their momentum to a larger, slower vehicle. Kare
10
Gerald D. Nordley and Adam James Crowl
will be needed - a gurative hand on the “off switch,” for
instance. However, this calculation illustrates the power of self-
sustaining exponential growth to acquire the energy needed for
star ight.
The arrays could be placed at one or both of the Sun-Venus
equilateral Lagrange points (Venus L4 or L5). A hundred
thousand terawatts of solar arrays, 2,000 kilometres across,
would stretch 100,000 kilometres across its orbit (a little larger
than the width of Jupiter) and would probably be visible as a
bright tiny lament in Earth’s sky. These threads in the sky
would grow longer and longer as the capability to send more
starships is added, eventually surpassing Venus in its brilliance
in the Earth’s sky.
The beam projectors would be reaction engines in themselves,
of course and affect their own orbits. They could be anchored
to asteroids; they might be attached to the solar energy stations
that power them; they might be scheduled so the recoil effects
cancel over an orbit, they might circulate masses [28] around
a planet or the sun to counter-recoil perturbations…many
solutions to this engineering issue may emerge.
The energy requirements for star travel are so high that it
is difcult to see how it can happen without self-replicating
systems. Also these systems, applied in other ways, would
have other profound implications for economics and social
organization. When considering the “cost” of interstellar travel,
one needs to consider the implications of automated replicators
and tens of thousands of terawatts. When humanity has what it
needs to go to the stars, its concerns in many areas will be very
different to those faced today.
In the fullness of time, one could do more than just travel
between local stars. One could look on the energy collection
system as the rst step toward a “Dyson sphere,” where a star
is totally enclosed by solar energy conversion systems. From
the standpoint of a Dyson sphere, the energy requirements for
interstellar travel are small; as a star like the sun puts out about
3.9 × 1026 watts. Imagine that the solar power factories run for
Fig. 9 Exponential growth of power supply.
11
Mass Beam Propulsion, An Overview
forty years. If ten thousand kilometres wide, the array they
could make would stretch across almost 30 degrees of Venus’
orbit. It would intercept an impressive 3.75 millionths of the
Suns output and produce 1.75 × 1028 Joules per year (about 20
million times what is produced on Earth today). This would be
enough for some 38,000 ights a year up to a Lorentz factor of
2.
It would also be enough to send several thousand-ton payloads
a year up to a Lorentz factor of 40,000. Assuming one could make
the appropriate beam projectors, that would get a ship to the M31
galaxy in Andromeda, in about 50 years of shipboard time.
Is it reasonable to think that humans will be able to build
such space-based self-replicating systems in the next 50 or 60
years? If so, they may be able to send out the rst relativistic
starships in the next century and be conducting a full-edged
interstellar commerce before its close. Given progress in life
extension, some people alive today may live to see it.
7. A NOTIONAL MASS BEAM
PROPULSION DEVELOPMENT SCENARIO
Figure 10 describes a notional rst-order technology roadmap
to a mass beam propulsion launch capable carrying passengers
to the Alpha Centauri System one hundred years from now.
In the beginning years, It builds on technological advances
in superconductors, robotics, self-replicating 3D printing and
space resource renement that are likely to occur for reasons
other than star ight, but will substantially contribute to
creating the technologies mentioned above.
In parallel with these developments, over the next twenty
years or so, the plan envisions an effort in design and
analysis directed toward mass beam propulsion with, initial
experiments performed in the 2030s with simple neutral atom
beam hardware descended from designs for missile defence
systems.
The current loops would not need to be superconducting if
the conductor cross sectional area is large enough. A toroidal
aluminium hull [27] for instance, would sufce.
Assuming that a more robust mass beam of some kind
(passive pellets, smart pellets, sailbeams, etc.) proves desirable,
accelerators for these could be developed over the next twenty
years, increasing in throw velocity over time as power is
installed. The rst stages of commercial asteroid mining efforts
are already in progress, and on a schedule fully compatible with
the notional timeline, if not actually a little in advance of its
projections. [43]
For a 2110 launch of a crew, one wants to know that a
deceleration system is in place. The robotic precursor system
that would build the deceleration system would likely have
to travel more slowly, with the capacity of an unassisted
deceleration. It is estimated that the robotic system achieves
a cruise velocity of 0.5 c and takes about 18 years to reach
the Alpha Centauri system, including deceleration time. This
allows three years to construct a system to deploy a low velocity
deceleration particle swarm for the human-rated vehicle and 4.3
years to report it. Backing away from 2111, the launch of the
precursor system should take place not much later than 2085.
The peak power for a 1000-ton payload to 0.5 c would
be about 3 PW (see Fig. 3) and the robotic precursor might
mass less than a third of the crewed ship. One petawatt
should be easily available by that time. The critical path (dark
arrows) nexus turns out to be the yby probe needed to get
the data to build and program the robotic precursor. To get
data back from this in the 2080 time frame, a ve ton 0.5
Fig. 10 A notional top level program plan for human staright by 2110.
12
Gerald D. Nordley and Adam James Crowl
1. R.L. Forward, “Roundtrip Interstellar Travel Using Laser-Pushed
Lightsails”, JSR, 21, pp.187-195, 1984.
2. M. Born, “Einstein’s Theory of Relativity", Dover, New York, 1965.
3. C.E. Singer, “Interstellar Propulsion Using a Pellet Stream for Momentum
Transfer”, JBIS, 33, pp.107-115, 1980.
4. E.F. Mallove and G.L. Matloff, “The Staright Handbook”, Wiley, New
York, pp.145-146,1989.
5. J.T. Early, “Space Transportation Systems with Energy Transfer and
Force Beams”, JBIS, 40, pp.371-372, 1987.
6. G.A. Landis, “Optics and Materials Considerations for a Laser-propelled
Lightsail”, 46th International Astronautical Congress, Oslo, Norway, 2-6
October 1989. Paper IAA-89-664.
7. R. Zubrin, “The Magnetic Sail”, Analog, 112:6, May 1992.
8. D.G. Andrews and R.M. Zubrin, “Progress in Magnetic Sails”, 26th Joint
Propulsion Conference, Orlando, Florida, 1990. AIAA Paper 90-2367.
9. G.Vulpetti, “Dynamics of a Field-Sail Spaceship”, Acta Astronautica, 21,
pp.679-687, 1990.
10. G.D. Nordley, “Relativistic Particle Beams for Interstellar Propulsion”,
JBIS, 46, pp.145-150, 1993.
11. G.D. Nordley, “Particle Beam Propulsion and Two-Way EML
Propulsion”, in JPL D-10673: NASA/OAST Fourth Advanced Space
Propulsion Workshop, ed. R.H. Frisbee, Jet Propulsion Laboratory,
Pasadena CA, p. 473,1993
12. D.G. Andrews, “Cost Considerations for Interstellar Missions”, JBIS, 49,
pp.123-128,1996.
13. C. Mileikowsky, “How and When could We be Ready to Send a 1000
kg Research Probe with a Coasting Speed of 0.3 c, to a Star”, JBIS, 49,
pp.335-344, 1996.
14. J.T. Kare and D.G. Andrews, “Propelling An Interstellar Probe With
Microsails”, 52nd International Astronautical Congress, Toulouse,
France, 1-5 October 2001.
15. J.T. Kare, “SailBeam: Space Propulsion by Macroscopic Sail-Type
Projectiles”, in Proceedings of STAIF 2001, ed. M.S. El-Genk, American
Inst. Physics, 2001.
16. J.T. Kare, “High-acceleration Micro-scale Laser Sails for Interstellar
Propulsion”, Final Report NIAC RG#07600-070, rev., 2002.
17. J.T. Kare, “Interstellar Precursor Missions Using Microsail Beams”, in
Proceedings of. STAIF 2002, ed. M.S. El-Genk, American Inst. Physics
CP-608, pp.313-317, 2002
18. G.D. Nordley, “Interstellar Probes Propelled by Self-steering Momentum
Transfer Particles”, 52nd International Astronautical Congress, Toulouse,
France, 1-5 October 2001. Paper IAA-01-IAA.4.1.05
19. F. Bishop, “Some novel space propulsion systems”, Aircraft Engineering
and Aerospace Technology, 75, pp.247-255, 2003.
c probe will need around 20 TW at end of acceleration. It
could go slower, earlier or faster, later, with about the same
results, thus the self-replicating solar power stations should
start about the year 2050. This allows about 39 years to
develop the particle, reector and beam driver technology,
with the major investments coming later (in the 2040’s) and
with signicant constituencies (rapid interplanetary travel,
resource exploitation and astronomical probes) other than
staright to provide near-term payoff.
8. CONCLUSIONS
Given plausible developments in robotics and use of space
resources, mass beam propulsion could let people travel to the
nearest stars in less than a decade, a time scale of perhaps only
two or three times that of early intercontinental ocean voyages.
This is based on known physics and anticipated extensions of
known. While new science might be helpful, particularly in the
area of superconductors, it is not needed.
That is not to say that there is not a lot of work to be
done, nor that some other system won’t prove superior in
the future. It would be remarkable if starship propulsion in
the 2100s looks as much like what is described here as the
Apollo project resembled Jules Verne’s space gun. However
by demonstrating that the question of interstellar travel can be
decoupled from the concerns of rocketry; exhaust velocities,
mass ratios, etc., the future of interstellar travel ceases to
be a marginal concern limited by rocket mass ratios to slow
“generation ships.”
By building small fractions of a Dyson sphere with
automated labour and space materials, civilizations could send
out thousands of starships a year at nearly the speed of light
and create an interstellar culture that could percolate through
this and other galaxies in a few hundreds of millions of years.
Humanity may happen to be the rst to do this. Or they may
nd others waiting.
ACKNOWLEDGEMENTS
This work was not funded other than by the authors. It would
not, however, have been produced without the insistence and
encouragement of Dr. James Benford. The authors thank Gayle
Wiesner for editorial assistance in preparing this document.
They would also like to thank Dr. Jordin Kare, Dr. Goeff
Landis and Forrest Bishop with help with the bibliography. Any
inadvertent omissions, of course, are the responsibility of the
authors.
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J.T. Early, “Space Transportation Systems with Energy Transfer and
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APPENDIX A: MASS BEAM PROPULSION BIBLIOGRAPHY
Not all of the items researched ended up providing citation in this text. As a service to the community of interest, the entire Mass
Beam Propulsion research bibliography is included below.
J.T. Kare, “SailBeam: Space Propulsion by Macroscopic Sail-Type
Projectiles”, in Proc. STAIF 2001, ed. M.S. El-Genk, AIP, 2001.
J.T. Kare, “High-acceleration Micro-scale Laser Sails for Interstellar
Propulsion”, Final Report NIAC RG#07600-070, rev., 2002.
A. Kantrowitz, “Propulsion to Orbit by Ground-Based Lasers”,
Aeronautics and Astronautics, 10, pp.74-76, 1972.
G.A. Landis, “Interstellar Flight by Particle Beam”, Acta Astronautica,
55, pp.931-934, 2004.
G.A. Landis, “Optics and Materials Considerations for a Laser-propelled
Lightsail”, 40th International Astronautical Congress, Malaga, Spain,
7-12 October 1989. Paper IAA-89-664.
G.A. Landis, “ Dielectric Films for Solar- and Laser-pushed Lightsails”,
in AIP/STAIF-2000 Conference Proceedings, Volume 504, Albuquerque,
NM, 2000.
G.A. Landis, “Small Laser-propelled Interstellar Probe”, 46th
International Astronautical Congress, Oslo, Norway, 2-6 October 1995.
Paper IAA-95-IAA.4.1.102
G.A. Landis, “Beamed Energy Propulsion for Practical Interstellar
Flight”, JBIS, 52, pp.420-423, 1999.
G.A. Landis, “Erosion Shields for Interstellar Dust,” Planetary Society
Conference on Practical Robotic Interstellar Flight, NY University,
1994.
B.A. Lebon, “Magnetic Shepherding of Orbital Grain Streams”, JBIS, 39,
pp.486-490, 1986.
E.H. Lemke, “Magnetic Acceleration of Interstellar Probes”, Acta
Astronautica, 8, pp.785-793, 1981.
E.H. Lemke, “Magnetic Launching in Outer Space”, JBIS, 35, pp.498-
503, 1982.
A.R. Martin, “Bombardment by Interstellar Material and Its Effects on
the Vehicle”, in Project Daedalus Final Report, JBIS, 1978.
E.F. Mallove and Gregory L. Matloff, “The Staright Handbook”, Wiley,
New York, p.145-146, 1989.
G.L. Matloff, “Use of Parabolic solar Concentrators to Improve the
Performance of an Interstellar Solar Sail”, JBIS, 49, pp.22-21, 1996.
G.L. Matloff,, “The Interstellar Ramjet Acceleration Runway”, JBIS, 32,
pp.219-220, 1979.
H. Morovec,”The Universal Robot,” in Vision-21: Interdisciplinary
Science and Engineering in the Era of Cyberspace, ed. G.A Landis,
NASA Publication CP-10129, pp.115-126, 1993.
V.G. Minogin, “Compression of Atomic Beams by Laser Radiation
Pressure”, Opt. Spektrosk (Russia), 60, 1061ff., 1986.
C. Mileikowsky, “How and When could We be Ready to Send a 1000
kg Research Probe with a Coasting Speed of 0.3 c, to a Star”, JBIS, 49,
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(Received 5 March 2012; Accepted 13 August 2015)
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