Institute of Atomic-Scale Engineering

Some Novel Space Propulsion Systems
 By Forrest Bishop

Copyright (c) 1997, Forrest Bishop, All Rights Reserved

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 mirrong [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

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 ‘tuner’ 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 author’s "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 author’s 1982 study (Appendix A.).

Constant closing speed with ship. Collision coefficient (1 = elastic, .5 = inelastic)

Skip 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 ship’s 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 6,=.0 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 spacecraft’s 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 ‘bare’ 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 ‘constant’

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:

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.

Figure 8. A spherical lightsail with film thickness less than the skin depth (‘partially silvered’) 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 ‘ribbon sail’,

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

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 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 ‘herding’ 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 ‘dumb’ 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 (‘tanker’) 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 ‘tanker’ 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 ‘dumb’ 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 (‘dumb sail’) 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.

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 author’s 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 ship’s 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.

References

Andrews, D., and Zubrin, R., "Magnetic Sails and Interstellar Travel", (1990), JBIS, Vol. 43, pp. 265-272

Andrews, D., "Cost Considerations for Interstellar Flight", (1994)

Bishop, F., "InterPlanetary Mass Driver", (1982), unpublished notes

Bishop, F., "The Construction and Utilization of Space Filling Polyhedra for Active Mesostructures", (1995),

available at: http://www.speakeasy.org/~forrestb

Bishop, F., "A Proposed MNT Active Cell", (1996a), available at: http://www.speakeasy.org/~forrestb

Bishop, F., "Starseed/Launcher (A Linear Accelerator for Interstellar Nanoprobes)",(1996b) available at: http://www.speakeasy.org/~forrestb

Bishop, F., "A Description of a Universal Assembler", (1996c), Proceedings of the IEEE

Joint International Symposia on Intelligence and Systems, ISBN 0-8186-7728-7

Bishop, F., "Open Air Space Habitats", (1997a), in "Tools for the Next Millennium",

CRC Press, Lance Chambers, Ed.

Bishop, F., "The Interworld Rapid Transit System (A Transportation Network for the Solar

System)", (1997b), submitted to The Journal of the British Interplanetary Society

Bishop, F., "Mesoparticle Beam Propulsion", (1997c), submitted to JBIS

Bishop, F., "The Graphene Crossbow", (1997d), submitted to JBIS

Cassenti, B.N., G.L. Matloff, and J. Strobl, "The Structural Response and Stability of Interstellar Solar

Chilton, F., et al, (1977), "Mass Driver Applications", in Space-Based Manufacturing from Non-Terrestrial


DeForest, S., "Electrostatic Charging on Spacecraft", in Solar System Physics, L. Lanzerotti, ed., North-Holland, New York,

(1979), Vol 3, pp. 358

Drexler, K. Eric "Design of a High Performance Solar Sail System", (1979), MS Thesis, Dept. of Aeronautics and Astronautics, MIT

Drexler, K. Eric "Nanosystems: Molecular Machinery, Manufacturing, and Computation" (1992a) John Wiley & Sons,

ISBN 0-471-57547-X

Drexler, K. Eric (1992b) "Molecular Manufacturing for Space Systems: An Overview", Journal of The British Interplanetary Society,

Vol. 45:401-405.

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pp. 261

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Forward, R. L., "Pluto-The Gateway to the Stars", (1962), Missiles and Rockets, Vol 10, pp 26-28

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Vol. 32, Apr.-June 1984a, pp. 221-226

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Rockets, Vol 21, pp 187-195

Forward, R. L., "Starwisp: An Ultra-Light Interstellar Probe", (1985), J. Spacecraft and Rockets, Vol 22,

pp 345-350

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Kantrowitz, A., "Propulsion to Orbit by Ground-Based Lasers", (1972), Aeronautics and Astronautics, Vol 10,

pp 74-76

Landis, G. A., private conversation.

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Congress, Torremolinos Spain, Oct. 7-13, 1989.

Landis, G., "Small Laser-propelled Interstellar Probe," Paper IAA-95-IAA.4.1.102, 40th IAF Congress, Oslo,

Norway, Oct., 1995.

Lemke, E. H., "Magnetic Acceleration of Interstellar Probes", (1981), Acta Astronautica, Vol. 8, pp. 785-793

Lemke, E. H., "Magnetic Launching in Outer Space", (1982), JBIS, Vol. 35, pp. 498-503

Matloff, G. L., "The Interstellar Ramjet Runway", (1979), JBIS, Vol. 32, pp. 219

Minsky, M., email communication, Oct. 31, 1997

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Sciences, Vol 25, pp 307-322

Nordley, G. D. "Relativistic Particle Beams for Interstellar Propulsion," (1994), JBIS, Vol 47, pp 145-150

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Spacecraft and Rockets, Vol 27, pp 589-596

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Acta Astronautica, vol. 2 pp 785-799

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pp. 107-115

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Rockets, Vol 27, pp 514-521

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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 spacecraft’s pusher plate. It may be currently feasible to build a version of this Solar sail particle beam system. In this proposal, a supply ship (‘tanker’) 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 ‘pusher plate’ at several km/sec. The ‘equivalent specific impulse’ 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 ‘proof-of concept’ 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 ‘bare die’ 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 ‘tanker’ Sunward, perhaps to a loiter orbit at the L1 Lagrange point between Earth and Sun, 1.5 Gm from Earth.

The lightsails, ‘tanker’ 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 Earth’s 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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