This section is a copy of : Fusion Rockets vs. Sailships: A Contrarian View. BENFORD, James. Sailships vs. Fusion rockets: a contrarian view. JBIS, 2017, vol. 70, p. 175-183.

James Benford, Microwave Sciences

In this paper, Benford argues for the use of solar sails with beamed power vs fusion powered rockets.

The problem with rockets for interstellar flight is that they will be very large, very inefficient and very costly. But starships need not be rockets, as Project Icarus design concepts are. Beam-powered sails–Sailships–are a better choice. Sailships, in principle, allow higher speeds than nuclear rockets.

The best feature of beaming energy is that the Beamer––with all its mass and complexity––is left behind, while the relatively simple ultra-light sail, carrying its payload, is driven far away. The Beamer can then be used for many sailship missions. For beam-driven sails, there are no physics issues. We have experimentally demonstrated flight, beam-riding and beam-induced spin for stability. The engineering requirement is for large assemblies of both modular sources of the photons, and large antenna/optic arrays. Engineering and cost are the major questions for development. Key issues for sailships are reviewed in light of the Star Shot project and relative merits of competing source technologies are assessed.

THE TROUBLE WITH ROCKETS

The thrust generated by rocket-powered spacecraft is limited by the speed at which gases leave the exhaust nozzle. All rockets only go about twice their exhaust speed–far too slow for interstellar travel. Future starships will need new and powerful forms of propulsion because they’ll be massive and need every possible efficiency.

The Rocket Equation says it all: thrust can only be applied so long as there’s fuel onboard, and rocket mass ratio depends exponentially on the exhaust velocity. Going three times the exhaust speed would require fuel 20 times the mass of the rest of the rocket (the ‘dry weight’) [1]. Chemical combustion of hydrogen and oxygen is simply too slow: solid fuel rockets have exhaust velocity v_e~2.5 km/sec, bi-propellant liquid fuels have v_e~4.5 km/sec. Note also that if the radius of our planet were larger, there could be a point at which a chemical rocket could not escape. That point would be reached if Earth’s radius were 50% larger. So super-Earths would need nuclear rockets to have space travel.

So, can rockets get higher exhaust velocity, and so higher speeds, by going to the Nuclear Option? Nuclear thermal rockets use a fluid– probably hydrogen because it’s lightweight–which passes through a fission reactor core, gets very hot, and exits the exhaust. The exhaust velocity is about 10 km/sec. Because nuclear rockets have high thrust, they could open up the Solar System to large, fast spacecraft, allowing harvesting of its resources. Much work was done in the Cold War on nuclear thermal rockets by both the US and USSR. There are now new efforts in the US, the best hope of getting high thrust at high speed soon. But they’re still not hot enough for interstellar flight.

To make interstellar missions possible requires higher speeds, using the nuclear reaction that drives the sun, fusion rockets, which are the mainline of starship concepts. They would be complex and large. Before Project Icarus [2], in 1978 the British Interplanetary Society produced the most detailed starship concept, called Daedalus, an inertial confinement fusion (ICF) rocket [3, 4]. ICF was then new, but 40 years later, ICF breakeven is as yet not demonstrated, despite strenuous efforts. Beams crush a pellet of fusion fuel—isotopes of hydrogen— from all sides, compressing the material into a tiny volume. Pressure rises enough for the hydrogen nuclei to fuse, releasing energy. The hot mass gives pulsed thrusts out of the nozzle. Daedalus was complex, as big as an aircraft carrier; it would dwarf the Saturn 5, our manned rocket to the Moon.

But fusion rockets must follow from achieving fusion here on Earth. Several big facilities are underway, but success has been elusive. So far, fusion hasn’t been conquered; fusion has conquered us for 60 years.

Most of interstellar propulsion thinking, as reflected in the Project Icarus concepts, is oriented toward rockets: huge expendable rocket, relatively small payload. But the one-vehicle approach to exploring a new planetary system results in an enormous payload, such as the 150-ton Icarus payload specification, 182 times that of the mass of Voyager 2.

The problem with rockets is that for interstellar flight they will be very large, very inefficient and very costly. For example, the kinetic energy of the Daedalus starship is enormous. With the final payload of 450 tons and a velocity of 13% of light, the kinetic energy of the starship is the equivalent of about 10 times all the energy in the nuclear arsenals of the world, several hundred thousand nuclear weapons.

For comparison, compare some key parameters of a US supercarrier, two Icarus design concepts [5], and a sailship [6]:

Aircraft Carrier 0.3 km, 105,000 tons, 0.01 T$

Firefly 0.75 km, 23,550 tons, mass fraction 0.0064, rocket cost 40 T$, 4.7%c

Ghost 1.2 km, 154,800 tons, mass fraction 0.0008, cost 0.02-34 T$, 6%c

Sailship 10 km, 10 tons, mass fraction ~0.1, Beamer cost 40 T$, sailship cost ~ 1B$, 10%c [operating cost, electricity to drive the Beamer at today’s rate (0.1 $/kw-hr) is 0.5 T$.]

And for a size comparison, see Figure 2. The ships that explored the oceans, such as the Santa Maria (19 m) and Kon-Tiki scale Polynesian rafts (45 m), as well as the Breakthrough Star Shot sail (~3m) could not be seen on this scale.

With the fusion rocket approach, the infrastructure, necessary to build such huge vessels and supply nuclear fuel, is thought of as a fixed cost, but is not easy to estimate in these designs. But the infrastructure cost of rockets will be quite huge. Only the final stage, with the payload, reaches the objective. The rocket is largely expendable: almost all of the expensive system is used as fuel or discarded later to get the payload into the new stellar system.

WHY BEAM-POWERED SAILS ARE A BETTER CHOICE

The rocket equation is:

m_f = m_oe^-Δv/ve (1)

where Δv is the final speed, m_o is the initial rocket mass, m_f the final mass and v_e is the exhaust velocity of the rocket. Integrating this, the final the kinetic energy at the end of the acceleration is given by [7]

E_R = ½ m_f V_e² (e^-Δv/ve-1) (2)

For a sail driven by a beam of power P and duration τ, from the equation of motion can be used to give the final kinetic energy:

m_f dv/dt = 2P/c

m_f Δv = (2P/c)*τ

E_B=½ m_f Δv c (3)

Where c is the speed of light.

A figure of merit is the ratio of the rocket energy to sail energy, R:

R=E_R/E_B=(1/cΔv)*V_e²(e^-Δv/ve-1) (4)

which is a function of only the exhaust velocity and the final speed.

At low velocities the ratio is just v_e/c, the rocket is favored because the rocket requires less energy to accelerate at low speeds. But at higher speeds the ratio begins to exceed 1 and the rocket is less efficient than the sail. The high-speed approximation (Δv/v_e >1) is

R≈(1/cΔv)V_e² e^-Δv/ve (5)

which is exponential in the mission velocity. Therefore the transition to a higher efficiency, favoring sails, occurs at approximately R=1.

The crossover velocity, where R>1, and beyond which sails are more efficient, is given as a function of the exhaust velocities. Fusion rocket is based on the Icarus Firefly concept.

The table shows the speed at which R=1 is reached for a variety of propulsion methods. Sails are certainly more efficient than chemical rockets (here hydrogen-oxygen, H₂ O₂) for speeds above 62 km/sec, comparable to solar system escape speed (Figure 3). They are more efficient than fission rockets above 155 km/sec, for missions within the solar system. For interstellar precursors >1,000 km/sec, beam-driven sails are more efficient than ion rockets, such as VASIMIR.

For interplanetary travel, fusion rockets will hopefully develop to higher temperatures over time, and should that happen, the exhaust velocity of fusion reactors will make a big difference. Sailships are more efficient than 10 keV fusion rockets past velocities of 16% c. And sailships will be more efficient than any fusion ships at velocities past 25.5 c. The Icarus designs, such as Firefly, are at ~100 keV and only aim for 5-8% c. So they qualify as efficient systems. But only because the velocity targets are not very high.

Figure 4 shows that starships need not be rockets.

• Sailships in principle allow higher speeds than nuclear rockets. As mentioned above, the kinetic energy of the Daedalus starship is the equivalent of about 10 times all the energy in the nuclear arsenals of the world, several hundred thousand nuclear weapons.

• A sailship can be extremely light, 10 tons or less. At a velocity of 13% of light it has a kinetic energy of 2 % of the world's nuclear arsenals. Now that's a lot of energy, about 500 average nuclear weapons. However, it's about 1/1,000th of the required energy for an interstellar fusion rocket.

• On the other hand, sailships are energy-inefficient; only a few percent of the energy radiated ends up as kinetic energy of the spacecraft, due to the nature of momentum exchange, which drives them, instead of energy exchange, which drives rockets.

• The best feature of beaming energy is that the Beamer –– with all its mass and complexity –– is left behind, while the relatively simple ultra-light sail, carrying its payload, is driven far away. The Beamer can then be used for many sailship missions. Like the 19th-century railroads, once the track is laid, the train itself is a much smaller added expense.

• Another fundamental attraction of beamed power for space is simple: electromagnetic waves can carry energy and momentum (both linear and angular) over great distances with little loss.

Here’s how beamed energy propulsion works:

The Beamer –– the source of the beam, including its powerful source and a large antenna or optic – projects a powerful laser, millimeter wave or microwave beam onto a large sail. The sail reflects the beam, picking up momentum, accelerating away from the Beamer.

Beamers would look like the microwave dishes we currently see listening to satellites, only very much larger. The expensive part of this utility is the Beamer, which wouldn’t be placed on Earth, but built in space from materials mined from the Moon or asteroids. Beamers might be positioned close to the Sun, where they could run on strong solar power.

In 2000, a team demonstrated first flight, in a vacuum vessel, of microwave-driven carbon sails using microwave beams to produce several gee accelerations as shown in Figure 4 [8]. Further experiments demonstrated beam-riding, stable flight of a sail propelled by a beam. Beam photon pressure will keep a concave shape sail in tension, and gives a sidewise restoring force. The beam can also carry angular momentum and communicate it to the Sailship, spinning the craft. This effect can help control by stabilizing the sail against drift and yaw in flight.

We can think of the context of the interstellar propulsion challenge the same way we've always thought of fusion: first get the physics solved, then the engineering, and finally address the economic feasibility.

For beam-driven sails, the physics is done, having experimentally demonstrated flight, beam-riding and beam-induced spin for stability [9]. The engineering requirement is for large assemblies of both modular sources of the photons, and large antenna/optic arrays. We have vast experience in such sub-systems in the microwave and progress is steady. There are clear paths forward to solving the engineering issues. The sailship cost is little; the Beamer cost dominates. Cost modeling has been done and shows a cost roughly equally divided between the cost of the beam generator and the cost of the aperture used to radiate the beam [6]. Millimeter-wave Beamers, for example, cost of the millimeter-wave sources will be roughly equal to the cost of the millimeter wave antennas.

We know that nuclear is much more expensive to research and then develop, compared with beam-driven sails. And what would a fusion rocket cost? That’s beyond this horizon at present.

KEY ISSUES FOR SAILSHIPS

Large-scale Space Construction

The sheer scale of the sailship, and the Beamer aperture which irradiates it, raises major issues. The Beamer aperture will be large numbers of independent radiators phase-locked to each other. Phase locking is a well-understood phenomena widely used in radars for many decades. Generally, phase locking uses amplifiers instead of oscillators, because amplifiers work better together in the large numbers of sources needed for power beaming. The Beamer is an assembly of replicated modules of power amplifiers and antennas, which can be something like the familiar microwave dish antennas or laser output optics.

The sail will likewise be assembled from modular elements and connected together. Many types of materials have been suggested; many geometries have been tried for assembling sails. To date, the largest solar sail prototype is 100 feet on a side, deployed in a laboratory. The 20 m IKAROS sail has been deployed and flown to Venus. Nevertheless it's an engineering challenge to see how large diaphanous structures, perhaps miles in size, can be assembled, as interstellar sailcraft will require.

Deployment and Maneuvering

First, an ultralightweight sailship must be deployed and maneuvered. The lightness and elasticity of new ultralight materials such as carbon fiber mats can tolerate virtually no external mechanical contact so that everything must be entirely “hands off”.

Practical requirements are to

• deploy, maneuver and control the very light structures with minimal mechanical contact,

• deploy from a minimum stowed volume (maximize packing fraction),

• provide for control after deployment.

One solution is to deploy by spinning up the sailship, which stabilizes the sail structure to both pitch and yaw. Spinning produces centrifugal force, which provides the required tension to hold the sail shape, preventing deformation and fluttering.

While spacecraft spinning up could be done with cold gas thrusters from unfurling to full deployment, it’s attractive to remotely induce spin, entirely hands-off. The other virtues hands-off methods have are reversibility, should spin grow too fast, and real-time control.

Beam-Riding and Stability

Riding on the beam, i.e., stable flight of the sail propelled by beam momentum, places considerable demand upon the shape of the sail. Some amount of beam jitter is to be expected. But even if the beam is steady, a sail can wander off the beam if its shape should become deformed. Little of the parameter space of possible sail shapes and materials, as well as beam shapes, have been explored to date. Generally, sails without structural elements cannot be flown if they are convex toward the beam, as the beam pressure would cause them to collapse. On the other hand, beam pressure keeps concave shapes in tension, so conical concave shapes are the natural answer to sailship configuration questions, essentially a circular cone. These shapes resist sideways motion if the beam moves off-center, since a net sideways force restores the sail to its original position. Simulations show that this passive stability works quite well. And it is made more stable if the sail is spinning, as it would be if spin deployment is used. Therefore the natural configuration of a sailship is a conical spinning shape with the payload hanging from the apex along a flexible tether. It is shaped like a parachute. Suspending the payload below the sail is the more stable shape configuration. Stability also depends upon the ratio of the sail mass to payload mass. Experiments have verified that beam-riding does occur [9].

Pointing the Beam

A very precise control system is required to keep the laser beam on the sailship at large distances from the beam aperture. Tailored phase variations in the beam sources, which are amplifiers, will give control of the beam direction, aided by retro-reflective signals returned from the sail itself. But round-trip transit delays will make such adjustments decline as the sailship flies away. Moreover, pointing and tracking systems have errors due to the system having imperfect optics, vibrations in the apparatus and other factors. This is called ‘jitter’ and varies the beam direction randomly. The beam center moves away from the sailship, and of course this increases with distance. The sailship acceleration should be done before this distance is reached. So there's a trade-off between the jitter and the distance over which the sail is accelerated: if limited by lower pointing accuracy, higher acceleration can make up for it.

The present capability in angular precision pointing in space, for example the Hubble Space Telescope, is about ten nanoradians, which is about a millionth of a degree. The pointing and tracking accuracy needed for sailship missions is of order the diameter of the sail divided by the distance over which it is accelerated. This will be microradians for interstellar precursors to nanoradians for 0.1 c starships and picoradians for 0.5 c sailships. This is a difficult requirement, but in the past century, pointing accuracies increased steadily and rapidly. so Future developments in astronomy may well make this requirement quite possible, a pointing control and transmitter optics will likely improve substantially in future.

Slowing Down

Another fundamental issue of an interstellar sailship will be deceleration as the sailship approaches its target star. Of course initial missions may be flyby, although the rapid advance of space telescopes may make that short look unnecessary. Flybys can gather local data such as plasma measurements that telescopes cannot. Still, short observing time undercuts flybys, so a number of methods have been suggested, including a magsail diverting inbound particle flux, as well as using the solar wind of the target star. Both require long deceleration times to get below escape velocity of the target star [10].

Robert Forward suggested the trickiest method: subdivide the sail to both decelerate to the target star and even return back to the Solar System: approach the target star, detach an inner part of the sail, the rendezvous sail, reflect a laser beam from Earth from the main sail, now a ring, to the rendezvous sail, decelerate it into the target system [11]. The main sail flies on. For return to Earth, detach an inner part of the rendezvous sail. Send a laser beam from the solar system to the rendezvous sail, which bounces off it and accelerates the return stage toward home. Approaching the Solar System, decelerate with the laser. The drawback: the pointing and power requirements for this scenario are far beyond what we know how to do.

LOW-MASS INTERSTELLAR PROBES: STARWISP AND STAR SHOT

Sailships’ very low mass makes them ideal for unmanned probe missions for flybys of planets orbiting nearby stars. The first low-mass end of the sailship spectrum is ‘Starwisp’, a Robert Forward concept for a lightweight, high-speed interstellar flyby robotic probe, driven by a microwave or millimeter-wave beam [12]. Starwisp is a large sail made of very thin wire mesh, with microcircuits at each wire intersection, each containing a node of a neural net. The sail would be pushed at high acceleration by the microwave beam; Starwisp reaches a fraction of lightspeed while still in the solar system.

Starwisp extrapolates nano-spacecraft and pico-spacecraft concepts to more sophisticated electronics, detectors, sensors and mechanical devices, reduced to near zero volume with very small mass. The payload would include small particle sensors and imagers of both optical and infrared and radio frequencies.

Forward’s minimal Starwisp design would be a 1–kilometer square mesh sail weighing 16 g and a few grams of microcircuits, a total of 20 grams. Wire spacing is 3 mm, less than the 30mm wavelength, mostly empty space, giving greatly reduced mass compared to solid laser-reflecting films. Revisiting Starwisp, Geoff Landis found ways to reduce it dramatically, with a mass of 1 kg, diameter 100 m, reaching 1% c [13].

Figure 7. Forward’s Starwisp concept: probe driven by an annular transmitter lens [12].

The low–mass approach has been taken to its extreme by the Breakthrough Star Shot project. This effort, sponsored by Yuri Milner, is a part of the multifaceted Breakthrough Initiatives, which also has projects on observations of nearby stars and SETI. Star Shot reduces the sail mass down to about a gram and sail scale to a few meters. The acceleration is driven by lasers with total power of 10-100 GW; accelerations are 10,000 100,000 gravities. Acceleration times are about 1-10 minutes [14, 15].

Constructing a laser light sail sufficient to propel a one-gram class spacecraft to 0.2c within a few decades using a laser beam director of approximately one-kilometer scale with a beam power of 10’s of GW requires thin and light-weight materials, perhaps metamaterials, meaning fabrication of meter-scale sails no more than a few hundred atoms thick. The material properties, which influence material choice and fabrication, are its reflectance, absorptance and transmittance, tensile strength in its areal density.

Stability of he sail on the beam is influenced by sail shape, beam shape and the distribution of mass, such as payload, on the sail. The laser system interacts with the sail through its power density distribution on the sail, duration of the beam, width of the beam, pointing error of the beam as well as its pointing jitter.

Other engineering challenges are:

• Building a ground-based kilometer-scale Beamer at high altitude in dry conditions, in order to reduce absorption and scattering in the atmosphere.

• Generating and storing terrajoules (gigawatt hours) of energy per launch.

• Launching a ‘mothership’ carrying thousands of nanocrafts to a high-altitude orbit, where it deploys the sailcraft.

• Using adaptive optics technology in real time to compensate for atmospheric effects for focusing the beam on the sail

• Protecting the payload from interstellar dust and gas collisions en route to the target.

• Capturing images of a planet, and other scientific data, and transmitting them back to Earth using a compact on-board laser communications system.

• Receiving data over interstellar distances over 4 years later.

MOVING TOWARD SAILSHIPS

Microwave, millimeter waves and lasers are all serious candidates for beaming to sails. Lasers are developing rapidly, in particular fiber lasers. The important distinctions between the types of radiation sources are as follows:

• Complexity Lasers have the tremendous advantage of short wavelength; therefore can be focused to a smaller sail from a smaller Beamer aperture. This is the approach that Breakthrough Star Shot has taken.

But lasers have a particular limitation: they are quantum devices, whereas the microwave and millimeter sources are classical devices. As a quantum device, lasers have a coherence length, the propagation distance over which a coherent wave maintains a specified degree of coherence. Wave constructive interference is strong when the paths taken by all of the interfering waves differ by less than the coherence length. A wave with a longer coherence length is closer to a perfect sinusoidal wave.

When the power laser is sufficiently high, nonlinear effects in the laser make the coherence length very short; the practical limit in today's fiber lasers, the current favorite in high-power laser technology, is about 1 kW per laser. Above that power coherence length shortens, making arrays are very difficult to build. On the other hand, classical devices can have very long coherence lengths because they are determined by the classical equations of electromagnetism.

The consequence of this is that laser arrays will involve very many lasers. For Breakthrough Star Shot, the required 10’s of GW means millions individual lasers which have to be built into an array.

Classical microwave and millimeter wave devices can operate at MW levels and therefore each GW of radiated power requires ~1000 sources per GW. Start Shot would require perhaps multiple thousands of sources rather than multiple tens of millions for lasers. Therefore a fundamental difference between the quantum and classical devices is their complexity.

• State-of-the Art For near-term research several factors favor microwave and millimeter waves over lasers, because they have practical advantages: Microwave equipment such as sources, anechoic rooms, antennas and diagnostics are commonly available than the emerging technology of high power lasers. That’s because microwave and millimeter wave sources, waveguide and supporting equipment, such as power supplies, are a developed industry. That means it is cheaper and faster to build systems. Lasers are developing fast, but at present are still expensive, ~100 $/W, and are produced in small numbers at slow rates. In fact, experiments to date have used spare microwave and millimeter wave equipment in lab experiments over distances of meters.

• Efficiency Microwaves are more efficient than lasers, typically 50-90%. Millimeter wave generation technologies now make it possible to generate wavelengths as short as 0.1 cm with relatively high efficiency (>40%). Laser efficiencies are near 40% now and have been slowly rising.

• Phased Arrays Microwave phased arrays of transmitters and apertures are relatively easily done, while phased arrays of laser beams, although possible in principle, subject to the coherence length constraint related above, are thus far little developed in practice. Work to date on laser phased arrays have been limited to small numbers of sources and modest power levels in laboratory experiments.

CONCLUSIONS

We should put more of our effort into beam-driven sails; we'll make more progress that way. The way to do that is beam-driven sail experiments and simulations, combined with the on-going parallel development of solar sails, which will tell us how to deploy and control larger sails. Large chambers capable of doing beaming experiments with sails already exist. Indeed, the experiments done to date have all used facilities built for other purposes.

To eventually have a power beaming capability, space infrastructure must exist to build upon [13]. How do we get from where we are now to a future when directed energy can be used for fast missions, including interstellar? By developing earlier applications of directed energy and solar sails in parallel with each other.

Beam-driven propulsion is more firmly grounded, more thought through and credible than nuclear fusion propulsion, as in Project Icarus. In today’s funding environment, that’s not likely to change: Due to Star Shot, sails are becoming near-term. Fusion rockets remain far-term, distant on a timescale of decades, if not centuries.

ACKNOWLEDGMENTS

I gratefully acknowledge discussions with Rob Swenny, Michael Lamontagne, Greg Benford and Paul Glister.

References

1. K. F. Long, Deep Space Propulsion, Springer, New York, p. 58, 2011.

2. Long, K.F, Obousy, R.K., Tziolas, A.C., Mann, A., Osborne, R., Presby, A., Fogg, M., "Project Icarus: Son of Daedalus ‐ Flying Closer to Another Star", JBIS, 62, pp. 403‐416, 2009.

3. Bond, A., and Martin, A. R., "Project Daedalus - Final Report," Journal of the British Interplanetary Society Supplement, 5-7, 1978.

4. Benford, James, “E-Beam ICF for Daedalus Reconsidered”, Project Icarus Research Note, 2012.

5. Freeland, Robert M. and Lamontagne, Michel, “Firefly Icarus: An Unmanned Interstellar Probe Using Z-Pinch Fusion Propulsion”, JBIS, 68, pp. 68-80, (2014).

6. Benford, James, “Starship Sails Propelled by Cost-Optimized Directed Energy”, JBIS, 66, pp. 85-95, 2013.

7. McInnes, C. R., Solar Sailing: Technology, Dynamics, and Mission Applications”, Praxis Publishing Ltd., Chichester, 275, UK, 1999.

8. Benford, James, “Space Applications of High Power Microwaves”, IEEE Trans. on Plasma Sci., 36, pp. 569-581, 2008.

9. G. Benford, O. Goronostavea and J. Benford, “Experimental Tests Of Beam-Riding Sail Dynamics”, Beamed Energy Propulsion, AIP Conf. Proc. 664, pp. 325-335, Pakhomov A., ed., 2003, Gregory Benford, Olga Goronostavea and James Benford, “Spin of Microwave Propelled Sails”, op cit. pp. 313-324.

10. Freeland, Robert M., “Mathematics of Magsails”, JBIS, 68, 306-323, 2015.

11. R. L. Forward, "Roundtrip Interstellar Travel Using Laser-Pushed Lightsails", J. Spacecraft and Rockets, 21, pp. 187-195, 1984.

12. R.L. Forward, “Starwisp: an ultra-light interstellar probe”, J. Spacecraft and Rockets, 22, pp. 345-350, 1985.

13. G.A Landis, “Microwave-Pushed Interstellar Sail: Starwisp Revisited”, 36th Joint Propulsion Conference, AIAA-2000-3337 2000.

14. https://breakthroughinitiatives.org/Initiative/3

15. Lubin, Philip, “A Roadmap to Interstellar Flight”, JBIS, 69, pp. 40-73, 2016.

16. J. Benford, ‘Sailships’, Ch. 9, Starship Century, ed. J. Benford & G. Benford, Lucky Bat Press, 2013.

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