Appendix A - Beamed Power Propulsion To The Stars
R. L. Forward
BEAMED POWER PROPULSION TO THE STARS
Dr. Robert L. Forward
Hughes Research Laboratories
3011 Malibu Canyon Road
Malibu, California 90265 USA
AAAS Symposium on Interstellar Communication and Travel
AAAS Annual Meeting
25-20 May 1986
Although it is possible to use fusion or antimatterrockets for slow travel to the nearer stars, it may be thatrockets are not the best vehicles for interstellar travel.Rockets consist of payload, structure, propellant, energysource, engine, and thruster. There is a whole class ofspacecraft that do not have to carry any energy source orpropellant or even an engine, and consist only of payload,structure, and thruster. These spacecraft work by beamedpower propulsion. Many examples of beamed power propulsionsystems have been published. Three will be discussed here.One is the pellet-pushed probe where small pellets areaccelerated in the solar system and guided to an interstellarprobe where they are intercepted and transfer momentum to thespacecraft. Then there is the maser pushed mesh probe. Thebasic structure is a wire mesh sail with microcircuits at eachintersection. The mesh sail is pushed at high acceleration bya microwave beam. The high acceleration allows the sail toreach a coast velocity near that of light while still close tothe transmitting lens. Upon arrival at the target star, thetransmitter floods the star system with microwave energy.Using the wires as microwave antennas, the microcircuitscollect energy to power their optical detectors and logiccircuits to form images of the planets in the system. Thepicture information is then beamed back to earth. A thirdbeamed power system is the laser pushed lightsail, where largesails of light-reflecting material are pushed to the stars bythe photon pressure from a large laser array in orbit aroundthe sun. The lightsail would reach relativistic velocity in afew years. Upon approaching the target system, a portion ofthe sail is detached from the center of the lightsail andturned to face the large ring sail that remains. The laserlight. from the solar system reflects from the ring sail whichacts as a retro-directive mirror. The reflected light decelerates the smaller rendezvous sail and brings it to a halt inthe target system. After the crew explores the system for afew years, a return sail is separated out from the center ofthe rendezvous sail. The laser light from the solar systemhits the ring-shaped remainder of the rendezvous sail and isreflected back on the return sail, sending it back to thesolar system. As the return sail approaches the solar system,it is brought to a halt by a final burst of laser power.
It is difficult to go to the stars. They are far away,and the speed of light limits us to a slow crawl along thestarlanes. Decades and centuries will pass before the stay-at-homeslearn what the explorers have found. The energiesrequired to launch a manned interstellar transport are enormous, for the mass to be accelerated is large and the cruisespeed must be high. Yet these energies can be obtained once we move our technology out into space where the constantlyflowing sunlight is a never-ending source of energy--over akilowatt per square meter, a gigawatt per square kilometer.There are many ideas in the literature on methods forachieving interstellar transport [See bibliographies byMallove, Forward, Paprotny and Lehmann, 1980; Paprotny and Lehmann, 1983; Paprotny, Lehmann, and Prytz, 1984 and 1985].In time, one of these dreams will become a real starship.
It is not easy to comprehend the distances involved ininterstellar travel. Of the billions of people living todayon this globe, many have never traveled more than 40 kilometers from their place of birth. Of these billions, a fewdozen have traveled to the Moon, which at almost 400,000 kilo-meters distance is ten thousand times 40 kilometers away.Soon, one of our interplanetary space probes will be passingthe orbit of Pluto, ten thousand times further out at4,000,000,000 kilometers. However, the nearest star at4.3 light years is ten thousand times further than that.
To carry out even a one-way probe mission to the neareststar in the lifetime of the humans that launched the probewill require a minimum velocity of 0.1 c (10% of the speed oflight) . At that speed it will take the probe 43 years to getthere and 4.3 years for the information to get back to us.The nearest star is called Proxima Centauri, part of a threestar system called Alpha Centauri. One of the stars issimilar to our sun. Further away are stars that are our bestcandidates for finding an earth-like planet. These areEpsilon Eridani at 10.8 lightyears and Tau Ceti at 11.8 lightyears. To reach these stars in a reasonable time will requireprobe velocities of 0.3 c. At this speed it will take nearly40 years to get there, plus another 11-12 years for the information to return to earth. Yet, although we need to exceed0.1 c to get to any star in a reasonable time, if we canattain a cruise velocity of 0.3 c, then there are 17 starsystems with 25 visible stars and hundreds of planets within12 lightyears [Forward, 1976]. This many stars and planetswithin reach at 0.3 c should keep us busy exploring while ourengineers are working on faster starship designs. Although itis possible to use fusion or antimatter rockets for slowtravel to the nearer stars, it may be that rockets are not thebest way to go to the stars.
You don't have to use the rocket principle to build astarship. If we examine the components of a generic rocket,we find that it consists of payload, structure, propellant,energy source, an engine to put the energy in the propellant,and a thruster to expel the propellant to provide thrust. Inmost rockets the propellant and energy source are combinedtogether into the chemical "fuel". Because a standard rockethas to carry its fuel along with it, its performance issignificantly limited. For missions where the final vehiclevelocity V is much greater than the exhaust velocity v, theamount of fuel needed rises exponentially as the ratio V/v.
It is possible to conceive of space vehicle designs thatdo not use the rocket principle and thereby avoid theexponential mass growth implicit in the design of a standardrocket. These are excellent candidates for starships. TheBussard interstellar ramjet [Bussard, 1960] is one example.The interstellar ramjet carries no fuel because it uses ascoop to collect the hydrogen atoms that are known to exist in"empty" space. The hydrogen atoms are used as fuel in afusion engine, where the fusion energy is released and theenergy fed back in some manner into the reaction products(usually helium atoms) which provide the thrust for thevehicle. Unfortunately, at this time, no one knows how tobuild either the proton fusion engine or the scoop (which mustbe very large in diameter as well as very low in mass)
There is a whole class of spacecraft that do not have tocarry along any energy source or propellant or even an engine,and consist only of payload, structure, and thruster. Thesespacecraft work by beamed power propulsion. In a beamed powerpropulsion system, the heavy parts of a rocket (propellant,energy source, and engine) are all kept in the solar system.Here, around the sun, there are unlimited amounts ofpropellant readily available, and the energy source (usuallythe abundant sunlight) and the engine can be maintained andeven upgraded as the mission proceeds. Many examples of suchbeamed power propulsion systems have been published inthe literature, three will be discussed here. All of theseversions can be built with "reasonable" extrapolations ofpresent day technology. The examples are pellet-stream-pushed, microwave-beam-pushed, and laser-beam-pushed vehicles.
In the pellet-pushed-probe concept [Singer, 1980], smallpellets are accelerated in the solar system and accuratelyguided to an interstellar probe where they are intercepted andtransfer momentum to the spacecraft. By using pellets, thefundamental physical limitation of the spread of an electromagnetic beam with increasing distance can be overcome byusing a particle beam rather than a photon beam for the momentum transfer. The pellets would be launched by a very longlinear electromagnetic mass driver. The accelerator would belocated in the Solar System and supplied by an energy sourceusing nuclear or solar power. The pellet stream would be verycarefully aimed immediately after launch and perhaps recollimated occasionally during flight. The pellets would beintercepted by the interstellar probe and reflected back inthe opposite direction, the process resulting in an increasein momentum of the probe.
The absolute pointing accuracy of the mass launcher isnot a serious limitation. The probe detects the incomingpellet stream and adjusts its position to stay in the stream.A series of course-correction stations could be located downrange from the launcher along the pellet stream. Each station, for example, would be three times farther downrange andwould produce one-third as much velocity adjustment. Thecoarser adjustments could be made electromagnetically orelectrostatically, and the finest adjustments could be maderemotely by light pressure from a laser or by interaction witha plasma gun or neutral atom stream.
One method for accomplishing the interception of the highspeed pellets at the vehicle is to vaporize them into a plasmawith a pulse of photons or particles and then reflect theresultant plasma from a magnetic field in a manner somewhatanalogous to the expulsion of plasma from a magnetic "nozzle"in a pulsed fusion rocket system [Hyde, Wood, and Nuckolls,1972]. The size of the magnetic reflector should be at leastas large as the radius of curvature of an incoming proton ion,which is 3 meters for an incoming pellet velocity of 0.1 c anda 10 Tesla magnetic field. Extensions of the pellet streamconcept include changing the pellet composition and velocityso that the pellets are fusion fuel that is captured at a lowrelative velocity, then used in a fusion engine foracceleration and deceleration. Deceleration at the targetstar system could also be accomplished by rebounding thepellets from an expendable unmanned lead ship to deceleratethe manned vessel at the target system. Of course, once the"interstellar highway" has been traversed, then a pellet-stream launcher can be constructed at the other end forrelatively easy two-way travel.
Starwisp is a light-weight, high-speed interstellar flybyprobe pushed by beamed microwaves [Forward, 1985]. The basic structure is a wire mesh sail with microcircuits at eachintersection. The mesh sail is pushed at high accelerationusing a microwave beam formed by a large segmented ringtransmitter lens made of alternating sparse metal mesh ringsand empty rings [see Figure 1]. Such a configuration of ringswill act as a crude, but effective, lens for a microwave beam.
The microwaves in the beam have a wavelength that is muchlarger than the openings in the wire mesh of the Starwispstarship, so the very lightweight perforated wire mesh lookslike a solid sheet of metal to the microwave beam. When themicrowave beam strikes the wire mesh, the beam is reflectedback in the opposite direction. In turn, the microwave energygives a push to the wire mesh sail. The amount of push is notlarge, but if the sail is light and the power in the microwavebeam is high, the resultant acceleration of the starship canreach many times that of Earth gravity. The high accelerationof the starship by the microwave beam allows Starwisp to reacha coast velocity near that of light while the starship is stillclose to the transmitting lens in the solar system.
Prior to the arrival of Starwisp at the target star, themicrowave transmitter back in the solar system is turned onagain and floods the target star system with microwave energy.Using the wires in the mesh as microwave antennas, the micro-circuits on Starwisp collect enough energy to power theiroptical detectors and logic circuits to form images of theplanets in the system. The direction of the incoming microwaves is sensed at each point of the mesh and that informationis used by the microcircuits to transform the mesh wires intoa microwave antenna that beams a signal back to Earthcontaining the picture information.
A minimal Starwisp would be a 1 kilometer mesh sailweighing 16 grams and carrying 4 grams of microcircuits. (Thewhole spacecraft weighs less than an ounce--you could fold itup and send it through the mail for the cost of first classpostage.) This 20 gram starship would be accelerated at115 gravities by a 10 gigawatt microwave beam, reaching 1/5ththe speed of light in a few days. Upon arrival at AlphaCentauri 21 years later, Starwisp would collect enoughmicrowave power to return real-time, high resolution colortelevision pictures during its fly-through of the system.
Because of its very small mass, the beamed power levelneeded to drive a minimal Starwisp is about that planned forthe microwave power output of a solar power satellite. Thus,if power satellites are constructed in the next few decades,they could be used to launch a squadron of Starwisp probes tothe nearer stars during their “checkout” phase. Once theStarwisp probes have found interesting planets, then we canvisit those planets using another form of beamed powerpropulsion, called laser sail propulsion. Although microwavebeams can only be used to "push" a robotic spacecraft awayfrom the solar system, if we go to laser wavelengths, then itis possible to design a beamed power propulsion system thatcan use laser beams from the solar system to send a starshipto the nearer stars, and then bring the starship and its crewback home.
One of the best methods for traveling to the stars woulduse large sails of light-reflecting material pushed by thephoton pressure from a large laser array in orbit around thesun [Forward, 1984]. With this technique we can build aspacecraft that can not only carry a large human crew atreasonable speeds to the nearest stars, but can also stop toallow the crew to explore, then return the crew back to earthagain within a human lifetime.
In laser sail propulsion, light from a powerful laser isbounced off a large reflective sail surrounding the payload.The lightsail is made of thin aluminum film stretched over asupporting structure that in turn is attached to the payload.The light pressure from the laser light pushes the sail andpayload, providing the needed thrust. The laser sail starshipis about as far from a rocket as is possible. The starshipconsists of nothing but the payload and the lightweight sail,which is both structure and thruster. The engine of ourstarship is the laser, the energy source is the Sun, and thepropellant is the laser light itself.
The sails that the laser craft would use would beadvanced versions of the Sun-pushed lightsails that have beendesigned by the NASA Jet Propulsion Laboratory for cometmissions and fast trips to the asteroid belt. The laserswould be advanced versions of t.he high power laser arrayspresently being studied by the Space Defense InitiativeOrganization of the Department of Defense. The importantthing to realize is that no scientific breakthroughs areneeded to build this starship. The basic physical principlesof the lasers, the transmitter lens, and the sail are known.All that is required to make the laser sail starship a realityis a lot of engineering (and a lot of money).
The lasers would be in space and energized by sunlightcollected by large reflectors. For pushing an interstellarstarship, the lasers would probably work better if they werein orbit around Mercury. There is more sunlight there and thegravity attraction of Mercury would keep them from being“blown” away by the reaction from their light beams. Theywould use the abundant sunlight at Mercury's orbit to producecoherent laser light, which would then be combined into asingle coherent laser beam and sent out to a transmitter lensfloating between Saturn and Uranus.
We will want a starship design that can carry outroundtrip missions to stars as distant as Tau Ceti and EpsilonEridani within a human lifetime. The lightsail would be builtin three sections [see Figure 2]. There is an inner payloadsail that is 100 kilometers in diameter. Surrounding that isan inner ring-shaped sail that is 320 kilometers in diameterwith a 100 kilometer hole. Surrounding that is an outer ring-shaped sail that is 1000 kilometers in diameter with a320 kilometer diameter hole. The total structure would mass80,000 tons, including 3,000 tons of payload consisting of thecrew and their habitat, supplies, and exploration vehicles.
The entire lightsail structure would be accelerated at30% of Earth gravity by 43,000 terrawatts of laser power.(Since the Earth only produces about 1 terrawatt of electricalpower, we would certainly want to use the free solar power inspace instead of trying to get our power from Earth.) Atthis acceleration, the lightsail would reach a velocity ofhalf the speed of light in 1.6 years. The expedition wouldreach Epsilon Eridani in 20 years Earth time and 17 years crewtime, and it will be time to stop. At 0.4 lightyears from thetarget star, the outer ring sail would be separated from thetwo inner portions. The inner portions would be allowed tolag behind while they are being turned to face the large outerring sail. The laser light coming from the solar system wouldreflect from the outer ring sail acting as a retro-directivemirror. The reflected light decelerates the two innerportions and brings them to a halt at Epsilon Eridani.
After the crew explores the system for a few years (usingtheir lightsail as a solar sail), it will be time to bringthem back. To do this, the smaller ring sail is detached fromthe payload sail and they turn to face each other. Providedsomeone back in the solar system remembered to turn on thelaser beam 12 years earlier, the laser beam from the solarsystem hits the ring-shaped sail and is reflected back on thepayload sail. The laser light then accelerates the payloadsail back toward the solar system. As the payload sailapproaches the solar system 20 Earth-years later, it isbrought to a halt by a final burst of laser power. Themembers of the crew have been away 51 years (including 5 yearsof exploring), have aged 46 years, and are ready to retire andwrite their memoirs.
It is difficult to go to the stars, but it is notimpossible. There are not one, but many different technologies, all under intensive development for other purposes,that, if suitably modified and redirected, can give the humanrace a flight system that will reach the nearest stars. Allit really takes is the desire and the commitment to a fewdecades of hard space engineering work and our first interstellar probe could be heading to the stars within ourlifetimes.
This work was partially supported by Air Force Contract 04611-86-C-0039.
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