There is a natural path through solar sails, which are now flying, toward beam-driven propulsion, and it’s a path Jim Benford has been exploring for the last eighteen years. In my Centauri Dreams book I described how Jim and brother Gregory ran experiments demonstrating that carbon sails could be driven by microwave beams back in the year 2000. We learned that the theory worked — a sail could indeed be propelled by a beam of photons — and moreover, we learned that the configuration of the craft and propulsion system allowed it to be stable.
Now we’re talking about beam-riding, which the Benfords were able to demonstrate in later experiments. For it turns out that the pressure of the beam will keep a concave-shaped sail in tension, and as Jim pointed out in a recent email, the beam also produces a sideways restoring force. His work showed that a beam can also carry angular momentum and communicate it to the sail, allowing controllers to stabilize the structure against yaw and drift. This is as far as our microwave-beaming experiments have taken us so far, but as solar sails become less an experimental than an operational technology, we can move to space-based experimentation.
Image: A near-term sail experiment under microwave beam. Courtesy of James Benford, Microwave Sciences.
Robert Forward’s name always comes up in such discussions. An old friend of Benford’s, Forward developed enormous interstellar mission concepts using beamed propulsion, ideas that physicists like Geoffrey Landis and Robert Frisbee were able to tweak, just as Jim did, to produce smaller systems. Jim went on to take a cost-optimized approach to the issue, understanding that even the most ingenious of starship designs will be driven by economics. His new paper discusses the matter and notes that a design project using his methods called Project Forward will be undertaken by Icarus Interstellar, the group that manages Project Icarus.
Benford’s notions are solid and based on long experience. As he wrote recently:
I feel beam-driven propulsion is more firmly grounded, more thought through and quantified than nuclear propulsion methods at present. We should put more of our effort into beam-driven sails in this era of little funding. The on-going development of solar sails will tell us how to deploy and control sails, so we will keep close links with that community. This will lead to beam-driven experiments and simulations. Let’s get on with it!
Let’s talk for a moment about the experimental work on beam-driven sails, which was enabled by the invention of carbon microtruss material that is both strong and absurdly light. The material from which a sail is made is critical given that a certain fraction of the power the beam provides the sail will be absorbed and must be radiated away. Given that acceleration is strongly temperature-limited, materials with low melt temperatures like aluminum, beryllilum and niobium are ruled out for beam-driven missions, no matter how useful they may be for standard solar sailing, which uses solar photons rather than concentrated beaming to drive the spacecraft.
Carbon mesh materials work admirably for beamed-sail experiments because carbon has no liquid phase and sublimes instead of melting, as Benford explains in his new paper. These materials allow a sail to operate at temperatures up to 3000 C, allowing them to be ‘launched’ in a vacuum chamber here on Earth without burning. The Benfords were able to push ultralight sail materials at several g’s of acceleration, with the sails reaching temperatures in the range of 1725 C from microwave absorption while remaining intact. Bear in mind that various mission concepts call for lower power densities than the scientists used here. Operating on Earth, they needed a powerful push to get the forces needed for liftoff within a gravity well.
Robert Forward’s interstellar concepts were awesome in their scale, but Benford points out that there is a path to be followed before getting to the interstellar stage. From the paper:
It’s important to realize that for large-scale space power beaming to become a reality it must be broadly attractive. This means that it must provide for a real need, make business sense, attract investment, be environmentally benign, be economically attractive and have major energy or aerospace firms support and lobby for it. Therefore, we include missions that could lead to Starwisp missions, from an infrastructure base developed for smaller-scale missions.
Starwisp was another Robert Forward concept that came out of a time when the scientist moved from laser propulsion ideas to microwaves, whose longer wavelength allowed the sail to be little more than a grid — the wavelengths involved are comparable to the human hand, as Benford told me in an interview some years back, whereas lasers operate at minute wavelengths. A microwave sail, in other words, could be far lighter than the sail required for a laser push because the microwaves are stopped by a conducting surface with gaps smaller than a wavelength. From this, Forward came up with the ultralight ‘starwisp’ design.
Imagine a wire mesh about a kilometer in diameter that weighs no more than sixteen grams. You’ll want data return from the spacecraft so Forward included microchips at each mesh intersection. The craft would be so light and insubstantial that it would be invisible to the eye, but it could be accelerated at 115 g’s using a 10 billion watt microwave beam, taking it to a cruising speed of 20 percent of the speed of light within a few days. Forward’s Starwisp paper included his usual love of gigantic objects, including a beaming lens 50,000 kilometers in diameter.
Geoffrey Landis has shown that the wrong materials would cause a Starwisp to be fried by the powerful microwave beam thus generated, which is why people like Benford are looking at entirely new sail materials as they explore closer and more practicable missions. And practicality — a realistic path forward through solar sails to beamed propulsion — is what I want to discuss on Monday, when I’ll run through the mission concepts Jim Benford has looked at from the standpoint of cost-optimization. Because if we’re going to move beamed sailing out of the realm of science fiction, we’ll need missions that are near-term and offer a clear and economical way to deep space.
The paper is Benford, “Starship Sails Propelled by Cost-Optimized Directed Energy.” I’ll post the link when this paper becomes available online.
I had no idea that these “sails” could to be accelerated to “several g’s” and higher. I’ve always assumed that even beamed sails accelerate at about the same orders of magnitude as solar sails. This completely transforms the economics and mission plans.
I look forward to reading the new Benford paper.
Paul – can you post links to earlier papers?
Alex, sure, I’ll post some links to earlier work on Monday when I resume the discussion of these sails.
“Forward’s Starwisp paper included his usual love of gigantic objects, including a beaming lens 50,000 kilometers in diameter”
I have read that such gigantic apature sizes are a must to keep a beamed sail accelerateing up to speeds in the region of 10%c. I confess I don’t understand the physics behind that (and could be mis-remembering could anyone enlighten me?) but if accurate the hurdle might not so much be bulding the probe as building the beaming system.
The problem with beamed propulsion for interstellar travel is that you are reliant on the people back home keeping the beam on (even if it costs them money to keep it on).
Awesome idea!
Very interesting. The fact that this is a near term technology with a potential for speed that significantly expands our list of potential stellar targets is of real importance in seeing it actually get done.
Support for an interstellar journey of discovery requires a destination of great interest that can be reached in a reasonable portion of a human lifespan. There’s no comparing the level of interest in a mission that would arrive at an Earth-like planet that shows signs of life in a matter of decades, to that of a mission to a star that is close by but of little interest to people, or one that might arrive at its destination in the distant future.
Reasonable trip times, a target of compelling interest, and the possibility of starting on the path to such a project now make for a powerful combination.
Sounds like we need to be thinking about extremely lightweight payloads.
Looking forward to Monday’s installment – and the links.
The problem with self-contained propulsion is that you’re reliant on the people in the ship being very careful where they point a rocket exhaust powerful enough to sterilize whole planets. If you’re in the ship you might want it self contained, but the people staying behind probably don’t.
I know that Konstantin Tsiolkovsky was the first (as far as I know) to propose solar sail , 1921 (took a number of years for people to think of space flight concepts that Tsiolkovsky didn’t think of!). In 1924 Friedrich Zander proposed solar sailing with thin sails , a tip of the hat to all those Russians who beat everybody to astronautics constructs , outside of fiction that is.
However , as far as I know, G. Marx first proposed beam energy for interstellar flight ,Interstellar Vehicle Propelled By Terrestrial Laser Beam, Nature, 211, 22 (1966) , Marx’s mistakes in this paper were corrected by J. L. Redding, Nature 213, 588 – 589 (11 February 1967).
Maybe it took the invention of masers and lasers for the idea of beamed propulsion to come to the fore, yet, unless someone knows differently , not sure why it took till 1966 to think of it. Solar sailing had a steady progression of development from Tsiolkovsky’s days.
I remember being gob smacked by Bob Forward’s paper in 1985 : Starwisp: an ultra-light interstellar probe. J. Spacecraft & Rockets, 22, 345-350.
Together with nuclear pulse propulsion , beamed propulsion, as far as I know, are the only fully realizable relativistic transport systems I know of (even if it’s ‘slow’ relativistic, .1 to .2 the speed of light), modulo all the really tough and expensive technological program needed.
The toughest problem with thin sails is that they are no match for the relativistic oncoming interstellar medium that they will encounter. Anything thin like that will be shredded in a very short time. In fact, I think “tough” is a severe understatement for the severity of this problem.
The gigantic lense would be necesary if we talk about a full-scale colony ship , incuding heavy stuff like radiation shielding and a totally different propultionsystem for braking at the targetstar. In this case the beaming would have to go on for a long time after the ship was out of effektive range of the earthbased beam , because of the scattering of the microwawe beam (this scattering might be less catastrofic in case of a laser beam ) .
A lense would then extend the effektive range .
On the other hand , a lense might not be necesary for a nearfuture lightweight interstellar probe , of the kind that would ZIP through the targetstarsystem at 10% lightspeed , and then spend a few more years transmitting the recorded data back to earth …
I suppose that the reason behind such a large aperture of the driver is keeping it’s beam pattern small enough (comparable to the sail size) out to the destination system. Probably it can be physically realized like an interferometer (think VLA but very much larger).
Why not take a different approach? Take a smaller aperture driver and fill its larger beam by a swarm of “nano sails”. I don’t have the exact numbers on this, but it might be that they can even be accelerated to greater speeds, be less prone to malfunctions en route, more fault-tolerant (depending on design), etc. And, everything can cost far less. Would be interesting to read the papers and see if this was considered.
There was a great paper published in JBIS in 1999 by Geoffrey Landis and Robert Forward that is worth a read: “Beamed Energy Propulsion for Practical Interstellar Flight, 52, 1999. The team involved in the work came up with a strawman interstellar flby misson which had the following characteristics:
4.2 light years distance
42.2 years total mission duration
30,000 km/s (0.1c) cruise velocity
1 km sail diameter
200 km lens diameter
100 kg spacecraft mass
33 kg payload mass
2.7 m/s^2 acceleration
25 GW power
1,100 AU thrust run
Laser beaming systems definitely need pushing forward as one of the candidates for the first unmanned mission in my view and its wonderful that Icarus Interstellar is now launching a project along these lines. But I wonder what the architecture (lens diameter, payload mass….) would like like for a crewed mission? Has anyone got any details on that. I’m not aware of any crewed studies for laser beaming systems so would appreciate any references to read.
Kelvin
So how does the ship decelerate at the destination?
djlactin>
Bob Forward’s treatment with diagrams appears here:
http://www.transorbital.net/Library/D001_AxA.html
It is a ‘staged’ beam sail configuration, and as one can see not only deceleration but the return stage too. I think it is the most ingenuous concept Bob ever thought of , among many of his inventions.
djlaktin
“So how does the ship decelerate at the destination? ” It doesn’t decelerate . What is being described here , is mostly relevant for a fast and reletively cheap probe that would zip through a starsystem without braking.
In order to decelerate , a very different design would be needed , perhabs one where the structural material of the sails becomes the fuel for another propulsion system . Or perhabs the sails will just be detached like an emty fuel tank.
Another posible role for beamed sails , comes from the scenario where we eventually reach the conclution that no other detectabe life exist in our galactic nabourhood . This could happen inside 30 years .In that case beamed sails might find a use in seeding the most fitting exoplanets with a broad spectrum of onecelled earthlife. Such ” seedpackages ” has been described by several SF writers , among them Lary Niven . The challenge would be to find a way of slowing down the life capsula without too much use of a heavy propulsionsystem . Lary Niven does this elegantly with a “stasis field” . We dont have one of those , but it seems logic that the incredible toughness of microorgannisms could somehow be exploited to make them survive a “hard landing” …One idea would be to disperse individual microbes atached to a strong elektrical charge above the exoplanet , in the hope that the planets magnetic field together with interactions with the “solar” wind , would somehow acidentaly be capable of slowing down a few of the bilions of individual life-particles to a velocity where a few of the few would suvive.
With all the talk of alternate mechanisms being needed to decelerate at the other end, I started to wonder if this was actually the answer rather than the problem.
Imagine a trip to Alpha Centauri by a manned Orion class vessel that had just used most of its mass in propellant to get to 0.1c, and that its remaining nuclear bombs represented only a small fraction of the remaining mass. Its potential for changing velocity at trips end could be boosted by wrapping ice around these remaining explosives, and canisters of such suitable material could be precisely targeted to rendezvous with the vessel by this method… Or could they?
A question: If the beaming station is located in space (is it?), then what will be done regarding the thrust driving the beaming station back? Are there some proposals?
http://www.niac.usra.edu/files/studies/final_report/4Landis.pdf
This is the other paper Landis wrote in 1999.
I think we could design an even lighter probe now.
Maybe we could do some new calculations
We may be closer than we think to launching with current technology and we can choose big or small. Small with the sails or big with H Bombs in Project Orion. Both routes seem to get us to 30,000 kps
First time post; absolutely love this site! I’d like to second Eniac’s question at 1232. At speeds of .20c how would you prevent the sail from being “sand blasted” by the interstellar medium?
Does anyone have an idea how to address the problem with the ISM? I hate to damp the enthusiasm like this, but we need to keep our eyes open.
It’s my first time posting on this site but I’ve been reading the articles for over two years. I was wondering about possibility of “engineering” the path of a similar craft. One that would use beam sails during the acceleration phase and mag sails to slow it down on the final approach. The path would be engineered using successive launches of charged smart dust. They would be slowly enough for the craft to overlap them just before it would reach it’s target system. In short the smart dust acts as a break for the mag sail. The biggest flaws I can currently think of are how much weight the mag sail would add, powering the mag sail, and removing enough of the smart dust and interstellar particles during the acceleration phase.Any ideas on how to improve?
I think that gravity lenses could help to focus the beam.
A special structure collector very near the Sun could generate a huge beam from solar energy to a new station far away in our solar system.
The second station would generate a new laser beam back using our star as a lense to focus the beam to the target star and move itself to be aligned at correct distance and aligned with our star and target star indefinitely.
But even with gravity lenses, probably the aperture of the laser beam will only focus a minimal part on the ship, so the laser beam will be some orders magnitude bigger than the energy captured by the ship. In the order of TW instead GW.
Fortunately, our star is plenty of energy. The first station could be huge in size, by light in mass. A constellation of satellites could be even a better solution for redundancy.
The structure could be huge, but it would allow a permanently channel of travel at near c speed. Reach another star in decades sound reachable without world ships. With some kind of suspended animation, where a pair of decades turn into two or three years of aging, sounds even “easy”.
Duncan Ivry, the power-sat would out mass the star-ship by an immense amount. The beam-sail described by Kelvin masses 100 kg and is driven by 25 GW of beam power. A power-sat typically masses 5,000 tons per GW produced in orbit, thus 25 GW laser power needing about about 100 GW of electrical power equates to 500,000 tons of power-sat to provide it. Reflected sunlight on the power-sat’s collector area should be enough to oppose the reaction force of the beam and then some. A perfect solar-to-beam conversion process would allow a power-sat to emit 50% incident power from one side and reflect 50% from the other, the resultant force balancing out.
Abelard Lindsey said on December 9, 2011 at 20:25:
“The problem with beamed propulsion for interstellar travel is that you are reliant on the people back home keeping the beam on (even if it costs them money to keep it on).”
LJK replies:
This is why we must build our propulsion space lasers on isolated planetoids near Sol or on Mercury. We then surround the facilities with automated-only defenses such as AI-controlled robots with suitable weaponry and programmed to support and defend the continuation of the interstellar mission no matter what happens with humanity, even and including its demise.
The designers just have to make sure the defense machines remain just defensive, though. We do not want a scenario where the AI decides that if the probability of humanity either stopping or dismantling the laser station is high if, say, a totalitarian regime comes to power and wants to use the laser as a weapon, then the AI determines that a pre-emptive strike on human civilization is warranted to protect the mission and it has got this honking big and powerful laser at its disposal….
Eniac
“Does anyone have an idea how to address the problem with the ISM? I hate to damp the enthusiasm like this, but we need to keep our eyes open.”
about the “sandblasing effect” ? as the whole idea of a microwawe sail is to be extremely lightweight , heavy shielding doesnt make sense . Automatic repairing by a spiderlike robot could deal with the damage done by the statisticly rare (we hope! ) particles big enough to cut a strand , but the effect of individal atomsize partikles at relativistic speeds on the sail material is one of the things to investigate as soon as possible . If it should be be impossible to make a resisstant material , again our hypotetical maintenance robot will be the answer , only this funktion wil be more demanding . One way would be for the robot to continously recycle and regenerate individual strands of the network . This would demand a reserve of extra material , that the spider could mix up with the regenerated part .
ljk: “defenses such as AI-controlled robots with suitable weaponry ”
Isn’t this a little bit over the top? And isn’t the whole thing complicated enough?
Eniac, the ISM is a non-problem. Beam-sails are too thin for much energy to be transferred between ISM particles and the sail, thus only a tiny area is damaged equivalent to the cross-sectional area of the dust particle. Over reasonable ranges of flight the cumulative damage is <<1% the sail's area. Another presenter covered this in one of the 100 YSS technical sessions that Jim Benford Chaired.
I’m confused. Is this greater part of this “sand blasting” problem due to dust, or the much more gradual “evaporation” of material due to relativistic gas particles?
Adam, I was not talking about the dust, but the gas particles. There are enough gas particles between here and Alpha Centauri to make a layer several micrometers thick if condensed along the line of sight. Any effective sail would have no more than comparable thickness. It should be expected that any proton hitting the sail at relativistic velocity will knock quite a few atoms out of their places and away for good. With a few micrometers having maybe 10,000 atoms, this will happen about 10,000 times during the trip, on each square Angstrom. Enough, I would think, to erode away the sail many times over. Repair is not really an option, either, as any spare material or repair “spider” will all be eroded away quickly, too.
It would not be too hard to get real data on relativistic particle erosion, I believe it is a well-understood problem in the design of accelerator targets.
Those who speculate about aliens sneaking into our solar system should consider the detectability of a 25GW beam aimed at us for 5 years, of a 1 km sail glowing at 3300 K, or better yet, of a nuclear fusion engine pointing at us while it decelerates. Unlike pre-1900, we’d certainly have no trouble seeing them now.
Duncan Ivry said on December 12, 2011 at 14:51:
ljk: “defenses such as AI-controlled robots with suitable weaponry ”
“Isn’t this a little bit over the top? And isn’t the whole thing complicated enough?”
Well, how would you defend the laser station against hostile elements? The whole point is that someone asked how would the station be kept running for decades or longer, especially if humanity undergoes less than ideal regime changes or even collapses. My answer is to entrust it to machines.
If you are going to build a giant laser station in space or on another world, what’s the big deal about a few defensive machines?
Eniac: “…erode away the sail many times over.”
This would depend on the duration of the acceleration phase. To avoid erosion one needs to accelerate fast and then, if the sail will be needed later, retract it out of harm’s way.
Eniac
“Repair is not really an option, either, as any spare material or repair “spider” will all be eroded away quickly, too.
Problems are made to be solved . A single spiderrobot cold aford to cary on its back a protection shield against the errotioneffect , and the shield could be excanged with a new one a limited number of times .
As for the aditional thickness of the material and quantity of spare material necesary for regeneration , it have to be seen in the perspective of a general system witch would EVEN with theese additions still be an incredibly lifgtweight sructure .
A more advanced solution would be to trap the indiwidual
OOOPS ! Power interruption !
( continued) … trap the individual atoms which got nocked loose from the sail material . If theese have an electrical charge they might be solidified elektrochemicly (vacum deposition) , so the material might be covered by a layer designed to be regenerated . Even this would eventually be destroyed , but it might buy enough time to reduce the weight increase to an acceptable lewel .
ljk, if some of our descendants decide to change their pritorities and use the material of the beaming station for something else (or dismantle it, because they just don’t like it), who are we, that we do not allow them to do this? And, above that, with force of arms?
And the other point: “what’s the big deal about a few defensive machines?” These are usually very expensive, and you should not underrate the problem of maintaining the weaponry in *addition* to maintaining the beaming station for a long time. It could very well be, that the weaponry will cost more than the beaming station alone.
But, any way, yo, peace, man :-)
RonS writes:
Exactly so. One of the scenarios Benford talks about is a .10 c sail being driven to cruise speed with a 1 AU ‘runway’ — an intense beam and high acceleration, after which the sail could be folded for cruise.
Folding the sail is an interesting option. The sail will start eroding during acceleration, though, which will set a limit on the velocity that is attainable. I do not know, but would not be surprised if such a limit were disappointingly stringent.
Duncan Ivry said on December 13, 2011 at 16:19:
“ljk, if some of our descendants decide to change their pritorities and use the material of the beaming station for something else (or dismantle it, because they just don’t like it), who are we, that we do not allow them to do this? And, above that, with force of arms?”
LJK replies:
I thought the entire point of defending the laser beaming station was to make sure the interstellar mission is never compromised at least at the Sol system end no matter what happens with or to humanity?
Duncan Ivry then says:
“And the other point: “what’s the big deal about a few defensive machines?” These are usually very expensive, and you should not underrate the problem of maintaining the weaponry in *addition* to maintaining the beaming station for a long time. It could very well be, that the weaponry will cost more than the beaming station alone.”
No they would not. Your defenses can be passive such as mining the entire area around the base with simple trigger bombs. Just ask the French about all those World War I and II mines they still keep finding buried in their soil that are just as deadly now as during those wars.
If you still are not happy with machines doing the job, then get generations of specially ordained monks to dedicate their lives to defending the station. No, I am not trying to be funny. If you are going to have a powerful laser station in space and you want it protected against various pitfalls, then you either shell out the proper funds for the proper defense or don’t bother building it at all.
“But, any way, yo, peace, man :-)”
Peace through ultimate firepower.
Re Al Jackson’s comment:
“…as far as I know, G. Marx first proposed beam energy for interstellar flight ,Interstellar Vehicle Propelled By Terrestrial Laser Beam, Nature, 211, 22 (1966) , Marx’s mistakes in this paper were corrected by J. L. Redding, Nature 213, 588 – 589 (11 February 1967).”
Jim Benford responds via email:
“First mention of beam-driven propulsion for interstellar purposes in the refereed literature was by Bob Forward, in Missiles and Rockets in 1962. Perhaps more people read the Marx paper in Nature four years later. He also considered only x-rays to drive the sail, because he flinched from large apertures for the antenna and sail. (Using such energetic photons is a doubtful matter; X-rays reflect only at grazing angles.) Marx didn’t pursue his idea further, at least not in the literature.”
ljk: “I thought the entire point of defending the laser beaming station was to make sure the interstellar mission is never compromised at least at the Sol system end no matter what happens with or to humanity?”
The end justifies the means? I hope you are not serious here. But you already said “No, I am not trying to be funny.” So, as far as I can tell, you are promoting homicide.
Paul Gilster, what do you say?
Duncan Ivry writes:
I’m sure Larry can defend himself against the charge of promoting homicide ;-)
But it’s a serious question, especially if lives are riding on it, as they would be in, say, Robert Forward’s scenario in Rocheworld, where a government is thinking about shutting down the beamer that would be used to decelerate a manned probe to Barnard’s Star. I would not aggressively defend a beamer for unmanned missions, agreeing with Duncan that a future society has the choice of what to do with the technology. But a trans-generational manned mission, if such ever occurs, would have to operate with the assumption of a working beam if needed for the flight to succeed. We’re talking about deceleration basically (using Forward-style ‘staged’ sails, I assume), because the acceleration phase is short enough in relative terms that huge social changes seem unlikely during that period. But I would want that beamer to work if we had a crew depending on it 40 years down the line, and I don’t think it’s homicidal to establish clearly defined safeguards on such a device. Those safeguards needn’t necessarily be lethal, but they have to be robust and readily understood by anyone hoping to shut down the beamer. In other words, shutting it down has to be really hard to do, to make sure that it’s not the act of a rogue group but rather the collective decision of humanity.
All I have to say to Duncan Ivry’s gross misinterpretation of what I said is “What the hell?!”
And no, Duncan, this does not mean that I think anyone who builds a space laser station is actually going to hell, okay?
An impenetrable defense system against our descendants? At the normal rate of progress in science and technology, in no more than one generation the defense system will be hopelessly primitive. It will also be rapidly dismantled if that is their desire. Pointless exercise, whatever the justification. As Paul says, accelerate fast and avoid the question entirely.
That’s why I figure any manned mission is going to be a mix of beamed propulsion on this end, (No energetic exhaust pointed at an inhabited system!) and magsail or reaction drive on the other end. Who is going to want to depend on the folks back home still paying the propulsion bill decades later?
I agree with Brett, I waited until the last comment to see someone talking about the real advantage of beam driving. The real advantage appearrs at relativistic speed, 1)almost all the energy is converted into kinetic one, 2) the sail experiences just a low temperature beam 3) no speed limit exaust, thus the optimal ratio momentum/energy is reached at … light speed.
But I agree with Eniac comment, even if he reall underestimate how man atoms are in a one micrometer square. ;)
I however have some questions about the article. I first figured it was really enlightening (oh oh) to say a microwave mirror, as a web, would weight much less than a dense matter light mirror, but finally I don’t really see how it resists to a scale factor argument. If someone has some maths…could be good.
Another silly question, does anone already heard about a billions watts beam generator (with references please)?????
Best.
Sorry for the y missing and the r doubling, I got a keypad problem.
Hi Folks;
I cannot believe that the background interstellar and intergalactic matter would erode even many of highly relativistic sail of micron thickness.
The diametrical cross-sectional area of our observable universe is close to 10 EXP 47 square kilometers and the mass of the total mass energy of the observable universe is only about 10 EXP 50 metric tons of which only 4 percent is baryonic, Thus an average column spanning the diameter of the entire visible universe would have an H2O STP matter thickness of only 25 micrometers for reactive matter.
But this is not a concern for the following reasons in addition to the one’s pointed out above by other commentors.
First, the sails could be replacable grid sails and driven by optical, IR, microwave or rF radiation. The mass of such sails can be reduced by many order of magnitude relative to monolithic sails that are only micrometer scales in thickness.
Second, sails having a very thick cable or thread like construction are conceivable where the cables or wires would be many times if not several orders of magnitude thicker than 25 microns. The sails could be mostly empty space or almost entirely empty space to reflect long wave rF phased array beams.
As for concerns about over burdening the conductive or super-conductive wires or cables used for such sails by extremely intense rF beams, well note that such reflective members could be very conductive to superconductive to thereby yield near perfect reflection. The EM energy that was not reflected would largely pass through the openings in the sail grid.
Second, a magnetic and/or electric field based scoup or antiscoup could divert the chargons away from the sail just as an extended electrodynamic scoup for an interstellar ramjet would. Electro-dynamic-hydro-dynamic-plasma-drive features could utilize the diverted plasma in a reactive and gainfull manner.
Third, who says the sail must be deployed in a manner that is orthogonal to the ship’s velocity vector? The sail might be parallell to the space craft velocity vector and driven obliquely from behind. This way, the effective thickness of the sail could be thousands of miles and the sail could include an electro-dynamic-hydro-dynamic-plasma-drive features.
Fourth, the above parallel sail could conceivably be made of negative refraction index materials would would be pulled forward by incident star light and highly blue-shifted CMBR, far infrared, and non-CMBR radio sources.
Fifth, the sail can simply be a deployed magsail or M2P2 type of sail or any other magnetic or plasma bottle sail. It is even possible that a plasma affixed to the space craft could be driven by rf radation, and even source based laser light upon attainment of extreme space craft gamma factors could be easily reflected by such sails. Plasma makes an excellent rF reflector even at only very small densities.
I have done a lot of writting on parallel sails such as negative refraction index monolithic and grid sails capable of extreme gamma factors.
Sixth, some sail materials such as any future forms of super-strong very conductive to super-conductive metallic hydrogen can be used as nuclear fusion fuel for fusion rockets upon degradation to useless levels.
Seventh, it has been proposed that very,very thin metallic very low gas density containing balloons might be used for nuclear warhead decoys and which could survive 100 meter proximity detonation to a one kiloton neutron bomb in the vacuum of space. The rate of radiative cooling would be tens of billions of Kelvins per second due to the extreme thinness of the baloon membranes and most of the nuetrons would pass right through the balloon without interacting or by only depositing an very small portion of the particles kinetic energy into the balloon and enclosed gas. Interstellar chargons are more reactive to electronic shell structures but not by that much.
Regards;
Jim
Paul, thank you for your response, and you have some interesting points.
For example this: “the acceleration phase is short enough in relative terms that huge social changes seem unlikely during that period. But I would want that beamer to work if we had a crew depending on it 40 years down the line”
What happend in the last 40 years? Serious financial, economic, and political crises happened, including big national debts in Europe and in America; more than 50 percent of the world’s population now lives in cities; several dictatorships were overthrown; many, many people are connected through the internet and use mobile phones; the world’s population will continue rising. And there is much more.
All this brought us severe social changes already and, to my humble opinion, will bring us many, many more. I seriously expect the world to be very different in 40 years from now — and it will not be so much like we space enthusiasts want it to be.
It will be interesting to see what the 100 Year Starship Project will tell us about this. I seriously expect that “they” — sadly — will not provide a solution. I hope that the opposite will come true.