Interstellar Sails and Their Precursors
Posted by ~Ray @ 2008-11-13 12:18:27
magazine. “Making Light Work” runs through solar sail basics for an audience that may seem surprising but I can tell you from my own flying days that as we used to wait in the pilot’s lounge for students to arrive we would often kick around outlandish concepts like deep space missions (and there were always a few dog-eared copies of
Friedman speculates about sails kilometers wide in the area of 0.1 microns in thickness ultralight films that would when the photons from sunlight lost their punch take advantage of huge laser installations that could be focused for interstellar distances. Now we’re into Robert Forward territory and also in range of feasible interstellar missions. For as Friedman notes solar sails are the only technology we currently have that could complete such missions in a single human lifetime:
What is exciting is that we know the way forward. We don’t have to invent some new physics (like matter/antimatter engines) and we don’t have to conjure up new technologies from science fiction (such as interstellar ramjets scooping up and using interstellar hydrogen molecules). Rather it’s all a matter of engineering—make the light sail materials thinner the spacecraft lighter and the lasers more powerful.
Of course the demands are still huge power on the order of 100 gigawatts which means power stations located in space assembled in the inner solar system where solar radiation is much higher than here on Earth (presumably sails would be involved in ferrying the needed materials). And then there’s the problem of sail construction conceivably handled by making the sail out of plastics whose evaporation would leave only the needed molecules to reflect sunlight and laser photons. Imagine a square kilometer sail weighing just a few kilograms its electronics sprayed onto the sail rather than flying as a separate payload.
Solar sail technology is no idle dream. After extensive study at Marshall Space Flight Center. NASA’s basic sail design has reached the point where space testing is the logical next step even as research continues in European venues like Germany’s. When we begin a serious push into solar sail technologies we’ll need to test these designs in near-Earth orbit and then move out into the Solar System. A logical mission for early sails will be as Friedman notes a replacement for the (ACE) a mission nearing the end of its lifetime.
ACE operates at a where the gravitational forces of Sun and Earth balance some 1.5 million kilometers from Earth. A sail mission that could monitor solar weather (and warn us of solar storms) could offer a new kind of station-keeping one that uses the momentum imparted by photons to stay in position closer to the Sun without the need of remaining at the libration point. Such a position would among other things allow greater early warning of potential ionospheric disruptions.
The range of sail missions available in coming decades will be huge but if we keep at it we may get to the point where building the kind of laser we’ll need for an interstellar mission becomes possible. Solar sailing is the kind of next-step technology that moves us from one-shot mission spectaculars like Apollo into the realm of a stable and long-term human presence expanding into the Solar System. For the short term we need to keep doing what Friedman and sail advocate Gregory Matloff are doing explaining and arguing for the needed steps to get sails into nearby space where their value for more complex missions will be obvious.
The solar sail concept is intriguing however; I see a possible technical problem with laser powered solar sails. Assuming that the laser is operated from some ‘off-earth’ position enabling it to take advantage of enhanced solar energy in some form to power the laser the laser itself will experience an acceleration in the opposite direction to the beam it emits (the laser acting as a ‘photon propulsion system’) will it not? Assuming instead that the laser is anchored to some ‘air-less’ planet or moon; and fired frequently to propel a variety of spacecraft eventually the lasers accelerations so produced may alter the orbit of its base. I wonder has anyone done investigated this ‘reaction’ problem?
Tim if the lasers are attached to the power collector system - solar of course - then the net thrust of the laser will always be less or equal to the energy received by the collectors. Thus the thrust will always be compensated for.
Of course the problem then becomes how do we stop the collector from being carried away by the incoming reaction force of the sun’s energy? All sorts of tricky configurations of counterweights extra solar sails and so forth can be used and the reaction force can even be used to turn the facility into a “statite” which is suspended by the reaction force against the sun’s gravity - i e it’s not in orbit anymore. This can have advantages for aiming the lasers and so on.
I would rather plant the lasers on the moon and use Helium-3 fusion instead. Of course nuclear fusion and in particular the He-3 variant first have to be mastered. But the advantages are 1) that you won’t need a huge solar power collecting area and 2) a solid base (surely laser pulse reaction won’t be a problem on a body as large as the moon) maybe even 3): He-3 mining on the moon (though not nearly as abundant as on Saturn and Uranus).
Yes laser sails are a technology for interstellar flight that ia already (almost) available but as I have stated before interstellar flight (and a lot more) probably hinges on our mastering of (Helium-3) fusion.
Nuclear fission engineers have 60 years of experience working with fission the process that powers atomic bombs and nuclear reactors. When the center of a radioactive atom is split apart the resulting charged atomic fragments fly away at 3 percent of the speed of light about 5,000 miles per second. Researchers led by George Chapline of Lawrence Livermore National Laboratory have designed a conceptual “fission fragment” reactor to harness those high-speed particles. Their reactor resembles a stack of vinyl records rotating into a cylindrical tower. Each “record” consists of graphite covered with radioactive fuel such as plutonium or americium. When the fuel spins into the tower it encounters additional radioactive material inside and triggers a controlled fission chain reaction. Powerful magnets around the reactor corral the resulting nuclear fragments so that they fly away in one direction producing an exhaust that could accelerate a rocket to 6 percent of the speed of light. To surpass 10 percent of the speed of light. Frisbee proposes building two fission rockets and staging them one on top of the other. The second stage effectively doubles the rocket’s top speed so the expanded version could zip along at 12 percent of the speed of light. Add another two stages to slow everything down by the end of the trip and you could pull into an orbit around a sister Earth in the Alpha Centauri system in 46 years. More-distant voyages would take more than a human lifetime however even using additional stages. To keep weight to a minimum the fission rocket would require a fast-decaying nuclear fuel such as americium. Americium is not a naturally occurring element so it would have to be processed from spent nuclear fuel. A mission to the next star would require roughly 2 million tons of americium not to mention a considerable amount of radiation shielding. Using cheaper uranium or plutonium would drive the fuel mass even higher. But the fundamental technology is ready to go.
Has anyone ever considered more exotic proposals for interstellar probes? I may not be putting this idea over very well! Plasma crystals which are basically particles of dust suspended in a plasma can be very well organised structures. If a plasma crystal could be made stable in deep space could it serve as the basis for a spacecraft? Information could be stored in the structure of the crystal perhaps recording the electromagnetic or gravitational enviroment around another star. If dust particles from that solar system could be pursuaded to incorperate into the crystal it could even perform a sample return mission. As it would mass very little and be made largely of empty space both the energy requirements and collision problems would be much releived. I dont know if a plasma crystal could be manufactured that would remain stable in space but has anyone heard of anything comparable or equally exotic?
Ion drives and all electric propulsion systems suffer from heat loss issues because the power source is separate to the rocket. Chemical nuclear thermal and pulse drives all lose their waste heat in their exhaust stream and have enough mass flow of propellant to use that for cooling if needed.
Ion drives are pretty inefficient losing about 40% of the electrical power as heat in the ion making & accelerating process. Plasma drives are pretty efficient depending on the propellant heating system - VASIMR and helicon thrusters both use RF heating that can be 90% efficient in converting electricity into heat. But the magnets in the VASIMR system do need cooling.
But the real heat loss problem is the power source - nuclear reactors for example require massive cooling systems and about 60-70% of the energy they make has to be dumped as waste heat. If super-efficient thermoelectrics could be invented the system is still limited by how well it can be cooled to create a thermal differential to extract power from.
What’s really needed for ion or plasma drives is a compact and highly efficient electrical power source. Robert Bussard’s fusor using p+B11 can get 95% of the ion energy converted into electrical power. He designed a 16 gigawatt system using two fusors to power a hybrid scramjet/rocket system for launching from Earth. To convert electricity into thrust high-powered electron guns would blast reaction mass into plasma and the exhaust channelled using magnetic fields.
Alternatively ultra-power density batteries - say using room-temperature superconductors - could power an all electric hybrid but the trick is whether such high-temperature superconductors can be made. The mass of batteries would be pretty heavy - say a 100 ton vehicle is flying into orbit. To get there about 9 km/s total delta-vee is needed - the equivalent of 40 megaJoules energy per kilogram of propellant for an air-breather. Experimental batteries can manage maybe 1000 Watt-hours per kilogram (most are a lot less) - 3.6 megaJoules per kilogram. A hundred-fold improvement would be needed to get a hyrbid vehicle into space double that for a pure rocket.
A Bussard fusor could power a VASIMR quite effectively but only once a working power-generating fusor is demonstrated. The next two fusors. WB-7 and WB-8 are being funded by the US Navy and it’s hoped they will prove Bussard’s scaling relations right thus allowing a power-producing fusor to be produced.
If high efficiency direct-conversion power is available then several different plasma rockets become viable. VASIMR being the best researched. VASIMR is basically a fusion rocket but without any actual fusion. Hooked-up to a Bussard fusor and that might change - direct fusion product propulsion being the most energetic rocket design exhausting the fusion plasma directly and getting up to 1,000,000 seconds Isp or more. The magnetic nozzle of a VASIMR would then be ideal but the helicon heater would be redundant.
The first demo flight of a VASIMR will be a small one attached to the ISS perhaps to be used for reboosting the Station against orbital decay. The next demo would be on a planetary probe using a few kilowatts from solar collectors. Manned missions will need fission or fusion reactors to produce megawatts/gigawatts of power or large laser collectors to catch beamed power.[ADVERTHERE]Related article:
http://www.centauri-dreams.org/?p=1597
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