Regarding Project Orion, another way in which fusion can be used to propel a craft that does not involve carrying the fusion fuel aboard the craft is obviously the so-called fusion runway.
As I like to envision it, pure nuclear fusion bomblets would be laid out in a spiraling trail in orbit around the Sun with a length as long as 40 light years. For 40 light year long trails, accelerations tangential to the trail and parallel to differential length elements of the trail could be as high as several Gs and angular craft accelerations as great as 100s of Gs might be sustained wherein the occupants of the craft would live in hydrostatically sealed pressure vessels to cancel the squashing affects of the G forces. The yield of the hydrogen bombs could be limited to a safe 1 kiloton or less, or perhaps have a higher yield as; the ship size, refractive properties of materials used in the construction of the ship, and/or the mechanical strength of such materials permits. Lorenz electrodynamic forces could perhaps keep the craft in solar orbit as it accelerated.
If the hydrogen bombs took the form of pure fusion bombs wherein almost all of the fusion fuel would be made to fuse, the 1 kiloton bomblets would have a paltry mass of only about 7 grams each. One kiloton is equal to about 4 trillion joules and so if such an atomic bomb was detonated behind, along side or within a space craft rocket chamber at a rate of one per second, in one year, the space craft could acquire (10 EXP 12)(3 x 10 EXP 7) Joules of kinetic energy assuming that the explosion conversion efficiency was 25 percent. Thus, a space craft with a mass of 300 metric tons could obtain a relativistic gamma factor of 1.001. If ten such devices where exploded per second, given the same conversion efficiency, the craft could obtain a gamma factor of 1.01. In one hundred years, the craft could obtain a gamma factor of 2 which is not bad for local star travel. The caveat here is the ability to shield the crew from ionizing radiation more than anything else, and this would be difficult using a space craft with a mass of only 300 metric tons. Higher gamma factors would be possible using more extreme systems.
If one kiloton fusion pebbles proved to be too powerful for a fusion runway driven space craft with a mass of only 300 metric tons, perhaps 0.1 kt pebbles or even 0.01 kt pebbles could be utilized in much more rapid detonation sequence. One caveat is actually being able to get most or all of the fusion fuel in these small pebbles to fuse. The actual G-forces of the craft due to acceleration could be arbitrarily ramped up to the extent that the quantity of fuel used per unit of time would not over burden the ships radiation shielding mechanisms, the ship’s mechanical strength properties, etc.
For a track that is 5,000 light-years long in distant orbit around the Sun, that is, a track that wraps around many times for which the average acceleration of the space craft is about one G in the tangential direction to each differential length element of the spiral length of the fusion runway, the gamma factor of the craft upon traversing the entire runway would be about 5,000. It is conceivable that an electrodynamic Lorenz force inducing mechanism could keep the craft in circumlinear motion around the Sun using a towed charged cable or by instilling a net electrical charge on the craft such that the craft would be pulled by the Lorenz force to maintain circular motion about the Sun. The mechanism used here would be the solar magnetic field as a reaction mechanism to produce the Lorenz force.
If the craft had a mass of about 3,000 metric tons and could harness the equivalent of 12,000 metric tons of fusion fuel converted into energy per year, then the quantity of fusion fuel required per year would be roughly 1,200,000 metric tons assuming that 1/4 of the fusion energy would be converted into ship based kinetic energy. To reach a gamma factor of 5,000 in 5,000 years, the craft would need to utilize 6 billion metric tons of hydrogen assuming that the efficiency of bomb energy to ship based kinetic energy conversion was 25 percent.
I could see that 0.01 kt pure fusion pebbles might make excellent mass driver or LINAC material for a mass and/or fuel beam driven craft being that they would have a mass of only 70 milligrams.
With all of the work currently being done in the field of nanotechnology, carbon fullerenes such as buckyballs and other molecular cages, and also reports as current as 2004 in main stream press reports that the U.S. Air force is studying the feasibility and applications of small quantities of antimatter, it is my opinion that essentially pure fission fusion bombs could by just around the corner.
Although such fission fusion bombs would probably find defensive military applications, I see the result as a potential boone for Orion type pulse rockets and for fusion pellet runway concepts.
In the event that a tiny amount of antimatter could be kept within a molecular cage or several proximate molecular cases within a mass of pure U-235, which could be surrounded by deuterium ice, or perhaps some sort of metallic dense deuterium material, then perhaps rupturing or opening the molecular cage could permit antiprotons contained within to leak into the nuclei of the U-235 atoms thus causing the atoms to fission, If enough fissions can be generated per unit of active cell within the overall U-235 mass, the whole U-235 sample could fission thus heating the deuterium to fusion temperatures with the result being the whole pure thermonuclear bomb explodes. Such a device with a yield on the order of 0.1 kt to 1.0 kt could make for an excellent Orion style space craft and also an excellent fusion runway pellet.
Perhaps a massive thick shield could capture dust particles and interstellar atoms, molecules, and ions in such a manner that the shield would gradually transmute into fission and fusion fuel which would then be used for propulsion. A good heat energy dissipation mechanism could recycle the KE of dust and atomic scale particle impacts while the shield increased in rest mass and in average atomic number.
I would like to think that mission parameters that far out do the Project Daedalus craft might be developed in the few short decades from now except based on the fusion runway concept. The great benefit of using a fusion runway is that the nuclear fusion fuel need not be carried along with the craft from the start of the mission thus reducing the rest mass of the vehicle from the start.
Regarding using fusion runway pellets, the circumlinear distribution of antiproton catalyzed fission bomblets wherein molecular cages containing antiprotons within the pellets of not only U-235, but also combinations of U-235 and U-238, or perhaps even pure U-238, would be opened up thus causing the anti-protons to be released thereby causing the fissions to proceed at a rate in which an effectively super-critical mass would be produced.
If such atomic fission bomblets can be produced, perhaps they can also be encased in the best performing nuclear fusion fuels for the purpose of propelling a space craft to relativistic velocities with nuclear fuel runway pellet runways having a reduced total rest mass compared to pure atomic fission pellet runways. It might be easier to use such a fission fusion mechanism than it is to use direct anti-proton based fusion. Note however, that direct anti-proton catalyzed fusion is also a theoretical mechanism that has been discussed for fusion runways in academia and other sponsored studies as is commonly known by the interstellar astronautics methods research community.
Note that perhaps the most significant caveats to this form of propulsion system are finding ways that permit the craft from being destroyed by mere dust particle impacts, and perhaps from space pebbles and rocks. Also, a means to protect the crew from radiation, not only from the atomic blasts, but also from the interstellar gas which would impact the craft with a gamma factor of 5,000 for the highly relativistic example given above.
Another major caveat is maintaining the high blast energy to ship based kinetic energy conversion efficiency especially as the craft reaches extremely relativistic velocities. In such cases, the bombs would need to be detonated within a large ship’s long aspect ratio reaction chamber, along side an energy extraction chamber, or near the back of the starship in a long aspect ratio rocket or pressure cone chamber. Problems of extracting energy would begin to present themselves due to relativistic length contraction of the ship with respect to the bombs reference frames. Perhaps the ship would need to tow very long very thin carbon nanotube fibers or some other material that could extract energy out of the nuclear blasts wherein the energy would then be converted to electrical energy by some unspecified mechanism. Note that the formula for relativistic length contraction is essentially as follows:
Delta L = L/gamma = L { [1 – [(v/c) EXP 2]] EXP (1/2)}.
The energy extraction system could be composed of highly heat conducting materials such as solid or perhaps wound diamond thread for strength or perhaps be composed of ultra strong carbon nanotube materials.
Note that theoretical studies of the effects of neutron bombs on space based balloons suggest that very thin film helium filled balloons could survive 100 meter proximity to the detonation of a one kiloton neutron bomb, simply because almost all of the resulting neutron radiation incident on the balloon would pass through the balloon without interacting with the balloon or the gas inside. Thus the reason for suggesting that high heat capacity highly refractive and radiation resistant strings might be useful for capturing at least some blast energy without being destroyed in the process.
If the balloon models are wrong, then perhaps much lower yield pure nuclear fusion bomblets could be used in a fusion runway; perhaps with yields as small as 0.001 kt or even 0.0001 kt. Note that the blast from the Saturn V manned Lunar rocket as it was lifting off the launch pad during its first second or so of the heightened thrust used to separate and clear the rocket from the from the launch pad was about 0.1 kt in yield or about the yield of the smallest nuclear weapons ever developed such as low yield battlefield nuclear weapons. By contrast, the bomb that demolished Hiroshima had a yield of about 12 kilotons which is about 120 times higher in yield. The largest nuclear bomb ever tested was done so by the Soviet Union in the early 1960s and had a staggering yield of about 59,000 kilotons or about 5,000 times the yield of the bomb the leveled Hiroshima.
Note that a circular trail pattern of fusion runway pellets located at between 30 and 100 AU from the Sun might be constructed within the next century or two and would have the advantage over a linear fusion fuel runway of the same total length, on the rough order of 100 to 10,000 for the examples given above, as a result of the fact that travel would not need to be far from the Sun in order to distribute the fusion runway: By definition, maximum travel distances for a given human and/or robotic distribution crew need only be at most 200 AU for a two way trip between the Earth and the runway. Multiple circumlinear arc sections of the fusion runway could be build simultaneously, in fact the entire runway could effectively be assembled simultaneously by crew working in parallel along different arcuate segments of the runway without any crew needing to travel any further than 100 AU from the Sun for the latter example.
The real point is that the folks of Project Orion have given us much food for thought in terms of potential ways of using nuclear energy to reach for the stars.
As a quick reminder, Project Orion was conceived as means by which interplanetary ships the size of ocean liners would be propelled forward by small nuclear bombs wherein about one bomb per second would be detonated behind a magnetically insulated pusher plate at a safe distance from the pusher plate. The idea was to eventually send humans to Mars and throughout the solar system by such a mechanism.
So when the reader is tempted to think negatively about nuclear energy, because of the unfortunate situations which lead to the Cold War as well as environmental issues of how to safely dispose of nuclear waste from commercial nuclear power plants, simply think about the many configurations of systems using nuclear energy that might help us get to the stars. I think if you think carefully and long enough, you will appreciate how nuclear energy can be a road for humanity to venture out among the stars.
Regards;
Jim
July 7, 2009 at 6:48 pm |
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