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So- we all know how traditional inertial confinement fusion devices work, right? You have an elliptical chamber with a high explosive or fission bomb at one focus, and you have a small deuterium-tritium (heavy hydrogen- "DT") ball at the other focus. The frozen (or liquid?) DT is encased in lithium which
also enhances beneficial neutron radiation. The entire vessel and casing is vaporized but lasts long enough to reflect shock waves onto the opposite focus point and DT ball, creating enough heat and pressure to cause a small fusion ignition there. In weapons this small ignition is used to set off all kinds of other stuff, but that is probably a process patented by General Electric so we won't go into it.
This design is great for a temporary energy release, but the idea is to use this small, lightweight design principal for a continuous propulsion engine.
The chamber would have to be much larger, stronger and heat resistant to withstand small fusion explosions. Here you can use your imagination concerning the latest high tech, high strength synthetic metals, matrixes, ceramics and cooling mechanisms.
DT fusion (perhaps like any fusion reaction) envelopes it's entire mass of fuel, even if the initial ignition is small. This is due to the shear intensity of the fusion reaction. You might say it is self propagating via it's own shock wave. So we are only really concerned with igniting the tiniest point of DT.
If we dispense with the pre-ignition explosion we can simplify and greatly lighten the entire mechanism, but another pre-ignition energy source would have be used. The intensity and energy of a laser is ideal. The laser or other energy source is the most difficult design aspect. I propose using a laser and increasing brightness and efficiency by using Gallium-Arsenide silicon whole wafers as the light source. It would require many wafers focused together. If they could be manufactured with a transparent substraight, perhaps it would be possible to "stack" them together at even wavelength intervals. Ga-As is very fast switching, and on-off cycles might be used to increase coherence beyond chemical lasers (which rely on very inefficient light sources). Perhaps Ga-As could also offer some advantages as the light source for chemical lasers. Perhaps even a gaseous semiconductor laser employing a structure maintained and based on the standing wavelength. All kinds of possibilites for higher efficiency light sources. A simple curved and focused array of wafers could also be used if intense enough.
If the lasers could be held sufficiently coherent and in phase, (or routed though reflectors), it may be possible to have nearly parallel laser beams totally negate each other's energy in a very small area and therefore concentrate a great deal of energy in a very small area. The biggest unknown of the entire concept is generating and concentrating the ignition energy. Masers, arcs or high energy plasma beams are all possibilities as well. Anything to get the maximum power into the smallest area.
That done, the rest is easy. The target is a small bullet of lithium encased DT fired from a multi-barrel gun many times a second. When arriving at the precise focus it is annihilated and ignites. Here you could just make the vessel open and hemi-spherical and you already have a small, crude fusion propulsion system. Granted- it's dirty, loud, toxic and highly radioactive (but only for 7 years). It's for outer space use only. But why keep firing off lasers? You might be able to save the expense and wear and make the chamber elliptical as in the original example. Fire the laser target at one focus. The other focus has a much larger fusion target placed similarly, but the entire end of the ellipse beyond it opens to an expansion chamber. Here only part of the explosion shock is reflected back inside the elliptical chamber back to the original focus point, where another small target is waiting to be ignited purely by the shock wave, and it starts all over again. Depending on the size of chamber and contained medium, we are probably talking about many thousands of times a second to take advantage of the resonance effect, but perhaps the rate is far too high to be of any use to mechanisms. I cannot comment on this.
This system is throttlable only by changing target mass (or combined target mass if multiple sources are somehow used) or changing the number of explosions over time- which would probably have to be at least 20-200 explosions per second to be useful. Thousands of times a second would be incredible and very smooth. Changing rate means killing any beneficial resonance, but perhaps fractional divisions of the resonance would also be efficient. Without resonance, just use lasers for all ignitions and use any shape chamber. Spherical chambers may also offer resonance advantages with a single explosion point and better harmonic efficiency points, but may also be subject to much greater stress.
Thrust and cooling might be enhanced by introducing some steam or other medium into the chamber to add reactive and thermal mass and act as propellant.
Fly nuclear!
A dissertation on Project Orion...
http://www.islandon...n/ProjectOrion.html
...probably the first nuclear spacecraft. [phoenix, Sep 03 2002, last modified Oct 21 2004]
Antimatter: Fission/Fusion Drive
http://ffden-2.phys.../fissionfusion.html
"A pellet of Deuterium, Tritium, and Uranium-238 (nine parts D-T for every one part U-238) is injected into the reaction chamber. First the pellet is compressed using ion particle beams, then irradiated with a 2ns burst of antiprotons. The antiprotons annihilate some of the pellet, producing enough energy to cause the U-238 to fission. In turn, the fission reaction ignites a fusion reaction within the Deuterium-Tritium (D-T) core. The fusion reaction produces the desired engine thrust. A new pellet is than inserted, and the process repeats itself." [phoenix, Sep 03 2002, last modified Oct 21 2004]
The British Interplanetary Society's Project Daedalus...
http://scientium.co...tarship/rockets.htm
..."uses a fuel consisting of pellets of solid deuterium and helium-3. These fuel pellets are sent to a chamber where they are struck by a high-energy electron beam, beginning the fusion process. A super-hot plasma is created, and directed for thrust using magnetic fields generated by superconducting coils. " [phoenix, Sep 03 2002, last modified Oct 21 2004]
Particle Bed Reactor engine using LiH as moderator
http://www.newworlds.com/mitee.html
See also George Dyson's book on Orion which covers similar research to above in the Fifties and Sixties [cloudface, Oct 04 2004, last modified Oct 21 2004]
[link]
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I practically understand this. Which is a first coz it invloves some science. |
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youve been thinking about this for a while havent you? have a croissant. a big one. |
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Certainly baked - as written - in SciFi (and I know [jutta] will want references, so I'll try to turn them up) as well in other forms. |
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I think you'll find the primary reason for not using large-scale nuclear power in spacecraft is that no one wants a nuclear reactor to fall on their head and it's banned by international treaty. |
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[phoenix] Great links! Thanks. My idea appears to be nearly as baked as radioactive exhaust. Maybe someone should build this thing already. Stage 2 is to make really small units that vent into a carbon filter and simply generate enormous amounts of heat for driving steam turbines or whatever (cars). Then we can start playing around with the high energy plasma to convert it directly to electricity, or create plasma pipe to simply replace electricity with something with much more energy density. |
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[unabubba]Jeez. I missed the laser port problem completely! I suppose only a .25" hole is needed. Low to medium detonation rate applications can use a mechanical shutter. Anything else means a quartz lens exposed to fusion, but I'm talking SMALL fusion detonations here, just repeated at a high rate. The stress and average temperature cannot exceed the limits of existing materials. I'm not talking Sci-Fi. I think this can be done today. It is basically a question of how large the vessel must be to contain the reaction without failing. |
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How will the fuel be dispensed so that the (probably) distorted shockwave makes a clean exit? I envision a “dud” pellet displaced whole or flattened to produce a residual window in place to be broken by a subsequent detonation. The disasterous effect of a small misfired fusion product flare could put an end to the mission of its craft. |
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The idea of tiny fusion explosions
reminds me of an offshoot of
project Plowshare - civilian uses
for nuclear weapons (not
technology, chain-reaction
bombs). One of them was to bui;d
an interstellar spacecraft that
would dump nuclear bombs out its
tail and ride the shockwave.
Another (which they actually
carried out) was to build an entire
saltwater harbour and entry
channel, capable of containing
several supertankers, using only
four nuclear weapons - 3 10 kTn
for the channel and 1 40 kTn for
the harbour. As I remember, all
the radioactive earth, etc. was
"safely washed out to sea". |
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And how thick will you have to make the thrust plate between the chamber and the rest of the ship? In order to survive being in the close proximity of a fusion blast, that plate will have to be so massive that the thrust imparted to the ship is so low you'd be better off going with a much safer and already pretty well understood chemical rocket system. This "bang-bang" proposal, which by the way was first voiced in the 19th century by Jules Verne (using chemical explosives) is the human technological version of God making a rock so massive He can't lift it. |
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instead of individual pellets, carefully meter a stream of fuel into the previously 'ignited' gases, to allow the heat & pressure of the shock wave formed thereby to trigger fusion in the fresh fuel - like a fusion ramjet. the fuel flow is controlled so that the shock wave creates a standing wave with respect to the ship |
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//DT fusion (perhaps like any fusion reaction) envelopes it's entire mass of fuel, even if the initial ignition is small. This is due to the shear intensity of the fusion reaction. You might say it is self propagating via it's own shock wave// |
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Not sure where you read this, try looking up the term "fizzle" in term of thermonuclear weapons. The fusion reaction is not self sustaining, but requires ongoing energy flux to expend all of the fuel. |
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This is why inertial confinement is so elegant. The "pellet" is so small, that it's immolation is complete. The casing of the pellet is completely vaporised, producing symmetrical implosion. If, say you were to ignite one end of a rod of DT, or whatever fusion fuel, there would be no garuntee that the reaction would cascade the length of the rod. there are other forces at play. |
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There is one thing that you may not have noticed about your geometery, Here is a bonus! If something continues through each focus it will eventually align with the two foci. You can make a very small hole and it should align rapidly and the thrust should have very little wasted or canceling vector of thrust. like 7 bounces and things are almost perfectly aligned. You would think it could go backwards and unalign the way it got there in the first place. I may be full of myself on this one but sit there and trace the lines on a few bounces. It shouldn't be near as many bounces as a typical laser. |
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As a matter of fact you would probably need a hole to make it continue to work. It might require half silvered mirrors to balance it. light easily refocused. |
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Shooting pellets to match the harmonic frequency sounds feasable Probably on the order of magnatude of 100th the speed of light would space them far enough apart that they could get to the focus of a huge space based chamber that would probably not have to be but a mile wide. |
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I know so little that I think this is possible to actually work. I don't see any point in complaining about problems I can't see. I was thinking about hollowing out mercury a bit anyway. I like the way you think. [+] |
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sp. 'substrate', I think, not 'substraight'. |
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Anybody else start reading this and instantly think "Vernon!"? |
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