h a l f b a k e r yRenovating the wheel
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Jim pondered rockets and figured the most potent fuel is well nuclear --- in anycase...
--- take a beam of unrefined fissionable material
--- heat until plastic
--- tap to create a regular interference pattern
--- drill severally through the beams length
--- fill sufficiently for
a controlled critical mass
...
acoustic tweezers
http://www.eurekale...08/ps-atc082809.php
Positioning tiny objects... [madness, Nov 16 2009]
Rocket
http://en.wikipedia.org/wiki/Rocket
First paragraph ... [8th of 7, Nov 16 2009]
Critical mass
http://en.wikipedia.../wiki/Critical_mass
Minimum critical mass in kgs at specific pressure [madness, Nov 17 2009]
[link]
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Yes, they do. But surely they would tend to unity, unless
you believe in that sort of thing? |
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[-] makes no sense...how does this make a rocket? |
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nuclear rockets were baked. |
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//drill severally through the beams length
// |
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Is "severally" a word? Did you mean several times, and thought you'd make up a word that means several-like or did you mean to say drill in a severe manner, ie severely? |
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Not sure if either option helps us here... |
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I think you're trying to say have a subcritical mass *in linear form* that is stable through having suitable 2-dimensional geomtery - kind of like some sort of extrusion whereby fission rates are managable at the original geometry. Then somehow you trigger the reaction by melting the business end, thereby achieving critical mass and producing (somehow) rocket-thrust. Presumably you're somehow controlling the rate at which heat is transferred along your fuel rod so that you can controll the progression of the melting and thus the reaction. Kind of like a big nasty radioactive road flare. |
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Interesting idea. Why not do the same by a) withdrawing one or more control rods from inside the monolithic fuel rod. or b) control a cooling system instead or better c) come up with an idea that is in some way inherently stable ie loss of control does not lead to a runaway reaction.. I understand that pebble-bed reactors are like this, ie a cooling failure *inherently* shuts off the reaction (because of water coolant slowing down neutrons and thus allowing them to react, so when the bed heats up the water expands and the density is wrong to slow down enough neutrons, or something like that). |
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I think [8th] might appreciate this idea, because it's kind of like a blueprint for a huge cataclysmic roman candle of death. Set one up upside down so the fission-ing end is on the top and the liquid fissionable materials flow down producing more reaction mass, etc. Be one hell of a way to herald in the raptures. |
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// huge cataclysmic roman candle of death. // |
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Assuming this is intended to be a Newtonian reaction drive, what's the working fluid ? Vapourized fissile material and fission fragments ? Not recommended anywhere in the vicinity of a habitable planet. |
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As a propulsion system for a deep space probe, there might be something here. You could use something like this round Jupiter or Saturn - which are seething with radiation anyway - without having the Environmentalists on your case. And it would offer extremely high power densities. |
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Control would be an interesting issue. |
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// unrefined fissile material // |
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The concept seems to be to create some sort of continuous-feed supercriticality. For this to work, the reaction products are going to need a surface to push against ..... any thoughts ? |
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Ummm... yes it is a rocket that yields a controlled (nuclear rather than chemical) detonation. And yes just like a conventional solid rocket it will burn to completion once ignited. |
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Why not do it differently (with some sort of clock work control rod)? Well that is project orion and it is baked and failed... But the idea is widely accepted as the highest yield propulsion method known (to man). |
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Control of the detonation relies on the ability to migrate (and position exactly) fissionable material within the rod/beam by setting up the right wave interference by simply tapping the beams surface (aka acoustic tweezers). |
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Drilling the beam through its length and filling it with conventional chemical propellant will be required because the small amounts of fissionable material will not attain critical mass without it. |
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... well that is the idea anyways ... |
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And like a conventional rocket there is no requirement for a fluid medium to push against. |
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// there is no requirement for a fluid medium to push against // |
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The reactants (working fluid) exit the rocket with momentum m x v and by this process impart momentum to the rocket itself but the law of conservation of momentum. But the "rocket" is not decoupled from the vehicle as a whole. The rocket motor generates thrust, which it transfers to to vehicle via its mounting structure. This thrust occurs because the rocket nozzle or combustion chamber experiences pressure on one side only from the reactants, the other "side" being the exhaust aperture. |
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A rocket does not require a fluid medium, u nlike a propeller or a gas turbine (turbojet) but it still needs a means of converting the thermal energy of the working fluid into translational energy. |
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// filling it with conventional chemical propellant will be required because the small amounts of fissionable material will not attain critical mass without it. // |
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Please, don't try to explain this. |
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yeah --- thats a rocket (your definition is superfluous (tell me what was the purpose of that anno?)) |
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Umm --- critical mass (no definition required (look it up)) |
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baked, and called, oddly enough, a "nuclear thermal rocket engine"; uses hydrogen. wikipedia. [mfd] |
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Orion didn't so much fail as scare people away, the concept is valid, but people object to launching quantities of nuclear material. Also it used detonations, not a controlled reaction. |
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The problem with the idea as (almost) written is getting thrust out of your reaction. Nuclear reactions are not directional, and can propagate away from the point of supercriticality rapidly. That is,there is little from preventing your entire fuel source from reacting at once, and nothing that will turn it into a rocket exhaust. Even if you get the reaction byproducts all moving in the same direction (commonly called a fusion torch, heavily pre-heated in science fiction), thrust is minimal, as the weight of exhausted material is minimal. |
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The nuclear thermal rocket uses a standard nuclear reactor to heat a seperate reaction mass, which then produces thrust. A much more practical approach, but one that unfortunately still requires high quantities of reaction mass. |
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What [MechE} said (more in sorrow than in anger). |
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I saw that film/Documentary/Niven and Pournelle Novel too.
[marked-for-deletion] No Idea, or a WTCTTTISIN..... |
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'Engagement' Anything that makes the fluid medium,
the rocket is
travelling in, thicker, is better right? A thrust against
a solid wall is going to give better forward motion
than a medium that the thrust can cut through. A
rocket, on a slight incline, up a flight of stairs versus
one straight up through the air. |
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Rockets do not push against anything, they throw something
out the back and move forward as a result. |
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//The problem with the idea as (almost) written is getting thrust out of your reaction// |
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The reason this is not a problem is because the quantity of fissionable material is such a small percentage of the total mass of the rocket(/metal bar). The actual problem with this proposal is obtaining a critical mass (not a lack of mass). |
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So what happens if critical mass is not achieved? Well the standard chemical propellant will burn producing a standard amount of thrust and the metal bar will simply get quite hot (and be subjected to fairly high pressure)... |
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If critical mass is achieved (ie there is enough heat and pressure to compress the fissionable material) then the metal bar will be ripped apart and the reaction mass will add to the output velocity/temperature and therefore total thrust. |
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What prevents the whole lot going critical all at once is again the relatively small percentage of fissile material given the temperatures and pressures that can be achieved with the chemical propellant. |
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//Orion didn't so much fail as scare people away, the concept is valid, but people object to launching quantities of nuclear material// |
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What was wrong with Orion was scale --- it was impossible at the time to make a small enough critical mass. Quite apart from any problems associated with the design working in a vacuum. The rocket had to be massive (to reduce acceleration) and the output radiation could not be reduced --- what is required is a vast number of nano-sized critical masses and sufficient dead weight to be accelerated. |
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// nano-sized critical masses // |
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The Prosecution rests its case, M'Lud. |
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For any given fissile isotope, the critical mass is a fixed value. It is possible to initiate fission in a subcritical mass by implosion, but this results in supercriticality, not a controllable divergent chain reaction. |
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A "nano-sized critical mass" is therefore a contradiction in terms. Any mass smaller than the critical mass is by definition not a critical mass. |
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It is possible to have "nano-sized" masses of fissile material but the physics and mechanics of combining them in a controllable way into a critical mass can best be described as "challenging". |
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Sigh --- you have miss understood. [link] |
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The critical mass of an element may be a 1kg sphere if the number of neutrons emited from the surface is one less than the total neutron decay. That is there is one more neutron produced by the mass than can be radiated by associated surface area. So a critical mass will become hotter. Clearly the ratio of mass to surface area is important. |
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A super critical mass produces more (than one) neutron than can be radiated and a sub critical mass produces less than one neutron than can be radiated by the surface area. |
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Pressure (amoung other things [link]) reduces the mass required to be critical. |
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I will accept that I should have said nano-sized super critical masses |
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There is no such thing as a small scale critical or super-critical mass. Critical mass is stringently defined by natural density and purity of your radioactive substance. |
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The only way to get a reaction in sub-critical masses is to increase density. In weapons, this is done by compressing the fissile material with conventional explosives, such that it's density is great enough that neturons probabilistcally encounter nuclei rather than flying off into space. Even this can only be done in highly refined radioactive material, since the upper limit of compression will not produce a reaction in un- or low- refined material. |
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Even if you got it to work, it is only possible in discrete units, not a continous string. At best you can produce a nuclear pulse rocket, which still suffers from the lack of directionality and the lack of thrust mentioned above. |
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Hey, [MechE], you fancy a pint ? We know a pub not far away with good ale and a lovely old brick wall to bang your head on - that's got to be more fun than this, right ? |
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Oh well, no new thought lines for the lightsaber. |
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To be honest its at times like this that i like to simplify things --- which to be honest tends to infuriate most people. |
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Take this bar of metal we have been discussing and place a charge of any description in the middle. When detonated the two halves will accelerate equally. |
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Now move the charge 2 thirds from one end and detonate. The acceleration of the smallest third will be twice the acceleration of the larger two thirds. That is rocket science. |
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With regards to discreet and continuous detonation it is all a matter of scale and your inability to descern each acceleration. And there is no requirement for any particular detonation device. |
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//place a charge of any description in the middle//
1.5 coulombs. |
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//severally// is a real word, but it means something like 'separately', which doesn't really help here. |
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//The acceleration of the smallest third will be twice the acceleration of the larger two thirds. That is rocket science.// |
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Actually, no, that's a grenade. Rocket science is when you figure out how to get it to keep doing it, and get the part you want to keep going where you want it to go. Both things that it's very hard to do with an atomic explosion. |
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[8th of 7] I know, but I keep hoping something will sink in. |
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//discreet and continuous detonation // |
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Not likely, perhaps "discrete." |
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// I keep hoping something will sink in // |
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You'll need special tools then, some sort of high capacity drill maybe ? |
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I'll try to sum up this idea in understandable language: |
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An otherwise normal solid rocket motor is modified with a liner between the structural case and the propellant core. The purpose of this liner is to hold a multitude of individual masses of fissile material. Each individual mass shall be of a barely subcritical size. |
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The theory of operation is that during combustion of the chemical fuel, the chamber pressure will increase, thereby applying pressure to the fissile material and increasing its density to the point that it becomes supercritical and undergoes fission. The energy released by the fission will add to the energy of the combustion gas, thereby greatly increasing the thrust of the rocket. |
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As nuclear fission produces such a large amount of energy per unit mass of fissile material, this could, in concept, be a viable option for enabling cheap interplanetary travel. |
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Now that I've vomited that up, let me explain why it WON'T work. |
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As several others have already mentioned, it takes a certain mass of material to initiate runaway fission. A lesser mass may be made supercritical by way of mechanically increasing its density through rapid compression. Heat, though a byproduct of the explosive compression, does not play a direct role in the 'ignition' of the fissile core of a nuclear device. |
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Three points to consider:
1. The farther from critical your mass is, the more compression it will take to become critical.
-- A barely subcritical mass may be made to undergo fission by hitting it really hard with a big hammer.
-- A weapon-sized subcritical mass requires a large, very carefully sculpted shell of high explosive in order to produce the high pressures required to compress the fissile material to levels where cascade fission will occur.
-- A "nano-sized critical mass" would require pressures typically only seen during impact in a particle collider.
No chemical rocket engine will produce sufficient pressure to cause any significant fission to take place in an otherwise subcritical mass of any size. |
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2: Even IF you could find just the right size of critical material such that your normal chamber pressure would initiate fission, the increased temperature would simultaneously cause expansion of the fissile mass, reducing its density and once again rendering the mass subcritical. |
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3: Even IF you could manage to get the material to undergo cascade fission, it would just fizzle, releasing very little energy. Without containment during the fission process, the critical mass heats up rapidly, expands, and goes subcritical. Even if pressure remained as it cooled and shrank, the mass would never pass criticality; it would self-limit by virtue of increased fission (and thus heating and expansion) as it cooled, and would find equilibrium. More likely, the fissile mass would go critical, self-heat and expand, melt before it got to nuclear-thermal equilibrium, and be blown out the back by the combustion products of the conventional solid fuel. |
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I applaud your creativity, but there are well-considered reasons that this concept just doesn't work. |
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At least it's wrong in an original way [ ] |
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Jim is an idiot and hasn't even applied heat to this idea, let alone baked it |
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//I applaud your creativity, but there are well-considered reasons that this concept just doesn't work.// |
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[Freefall] Thanks for that --- it is nothing more than a mere bagatelle. I accept your arguments, although there is nothing there that represents a theoretical limitation. |
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The thought that immediately pops to mind is that you have described the problems of initiating a fission event using chemical accelerants. Which is, as I understand, already well understood. |
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And that you have not entertained the notion that once the first fission event occurs this will/should/could trigger a subsequent event. Ideally each fission event will increase the existing 'chemical' pressure enormously/exactly sufficently in the rocket chamber to initiate a subsequent event. |
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Jim, exactly, that's the major problem. Once the first reaction goes, it will do one of two things. It will either blow apart, a fizzle (read further down the article, also notice in the article that pressure is important because it affects density, which we've already discussed), or it will trigger more spheres, which will trigger more, etc. The problem is that if this chain reaction occurs it will trigger all the spheres in a time on the order of microseconds. This will most likely blow your rocket chamber apart, and even if not, it will provide a single pulse of thrust and sputter out, since your conventional fuel will also be blown out of the chamber at the same time. |
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A theoretical system where fuel was fed in, a single nuclear charge near critical mass was fed in, the fuel was detonated, triggering the nuclear reaction, producing a pulse of thrust, and then this was repeated might theoretically be possible. Turning that theory into practice would be difficult if not impossible, since your firing chamber, feed mechanisms, and fuel ignition systems would all have to be perfect and maintain that perfection through multiple nuclear blasts. (The fuel would have to be mixed perfectly with air and detonated uniformly around the nuclear charge). Your repetition rate would also be low, because the heated materials from the first blast would have to clear the chamber before the next could be injected. |
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You'd be better off tossing the nuclear charge into your firing chamber with a standard conventional explosive shell, which is exactly what Project Orion proposed, so that is not an original idea. |
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