lambda0 le 28 Août 2007 07:36
Gilgamesh a écrit:
lambda0 a écrit:
Effectivement, dans sa forme actuelle, un générateur de 1 GW basé sur ce principe nécessite la construction à côté d'une usine capable de retraiter environ 1 million de tonnes de déchets radioactifs par an.
Gloups.
Un million de tonnes par irradiation ?
C'est la production totale de déchets pendant un an. Chaque tir génère entre 50 et 200 kg de déchets irradiés, la cadence étant de 0.
1 à
1 Hz...
Gilgamesh a écrit:
Merci pour la correction. J'ai l'impression que ce sont les "2 milliards de degrés" qui séduisent, et puis le fait que ce ne soit ni militaire (fusion inertielle par tir laser) ni institutionnel (tokamak ITER).
Oui, c'est ça. Beaucoup de confusions entre température, puissance, et énergie. De plus, si on y regarde de près, ce système a de nombreux inconvénients, dont le moindre n'est pas cette monstrueuse production de déchets radioactifs (patatra pour ce mythe de la "fusion propre" !), et est de toute façon très en deça des performances des tokamaks.
Gilgamesh a écrit:
lambda0 a écrit:
Pour l'instant, Bussard a bien du mal à trouver des financements, et son système n'est connu que par quelques blogs.
Dommage.
Je suis assez mal à l'aise par rapport à ça. Est ce que Bussard exagère dans sa lettre ouverte (publiée sur fcs) ? Est ce que c'est l'aveuglement du solitaire ou de la pure lucidité ?
Difficile à dire. Je lui accorde le bénéfice du doute parce que ce n'est pas le premier charlatan venu, il a un CV impressionnant, travaille dans le domaine de la fusion depuis des décennies. Mais pour l'instant, on a quand même pas grand chose à se mettre sous la dent à part des articles de vulgarisation, ou traitant des applications. Aux dernières nouvelles, il serait en train de rédiger un article d'une centaine de pages décrivant de façon plus approfondie la théorie de son dispositif. J'attend ça avec impatience, si cet article est accessible.
De plus, il y a des centres de recherche qui travaillent sur différentes variantes des réacteurs IEC, donc a priori, c'est de la bonne physique.
Gilgamesh a écrit:
lambda0 a écrit:
Cependant, les gens de Los Alamos travaillent sur une autre variante très intéressante...
Tu en dis trop ou pas assez, là
. Sur le même principe de confinement électrostatique ?
a+
Oui, c'est aussi un réacteur IEC, d'une géométrie un peu différente.
http://www.lanl.gov/p/rh_pp_park.shtmlVoici une thèse sur le sujet
http://ssl.mit.edu/publications/theses/ ... Thomas.pdfPar contre, je ne jurerais pas que les applications visées sont vraiment pacifiques...
A+
Gilgamesh Moderator le 09 Décembre 2007 12:16
Salut,
Une nouvelle intéressante piochée sur Slashdot : une équipe de l'Université de Californie a trouvé que des bactéries pouvaient induire la formation de nanotubes.
Shewanella est une bactérie reductrice, capable de former ici un sulfure d'arsenic à partir d'As et de sulfate S2O3. Une fois le composé formé sous forme amorphe, après incubation, il se forme - selon un processus à éclaircir - des nanotube de 30 micron d'As-S. Il est envisagé que les polysacharides bactériennes jouent le rôle de "patron" pour la croissance des tubes.
L'intérêt manifesté par les chercheur c'est de produire un composé ayant des propriétés intéressantes en microélectronique qui n'est pas encore produit par des processus abiotiques.
Dans le cadre de la physique de l'Arche c'est l'inverse qui m'intéresse. Cette news indique qu'il n'est pas absurde d'imaginer qu'une cellule végétale modifiée puisse produire une parois incluant des nanotubes de carbone et d'augmenter de cette manière la résistance à la traction.
Published online before print December 7, 2007
Abstract:
Biogenic formation of photoactive arsenic-sulfide nanotubes by Shewanella sp. strain HN-41Microorganisms facilitate the formation of a wide range of minerals that have unique physical
and chemical properties as well as morphologies that are not produced by abiotic processes. Here, we report the production of an extensive extracellular network of filamentous, arsenic-sulfide (As-S) nanotubes (20–100 nm in diameter by ~30 µm in length) by the dissimilatory metal-reducing bacterium
Shewanella sp. HN-41. The As-S nanotubes, formed via the reduction of As(V)
and S
2O
32-, were initially amorphous As
2S
3 but evolved with increasing incubation time toward polycrystalline phases of the chalcogenide minerals realgar (AsS)
and duranusite (As
4S). Upon maturation, the As-S nanotubes behaved as metals
and semiconductors in terms of their electrical
and photoconductive properties, respectively. The As-S nanotubes produced by Shewanella may provide useful materials for novel nano-
and opto-electronic devices.
Gilgamesh Moderator le 22 Décembre 2007 13:41
Je remet ici l'article sur le concept de Ice Rocket de Vos Post
Hydrogen Ice Spacecraft for Robotic Interstellar Flight
by Jonathan Vos Post, F.B.I.S. 1
(...)
2.0 DESIGN OF HYDROGEN ICE SPACECRAFT
(...)
The ideal spacecraft can be lightweight, inexpensive, and fuel efficient by using balls of hydrogen ice as both structure and fuel. Hydrogen may be "exotic" in its structural function, because it has the tensile strength of butter, but it accounts for over 75% of all matter in the universe and costs under $10 per pound. The material can be stiffened with the admixture of carbon or boron fibers, or various particulates.
Balls of modified hydrogen ice first serve as structure, then are detached, liquified or turned into slush, and channeled into a fusion reactor as fuel. In this way, almost all non-essential parts of the spacecraft are consumed during the mission. (...)
This type of "autophage" (self-consuming) spacecraft achieves an extremely low dead-weight fraction, which is a critical parameter for optimizing the performance of interstellar spacecraft. (...)
To reduce the volatility of hydrogen ice, a particular self-refrigerating structure was invented by James Stephens, analyzed quantitatively by James Salvail at the University of Hawaii
Concentric spheres of very thin metal (i.e. lithium) or metallized mylar coating thicker concentric spherical shells of hydrogen ice are connected to each other by at least two rods made of a material that has very low thermal conductivity. This is necessary so that the spheres above the instantaneous level of the subliming ice surface do not move relative to each other. The outer shells are highly reflective, thick enough to provide reasonable structural integrity. The inner spheres are made of the same materials, but much thinner (is much less than 0.1 cm), as they are merely radiation shields.
The radiation shields and outer hulls must contain enough sufficiently sized holes or pores so that sublimed hydrogen molecules are quickly lost into space. The evacuated spaces between the slowly receding ice surface and the outer hulls thus have negligible gaseous heat conduction because the gas is very rarified. Gas flux is small enough that heat convection is also negligible.
Under these conditions, the escaping sublimed hydrogen expands and refrigerates the remaining concentric spheres, maintaining a temperature well below the 20o K melting point of hydrogen; the nominal system temperature is 5o K.
The system as a whole as conceived by James B, Stephens includes: (1) ice embedded insulation, (2) vapor cooled insulation, (3) isomer conversion catalyst integral with insulation, (4) Infrared photon reflective and vapor conductive insulation, (5) vapor cast crystalline hydrogen ice using nuclear magnetic resonance heating of non-crystalline ice, (6) self-forming filamentary insulation from dispersed particles in the ice that cohere due to ice cleaning.
The attributes of the system include: (1) unitized design -- hydrogen ice is the cryogen, propellant, shielding, absorber, power source, window, and insulation support during launch; (2) superconducting temperature cryostat (less than 5o K for hydrogen); (3) self-insulating solid cryogen; (4) long lifetime in Earth orbit; (5) low cost material (less than $10/pound); (6) low cost fabrication (casting process); (7) low launch cost (withstand high acceleration forces); (8) low cost operation (efficient superconducting solid state system); (9) acoustically quiet (no moving parts); (10) thermally stable (large thermal capacity well insulated); (11) high density ice vapor cast and used at same temperature avoiding shrink stresses in insulation and components embedded in ice.
Stephens also emphasized neutron absorbing properties of hydrogen ice, microwave reflection or absorption, laser-tough shielding, neutral and charged particle beam tough shielding, radar stealth, and a wide range of capabilities for embedded avionics, including: phased-array radar, solar-powered ion rocket and superconducting magnet power generator/storage, and superconducting guidance and control.
As the concept was extended by this author, individual hydrogen ice spheres can be orbited by small boosters, and later assembled into a large spacecraft. Solid hydrogen is inherently safer than liquid hydrogen. The spheres can have embedded avionics, providing distributed redundant capability for the spacecraft at superconducting temperatures. Once assembled, the low accelerations typical of ion, fission, or fusion propulsion would not endanger the inherently low compressive and tensile strength of hydrogen ice as a structural material. An acceleration of 0.0485 gravities is used in Section 3.1. The hydrogen ice spheres would be between the payload (or crew) and the nuclear propulsion, providing neutron-absorbant shielding at no extra cost.
(...)
Hydrogen ice by itself is imperfect as a structural element; various methods of stiffening by the admixture of carbon or boron fibers have been explored, as well as admixtures of particulates such as montmorillonite clay.
A survey of cryogenic ices and slushes has been presented in an earlier article by this author.4 For this paper it suffices to note that hydrogen ice has a density of 70.6 g/l at -262o C, melts at 20o K to become a liquid with density 70.8 g/l at -253o C, and that slush is intermediate in density but has various advantages over both solid and liquid.
(...)
2.2 Lithium or Boron in Hydrogen: Icy Isotopes
In one sense, ordinary hydrogen (protium) is the ideal structure/fuel, as it is extremely cheap and has the lightest molecular weight of any material exhaust. But the fusion reaction attainable 20 with ordinary hydrogen fuses two protons to produce a deuteron (deuterium nucleus), a positron (anti-electron), and a neutrino, at an energy of 0.42 Mev (million electron volts). This yields 2.0 x 1013 Joules per kilogram of fuel.
p + p -> D + e+ + v But this is irrelevant, since the reaction involved is not true nuclear fusion. As revealed by the emission of the neutrino, this is a "weak force" reaction, rather than a "strong force" reaction. Too much of the energy is carried away by the neutrino. The reaction is too difficult to initiate. The total energy yield is (relatively) low. And for little more effort, with more sophisticated fuel, we can get better results.
If our hydrogen ice is made of equal proportions of protium and deuterium, we can fuse the two to produce Helium-3 and a gamma ray, with 5.49 Mev energy, corresponding to 1.75 x 1014 Joules per kilogram of fuel.
p + D -> He3 + gamma But this is not a good idea either. The gamma rays would be emitted in all directions, and tend to fry the payload. We might as well eliminate protium completely, and use either pure deuterium ice or a deuterium/tritium mixture.
Pure deuterium ice would result in two different reactions, yielding a combination of Helium-3, tritium, protons, and neutrons.
D + D -> He3 + n 3.27 Mev (7.8 x 1013 J/kg) D + D -> H3 + p 4.03 Mev (9.65 x 1013 J/kg) Deuterium is easily obtainable in massive quantities, since it makes up roughly 1 part in 6000 of the hydrogen in water here on Earth. D2O, heavy water, costs from $0.06 to $1/gram depending upon quantity and purity. The deuterium-deuterium fusion reaction is moderately easy to initiate, requiring a temperature in the 10 million degree range. But the neutrons in the output are nasty. Since they are uncharged, they tend to fly in all directions, uncontrollable by electric or magnetic fields, frying and/or rendering the payload radioactive. Nonetheless, this is the reaction and fuel used by default throughout the remainder of this paper.
An energetic deuterium-tritium reaction seems at first to have certain advantages. This is the most studied reaction today, because of the low ignition temperature of roughly 10 MK.
D + H3 -> He4 + n 17.6 Mev (3.37 x 1014 J/kg)
This is actually the easiest fusion reaction to ignite, and may thus be the first used for terrestrial fusion power. But tritium is quite radioactive, decaying in about a decade, and that neutron is still trouble.
There are several interesting reactions involving Helium-3 in the fuel, but we disregard them here for two reasons. First, it's hard to obtain, although it might be extracted from the upper centimeter of lunar regolith where it has accumulated from solar wind. Second, the self-cooling approach described in my articles for hydrogen doesn't work as well for helium isotopes, which have to be cooled to below the background temperature of the universe. Frozen helium is just too volatile.
This leaves us with several more exotic reactions. We consider Lithium. Lithium occurs in nature21 with an abundance ratio of 7.39% for the isotope Lithium-6 (Li6) to 92.61% for Lithium-7 (Li7). Lithium melts at 180o C, and boils at 1,326o C. If we built the spacecraft out of equal proportions of protium and pure Lithium-6 isotope, we have:
p + Li6 -> He4 + He3 3.90 Mev (5.53 x 1013 J/kg)
We would be using hydrogen ice with lithium foil in the self-refrigerating concentric structure, plus walls and girders of lithium. Lithium is a soft metal, but at cryogenic temperatures (and away from water) it is strong enough without brittleness to suffice for structural purposes. Unfortunately, this is a difficult reaction to ignite.
We get somewhat more bang for the buck if we use isotopically pure Lithium-7, for a reaction yielding an electromagnetically focusable stream of alpha particles.
p + Li7 -> He4 + He4 17.00 Mev (2.0 x 1014 J/kg)
Again, this is a hard reaction to ignite.
We can use deuterium ice and pure Lithium-6, again getting an all-alpha output:
D + Li6 -> He4 + He4 22.30 Mev (2.67 x 1014 J/kg)
Or even consider protium plus Boron-11 for the so-called Boron-fission reaction:
p + B11 -> He4 + He4 + He4 8.80 Mev (7.0 x 1013 J/kg)
But this is even less studied, and also extremely difficult to ignite, requiring perhaps 1,000 times the ignition temperature of Deuterium. Lithium or Boron fusion might be initiated by incoming protons from interstellar space when rammed into at over 0.02 c, which might be useful for upper stages of a staged interstellar spacecraft.28
If we use fibers of Boron-11 to stiffen deuterium or tritium ice, it might be okay to let those boron fibers go right into the rocket engine, vaporize, and partly engage in nuclear reactions. The unreacted boron would reduce the energy yield somewhat, and merely be expelled as part of the reaction mass.
There is a clever way to get the lithium mixed in with the hydrogen. Lithium is very soluble in anhydrous ammonia (NH3 with no water). The resulting solution is the lowest density liquid known at room temperature, with a density of only 0.511 g/l.22 Regular ammonia, NH3, has a molecular weight of 17.03, a density of 0.7710 g/l, and melts at -77.7o C, while Trideutero ammonia, ammonia-d3, ND3, has a molecular weight of 20.05 and melts at -74o C.23 Lithium solutions in ammonia have metallic conductivities above 9 Mole percent metal. There is a eutectic at 22 Mole percent metal at 88o K., and at lower temperature is a stable solid compound, perhaps Li(NH3)4.
We can mix up batches of Lithium-6 or Lithium-7 in ordinary anhydrous ammonia, or Lithium-6 in fully deuterated anhydrous ammonia, freeze the stuff in the concentric perforated lithium-foil configuration, and build our spaceship out of that lithiated ammonia ice. This does leave us with a certain amount of useless nitrogen, which would contaminate the fusion reaction, unless separated out and expelled as unreacted exhaust mass. But lithiated anhydrous ammonia might be worth investigating as an exotic chemical fuel for liquid oxygen combustion.
Where does this leave us? We don't have a clear idea of a spacecraft fusion reactor that burns lithium or boron.31 So we may have to bite the bullet on the neutron radiation problem and build our spacecraft out of deuterium or mixed deuterium-tritium ice. The rest of this paper makes that assumption. Nordley points out "that as soon as the main reaction happens, the products become available for side reactions. While the output of particles from these side reactions may be several orders of magnitude below the output of the main reaction, and thus not worthy of interest regarding the kinematics, they will still be very significant (especially the neutrons) to electronics and biological components at the power levels needed for interstellar flight."
We note that Lithium can trap neutrons, heating up, and transferring that heat to melt or slushify hydrogen ice. Future considerations include analysis of the limits of neutron-hardened payloads through redundancy, self-repair, or even nanotechnology.
Gilgamesh Moderator le 22 Décembre 2007 14:20
Architecture du palier (transmission de la poussée depuis la corole à l'habitacle tournant, au travers du mât central).
Premier niveau : palier magnétique
La force magnétique F s'exerçant entre deux surface est :
LaTeX :
F\ =\ \frac{B^2S}{2\mu_0}
avec :
B le champs magnétique (en tesla)
S la surface de l'entrefer
μ
0 la perméabilité magnétique
kg·m·A
-2·s
-2LaTeX :
\mu_0\ =\ 4\pi 10^{-7}
Un aimant permanent produit un champs B d'environ
1 tesla. Soit une pression résultante (P=F/S) de 4 bars (4.10
5 Pa). Il faut supporter une force d'environ 10
11 N (~ 10 mégatonnes de poussée).
Soit une surface de S = F/P de 2,5.10
5 m², ce qui représente un disque de 280 m de rayon. C'est quand même très grand, sans être totalement stupide comme ordre de grandeur.
On peut réduire la surface en augmentant l'intensité du champs, mais alors il faut utiliser un supraconducteur, et le système n'est pas aussi passif qu'avec un aimant permanent. Pour B=18 T (un champs vraiment costaud mais qui reste dans les ordres de grandeurs d'un champs produit par ce type de dispositif) la surface du disque se ramène à 12-13 m.
La palier est une pièce maitresse et on doit donc le rendre très redondant sur le plan de la sécurité.
Deuxième niveau : redondance
Les paliers supraconducteur magnétique sont disposé 12 segments disposés en étoile, par unité de 3 éléments, soit quatre jeu complets travaillant par roulement. Un premier travaille. Le second est opérationnel immédiatement au cas d'une interruption brutale du premier ou travaille de concert avec lui ce qui revient au même. Un troisième peut être examiné et démonté pour maintenance. Un quatrième est inactif mais en état opérationnel pour se substituer en cas de défaillance des deux éléments actifs.
Troisième niveau : palier à aimant permanent
En cas de ruine total du palier central ou d'interruption brutale et imprévue de l'alimentation électrique ou du refroidissement, un second niveau palier, de surface beaucoup plus vaste, formée d'aimants permanents et placé en amont de la poussée prend passivement le relais. Un aimant permanent possède toute de même une faiblesse. Si la température, suite par exemple à un frottement des surfaces grimpe au delà de la température de Curie (700°C pour du fer) le champs magnétique s'annule.
D'où un :
Quatrième niveau : palier mécanique
formé de deux surfaces de type "Teflon" séparées par de la glace et circonscrit par un joint résistant à la pression et qui ne travaille pas en temps normal
En fonctionnement nominal les palier de sécurité 2 et 3 ne travaillent pas. La poussée est transmise directement par un système qui romps mécaniquement dès que le cisaillement dépasse un certain seuil.
Le
cinquième niveau est l'arrêt moteur ; tout ce qui précède doit permettre d'interrompre la poussée en disposant d'un peu de temps en cas de rupture brutale de premier niveau.
La géométrie des paliers est sphérique afin d'éviter le déboitement des éléments.
a+
Gilgamesh Moderator le 23 Février 2008 20:47
Je republie ici en LaTeX les formules de calcul des paramètres du trajet de la fusée relativiste parues dans la Usenet
Physics FAQ.
Dans l'hypothèse de la fusée relativiste, l'accélération
a est constante tout le long du trajet.
La vitesse atteinte finale est
v.
La longueur du trajet est
d. Sa durée est
t dans le référentiel au repos et
τ (tau) dans le référentiel du voyageur.
M/m est le ratio carburant/masse utile dans l'hypothèse optimale d'un propulsif entièrement converti en photons parfaitement colimatés derrière l'engin. La masse du système propulsif (moteur, réservoir d'antimatière...) est négligée.
Mode de calcul des fonctions hyperboliques utilisées.
Sinus hyperbolique :
Cosinus hyperbolique :
Tangente hyperbolique :
Paramètres du trajet relativiste Rappel : a=cte
Durée du trajet :
(dans le référentiel au repos)
Distance parcourue :
(dans le référentiel au repos)
Vitesse atteinte :
(dans le référentiel au repos)
Durée du trajet (temps propre du voyageur) :
Facteur de Lorentz :
Ratio carburant (M) / masse utile (m):
Rappel : conversion Mc² à rendement
1Formules dans l'article original:
The Relativistic RocketUpdated by Don Koks 2006.
Fuel numbers added by Don Koks 2004.
Updated by Phil Gibbs 1998.
Thanks to Bill Woods for correcting the fuel equation.
Original by Philip Gibbs 1996.
Code:
t = (c/a) sh(aT/c) = sqrt[(d/c)² + 2d/a]
d = (c²/a) [ch(aT/c) - 1] = (c²/a) (sqrt[1 + (at/c)²] - 1)
v = c th(aT/c) = at / sqrt[1 + (at/c)²]
T = (c/a) sh-1(at/c) = (c/a) ch-1 [ad/c² + 1]
gamma = ch(aT/c) = sqrt[1 + (at/c)²] = ad/c² + 1
M/m = gamma(1 + v/c) - 1
= cosh(aT/c)[ 1 + tanh(aT/c) ] - 1
= exp(aT/c) - 1
Code LaTeX :
Code:
sinh(x)\ =\ \frac{e^x\ -\ e^{-x}}{2}
cosh(x)\ =\ \frac{e^x\ +\ e^{-x}}{2}
tanh(x)\ =\ \frac{sinh(x)}{cosh(x)}
t\ =\ \frac{c}{a}\ sinh\left(\frac{a\tau}{c}\right)\ =\ \sqrt{\left(\frac{d}{c}\right)^2\ +\ \frac{2d}{a}}
d\ =\ \frac{c^2}{a}\ \left[cosh\left(\frac{a\tau}{c}\right)\ -\ 1\right]\ =\ \frac{c^2}{a}\left[\sqrt{1\ +\ \left(\frac{at}{c}\right)^2}\ -\ 1\right]
v\ =\ c\ tanh\left(\frac{a\tau}{c}\right)\ =\ \frac{at}{\sqrt{1\ +\ \left(\frac{at}{c}\right)^2}}
\tau\ =\ \frac{c}{a}\ sinh^{-1}\left(\frac{at}{c}\right)\ =\ \frac{c}{a}\ cosh^{-1}\left(\frac{ad}{c^2}\ +\ 1\right)
\gamma\ =\ cosh\left(\frac{a\tau}{c}\right)\ =\ \sqrt{1\ +\ \left(\frac{at}{c}\right)^2}\ =\ \frac{ad}{c^2}\ +\ 1
\frac{M}{m}\ =\ \gamma\left(1\ +\ \frac{v}{c}\right)\ -\ 1\\
\frac{M}{m}\ =\ e^{\frac{a\tau}{c}}\ -\ 1
article Wiki Relativistic rocket
http://en.wikipedia.org/wiki/Relativistic_rocket