Enthalpy

People do recover isoprene from used tyres. I'd guess isoprene is a molecule small enough to separate well from sulphur, this avoiding the real hurdle. Once you have isoprene, you can make pretty much anything of it, including new rubber, better replacements for Diesel oil, kerosene, rocket fuel (I contemplate Farnesane and Phytane here)... The real difficulty is that latex is a very cheap source of isoprene, and paper factories offer terpenoids nearly for free as by-products (turpentine, pinene, carene, pinane, myrcene...). Crude and refined oil are also dirt-cheap, though they're getting expensive for the uses we make of. So a material from tyre recycling would face very hard competitors.

If you know a coating that converts infrared to visible, please tell. Mankind would make good use of it. The mirror for infrared only is one topic suggested here and has been experimented by the industry; it works less well than hoped, for instance because the mirror is directional and needs the filament to stay right at the center of a spherical bulb, so a lot of engineering is needed before this physics produces a better bulb. Time is running out for filament bulbs because fluorecent bulbs are already here and LED improve quickly, so filament bulbs should improve on the short term to stay in the race. Heat from a light bulb is sometimes useful... But not in Summer when one spends 3W electricity to remove 1W heat from a room, and anyway, electric heating is expensive. Have power-saving computers for the same reason.

The elastomer I used (NBR?) in my trial did not slip, but a perfluoroelastomer might more easily - or a fatty environment or high speed might lead to slip. Two films wound around the viscoelastic element would prevent slipping shall this occur - with more turns than on the sketch below... Natural candidates are polyester or polyimide films, maybe a thin elastomer film containing fibres like a tyre does. The films don't need to wrap the whole width of the viscoelastic element nor cover the width in one single piece. Threads are an alternative to films; instead of spirals, they can be wound as helices. I give the idea, someone else shall find out how to assemble the thingy, especially with many viscoelastic elements... Split the shell in two parts, like a steam turbine? Hopefully brakes and dampers won't slip more than my prototype did. Corrugated running surfaces must help, high friction coefficients as well: aluminium bronze, nickel layer, ceramic layer... About assembling: when the elastomer is compressed, it gets also longer. In order to keep a nice toroid shape, it shall better be shorter initially than in place. Marc Schaefer, aka Enthalpy ============================================================================== Elastomer damping depends on temperature. Some materials are better, especially silicone rubber, which is used as a damping material on spacecraft for that reason. One may also combine several materials (in separate elements) whose highest damping lie at different temperatures, so that damping depends little on the temperature. Some uses demand a constant damping, like the landing shock absorber of an airliner, other uses less so. Whether a satellite antenna takes 2s or 30s to deploy isn't important. Marc Schaefer, aka Enthalpy

Sketch of the shock absorber: The slopes (which need not be uniform) squeeze the viscoelastic elements increasingly over the stroke, so the braking force doesn't have to drop as the speed does. More viscoelastic elements can also be brought in the narrow space as the piston travels. As it now looks, using many viscoelastic elements can make this dry shock absorber as small as, or maybe smaller than, a hydraulic one. Heat is a remote limit (silicone rubber: 1500 J/kg/K, 1400 kg/m3), and the braking force relates indirectly with the squeezing pressure and with the larger wall's area instead of the head area as in a hydraulic brake. The original intent was spacecraft, for instance to land on celestial bodies, but other use can be thought of - even to land airliners if this absorber gets more reliable or lighter than a hydraulic one. Merry Christmas! - ¡Feliz Navidad! - Frohe Weihnachten! - Joyeux Noël! - Feliz Natal! - Buon Natale! Marc Schaefer, aka Enthalpy

A shock absorber can be built a similar way, especially if the compression of the viscoelastic element(s) increases over the stroke to compensate the decreasing speed. The compression can be small in the waiting position, so the viscoelastic element creeps little - but beware the element must roll, not glide, soon at the stroke's beginning if not right from the beginning. Clean, free of maintenance... like the brake. Marc Schaefer, aka Enthalpy

Panels extruded the same way and welded can make payload fairings. Reasonably light: AA6005 with t1=t2=1mm would weigh some 3t for D=5m and h=10m. To my eyes, such a fairing is low-tech and looks easier to make than with composites. It should be stiffer. It won't dissipate any acoustic power as is, but its stiffness may provide a good sound barrier. Add foam? Marc Schaefer, aka Enthalpy

Dielectric coating equipment is presently inherited from semiconductor technology, making it expensive. I believe it would be cheap if a volume industry demanded it. Look: candy films are presently polyester, metallized on one face. Expensive if made in an apparatus for semiconductor processing, but they developed the same metal evaporation process for complete rolls of polyester foil, like W=1m and D=0.5m and t=50µm, or 4000m2. They introduce a new roll, a mandrel, pump to vacuum once, and pass the 4000m2 in front of the evaporator in a continuous operation. With some development, a similar machine could deposit multiple layers of dielectric in a single pass on a thin refratory metal film, which can be then glued on the mirror. I doubt it's any cheaper than a metal coating. Covering the whole visible spectrum at varied incidences must be possible, though less common than a narrow band.

Alas! Ceramics do evaporate. Among all metals and ceramics, the material with the smallest vapour pressure is tungsten, that's why it's used. What might perhaps be possible is use a quite lower temperature, combined with a material (just a coating perhaps) that emits little IR, and compensate the lower temperature with a bigger area. A little bird tells me that this straightforward idea is already considered and abandoned. A different direction would use higher temperatures, but in a gas and for a short duration. Pressure pulses propagating in a (noble) gas can produce a huge temperature that makes them very bright. These lamps are powered by explosives, but maybe a different actuator could let them work continuously. Though, the indirect process shall stay efficient from beginning to end...

As these question are straight application of the course, I'm not going to tell more neither. Just that for the rivet, shear strength isn't necessarily the limiting factor. Bending stress uses to be much worse, especially for a hard dowel.

Hi Anders! I want to take my time before trying any answer - sorry for the delay. Marc

Field concentration at the pointed electrode only. It's similar to two pointed electrodes at twice the distance, except that the positive and the negative electrodes have different roles. A negative very sharp electrode produces a local corona discharge that doesn't generalize over the full path, while a positive sharp makes most often a full spark. Or it's the opposite if I mixed it up...

I expected each virtual electron of the pair created from nothing to have a kinetic energy of -511 keV, so the annihilation after the creation by fluctuation releases no energy for photons. Unless an other source brings energy, typically a photon >1022 keV, and the pair has good reasons to separate, like the intense electric field near a heavy atomic nucleus, and then the new electrons with a positive kinetic energy can last. And if the photon has just little less than 1022 keV, the virtual pair lives a bit longer before recreating the photon, which can be deflected by a nucleus because the stronger field close to the nucleus eases the temporary creation of a slow pair which recreates the photon with a delay. Did I get this wrongly?

The V antenna is more directive than a dipole, hence the wave that leaves it is wider than a half wavelength. Consequently, synthetic beam formation alone can't achieve the narrowest focus; something like a lens, individual or collective, is necessary, possibly in addition to phasing the array.

I'd rather say that nerves exchange molecules, and electricity is a side effect because these molecules are charged or polarized. Humans produce tiny amounts of electricity, for instance at their muscles. This is what an electrocardiogram or EEG measures. Too little to propel you bicycle, but enough to be measured by sensitive electronics. Experiments under way would liek to control computers by thought, just with electrodes in a helmet.

The potential increases with distance, but more slowly than the distance, ie not linearly. This holds for all insulators. So a "breakdown field" holds for a (very) limited range only. It also decreases with the area. The breakdown field seems more related with the insulator's volume. The shape of the electrodes is paramount, as these concentrate the field in their vicinity, and breakdown anywhere on the path is likely to propagate over the whole distance - except if the field is much weaker farther away, in which case the discharge can be local, as in corona effect. A lower pressure, say in an airplane, lowers the potential a lot. Used in fluorescent lamps, which can be metre long. So 20kV over 1cm with spherical electrodes (the only reproducible ones) can easily convert in 10kV with sharper ones.

Said water or easily vaporized liquid could be in a closed circuit, eliminating the need to replenish it. Then the heat sink could be farther away from the heat source, hence bigger and more easily blown with fresh air. An especially interesting shape of closed circuit is a tube whose inner faces are corrugated, possibly sintered, so the liquid wets them well. This would enable vapour to flow at the middle of the tube and liquid to flow back at the wall, even in seemingly less favourable orientations. The tube being hermetically closed, the pressure could adapt to the operating temperature. We're on the verge of inventing the heat pipe.

A V antenna is wideband, and it can produce naturally the asymmetric pulse. The source injects a current whose longitudinal components in both arms of the V compensate another's effect, and transverse components add. As only a fraction of the current radiates, the V antenna is much a transmission line; its line impedance should better be kept uniform so the current is as well: the conductors on the sketch are thicker where wider apart - or they can be sheet of variable width, or several wires packed closely at near the source and spread at the far end. Inject a current pulse that raises in 70 ps for instance - easier than the previous 30 ps. It propagates in the conductors essentially at the medium's transverse electromagnetic speed, say 350 ps for some 100mm length, so the A field (magnetic vector potential) emitted near the source arrives at the on-axis target retarded by that additional delay, while near the tips, the field is emitted later but without the additional delay. The radiation cumulates over the length, with the net effect on-axis to radiate a 70 ps pulse as would the same current do over the width of the antenna, but over a broad band. Off-axis, the delays compensate badly so radiation is weaker. This is very similar to an ultra-relativistic particle that radiates when deflected. If the V is 2*10 mm wide for instance over 100 mm, the discrepancy is less than 2 ps. After some 350 ps for 100 mm length, the pulse arrived at the open conductor tips is reflected and the current stops, beginning at the tips and extending to the source over the 100 mm in 350 ps more. Because the reflected pulse travels away from the target, its effect created near the source arrives at the target 2*350 ps later than the effect created near the tips, or 700 ps, so the A field takes this time to decrease. The induced electric field, which is the variation speed or A, is made asymmetric here, again more easily than with the butt dipoles. Other antennas have a wide band (log-periodic, cigar) or rather wide (helical, Uda-Yagi...) but the V has naturally the uniform propagation time that takes care of a pulse's shape. ----- The electronics can preload the antenna and discharge it with the 70 ps current rise time in a switch transistor near the antenna, where the current lasts for 700 ps - or the pulse can be produced by other wideband means. A propagation line can also reach a more distant component, say one output of an integrated circuit, but not too far since this prolongs the conduction time and worsens the achievable repetition rate. A distributed amplifier is an option. An array of such antennas and sources brings the needed signal strength to the target. Arranging them on a section of a sphere remains good, and as the V antennas are much more independent, electronic steering and beam forming fits better here, especially if the integrated circuits can make the variable delays. Now the focus can follow the patient's movements. Or if one (or few) source has enough power, a single antenna can feed a concentrating reflector or lens. With individual or collective lens, the antenna V can be wider and still focus to a narrow target. Immersing the array in a water-like or brains-like material brings the advantages described for the butt dipoles. Electronic steering eases the compensation of diffraction at the skull to achieve full resolution in the very permittive medium: very few mm, even deep in the brains. Marc Schaefer, aka Enthalpy

The Wöhler curve depends essentially on each alloy and its condition, so a few formulas like on the cited website don't make sense. Especially an austenitic stainless steel behaves very differently from a classical tempered steel. Fatigue resistence also depends very much on surface hardness and smoothness of the part, so any attempt to relate it with the material's bulk properties would be futile. It depends also on the thermal history of the part, for instance welding. This is a very variable quantity, where quick estimates are unusable. That's why people measure it. To organize a test, you must know: - How many cycles are needed in the projected use, if you design a part; - Or how many cycles you're willing to test; - And have an idea of the corresponding stress. - Alternately, you can know the stress and measure the corresponding number of cycles, but to project the test, you must know approximately when the sample will break or how long it needs to resist. Depending on the desired number of cycles, you can use one apparatus or need an other one, so yes, you must check the material in advance, by comparison in case yours isn't well documented. One first split among stainless steel: austenitic / ferritic / martensitic, and less commonly duplex / martensitic with precipitation hardening / austenitic with precipitation hardening.

Transcranial Magnetic Stimulation isn't accurate enough for that purpose, if I understand properly its capabilities. The induced field spreads over 50mm * 40mm or more, which covers more than a specialized zone (vision, audition, decision...) of the brains, and certainly more than a precise function of such a zone (say, bending the second joint of the left forefinger, as functional Magnetic Resonance Imaging (MRI) shows such a degree of brain specialization). Thoughts must result from sequences in brain activity and not just the activation of one area, even if it could be targeted accurately. TMS seems to inhibit only the activity of the target zone when acting on it, and strengthen its work in the future. That's less than suggesting a precise idea. It's little more than the sought effects of an electroshock, with more adequate means that have fewer unwanted effects, and vaguely targeted. Up to now, TMS works only at contact distance, and this can't improve with an essentially magnetic field. My latest (=electro-magnetic) version here but improves that, as it needs an antenna as wide as distant to obtain a ~20mm target zone. Improving the bad field concentration has been a constant desire for TMS since TMS began to work 30 years ago, and this thread tends to that right now, in addition to making the apparatus more convenient.

With pulses like 70 ps long we leave the simpler near-field operation and magnetic coils. This is electromagnetic wave with antennas. A strong short pulse followed by a weaker and longer compensation needs a wide band antenna, nevertheless directive and powerful. Here's one possibility I suggest (click to enlarge), where the rectangles are conducting elements meant to radiate: The antenna comprises many dipoles side by side and butt, together with driving electronics. The shape should better approximate a section of a sphere; the dipoles can be driven with a slight time difference only, for fine steering, to compensate small tolerances, or to compensate diffraction at the target. The antenna has no reflector but can be backed by an absorber at some distance. The dipoles are charged before each pulse; here resides the emitted energy. Each dipole has locally its own fast transistors to discharge it, for instance two BFG425W. The switch could also be behind a line, and possibly shared among several dipoles, but bigger transistors are uncommon at that speed. Take dipoles 2* 20 mm long, wide to have 80 ohm wave impedance, charged at +-4 V. The transistors can reach the 50 mA in 30 ps; this constant current widens to 2* 20 mm within 70 ps, which defines the duration of the strong short plateau of induced electric field at the target. 10*12 dipoles at 200 mm distance would create in air the same field as present TMS apparatus with 3 GA/s. Permittivity at the brains and the cranial liquid reduces the field, and so do reflections at the scalp and the skull, but R2 times more dipoles, R times more distant, create a field R times stronger. When the current pulse reaches the ends of a dipole, fast diodes (for instance three BAT62 per dipole) allow the current to continue flowing, due to the antenna's inductance, this time between butt dipoles. The current decreases more slowly, first due to losses in the transistors and diodes, say 1.2 V versus 8 V accelerating voltage, and second due to the finite length cumulated by butt dipoles. This defines the weaker longer plateau of induced electric field, maybe 6 times weaker here than the strong plateau. Less asymmetric than before, but still better than present TMS apparatus. ----- The dipole width or diameter can adjust the wave impedance a bit to match the components' current capability. They can't be too close, or their interaction will limit the current. The examples given are not optimum, especially the old diode is capacitive and slow (which cancels out partially). Stronger voltages would be very useful but MOS seem unavailable at this speed. I haven't checked other FET. Driving the bipolars is difficult. I expect no charge gain per stage at 30ps, so the gain of the driving tree shall result from impedance transformation, but on a wide band and with floating voltages... Striplines similar to a gamma match, with ferrite for the bandwidth, and tapered to match the impedance? ----- 70 ps make a pulse 20 mm long in air, which relates to the best field concentration at the target. This speed is difficult for components, and isotonic water has only 49 mm penetration depth at 5 GHz and 22 mm at 10 GHz. Operation immersed in a water-like (or brain-like) material would improve a lot. It could use a liquid gel at the hair and a solid gel on the way from the antenna. 2 to 5 GHz limit the losses; the ~150 ps pulse is easier to produce even if stronger and is only 5.5 mm long in water, which improves the field concentration at the target and the size of the dipoles. Immersed operation also reduces reflections, as only the skull has strong interfaces, and reduces imprecision due to refraction. Subtle beam synthesis by electronic steering can compensate the aberrations due to the skull, but this difficult option is more futuristic. If the antenna doesn't touch the skull, the beam can follow the optically observed head's movements, by limited electronic steering, or by automatic control of the antenna's position. Marc Schaefer, aka Enthalpy

To decide on your apparatus, you need knowledge about the material to be tested. It's called a Wöhler curve and relates the number of cycles with the admissible stress. Deforming a solid, even elastically, take much energy (and force) which is impractical to produce, store and give back many times, so machines for such trials bend the probe and rotate it so the compressed and extended sides alternate. Other processes use to reduce the number of cycles. http://en.wikipedia.org/wiki/Fatigue_(material)#The_S-N_curve Books consider the frequency isn't important. Useful to conduct tests more quickly.

No one answers? My two cents then: Xylene isomers have very different melting points. Other properties differ less. Hope someone more knowledgeable will jump in!

At 100°C, aramide (Para or Meta), like in bullet-proof jackets. Maybe Liquid Crystal Polymers, I don't remember what temperature they accept. More constraints, like transparent?

Decent cryptographic coding wants to make brute-force attacks impossible whatever the unreasonable processing power available. On symmetric codes, 128b would be more than enough IF no other attack were possible, which is generally impossible to prove. Then you have asymmetric codes, or "public key codes", which demand much longer keys, oftens chosen as 2000 bits presently. Signature, or authentication, demands a bit more than 128 bits, like 160 or 192. Beyond key length, authentication is in serious trouble because all traditional codes (MD, SHA, maybe SHA-1...) have been broken BUT are still used. ----- And then you have all codes that have been voluntarily botched because governmental agencies obtained this to spy their citizens. Cell phones have a too short key. The code was ill-designed, had initially short 64 bits keys, every participant to the standard had realised the code was weak, but the French reduced the key length to 40 bits in addition. The French bank card has a too short public key, something like 320 bits. Consequently, it was broken by Serge Humpich as a demo. He was condemned, his software and knowledge stolen by the spooks... and the short key remains. All Bluetooth codes, including the more recent ones, are very weak. Crack software exists on the Web to break the older code in few seconds on a PC: it gives the clear text AND the key. You know Windows' Protected Storage Area? It stores your session password, your Outlook message password, among others - though now MS tells it's not for new design. On French Windows Nt4-2k-Xp, all users have the same one "secret" key to protect the PSA. Many years after the French law had evolved and easily allowed one key per user - W2k had received since the law 4 service pack, one SR, many patches, Xp had received 2 SP... - Microsoft issued the KB955417. But not as a security patch: as a functional improvement which isn't downloaded automatically... Not bad neither: in Linux secure servers (was it Apache?) which make over 90% of all "secure" https sites, a "bug" limited the number of different keys to 65536. Perfectly visible in the open source code, but it stayed there for several years. Every https link could be cracked by trying 216 keys, not 2128. So open-source doesn't mean "safe because everyone can check it" but rather "someone else should have checked it". The Italian government at some time accused Skype of being used by the mafias, so you can suppose Skype is botched meanwhile. Since the former French government, every Internet site is requested to deliver all your identities and passwords to the police on request. Trouble: the sites are supposed to ignore your password, which is encoded by your machine even on the first time and never passes in clear text over the line. The only possibility I see is that all sites in France use phishing versions of their pages to steal you password when requested. You may ask if this is the country's interest, as weak crypto is broken by foreign spooks and criminals as easily as by French ones. Or if you don't ask it, I do.

ESA as well considers this possibility, and frankly, I'm not enthusiastic about the report I've seen. I hope ESA has other better work than the report found on the Web. It needs some 5T in a big volume, at least 3 times the habitat diameter, which would be created by MgB2 superconductor. - The report states "no loss because superconductor", which is grossly false. People who wrote this just ignore the topic. Very bad start. - Consequently, it doesn't tell how helium is liquefied... Only heat leaks through insulation are considered, not ohmic losses in the type II superconductor. - Why not ask the LHC's designers to make the report? At least these people achieved something, and have seen a quenching. - Travellers are supposed to live within half a tesla! This prevents absolutely every so small amount of ferromagnetic material, which means no technology on board. Even metal parts would be hard to rotate, and currents would create torques... - What happens when the electromagnet quenches? This happens quite a few times on Earth with dramatic consequences - devastating boom. In space it's deadly. This risk alone looks bigger than cancer risk. My impressions: - Wrong people were asked to make this study. - The task was understood as an intellectual walk, not to make a usable design. - Maybe the authors didn't even notice the impossibilities. - Completely impossible until many huge hurdles are solved.