Enthalpy

Electricity replacing hydraulics: this is very tempting, because: - Hydraulics needs maintenance but still fails. Electricity hopefully improves that; to be seen. - Electric actuators may become lighter than hydraulics. For slow movements, and after an industrial effort. - With big batteries, the aeroplane can shut down its power generator on the ground, which more and more airports demand. Though, this switch is less than easy, and aircraft producers and operators are conservative, by nature and for some good reasons. The Dreamliner is the first to push electric actuators so much, more so than the contemporary A-380 for instance, which could change the chemistry of its smaller batteries. The Dreamliner keeps some hydraulic actuators, but I suppose electric ones will replace them all on future aeroplanes. The aeroplane industry has pushed for quick electric machines for two decades, especially with a steel sleeve to retain permanent magnets; my suggestion with graphite fibers or with wound steel band should improve http://www.scienceforums.net/topic/73798-quick-electric-machines/

At 25µm wavelength, CCD detectors would be unaffordable. Looks like gratings are out.

One plant in China uses residual heat to distribute hot water to homes. Which is not something I'd like to see everywhere, due to the obvious risks made worse by the connecting pipes. Many people are tempted by the residual heat of fission waste after the elements are separated, especially strontium 90. http://en.wikipedia.org/wiki/Strontium-90 Some radioisotopic thermal generators (RTG) were built in the Soviet Union using 90Sr. http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator Though, it emits gamma rays, by bremsstrahlung and by the beta emission of the daughter 90Y, as opposed to the painstakingly produced 238Pu which is a pure alpha emitter. http://en.wikipedia.org/wiki/Plutonium-238 These 90Sr RTG were used to power remote lighthouses and they created accidents, for instance two Siberian hunters who slept near one for warmth and died few days later. One Lunar exploration programme by South Korea plans to use a 90Sr-powered generator, and frankly, I would not use an RTG on the Moon. Risks before and during launch are just too big; Solar panels work well on the Moon. There are supposedly reasons like operation during the long night, but I'd choose differently. Probes have used Sunlight at the asteroid belt recently; our Moon is 6 times easier.

It's the idea behind it. But how difficult is signal processing for it? The source is essentially random and diffuse, and all the background reflects noise, in addition to the target that tries to be acoustically banal. Discrimination by the echoes' direction only will supposedly show nothing. I suppose that in this mode, the passive sonar discriminates the target from the background through the distance also, but this demands to correlate the echoes with the source noise, which spreads over all distances. Less than obvious! In addition, submarines use to conceal themselves near an irregular ground.

Maybe this question is decideable, thanks to functional NMR imaging. When a human sees a shape, f-NMR shows the activity at his occipital cortex, and more surprising, this activity reproduces the pattern observed by the subject. Now, you imagine the difficulty of the protocol... It would need an animal with eyes and echolocation, put it in an NMR machine and convince the animal this is normal life... Then show it an optical image only (light on a screen), observe its brains activity. Give it a sound pattern only, observe its brains activity. Compare both. Not obvious, is it? And as echolocation is naturally 3D, sight is less rich. ----- Sonars for submarines, surface boats, helicopters do get the data for a complete 3D image from a single ping. You might use colour to display the distance if you wish. It's just that... They don't ping any more! Except at the very last minute, to fire a torpedo or grenade precisely at the target. Because right after the ping's echo comes the submarine's own torpedo - at some 500m/s. So sonars use to work passively now: they only listen. Can an image be built just from listening? I've read that. With much signal processing, echoes from natural noise sources, especially waves at the surface, are said to give a true image of all items in the sea: the ground, the submarines, the big animals. Is it true? No idea. ----- Maybe well-seeing humans link sight and touch in some part of the brains. For sure, toddlers learn to correlate both senses - you know, at the age of grasping the flowers from the wallpaper. We can make a mental image of a shape, or of our environment, from touching much like from seeing. This is something that well-seeing humans learn little with sound. That's one reason why I suggest to build a laser ranging blind's stick with a moving actuator instead of an audio signal: http://www.scienceforums.net/topic/74606-blinds-stick-with-laser-tactile/

The scenario I propose for Jupiter brakes there using the Solar thermal rocket. Same for the asteroids and the Jovian Troyans, which are return missions. I didn't check for Uranus and suppose it's impossible at Neptune. I know from previous scenarios that Mars is an easy target for the Solar thermal rocket. Work is in progress for Saturn, I'll try a bang-and-whistles mission like at Jupiter, if possible.

No photon emitted by some radioactive decays for instance. Electron neutrinos appear each time a protons and neutrons transform in an other. None at alpha emission for instance. Lithium blankets are necessary (but not enough!) to recreate the tritium used by the reactor and unavailable on Earth. For other purposes, and material would fit. Breeding tritium is a huge weak point of tokamaks. Maybe this will be possible, maybe not (adios tokamaks then), and I claim this step would be as polluting as a uranium reactor (hence abandon fusion, I say).

10-30s is no known time for electrical and thermal conductivity. But 1fs is encountered. The wavefunctions accessible to electrons in crystals are computed by summing individual atomic wavefunctions... - With a constant phase hop between adjacent atoms, corresponding to k in the "metallic" electron's propagation, not just with +- signs for each atom - Textbooks do it with s functions and linear or cubic crystals. Serious work take the real atoms' wavefunctions (several ones, not just the lowest available) and the real atoms' positions, and they do obtain band diagrams that match measurements more or less. - The true wavefunction is necessary to explai why diamon, silicon and germanium have very different band structures. - Aluminium, as a metal, has no simple conduction band. It has many bands that overlap hence are partly filled. Electrons, or available states, exist forever and can be occupied in theory for ever. Deep states are indeed occupied for very long. Relaxation times tell how long an electron near the Fermi level stays in a state before something (for instance a phonon) sends it to an other state. people avoid this subject and introduce a time ex nothing. There is more. It seems that electrons interact very often with an other, but because this interaction conserves the momentum sum and the current sum is proportional to the momentum sum as soon as the electron mass doesn't vary (which would happenin a solid far before relativistic speed), models neglect completely the interactions between electrons and care only about the crystal. One exception are "hot electrons" whose mass is uncommon. With the band structure, you get only state densities and electron masses. To evaluate electric and thermal conductivities, you also need the mean free path or the time of free flight for electrons, which depends on temperature, alloying elements, metallurgical state. Reasonable Some people claim to model resistivity against temperature... but superconductivity isn't well explained, so resistivity model can't be complete. Why choose aluminium? It's a complicated case. Silver is easier.

From distance, ET could pick its radiowave emissions. Human technology can but receive them from Earth with huge antennas and because the transmitting antenna is well directed, but others may have better receivers. It depends from what distance. Far probes are half a light-day from Earth, but 5 light-years from the next star. Square that to estimate the received power.

No probe has orbited Uranus; only Voyager 2 passed by in 1986, and knowledge improved slowly since then. http://en.wikipedia.org/wiki/Uranus http://en.wikipedia.org/wiki/Exploration_of_Uranus http://en.wikipedia.org/wiki/Voyager_2 We won't wait one and a half decade for a Hohmann transfer. The Solar thermal rocket shall send the probe there in 6 years, which means 12858m/s relative to Earth and 12355m/s relative to Uranus, without the assistance of other planets. [xls file joined] An Atlas V v551 (or an Ariane V Me - something with a wide fairing) shall put 5589kg to 4300m/s above Earth's gravity. Four D=4.57m Solar engines add 8558m/s to the remaining 2806kg as they eject 2783kg hydrogen in 42 days. The Solar thermal stage is thrown away before braking: 155kg tank, 300kg truss, 120kg engines, leaving 2231kg. A chemical rocket brakes by Oberth effect at the final orbit's periapsis: Small radius 30Mm and 19173m/s Big radius 600Mm and 959m/s Period 5.34 days Orientation as possible The escape speed is 19646m/s at 30Mm, so the probe arrives there with 23208m/s and must lose 4035m/s. The rocket burns 1282kg of 700:100 O2:H2 at 25 bar expanded to 84Pa in four D=0.8m nozzles to achieve 4699m/s=479s isp. Electric pumps take 14kW during 15min from a 28kg Li-polymer battery. http://www.scienceforums.net/topic/73571-rocket-engine-with-electric-pumps/ The pumped engines weigh 40kg with driver, the tanks 40kg, their supporting truss 100kg, leaving 769kg for the probe's frame, instruments, equipment including the battery. Few orbits are possible; this one looks useful. Landmarks: Uranus' radius is 25Mm. Are radiations known at 30Mm? Voyager 2 passed at 81.5Mm and radiations were easy there. The main rings span from 38Mm to 98Mm. They are equatorial, that is >90° to the ecliptic. http://en.wikipedia.org/wiki/Rings_of_Uranus The major moons are equatorial and span from 129Mm to 583Mm http://en.wikipedia.org/wiki/Moons_of_Uranus The closest known moon is at 50Mm. Of 27 known moons, 7 are pictured. Marc Schaefer, aka Enthalpy ======================================================================== Neptune resembles Uranus: similar mass and diameter, no orbiter ever, Voyager 2 passed by in 1989. http://en.wikipedia.org/wiki/Neptune http://en.wikipedia.org/wiki/Neptune#Exploration http://en.wikipedia.org/wiki/Voyager_2 Sending an orbiter there is about the same, just farther. An 8 year travel needs 14380m/s versus Earth and 15771m/s versus Neptune. The Solar thermal rocket starts again with 5589kg at 4300m/s above Earth's gravity, ejects 3106kg hydrogen in 47 days through four D=4.57m Solar engines to add 10080m/s, ending at 2483kg. 590kg of Solar thermal propulsion are thrown away. The probe arrives with 1893kg at Neptune to brake chemically by Oberth effect to the final orbit: Periapsis 28Mm, apoapsis 770Mm, period 7 days, orientation as possible. Cf equator 24.8Mmn, Voyager 29.2Mm, rings 41-64Mm, Triton 355Mm, known moons 48Mm-far At 28Mm, escape speed is 22091m/s, elliptic orbit 21700m/s Rings http://en.wikipedia.org/wiki/Rings_of_Neptune Moons http://en.wikipedia.org/wiki/Moons_of_Neptune Magnetosphere http://en.wikipedia.org/wiki/Neptune#Magnetosphere The probe arriving at 15771m/s dives to 27143m/s at 28Mm where it brakes by 5443m/s in 15min. This consumes 1299kg of O2 and H2 in the same 479s engine as at Uranus, leaving 594kg in orbit. 170kg for propulsion allow 386kg for frame, equipment, instruments. =============== One fascinating option is to send sistership probes to Uranus and Neptune. Share the design, the production, the spare, the operation team, the science team - and launch both at the same epoch. Then, as teams are occupied by Uranus after 6 years travel, a 9.8 year travel to Neptune is more acceptable. This takes 13230m/s from Earth, just 3% mass difference with the acceleration toward Uranus, and arriving with 13201m/s at Neptune takes a 4035m/s insertion kick at the described orbit - same kick as at Uranus, same 769kg available. Marc Schaefer, aka Enthalpy

The high specific impulse permits a Jupiter mission that orbits several moons successively and observes in between the planet from varied distances. http://en.wikipedia.org/wiki/Jupiter http://en.wikipedia.org/wiki/Moons_of_Jupiter The Galileo probe had 200kB of Ram and a magnetic tape, so a new design could carry improved instruments. http://en.wikipedia.org/wiki/Galileo_spacecraft Atlas V 551 (or Ariane V Me) can put 8900kg on a transfer orbit 1804m/s below geosynchronous, so it shall put 5459kg at 4500m/s above Earth's gravity. The Solar thermal rocket makes the rest, beginning with a 2 years 9 months Hohmann transfer (my mistake at the Trojans). 1596kg hydrogen add 4294m/s to Atlas' 4500m/s and Earth's 29785m/s, leaving 3863kg heading to Jupiter. 1411kg hydrogen accelerate by 5643m/s and leave 2452kg just above Jupiter's gravity well. Twelve D=4.57m engines take 175 days to brake at quasi 5.2AU, lengthening by almost 3 months; their collective consumption there is 93mg/s = 8.04kg/d. Similar concentrators can provide each ~500W electricity as I describe there: http://saposjoint.net/Forum/viewtopic.php?f=66&t=2051&start=20#p23867 They resemble the high-gain antenna as well. The craft falls to 11.5Gm distance to Jupiter and acquires 4705m/s. 321kg hydrogen brake by 1378m/s in 40 days, leaving 2131kg on the orbit (3327m/s) of one moon of the Himalia group that has four or five members. http://en.wikipedia.org/wiki/Himalia_group The orbit is tilted by 27.5° so the wide launch window for this goal opens every six years. A part of the upper Hohmann kick would better combine with the capture to benefit from some Oberth effect. I suppose the craft must arrive below the Himalia orbit and brake over half an ellipse or nearly 125 days while rising to the Himalia orbit. Someone else shall develop the theory or guidelines for crafts with weak accelerations. The other small moons have unrelated orbit inclination; maybe some is accessible. The probe sinks from the Amalia orbit (3327m/s tilted 27.5°) to a circular untilted 6890m/s orbit, of radius 4.29 times smaller, by a Hohmann transfer. 321kg hydrogen give in 40 days the 2035m/s upper kick that de-tilts (1582m/s) and brakes (1281m/s), leaving 1809kg on the 61d elliptic transfer. 255kg hydrogen in few kicks totalling 32d brake 1884m/s, leaving 1554kg on the circular 6890m/s orbit. There, ten engines are thrown away (250kg), a 3905kg hydrogen tank (200kg), the truss that holds it (200kg) - though most of the truss could have been thrown right after the chemical propulsion. The probe continues at 904kg, braking slowly over many orbits as it consumes 15.5mg/s=1.34kg/d. 91kg hydrogen let dive by 1310m/s in 68 days and leave 813kg at 8200m/s around Jupiter, that is at Callisto. 158kg hydrogen let dive by 2683m/s in 118 days and leave 655kg at 10883m/s around Jupiter, that is at Ganymede. 135kg hydrogen let dive by 2862m/s in 101 days and leave 520kg at 13745m/s around Jupiter, that is at Europa. Here the probe might split a lander, throw away an engine or a tank... The following doesn't do it. 130kg hydrogen let dive by 3581m/s in 97 days and leave 390kg at 17326m/s around Jupiter, that is at Io. The engines (50kg), the 514kg hydrogen tank (55kg), a truss around it (45kg) leave 240kg for the probe's frame, equipment, instruments. Marc Schaefer, aka Enthalpy

I have nothing to remember. The question was "use Earth's magnetic field to hover". Nothing more. You answered "Not stable because of Earnshaw's theorem". You were wrong because you forgot Earth's gravity which does stabilize levitation in banal experiments.

Thanks for the corrected address!

Parallel connection depends on how sensitive the battery current reacts on the voltage. With 1.5V alkaline of nearly the same history, they may get a bit warm, you may lose some capacity, nothing spectacular. With Cd-Ni accumulators: boom, either at the cells or at the cables, and that's brutal. So in case of doubt, it's better not to try. A single cell is many locations in parallel, sure... but - They were produced at the same time from the same material - They have always been charged or discharged together so no imbalance has a chance to develop during use. As for the Dreamliner, the dangerous kind of lithium accumulator was the wrong choice, but now they're stuck because this bird relies so heavily on electricity to replace hydraulics, and safer chemistries weigh more. The ironic bit is that passengers aren't allowed to board with dangerous materials, whose list includes many lithium batteries. The response by the manufacturers has been to wrap the batteries in a fireproof envelope: less than perfect, but it's a response, and it should have been done from the beginning. Some design mistakes are just bad luck, this one was a bit coarse to my taste. One 787 has caught fire recently, after the corrective measure was recommended, yes... But I haven't read whether this particular plane had already received the modification, nor if the fire relates with the battery. Also keep in mind that competition is rude.

A clock launched straight up at 8.5km/s was atop a multistage rocket. There is little choice for that: a hydrogen gun might achive the speed under the best conditions, but the clock wouldn't survive, and that speed isn't sustainable through the atmosphere. 8.5km/s is nearly Earth's liberation speed (11km/s). Zero gravity would result from the extinction of the rocket and the lack of atmosphere above a few 10km. Hydrogen masers can be rugged, more easily than atomic clocks, and serve in (some, not all) GPS satellites for instance.

Why an avalanche photodiode? They use to serve for very faint light. Besides being less convenient to use, they would be flooded with light in a normal ambient. In case your design relies on signal processing to extract the echo from ambient light, rather than from heavy filtering on wavelength and direction, then a less sensitive detector would be better.

John Cuthber, you made the mistake. The question was "hover over Earth", that is in a gravitation field. You pretended to use Earnshaw's theorem, despite it does not apply because of the gravitation field.

Second order diffraction happens, but may be out of the range of the instrument, depending on the grating's spacing and the possible incoming wavelengths. With near infra-red and 2.5µm spacing, things look rather good. It limits the instrument's bandpass, though.

"Theorems" about technology and physics use to be grossly false, because we don't live in a mathematical theory. "Proving" something impossible uses to suppose that humans be silly, which they aren't. Often, a "proof" of impossibiilty ony proves the lack of imagination of its author. Long ago, I said "Reagan's directed energy weapons will never work because of fundamental physical limits" and meanwhile they work, so I refrain from such assertions now. Incidentally, Earnshaw's so-called "theorem" suppose no other action in the world. Just a gravity field (something difficult to get rid of on Earth) makes this "theorem" unapplicable, and indeed magnets float over superconductors, stably thanks to gravity. As well, an AC field provides stable positions, as in a Paul trap: http://en.wikipedia.org/wiki/Paul_trap Just for fun: my preferred "proof" of impossibility relates to captchas http://en.wikipedia.org/wiki/CAPTCHA which many people wanted to "demonstrate" are too difficult to hack. One ingenious swindler let his robots collect captcha images from the sites he wanted to hack automatically. He had his own website to display the collected captcha images, together with pictures of pretty women who took off their clothes when human visitors of the website solved the captchas. Then the swindler's robots could insert the solutions in the sites to hack them. Well done, isn't it?

Phosphorescence is not reversible. Many Leds are nearly 100% efficient, without phosphorescence, but the reverse operation, which is essentially a photovoltaic cell, is not efficient. A few reasons: - Sunlight is a broad spectrum, but photons below some energy are not converted, the others above are converted only to this energy. - The voltage at which the resulting current is available can't equate this energy. - And then you have the usual nasty technological reasons, but this time they aren't the worse ones.

Metals conduct electricity by their electrons, under normal conditions. So much that there is a fixed relationship between thermal and electric conductivity http://en.wikipedia.org/wiki/Wiedemann%E2%80%93Franz_law because electrons conduct and kT at the same time.

Don't expect a simple formula! Essentially, you have to compare the rope's heat capacity (take its linear mass and steel's capacity per kg) with the heat transfer to water, and this latter is complicated as it depends on a flow. You may try your luck with engineeringtoolbox, but beware heat flows depend essentially on speed any many conditions, like roughness. What about experimental determination? Or could the context give an intuitive obvious solution, that is "very quick" or "too slow"? If for instance the rope's radial conductivity limits the heat transfer, and not the water, then the question gets simpler again. Only figures would help that.

This depends fully on the detailed time behaviour of all components in the loop. What you describe - in this case with light, others use only electricity, or hydraulics... - is a feedback loop, which is commonly used in servomecanisms. For instance all control surfaces of an aeroplane, like flaps etc., are moved by actuators because the pilot's muscles wouldn't suffice, and their position follows the steering stick thanks to such a feedback, which compares the input by the pilot with the flaps' position measured by a sensor. Very complete theories exist to tell if the loop is stable, in which case the output follows the input, or unstable, when the output "flickers". Rather easy for electronics or hydraulics, they become more cumbersome if aerodynamics is involved. In the case you suggest, a laser diode "can" be controlled to emit an intermediate power of light (not easy), making stability possible - this depends on details of the loop. If the laser control is only on/off, then straight stability is impossible and the light will flicker; though, in some uses, you may be interested in the mean power of this flicker, with the proper loop design, this mean power can be controlled by the input. Such an operation is called sigma-delta in electronics (for add-subtract) and used for metrology and audio.

Snowbird has been built by AeroVelo, a team of students at the university of Toronto The previous (young strong light wo-) man-powered one relied on ground effect to hover little above the ground. Toronto's team has obviously improved a lot, since their flies higher, not using the ground effect (winning as a consequence the Sikorsky prize, which had been proposed so long ago), and since the pilot doesn't even look exhausted. Fantastic achievement!

Piezo drivers exist with varied range, depending on the need. Their active element can work by bending for instance, in which case it amplifies the movement as a bimetallic actuator does. Some piezo materials are transparent to the IR; quartz is to 3µm. The piezo could also be outside light's path, but then its index change won't act. Cd-roms have tracks spaced by 2.5µm so they won't need an order sorting filter; the tracks could even be too close. I certainly agree that FT has advantages, but we're speaking about do-it-yourself, which favours a grating. Maybe one process to make with a lathe a grating free of transmission losses: - Turn flat a piece of aluminium, preferably 6000 or 1000 series alloy. Get 0.3µm smoothness with little skill, much better is easy. - Leave the part in place, take a sharp 45° angled HSS tool, choose the desired feed according to the wavelength range. 10µm is a small but common value, less is available. Turn "flat" to make a groove. - Anodize the face black. - Grind away the anodization from the hills, leave it in the grooves. (Turning it away would need to anodize within the lathe; processes exist for big parts without a bath) The grating is circular, but this could be an advantage. It results in a lens whose focal length varies according to the wavelength. The lens has spherical aberration, but maybe a sector of it can approximate an ellipsoid if the source and the detector are offset in the proper direction, as in a Jolo telescope. This would focus light on a CCD line detector without materials transparent to IR.