Enthalpy

This site tells that automatic air brakes are standard in Canada, and act when pressure drops: http://www.railfame.ca/sec_ind/technology/en_2002_AutomaticAirBrake.asp Wagons have also valves to isolate them from the brake line, and to purge their pressure reserve so they can be moved if isolated, for instance to sort wagons using a small hill.

Some spectrometers use a transparent wedge to make intereferences whose spacing depends on wavelength. Fringes are observed by a linear CCD. Software de-fourierizes the signal to tell what the light components were. Looks easier for DIY than the moving mirror, as only the wedge is difficult. A different approach would use a piezo transparent material instead of moving a mirror. But why not a grating? A CD sector could be enough. Long ago, amateur astronomers made their own gratings.

Not only does it smell badly, it's also a poison. If your chemistry knowledge doesn't suffice to produce it, I recommend not to do it at all. Same for hydrogen sulphide.

It's as usual a matter of how much. Earth's North pole does, as any magnet, repel a magnet with North under, and so would Earth's NS field repel a NS magnet, but... Take 1m3 of best magnets with 1MA/m. If Earth's field drops from 40µT to 20µT over 5000km altitude, it means 10J change or 2µN, which doesn't lift 5t. No hope with superconductors neither. The Casimir effect is attractive. "Repulsive" Casimir is little more than cheating with words. It can't be amplified neither, works only at really tiny distance, and we have very little action on it.

Hi! An attempted explanation for the tragedy of Lac-Megantic is that the only running locomotive was shut down by the fire brigade when extinguishing it, and as this locomotive stopped to provide pressurized air to the train, the wagon's brakes opened and the train ran away. Which I can't understand, because at least here in Europe, for over a century and exactly for the cited explanation, all brakes are pressed by a spring and need pressure to open. Could this be any different in Canada or the US? Thanks!

Accurate dating depends on the production rate on 14C in the atmosphere. Recent papers suggest this rate may have varied slightly in ancient times, but certainly not by many %. Anyway, dendrochronology, which is accurate to one year in many places on Earth, can recalibrate 14C dating. Future archaeologists won't have this chance, because nuclear tests have injected man-made 14C and made the method unusable to date items past mid-20th century.

The Pioneer and Voyager probes have seen interesting physics near 90 Astronomical Units. Grass isn't greener there, but it's something like the Termination shock, where our Sun doesn't fully define the medium any more. As well, we could better measure star distances by parallax, and check on the way the Pioneer anomaly and some general physics. Though, the probes were meant and equipped to investigate none of these, and took four decades - so shall we have a new probe there within ten years? This needs 42780m/s above Sun's gravity (or a bit less), meaning a start at 30252m/s over Earth's gravity and own 29785m/s. An Ariane 5 with the Esc-B (public data is incomplete) shall provide 4346m/s over Earth's gravity to 7345kg, oriented as 3199m/s forward and 2941m/s sunwards, so the perihelion is at 0.98 AU and gives 19+15 days to accelerate under 0.99 AU conditions of Sunlight and speed. A first stage ejects 4756kg hydrogen through fifteen D=4.4m engines to bring 12953m/s. The tank weighs 230kg, a truss 210kg, 11 engines 330kg, leaving 1820kg after separation. More stages would improve. The second stage ejects 1178kg through the four kept engines, to bring 12953m/s more. 95kg tank, 55kg truss, 120kg engines leave 372kg for the frame, equipment, instruments. Is a very low perihelion better? The heavier chambers reduced the payload in the case I checked. And if a mission needs a low circular Sun orbit, or a circular polar one, I feel a Solar sail better. Maybe astronomers would share the Square Kilometer Array, if it's for good science, and if the probe transmits as short bursts, or if a separate frequency wastes no observation time. Marc Schaefer, aka Enthalpy

Mercury needs 7533m/s then 9611m/s in a Hohmann transfer from Earth, that's why chemical rockets make detours by Venus and Earth and take 7 years with a big launcher. The Solar thermal rocket goes there in three months. Start as usual with 2793kg at 4881m/s above Earth's gravity: 537kg hydrogen provide the remaining 2652m/s at aphelion, leaving 2256kg. Two 4.6m concentrators take 15 days. 1216kg hydrogen give 9611m/s at perihelion, leaving 1040kg above Mercury's gravity. The two concentrators take 10 days. 112kg hydrogen used close to Mercury give 1245m/s to achieve if desired a low circular orbit (escape: 4250m/s). If pushing for 1/7 of the orbital period, where the kicks are about 88% efficient, this takes around one week and leaves 928kg. Mercury's orbit is eccentric (and tilted). The Hohmann transfer, from Jarret Mathwig's thesis, supposes circular orbits, so a slick mission planner would save performance. Some Oberth effect is also possible; trade isp for thrust there. Each concentrator may weigh 17kg and the stronger chamber 87kg, so both engines take 208kg. The 28m3 tank with thin steel, foam, multilayer insulation and polymer belts takes 130kg. This leaves 590kg on low Mercury orbit for frame, equipment and instruments. Orbit changes are possible. Marc Schaefer, aka Enthalpy

Samples from Phobos and Deimos, Mars' moons, are a mission easier than asteroid samples: http://en.wikipedia.org/wiki/Phobos_(moon) http://en.wikipedia.org/wiki/Deimos_(moon) From the Hohmann transfer, a supplemented Falcon-9 provides all the 2945m/s above Earth's gravity to 3414kg spacecraft, while Earth's atmosphere makes all the braking on the return leg. The Solar thermal rocket provides Hohmann's 2649m/s near Mars, then 2139m/s to descend to Phobos, some 216m/s to sample a dozen sites there, 789m/s to climb to Deimos, some 216m/s to sample a dozen sites, 1350m/s to climb and escape Mars' gravity, and 2649m/s to come back. This amounts to 10260m/s, easy for the Solar thermal rocket. The craft weighs 2758kg as is hovers at Phobos. At the moons, the engines operate at 2.1* thrust, or isp=800s, so 2* 12 hops of 1h each at 5mm/s2 comsume as much hydrogen as 684m/s at full isp. This needs only five 4.6m concentrators then. More thrust increase would improve. The craft weighs still 1494kg when arriving near Earth. Since thick regolith covers the moons, lighter samples should suffice, permitting a smaller launcher and craft. Marc Schaefer, aka Enthalpy

Yes. They fuse D-T to helium for years. Why shouldn't you read before posting? No. Humans fuse D and T to 4He and n in tokamaks. The energy leaves as neutron speed mainly. Again, why not read a little minimum? Wiki, you know.

A mission similar to the main belt asteroids can bring back samples from Jupiter's Trojans: http://en.wikipedia.org/wiki/Jupiter_Trojan The main belt return trip leaves room for extras, like sampling both at 2.38 AU and 2.58 AU (+764m/s if you're patient); but Jupiter's orbit needs to care more about speed changes and mass. The 2-year Hohmann transfer needs 8793m/s over Earth's gravity at perihelion and 5643m/s at aphelion. If again a supplemented Falcon-9 puts 2793kg at 4881m/s, the trip takes 15198m/s. If the parts thrown away only compensated the samples, the craft would arrive at Earth at 822kg. 754, 744 and 472kg hydrogen are ejected; braking over 120 days at 5.2 AU takes 9 engines with 4.6m concentrators, and accelerating there only 6 engines. Concentrators weighing 1kg/m2 seem a reasonable effort, so each engine could weigh 30kg. Tanks of 100µm steel, 20mm foam and 50 layer insulation, hold by polymer straps, weigh 110kg for the first 754+744kg hydrogen and 55kg for the last 472kg - in case the first is thrown away. The sampling drill can stay there as well. That's room for samples, and for the ability to visit several Trojans. This would leave some 400kg for all craft functions and the re-entry capsule, not including the tanks, engines and 180kg souvenirs. Bonanza! Missions to Jupiter use the Oberth effect to brake there, but I suppose the Trojans are too far away. Bigger launchers, like Ariane 5 with the Esc-B, would give a stronger initial kick to a heavier payload, for instance both missions to the main belt asteroids and to the Jupiter Trojans at once. Marc Schaefer, aka Enthalpy

The Solar thermal engine's isp=1267s=12424m/s can bring asteroid samples to Earth . Take a Falcon-9 for its wide fairing; it puts 10t at 400km Leo. A Leo-to-Gso stage as I describe there http://www.scienceforums.net/topic/73571-rocket-engine-with-electric-pumps/#entry736092 used as an escape stage injects 2790kg at 4880m/s above Earth's gravity. A Hohmann transfer to 2.58UA in the main asteroid belt takes only 15 months http://en.wikipedia.org/wiki/Asteroid_belt it needs additional 1093m/s near Earth and 4684m/s in the belt, while the return leg takes 4684m/s in the belt, and a capsule aerobrakes for free from 12.2km/s. Cumulated 10461m/s would leave 1200kg at reentry - but meanwhile samples were taken, and the craft has made manoeuvres. Braking 4684m/s takes 804kg hydrogen; over 30 days, it needs 7 concentrators of D=4.6m at 2.58UA. These are oversized at 1UA, and the diameter of a secondary or third mirror can limit the power if desired. The chambers for 2.58UA are lighter that previous estimations, and I'm confident that the concentrators can weigh <<3kg/m2, so each engine is maybe 30-50kg. This leaves mass for equipment and samples, all with a single stage. Fascinating: the craft pushing 0.4N per 100kg at 2.58UA can lift off a D=10km asteroid by its Solar engines, and hop from one asteroid to an other, taking samples at each one - maybe capture tiny asteroids provided it sees some with a reasonable speed. Marc Schaefer, aka Enthalpy

It's much guessing, and I should have made it clearer, my fault... What I'm sure is that recoil would permit imperfectly opposed paths and give usable early information about the angle difference to 180°. What does literature say about "momentum-entangled particles"? There is more - and again, much guessing here: the position of the fringes needs a well-positioned light source, which implies that the light momentum (=direction) is imprecise. With one single photon, that would be just a Heisenberg relation. Could it be that the momentum entanglement between both photons suffers the same uncertainty? That is, the more precisely opposed their direction, then the less precise the position of the emission, or the positionS of the emissionS? I suppose it's not a matter of position-momentum undetermination of the emitter, because an atom is heavy so its own momentum would have little effect on the photon pair's one. It's more that one atom could emit two linked photons in directions little correlated, and absorb easily the resulting momentum. That looks better: take an atom as a source. A 3s -> 2p -> 1s through the peanut-shaped 2p would link the second emission directions but only weakly, like a short antenna, that is like sin(angle) between the 2p peanut direction and the photon direction, while the 2p peanut direction defines the first emission direction. To get a more precise direction for the second photon, it takes an emitter wider than a single atom, and then the position of the emission must be unknown enough. Or the positionS of the emissionS - but one uncertain position common to both looks enough. Same uncertainty dx*dp here as with one single photon. Maybe you're on the path to applying uncertainty relations to the entanglement precision of several particles. Here's a paper about the relation between the precision of momentum entanglement and position precision: http://arxiv.org/ftp/arxiv/papers/0905/0905.4830.pdf First, they use a crystal to produce the photons, so the emission position is imprecise. Page 2: "two-photon position separation Δ(x1-x2) and the two-photon momentum sum Δ(px,1+px,2)" people are aware of uncertainties in the correlation. Page 6: "Assuming a perfect spatial correlation with x1 = x2, ∆ , , (is limited by the emission area in the nonlinear crystal with radius w1,2"

It is because the detector at A and the experimenter there do not influence the other particle at B, I maintain. The particle pair decides its behaviour, not the detector. And you'll keep the same fringes at B - only the observation correlated wth the detection at A changes. [ That is, fuzzy at B when the other photon is at Ax. Fuzzy at B when the other photon is at Ay. Fringes at B, resulting from the phased superposition of both fuzzy images at B, when the other photon is indifferently at Ax or Ay. ] [ Meanwhile I doubt that last one ] ---------- There is a difficulty in the proposed setup ---------- The initial description said "Two entangled photons" and I challenge that. Their side momentum, or angular direction if you prefer, is linked with a third particle which is the emitting atom or crystal. Through its recoil, the atom absorbs momentum and allows the photons to have directions not opposed. Depending on the atom's recoil, even if the observer knows that one photons has passed through slit Ax, the other photon can still pass through Bx or By. A random recoil direction and intensity would fully blurr the experiment. There is more. This recoil is observable, and the observation can be transmitted on time to both observers. Such an observation might even destroy completely the link between both photons; as an other possibility, observing the atom only reduces the entanglement to the photons, with a shift in their relation that depends on the atom's measured momentum. I have no opinion about that. This situation differs from experiments that use photon polarization. If an electron falls 3s -> 2p -> 1s as an idealized example, the atom is isotropic in 3s and 1s so both photons are correlated only among themselves. I still don't know if this is an experimental or theoretical difficulty. If you find something that emits two photons in opposite directions necessarily, fine. Even pairs of 511keV gammas depend on the electrons' momentum.

Why shouldn't you read a bit about tokamaks? ITER's site for instance provides much explanations, Wiki as well. Fusion creates little gamma, certainly nothing capable of splitting a helium nucleus. Absorbing them by technological means is no big worry. It's done every day. Heavy elements have their 1s shells 100keV deep, not 1MeV. Or maybe the old Andrei had on this occasion a reasoning even more accurate than yours? Who knows?

Escaping Earth or an other body is best done with one or few kicks at the perigee. The Solar thermal engine is bad here: its faint thrust used briefly at each orbit would take too long and demand too many concentrators. Escaping by continuous rise of a circular orbit is inefficient, letting a Solar thermal engine use as much propellant as an oxygen-hydrogen engine. Chemical engines shall escape planets. Providing little more speed at perigee than the escape minimum leaves much speed after escape because energy goes as speed squared http://en.wikipedia.org/wiki/Oberth_effect so I checked for a high-energy mission how the chemical and Solar engines shall share optimally the provision of speed beyond escape. From 200km above Earth, where speed is 7791m/s and needs 3227m/s more to escape: An oxygen and hydrogen engine with isp=465s best brings 4054m/s = 3227 + 827m/s, leaving 4346m/s beyond escape; An oxygen and dense fuel engine with isp=395s best brings 3789m/s = 3227 + 562m/s, leaving 3561m/s beyond escape. The wide optimum adapts to practical arguments. The needed performance nears a Mars transfer or a direct injection in Gso. Marc Schaefer, aka Enthalpy

If you're willing to spend a few years measuring and modelling many compounds, then you can create your own model. Presently there are very few ones (like AM1, PM3, MND0...) because the task is difficult. If you hoped an answer like "it's cosine times polynomial", just forget it and use the standard models cited above. Depending on how serious your work shall be (MD tends to be extremely simplified to be able to run) you can define more rudimentary models, like atoms being hard spheres, or interaction potentials as polynoms of 1/R, and bonds having some stiffness... The detailed knowledge about such atoms behaviour is concentrated in the models cited above - no idea if someone has published it on paper, as this would make much volume and a boring read. So you might observe what AM1 tells and deduce a simplified model for each bond or intermolecular force... Knowing that many such force fields are themselves deduced from orbital simulations (aka "first principles"), one may wonder how much science is in a model cascade of nothing measured. Let's see if more optimistc forum members pass by...

And I believe that the presence of a detector at A does not change the observation at B. The observation at B may depend on what gets measured at A - and this result is decided by the particles, not by the observer nor the detector. So no information can be transmitted. And if summing on all possible measures at A, one would get the standard observation at B, that is the fringes.

Are you sure? I thought it would take forever as observed from the swallowed object, not as observed from Earth.

Antimatter is so difficult to keep for some time in our world of matter that nobody cares to theorize why they should coexist.

Superconductivity is still an open question, at least in some materials where the previous theory fails. Can the theories about resistivity be true if they fail to explain the absence of resistivity? The honest answer should rather be: "this is still ignored". Some mental models we have for metallic electrons must be grossly misleading, especially the electron gas. Metals' heat capacity would tell that few electrons are mobile, but Hall effect tells that many electrons move.

I bet detection at one slit on side A does not destroy the interferences at side B. One general idea is that acting one one particle does not influence the other - and much less, influence it at distance, which would transmit information faster than light. Entangled states only introduce correlations between the behaviour of particles, not the behaviour of each one. Please take with care. ----- If the detector is after the slit at A, then the photon is still undetermined when passing the slits of A, and so is the entangled photon when passing the slits of B. I suppose a detector behind one slit at B would be correlated with the detector behind one slit at A, but not a detector at the fringes of B. It could be something like: over enough photon pairs, you see fringes at B whatever you do at A. At B, you see photons only in bright areas of the fringe, and these areas are accessible by photons through either slit of B. When you detect a photon at B's fringe and you observed at A that the entangled photon passed through one slit, it only means that the photon passed the screen at B through the corresponding slit, fine. ----- One other possibility, maybe better: - Fringes at B whatever you do at A, if you observe B without condition on the measure A - If you observe B only when seeing a photon at Ax, no fringes - If you observe B only when seeing a photon at Ay, no fringes - But if you observe B over all results of Ax and Ay, then the two uniform lightings at B interfere and give fringes. This second possibiilty is not a transfer of information neither, because the detector at A does not influence the behaviour at B. When the photon pair "decides" to be seen at Ax - independently of the presence of the detector - then it behaves differently at B as well.

Engineeringtoolbox is always an excellent address... The head loss estimate is for a straight tube, but THX-1138 told about bends, so losses will exceed it. By how much, depends on many details. If someone has made comparison trials by himself with a sacrificial anode, he shall please tell us! My corrosion trials showed instead no significant effect by corrosion couples. I know that water is used to cool electric wires and coils... But expect it to be difficult.

For having played myself with explosives and rockets: (1) Be paranoid with these things. (2) Fire them only with thick soil separating them from you. Distance isn't enough. Don't look at them: use mirrors or a camera. (3) Use electric igniters, nothing else. Seek a way to remove them while you stay at your protected location. (4) Slowly igniting rockets are dangerous. (5) How do you hold the last propellant remnants to the walls? If they fall in the throat you've lost. (6) Inspect the propellant for cracks. If any, don't burn it within the rocket casing. (7) Your drawing is imprecise. You won't ignite deep into the propellant without a gas escape, will you? And you must think at the propellant shape to get the desired thrust profile over time, first to get enough thrust. Remember a hand grenade contains about 100g of explosive.

Kernighan and Ritchie is the reference book, but: - It is not a beginner's book! - It describes straight C. Meanwhile C++ was introduced and it influenced Ansi C a lot. Ansi C has significant differences with K&R. Also, C makes often Unix-style programs while C++ makes more often programs with a Graphical User Interface, like the ones common on Windows. Though, do not begin with C++. I see no reason to begin with an other language. C may not be the easiest one, but it's similar to the rest of the family, and it's the most useful. Once you're easy with C or a similar one (which includes Fortran and more), I strongly suggest to learn one very different language like Prolog, Smalltalk or especially Lisp. Even if you don't use an exotic language for daily programming, their radically different methods and algorithms opens you to other programming styles which are usable and useful in C. One assembly language (simple, not Pentium) also brings benefits. Instead of C books, you might search for programming tutorials, and pick one that uses C.