Enthalpy

These miniature power adapters use to be switched mode power supplies, with an active regulation. If you put them in parallel, only the one imposing the slightly higher voltage will supply current. In the best case, the other is useless; in the worst case, you may burn one or both, though this is not very likely. In series, you probably get twice the voltage and once the current - but if the output doesn't float, you just get a short-circuit. You can test this by checking the insulation resistance between the output and the mains without plugging in, but this test is far from perfect. Note 1: we're speaking about active regulated circuits, that's not the same as abstract circuitry or resistors. But expect worries (zap and boom) with accumulators as well in a parallel connection. Note 2: such an adapter is so cheap, for instance on eBay! Buy the proper one, don't take risks nor waste time.

Despite using only integer numbers and counts, the ultimate result is not rational - it's not a quotient of two integers. As you take more and more numbers, you get a quotient (rational) that is more and more accurate (it converges) but the limit, Log(10)/Log(2) is not a quotient. It's a real number, which can be approximated as precisely as desired by a rational number, but is equal to none. This is one method to introduce (to construct) the set of real numbers, as "the limits" of sequences of rational numbers, because these limits exceed the set of rational numbers - where "the limits" needs some mathematical artifacts.

I've just made the experiment with a smooth glass full of water (do it with oil if you want) and the paper doesn't get translucent. So rough paper surface is not the proper explanation for water. Optical index matching is a better one.

Poincaré only conjectured that three bodies made no stable orbits. Meanwhile many such systems have been observed so we know they're stable. Find a newer book. And, yes, many people work hard on this - even though you're not aware of. Because physicists are honest and open-minded, figure that.

Some moons are really heavy with respect to the planet, more so than our Moon. Have a look at Pluto with its moons Charon and at least two more. Wobbling is observed on the star's light spectrum and would result from the combined mass of the planet and its moons. It's the resulting centre of mass of both objects that orbits simply the star, so any effect of the mass being split between the planet and a moon would be of second order, or far smaller than our detection possiblity. Presently, only planets rather heavy, around stars bright enough to us hence near, are candidates for detection through wobbling.

Some cars with piston engines have an integrated starter-generator http://en.wikipedia.org/wiki/Integrated_starter-generator_system As the name tells, this electric machine does both, and the difficulty is that it runs together with the piston engine all the time (and at the same speed), so torque is needed as a starter but the resulting big diameter results in a high linear speed after starting, when the piston engine runs fast. Imagine a D=300mm starter-generator that must survive if the engine runs at 10,000rpm: that's 157m/s, which produces a brutal centrifugal force. For them as well, winding graphite composite around permanent magnets - or possible around a squirrel cage or rotor coils - is a simple means to withstand the centrifugal force, simpler than a steel sleeve. Several turns of thin steel band, possibly as wide as the magnets or rotor, is an other one, where gluing or brazing is easier than slipping a sleeve on and can cumulate more thickness. ----- The integrated starter-generator saves a few parts; it also starts the engine quicker, easing features like the start-stop system http://en.wikipedia.org/wiki/Start-stop_system with more steps possible towards the hybrid electric vehicle http://en.wikipedia.org/wiki/Hybrid_electric_vehicle One little ambitious step I'd enjoy would replace the mechanical reverse gear by the electric motor. An other one would complement or replace the first gear with the electric motor so the car has no minimum speed in a traffic jam. Smooth control of low speed in both directions would also help to park a car. Marc Schaefer, aka Enthalpy

Maybe the surface of oil stays flat while water's surface closely follows that of paper. That's only a suggested explanation, strictly nothing sure. An other possibility: oil's index matches that of paper more closely, avoiding diffusion at the many interfaces. Suggestion: try with paper in a glass bottle full of oil and an other one full of water. That will tell if the explanation by smooth surface holds.

Fuels cells limit the power more than the energy. In the following perigee-apogee stage that accelerates only 1m/s2, a 100kW Honda cell for marketed cars is to weigh <100kg http://en.wikipedia.org/wiki/FCX_Clarity http://www.ansoft.com/workshops/altpoweree/Andy_Bosco_GM.pdf and it replaces 598kg of Li-polymer batteries with 21kg of H2 and O2 ejected continuously. The stage starts with 15t on a 200km 30° orbit, as for instance the H-IIA achieves from Tanegashima, and provides the following 4330m/s. 15kN thrust need many perigee and apogee kicks but let 100kW achieve 63bar in the chamber (mass 750:100, H2 pump 55%, O2 pump 70%, injectors 1.2), and four 1.0m nozzles combine to isp=490s including the 236pppm used in the cell. The ellipsoidal H2 tank weighs 64kg with 120µm brazed steel, foam, multilayer insulation and the holding polymer belts. The prismatic toroidal O2 tank weighs 198kg of already described AA6005 extrusion, the payload adapter 66kg of same material. The four-chamber engine shall weigh 200kg, the sensors and control 200kg, unaccounted items 100kg, for a 828kg dry stage. The longer interstage of AA6005 extrusion is 349kg heavier, the fuel takes 8706kg, leaving 5.1t payload in GSO. One added fuel cell for more pressure wouldn't pay for its mass. A single deployable nozzle can replace the four but needs verniers. The H-IIA would better put a lighter stage on a higher orbit. The hydrogen throughput is little for a centrifugal pump but much for a screw one; maybe a single axial pump, like at booster pumps but faster. Marc Schaefer, aka Enthalpy

Because quaternion multiplication doesn't commute, you have to distinguish between differentiation to the right and to the left. Which my old book on that topic briefly commented with: "this must be one reason why differential calculations on quaternions are little developed".

Black holes unobservable? That was 30 years ago. Meanwhile they're observed. Find a newer book.

The thread's title is "dark matter" but the first sentence "dark energy". These two notions differ; which one is meant? To my feeling, "could it be playing a role" is too vague. Do you propose a precise mechanism? As an element of answer, dark matter is not concentrated at stars, because the total mass of stars - especially our Sun - is known and it does not explain gravitation effects of galaxies. As well, gravitational lensing is observed in some galaxies where ordinary matter is not. So dark matter would be very diffuse, like still unobserved particles that accumulate at galactic scale only, or might perhaps be concentrated - though many size scales have been rejected by observation - but separated from normal stars and gas clouds. That said, dark matter might interact with our Sun's chromosphere even if very diffuse. But how? To stay diffuse, dark matter has to interact very little with ordinary matter; a common representation is that they interact only by gravitation. And could you tell more about the chromosphere observed at an other star?

An efficient upper stage for the KSLV is sooner developed if electrically pumped. To achieve 202kN and isp=369s, it burns RG-1 and oxygen at 140bar and expands in a D=1.45m nozzle; rolls at the vernier thrusters guide the interstage during separation. The stage begins with 22.6t and ends with 5.5t. Axially oriented AA6005 extrusion, with t1=t2=1mm, a=45°, B=22mm, as described there http://www.scienceforums.net/topic/60359-extruded-rocket-structure/ makes the outer cylinder, interstage, payload adapter and maybe the fairing. The rest is AA7022 sheet and tube; all is welded together. The RG-1 tank is a D=2.4m d=1.7m t=1mm ellipsoid hold by two 45° t=0.5mm cones. The oxygen tank is hold by two t=0.6mm cones; it's a D=2.8m t=1mm sphere covered with 10mm foam and multilayer insulation. The sphere's diameter and height shrink at cold, and the cones less so; their best angle is near 45° to reach the cylinder without stress. The tanks and cylinder weigh 490kg, the payload adapter 65kg, the engine with pumps and truss 375kg, the Li-polymer batteries 850kg, sensors and control 200kg, unaccounted items 100kg. The earlier separated interstage and fairing weigh 330kg and 570kg. The empty stage weighs around 2074kg, leaving 3450kg payload at 4400+5100m/s performance. 74% efficient centrifugal pumps need 836kW and 376kW shaft power. The oxygen impeller can be 85mm wide and 8mm high running at 586Hz, its electric motor 152mm wide and its poles 100mm long. Marc Schaefer, aka Enthalpy

Turbofans fit faster flight, and an electric motor can rotate them too. The example shall be a Trent WXB, please: D=3.0m fan receiving 70MW at 2683rpm (taken from varied versions, hence not fully consistent). The 40mm thick neodymium magnets (here Thyssen 300/110) on OD=1.0m move at 140m/s. Thin narrow bands of permeable hard steel wrap them, totalling 7mm thickness to limit to 3mm the radius variation due to centrifugal force. The bands can be brazed (especially if covered with nickel) or glued; chemical etching can bevel their ends. The air gap is 6mm outside the magnets at idle; some elastomer or springs are required at the magnets' inner side. If feasible, pre-stressing the steel bands when winding improves that. Magnetic leaks in the bands widen the transition between the poles but loose no induction at the plateaux. The motor is 404mm long, its 16 poles have 7 active slots of 8 at any time, each 11mm wide and 44mm deep to host one phase that comprises 4 turns of square copper wire, 9mm*9mm large with a 3mm*3mm central hole. Induction at the gap is flat 1.09T, current per slot 10.1kA, the inverter's supply is 4kV. Ohmic losses total 730kW or 1%. Fe-Co laminations at 2.0T loose 70kW. 18dm3/s cooling oil flow at 5m/s in parallel through all wires, and some more through iron. The bare electric motor weighs 2030kg, a complete original Trent XWB 6600kg. The stator has OD=1.33m, or the mean diameter of the Trent's primary air inlet. Huge strong magnets are a real danger for mechanics; a squirrel cage design would improve it. Wrap the aluminium or copper cage in steel or graphite composite to resist the centrifugal force, including the shorting ends. Well, as soon as we have 70MW electricity on board... Marc Schaefer, aka Enthalpy

I wouldn't be too pessimistic about losses - but it all depends on the distance! While normal glass is opaque after very few decimeters, PMMA (like Plexiglass and other brands) is very transparent over several meters. I observed it over nearly 10 meters at visible light: objects would still be seen with all colours. Do I remember that silica lacks the colour centers that absorb light in glass? So if the goal is to spread Sunlight from a roof to rooms one or two floors below, it could work (mirrors at a shaft being a strong economic competitor). From Florida to Alaska, no hope, agreed.

In such transcendental equations, you can often find a simple loop algorithm that runs on a calculator without any programming - the advantage over Newton and the like. Here: x = exp(1/x) gives without long rewriting two loops: -> Log -> Reciprocal (which after short trying doesn't converge) -> Exp -> Reciprocal (which does converge, starting with 2 for instance) Someone interested in maths could try to prove that one of both directions of the loop must converge. At least I saw it work about every time, if the initial value is lucky. More generally, x can act, after a transformation like exp(), at more than two terms; then you have more than 2 loop directions, as you choose which term shall return x, the other terms being used in the forward direction. Marc Schaefer, aka Enthalpy

A first general difficulty with fibers is to inject the light in, because fibers are small and accept light from a limited angle. If you consider visible light and the near infrared, which contain all the heat of Sunlight, and a distance of 10m, you will find fibers that lose no significant power. In a first check, you can split fibres into families: plastic (always wide), multimode silica (=wide), monomode silica (=very narrow).

The unducted fan or propfan has two counterrotative propellers so the downstream air doesn't rotate. A bigger blade pitch then remains efficient, so the propeller can be used for faster flight. http://en.wikipedia.org/wiki/Propfan Electric motors combine with counterrotative propellers more easily than turbines do: The version at the fuselage has concentric shafts. The detached version needs the pylon between the propellers, but helps dynamic stability and the bearings; I expect propellers wider apart to be less noisy. Electric motors can have the rotor outside - not depicted here, please look at a processor cooling fan. This enables more layouts that combine well with propellers but are less favourable to the bearings nor the protection against dust and rain. Marc Schaefer, aka Enthalpy

This design (click to enlarge) with electric pumps everywhere puts for instance a Soyuz spaceship in orbit, but is smaller and lighter (201t) than the venerable Soyuz launcher (315t). The 4m body accommodates efficient 2.6m nozzles that deploy like Vinci does; 140 bar, 140 bar and 100 bar make the rest of the Isp. Rolls near the Vernier thrusters guide the previous stage during separation, as on Zenit's Dm-slb, to prevent collisions. The whole cylindrical body, plus the payload adapters, the third stage's oxygen tank and possibly the fairing, consist of assembled aluminium extrusions as I suggest in http://www.scienceforums.net/topic/60359-extruded-rocket-structure/ here of AA6005 with t1 = t2 = 1mm, a = 45°, B =22mm, oriented axially, to resist 8MN compression and 8MN*m bending moment, twice the expected stress. Mastering thinner extrusion of stronger weldable alloys (AA7022) would improve, as would magnesium alloys. Cylindrical tanks are wrapped tightly in welded bands of AA6082, 0.3mm at the top, up to 3.0mm at the bottom of the first stage's oxygen. Tank heads are of AA7022, as is the second kerosene tank and its holding cones, as well as all trusses - all welded together. At the third stage, the structural fuel tank is a prismatic toroid of the aluminium extrusion, while the balloon oxygen tank is of 250µm brazed steel, plus foam, hold to the toroid by polymer belts. Per ton of combined propellants, the structural sets of tanks weigh 34.1kg, 28.9kg and 28.1kg. Li-polymer batteries weigh 49.2kg, 49.2kg and 35.1kg per ton (throttling unaccounted), but the dry stages keep at 121kg, 109kg and 108kg per ton. The first oxygen pump takes 15MW at 13800rpm; its motor can be 360mm long, 530mm wide and hollow, to weigh 380kg. A two-stage design would weigh 330t, with poor capability for slightly higher orbits. An optional fourth stage, scaled from my description for Zenit, or from the present third stage, brings payloads to geosynchronous orbit from Baikonur or the Equator, or towards Mars... A 2.6m nozzle would improve further, as would strained fuels and the smaller thrust. Marc Schaefer, aka Enthalpy

What if you put a drop, say, on a sheet of paper you place outside, and wait? Maybe most of your material stays and the odour vanishes or diminishes? This would be a first hint that a minor product makes the smell.

Independently of the inverter, a battery stores electricity by a chemical reaction of its constituents. Some lose electrons, others (ions) get one, and when the battery is well thought, this happens only when an electric current flows outside the battery. The electrochemical reaction goes in opposite directions during charge and discharge. Linking the reaction completely with the external current is the difficult part; only perhaps 100 practical cell chemistries have been widely used since electricity exists. http://en.wikipedia.org/wiki/Battery_(electricity) http://en.wikipedia.org/wiki/Rechargeable_battery

The intensity of the doubled frequency relative to the fundamental is variable (experimenters would say, erratic) but the intensities of the products of nonlinearity are fully linked. Take a second-order nonlinearity, since even powers in Taylor's expansions produce the even harmonics: [a1cos(w1t)+a2cos(w2t)]2 =a12cos2(w1t) + 2a1a2cos(w1t)cos(w2t) + a22cos2(w2t) =0.5a12cos(2w1t) + a1a2cos[(w1+w2)t] + 0.5a22cos(2w2t) plus some terms at w=0 and a w1-w2 so the field amplitude at w1+w2 is double (power is quadruple) the geometric mean of the amplitudes at 2w1t and 2w2t. This has practical implications in radiocomms. Nonlinearity is sought for mixers of heterodyne receivers http://en.wikipedia.org/wiki/Heterodyne_receiver but nonlinearity isn't desired among the many signals arriving at the mixer from different transmitters. To improve mixers, designers prefer parts with even nonlinearity http://en.wikipedia.org/wiki/Intermodulation and combine them in symmetric circuits so the mixer is as linear as possible against the input signals.

You can magnetize some spherical material so that its axis points everywhere toward the center - or rather, assemble many magnets on a sphere. But the net result will be zero magnetic field. This is up to now observation; magnetic field flows only as loops, not from or towards a point or region (=the center of your proposed shape). The same holds for coils: you could easily wind several spherical coils around an other, with varied directions, and adjust the currents to obtain a field with sperical symmetry - but this field will be zero. Laws for electromagnetism formulate it as div(B)=0, or "no divergence", or "flux is conserved". Of course, physical laws are here to reflect observation; so many people tried for so long time that more attemps with coils are vain. In a different attempt, some people hoped to observe "magnetic monopoles" particles in colliders, but none has been seen. As for magnets, their poles use to come in pairs, even if shaped as a sphere. At a motor, a magnet can have for instance 6 poles: North at 0, 120°, 240°, and South at 60°, 180°, 300° - even when the magnet is a single part. You can even have poles distributed as NSN, for instance at an E-shaped electromagnet, but because the flux is conserved, the lone South pole must have a bigger area than the two North, or the induction must be stronger there.

There are different models of dark matter - enough ones that, if we're lucky, one will be seen to fit some day... Several models (the most common, not the only ones) invoke wimps to constitute dark matter. These Weakly Interating Massive Particles do not respond to the strong force. Interacting (only?) by gravity, they would make rather diffuse halos in galaxies, or possibly have constituted black holes. A new force, felt only by dark matter, that would make clumps of it? This model would have the same limit as the ones that propose clumps of ordinary matter to explain the missing mass: such clumps would be detectable for they would bend light; most possible scales for such objects (brown dwarves, extinct suns, stellar black holes...) have been disproven experimentally. What's sure: the missing mass, for which a dark matter explanation is wanted, can't reside at normal matter, because the mass of observable matter is known (...well enough...) and this mass doesn't suffice. We need mass elsewhere, because we know the one of visible objects. Even more precise: gravitational lensing, which tells where matter is in galaxies, is sometimes observed where normal matter is not. This is one strong argument for dark matter and against modified gravitational laws.

No self-regulation nor correction. For instance Mercury's orbit rotates slowly - that is, its major axis changes its direction, as does its perihelion. This is a response to various perturbation, and to a minor extent, to a relativistic effect. Comets that get diverted from their parent cloud don't go back there. Spacecraft that humans send near Jupiter to exploit the powerful perturbation keep the new trajectory.

Hey Knumbnuts, did we meet at an other forum ? Your illustration picture is unmistakeable. (Sorry for polluting the thread, I couldn't use the message function)