Enthalpy

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About Enthalpy

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  1. Sure. Failed designs show much bigger examples at Olkiluoto, Flamanville, Tianshan and next Hinckley Point. What I like less at explanations by failed attempts is that they offer so many variants that they can explain anything. Or if you prefer, they are not refutable. It does not mean that they are wrong, only that reasoning on them is virtually impossible. This is a seducing interpretation, but it has some difficulties. Why go so deep in the cave? 50m would suffice, they walked 300m from the entrance in a difficult terrain. That's dangerous and lengthy. For a safe water supply available only there, I'd do it. To sleep comfortably, I'd prefer a nearer location. The artefact is (at least today) in a location low and inundated. That's where I would not put my camp. The "walls" follow contours of constant altitude. Perfectly justified for a reservoir, while a camp wall would have straight walls where the terrain drops. Traces interpreted as fire remnants are everywhere, including on the "walls", rather than at the centre. Their skull volume exceeded our. But so does elephants' skull too. The surprise to archaeologists is that they had seen no construction by Neanderthals prior to this one. And what is intelligence? Intelligence is specialized. My cat was intelligent to interact with humans and exploit us, but he never made something with an object, except if the object represented a prey. He wouldn't even carry an obstacle away from his bed. Bruniquel is the first time archaeologists find indications of Neanderthals going deep in caves. 175,000 BP was during a cold period, yes https://en.wikipedia.org/wiki/Ice_age Good point, the problem of wood transport. =========================================================================== The human bones in Jebel Irhoud, presently attributed to Sapiens Sapiens, were dated to 300 000 BP in 2017, that is, after the paper about the artefact in Bruniquel. https://fr.wikipedia.org/wiki/Djebel_Irhoud https://en.wikipedia.org/wiki/Jebel_Irhoud This half-opens the alternative possibility that Sapiens Sapiens made the artefact in 175 000 BP in Bruniquel. No evidence exists of Sapiens Sapiens in Europe at that time. From now-Morocco to now-France, humans would have needed to cross the Gibraltar straights (deep water during the ice age too). But few years ago, archaeologists saw only Neanderthals in Morocco at that time. They may change their mind for Europe too. So what's more difficult to accept: first evidence of Neanderthalensis making artefacts deep in caves, or first indication of Sapiens Sapiens in Europe at that time? Please take with mistrust, as here I'm very far away from anything I imagine to understand.
  2. At Bruniquel cave's artefact, the paper's authors recorded a strong magnetic gradient that excludes many origins. The data is mapped on fig. 5 with explanations http://www.nature.com/nature/journal/vaop/ncurrent/fig_tab/nature18291_SF5.html (it was there) they saw +-12nT/m at several places, up to +-24nT/m. I didn't see the measurement altitude and suppose it was 1m over the flat bottom, maybe 0.5m over the construction's peaks. The apparatus takes the difference between two Geometrics G858 sensors http://www.geometrics.com/geometrics-products/geometrics-magnetometers/g-858-magmapper/ which measure the total induction by caesium vapour cells and are stacked vertically with 0.22m separation, so "gradient" means the vertical gradient of the total induction. ---------- I compare with what a small magnetic dipole achieves: the strongest case is the polar component in the polar direction, B = 2*10-7mR-3 where m is the magnetic moment in A*m2, B in Tesla, R in m http://www.phys.ufl.edu/~acosta/phy2061/lectures/MagneticDipoles.pdf https://en.wikipedia.org/wiki/Magnetic_dipole grad(B) = 6*10-7mR-4 (put signs as you like) 24nT/m would need 2.5mA*m2 at 0.5m or 40mA*m2 at 1m - or half as much for 12nT/m. We can already note that today's best permanent magnets of FeNdB provide 1100kA/m, so at 1m such a supermagnet would need to be 2mm3 big. ---------- The magnetic susceptibility X of the artefact's materials deform the geomagnetic field, but how much of which materials does it take? At Bruniquel, the total geomagnetic induction is 46µT, tilted 58° from the horizontal https://upload.wikimedia.org/wikipedia/commons/f/f6/World_Magnetic_Field_2015.pdf https://en.wikipedia.org/wiki/File:World_Magnetic_Inclination_2015.pdf and the total geomagnetic field 37A/m. I take everywhere SI conventions, where X is dimensionless and 1+X is the relative permeability µr: Induction B (T) = (1+X)µ0H (H field in A/m) with µ0=4pi*10-7 by SI definition A volume L*S of material with susceptibility X lets pass the same induction as vacuum if we add a current H*X*L around it and then the external distribution is the same as for vacuum. This current has a moment H*X*L*S proportional to the item's volume. And since the added current compensates the item's presence, the item has the same external effect as the added current without the item. Yes, this can be done better. An algebraic solution exists for a sphere, and by packing spheres of varied radii, we can fill any shape. Put signs as you prefer. The added current depends rather on the reluctance which varies with 1/(1+X), so for big X at ferromagnets the computation differs. Now, the far effect of a small dipole is proportional to the magnetic moment, so for any small distant shape with uniform susceptibility X, only the volume counts. The item acts by m (A/m) = H*X*V, and if the item isn't small, integrate this with varying distance. If the item were magnetically soft iron, (0.1m)3 at 1m would make the observed 24nT/m. Wow! ---------- How much? Let's take arbitrary 1dm3 of materials with X=10-5 at 1m. In the 37A/m geomagnetic field, it's equivalent to a dipole of moment m = 370nA*m2 that creates a gradient of 0.22pT/m. For the materials expected at the artefact, some experimental X are: http://www.irm.umn.edu/hg2m/hg2m_b/hg2m_b.html https://en.wikipedia.org/wiki/Magnetic_susceptibility -0,9*10-5 Water -1,3*10-5 Calcite (CaCO3) +2,7*10-4 Montmorillonite (clay) I had suggested here on 31 May 2016, 09:59 pm that the mere height of the contruction could explain the magnetic perturbation. But even 20dm3 of calcite at 0.5m create only 0.07nT/m. Wrong! The authors observed that calcite's X doubles after heating in a fire. But 1dm*6dm*6dm of heated calcite at 1m create insufficient 0.02nT/m. Raw clay has a bigger X that varies much with the iron contents. Taking 27*10-5 for 1dm*6dm*6dm at 1m explains 0.2nT/m, still 100* too little. Clay heated by fire has a stronger effect. From M. Kostadinova-Avramova and M. Kovacheva https://academic.oup.com/gji/article/195/3/1534/622882 table 3 their most reactive sample, heated to 700°C and cooled in 50µT, has a susceptibility as above essentially, but also a permanent magnetization of 22A/m if I read properly, almost as strong as the geomagnetic field. 0.05m*0.2m*0.2m of this heated clay suffice to create 24nT/m at 1m. Or (50mm)3 at 0.5m. ---------- Maps of the gradient at different heights would let infer the altitude of the cause. It's an inverse problem, difficult to solve and with many solutions, needing some assumptions like "isolated small sources". I'd be glad to know how much clay was available in this cave, and whether the artefact accumulates abnormally much of it. And whether bare calcite from this site gets the observed colour from mere heating, or if it needs to absorb clay. R=50mm get hot to the centre in 2min only: are the stalagmites brown to the core? My gut feeling is that the colour is too uniform for a fire, I imagine a prolonged contact with clay more easily. Or could water drops bring the clay too? Marc Schaefer, aka Enthalpy
  3. Woodwind Fingerings

    Some fingerings and keys systems I proposed need two buttons at some fingers, notably for the bassoon. I had suggested to operate the buttons at different phalanges. The fingertip could also slide between the buttons. If bending the finger suffices thanks to the relative positions of the buttons, this movement may be easier and faster. Experiment shall decide. On the sketch, the proximal button is higher than the distal one and rounded, so the finger slides to it more easily, over its natural slope, and can descend to the distal button. At least at the thumb's buttons of existing bassoons, I prefer no rolls. The oboe, contrabassoon and many woodwinds have small covers and displacements, but the proposed arrangement may not fit a baritone saxophone. Or is it less bad than presently for the saxophonist's right pinkie? Marc Schaefer, aka Enthalpy
  4. Solar Thermal Rocket

    Accelerating nearer to the Sun than Earth is would be more efficient, but if a probe must first brake for that, it gains knowingly nothing. So what if Jupiter sends the probe back to 0.3AU where it accelerates towards the terminal shock? Alas, the flyby must reduce the eastward speed and lose energy. The leg towards the Sun takes time as well. And the net result is: better than shooting from Earth directly, but not quite as efficient as leaving just by the slingshot at Jupiter. TerminationShockJupiterSun.zip Having to operate at 0.3AU in addition to 90AU, the probe becomes much harder to design. It also needs a cryocooler for the hydrogen that makes the second kick, while the simple Jupiter slingshot may conditionally live without. Are there advantages? The same launch can send an other probe in the opposite direction, after Jupiter puts it on a retrograde orbit. Nice, but it takes about 3 years more, and if waiting 5.4 years, a simple slingshot at Jupiter can target the opposite direction too. The deflection by the Sun can vary more, by adjusting the perihelion or by pushing earlier or later. My spreadsheet pushes early. Better: Jupiter can send the probe out of the ecliptic plane, up to polar solar orbits, so the probes can reach the terminal shock in any direction. My spreadsheet computes only prograde and retrograde orbits, but the tilted performance is in between. As the Sun's movement apex is far from the ecliptic plane, reaching this direction may justify the extra engineering and waiting. Saturn seems to achieve retrograde orbits better than Jupiter does, but the legs to it and back take longer. Flybys at Venus, Venus, Earth before Jupiter may save propellant and achieve better angles. I checked none. Marc Schaefer, aka Enthalpy
  5. Solar Thermal Rocket

    Of the huge speed needed to reach the Sun's termination shock at 90AU in bearable time, a slingshot at Jupiter can give 5 to 6km/s for free TerminationShockJupiterSlingshot.zip so it's time to update the suggested mission of Jul 07, 2013 http://www.scienceforums.net/topic/76627-solar-thermal-rocket/?do=findComment&comment=755396 including as well the improved script for Earth escape of Jul 27, 2014 http://www.scienceforums.net/topic/76627-solar-thermal-rocket/?do=findComment&comment=818683 The future Vega-C puts 3.1t at naturally inclined 400km Leo. Saved 50M$ over Ariane. Sunheat engines raise the apogee to 127Mm radius in less than a year, tilt as needed, end with 2391kg. An O2+H2 engine kicks at perigee to send 1888kg at escaped 4233m/s. The optimum is broad, and more speed saves volume under the fairing. 50kg of chemical engine and oxygen tank are dropped. 1838kg remain. Sunheat engines add 5km/s for 9233m/s. End with 1229kg. 300kg of hydrogen tank and sunheat engines are dropped. 1538kg remain. Sunheat engines add 8367m/s for 17600m/s versus Earth. End with 784kg. Jupiter reached in 0.8 years brings the asymptotic speed to 35km/s versus the Sun. The probe reaches 90AU in 13 years after Earth escape. The remaining 6 D=2.8m engines eject the 754kg in 17 days. They weigh 70kg and the remaining tank 140kg, leaving 574kg for the bus and the science. That's half more than previously with a cheaper launcher - or send 6* more than Vega-C with Ariane. Several smaller probes could observe the turbulence of the medium, sense it by radio transmissions, explore different directions... Jupiter can spread the probes, including out of the ecliptic plane. The probes can also leave Earth's vicinity at different dates. The concentrators can serve as antennas. Each can also collect 1W sunlight at 90AU. If relying on propoer orientation, this is more than enough heat and electricity for a probe that makes a measure and transmission per week, but is little for a parallax measurement probe. Spin stabilisation is an interesting option. Such a mission is the perfect opportunity to test the Pioneer anomaly http://www.scienceforums.net/topic/79814-pioneer-anomaly-still/ Marc Schaefer, aka Enthalpy
  6. Solar Thermal Rocket

    Here's a sketch of the Saturn flyby to join 2015 BZ509. Probes use to overfly planets forwards to get speed, but this script overflies backwards to obtain the retrograde speed. This loses speed, so the propulsion must provide more, in amounts compatible with the sunheat engine. Phi is as in the spreadsheet, Psi is the deflection as in Kate Davis' article http://ccar.colorado.edu/imd/2015/documents/BPlaneHandout.pdf The figures are one example from the spreadsheet, where the probe leaves Saturn with no sunward speed for a cheaper but lengthy strict Hohmann transfer to 2015 BZ509. Other cases can be more realistic, other choices better. Saturn has advantages over Jupiter: The probe arrives with less Eastward speed in the Sun's frame, that's more Westward speed in the planet's frame. Saturn is slower, so the probe loses less speed. The probe gains speed when falling from Saturn to BZ509. Saturn isn't synchronous with BZ509. The launch date and speed adjustments let the probe meet the asteroid, while arriving on time isn't easy with Jupiter. But Saturn moves too slowly to let pick a position, so the probe will improbably arrive at a nodal point of the asteroid's orbit. The correction may demand additional speed which the spreadsheet doesn't include. Combining the correction with the tilt kick or the arrival kick linders the cost. Mass estimates may come some day. Marc Schaefer, aka Enthalpy
  7. Solar Thermal Rocket

    The asteroid 2015 BZ509 is on a Sun orbit as big as Jupiter but retrograde, elliptic and titled https://en.wikipedia.org/wiki/(514107)_2015_BZ509 A recent study suggests that the asteroid is of extrasolar origin, the only matter known in our solar system. So shall we bring samples back from 2015 BZ509? With a reverse orbital speed around 13km/s, the task seems impossible, but the sunheat engine and a flyby at Saturn would enable it, according to my estimates. A flyby at Jupiter instead lets difficulties arise, but I didn't try hard enough: reach BZ509's orbit, synchronize with the asteroid, wait for favourable positions... A solar sail doesn't look good: at one Sun-Mercury distance it takes 18 years to tilt its orbit by 163°, and at one Sun-Jupiter distance it's not autonomous. The probe leaves Earth much faster than for a Hohmann transfer, so it arrives at Saturn in only 2.3 years with much speed in a direction mainly outwards, and passes before and above Saturn which deflects it in a reverse orbit in the same plane (tilt 180°). The minimum global speed cost puts the probe in a Hohmann transfer from Saturn to BZ509, but other options would save time. At mid-transfer, the probe corrects the orbital plane by up to 17°, which is costly, and near the asteroid it makes the final push to accompany it. The Earth-to-Saturn leg is much exaggerated in the sketch: displayed too straight, with the Earth at the wrong place. Reversing the path to come back would need a doubtful combination of positions and cost much speed, so the probe shall head to Earth directly and aerobrake despite the reverse direction. That's about 68km/s, just above the present record, wow. The path is in BZ509's orbital plane, and braking more than Hohmann needs shall let the trajectories cut an other. BZ509overSaturn.zip The spreadsheet contains my estimates. A few uncertainties remain: I computed for Saturn at perihelion or aphelion only, but this makes little difference and the flyby copes with various angles, so intermediate cases should be fine; and I computed for BZ509 at perihelion or aphelion only, but we can't wait until Saturn is at the proper place, and for BZ509 I'm not sure that the intermediate cases are as favourable as the extreme ones. More details and explanations may come some day. Marc Schaefer, aka Enthalpy
  8. Avoid Cookies Warning

    Hello Internet users! (Should be 100% of the members here) You noticed that all sites, including the very best forums, put for a couple of months a warning that they use cookies - at least for users in the EU. It's not that you were supposed to ignore it. Nor that cookies are deleterious. It's just because the EU has decided that you should not ignore this horrible threat to your private life. Worse: if your reasonable security settings let the browsers erase all cookies at closing, then you get the warning about cookies each and every time. A bit like if the law wanted you not to erase the cookies. The politicians make laws to force companies spy individuals out, for instance reveal all passwords to governmental agencies hence know and store the passwords, which enabled hackers to steal them all at Yahoo. Or to put backdoors in communications, operating systems and so on, making the countries perfectly vulnerable to the alleged enemies. We may wonder if the politicians really should warn us about the companies, or rather the other way. Anyway, I'd to share with you the notice that made my use of Internet a little bit easier: Someone has written for Firefox and others an extension to block the popup warning about cookies It's available there: https://addons.mozilla.org/en-GB/firefox/addon/i-dont-care-about-cookies/ so nice! Enjoy!
  9. Solar sails, bits and pieces

    More about the Venus flyby for the Mercury samples mission. It brings much more than I hoped. But as these computations are slippery, I may have made mistakes. Kate Davis gives an angle in relation with the asymptotic speed http://ccar.colorado.edu/imd/2015/documents/BPlaneHandout.pdf which I rearrange slightly with an escape speed at the periapsis 50m/s lost in drag need 10-10kg/m3 residual atmosphere, 0.5µm erosion about as much, and 400W/m2 a bit more. Having to atmosphere table for Venus, I made horrible scaling factors with Earth and decide the periapsis shall be at 683km over the 6052km planet's radius. The escape speed is 9825m/s there. And here's an angle-versus-speed graph for this periapsis at Venus Ariane 5eca can put 4550kg at 3475m/s above Earth's gravity, Ariane 6 hopefully too. Aimed 38° Sunwards, or outwards in an other launch window, the probe arrives at Venus with 4713m/s and 60° sunwards and forwards. Well, if Venus were in the ecliptic plane, sorry folks. The Venus flyby can give the probe 4066m/s polar speed, zero radial speed (this isn't optimized) and 2365m/s below Venus' orbital speed, so the probe's orbit is in Mercury's plane (...if Venus were in the ecliptic) and with 0.648AU semimajor axis. VenusFlybyNum.zip From there, the probe sails to Mercury's orbit, which costs roughly 10760m/s. At (mean!) 0.648AU, the sail accelerates 4550kg by 55µm/s2, so this step costs 4 years. Or a bit more, since Mercury's apsis line is not exactly in Venus' orbital plane, and so on. The rest of the mission stays the same, but with more mass. 1100kg sail leave 700kg for the bus and 2750kg as the descender starts: four times more mass than previously. The ascender has now a comfortable size, and since the electricity doesn't scale up, it brings more than 40kg samples back. Two prospectors look possible. The return path, with 1840kg, takes only 1.5 years to accelerate towards Venus. Marc Schaefer, aka Enthalpy
  10. Woodwind Materials

    Deposited metal can make corrugated walls, at a straight tube or any shape. The walls get stiffer against bending while keeping lightweight. Let's take a example tube of D=19mm, rho=10370kg/m3, E=85GPa, and e=0.2mm with the mean fibre corrugated to 1mm peak-to-peak. It weighs 2.1kg/m2 but is as stiff as if it were 0.67mm thick. Its first oval mode resonates at 7.1kHz instead of 2.6kHz for 0.45mm smooth walls twice as heavy. These walls can conduct heat oscillations over much of their thickness, in case this matters. A sandwich with a core of foam, balsa or honeycomb can't. Corrugations increase some losses, both aerothermal and by heat conduction. This is expectedly a big drawback at a flute, but an advantage for instance at The bell of clarinets, especially the bass, contralto and contrabass The bell of saxophones The entrance of the neck of saxophones The bell of adaped oboes, especially the English horn, baritone and heckelphone Marc Schaefer, aka Enthalpy
  11. Solar sails, bits and pieces

    General figures about the thermal design for the Mercury samples mission. Far from Mercury but at 0.307AU (Sun-Earth distances), sunlight is 14480W/m2 strong. It heats to 300K a perpendicular second-surface mirror that absorbs a=0.03 of it and emits e=0.95 infrared from one side. Shaded areas are cold. Staying close to Mercury's noon area at 600K (700K are reported) and parallel to it, an ideal flat area absorbing infrared from both faces but no sunlight attains 505K=+232°C, a conducting sphere too. The probe's outer faces should survive >600K but can't cool the equipment when facing Mercury. A possible design puts several cooling faces at the probe body and makes a thermal contact (by a fluid or a deformation) with the coldest one at each time, when it doesn't face Mercury nor directly the Sun. At 60° incidence but emitting from a single side, a second-surface mirror reaches 300K. Averaging with Mercury's night side helps a lot, especially at the batteries. Active steering of cooling surfaces is less reliable. The probe doesn't need a general fridge cooling cycle. ---------- A thin polyimide film for the sail (arbitrary a=0.3 e=0.9 both sides) facing the Sun reaches survivable 454K=+181°C and radiates infrared to the probe's body but from a limited solid angle. Choose well the polymer deposited after the metal. Cooling the telescopic booms is difficult. ---------- The solar cells could populate sparsely a Sun-facing area. If they're transparent to the near-IR (a=0.7), cover 1/4 of the area, the rest offering a=0.03 and e=0.95, and both panel's faces emit (e=0.95), sunlight alone heats to 436K=+163°C, 600K Mercury in the back 550K=+277°C shortly. A single emitting face must be less populated. Few mm thick aluminium foil spreads the heat of small 20mm*20mm cells without heatpipes. Wide bandgap cell semiconductor is the obvious choice. The soft bond with the support conducts heat away easily but must resist the peak temperature. ---------- The surface of Mercury is still hot when the Sun sets. 200h night cool very roughly 0.2m soil whose surface gets chilly but is blasted clear by the landing engines, so insulating the prospector's bottom seems better. Deep soil probes would come warm. ---------- De-orbiting and landing on Mercury consume about 4.3km/s, taking off and navigating to the orbiter 4.0km/s. Two stages of hydrogen and oxygen shall do it. The descender starting with 700kg burns 0.94+0.14kg/s at 30bar to push 5kN in 1*D0.6m. Including 2.2g/s in the 15kW 8kg fuel cell, the Isp=4622m/s=471s lands 276kg, of which 50kg are abandoned, so the unpolluted crawler+ascender weigh 226kg. Minus the crawler with instruments but plus the samples, the ascender takes off with 100kg to meet the orbiter with 42kg only, leaving maybe 10kg for the packaged samples. Check my lightweight boxes http://www.scienceforums.net/topic/85103-mission-to-bring-back-moon-samples/?do=findComment&comment=823276 Multiple prospectors look hard. The Earth reentry capsule must stay with the orbiter. A heavier and slower mission injected to Venus flyby by the launcher would be much better. Without the descender+crawler+ascender, the return leg isn't as long. Hydrogen needs active cooling at least at the ferry-orbiter. I won't detail the easier oxygen, and as mass at the ferry and the descender is easier, I concentrate on the crawler+ascender. 7.5kg hydrogen for the ascent plus 5.0kg for mean 50W over 70 days fit in D=0.75m. Polymer belts holding the tank leak negligible heat, but from the 300K surrounding ascender, 50 plies of e=0.02 multilayer insulation leak 300mW. A cryocooler, third-Carnot efficient from 20K to 250K, uses mean 12W, rather with stronger intermittent runs and a mechanical separation at idle; its heat dump operates in Mercury's shade, when awaiting the orbiter as well. Or let the hydrogen evaporate at the crawler+ascender. 300mW evaporate 4.2kg over 70 days, so the fuel cell taking hydrogen mainly gaseous would keep the liquid cold. That's marginal with the present figures but feasible at a bigger prospector. I wouldn't burn methane in the engines and crack it for electricity. Lose 20% mass at the descender, 20% at the ascender, double the oxygen+fuel consumption for electricity, add complexity and mass. Marc Schaefer, aka Enthalpy
  12. Woodwind Fingerings

    Here's an other mechanism that responds to the closed-to-open transition in the direct holes to close some consequent holes. As opposed to the sketch on Jan 14, 2018, lateral axles carry the direct covers or rings, while the transition shafts run at the centre. This is easier at the oboe family and parents, as the fingerings I proposed on Jun 03, 2018 need only 5 transition shafts that can make concentric pairs. The sketch displays only two transition shafts acting each on two consequent covers arbitrarily spaced. Here the axles of the resultant covers are at the sides. The pairs of register keys are not displayed; articulated at the sides, they may be slightly simpler than on the Jan 14, 2018 mechanism. The rest is about as complicated, so silence, ease of adjustment, assembling and fabrication can decide. Marc Schaefer, aka Enthalpy
  13. Solar sails, bits and pieces

    More speeds and delays for the Mercury samples mission. Still not quite accurate. A cheaper launcher like Vega-C can put the probe on a 400km Earth orbit (or higher for the drag), preferibly polar 6h-18h. The sail then adds 7675m/s in spiral to escape in some lengthy 7 years. But if a mid-heavy launcher injects the probe in Venus transfer, the mission saves time. I have still no firm data about a Venus slingshot, so I take arbitrary 1.5km/s backwards from Venus' speed. The resulting trajectory is elliptic (but Mercury's orbit is too) with 0.66AU mean radius, from where the sail brakes to mean 0.39AU in only 1.8 year. The flyby can even more usefully tilt the orbit to Mercury's plane. This estimate in inaccurate, but 4 years saved on each leg is a lot - or the mission can be heavier and land on more sites. I had computed an elliptic capture at Mercury, but the probe will rather spiral down. Twice the Delta-V but for 2/3 of the time, same duration hence. A 1100kg D=203m sail isn't agile (2*106kg*m2), even at Mercury (30.4µPa incoming pressure). Two 100m2 on-off control surfaces at 45° provide 0.2N*m. The peak angular acceleration is 100nrad/s2, so a sine oscillation of peak pi/4 takes a period of 5h (and 32h at Earth). Compare with 2h orbit around Mercury: the sail can only rotate regularly around itself, and the controls steer slowly this rotation or drift the orbital plane to follow the Sun's direction. On a 6h/18h orbit, the sail can stay at 45° to sunlight. Or the sail can rotate by 180° per equatorial orbit to push flat away from the Sun at one point and show its profile at the opposite point. Consequently, don't expect the sail to cast shadow on the bus. The Sun's tidal force is strong on Mercury orbit and might let a sail's adequate orbital plane rotate to catch the prospector rising back from the surface. Or not. I didn't check. From a Venus slingshot, the probe would reenter Earth's atmosphere at 11.5km/s while a spiral transfer without slingshot (+4 years) lets brake the probe to 7.9km/s around Earth (+7 years). The lighter heat shield doesn't justify it to my eyes. Marc Schaefer, aka Enthalpy
  14. Woodwind Fingerings

    In the automatic cross-fingering clarinet system I proposed here on Jan 07, 2018, opening the left index cover is already a closed-to-open transition: with an implicit direct hole just above, always closed. This transition does open consequent holes for the 5+3 and 9+5 modes. Some fingerings in the upper first 12th open the left index cover, on Jan 07 as on May 10. This would open the two highest consequent covers, contrary to both sketches, and emit the wrong note. In one possible correction, an added register button closes only the two highest consequent covers and serves at the low end of the upper first 12th. This button must be accessible together with the low F/C button. Marc Schaefer, aka Enthalpy
  15. Solar sails, bits and pieces

    Bringing samples back from Mercury looks feasible with the sizes and masses described here on Aug 25, 2013 and followings. A sail of 10 sectors and 3.25hm2 tilted by 45° to the Sun's direction pushes 105mN at 1AU (Sun-Earth distance). Weighing 1100kg plus 1300kg for the bus and the science, it accelerates by 44µm/s2 here. Starting just escaped from Earth, with 29.8km/s speed around the Sun, it takes 5.5 years to spiral in the ecliptic plane to 0.387AU, the mean Sun-Mercury distance. Mercury's orbit is tilted by 7° so its 47.4km/s have a 5.8km/s polar component. Combining this quadratically with half of the 17.6km/s spiral delta-V result in 19.3km/s need, or 10% more, lengthening the travel to 6.0 years. Mercury's elliptic orbit needs little performance more. The return leg takes as long. The launcher injecting the craft towards Venus for a braking slingshot there would save much time, with the sail operating only near the Sun. Same for the return path. I won't detail this option now; later maybe perhaps, or not. Capture by Mercury and orbit lowering is trivial. A low orbit with 4km/s escape and 2830m/s orbital speeds takes 1170m/s to join. 290µm/s2 at 0.387AU acting 1/3 of the time achieve it in 138 days. There, a 900kg prospector (or several smaller ones if possible) separates and lands on the night side. It gets electricity from liquid oxygen, hydrogen and fuel cells. 100W for 50 days (less than 88 days, one local night) at 50% power efficiency consume 64kg propellants. 100W shall feed the experiments and sampling activities, transmissions and housekeeping, and is enough to keep the probe lukewarm. The prospector uses also oxygen and hydrogen to descend to the surface and to climb to the orbit, all from the same superinsulated balloon tanks described elsewhere. Details later maybe, or not. Mercury rotates by 1 turn in 58.6 days versus the distant stars, so in 50 days it turns by 180-26.4°. The prospector can't regain the same orbital plane unless we restrict the landing site to the equator. Though, the orbiting sail can change its plane meanwhile: turning 2830m/s by 26.4° costs 1305m/s which, at 470µm/s2 (900kg less) acting 1/3 of the time, is achieved in 96 days. The prospector shall wait in orbit before descending or after climbing. Raising the apoapsis for the manoeuvre and lowering it afterwards may speed it up. More orbit changes permit several landing sites at different longitudes. After the symmetric return leg, a capsule brings the samples to Earth's surface. Marc Schaefer, aka Enthalpy