Enthalpy

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Enthalpy last won the day on March 29 2015

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About Enthalpy

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  1. Solar Thermal Rocket

    http://www.scienceforums.net/topic/76627-solar-thermal-rocket/?do=findComment&comment=1007124 Where did you see such a thing? A strong electric engine? Why?
  2. On Mars, oxygen would be produced from the CO2 atmosphere, for the crew and maybe the engines. Demonstrators exist already that electrolyse CO2 in a hot ceramic. Trials on the Moon would be too different. Short stay: your choice, your good right. Methane can't be liquid above 191K, whatever the pressure. It demands active cooling on the too warm Mars. Storing gases is excluded because the tanks are too heavy for space travel. Even if burying in the chilly Martian soil, some fuels like RP-1 "kerosene" would freeze; concentrating sunlight is a better option, and low-freezing fuels a safer one. The density of rocket propellants isn't a primary concern. Hydrogen serves on most launchers, even at the first stage on Delta. Produce methane on Mars: smart people for sure, but nevertheless I say it's a bad idea. Very difficult, and bringing a bit more hydrogen is much simpler, safer, probably lighter. Remember hydrogen must be brought and stored to produce methane anyway. Glider and escape: have a look at the IXV. Put it atop a launcher without fairing but with an escape rocket on top. Accurate landing: a mission to Mars was lost recently because of inaccuracy. 18 successes on Earth prove less than 95% reliability for that single failure cause. "We landed the crew too far from the return vessel and can't send them anything timely" would be disastrous. Zubrin's team foresees a rover but neglects the lack of highways and the insufficient radiation shield. Other companies exist outside SpaceX, with engines for varied fuels. The RD-170 pushes 8MN with RG-1. Methane is negligibly more efficient but much more dangerous, while RP-1 doesn't catch fire with a lighter. And please forget ammonia, it is t-o-x-i-c, didn't you know? "If you're saying that chemical rockets are more efficient than solar thermal for LEO to staging area": No, I didn't.
  3. Alternating electrodeposition...

    Hi both, It is done for nickel-cobalt alloys. These elements have very close properties, including the redox potential. Instead of pulses, I'd use currents, which can be regulated exactly too. I had hoped, but never tried, to run the multiple-sources deposition for long enough that the bath gets richer in the metal more difficult to deposit due to the difference in redox potential. At some point, and if the metals aren't too different so some equilibrium is reached, the proportion of dissolved source must impose the proportion of the deposition. I'd have put the desired target only then. Though, I've heard only of Ni-Co, not even Ni-Cu, so the process can't be easy. What does exist is a deposited layer that incorporates other elements from the bath, and not only metals. Contractors can embed particles of Ptfe in Ni for instance.
  4. GaPSi

    Hello dear friends! Heterojunctions are produced by deposition (often epitaxy) of a semiconductor on an other to achieve excellent components. Silicon being the most commmon material, Si1-xGex is used as a material with a different bandgap, despite the drawbacks of germanium: lattice constant very mismatched, main electron valley in <111> direction versus <100>... As opposed, the GaP crystal is known to resemble Si closely: same zincblende lattice (called diamond when the atoms are identical), lattice constants matched to 500ppm, main electron valley in <100> direction too. GaP's bandgap is also bigger and differs more from Si, nice. http://www.ioffe.ru/SVA/NSM/Semicond/Si/index.html http://www.ioffe.ru/SVA/NSM/Semicond/GaP/index.html http://www.ioffe.ru/SVA/NSM/Semicond/Ge/index.html So it is seducing to produce epitaxies of GaxPxSi1-2x on Si. The less distorted crystal should conduct better, and the valley in <100> direction should accept a bigger proportion x. Since these heteroepitaxies aren't common up to now, many difficulties must arise; I try to address one here, the exactly equal amount of Ga and P. It won't happen naturally because Si crystals readily host the small P atoms but not Ga, and the resulting doping in Si must be so huge that no component can be built. My suggestion is to use an already formed GaP single-crystal as an epitaxy source. Semiconducting GaP is exactly balanced, perfect for a usable epitaxial layer. I imagine something like a short-pulsed laser could evaporate GaP to keep the proportion, and evaporate Si from an other source, for simultaneous deposition on the epitaxy target. The GaP source may evaporate one element more easily at the beginning, despite the laser pulse atomizes a volume of solid with almost no selectivity. The initial surface of GaP may also be less clean. The answer is to have a shutter and begin the deposition some time after beginning the evaporation. If the evaporated P and heavier Ga atoms fly to the epitaxy target for 100µs and 150µs (as an example) after the laser pulse, the epitaxy composition may fluctuate over the time hence the depth. The answer is a pulse repetition rate much faster than said 100µs. As 100µs flight time may spread as a 50µs Gaussian, 50ns repetition rate will smooth the distribution to far better than 1ppm. The evaporation rate of Ga and P differ at the epitaxy target, which shouldn't hence be too hot. Well, maybe. Marc Schaefer, aka Enthalpy
  5. Solar Thermal Rocket

    To accelerate a craft for weeks, the sunheat engine uses an energy amount impossible to store. But to raise or lower an apoapsis, escape a celestial body or get captured, the smaller energy for short kicks can be obtained during idle time and stored, as already noted on Jun 10, 2014: http://www.scienceforums.net/topic/76627-solar-thermal-rocket/?do=findComment&comment=810218 More details now. ========== Lithium-polymer batteries can store 475kJ/kg more or less. Before a kick, the concentrated light can be split by wavelength and sent to small solar cells of varied bandgaps for >40% conversion. http://www.scienceforums.net/topic/76627-solar-thermal-rocket/?do=findComment&comment=826983 Other uses welcome the efficiency and collecting area far from the Sun. To raise the apoapsis in Earth orbit, each sunheat engine without storage consumes 20kW during 1200s. A battery providing as much energy to double the force weighs 50kg. That's worse than adding one ~30kg engine. But: Fewer engines are easier to deploy. Operation during an eclipse season gains time and provides flexibility. Electric pumps for the propellants making the escape chemical kick justify 100-150kg batteries if starting with 18.5t at Leo, so 8 sunheat engines get 30% more energy per kick at zero extra mass, and the added thrust can concentrate on the best 1000s. At Mars (1.52AU) the battery is already lighter than the additional engine, at Saturn (9.58AU) the battery advantage would be 50*, if 1200s kicks were meaningful there. Other uses cherish the good electricity production at the outer planets. 700W from 8 concentrators at Saturn. The combination isn't a resistojet, because direct heating by sunlight is kept, as it avoids the wasteful conversion to electricity. Far better for the long pushes. The engine can use the same chamber for both modes or not. The described regenerative insulation methods apply. ========== Melting a metal stores heat too. Simpler than making electricity first, but heavier, and they probably dissolve their tungsten container. Tmelt Hmelt K kJ/kg ------------------- 3290 Ta 159 2896 Mo 286 ? 2750 Nb 288 2506 Hf 143 ------------------- One hope is to find some eutectic of tungsten with one or several other elements giving the desired melting point, as inspired by Sn-Pb-Ag solder not dissolving Ag. I didn't find credible data about Hf-W nor Nb-W eutectics, only calculated data; Mo-W and Ta-W seem to make no eutectic and be fully miscible. ========== Melting a ceramic stores more heat. Beware data is inconsistent. BeO is toxic, and B is expected to corrode W. More complex formulas are possible. Mixes shall bring missing melting points. Tmelt Hmelt Hform K kJ/kg kJ/mol --------------------------- WO2 -285 Ta2O5 -409 NbO -406 --------------------------- 2988 ZrO2 706 -550 2915 MgO 1920 -602 2703 SrO 674 -592 2500 Y2O3 463 -635 2318 Al2O3 1071 -559 2247 ZnO 860 -351 2218 MnO 767 -385 --------------------------- The last column shows the heat of formation per mole of oxygen atoms. If the melt's metal binds more strongly with oxygen, it could be compatible with W; computing the chemical equilibrium would be better if data exists. For water propellant at a lower temperature, container candidates are Nb or better Ta, possibly with W coating on the melt side, and some ceramics. As heat storage keeps the engines hot for months, the temperature should rather be 2700K or 2600K. With the ceramic fully molten, 2800K remains possible during week-long accelerations. MgO stores 4* more energy than a battery and is 2* lighter than an added engine for perigee kicks. The advantage increases at the outer planets. ========== The freight transport to the Moon took 10 months of perigee kicks with 10 sunheat engines http://www.scienceforums.net/topic/76627-solar-thermal-rocket/?do=findComment&comment=1007124 Let's replace 4 sunheat engines by 120kg of MgO heat storage: the 1.55* stronger kicks gain 3.5 months, and thanks to the shorter kicks' efficiency, we land 2% more mass. Marc Schaefer, aka Enthalpy
  6. Well, there are many possible options (plus the ones still not thought at), and even more combinations. I doubt that propellants produced on the Moon and brought to Earth-Moon Lagrange point are any cheaper than if brought from Earth. And until I see a convincing proposal, I consider all cannons and variants are unusable. You had initially the intention to evaluate a short stay mission. Its big delta-V constraints everything else, so you might begin with these figures and their implications on the propellant mass and so on. The speeds differ a lot from the Hohmann transfers illustrated in the second post. Please note that my xls supposes no flyby at Venus, but most scenarios seem to gain from it. And... I don't see any advantage in a short-stay mission, while the drawbacks are huge. Are you the last person putting time in it? Methane and oxygen need active cooling to store in the Martian atmosphere, and I suppose in Earth and Mars orbit too. Once you have active cooling (a vital technology for space exploration, we should have had it for decades, wake up!), hydrogen is possible too; it's only a matter of tank mass and cooler difficulty. Here's a well insulated tank http://www.scienceforums.net/topic/60359-extruded-rocket-structure/?do=findComment&comment=761740 But if sticking to dense fuels, some are about as efficient as methane, safe on Earth and won't freeze on Mars http://www.chemicalforums.com/index.php?topic=56069.msg297847#msg297847 http://www.chemicalforums.com/index.php?topic=56069.msg272080#msg272080 (images only if logged in) while the toxic oxidizer Mon-30 is storable on Mars. Aerobraking brings an awful lot, both at Mars and Earth. You must decide if aerobraking from the transfer speed or from orbit. If aerobraking from a fast transfer, you need wings to achieve downlift so the vessel stays in the atmosphere long enough to permit a bearable deceleration. It's probably not compatible with sunlight concentrators, so some choices are hard. But it has already been combined with a capture by Mars rather than landing. In my scenario, the crewed vessel aerobrakes, the preset modules don't. And you must decide whether the capsule or plane serves again at Earth. If presetting hardware at Mars, you must decide whether on orbit or on the ground. Meeting on the ground suppose to land accurately, which is a risk, while orbiting hardware can be redundant and landed where and if needed. My opinion is very clear. If producing propellants on Mars, you should check what power it takes over how long just to make oxygen. Ouch. Solar energy would need many big concentrators. And I consider methane is far too difficult and risky, while a little bit more hydrogen can be brought from Earth and consumed at the engines. Much safer, probably lighter than the methane plant. http://saposjoint.net/Forum/viewtopic.php?f=21&t=1953#p36418 If meeting at Lagrange point (why? Isn't an elliptical Earth orbit better?) I'd bring much mass there by the craft with sunheat engine and little by the vessel with chemical engine: the astronauts, their radiation shield, their beds and suits, water and life support - but probably not propellants. And I have nothing against the Raptor, but it may be oversized for Earth escape, whose kick can last 600s: compute with the consumption rate. I found a few RL10 are strong enough, they burn the same hydrogen as the sunheat engine and offer a better ejection speed than methane. A chemical engine is more efficient to escape Earth (it gains like 20-30%). I've evaluated the best share there http://www.scienceforums.net/topic/76627-solar-thermal-rocket/?do=findComment&comment=818683 which you can compare with the sunheat engine alone there http://www.scienceforums.net/topic/76627-solar-thermal-rocket/?do=findComment&comment=1009859 it gains 20-30%, interesting. The same holds at Mars! You choose if you use the same engines at Mars. And for a Hohmann Earth-Mars, the optimum is "nearly all' the transfer speed by the chemical engine, which translates to "all" since the optimum is wide, the sunheat engine only raising or sinking the apoapses. But an accelerated or a lowered perihelion transfer does use the sunheat engine during the transfer.
  7. I've re-read my documents, and the sunheat engine raises and lowers apoapses with kicks at periapses. This is more efficient than spiralling, but longer. In my scenario, the manned vessels escape Earth or Mars by chemical propulsion (oxygen and hydrogen) to go fast, they accelerate by sunheat engines, and aerobrake at both destinations. So the transfer differs from presetting the return vessels and the descent-ascent modules on Martian orbit, which is slow. The manned vessel meets the preset hardware on Martian orbit. I don't use any Lagrange points nor in-situ propellants. My scenario depends on no prior activity on Mars nor on landing at an accurate location, and offers redundancy. Maybe other scenarios save some launch mass, but check the risks too. If planning to use propellants produced on Mars or the Moon, consider burning hydrogen brought from Earth and producing only oxygen locally. It's almost as efficient as producing methane if the yield were 100%, and it's way simpler. No overcomplicated heavy plant brought to Mars. Electrolysis of dioxide in hot ceramic produces the oxygen, proven technology. http://saposjoint.net/Forum/viewtopic.php?f=21&t=1953#p36418 obvious choice to my eyes. ---------- Hydrogen versus methane: the transit time improves very slowly with the delta-V and the specific impulse. It's better to save mass instead. Aerobraking, sure. Even more so if the transfers are accelerated. Transfer times: the figures by SpaceX are surprising. They may correspond to very favourable configurations of Earth and Mars. Anyway, don't compare them with other estimates that base on circular orbits. It would be wise to run a computation with SpaceX' figures, to check by how much they cheated optimized their example. Please keep in mind that the announcement by SpaceX was part of a show. Nobody knows outside the company if they're serious about a Mars mission, nor whether the Raptor engine is meant for the BFR, or rather for a smaller launcher - nor how much soot plagues their gas generator at the Raptor. For Tesla's gigafactory, Musk had said "car batteries" but meanwhile he sells them for home and grid storage. About landing the Falcon 9's first stage on a barge, which is an overclever enabling solution, they didn't tell in advance, and once everyone saw it, they told "temporary solution".
  8. Manned Mars Mission

    And because the attached Xls have been lost here, I upload them again. EarthCargoDeparture.xls MarsCargoArrival.xls
  9. Non-Hohmann to Mars

    The lowered perihelion transfer for a "short stay mission" is long known, also as "opposition-class" mission type. It is considered for instance in 2009 in Nasa's NASA/SP–2009–566 "Human Exploration of Mars, Design Reference Architecture 5.0" https://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf Because a short stay gathers less science and needs too long transfers (rays, microgravity) in wasn't favoured in 2009. Meanwhile, sensors on Mars' surface have shown that rays are rather benign there, and the astronauts can shield their house anyway http://www.scienceforums.net/topic/83289-manned-mars-mission/?do=findComment&comment=895610 so the long stay is a clear choice now. My corresponding spreadsheet here is NonHoMarsRadiusSlice5. ---------- And because the Xls are lost here, I upload them again. NonHoMarsRadiusSlice5.xls NonHoMarsInwardsOpposition2.xls
  10. Solar Thermal Rocket

    Electric thrusters need so much power that solar panels feed only a faint push. Operations near a planet, which takes stronger engines, lead to use a nuclear reactor which can't be very light and needs a big radiator. The cancelled Jupiter Icy Moons Orbiter Jimo (not the Explorer Jime) was such a project, and this drawing (thanks to Nasa) illustrates the oversized feeder for the electric engine: The reactor is at the tip, the "wing" is the 422m2 heat radiator, and from https://en.wikipedia.org/wiki/Jupiter_Icy_Moons_Orbiter the science payload orbiting Europa, Ganymede and Callisto would have been 1500kg, similar to what the sunheat engine promises - but Jimo would have needed chemical engines to leave Earth, so despite the electric thruster's higher Isp, it would have weighed 36t in Earth orbit launched in three heavy flights and cost 16G$.
  11. mini motor

    Yes it's possible, sure. For instance the turbocharger of a piston engine is about half as powerful as the piston engine and is very small. A gas turbine alone would only be twice as big as the turbocharger. If electric engines are allowed to run as quickly as a gas turbine, then they're even a bit smaller then them. You can check this at a turbo-alternator: https://en.wikipedia.org/wiki/File:Turbogenerator01.jpg the three yellow casings host three pairs of low-pressure vapour turbines, the high pressure turbines are before, and the alternator (of same power necessarily) is in the small red casing at the rear. Then, you may ask if an electric motor running at 100m/s or more makes a convenient transmission to the wheels. Helicopters have such a gear. Intermediate situations exist. Electric cars have presently powerful motors that rotate not so fast and are typically located at the wheels. Find some drawings.
  12. Gyroscopic contraption...

    Hi Externet! Identical parallel wheels, same angular speed but opposite direction: no global torque is required to topple the frame BUT each wheel creates a strong torque, so everything but be sturdy enough. Two wheel axes perpendicular: nothing special, the net torque is the sum of both. The gyroscopic torque is the vector product or the wheel rotation (put inertia moments if you wish, I don't care presently) with the toppling rotation, hence perpendicular to both axes. For instance if the toppling axis is perpendicular to both wheel axes, you get two gyroscopic torques which are perpendicular to an other and add vectorially. Three wheel axes perpendicular: nothing special again. For instance if the toppling axis is parallel to one wheel axis, this wheel creates no gyroscopic torque, and the two other wheels do like in the previous case.
  13. Can an EMP Reach Ocean Floor

    And they're made up of fibre optics.
  14. Extreme high temperature insulation materials

    No material withstands 8500°C. Either the plasma radiates little enough, is far enough, has a density small enough... and the walls have naturally a much lower temperature, or you'll have to cool the walls actively, whether you like it or not. Then, you must check if the plasma remain so hot despite the contact with the walls.
  15. Quasi Sine Generator

    Hello you all! Here's a means to produce a sine wave voltage, very pure, with metrologic amplitude, whose frequency can be varied over 2+ octaves in the audio range - this combination may serve from time to time. It uses sums of square waves with accurate shape and timeshift. A perfectly symmetric square wave has no even harmonics. Adding two squares shifted by T/6 suppresses all 3N hamonics as the delay puts them in opposition; this makes the waveform well-known for power electronics. Two of these waveforms can be added with T/10 shift to suppress all 5N harmonics, then two of the latter with T/14 shift, and so on. A filter removes the higher harmonics as needed. The operation makes sense, and may be preferred over direct digital synthesis, because components and proper circuits may provide superior performance. Counters produce accurate timings. If a fast output flip-flop outputs a zero 1ns earlier or later than a one, at 20kHz it leaves -90dBc of second harmonic and at 1kHz -116dBc, but at 1MHz less interesting -56dBc. If the propagation times of the output flip-flops match to 0.5ns, at 20kHz they leave -100dBc of third, fifth, seventh... harmonic and at 1kHz -126dBc. 74AC Cmos output buffers have usually less than 15ohm and 25ohm impedance at N and P side. On a 100kohm load, the output voltage equals the power supply to +0 -200ppm. 5ohm impedance mismatch contributes -102dBc to the third harmonic, less at higher ones. Common resistor networks achieve practically identical temperatures and guarantee 100ppm matching, but measures give rather 20ppm. This contributes -110dBc to the third harmonic, less at higher ones. This diagram example would fit 74AC circuits. Programmable logic, Asic... reduce the package count and may use an adapted diagram. To suppress here the harmonics multiple of 2, 3, 5 and 7, it uses 8 Cmos outputs and resistors. As 3 divides 9, the first unsqueezed harmonic is the 11th. A counter by 210 has complementary outputs so that sending the proper subsets to 8-input gates lets RS flip-flops change their state at adequate moment. Programmable logic may prefer GT, LE comparators and no RS. I would not run parallel counters by 6, 5 and 7 instead of 210 as these would inject harmonics. The RS flip-flops need strong and fast outputs. Adding an octuple D flip-flop is reasonable, more so with programmable logic. I feel paramount that the output flip-flops have their own regulated and filtered power supplies, for instance +-2.5V, and the other logic circuits separated supplies like +-2.5V not touching the analog ground. That's a reason to add an octuple D flip-flop to a programmable logic chip. For metrologic amplitude, the output supplies must be adjusted. All the output flip-flops must share the same power supplies, unless the voltages are identical to 50ppm of course. A fixed filter can remove the higher harmonics if the fundamental varies by less than 11 minus margin, and a tracking filter for wider tuning is easy as its cutoff frequency is uncritical. The filter must begin with passive components due to the slew rate, and must use reasonably linear components. ---------- I tried almost three decades ago the circuit squeezing up to the fifth harmonic, and it works as expected. Squeezing up to the third is even simpler, with a Johnson counter by 6 and two resistors. Measuring the spectrum isn't trivial, for instance Fft spectrometers can't do it; most analog spectrometers need help by a linear high-pass filter that attenuates the fundamental. Marc Schaefer, aka Enthalpy