At one, the only contribution to the body drag coefficient is from skin friction.
I've been trying to figure out the best way to crack CO2 for CO/LOX in LMO. So far, the three best options I've arrived at are electrolysis, direct reduction via zirconia, and reverse water-gas shift. They are as follows:<snip>2. Direct reduction via zirconia... heat CO2 from -250C (ambient mars@100km, guesstimate) to 1100C, which dissociates ~30% of CO2 into CO + O2, electrochemically pump O2 across zirconia for collection. Heating requirement reduced by exchanging heat from hot CO/O2 output exhaust with cold CO2 input. -Advantages : relatively low power requirements -Disadvantages: zirconia tubes are brittle, have low output, requiring large numbers. least mature.
For reference, Daniel Eder (AKA danielravennest) on his big space infrastructure wikibook https://en.wikibooks.org/wiki/Space_Transport_and_Engineering_Methods/Resource_Extraction has a interesting design for a 400Km orbit type monolithic ramscoop. Notably, the solar panels remain in the collector frontal area drag shadow.
Quote from: Asteroza on 01/06/2015 10:20 pmFor reference, Daniel Eder (AKA danielravennest) on his big space infrastructure wikibook https://en.wikibooks.org/wiki/Space_Transport_and_Engineering_Methods/Resource_Extraction has a interesting design for a 400Km orbit type monolithic ramscoop. Notably, the solar panels remain in the collector frontal area drag shadow.I am surprised that they do not use a turbomolecular pump type design (crude looking fan rotor on front like original Star Trek Enterprise) to prevent incident gas from exiting the inlet (1 stage of turbomolecular pump can have a pressure ratio of >100). Could be made from aluminium foil on a magnetic bearing, almost zero power consumption (rotation is at least partially driven by incident gas molecules).With orbital speed ~7.5km/s and thermosphere gas molecular velocities of probably 300-500m/s average the flanks would need to be tapered at about 3-4° to prevent molecules hitting the sides of the scoop ie length to width of only about 6. That inevitably limits the available surface area for solar panels - making beamed power, tether power or nukes very attractive.It would be possible to add trailing scoops in wake of primary one to collect any laterally incident molecules (you are already paying drag price for them hitting as I would intuit that they bounce off at speeds of only a few hundred m/s) so that would increase area available for solar panels to an almost arbitrary degree by reducing their drag penalty.Another problem is that a long skinny scoop design is unstable in pitch due to tidal forces. A pair of scoops coupled together by a vertical tether so that they fly in formation a few hundred vertical meters apart would be a nice solution to do alignment, these tethers could be tiny in cross section, and could maybe also be used for propulsion. But the turbomolecular pump approach can also greatly shorten the scoop. Alternatively a vertical array of scoops could work.
Just doing some thread necro... Quote from: Hershey on 09/01/2013 02:31 amI've been trying to figure out the best way to crack CO2 for CO/LOX in LMO. So far, the three best options I've arrived at are electrolysis, direct reduction via zirconia, and reverse water-gas shift. They are as follows:<snip>2. Direct reduction via zirconia... heat CO2 from -250C (ambient mars@100km, guesstimate) to 1100C, which dissociates ~30% of CO2 into CO + O2, electrochemically pump O2 across zirconia for collection. Heating requirement reduced by exchanging heat from hot CO/O2 output exhaust with cold CO2 input. -Advantages : relatively low power requirements -Disadvantages: zirconia tubes are brittle, have low output, requiring large numbers. least mature.I'm surprised to see that "ambient" temperature mentioned. I had expected that gas collected via collisions at multiple km/s would end up at high temperatures at the throat of the nozzle. A craft doing entry needs a massive heatshield to cope with high temperatures from gas collisions.PROFAC will operate at similar orbital velocities, just at lower pressures. ISTM the collected gas will be heated via the same effect? Is the nozzle that efficient at bleeding away collision energy that the gas ends up very cold? Cheers, Martin PS can that -250C really be correct? That's apparently colder than the surface of Pluto out around 40 AU.
Quote from: RobLynn on 01/07/2015 10:48 amQuote from: Asteroza on 01/06/2015 10:20 pmFor reference, Daniel Eder (AKA danielravennest) on his big space infrastructure wikibook https://en.wikibooks.org/wiki/Space_Transport_and_Engineering_Methods/Resource_Extraction has a interesting design for a 400Km orbit type monolithic ramscoop. Notably, the solar panels remain in the collector frontal area drag shadow.I am surprised that they do not use a turbomolecular pump type design (crude looking fan rotor on front like original Star Trek Enterprise) to prevent incident gas from exiting the inlet (1 stage of turbomolecular pump can have a pressure ratio of >100). Could be made from aluminium foil on a magnetic bearing, almost zero power consumption (rotation is at least partially driven by incident gas molecules).With orbital speed ~7.5km/s and thermosphere gas molecular velocities of probably 300-500m/s average the flanks would need to be tapered at about 3-4° to prevent molecules hitting the sides of the scoop ie length to width of only about 6. That inevitably limits the available surface area for solar panels - making beamed power, tether power or nukes very attractive.It would be possible to add trailing scoops in wake of primary one to collect any laterally incident molecules (you are already paying drag price for them hitting as I would intuit that they bounce off at speeds of only a few hundred m/s) so that would increase area available for solar panels to an almost arbitrary degree by reducing their drag penalty.Another problem is that a long skinny scoop design is unstable in pitch due to tidal forces. A pair of scoops coupled together by a vertical tether so that they fly in formation a few hundred vertical meters apart would be a nice solution to do alignment, these tethers could be tiny in cross section, and could maybe also be used for propulsion. But the turbomolecular pump approach can also greatly shorten the scoop. Alternatively a vertical array of scoops could work.Turbomolecular pumps have low efficiency compared to the theoretical minimum needed to do the compression. That imposes a major penalty on designs that use these. I also imagine that the gas cooling will involve a lot of energy. However, electrostatic propulsion without any kind of confinement or compression of the gas also has huge problems (I can't remember specifically right now). The most promising looked to be the plasma confinement, where the gases are mostly self-compressed, they are not cooled, and they are heated by microwave to get good specific impulse from your thruster. The funnel idea also seems to be bunk. The logic is that these could work, but only below the Karman line. Otherwise the densities are too low. The air molecules are primarily not hitting each other. Unless you can get them to a high compression ratio just by wall ricochet geometry, you'll have almost all molecules bounce back out. That's why the promising proposal is a honeycomb series of tubes oriented in the direction of motion. The high degree of collimation makes it easy to get in, but after a single collision the angles are random and it's very hard to get back out because there's only a few degree window.I strongly agree with the statement that "flanks would need to be tapered at about 3-4°". In fact, I would like to see analysis of this done, and I'm thinking about returning to that myself. You have the Gaussian thermal angular distribution, and with this you can find the fraction of molecules that would hit the flank at any degree of tapering. With that angle, you can find an acceptable ratio of solar panel area to collector area, which then gives the power per area you can get, which then gives the altitude you can operate at. That "triangular" collector seems to be the best alternative to the tethered concept.Stability should be dominated by aerodynamic forces (at least for the altitudes I have in mind), not tidal forces. That's a big difference between VLEO and LEO. In the triangular design, I'm mildly hopeful because any rotation will expose a surface to more air pressure, which would give a stabilizing torque. However, this depends on the relative locations of the center of mass versus the center of pressure. If the heavy equipment was all located near the mouth (at the front), then the outlook for stability looks good.Heck, you could even have control surfaces, making the concept weirdly resemble aircraft while being totally not at all the same physical thing. The problem is that these surfaces would impose a major energetic penalty, but they could still be used for quick attitude adjustments. You would just need some way to either fold them up when not in use, or have them integral to the sides of the craft.
Turbomolecular pumps have low efficiency compared to the theoretical minimum needed to do the compression. That imposes a major penalty on designs that use these. I also imagine that the gas cooling will involve a lot of energy. However, electrostatic propulsion without any kind of confinement or compression of the gas also has huge problems (I can't remember specifically right now). The most promising looked to be the plasma confinement, where the gases are mostly self-compressed, they are not cooled, and they are heated by microwave to get good specific impulse from your thruster. The funnel idea also seems to be bunk. The logic is that these could work, but only below the Karman line. Otherwise the densities are too low. The air molecules are primarily not hitting each other. Unless you can get them to a high compression ratio just by wall ricochet geometry, you'll have almost all molecules bounce back out. That's why the promising proposal is a honeycomb series of tubes oriented in the direction of motion. The high degree of collimation makes it easy to get in, but after a single collision the angles are random and it's very hard to get back out because there's only a few degree window.
Uninhabited Aerial Vehicle (UAV) and drone options may use nuclear ramjets or rockets..Let’s go to the stars, as quickly as possible.
Quote from: AlanSE on 01/07/2015 02:49 pmTurbomolecular pumps have low efficiency compared to the theoretical minimum needed to do the compression. That imposes a major penalty on designs that use these. I also imagine that the gas cooling will involve a lot of energy. However, electrostatic propulsion without any kind of confinement or compression of the gas also has huge problems (I can't remember specifically right now). The most promising looked to be the plasma confinement, where the gases are mostly self-compressed, they are not cooled, and they are heated by microwave to get good specific impulse from your thruster. The funnel idea also seems to be bunk. The logic is that these could work, but only below the Karman line. Otherwise the densities are too low. The air molecules are primarily not hitting each other. Unless you can get them to a high compression ratio just by wall ricochet geometry, you'll have almost all molecules bounce back out. That's why the promising proposal is a honeycomb series of tubes oriented in the direction of motion. The high degree of collimation makes it easy to get in, but after a single collision the angles are random and it's very hard to get back out because there's only a few degree window.Unless there is a sufficiently large flux of incident gas molecules to collide with and push gas molecules inwards I don't see how the honeycomb collimation would work to prevent molecules leaking out (unless mean free path << honeycomb length). My guess is would depend on the ambient gas density and would only work below some cutoff altitude. But as already established the power requirements per frontal area set limits on how low you can operate - and solar panels cannot provide enough power below a certain altitude.I think turbomolecular pump inlets could work at any altitude (particularly higher orbits with lower densities) - providing a definite one-way-valve effect at inlet.http://www.pfeiffer-vacuum.com/know-how/vacuum-generation/turbomolecular-pumps/design-operating-principle/turbomolecular-pump-operating-principle/technology.action?chapter=tec2.8.1.1I can see how efficiency of the turbo molecular pump is an issue (though I would guess it increases with blade speed and so stage pressure ratio), but at least it is possible to make an operable solution as can raise the altitude until the panel area is sufficient for the inlet area, and tech is reasonably conventional - perhaps even off-the-shelf.
...rotovator atmospheric collection. I like it.BTW, Jon Goff, isn't there some nice closed-form equation for the specific strength needed for a rotating tether of a certain velocity?
The rotovator air scoop appears to address some difficulties with hypersonic heating and parasitic drag. The system still needs thrust to accelerate the collected air into orbit.