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#22
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VASMIR plasma engine's power source?
Ian Stirling wrote in message ...
Alex Terrell wrote: (Allen Thomson) wrote in message . com... (Henry Spencer) wrote The usual assumption is that power would be from a reactor. It just about has to be. If you look around for relevant numbers, you can find things like, Why? Out as far as Mars, solar power is ligher than nuclear power for a given power level. Why go nuclear? A look at http://www.entechsolar.com/SpacePaper4.pdf may be informative for those that have not been keeping up with the state of the art (it surprised me). 183W/Kg (at one sun) from a solar panel with silicone lenses. I don't know what nuclear can do. I've seen a figure of 30 tons per MW, which is about 1/6th of the performanve you cite. |
#23
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VASMIR plasma engine's power source?
John Schilling wrote:
Sander Vesik writes: Ian Stirling wrote: Joseph S. Powell, III wrote: I was just glancing at some sites mentioning VASMIR plasma engines, and was wondering if the power source was nuclear (reactor or RTG?) or solar or some other source? Whatever can deliver enough power, and can be approved for launch. RTGs deliver some 400W/Kg (of radioisotope) of thermal power (80W/ Solar arrays can achieve some 40W/Kg of electrical power. http://www.entechsolar.com/SpacePaper4.pdf reports on a solar array that does some 180Wk/g. cells giving you 600W/kg exist - they use aluminium foil as substrate - and designs using say kapon as the substrate could get up to at least 2KW/kg. Sure, this will degrade over time in space and is distance sensitive. Be careful; solar *cells* are not solar *arrays*. To get power on a right, my bad. spacecraft, you need solar cells, interconnects, wiring harness, cover glass, a structure to tie it all together, slew mechanism to point it at the sun, and probably a power regulator. But are all of these - and all of these being heavy - really necessary? The glass covers of the solar panels are going to last much longer than the mission time for most missions. Same applies to the large rigid structure they are built in. In most cases, building solar panel structures that are going to last for centuries is utter waste - anything beyond 2x mission time really is. so instead of glass you could use slowly UV-degrading transparent plastic, unroll a fabric of the solar panel like a sail instead of having it be fixed on four sides, use plastics with UV-blocking materials for parts of the structure and so on. So the panel would look like something like this: || | || | || | |+-------------------------+ || | || | || | where the first double line is a roll of "fabric" at the start that will be unrolled towards the end. over which you would then unroll the fabric, attaching it at the 'beams' while unrolling using a cable/rope with attachments at the side of the fabric. The panel also doesn't need to handle extra stress during unfolding The lighter teh panel is the less power you need to unfold and turn it. 180 W/kg really would be quite good. 40 W/kg is typical. I believe the 180W/kg figure is for an inflattible solar panel (which would also not have a glass cover over the silicon, etc). -- Sander +++ Out of cheese error +++ |
#24
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VASMIR plasma engine's power source?
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#25
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VASMIR plasma engine's power source?
(Christopher M. Jones) wrote in message om...
(John Schilling) wrote in message ... Why go nuclear? Because VASIMR wants megawatts. You seem to be assuming that nuclear and solar power scale linearly, that a one-megawatt solar array or nuclear reactor will have a hundred times the mass of a ten-kilowatt solar array or nuclear reactor. This is not the case - solar power starts to have problems above ~100 kW, and nuclear at *less* than ~100 kW. Either could be made to work for a ~megawatt VASIMR, and solar might have to be made to work if the nucleophobes have too much inflience that year, but nuclear is clearly preferred. You could maybe do a sub-megawatt VASIMR demonstration with solar, but it would be a challenge. Why? As the other post points out, six times lighter than nuclear. And then you've got the question of where to go. Go to Mars and you drop your power by 1/2, there's no really good destination in the inner Solar System. Except maybe NEOs. Probably the best destination in the solar system, at least for the next 50 years. |
#26
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VASMIR plasma engine's power source?
In article ,
Sander Vesik wrote: spacecraft, you need solar cells, interconnects, wiring harness, cover glass, a structure to tie it all together, slew mechanism to point it at the sun, and probably a power regulator. But are all of these - and all of these being heavy - really necessary? You basically can't avoid having *some form* of most of those. The slew mechanism may be unnecessary if you point the whole spacecraft, but that's about the only optional item. The glass covers of the solar panels are going to last much longer than the mission time for most missions. Bear in mind that we're not talking about sheets of plate glass here. These are very thin layers of glass applied directly to the cells; half a millimeter is about the *thickest* used, and a tenth of that is not unheard-of. The main reason for the glass, by the way, is radiation protection for the cells. (Do note, also, that if you make the cell mounting material thin and flexible, you have to think about radiation shielding for the *backs* of the cells.) Same applies to the large rigid structure they are built in. Large solar arrays actually are often quite flexible. Great amounts of effort have been invested in trying to make these structures lightweight, which includes making them no more rigid than necessary. ...so instead of glass you could use slowly UV-degrading transparent plastic... Um, why exactly would this be an improvement? I don't see any reason why it would be any lighter. unroll a fabric of the solar panel like a sail instead of having it be fixed on four sides... Often done already. use plastics with UV-blocking materials for parts of the structure Composite structures are already in use. So the panel would look like something like this: || | || | || | |+-------------------------+ || | || | || | where the first double line is a roll of "fabric" at the start that will be unrolled towards the end. Ever seen pictures of the original Hubble solar arrays? People were doing exactly this sort of thing a quarter-century ago. -- MOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | |
#27
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VASMIR plasma engine's power source?
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#28
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VASMIR plasma engine's power source?
Sander Vesik writes:
John Schilling wrote: cells giving you 600W/kg exist - they use aluminium foil as substrate - and designs using say kapon as the substrate could get up to at least 2KW/kg. Sure, this will degrade over time in space and is distance sensitive. Be careful; solar *cells* are not solar *arrays*. To get power on a right, my bad. spacecraft, you need solar cells, interconnects, wiring harness, cover glass, a structure to tie it all together, slew mechanism to point it at the sun, and probably a power regulator. But are all of these - and all of these being heavy - really necessary? Pretty much, yes. The glass covers of the solar panels are going to last much longer than the mission time for most missions. Same applies to the large rigid structure they are built in. In most cases, building solar panel structures that are going to last for centuries is utter waste - anything beyond 2x mission time really is. Believe me, nobody is building solar arrays to last for centuries. The issue with solar array structure is not some gradual weakening or erosion of the structure, but dynamic failure. Either you've got enough rigidity and internal damping to keep the thing from shaking apart in its first few hours, or you don't. If you do, you've got something pretty close to the present state of the art in solar array design. And the cover glass, the issue there isn't how long the glass itself will last but whether it is thick enough to stop the deluge of energetic electrons and protons that would kill the solar cells. If you've got a cover glass that is half as thick as it needs to be to stop most of the radiation, yes, your cover glass will still last decades. But the solar cells will be dead in weeks to months depending on the environment. There is, at the margin, a mass vs. lifetime trade. But most of what can be eked out of that trade already has been, and it's going to take Extreme Cleverness to get any more substantial reductions in the mass of the support systems without killing the arrays in very short order. And people are working the Extreme Cleverness angle from several directions, but that's not the sort of thing you can count on paying off. -- *John Schilling * "Anything worth doing, * *Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" * *Chief Scientist & General Partner * -13th Rule of Acquisition * *White Elephant Research, LLC * "There is no substitute * * for success" * *661-951-9107 or 661-275-6795 * -58th Rule of Acquisition * |
#29
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VASMIR plasma engine's power source?
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