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Post by airc777 on Jul 27, 2019 15:22:57 GMT
...for producing electricity at the target, not thrust directly.
It's occurred to me that I didn't know how this process is actually done when I was trying to calculate efficiency losses.
Original context was a hypothetical carbon neutral commercial aircraft via a method of a geostationary solar power plant and laser beamed power and electric ducted fans. When doing the math I didn't know how best to get the power out of the laser while preserving the trust to mass ratio of the system. I assume it's too much energy density to use photovoltaic receivers, and too mass inefficient to use a target thermal mass attached to a Stirling engine.
Would high directivity microwaves be better for this application then lasers? My assumption was that the microwave receiver target area would be just entirely too big to be used to power a single aircraft that was intended to transport passengers and potentially fly over populated areas.
It's also occured to me that there are just too many steps in the above described process and it would probably be better to have the aircraft propelled via a thermal-ramjet. Then you could also probably use the engine housing as your target thermal mass with some heat pipes equalizing temperature around the combustion chamber. (Is that the right world to describe the inside of a thermal-ramjet?) Because it's a ramjet it would need to be accelerated up to speed to function, my proposed carbon neutral solution to that was just massive linear actuator catapults in the runway.
Unrelated: but can we get some beamed laser resistojet missiles in CoaDE? I suspect it could help my laserstars time-to-kill after the targets ciws has been disabled.
Second edit: It's occurred to me that 'thermal-ramjet' was a more accurate descriptor then 'resisto-ramjet'.
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Post by luxardens on Nov 23, 2019 23:01:59 GMT
Wel, they all have some advantages and disadvantages.
The photovoltaic cells have the distinct advantage that you can pick a spectral range for your space-based emitter and a fitting cell to get relatively high efficiency (Watt electricity per Watt of light absorbed) when compared to say, a photovoltaic cell that has to absorb sunlight (because sunlight is a very broad spectrum). This also means you can stick to cells with only one p-n junction, without losing efficiency, which is cheap.
Also, the limit on what some photovoltaics can take before dying from the thermal effects is very appreciable. I've seen numbers ranging up to hundreds and even thousands (!) of times the irradiance of sunlight near Earth (1 KW/m^2). Now the normal near-earth irradiance you get from sunlight is not enough to power any useful aircraft, but with 10 or even 1,000 KW/m^2 you could theoretically design something useful for sure.
A thermal engine would have the advantage of even higher maximum irradiance, since often it's just a mirror focusing light to heat/melt something. But a thermal engine is always going to contain a shitton of parts and is therefore likely to be heavier than just photovoltaic cells and electric engines. It also has the distinct advantage of being able to utilize a broad spectrum of EM. I think this is the more 'reasonable' concept, because the space station could redirect and focus sunlight to the plane instead of having to convert it into electricity first, and then into a laser beam. That would limit your plane's mirror to a 'max temperature' of ~5800K (because thermodynamics) but
that's still very decent.
The disadvantage is mostly in the emission and transmission. Assuming that the station power comes extremely cheap, and you effectively have Near-Infinite Station-Beamed Power (NISBP) (yes I just made that up), then that's not as much of a concern, but if it's limited... well, you either have to direct laser beams directly towards the craft, or more broadly illuminate the entire area. I don't think lighting up the entire area around it is feasible, because you lose a shitton of power to nothing and anyone standing underneath the airplane looking up will have his retinas burnt out (or have his hair set on fire, or his brain vaporized. Depends on the intensity). So you need to point the lasers directly at the craft. That's kind of hard, but not improbable I guess.
You're always gonna have the standard losses due to Mie, Rayleigh, absorption, et cetera. Losses increase up to a factor ~11 as the station appears lower near the horizon. I did some research on optical satellite communication a while back and found that under light conditions (no clouds, moderate angle above the horizon), the power loss from space to ground was still ~50% to atmosphere alone (this was in the 780nm range). This was for an incoherent light sources, but I believe the same sort of numbers are applicable to lasers. Of course that's dwarfed by the need to produce the laser beam in the first place, most types of which have atrocious efficiencies, so if that's a given and considered wholly acceptable then none of that matters anyway.
I've got very limited knowledge on microwaves so I can't speak for the viability of those.
So yea, in conclusion: given NISBP it's viable either way, because you can light up the plane with a shitton of W/m^2 if you want. But that doesn't mean it's a sensible thing to do though. If I had access to NISBP, and wanted to be CO2-neutral, I'd have it redirect solar light to a ground-based thermal engine to produce near-infinite electricity, and then use that to do good stuff like make hydrogen and then Sabatier the crap out of the CO2 in the sky to produce sweet Methane to fuel aircraft with, rather than having to build solar-powered aircraft with heat engines or hauling photvoltaic cells into the sky. Aircraft dry mass comes at a premium and while you might get something working "good enough" with sufficiently advanced tech, I don't think beamed power is going to beat simple chemical fuels (inside the atmosphere).
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