[vansig]

Brian H wrote:

i bet that a pinch near atmospheric pressure is possible
(but probably at much higher voltage).

?? I think that’s around 760 torr, quite a stretch from 40! Why would you want to?

you might not want to. but, the existence of ball lightning is kind of a giveaway.
who knows? increasing the pressure could be a good thing.

[vansig]

seems like we’ll want everything radiation-hardened. we have neutrons and xrays in the DPF, and plasma in the van allen belts as well.

plus,
it’s looking like these MTPV cells like it hot: if i’m reading this right, that’s 1000 – 1500 °C; but they generate 5-10 watts/cm².
http://www.economist.com/science-technology/technology-quarterly/displaystory.cfm?story_id=15582193&fsrc=rss

so we set them up to blanket the high temperature equipment, and scrounge 100 kW for each m² they cover.

[vansig]

well, where ever they end up, it looks like you want them to mass < 500 mg for each watt they generate.

[vansig]

zapkitty wrote:

micron-gap thermal photovoltaics

This could be excellent if it compares favorably with the equivalent increase in power/radiators in mass and volume.

But the recovery of 80% of the thermal from reactor and drives as electric would do great things for radiator size etc if the mass of the dots gear doesn’t eat the margin gained.

With the current stats that would give us a DPF box running at .35 MWe (!) and a double-faced radiator of 66 m2… a square just over 8 meters on a side that masses 1.3 tons…

I like it!

So what you need, then, is for your thermo-photovoltaics to weigh in at < 500 kg total. I think these could be made quite compact, especially if they can be built into the radiator structure.

By the way, One of the links to their description was broken, but here’s another,
http://www.treehugger.com/files/2009/01/thermal-photovoltaics-solar-power-mtpv-higher-efficiency.php
seems to me, that the vibrating mushroom cap generates an alternating current, which is rectified. Since heat is broad spectrum, the sizes of these would vary along the temperature gradient, making them extremely efficient emitters, too (e >.99).

[vansig]

At current energy prices, it is considered uneconomical to recover CO2 for sequestration by making dry ice, due to the expense of compression and refrigeration.
“At temperature of 197.5 K (-78.5°C), the vapor pressure of solid carbon dioxide is 1 atm (760 torr). At this pressure, the liquid phase is not stable, the solid simply sublimates.” — http://science.uwaterloo.ca/~cchieh/cact/c123/phasesdgm.html

So, what you’ll want to do, then, is to use cheap electricity to bring down the price of CO2.
Then, sequester as much carbon as you like, via biomass. Carbon recovery by photosynthesis dramatically increases when plants are fed a rich supply of nutrients including high CO2 concentration.

However, this bio-recovery process will probably appear, first, via pumping the effluent from coal plants directly to algae tanks.

[vansig]

Sorry, that’s a ridiculous misfeature. Not-only are posters approved by you, (and therefore moderately trusted), but banning parentheses does nothing to stop cross-site scripting.

proof: click here
http://tinyurl.com/4g5c7t

please disable the ban on use of parentheses.

[vansig]

Getting seriously down to the topic of electrode erosion, here…
if i remember correctly, as of today it’s measured in the microns per pinch. Obviously this wont do for production. But the electrodes in use, today, are copper, rather than beryllium, which absorbs more xrays.
Are they also not as aggressively cooled as planned?

Cu
m.p. = 1357.77 K
heat capacity = 24.440 J·mol−1·K−1
thermal conductivity = 401 W·m−1·K−1
electrical resistivity = 16.78 nΩ·m

Be
m.p. = 1560 K
heat capacity = 16.443 J·mol−1·K−1
thermal conductivity = 200 W·m−1·K−1
electrical resistivity = 36 nΩ·m

What’s gained in higher melting point by using beryllium could be lost via increased electrical resistance, lower heat capacity, and lower thermal conductivity. On the other hand, with pulse timing in nanoseconds, the skin effect comes into play, as well. The beryllium should have a deeper skin depth (~9..90 μm through the pulse, as opposed to ~6..60 μm for copper).

Even so, unless a thin coating of something like tantalum-hafnium-carbide (m.p. 4488 K) can totally eliminate erosion, i’m not seeing an easy way to keep down xray absorption. Will it?

[vansig]

Evan Carew wrote: In science, a crackpot is often described as someone claiming to be misunderstood, and who refuses to submit their work to peer review. See the link here to a humorous page in Wikipedia on the subject.

Evan,
your url came out garbled. unfortunately, the forum software, here, seems to prevent me from correcting it fully..
http://en.wikipedia.org/wiki/Crank_(person)

[vansig]

Ok. i’m squinting at the image, here
http://en.wikipedia.org/wiki/File:Vasimr.png

the ICRH antenna, situated aft of the magnets, generates most of the heat. there is already a vacuum between the plasma and the superconducting magnets. yes, the magnet cores touch parts of the engine chassis.. is that a thermally-insulating ceramic?

i assume the magnets have low-temperature coolant running through their cases, which are also given mirror-reflective surfaces?

[vansig]

I’m guessing the magnetic cores themselves may consume 5%, (~10kW) per engine. From a standing start, a thermal mass such as methane at or near its triple point, 90.67 K (-182.48 °C), 0.117 bar, seems like a good way to manage temperature there (CH4 heat of vapourization (~500 kJ/kg) is almost 1000 times greater than argon’s).
And switching YBCO out, in favour of the current high Tc record holder (Hg·Ba·Ca·Cu·O, Tc~135 K) would let the system run hotter, overall. This reduces radiator size by a factor of 4.

But I’m starting to favour helium in the radiator itself, since we wont have to worry about it freezing when the engines are down.

If we get smart about radiator design, we seem to have quite a temperature gradient available, based on what parts of the system run the hottest. Because of that, hotter areas will radiate faster than cooler areas; so, can some parts of it bypass? –returning higher-temperature fluid to less-sensitive equipment?

The low-temperature fluid return (~85 K ?) then needs ΔT·flow ~ -2 kg·K/s for the magnetic cores only, for each engine; while the high-temperature return (~270 K ?) is doing the bulk of the cooling for the system, ΔT·flow ~ -200 kg·K/s.

[vansig]

after meditating on what’s been gleaned and slopped into the wikipedia article,
http://en.wikipedia.org/wiki/VASIMR#Research_and_development
i’m thinking i may have overestimated the refrigeration requirements for the magnets, by as much as a factor of 8. so with four engines, that reduces the coolant problem to only 16 times too much. :-/

ideas:
liquid N2 maybe gives a bit more head room, there?
further progress on high Tc superconductors?
make use of a thermal mass, (such as solid CF4 or propane?) and pulse the thing?

[vansig]

vansig wrote:
Not sure how to calculate the amount of argon needed to cool the magnets, but this will feed into the equation for optimal Isp.

It seems 80 kW waste heat per engine would boil .5 kg/s Argon at ~85 kelvin; but that’s much more than we want (by a factor >100), so most of that heat will have to be rejected another way. Keeping 80 kW at below Tc~93K implies a hectare-size radiator.

Yikes! Please tell me those magnets wont produce that heat.

[vansig]

Lots of fun!

A VX-200 engine is predicted to have 5 N thrust at Isp=5000s. Let’s run YBCO tape (Tc~93K) to make the superconducting magnets (2 Tesla per engine), and cool them with liquid argon, which, after it also cools the hot parts of the whole system, becomes propellant. The hotter the Argon gets on its way to the nozzle, the less work you have to do to heat it to plasma temperatures before acceleration.

Not sure how to calculate the amount of argon needed to cool the magnets, but this will feed into the equation for optimal Isp.

[vansig]

okay, then.

but those moon buggy tires will be mine, soon enough..
http://www.youtube.com/watch?v=Vv8rWd1c0b8

🙂

[vansig]

In so much as wikipedia wants to rely on verifiable secondary-sourced material, the absence of controversy from the article suggests that the ongoing mud-slinging here may be ordinary posturing among arrogant specialists. Who really has a better grasp of the concept and where it’s applicable? I certainly don’t.
But they are using language that seethes with contempt. I’ll have to study the concept some more, myself, but so far i’m reading that the concept of reconnection is being misused by one of these two experts..

kind of like comparing a broken circuit
to two or more circuits becoming one.

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