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Post by princessj on Aug 13, 2019 22:01:20 GMT
A lot of radiator designs in this game have a very high operating temperature, heated to incandescence to minimize the required surface area. This heat would probably cause thermionic emission, which increases exponentially with temperature of the material. Since the spacecraft is in a vacuum, continuous emission would create a negative space charge surrounding it. Could this be dangerous? Is there any way to reduce the charge?
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Post by airc777 on Aug 14, 2019 1:00:48 GMT
Throttle down the reactors when not currently firing the lasers or mpdts?
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Post by airc777 on Aug 14, 2019 6:07:03 GMT
So according to the Wiki page on Thermionic emission it's only significant at operating temperatures above 1,000k, so for as long as you're aren't full powering the reactor and the laser you should be fine.
So according to the Wiki page on Thermionic converters there is a way to recover Thermionic emission at 5 - 20 % efficiency at temperatures of 1,500k - 2,000 k into useful electricity. So there's immediately one of two possibilities: either adding Thermionic converters to existing radiator designs improves the total power output and mass efficiency from the reactor which is strictly a good thing, or adding Thermionic converters to existing radiator designs adds so much mass and complexity that it reduces the overall mass effectiveness of the reactor.
I'd assume the latter is probably true because thermodynamics are usually uncooperative whenever engineers are trying to do something cool, so there are immediately two possibilities: either ~1 minute of full power does not result in enough Thermionic emission to cause catastrophic failure in which case the designs are probably fine as is, or ~1 minute at full power will cause catastrophic failure.
Assuming the latter is true we need to build 1,000k exhaust reactors and redesign our ships. Assuming the former is true then all we need to do is add a small Thermionic converter to de-ionize the radiators after the ~1 minute full power run, right?
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Post by AtomHeartDragon on Aug 14, 2019 6:36:43 GMT
If waiting for charge to equalize with solar wind plasma is not an option, then just apply conductive coating to your radiators and have some ion guns to dump unwanted charge?
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Post by cipherpunks on Aug 14, 2019 11:32:54 GMT
[...] ~1 minute at full power will cause catastrophic failure. Assuming the latter is true we need to build 1,000k exhaust reactors and redesign our ships.
Funny, me not knowing a solution to this thermionic woe (and not having time to learn some) was the reason that I refused to build radiators operating above 1070K for my electric-driven ships, for the sake of so-called "realism". Just ¢2paging matterbeam : any thoughts? |
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Post by matterbeam on Aug 30, 2019 1:06:35 GMT
[...] ~1 minute at full power will cause catastrophic failure. Assuming the latter is true we need to build 1,000k exhaust reactors and redesign our ships.
Funny, me not knowing a solution to this thermionic woe (and not having time to learn some) was the reason that I refused to build radiators operating above 1070K for my electric-driven ships, for the sake of so-called "realism". Just ¢2paging matterbeam : any thoughts? The thermionic current density for a graphite radiator in vacuum at 2500K is on the order of 300 A/m^2. But, it might be such a major effect since there is no positively charged collector for the electrons: www.quora.com/Does-thermionic-emission-occur-in-light-bulbs-used-at-home |
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feld
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Post by feld on Sept 18, 2019 15:32:43 GMT
princessj,
The initial problem formulation contains one un-real assumption that actually might matter for your answer. The space environment is not a vacuum but a plasma. That plasma is quite diffuse (average ~5 particle per cubic meter within the solar system though often higher near the bodies where we operate in CoaDE) but the charges are so numerous and so mobile (and electric fields so long ranged) that I think any thermionic charge buildup will generally be naturally neutralized.
However, I think you are right to be concerned about it for other reasons:
1. The plasma consists of electrons, protons, and helium nuclei. You might be concerned about long term wear on your radiator surface coatings or pitting of the radiator surfaces from proton/alpha particle impingement. Both would reduce the lifetime and performance of your radiators.
2. In general, when flying thru plasma, different materials build up charges and different rates and so maybe can lead to differences large enough to create an arc within the vehicle. I don't think that's a concern here. This might pose a problem transitioning through radiation belts.
v/r
feld
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Post by AtomHeartDragon on Sept 18, 2019 16:12:58 GMT
1. The plasma consists of electrons, protons, and helium nuclei. You might be concerned about long term wear on your radiator surface coatings or pitting of the radiator surfaces from proton/alpha particle impingement. Both would reduce the lifetime and performance of your radiators. Given that the radiators are going to gain positive charge via thermionic emission I would expect it to alleviate the concerns about impacts. Also, I am not sure about deteriorating performance (as opposed to just lifetime). You can place conductors to prevent that. |
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feld
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Post by feld on Sept 18, 2019 16:33:18 GMT
Atom, Agreed on the conductors. Indeed, that's what is sometimes done on s/c today. There's also "plasma contactors" that maintain voltage differentials on solar powered vehicles like ISS. ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040110836.pdfReal space qualified thermal control assemblies (like radiators) usually have some kind of surface coatings or finish to tune their emissivity. Over s/c life these coatings often degrade due to exposure to solar UV, radiation belts, or just the ambient plasma. In this case, continuous lifetime impact of solar wind material at ~200 kms relative is going to cause a little bit of surface sputtering on your radiator surfaces. Not sure if it will matter much over s/c life because, well, the conceptual studies we've done get into detail but we're not at the point yet at NASA/ROSOCOSMOS/ESA/JAXA/etc of really designing large long life *deep space* vehicles. I am assuming that CoaDE thermal controls surfaces will need to be similarly prepared. Though "military grade" surfaces would probably be rougher anyway, accepting a performance hit for redundancy's sake. Anyway, that's why "plasma impingement" says "performance deterioration" to me. v/r feld |
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Post by EshaNas on Sept 19, 2019 2:19:38 GMT
But what does this thermionic emission *do*? Turn the radiator into a spark plug? Is that really something that's a roadblock to radiating?
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feld
New Member
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Post by feld on Sept 19, 2019 3:16:28 GMT
It's not a road block to radiating. It's just a potential hazard to other electrical bits on the spacecraft. Surface charging can get up very high causing R2-D2-hit-by-Jawas-type effects.
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GSUI5051
New Member
Waiting 4 next CoaDE update
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Post by GSUI5051 on Sept 20, 2019 6:54:13 GMT
Please consider sublimation of materials.
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Post by AtomHeartDragon on Sept 20, 2019 16:39:36 GMT
Which solid has the lowest vapour pressure in, say, 1100-2630K range?
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feld
New Member
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Post by feld on Sept 20, 2019 18:54:27 GMT
Please consider sublimation of materials. Why, given that the OP was concerned about thermionic effect? Do you mean "the radiators in this game routinely live unrealistically close to their thermal limits" ? -if so, then I agree  |
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