Came across this thread while looking to use MLI on mid-altitude balloon flights for battery thermal management for my business. What Exeric cautioned about is indeed the case. At low altitudes (<10,000ft), MLI does a horrible job of insulating due to most of the heat flow being dominated by conduction and convection. A 10-layer MLI was so bad we never even flew it for a mid-altitude (~10,000-60,000ft) as the batteries would already be too cold by that point. In short, 1/2" of XPS worked far better in our test with a temp difference of ~60°C and a surface area of ~10sq in. For high-latitude to space-like environments its a great solution. Unfortunately, as Exeric pointed out, anything other than a vacuum between layers greatly degrades this type of insulation. There is a reason it is often recommended to have ~3/4" air gaps with radiant barriers on houses (since most are build with atmospheres around them) instead of MLI's 20-60mil gaps.
We're toying with the idea of a mixed system where a thinner MLI blanket surrounds the batteries and is housed in an XPS external box, but its not a high priority as it adds weight. This is different than layering the insulation in the gaps which would render MLI useless as a radiative barrier.
Quote:
Originally Posted by Exeric
insulating materials invariably have a higher emmisivity than a vacuum. So installing insulation between layers is not a solution. It's the touching of molecules to a radiant barrier that conducts (conductivity?) heat away. Of course, some molecules conduct heat better than others, but there isn't any molecules that conduct heat less than no molecules.
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