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I was confused as to what your post meant; are you recommending a mod? an outfit? questioning it's existence? ...

recommending an outfit

here is a description the outfit:

  <description>While the process of refining fuel for space ships is possible using cheap available materials with the adequate machinery, production is usually limited to large immobile stations or astral bodies. By sacrificing machinery size and complexity, the Fuel Recycler is able to synthesize fuel in a very compact form. The downside is that the device ends up significantly reducing efficiency and requiring large amounts of raw materials to generate fuel. Despite this, such generation can be useful for long space journeys or when in a pinch.</description>

  <desc_extra>#gAllows converting space resources to fuel.#0</desc_extra>

I know it exists. 

I just don't know what planet sells it,

So... the fuel recycler outfit exists and was added by Bobbens more than a year ago (all according to the git repository), it was last edited a years ago but possibly just because Bobbens was updating every file that contained a reference to something being updated.

It is not lists in the file containing all outfits in use... which is a bad sign. Neither is it contained in any of the subset files that are normally used for planet shops, neither is it obviously used in any of the ship configuration files that might contain the outfit (AI ships don't tend to have things they don't need so military ships are very unlikely to have this equipped... in fact I'd have guessed only cargo or mining ships since most other ships have little use for it and none of the standard ones have it.

That leaves only quest specific ships that have one-off designs and... that's it. I can't login right now so I can't search the git codebase for Naev but I would suspect very strongly that this was an outfit created to serve a purpose that never materialised or is still planned but yet to be implemented (if it is not in git it won't even be in the nightlies). I have to admit I only checked main so you might find something in one of the branches but unless it is in is main then it won't ever be released... some branches will make it but others (currently) look like they never will being mostly abandoned or so out of lockstep with main that it would take more effort to update them than to start from scratch.

Since Naev is an evolving game you will always find stubs in there that represent new, abandoned, outdated, deprecated or forgotten about things, if only because the original dev left it behind and no-one made use of it or took it on. I've done a bit in my time but because I'm short on time I generally work on things that are already there and sharpen them up or shift old code towards new standards and replacing deprecated code (with a byline in proofreading that comes from reading through the files) - there's always something to do besides taking on a new project that never gets finished because the codebase moves on before I find the time to finish it (though others have adopted some of my ideas from discussions and small hints I've added here and there :) - every little does indeed appear to help! :)

So, if you have a desire to use this outfit then code it into a few places where it would fit - out of the way mining areas, maybe associated with the Mining Vrata, would be something that makes sense since it'd help miners from running out of fuel while hinting the good stuff out at the edge of known space. You might prefer to code it into a unique ship or create a new AI ship (a mining one maybe) that uses this outfit making it possible to beg, borrow or steal! :) :D