First off, really cool mod. Genuinely really great ideas and the runtime particle systems and all its mechanics is very impressive. I appreciate the amount of work it must of taken to implement all that from scratch without any proper engine support. (Thanks for all your other mods and modding contributions too)

I played through the moon yesterday over a couple of hours and since then have been thinking about it a lot, and I've come up with some specific ideas for quite a few things. I want to point out right at the start here, that this is only my subjective opinion, and I completely understand if you disagree with any or all of my suggestions.

As a summary for the first few ideas here:

I think there's room to add some post unlocking cargo drop progression to flesh out the planet a bit more in ways that could have beneficial side effects that reduce the runtime code needed (I looked through some of the source code and wow, there's a lot).

1. It's been suggested before, but a way to make plutonium-239 after completing the planet and without the solar wind would probably be a good idea both for steady production and performance.

   A way you could do this without invalidating the existing game design or devaluing the very impressive particle system is to have a kovarex style recipe for plutonium-239, but it requires a special catalyst that is returned with the recipe. This catalyst could require a huge amount of plutonium-239 to craft (a few thousand maybe? needs balancing based on a good setup). This would mean the player does need to optimise production using the particle system to produce the initial amount, but is then rewarded with another way of making it that is more UPS friendly and allows disabling the solar wind.
   If you want to add in some more complexity you could even make the catalyst degrade (spoilable) and need "recharging" to fresh with some plutonium-238.

2. The damaging gamma radiation from the reactor is very cool, but as far as I can tell has pretty limited gameplay implications. You surround it with water tanks or put on an energy shield and it's a solved problem.

   My main idea to improve this is to add another stage of progression where you build shielding that stops the radiation. To make this interesting you could need to bombard holmium plates with the gamma radiation in order to turn it into the material need for shielding. This makes further use of your cool particle mechanic in a neat little gameplay loop. I don't think you need new graphics for this, just stopping the constant particles would be pretty obvious (and also good for performance).

3. It would be nice to be able to move all the non-reactor buildings in the late-game and after completing Cerys. The fulgoran cryo-plants are the easiest, and the ability to move them could unlock with the normal cryo-plant tech. I think this makes intuitive sense. The engineer learning to make something like them, gives them the knowledge needed to safely move the fulgoran ones.

   The crusher I'm less sure where the unlock would make sense. It could unlock from excavating the reactor, be its own research for after bombing the magnetosphere, or be another cryogenic tier research unlock.

   A positive side-effect of this is you could implement more standard recipes for upgrading the quality rather than needing all the runtime code. The recipe could be inputting the building and an amount of upgrade materials, and use quality modules on the machine like normal. Then it would output only the building (at a potentially higher quality).

These next few ideas start to get a bit more subjective about balancing and the feeling of the mod.

• 4. The crushers felt a bit lacking to me and I think expanding their functionality a bit wouldn't hurt. A big thing I noticed was how damn slow recycling advanced circuits is compared to everything else. A solution for this would be to have a manual "recycling" recipe for something like 10 advanced circuits back to 25% of its materials. This could be restricted to only the fulgoran crushers and would be a nice extra use for them.

  My other idea is to give the crushers a faster engine recipe since the default is so slow. I think the materials are balanced well, but a 2 second base recipe for engines would be a nice extra incentive to get more crushers instead of spamming more assemblers. Of course this does raise the issue of how is a crusher making things? It causes a bit of a conflict with the current graphics but I think you could change it from a crusher to something like a fulgoran manufactory. It's basically an alien black box and looks nothing like normal crushers already. It being some advanced manufacturing machine that can both crush and build things seems very plausible.

• 5. Like some other feedback, I felt the iron output was a bit lacking. I definitely could have made it easier if I had setup asteroid reprocessing to get more that way, but players may not have done Gleba to unlock that tech before they visit Cerys.
  My suggestion here is to give the nitrogen-rich minerals a bit more use. We already know they contain iron, so what about another recipe that converts it straight to iron without the ammonia? It could be fairly lossy e.g. 4 to 1 and/or require a crusher to do it.

Now we get to the very subjective part that I realise you may strongly disagree with

Thank you for allowing construction bots and providing a sandbox mode option in the settings. I really appreciate you being understanding about other people having different ideas of fun and giving them the option. That said, I think there's room for a more streamlined but guided experience between "nothing but construction bots" and "everything's allowed". Even with construction bots the actual progression was fairly tedious at points for me since it was often just hand feeding stuff into chests. I personally don't enjoy that early stage of factorio and it was the least fun part of the mod to me.

To be clear, I'm not suggesting changing the default behaviour of the mod. I actually think this would probably be best as a separate mod to help clearly communicate the differences in intent and playstyle. This would allow clear warnings about potentially breaking the intended balance and needing to self-police in order not to do so. I do understand the desire to lock everything down to prevent anyone breaking the intended progression, but I think sometimes it's worth letting players decide for themselves what's fair instead of heavily restricting them and reducing their ability to creatively solve things using different approaches.

As for the actual changes: I think whitelisting some more stuff that could be cargo dropped could really cut down on some of the bits that can be annoying.
The things I really noticed missing and would have liked to have dropped in were logistics bots, roboports, logistics chests, inserters, modules, beacons and my own higher quality recyclers. Yes this would break the balance if people just dropped them in and recycled them but it still provides a more guided experience than just allowing everything. The modules are the most breaking and could even be a setting that defaults to off.

I know the bots might sounds terrible but the planet is kind of the textbook use case for logistic bots, low throughput, small amounts of items. I don't even use bots much and kept feeling they would be perfect solution to reducing the amount of hand feeding chests and machines. The increased energy multiplier for bots could also lead to some interesting decisions about power management.

Potentially creating my own tweak mod

I'm not going to do anything unless I have approval, but I can probably implement ideas 4 and 5 along with the reduced drop restrictions in a tweak mod that conforms to the licensing restrictions, because it wouldn't need to reuse any source code or assets. I would like to do what you want, rather than what technically conforms to licensing, so please let me know if you are okay with me making such a mod or not.

1,2 and 3 are trickier since they can only be partially done without source access. I'll probably implement all of these privately because I like the mod and the changes would make it easier to justify keeping it in my save. Heavily modded space age is brutal on the UPS even on a very high end system, and seeing how much my maximum UPS has been dropping already has me worried. I might do an optimisation pass for fun too (yes I know I'm weird, testing 10,000 trains with an API I've been writing yesterday made me way too excited).

Final words

If you've somehow read this far then thank you for taking the time to do so.

Thanks again for the great mod!

You may already know about this but I decided to do some experimenting with heat modding after seeing your implementation of manually subtracting temperature to get much higher heat losses on Cerys. I was very curious if it was possible to do without any ongoing code.

It does seem possible to replicate the mods behaviour using invisible "heat sink" entities rather than needing to use runtime code to manually remove heat. This could remove nearly all the runtime overhead and instead use the base engines multi-threaded heating system.

I experimented with a few different entities that could work as heat sinks and found furnaces to be best option since you remove all the inventory slots and they support the heat_energy parameter.

The heat pipe temperature system would be easy to fully replace with invisible heat sinks but heat towers are a bit harder. I did find it is possible to do a reactor, on a heat pipe with a custom radius, with a heat sink to replicate the behaviour of the current heating towers. The heat pipe would still need to be swapped with different heat radius version for the nice animation, but there could be a setting to disable the animation and have no periodic runtime overhead for the heating towers.