You can only build one.

No, because you require two durantium. The only way to build that ship is to either acquire a second durantium node, or to improve your mining techs, so you can mine more resources from the node you have.

According to the patch notes, it is supposed to be use for missile and point defense augments. However, that doesn't seem to be the case. Probably a bug.

Yes. However, now that it got destroyed you can build another one. You don't lose the resource, if you build a ship using it. It is just "in use" as long as the ship exists. As soon as the ship gets destroyed or decommissioned, the resource is available to use again.

It's too early to tell. I can't build an opinion on them without any feedback on how they change combat-performance.

I think this boils down to the question: "what happens to your ships, if you no longer have enough resources?" As far as I can tell from playing the game, they stay around. So selling them would be possible. You just wouldn't be able to build ships using durantium of your own, until you have more than 10 of the resource. I, on the other hand, would be able to build more ships, because I no longer have any that require durantium.

If this is intentional or not, I can't really tell. It does seem exploitable on both sides though. As it is, you could sell all your resource-using ships to an ally, and keep building more of them. Repeating this infinitely. However, if no longer having enough resources to support your ships meant losing those ships, then all your enemy needs to do is to destroy your mining starbases, and your fleets will be much reduced. That doesn't seem fair to me.

This sucks. Augments aren't good enough to have a trade off of damage, and having one ship at a time is disgustingly underpowered.

We aren't completely blind on combat and doing basic math gas shown its not worth it. It needs to be revalued. Make it so.

DARCA