I thought it was just once per turn like every other ability. I'll change the description to say "Once per year"
I am actually going to combine the families in v2.0 for a lot of things making them upgradeable. I'm combining all the reactors into one family as well as the engines. I'll do the same for the cities since Arcologies are on the high end of the Urban tech anyway and they do the same thing.
I've been checking on the AI periodically to see how it's doing. The Abaddon seem to be making ships well and playing strongly. The Sergetti on the other hand are making ships with engines, but not reactors. The Praetorians seem to be up to the same foolishness. Another thing of note is that they are putting multiple navigational shields on their designs as well. I'm not sure if the benefits for the nav shield stack or it might be a good idea to find a way to make that component limited to 1 per ship.
By the way I'm enjoying the PBW game we've got going with the older mod.
Some of the races don't have the ai done yet, so they're building ships according to the stock data files, which means they don't build ships with reactors and use too many engines.
They're using multiple nav shields because that's the only shield component they have.