The serria rail gun.
I know that's going to be incredibly contraversal for any space enthusiasts and I get and understand why building it was important.
A conventional space launch costs 20 million dollars, to send 2,000 tons into the air, the serra rail gun can send a ton into space for the cost of about 200,000 dollars per launch and this includes the price of labor, the reusable capsiles, and maintaining the nuclear plant and the three rail guns that are used. Thing is I live in that area and I have to listen to it being used.
Imagine a sonic boom going off once an hour every hour, and it does not stop. Sure its great for nasa, its great for space tourists, and for any one in the industry but for the people who live around the mountain they used to make the thing? It sucks hardcore.