A railgun set of parts that you can use to accelerate objects. Maybe detachers kinda work? But they don't work between crafts. Rocket engines kinda work? But they risk blowing everything up.
This could be helpful for if you set up a moonbase, for example. Or when you're in deep space, and you need to launch a craft but lack the fuel. Railguns could drain your whole battery supply in exchange for a lot of speed. Where normal rockets would have a hard time getting...
Maybe it could be something that can be developed alongside futuristic fuels like metallic hydrogen.
Imagine a big biiiig spaceship with a long rail, and it shoots drones out of it. How cool would that look like.