Dug into the code a bit, and it looks like the issue is that the mod is spawning the rocket part some 500 tiles ahead of the ship. I'm not sure if spawned entities in space always move downward, but even if so, it'd take ages to cover those 500 tiles, which is why it seems to only appear when the ship starts moving. I adjusted the mod code locally to spawn it between 75 and 150 tiles ahead of the ship instead, and it appeared as expected in space next to and a bit ahead of my ship and slowly drifted into range of my turrets. I suspect it was placed so far ahead either anticipating movement, or just expecting it to drift downward over time (but a looooong time, asteroid drift is very slow when the ship is stationary), or just to avoid spawning it inside a particularly long ship, since it doesn't do any collision detection, but the result is that, for practical purposes, currently the rocket only shows up when the ship it spawned on moves.