This memory randomly came back to me when I woke up in the middle of the night and it's been bothering me since. When I was a kid in the early 90s, I remember telling my dad about a rocket concept where the rocket would scoop up dust and gas from the space environment around it and use that as a propellant. His response was to make a face and go "cool" and that was the last time I thought about it. I think I might have seen it talked about in a library book and have a vague memory of a cartoonish illustration of a rocket with a funnel on the front.

At the time, the concept exited me, but it felt like a really long stretch to make something like that work. I haven't really thought about it since. Is this concept still bouncing around? It doesn't really sound that viable, but I don't know crazy ideas sometimes work.