Partly, the obvious problem was figuring out a plausible design that could use collisions occurring primarily on the front of the craft for propulsion. I was certainly assuming a much lower background matter density (I was thinking a similar number per square meter or so rather than cc, and probably guesstimating less energy loss with each collision, I haven’t done any actual math :p). This is all much less of a problem for how I was actually intending to use the basic idea in the story, the idea of using it in a remotely plausible, hard sci-fi ish space probe was a separate one.