One of the fundamental differences between modern rockets and those used in the nineteenth century and earlier is the presence of converging-diverging nozzles, which massively improve exhaust velocity and thus performance. Could the principles be discovered and implemented much sooner, and if so, what impact could they have on rocket artillery? Could rockets become a much more important weapon of war sooner? Could they become more common on battlefields and aboard warships?
What other improvements could conceivably be made to drastically improve the state-of-the-art before 1900?