User blog comment:David Rain/Steampunk in space/@comment-32656-20131024093408/@comment-32656-20131025131957

Same rough neighborhood. The fuel in a rocket would be replaced by the explosive charge. Bit smaller, but... not by all that much.

With the cannons, you're limited by the size of the cannon. To get into space, you need a truly massive one - and it is something that would be viewed as a massive threat by a country's neighbors.

http://en.wikipedia.org/wiki/Space_gun

Still need some way of dealing with it in orbit, mind. That's the problem.