I thought I was pretty clear that I don't see this happening for hundreds of years at least.
The engineering problem is insurmountable today. But there doesn't seem to be any reason it couldn't be done eventually, given our technological trajectory, unless we believe we are truly on the precipice of severe diminishing returns in most science and engineering fields, and I just don't see that right now.
George Cayley figured out how to build an airplane in 1799, but it wasn't for another century until materials science and high power-to-weight ratio engines made the Wright Flyer possible.
There are plenty of depths to plumb in space systems engineering that we haven't even really had a proper look at yet. A Mars mission with chemical propulsion is hard, but could be made substantially easier with nuclear thermal propulsion - something we know should work, given the successful test fires on the NERVA program back in the 60s. First stage reusability was fantasy 15 years ago, today it's routine.
Obviously, I'm extrapolating a long way out, and maybe at some point we'll run against an unexpected wall. But we'll never know until we get there.
> Obviously, I'm extrapolating a long way out, and maybe at some point we'll run against an unexpected wall.
GP has set the 'low bar' of providing a material that survives a series of nuclear blasts whilst generating useful thrust. I'm not qualified to judge whether or not that requires new physics but it seems to me that if we had such a material that we'd be using it for all kinds of applications. Instead, we rely on the physical properties of the materials we already know in configurations that do not lend themselves to the kind of use that you describe.
That's the difference between science and science fiction, it is easy to write something along those lines and go 'wouldn't it be nice if we had X?'. But if 'X' requires new physics then you've just crossed over into fantasy land and then further discussion is pointless until you show the material or a path to get to the material.
See also: space elevators, ringworlds, dyson spheres etc. Ideas are easy. Implementation is hard.