(Henry Spencer) wrote:

Personally, I am skeptical of the argument that suborbital incrementally
grows to orbital, because I don't see enough market for the intermediate
steps. You don't gradually improve suborbital vehicles to make orbital
ones -- it's a substantial jump up in technology.

I think thermal protection for re-entry is a good candidate for such
incremental improvement. Making higher and longer suborbital flights
requires better TPS. At some point it'll be good enough for re-entry
from orbital velocity. Then the only significant technical difference
is propulsion.