Doug,

Philosophical arguments aside, I would say an aircraft ceases to be experimental and is operational when it enters mass production and is operated by regular squadrons in the military or in regular freight and passenger service in civilian flight operations. In short the test pilots leave and the regular pilots take over.

So I would consider the ME-163 as non-experimental as over 400 hundred were built and it served in operational squadrons. Chris Vance however seems to believe Germany’s ME-163 was still an “X” craft for some odd reason. I suspect its only because it provides strong empirical proof for Bell’s arguments if you accept it as an operational rocket plane. Which brings us back to Bell’s article and the key question the space tourist industry future depends on. Are rocket planes finally, after 70 plus years of experimental use, ready for prime time? Or will Bell’s predictions come true?