I had assumed, in my ignorance, that the Merlin engines were not an important advancement. I don't think that specific impulse is necessarily the important sen

I had assumed, in my ignorance, that the Merlin engines were not an important advancement. I don't think that specific impulse is necessarily the important sense of "efficiency" (though I do assume that it correlates with the important sense). The relevant efficiency, to me, is "awesomeness achieved per human effort and non-renewable resource", and the useful proxy for this is "awesomeness per price". Arguably SpaceX rockets are cheaper (per given mass to given orbit, obviously) that most or arguably all of the competition. I say "arguably cheaper" because it's a little tough to figure out how to price the shoulders of giants on which they stand. And really I only picked the word "efficiency" in the context of Stephenson's "efficiency or reliability". Anyway, hopefully this explains why I think that true reusable rockets, Musk's often-stated ambition, qualify as a "very large improvement" in efficiency. Specific impulse is not expected to significantly improve, right? Also, I apologize in advance for my legion failings in the area of comprehending aerospace engineering.