If you haven't already check out Chad Rigetti's talk from last year's Startup School. Excellent perspective on the challenges and rewards of working on "hard tech"
I think the problem can be broken down into specific steps fairly easily.
Executing those steps is not easy though - it is rocket science after all.
YC doesn't seem to shy away from ideas that are really out there. I'm wondering if the lack of rocket "startups" is due to something from YC's end or if it's a matter of numbers.
That is, the number of people with skills to build a rocket ship startup is so small that there isn't a critical mass of founders starting these companies. As a result, people are not seeing enough of them to be accepted into YC.