As has been commented elsewhere, real engineering is hard. And it costs money. I'm sure that in the X-prize case the barrier is money. If that were plentiful, one or more of the concepts could be built, tested and succeed.

But money is not plentiful. Sad that not one of the thousand or so billionaires on the planet gives a shit about manned spaceflight. We have several options: continue the so-far futile attempts to interest such billionaires in space, promote space so that a young billionaire-to-be catches the space bug and funds stuff, or you young-uns go earn a few billion and fund it yourself.

-philw (option 3 Justin, remember your bbs buddies)