Well, first off all the problem with discoveries tank is due to a manufacturing problem with the stringers, not a design flaw.

Second of all, why use SSME's? They are designed for re-use, and have restart capability that will not be needed. A better choice would be the rocketdyn's RS-68, single use, cheap as fuck, provides more lifting power.

The theory goes that a mars sized planetoid, named Theia, had formed at earths L4 or L5 Lagrangian points. As it's mass grew due to impacts, it was no longer stable at that point, and slammed into earth. The resulting debris cloud came together to form the moon.

See http://en.wikipedia.org/wiki/Giant_impact_hypothesis