When something as simple as this (practically a fuel tank with wings on it) can be an SSTO, I think the RAPIER is a little OP. Also, it can get up to 2500 Isp in atmo at the right altitude. It's hard to think of a situation where I wouldn't use the RAPIER as the first stage. Edit: This one's even better. Made it to LKO w/o wings: http://i.imgur.com/bfmb59O.jpg (posting as link to save screen space)