First concept for space shuttle
Earlier concepts
(You have to visit this side!)
Just avoiding vertical launch is a huge advantage. The ability to bring the launcher to 40kfeet and a few hundred mph saves a huge amount of fuel.
One idea is to literally tow it into the air. I understand there was a test with an F-106 towed into the air by a C-141 at Edwards AFB that represented towing a high speed vehicle up to altitude. Worked fine.
I've got to wonder what the performance would be of a scramjet/rocket vehicle launched from the top of an XB-70 class aircraft like the drone that launched from the top of the SR-71. It seems hard to believe, but the XB-70 held the absolute weight lifting record in the days before the 747 and C-5. I'm sure it could have lifted a hefty vehicle on it's back. Don't know how fast it could have accelerated it, although it was designed to cruise at Mach 3+. I'm not sure if you can light a scramjet at Mach 3, but I think there are some combo scramjet/ramjet designs out there.