Since the Orbital ATK Pegasus already exists and already has a customised plane it launches from, can we assume launching Pegasus is just phase 1 for this plane? If that's all it's going to do there would be no point. So presumably this has capabilities the current Pegasus launch plane doesn't have, and eventually we'll see it launching something like a Pegasus XL Plus? Do we have any indication how big a rocket this thing can carry?

Also, I wonder how this compares in capability to the XS-1 currently under development, which will be a vertically launched first stage rocket that can fly back for a runway landing.

Edit: just found the link below from last year on Space.com.

http://www.space.com/32680-stratolaunch-rocket-vulcan-aerosp...