As an aerospace engineer do you know what kind of thrust they'd need and why they can't just use something like the https://en.wikipedia.org/wiki/Pratt_%26_Whitney_J58#Specific...? My guess would be that the military engines are not at all designed to be economical on either fuel or unit cost.
Those haven't been made for a long time and there's nothing presently in use like them. Also those were turbojets; Boom wanted to use turbofans for fuel efficiency.