And I quote from Space Shuttle by Dennis Jenkins (1996 ed.):
" a vehicle with a 100,000 pound gross weight could obtain Mach 6.4 at 660,000 feet and have a sustained seed of Mach 5.3 for over one minute. The vehicle would be rocket-powered, burning liquid oxygen and water-alcohol propellants. Using this as a carrier aircraft, a vehicle the same general size and weight of the Bell X-2 could be launched at Mach 3.0 and 50,000 feet, attaining speeds up to Mach 10.0 at altitudes approaching 1,000,000 feet. It was felt that a duration time of one minute over Mach 8.0 would be achievable by this second stage."