[RightWhale]

They need to be moving several Mach numbers before they can light off the scramjet. This would probably require a rocket. Whether they launch that assembly from an aircraft rather than straight from the ground would be up to the design engineers, but a political decision from higher up might require as much reusable gear as possible without regard to cost.

[ArrogantBustard]

They need to be moving several Mach numbers before they can light off the scramjet. This would probably require a rocket.

Or an SR-71.