In terms of payload, and as someone asked below, "ordinance and yield" Would it actually need any? Smashing mach 20 into a target would yield tremendous kinetic force without explosives.

Same principle as a http://en.wikipedia.org/wiki/Railgun where projectiles travel at 5,400 mph, and the gun can be situated on an aircraft carrier.

On that note, the article above mentions railguns as way to launch spacecraft into orbit: applicable for this Mach 20 plane.