For air-breathing hypersonics, it lies with materials, propulsion and thermal management for sure and has to get a decent range and be reliable. When I worked on DARPA RASCAL, our F100 propulsion system used MIPCC (Mass Injection Pre-Compressor Cooling) and was rated to M3.5 up to 150K feet altitude with full "sea level" performance from the engines. For a theater strike platform, you could use MIPCC and get very high speed at theater strike altitudes and definitely do some serious supercruising but you have to carry water on-board unfortunately, nothing is free. We used a mix LOX and water via an inlet spray bar system but after test cell propulsion testing completed, could use atomized water alone and performance met predictions. Would be nice to have an VLO/LO, evolved F-111 type for this type of strike mission, we still need a good mix of platform types.