I believe the second issue is that engines which work well at supersonic speeds don't work well at slower speeds and visa versa "A typical air-breathing DMRJ p

I believe the second issue is that engines which work well at supersonic speeds don't work well at slower speeds and visa versa "A typical air-breathing DMRJ propulsion system can only begin operating when the vehicle achieves supersonic speeds greater than Mach 3. In fact, since ramjet (and scramjet) engines don’t have rotating parts (compressor and turbine) as turbojet and turbofan engines, they rely on specially designed air inlets to compress the air to the pressure required for the combustion. The efficiency, however, only gets sufficiently high when the speed reaches at least Mach 2, due to the higher temperature and pressure obtained in the combustion chamber. This is why ramjets usually need other means to reach the minimum speed for ignition, such as rocket boosters or, in the SR-71 Blackbird, dual cycle turbo-ramjet engines." from https://theaviationist.com/2023/12/16/ge-aerospace-demonstrates-hypersonic-dual-mode-ramjet/ https://theaviationist.com/2023/12/16/ge-aerospace-demonstra...