Ramjets do not require supersonic flow speeds. They require positive speed (they don't work at a standstill), and are usually tuned for a sweet spot. This is us

Ramjets do not require supersonic flow speeds. They require positive speed (they don't work at a standstill), and are usually tuned for a sweet spot. This is usually supersonic because most applications are military, where the supersonic capability is desired. That said,they usually don't produce much thrust until you get over M0.5.