Which type of engine needs to travel very fast through the air to function properly?

The ramjet and scramjet engines are designed to operate by taking advantage of the high speed of the vehicle through the air. Unlike traditional jet engines, which rely on a mechanical compressor to compress the incoming air, ramjets and scramjets utilize the high-speed airflow itself to compress the air before combustion.

In the case of ramjets, they can function efficiently at supersonic speeds and require the vehicle to already be moving at a considerable velocity to provide the necessary air pressure for combustion. Scramjets, on the other hand, operate at hypersonic speeds and are designed to handle airflow at supersonic speeds throughout the entire combustion process.

Thus, the requirement for significant speed through the air to operate effectively sets ramjets and scramjets apart from other engine types. Turbofan and reciprocating engines can function at a variety of speeds and do not rely on high-speed airflow in the same manner, while propfans, though efficient, still do not have the same operational reliance on high-speed flight as ramjets and scramjets do.