Ever heard of the Aerospike jet engine design? Polaris Aerospace is preparing to fly two new prototypes jets for its MIRA supersonic/hypersonic aerospike spaceplane platform

The aerospike engine in theory, should be effective from sea level all the way up into space. 

The aim of the MIRA project is to develop a cargo and/or passenger-carrying spaceplane running a single stage to orbit (SSTO), that can takeoff and land on runways, and that's fully and rapidly reusable.

https://newatlas.com/aircraft/mira-ii-aerospike-rocket-flight-test/