Those look like they’d be useful for a rail up a mountain space vehicle launcher. Don’t know why we haven’t developed that yet. The vehicle would already be a couple of miles high and moving at great speed when it finally needs to turn on the burners.
As with any ramjet, the engine only produces thrust when the vehicle is moving at high speed. The proposed Supersonic Low Altitude Missile (SLAM) would have been boosted with solid rocket motors. Static tests of the engine were performed by preparing huge quantities of compressed air and shoving said compressed air down the engine’s throat, simulating high speed flight.