Jet engine design shifts significantly between supersonic and subsonic aircraft. Supersonic flow behaves very differently from subsonic flow. Aft-swept wing settings are used in high subsonic and supersonic flight. Its operation relies on the different properties of gases flowing at subsonic and supersonic speeds. It represents the boundary between subsonic and supersonic infall. Both subsonic and supersonic designs would be considered. Early jet aircraft used turbojet engines that were relatively inefficient for subsonic flight. Rocket assist is most effective with subsonic artillery projectiles. Two performance classes of .22 rimfire subsonic rounds exist. Brittle failure may occur at subsonic velocity. Pitot inlets are used for subsonic aircraft. The shock waves generated slowed the air to subsonic speeds relative to the engine. Pellets are designed to travel at subsonic speeds. These are designed from the start as dedicated subsonic projectile systems. The bomber itself will fly at subsonic speeds.