[Type] Precooled jet engine / LACE (combined cycle with rocket) [Advantages] Easily tested on ground. High thrust/weight ratios are possible (~14) together with good fuel efficiency over a wide range of airspeeds, mach 0-5.5+; this combination of efficiencies may permit launching to orbit, single stage, or very rapid intercontinental travel. [Disadvantages] Exists only at the lab prototyping stage. Examples include RB545, SABRE, ATREX [Description] 
Intake air is chilled to very low temperatures at inlet before passing through a ramjet or turbojet engine. Can be combined with a rocket engine for orbital insertion.