Flameout

A flameout is when an engine is deprived of any non-fuel resource needed to function. For example,a jet engine flames out when the air concentration reaches a certain point,and a liquid fuel engine will flame out if the oxidizer runs out. SRBs(Solid Rocket Boosters) and RCS thrusters cannot flame out,since SRBs come packed ready,and RCS only uses monopropellant.

The best way to avoid flameouts is to add more intakes(jets), or check if the fuselages you are using are for jets or spaceplanes (Handy Hint:Wings and the Mk0 Fuselage do not hold any oxidizer).