We are having a discussion about this on our group DIY homebuilt turbines and wonder if anyone has a definitive answer .

 

i am going with,,, jet is interal, icbm is exteral, lol, fireworks rocket is internal.

 

From Wikipedia:

The internal combustion engine is an engine in which the combustion of a fuel (generally, fossil fuel) occurs with an oxidizer (usually air) in a combustion chamber. In an internal combustion engine the expansion of the high temperature and pressure gases, which are produced by the combustion, directly applies force to a movable component of the engine, such as the pistons or turbine blades and by moving it over a distance, generate useful mechanical energy.

The term internal combustion engine usually refers to an engine in which combustion is intermittent, such as the more familiar four-stroke and two-stroke piston engines, along with variants, such as the Wankel rotary engine.

A second class of internal combustion engines use continuous combustion: gas turbines, jet engines and most rocket engines, each of which are internal combustion engines on the same principle as previously described

 

Internal, as z28pete mentioned. Even though you can "see" inside the combustion chamber, one exists - and that is where the propulsion energy is created. There is no external energy applied to an internal "working fluid" that creates motion (e.g. burning coal to heat water to create steam; or in a more modern sense, "burning" uranium to do the same).