So every (non-technical) explanation of ramjets I've seen talks about how the intake slows and pressurizes the incoming air before combustion but then takes for granted the fact that burning fuel should produce net thrust. Why? Why don't the expanding gases created by combustion (via added gases and heat) create just as large a force in the forward as backward direction?
I presume this has something to do with the shape of the ramjet. After all if one had an enging that looked like this:
With the *'s indicating the location of combustion one would expect it to generate drag rather than thrust. The expanding gases produced by combustion would push against the narrowing sides of the engine and create a backward force on the plane. To produce net thrust does the inlet have to be narrower than the outlet or what?
I'm looking for an answer that can be translated back into basic Newtonian mechanics so if you're answering in terms of some derived equation about pressure and velocity please relate it back to basic concepts.