Firefox just quitted me, now I can re-type the whole sory

What I did to arrive at this was take an engine I had the heat balance on and do an analysis just like the cooling system was a ram jet.

Do I understand correctly that you took the exhaust heat into account as well? (Since that's the majority of the engine heat)

Another sacred cow is that you must have a boundary layer diversion scheme.

I've never fully understood the need for it. First of all, in our region (up to 250 mph or so), boundary layers are thin and thus, momentum loss is pretty small. Also, like you said, you're re-energizing that same boundary layer again. Thirdly, having a "lip" to not ingest the boundary layer has a lot of extra area, producing drag as well.

Your comments about a ramjet got me thinking. I found a nice tool of NASA, doing all this automatically ;-)

EngineSim 1.7a beta
So, what did I do?

Input:

Speed; 250 mph, sealevel, Inlet recovery, 0.9, and otherwise, everything default.

Then, we've got the inlet/outlet ratio. I set it to 1.

Then we can fiddle around with the inlet area, until the fuel flow seems correct. I used the fuel flow to "tune" it, because that's exactly what defines HP. Since about 60% of the fuel is transferred to heat, you take 60% of your total fuel flow. Note that many engines (especially the conti/Lyc) are running rich and thus producing less heat than you'd expect. Hence I use the 100HP Rotax 912, which burns 20 liters/h @ 75%. So that's 33 lbs/hr for 75HP.

An inlet area of 0.1 sqft gives this fuel consumption. It produces a net thrust of 177 lbs and has an (internal?) drag of 45 lbs. That's 131 lbs, or 600N.

250 mph is 112 m/s. 600N @ 112 m/s is 67 kw, or almost 90HP. 60% of this is 40kw, or 54HP of net thrust. Total thermal energy was (0.6/0.4)*75HP=112.5 HP, so the "thermal propulsion" is only 50% efficient.

So what goes wrong in my calculations/assumptions?

I suspect, the "exchange efficiency" of the heat is very low, because of the very small temperature differences.