Can these be 'stacked' - Can you have a turboFan in line with a turboProp such that the output of the wash of the Fan feeds into the Prop, but with a portion of the wash spinning to thrust on the outer ring of output.=, via a design in the cowlings which is hyper directed thrust vents (think the grid of straws used to funnel water into a cohesive column, which can be directed)
Imagine a small diameter turboprop behind a much larger turbofan
Technically yes but it would make performance worse. Props are efficient because of their large size, which allows them to push a lot of air, if you shrink it then you give up that advantage. If you had extra power left over to turn a prop, you'd want to use it to turn a bigger fan, or just leave that power in the exhaust.
As far as has ever been discovered - no, this won't work. You can think of a fan as just a prop enclosed in a housing (jet engine). A prop loses effectiveness at the speed of sound because the air passing the prop gets a high pressure shock wave that effectively makes the prop not prop-shaped, so it can't move air. Picture it as using a hammer on glass instead of pushing on glass with your hand, one works way better than the other for generating thrust. The only way (so far) to have an effective prop is to have the prop tips move slower than the speed of sound. Engine designers worked around this by slowing the air down around the prop. To do this, they moved it inside a tube. They slowed the air down so that the prop (fan) can travel more slowly, and then they heat the air behind the prop to gain excess pressure and thrust. And this is exactly what a mach 1+ jet engine does. The opening at the front of the engine forces air in, and the design of the inlet slows the air down so it is subsonic, along with a corresponding increase in pressure. Some fans (props) which are now effective because they spin at subsonic speeds compress it more, so more air can enter the engine. Then they burn fuel to heat up the air, increasing the pressure. After that they have a few more fans that run in reverse to drive the fans at the front of the engine, and finally exhaust this hotter, bigger, more high pressure air out the back of the engine to produce thrust.
So hopefully you see how your question is an interesting one, and one that has already been sorta done. Turbofans and turboprops are really quite similar, but at mach speeds only the turbofans have the right environment to be able to work efficiently. Your idea would have the prop in a supersonic air stream, which would make it effectively useless.
>a high pressure shock wave that effectively makes the prop not prop-shaped, so it can't move air.
Leading micro eddys can solve this.
If the induction is a straight stream, it will fail - you need to direct micro eddys
If you do this with mechanical means (deflection cowlings) you will hit a limit.
The ideal design is in the funneling of eddys as they traverse in a super spiral between the front eddy and as it spirals to the thrust vector.
however, pre-ionizaton, and then magnetic ion direction can swirl the eddy to the desired output. However, AIR is not the thrust component at this time, its ionized energy which is being "thrust" (thrust is typically thought of as a 'push' - but this is actually a 'pull'
Well, samstave's original question was one of adding power with an inline stack. As I understood it, that isn't the purpose for he Tu-95's pair of props since they share a power source. The below explanation [0] has some interesting analysis based on Russian language documentation about how torque is divided between the prop pairs. Additionally the paper linked from the Wikipedia contra-rotating prop page "Analysis of a contra-rotating propeller driven transport aircraft" [1] has a great section on fuel savings, which probably has contributed to the Tu-95's success.
Thus, the front prop gets almost 20% more torque than the rear prop.
Another aspect to consider is changing engine characteristics for different conditions. An example is changing the structures guiding air into the engine.
"can" vs "can come up with an implementation that provides any performance benefits in any set of real world circumstances"
You could. But there's no way the efficiency and complexity penalty having props feeding fans or fans feeding props comes out ahead of "pick one and make it bigger"
Can these be 'stacked' - Can you have a turboFan in line with a turboProp such that the output of the wash of the Fan feeds into the Prop, but with a portion of the wash spinning to thrust on the outer ring of output.=, via a design in the cowlings which is hyper directed thrust vents (think the grid of straws used to funnel water into a cohesive column, which can be directed)
Imagine a small diameter turboprop behind a much larger turbofan