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Those micro jets, they are very inefficient in part due to little or no bypass. 

As basic jet engines they function as ‘gas producers’ which means they create high KE high velocity gas exhaust. What can be done here to boost thrust and payload and thus increase weapon payload and/or fuel payload is to attach a slower RPM downstream turbine fixed directly to a prop.  What we get is more thrust per unit fuel, so a larger drone is possible. Cost per unit of thrust should not increase because the added part should be quite cheap, in fact it should decrease.

This is how most turbo props work. 

The prop can also be placed in a ducted fan at the back, which may reduce RCS and be based on absorbing materials with carbon black additive which also make the turbine less visible. 

So the principle is this – high velocity gas leaves unusuable kinetic energy in the exhaust stream. Taking some of that K.E. and using it to turn a prop results in higher mass flow at lower exit velocity which increases propulsive efficiency. 

The added turbine does not create significant jet engine back pressure, this is known since the days where 2 strokes were fitted with compounding turbines. 

It also will not create a counter rotational torque on the aircraft. 

You want to get a by pass ratio well up above 3. Since we can tell the max speed is subsonic on this design, the other things to optimise are duct inlet shape which can slow the inlet air, and prevent prop tips going transonic, so the design would probably better utilise more blades going at a slower rpm. 

Blending into the fuselage has additional advantages, because the fuselage speeds up the air at the boundary, slower air is ingested to the ducted fan or open prop. This is more efficiently accelerated so you can get better thrust per unit fuel. 

The add on system does not alter the main jet engine. I would suggest that UA considers building these as add on kits with zero or low bypass microjets, using blisk based turbine/s. 

Two such blisks, counter rotating, can power two seperate directly attached, counter rotating props or fans. This removes swirl and can further improve efficiency, but requires a lot of aero analysis. 

Because the downstream turbine rotates comparatively slowly, it does not need to be made from exotic high temp alloys that resist stretching under centrifugal loads.