Can motorjets be a viable means of propulsion? A motorjet is a very early type of jet engine where intake air passes through a compressor driven by a separate piston engine, and is then mixed with fuel and burned to provide thrust. They were used in a handful of designs around the time of World War II, but fell out of favour when gas turbine metallurgy had advanced to the point where it was feasible to drive the engine compressor from a turbine in the exhaust stream without having to rebuild the engine every few flights; all else being equal, a motorjet, requiring an entire second engine to drive the compressor, is heavier, and, thus, has a lower thrust-to-mass ratio than a turbojet or turbofan. Are there any situations where a motorjet would still be a better choice than a turbojet or turbofan?

I can think of a few maybes:


Aircraft using fuel that produces refractory oxides when burned (such as boron-containing fuels and most organometallics), or operating in air heavily contaminated with particles of same (such as volcanic ash clouds), as these are very hard on turbines unless the exhaust gas temperature (EGT) remains high enough to vapourise these oxides all the way out the tailpipe; a motorjet has no turbine blades in the exhaust stream to suffer ill effects from this (unlike a turbojet or turbofan), does not need an onboard supply of oxidiser (unlike a rocket), and produces reasonable thrust even at zero airspeed (unlike a ramjet).
Aircraft with very severe length restrictions, as a turbojet or turbofan's compressor(s) and turbine(s) all have to be in line with one another, whereas a motorjet's compressor motor could be stacked vertically above the compressor and combustion chamber.
VTOL aircraft, where the torque and gyroscopic effects from all that heavy, rapidly-spinning turbomachinery can cause control problems while hovering or moving at very low speed (due to the control authority of aerodynamic control surfaces being very low at very low airspeeds, and nil at zero airspeed), as pointed out in this comment by Peter Kämpf on an answer to one of my earlier questions.
Internal combustion engines (ICE) have great thermal efficiency, so driving a fan or compressor using an ICE is on paper very appealing indeed.


  It is well known that internal combustion engines are capable of achieving higher thermal efficiency than gas turbine engines.


NASA even looked at it in 1976. The proposal was to substitute a jet engine's combustor with an ICE. Two problems with that (according to Rolls-Royce) are the need for a gearbox and "component aerodynamic mismatching under certain engine operating conditions" due to the single-shaft design.

R-R filed a patent in 1995 that according to them fixes the problems with NASA's engine. So far we know why motorjets will have a hard time making a comeback.

R-R's solution is, more shafts, of course. They propose to have an ICE in place of the combustor, but this time it has its own shaft, fed by its spool's compressor, and the exhaust would drive a turbine (too many parts already) that drives a ducted fan (think turbofan but with ICE thermal efficiency).


  According to the present invention, an aircraft compound cycle propulsion engine comprises a propulsive fan and a core engine to power said fan core engine comprising an air compressor downstream of said fan, at least one rotary internal combustion engine configured to receive compressed air from said air compressor, a power turbine positioned to receive and to be powered by the exhaust efflux of said at least one rotary internal combustion engine, a first shaft drivingly interconnecting said power turbine and said propulsive fan, and a second shaft drivingly interconnecting said at least one rotary internal combustion engine and said air compressor.






Source: https://patents.google.com/patent/US5692372A/en