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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.

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    $\begingroup$ en.wikipedia.org/wiki/Liquid_air_cycle_engine uses similar solution and I guess would qualify as a modern version of a motorjet? So potentially of interest despite not being really relevant to the question, I hope. $\endgroup$
    – Ville Niemi
    Dec 28, 2018 at 6:58

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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.

enter image description here

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

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    $\begingroup$ This configuration is not the OP's motorjet, which does not have a turbine. $\endgroup$
    – Pilothead
    Oct 4, 2018 at 1:16
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    $\begingroup$ NASA and RR use of a combustion engine in some contraption other than a motorjet is interesting but has no bearing on the question. The reference, description, and examples all contrast a motorjet with a turbofan/jet, the operative difference being the presence of a turbine which was metallurgically difficult at the time this was under consideration. $\endgroup$
    – Pilothead
    Oct 4, 2018 at 1:38
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    $\begingroup$ @Pilothead - The way I see it (opinions may vary): motor ✓, shaft ✓, compressor ✓, exhaust ✓, NASA and R-R choosing to use the exhaust to drive an efficient fan, which makes sense for subsonic applications, is the difference you're noting. With the more efficient fan, we still haven't heard from the design. $\endgroup$
    – user14897
    Oct 4, 2018 at 1:45
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    $\begingroup$ "Internal combustion engines (ICE) have great thermal efficiency..." Huh? ICEs seem to be terribly INefficient: Toyota brags about getting 38% efficiency from its gasoline engines, diesels are a bit better, but only get above 50% in the large ones used in ships and the like. And as a practical matter, you find ICEs only rarely used for power generation (usually for standby or emergency power). Even where an ICE could easily be used, as with natural gas, gas turbine generation is preferred. $\endgroup$
    – jamesqf
    Oct 4, 2018 at 4:00
  • $\begingroup$ @jamesqf - see added quotation from R-R's patent. $\endgroup$
    – user14897
    Oct 4, 2018 at 4:05
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A motorjet is not a useful configuration.

The purpose of a motorjet is to generate pure jet propulsion without the difficulties associated with use of a turbine. The elimination of the turbine is not the material factor in determining whether this configuration still has a useful purpose. The important thing to identify is whether or not pure jet propulsion is useful.

The trend in aviation propulsion is towards ever higher bypass ratios, due to the increased fuel efficiency of a fan or prop compared to that of a pure jet. This means ever more power is mechanically extracted from the core to drive a fan or prop.

enter image description here

An internal combustion engine already has all its useful power available in mechanical form, which can then be used to drive a prop or fan. Diverting energy to drive the compressor and combustor in a motorjet is exactly opposite of the trend towards fuel efficiency, making it useless in subsonic applications.

The ICE is also a very heavy substitute for a turbine, so even supersonic applications that benefit from low bypass operation won't use it either.

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  • $\begingroup$ isn't motorjet infinite bypass ratio by definition? $\endgroup$
    – user3528438
    Dec 29, 2018 at 23:53
  • $\begingroup$ @user3528438 A motorjet has a zero bypass ratio. All the air goes through the combustor where it is heated to produce jet thrust. There is no power extraction to drive a fan or prop. $\endgroup$
    – Pilothead
    Dec 30, 2018 at 23:50

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