Q:

Is a subsonic scram-jet reasonable?

By placing a de laval nozzle at the inlet, can the resulting supersonic airflow allow a scram jet to operate efficiently?

A:

First let's clear up some things:

A SCRAM jet is a Supersonic Combustion RAm Jet, where the flow speed in the combustion area is still supersonic. Efficient combustion needs low speeds, hence a supersonic combustion is not a good solution to maximize efficiency. It needs rapidly mixing and burning fuels like hydrogen, lest most of the thermal energy release happens when the gasses have left the engine.
The operating principle of all aircraft engines with continuous combustion is that the kinetic energy of the flow is converted into potential energy, namely pressure. This pressure is needed to convince the hot combustion products to leave the engine at the rear end, where the pressure drops as the flow is accelerated again. If the engine flies at subsonic speed, the first area where supersonic flow is possible is in the convergent-divergent nozzle. Normally, pressure recovery is too small at subsonic speed, and either some turbo machinery or an oscillating pressure wave is needed to increase compression.

A de Laval nozzle (first developed by Ernst Körting in 1878) is a fixed convergent-divergent nozzle and needs high inlet pressure to function. Placing it at the subsonic inlet would require a near vacuum at the outlet to make it work. Using it in combination with a Ram jet would result in separated, subsonic flow with little pressure recovery in the combustion area, and very little thrust could be produced. In principle such an engine would run, but I doubt that it will even produce enough thrust to result in a net thrust gain. Efficiency would most likely be negative.

