Like a gun, which 'reacts' with a quick backward movement (i.e. the recoil) when a projectile is shot forward at high speed, a jet engine derives its thrust by reaction to its high-speed ejection of combustion products, and by the expansion of heated air, which is pushed out at a higher speed than when the air is drawn in. (After all, ordinary propellers work in a similar way. They accelerate backwards the air mass that moves through their rotating blades.) These introductory remarks are meant to help our visitors gain an insight into the workings of Ing. Campini's thermojet. It is, in effect, to be considered more as a hybrid than as a jet engine proper. An internal combustion engine characterized by reciprocating motion of pistons in its cylinder - in our case, a 900 hp Isotta Fraschini L. 121/R.C. 40 engine - drove a compressor incorporating 2 ducted propellers and a propeller designed to direct the flow and minimize the breakdown of the smooth airflow. A ring of injectors (i.e. the burners) introduced kerosine, whose combustion increased the volume of the thermojet and the exhaust velocity.
It was an interesting application, albeit structurally different from German turbojets - like those fitted to the Messerschmitt Me 262 aircraft - which, in place of the reciprocating engine, featured a turbine-driven air compressor. The turbine was to be found past the blast chamber, and the air under pressure was heated not through the injectors, but through several blast chambers that heated the air by conduction. Which was a more effective solution. What's more, the higher the altitude and speed, the better the performance of turbojets. On the other hand, endothermic reciprocating engines - like the one used by Ing. Campini - attain top efficiency at sea level, while they call for an extra compressor to operate at high altitude where the air is rarefied.
The engine designed by Ing. Campini had many other drawbacks. That is, it was heavy and bulky, the type of engine used to drive the compressor was rather complex, the efficiency of the burner was low - although it came close to the best possible performance of the day -, and maximum power was considerably limited. This is why the German design - which came into use when World War II was drawing to an end, and was partly due to the British research work carried out in parallel - is the forerunner of the modern jet engines, while the Italian version has a purely historical value.