by Philippe Ballarini
Translation Marc Binazzi

The ramjet
Past, present or future ?

Imagining a propelling device reproducing the classical thermodynamic cycle compression-combustion-expansion, and this without any moving part, this is what engineer René Lorin achieved right from the beginning of the 20^th century. The originality of this principle was to reduce the propeller to a mere tube. His discovery came far too soon. Indeed, for his ramjet engine (or, as René Leduc would eagerly name it, his thermal jet engine nozzle) to be operational, air speed by the intake of the jet nozzle should (in order to obtain the required compression) reach a speed that no machine at that time would possibly generate (something like 200 kph). Therefore René Lorin never reached the operational phase.

burnt gases exhaust

fuel injectors

We had to wait till René Leduc « re-discovered » the process for the thermal jet engine nozzle to come to real life. The ramjet engine could appear as the ideal engine, in particular to conquer high speeds. Truly speaking, its advantages were far from insignificant. The absence of moving parts, a rather low cost induced by its simplicity of manufacturing, a fuel consumption much lower than that of rockets, not to mention its adequation to high speeds, all these elements pleading in its favour and possibly accounting for René Leduc's relentlessness to impose it in the field of aeronautics.

Yet the ramjet engine suffers a major flaw, one that is not easily by-passed : its inability to make any flying object take off by itself. An aeroplane equipped with that propulsion system will be either piggy-backed from another aeroplane or equipped with a dual propulsion ensuring its take-off, or even will be catapulted from a specially devised trailer, which, one will admit, makes its implementation highly intricate.

Nevertheless, the ramjet engine did not vanish from the aeronautical scene with the Leduc and Nord-Aviation's « Griffon », the latter being equipped with a dual propulsion, thermo - ramjet engine.

Corporations such as Sud-Aviation, Nord-Aviation or ONERA quickly understood the advantages they could gain from this process, speeds above Mach 3,5 not being within easy reach while using a sole turbojet. Right from the beginning of the fifties, various tactical missiles with ramjet engine propulsion accelerated by powder rockets were designed. An example of that system : Matra-Aerospatiale's current ground-air ASPM (Average Range Ground - Air) missile.

Still, ramjet engine's future will perhaps not be limited to missiles alone. On both sides of the Atlantic ocean, many researchers seem to be thinking that a bright future can be expected for engines combining take-off turbojet, broadly hypersonic speeds ramjet engine, and rockets for out-of-atmosphere forays. The ramjet engine would operate for speeds in excess of Mach 3 (it should be noted that so far the fastest atmospheric propulsion machine is SR-71 at Mach 3).

This concept has been studied in particular for designing « orbital » planes able to free themselves on their own from the atmosphere and the vacuum that delimits the new frontier for the aviation of the 21^st century, aiming at operating without the extremely expensive rocket launchers.

In spite of Leduc's relative failure, the ramjet engine gained widespread acceptance. It should be noted that even though the Americans and Russians are closely investigating that type of engine, France, which has always been highly innovative in the ramjet engine area, of which it remains the cradle, still holds a dominating position in that field where projects abound. Therefore, the end of the Leduc should not be interpreted as an extinction for ramjet engines which should enjoy a bright future in the area of hyper speeds (superior to Mach 5), where turbojets are ineffective.

© Aérostories, 2001