present context the term u201c helix u201d is used broadly possibly even covering a line presenting a variable angle relative to the central axis x and thus presenting a variable pitch between spires in yet another embodiment shown in fig4 a and 4b the concentric first and second helical channels 204 and 205 are all connected to the inlet of the second propellant e 2 it is thus possible to inject the second propellant e 2 via a plurality of concentric rings 210 of helical channels 204 205 thereby obtaining a better match to a desired flow rate for the second propellant e 2 a third channel 206 of annular section and connected to the inlet for the first propellant e 1 serves to inject the first propellant e 1 this third channel 206 is situated outside the helices 204 a 205 a of the first and second helical channels 204 205 the remaining elements of the injector element 201 shown in fig4 a and 4b are given the same reference numbers as the corresponding elements in the preceding figures nevertheless as an alternative the third channel 206 may be situated inside the concentric rings 210 of the first and second helical channels 204 and 205 as in the embodiment shown in fig5 in.