of which is coupled to a spool fastened to the piston said cable being wound onto the spool according to a variant of the invention the axis of the spool is offset relative to the axis of the shell which prevents interference between the piston and the payload in the event the cable breaks according to another variant the shell possesses at least two cables wound onto at least two spools distributed at regular angular intervals on the piston see fig5 and 7 according to another variant the axis of the spool is identical to the axis of the shell see fig6 the spool or spools can consist of one or more cylindrical extensions of the piston according to an advantageous embodiment the spool is retained in a temporary manner for example by a spot of glue on a countersink present on a collar coupled to the head the piston having means allowing it to be held on the spool see fig4 the means by which the piston is held on the spool can comprise a deformable flexible ring placed in a groove of the piston preferably the spool will have a conical profile intended to cause compression of the flexible ring in its groove when the piston is put.