with reference to fig1 , a projectile 1 incorporates several deployable fins 2 at its rear part , only one of which is shown here . the fin is in the folded position , pressing against and along the projectile body 1 and oriented substantially along axis 3 of the projectile . during firing , it pivots , thanks to a deployment mechanism 4 , which is not described further here since it is not the subject of the present invention . this deployment mechanism may be analogous to that described in patent fr - 2860577 to which reference may be made for further details . when the fin is deployed , a rear part 5 of the fin penetrates into a radial groove 6 arranged on a support 7 integral with the projectile body 1 . the support 7 may be more clearly seen in fig2 . the support 7 comprises two elements 8 and 9 which are arranged on either side of the groove 6 . these elements will be made integral with the projectile body 1 , for example by screws ( not shown ). one of these two elements ( element 8 ) is made of a plastic material and incorporates an extension 10 which constitutes the bottom of the groove 6 . thus , when the fin 2 is deployed , the shock of the fin against the bottom of the groove 6 is absorbed . element 9 is made of metal ( for example aluminum ). it has a recess 11 in its upper part . in accordance with the invention , a tongue 12 is fastened to element 9 . this tongue is constituted , for example , by a sheet of steel a few tens of millimeters thick with square edges . the tongue 12 is pinched between element 9 and a block 13 made , for example , of aluminum . fastening is ensured , for example , by two screws , only the axes 14 of which are shown in the figures . in this way , the link between the tongue 12 , element 9 and the block 13 can be assimilated to an embedding allowing neither slippage nor pivoting of the tongue 12 . because of its material and its thickness , the tongue 12 has certain flexibility . it can bend upon the passage of the fin in the direction of the opening without opposing any resistance whilst still remaining within the range of elastic deformation . it is arranged such that its end 12 a protrudes into the groove 6 . this part of the tongue has square edges so as to avoid any subsequent relative slippage with the fin 2 when this tends to rise up out of the groove 6 . the tongue 12 thus protrudes by around 0 . 5 mm inside the groove 6 . the recess 11 , to the side of the groove 6 , enables the tongue 12 to bend in direction i thereby enabling the butting of the mechanism , thus preventing the fin from rising up in a direction e opposite to the first direction i . fig3 shows the fin 2 engaged in the groove 6 . when it enters the groove 6 , the fin 2 pushes the tongue 12 with no difficulty causing it to bend easily and freely deform in the recess 11 , allowing the fin 2 to pass . once in place , it is no longer possible to remove the fin 2 from the groove 6 . indeed , the end 12 a of the tongue 12 presses on the lateral surface 2 a of the fin without slippage , thanks to the square edges and to the judicious dimensions of the tongue . by reaction , the tongue 12 presses the fin 2 onto the face 6 a of the groove ( integral with element 8 ). indeed , the tongue works in compression under the action of the stresses communicated by the fin attempting to exit the groove . the adherence stress of the fin 2 on the face 6 a is greater than the external forces tending to raise the fin . the fin 2 is thus firmly immobilized by the adherence force at face 6 a , and following the principle of buttressing . the device is all the more effective in that the external forces are substantial . it is thus impossible for the fin to rebound , any backward movement being prevented by the tongue 12 . the fin 2 is immobilized immediately , reliably and with no play , and the fin is no longer able to exit its groove . additionally , by way of a variant , it is possible for a shock absorbing material ( for example a layer of rubber ) to be positioned in the bottom of the groove 6 . additionally , elements 8 and 9 of the support 7 may be made of the same material . these elements may be constituted of a single piece 7 . the case of the deployment of stabilizing fins has been taken here as an example to describe the invention . it is naturally possible for the invention to be applied to other types of deployable surface ( elevons , wing , etc ). it is moreover possible to implement the invention using a deployment device of a different structure to that described in patent fr - 2860577 .