GB2265443A - Fin assembly for a projectile - Google Patents

Abstract

A fin assembly 12 includes four tail fins 14 pivotally mounted on an axially slidable block 24. The fins are deployable from a stowed position, in which they lie generally parallel to the longitudinal axis of the associated projectile, to a laterally extended position. Each fin is connected by a link 30 to a pivot point 28 which is axially fixed with respect to the projectile, whereby rotation of the fin moves the link to draw the block 24 forwardly. Deployment of the fins is therefore accompanied by a forward shift of their pivotal axes 26. <IMAGE>

Description

FIN ASSEMBLY FOR A PROJECTILE This invention relates to a fin assembly for a projectile.

We have found that in certain forms of projectile the stability requirements following launch may be incompatible with those required at a terminal phase of flight of the projectile. For example it is often required that the projectile have a high degree of stability for an initial period following launch where little or no manoeuvring of the projectile is required but that it should have a lower degree of stability during the final phase of flight to allow it to manoeuvre to acquire and destroy a target.

In other circumstances it may be that the system design, rather than stability considerations, dictates that the projectile is "over-stable" and thus, unless the stability is reduced, relatively high lateral acceleration must be applied to the projectile to produce a given radius of turn.

According to one aspect of this invention, there is provided a fin assembly for a projectile, comprising a plurality of fins pivotable from a stowed position to a deployed position and arranged such that deployment of the fins is accompanied by a forward shift of the pivotal axes of the fins relative to the projectile.

Further aspects will become apparent from the following description of a non-limiting example of this invention, reference being made to the accompanying drawings, in which: Figure 1 is a schematic side view of an aft end portion of a projectile fitted with an example of fin assembly of this invention but with only a single fin shown; Figure 2 is an aft end view on the projectile of Figure 1; Figure 3 is a part-sectioned side view on the part of the projectile of Figure 1 but with the fin member and correcting link removed for clarity; Figure 4 is a section view on lines IV-IV of Figure 3 when the fin members are in the deployed position, and Figure 5 is a view similar to Figure 1 but illustrating the fixed centrode swept by the fin assembly linkage as the fin erects over the last 50 degrees of travel.

The projectile illustrated in the drawings incorporates a means for reducing the aerodynamic stability of the projectile by sliding forward a decoupled fin assembly thus enabling the projectile to experience high lateral acceleration with minimum radius of turn while at the same time permitting a self-erecting fin assembly.

The projectile includes a main body 10 which incorporates a lateral thruster system or similar system (not shown) for imparting to the projectile a lateral acceleration vector in accordance with commands issued from a guidance system or autopilot. The aft end of the body portion supports a fin assembly 12 which is despun from the remainder of the projectile body and which includes four offset tail fins 14. The fin assembly 12 comprises a collar 16 having a forward end connected to the aft end of the projectile main body 10 and extending rearwardly away from the main body 10. A tube 18 with external splines 19 fits over the aft portion of the collar 16 and is rotatably mounted on the collar 16 by bearings 20 to allow rotation of the tube 18 about the longitudinal axis "L" of the projectile.A fin mounting block 22 having internal splines is slidably mounted on the tube 18 for movement parallel to the longitudinal axis "L" of the projectile. The block 22 has an aft portion 24 of square external section and each face of the block pivotally supports on a pivot 26 a fin 14 for pivotal movement against the face in a respective plane parallel to and spaced from the longitudinal axis "L" of the projectile.

The forward portion of the tube 18 is provided with four equispaced pivots 28 and a respective connecting link 30 interconnects each pivot with a pivot 32 near the inner leading edge of the associated fin 14. Each connecting link, in use, moves in the same plane as the fin 14 to which it is connected.

In the stowed position, each fin 14 is stowed parallel to the main body of the projectile so that the block 22 is located at the aft end of the tube 18 and the connecting links 30 lie parallel to the longitudinal axis of the missile.

The four offset fins erect by jack-knifing on the pivots 26 on the block 22. As the fins 14 erect, the connecting links 30 cause the block 22 to slide forward on the tube 18. Thus the fins are erected some distance forwardly of the stowed position and swept back. In one example, the erected fins are 70 mm forward of the stowed position and swept back through 300. The tube 18/block 22 splining prevents the connecting links 30 from being twisted. The fin assembly is de-coupled in roll by the bearings 20. The linkage arrangement initially causes the block 22 to move rearward before moving forward.

A possible buffer/stop position is indicated at 34 on Figure 3 for use where the fins are deployed at one specific velocity. In another example of assembly (not shown) the tube 18 could be shortened and the space within the tube 18 used to house a dashpot assembly. This could be actuated by a cross-pin in the block 22 working through slots in the wall of the tube 18.

In use, the fins 14 will need initial lifting into the airstream to start deployment. During deployment, aerodynamic loading is predominant causing the fins to accelerate rapidly away from the stowed position. Figure 5 shows the positions of the fin over the last 500 of deployment in increments of 5 and the corresponding positions of the fixed centrode swept by the fin linkage. It will be seen that the instantaneous centre moves outwards from the main body 10 so that, towards the end of deployment, the instantaneous centre of the linkage is well out from the projectile body, approaching half the semi-span of the fins 14. This means that the aerodynamic forces on the fins are almost balanced around the instantaneous centre towards the end of deployment, thus reducing the deployment energies so that the deployment velocity is gradually reduced, especially towards the fully deployed position with consequent benefits in reduction of shock loadings etc.

In another example (not shown) it would be possible to provide an arrangement which included two fins mounted on a block'as -in the illustrated arrangement and two fins pivoted on the projectile body.

Claims (9)

1. A fin assembly for a projectile, comprising a plurality of fins pivotable from a stored position to a deployed position and arranged such that deployment of the fins is accompanied by a forward shift of the pivotal axes of the fins relative to the projectile.

2. A fin assembly according to claim 1, wherein the fins are pivotally mounted on a fin mounting block slidably mounted with respect to the main body portion of said projectile.

3. A fin assembly according to claim 2, wherein each fin is connected by a respective connecting link to an associated pivot point which is axially fixed with respect to said projectile, whereby rotation of said fin member causes said connecting link to be moved to draw the fin mounting block forwardly.

4. A fin assembly according to any one of the preceding claims wherein each fin is stored in its stowed position with a major portion of the fin lying forwardly of the pivotal axis of the fin.

5. A fin assembly according to any one of the preceding claims wherein means are provided to allow rotation of the fin assembly with respect to the projectile.

6. A projectile incorporating a fin assembly according to any one of the preceding claims.

7. A projectile according to claim 6, which further includes control means for imparting a lateral thrust to said projectile.

8. A fin assembly substantially as hereinbefore described with reference to, and as illustrated in, any of the accompanying drawings.

9. A projectile substantially as hereinbefore described with reference to, and as illustrated in, any of the accompanying drawings.

9. A projectile substantially as hereinbefore described with reference to, and as illustrated in, any of the accompanying drawings. Amendments to the claims have been filed as follows CLAIMS

1. A fin assembly for a projectile, comprising a plurality of fins each pivdtable as a whole about a respective pivotal axis from a stowed position to a deployed position and arranged such that deployment of the fins is accompanied by a forward shift of the pivotal axes of the fins relative to the projectile.

2. A fin assembly according to claim 1, wherein the fins are pivotally mounted on a fin mounting block slidably mounted with respect to the main body portion of said projectile.

3. A fin assembly according to claim 2, wherein each fin is connected by a respective connecting link to an associated pivot point which is axially fixed with respect to said projectile, whereby rotation of said fin member causes said connecting link to be moved to draw the fin mounting block forwardly.

4. A fin assembly according to any one of the preceding claims, wherein each fin is stored in its stowed position with a major portion of the fin lying forwardly of the pivotal axis of the fin.

5. A fin assembly according to any one of the preceding claims wherein means is provided to allow rotation of the fin assembly with respect to the projectile.

6. A projectile incorporating a fin assembly according to any one of the preceding claims.

7. A projectile according to claim 6, which further includes control means for imparting a lateral thrust to said projectile.

8. A fin assembly substantially as hereinbefore described with reference to, and as illustrated in, any of the accompanying drawings.