US20250334386A1

US20250334386A1 - Deploying fins for a gun-launched projectile

Info

Publication number: US20250334386A1
Application number: US18/409,403
Authority: US
Inventors: John A. Condon; Bryant P. Nelson
Original assignee: US Army Research Laboratory
Current assignee: US Army Research Laboratory
Priority date: 2024-01-10
Filing date: 2024-01-10
Publication date: 2025-10-30

Abstract

A fin deployment apparatus for stowable fins that comprises a sub-caliber fin hub configured to be rotatably coupled to an aft portion of a projectile, the fin hub having a plurality of flat, planar exterior surface regions, each surface region having pivotably coupled thereto a respective set of fins. Each fin in a stowed position is disposed upon a respective exterior surface region of the fin hub and a respective exterior cavity or recess of the projectile. Each fin is urged toward a deployed position by a respective torsion spring, which operates to deploy the fin after the projectile has left the barrel or tube. The sub-caliber fin hub is further configured to be keyed to the aft portion of the projectile during storage and while within the bore of the barrel or tube in response to urging from a propellant, the fin hub when keyed thereto being rotatably locked with the projectile forebody

Description

GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to ordnance having stowable fins, and, more particularly, to a deployment apparatus for stowing and deploying the fins.

BACKGROUND

Fins are typically added to a projectile to add/provide aerodynamic stability in free flight but may be overly complex in design and expensive to produce. They also may be difficult to apply in a gun-launch or tube-launch environment, wherein the fins may need to be super-caliber (i.e., greater than the caliber diameter of the projectile) and may occupy an unacceptably large internal volume in the projectile and negatively impact projectile payload-carrying requirements.
Mechanisms exist to retain fins in a stowed position within a projectile in a barrel or tube, and to deploy super-caliber fins when the projectile is in flight. Such deployment mechanisms are complex, costly, and occupy more internal space/volume.
For example, U.S. Pat. No. 8,415,598 provides extendible fins for a tube-launched projectile by maintaining the rear fins in a stowed configuration until the entire projectile has exited a gun tube muzzle. After exiting the muzzle, the fin deployment apparatus may function by using the differential pressure between a pressurized gas cavity of the fin deployment apparatus and the atmosphere surrounding the fin deployment apparatus. The differential pressure may shear a bolt and/or push a plug out of and away from a base, thereby releasing and deploying the stabilizing fins. The plug may have an aerodynamic shape to ensure that it does not return to the gun site and inflict damage to the crew or equipment. Once released, the fins may then mechanically lock at a desired angle from the base.

SUMMARY OF THE INVENTION

Various deficiencies in the prior art are addressed below by the disclosed fin deployment apparatus and system comprising a sub-caliber fin hub for stowing and deploying fins on a gun-launched or tube-launched projectile. Each fin is deployed via a respective torsion spring, and in a stowed position occupies a portion of the very outer sub-caliber peripheral space on the rear part of projectile and a portion of the forward part of projectile, which maximizes internal volume to provide increased payload capacity. A coupling/de-coupling feature allows the fin-set's hub to spin relative to the projectile's forebody to minimize roll damping thereby increasing maneuverability for bank-to-turn control schemes.
The fins are retained by the cartridge case during storage. During launch (in-bore) the fins are retained by their geometry. The center of gravity of each fin is located inside of the pivot point such that the launch acceleration retains the fin in its stowed position (i.e., the launch induced torque is greater and opposite to the torque provided by the torsion spring). Upon muzzle exit, the munition is no longer exposed to the launch acceleration allowing the torsion spring to deploy the fins.
Various embodiments provide a deployment apparatus for stowable fins that comprises a sub-caliber fin hub configured to be rotatably coupled to an aft portion of a projectile, the fin hub having a plurality of flat, planar exterior surface regions, each surface region having pivotably coupled thereto a respective set of fins. Each fin in a stowed position is disposed upon a respective exterior surface region of the fin hub and a respective exterior cavity or recess of the projectile. Each fin is urged toward a deployed position by a respective torsion spring, which operates to deploy the fin after the projectile has left the barrel or tube. The sub-caliber fin hub is further configured to be keyed to the aft portion of the projectile during storage and while within the bore of the barrel or tube in response to urging from a propellant, the fin hub when keyed thereto being rotatably locked with the projectile forebody. The fins generally comprise elongated fins having proximal and distal ends substantially axially aligned with the projectile in the stowed position. The proximal and distal ends are substantially radially aligned with the projectile in the deployed position.
Various embodiments provide a sub-caliber fin hub that is configured to be rotatably coupled to an aft portion of a projectile and comprises a plurality of flat, planar exterior surface regions, each surface region having pivotably coupled thereto a respective set of fins, each fin in a stowed position disposed upon a respective exterior surface region of the fin hub and a respective exterior recess of the projectile, each fin being urged to a deployed position by a respective torsion spring. The sub-caliber fin hub configured to be keyed to the aft portion of the projectile during storage and in-bore in response to urging from a propellant, the fin hub when keyed thereto being rotatably locked with the projectile forebody. The fins may comprise elongated fins having proximal and distal ends substantially axially aligned with the projectile in the stowed position. The proximal and distal ends may be configured to be substantially radially aligned with the projectile in the deployed position.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the present invention.

FIGS. 1A and 1B illustrate side views of a projectile and fin deployment apparatus in a stowed fin position and a deployed fin position, respectively;

FIG. 1C depicts a rear view of the projectile and fin deployment apparatus of FIG. 1B;

FIGS. 2A and 2B illustrate side views of a projectile and fin deployment apparatus in a stowed fin position and a deployed fin position, respectively;

FIG. 2C illustrates the locking feature which prevents in-bore rotation between the fins and forebody;

FIG. 3 depicts a perspective view of an integrated munition according to an embodiment; and

FIG. 4 depicts a detailed view of an individual fin deployment mechanism with a fin in the deployed position. The shoulder screw provides a bearing surface for the fin deployment.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the sequence of operations as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes of various illustrated components, will be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments have been enlarged or distorted relative to others to facilitate visualization and clear understanding. In particular, thin features may be thickened, for example, for clarity or illustration.

DETAILED DESCRIPTION OF THE INVENTION

The following description and drawings merely illustrate the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its scope. Furthermore, all examples recited herein are principally intended expressly to be only for illustrative purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art and are to be construed as being without limitation to such specifically recited examples and conditions. Additionally, the term, “or” as used herein, refers to a non-exclusive or, unless otherwise indicated (e.g., “or else” or “or in the alternative”). Also, the various embodiments described herein are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.
Various embodiments contemplate a gun-launched, space-saving, rear fin-set deployment apparatus and system providing stability to a ballistic trajectory for spinning or non-spinning ordnance (e.g., munition or submunition), unmanned aerial vehicles (UAVs) or drones, and/or other gun-launched or tube-launched projectiles while minimizing impact to the projectile's payload requirements (e.g., form factor, size, footprint, etc.). For example, the use of the disclosed rear fin-set increases stability (therefore accuracy and lethality) and payload capacity of various ‘enhanced’ ordnance, such as 40 mm, 155 mm, and/or other larger-caliber munitions, including munitions configured for guided/extended-range applications.
Simultaneous fin deployment is desirable because it minimizes asymmetric load which will perturb the trajectory of the munition. Many precision munitions are designed to be marginally stable to allow for enhanced maneuverability throughout the flight and in the terminal flight phase. Aerodynamic fins provide stability in pitch, roll, and yaw. However, for bank-to turn control schemes (a technique where the airframe rolls its body into the desired plane perpendicular to the desired direction then performs a pitch maneuver; commonly seen in high-maneuverability aircraft) roll stability decreases overall maneuverability by reducing the speed of such a maneuver. Aerodynamic fins provide stability in pitch, roll, and yaw. However, for bank-to turn control schemes (a technique where the airframe rolls its body into the desired plane perpendicular to the desired direction then performs a pitch maneuver; commonly seen in high-maneuverability aircraft) roll stability decreases overall maneuverability by reducing the speed of such a maneuver.
FIGS. 1A and 1B illustrate a projectile and fin deployment apparatus in a stowed fin position and a deployed fin position, respectively. FIGS. 2A and 2B illustrate a projectile and fin deployment apparatus in a stowed fin position and a deployed fin position, respectively.
Specifically, an elongated projectile 1 of a predefined caliber (e.g., 40 mm, 155 mm, etc.) has rotational and translational degree of freedom and is keyed to a sub-caliber aft or rear portion SCR thereof a sub-caliber fin deployment assembly 2 comprising a fin hub 3 having a plurality of external surface regions HS configured to align with and mechanically cooperate with corresponding external surface regions PS of the sub-caliber rear portion SCR of the projectile 1 to enable stowage therein of respective fins 5.
As depicted in FIG. 1A, a fin 5 is pivoted about hinge 4 into a stowed position in substantial alignment with an axis A of the projectile 1 that extends over both a respective hub surface region HS and a respective projectile surface region PS so as to occupy a sub-caliber peripheral space between those two surface regions HS/PS and an inner bore surface of a gun or tube. This is the position of each of the fins 5 when the projectile 1 is within the gun or tube bore.
As depicted in FIG. 1B, a fin 5 is pivoted about hinge 4 into a deployed position in substantially perpendicular alignment with an axis A of the projectile 1. This is the position of each of the fins 5 when the projectile 1 is in flight.
The sub-caliber fin deployment assembly or apparatus 2 is rotatably and slidably attached and keyed to a sub-caliber aft or rear portion SCR of the forebody of the projectile 1. The keying will be described in more detail below with respect to FIG. 2C. Specifically, in a keyed position one or more protrusions P formed at a front facing portion of the portions of the fin hub 3 are received by a respective one or more cavities or recesses R formed at a rear facing portion of the projectile 1 such that, when the protrusion(s) P are urged into engagement with the recess(es) R the fin hub 3 (therefore the entire fin deployment assembly or apparatus 2) cannot spin independently of the projectile 1.
An initial urging resulting in such keying of fin hub 3 into projectile 1 may be provided during an initial assembly/fitting together of a projectile 1 and fin deployment assembly 2 to provide corresponding factory-integrated ordnance, or a field integration of a fin deployment assembly 2 to a projectile 1 to provide corresponding field-integrated ordnance. This keyed or integrated position of projectile and fin deployment apparatus (with fins stowed in an external cavity or recess of the projectile) is maintained during launch of projectile 1 by the force of propellant gasses.
A retaining screw 10 is axially oriented with respect to the fin deployment assembly 2 and projectile 1 passed through an aperture formed by an inner bushing 12 to engage with corresponding thread in the rear portion of the projectile 1. The inner bushing 12 is housed within an aperture of an outer bushing 14 of the fin hub 3. The inner 12 and outer 14 bushings are arranged to allow the decoupling of fin hub 3 from the projectile 1 forebody such that the fin hub 3 is able to rotate about the retaining screw 10 independent of any rotation of the projectile 1 when not keyed to the projectile 1 (e.g., separated by a distance d).
In one embodiment, the inner bushing 12 comprises a metal bushing and the outer bushing 14 comprises a plastic bushing are arranged to allow the decoupling of fin hub from the projectile forebody. Inner and outer bushings spin relative to one another via low friction contact. FIG. 2C illustrates a locking feature which prevents in-bore rotation between the fins and forebody. Specifically, upon muzzle exit, the fin section of a projectile translates rearward by a distance d, thereby disengaging the locking feature and allowing the fins to rotate independently.
Referring to FIG. 1A, the two faces are shown in the keyed K position, wherein the distance between the two keyed faces is substantially zero (the faces butt up against each other). However, referring to FIG. 1B, the two faces are shown as separated by a separation distance d after the munition has been fired and exits the barrel.
The separation distance d is limited by the retaining screw 10; that is, the fin deployment apparatus 2 may move or slide (with or without rotation) along the retaining screw 10 from a keyed (abutting) position with respect to the projectile 1 up to the allowed distance d from the projectile 1.
Deployment of the fins after the integrated ordnance leaves the gun or tube results in sufficient aerodynamic drag so at to exert a force upon the fin deployment apparatus 2 sufficient to enable it to slide rearward via the retaining screw 10 until the distance d between the fin deployment assembly 2 and projectile 1 is reached, at which time the fin deployment assembly 2 may rotate independently with respect to the projectile 1 and provide aerodynamic stability thereby. Additionally, the deceleration at muzzle exit works with aerodynamic drag to separate fin deployment assembly 2 from projectile 1.
FIG. 1C depicts a rear view of the projectile and fin deployment apparatus of FIG. 1B. Specifically, FIG. 1C depicts a rear view showing the deployment of a four fins variant (fins 51 through 54), each of the fins 5 being mounted by a respective hinge assembly (hinge assemblies 41 through 44) to a respective external hub surface region (HS₁through HS₄) of the fin hub 3. As depicted, the fins 5 deploy at substantially 90-degree angles with respect to each other, which angle may be adjusted by changing the location of bump stop 30 when using more or fewer fins.
It is noted that while the various figures and descriptions thereof are generally directed to 4-fin embodiments, it will be appreciated that various embodiments may have more or fewer fins (typically 3-8+ fins depending on the projectile and its purpose).
Further, while the various embodiments are generally described within the context of rear mounted stabilization fins, various embodiments may also be used within the context of projectile steering fins (typically fixed to a forward portion of a projectile such as via a canard). Thus, in some embodiments, munitions such as projectiles may include a separate forward canard actuation system that functions in conjunction with a rear fin-set.
Thus far described is a set of four substantially identical, independently-driven, sub-caliber fins (e.g., metal or composite material) that are connected to and initially stowed via a fin hub (e.g., fiber-reinforced plastic material) which is ‘keyed’ to the rear of a projectile (i.e., projectile forebody) so as to prevent spinning of projectile's aft end and the fin hub relative to each other during travel within a rifled or unrifled gun barrel or tube. The set of fins remain stowed within an external cavity or recess of the projectile upon projectile launch and during travel and acceleration within the gun barrel or tube, whereupon the fins are then deployed via urging from respective integral-to-hub metal torsion springs upon exit from barrel/tube muzzle. That is, the fins are retained by the cartridge case during storage. During launch (in-bore) the fins are retained by their geometry. The center of gravity of each fin is located inside of the pivot point such that the launch acceleration retains the fin in its stowed position (i.e., the launch induced torque is greater and opposite to the torque provided by the torsion spring). Upon muzzle exit, the munition is no longer exposed to the launch acceleration allowing the torsion spring to deploy the fins.
FIG. 3 depicts a perspective view of an integrated munition according to an embodiment wherein the fins 5 are in a deployed position and fin assembly 2 is separated by distance d from projectile body 1.
FIG. 4 depicts a detailed view of an individual fin deployment mechanism with a fin in the deployed position. Specifically, each fin 5 is hinged via a shoulder screw 4 or similar passing through the fin 5 and pivotally securing the fin 5 to a corresponding external surface regions HS of the fin hub 3. A torsion spring 20 is connected, at one of its ends, to the fin 5 by penetration of the end of the torsion spring 20 through a torsion spring aperture 22 formed in the fin 5 such as depicted in FIG. 4 . The other end of the torsion spring 20 is received via a respective torsion spring cavity 24 within the fin hub 3.
The torsion spring is positioned to urge the fin 5 from a stowed position to a deployed position, which urging is sufficient to cause deployment after the integrated ordnance is fired and exits from the barrel or tube. For example, the coiled center portion of torsion spring 20 as depicted is positioned within, and allowed to rotate with-respect-to, the main longitudinal axis of the shoulder screw 4.
Each fin 5 has associated with it inertia and center-of-gravity properties designed such that, during acceleration in-barrel, the fin 5 stays in the stowed position. The stowed fins are circumferentially located on the fin hub 3 exterior and a rear portion of projectile 1 exterior and are initially enclosed/contained by a standard cylindrical cartridge case with integral standard propelling charge or Kevlar string. Upon firing, the propelling charge gases expand and accelerate the coupled fin hub and projectile forebody down the barrel, separating from the cartridge case or breaking/burning the Kevlar sting in the barrel. Upon muzzle exit, fin deployment, and introduction of aerodynamic loading upon the fins as they transition from stowed to deployed states, the hub partially separates from the rear of projectile and is free to spin relative to the rear of the projectile.
In one embodiment, four commercially available metal set screws, one commercially available metal machine screw, one metal inner bushing, and one plastic outer bushing are arranged to allow the decoupling of fin hub from the projectile forebody. Inner and outer bushings spin relative to one another via low friction contact.
In one embodiment, each fin has a notch or bump stop guide 32 formed thereat and positioned so as to receive, upon fin deployment, a bump stop 30 formed or positioned on the fin hub 3 such that each deployed fin 5 is held in an aerodynamically appropriate position.
Advantageously, the various embodiments for stowing and deploying fins on a gun-launched, tube-launched projectile integrating simple commercially available torsion spring and utilize as a storage cavity or recess the very outer sub-caliber peripheral space on rear of projectile as well as a portion of the forward part of projectile in the stowed configuration, which further maximizes internal volume providing increased payload capacity.
Various embodiments provide a deployment apparatus for stowable fins that comprises a sub-caliber fin hub configured to be rotatably coupled to an aft portion of a projectile, the fin hub having a plurality of flat, planar exterior surface regions, each surface region having pivotably coupled thereto a respective set of fins. Each fin in a stowed position is disposed upon a respective exterior surface region of the fin hub and a respective exterior cavity or recess of the projectile. Each fin is urged toward a deployed position by a respective torsion spring, which operates to deploy the fin after the projectile has left the barrel or tube. The sub-caliber fin hub is further configured to be keyed to the aft portion of the projectile during storage and while within the bore of the barrel or tube in response to urging from a propellant, the fin hub when keyed thereto being rotatably locked with the projectile forebody. The fins generally comprise elongated fins having proximal and distal ends substantially axially aligned with the projectile in the stowed position. The proximal and distal ends are substantially radially aligned with the projectile in the deployed position.
Various embodiments provide a sub-caliber fin hub that is configured to be rotatably coupled to an aft portion of a projectile and comprises a plurality of flat, planar exterior surface regions, each surface region having pivotably coupled thereto a respective set of fins, each fin in a stowed position disposed upon a respective exterior surface region of the fin hub and a respective exterior recess of the projectile, each fin being urged to a deployed position by a respective torsion spring. The sub-caliber fin hub configured to be keyed to the aft portion of the projectile during storage and in-bore in response to urging from a propellant, the fin hub when keyed thereto being rotatably locked with the projectile forebody. The fins may comprise elongated fins having proximal and distal ends substantially axially aligned with the projectile in the stowed position. The proximal and distal ends may be configured to be substantially radially aligned with the projectile in the deployed position.
While the disclosure has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made, and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt a particular system, device or component thereof to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure is not limited to the particular embodiments disclosed for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another.
In the preceding detailed description of exemplary embodiments of the disclosure, specific exemplary embodiments in which the disclosure may be practiced are described in sufficient detail to enable those skilled in the art to practice the disclosed embodiments. For example, specific details such as specific method orders, structures, elements, and connections have been presented herein. However, it is to be understood that the specific details presented need not be utilized to practice embodiments of the present disclosure. It is also to be understood that other embodiments may be utilized, and that logical, architectural, programmatic, mechanical, electrical and other changes may be made without departing from general scope of the disclosure. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims and equivalents thereof.
References within the specification to “one embodiment,” “an embodiment,” “embodiments”, or “one or more embodiments” are intended to indicate that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. The appearance of such phrases in various places within the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Further, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not other embodiments.
It is understood that the use of a specific component, device and/or parameter names and/or corresponding acronyms thereof, such as those of the executing utility, logic, and/or firmware described herein, are for example only and not meant to imply any limitations on the described embodiments. The embodiments may thus be described with different nomenclature and/or terminology utilized to describe the components, devices, parameters, methods and/or functions herein, without limitation. References to any specific protocol or proprietary name in describing one or more elements, features or concepts of the embodiments are provided solely as examples of one implementation, and such references do not limit the extension of the claimed embodiments to embodiments in which different element, feature, protocol, or concept names are utilized. Thus, each term utilized herein is to be given its broadest interpretation given the context in which that term is utilized.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
The description of the present disclosure has been presented for purposes of illustration and description but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope of the disclosure. The described embodiments were chosen and described in order to best explain the principles of the disclosure and the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.

Claims

What is claimed is:

1. A deployment apparatus for stowable fins, comprising:

a sub-caliber fin hub configured to be rotatably coupled to an aft portion of a projectile, the fin hub having a plurality of flat, planar exterior surface regions, each surface region having pivotably coupled thereto a respective set of fins;

each fin in a stowed position disposed upon a respective exterior surface region of the fin hub and a respective exterior recess of the projectile, each fin being urged to a deployed position by a respective torsion spring;

the sub-caliber fin hub further configured to be keyed to the aft portion of the projectile during storage and in-bore in response to urging from a propellant, the fin hub when keyed thereto being rotatably locked with the projectile forebody.

2. The deployment apparatus of claim 1, wherein each of the fins comprises an elongated fin having proximal and distal ends substantially axially aligned with the projectile in the stowed position.

3. The deployment apparatus of claim 2, wherein the proximal and distal ends are substantially radially aligned with the projectile in the deployed position.

4. The deployment apparatus of claim 1, wherein the fins comprise a metal or composite material.

5. The deployment apparatus of claim 1, wherein the fin hub comprises a fiber-reinforced polymer.

6. The deployment apparatus of claim 1, wherein the sub-caliber fin hub is rotatably coupled to the aft portion of the projectile via a metal inner bushing rotating about an outer plastic bushing via low friction contact therebetween.

7. The deployment apparatus of claim 1, wherein each fin has formed thereat a bump stop guide positioned to receive, upon fin deployment, a respective bump stop formed on the fin hub and positioned to hold the fin in an aerodynamically appropriate position.

8. A sub-caliber fin hub configured to be rotatably coupled to an aft portion of a projectile, comprising:

a plurality of flat, planar exterior surface regions, each surface region having pivotably coupled thereto a respective set of fins, each fin in a stowed position disposed upon a respective exterior surface region of the fin hub and a respective exterior recess of the projectile, each fin being urged to a deployed position by a respective torsion spring;

the sub-caliber fin hub configured to be keyed to the aft portion of the projectile during storage and in-bore in response to urging from a propellant, the fin hub when keyed thereto being rotatably locked with the projectile forebody.

9. The sub-caliber fin hub of claim 8, wherein each of the fins comprises an elongated fin having proximal and distal ends substantially axially aligned with the projectile in the stowed position.

10. The sub-caliber fin hub of claim 8, wherein the proximal and distal ends are configured to be substantially radially aligned with the projectile in the deployed position.

11. The sub-caliber fin hub of claim 8, wherein the fins comprise a metal or composite material.

12. The sub-caliber fin hub of claim 8, wherein the fin hub comprises a fiber-reinforced polymer.

13. The sub-caliber fin hub of claim 8, wherein the sub-caliber fin hub is rotatably coupled to the aft portion of the projectile via a metal inner bushing rotating about an outer plastic bushing via low friction contact therebetween.

14. The sub-caliber fin hub of claim 8, wherein each fin has formed thereat a bump stop guide positioned to receive, upon fin deployment, a respective bump stop formed on the fin hub and positioned to hold the fin in an aerodynamically appropriate position.