Pressure relief fins for improved roll control of precision projectiles

Pressure relief holes are sized and positioned on each fin of a projectile to allow high pressure from one side of the fin due to canard and fin interaction to bleed to a low pressure area on the opposite side of the fin. The location of the pressure relief holes target pressure differences due to canard roll control downwash while not affecting pressure differences due to projectile attitude. The fin planform area remains small enough to meet packaging and fire requirements for this type of munition and large enough to aerodynamically stabilize the airframe thereby enabling a more robust pitch and yaw autopilot design.

STATEMENT OF GOVERNMENT INTEREST

The inventions described herein may be manufactured, used and licensed by or for the United States Government.

BACKGROUND OF THE INVENTION

The invention relates in general to precision guided weapons and in particular to precision guided weapons with deflectable canards.

Precision munitions that use deflectable canards to create maneuvers experience a reduction in their control authority in the roll direction due to downwash effects on their fins. Prior solutions to this problem include using decoupled bases/fin hubs, very large fins to overwhelm the effect, using deflectable tail fins instead of deflectable canards and using very small fins to reduce the interaction effects and thereby essentially rely solely on canards for active control of the missile.

However, the previous efforts are not suitable for certain precision guided weapons. Decoupled bases are more mechanically complex. Deflectable tail fins require control electronics collocated at the rear of the projectile or lengthy communication paths which may not be feasible for all weapons. Enlarging the fins necessarily creates a larger cross sectional diameter of the weapon which may not be compatible with desired packaging and gun tubes requirements. Finally, reducing the size of the fins to interaction effects may reduce the level of control to an unacceptable level for certain munitions.

Specifically, there exists a need for a precision guided weapon which can mitigate roll control issues caused by deflectable canards while maintaining canard control of the weapon without drastically changing the size of either the canard or the fin.

SUMMARY OF INVENTION

One aspect of the invention is a precision guided weapon having rear fins with pressure relief holes sized and positioned to allow high pressure from one side of the fin due to canard and fin interaction to bleed to a low pressure area on the opposite side of the fin.

The invention will be better understood, and further objects, features and advantages of the invention will become more apparent from the following description, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION

A precision guided munition with fins comprising pressure relief holes allows for relief of pressure differences due to downwash which significantly reduces the canard/fin interference effects. Accordingly, roll control of the projectile is restored. The precision guided munition is an easily controlled aerodynamically stable projectile configuration that is packageable and has a significant reduction in the canard/fin interaction, which increases the roll control authority of the projectile. Advantageously, the pressure relief holes may be a modification to standard fin platforms currently in use thereby negating the need for further modifications to the projectile or its weapon platform. Additionally, the fin planform area which contributes to projectile stability is not altered.

FIG. 1is a side view of a munition with fins comprising pressure relief holes, in accordance with an illustrative embodiment of the invention. While throughout this specification, the projectile is described in terms of a munition, the projectile is not limited to a munition. The projectile may be a non-military projectile and throughout this specification, the terms munition and projectile are used interchangeably. In the embodiment shown, the projectile10is artillery mortar projectile. For example, the projectile10may be a 120 mm mortar projectile. However, the projectile10is not limited to artillery projectiles and may be any projectile10, which experiences downwash due to canard and fin interaction. Further, the projectile is not limited to airborne projectiles and may be any projectile which is configured to travel through a fluid medium including fluids in the gaseous or liquid state or some combination of both.

The projectile10comprises a nose12, a body14, a canard set16and a fin set comprising a plurality of fins18. The nose12is the front and round part of the projectile and is the first part of the projectile to touch the incoming air stream. The body14of the projectile contains the payload and guidance electronics. The canard set16controls projectile flight by deflecting in a coordinated way to produce aerodynamic moments about the projectile center of gravity. The fin set provides stability to the projectile. Each fin18of the fin set comprises one or more pressure relief holes20.

FIG. 2is a side view of a fin with pressure relief holes, in accordance with an illustrative embodiment of the invention. Each fin18of the fin set comprises a leading edge181which is the first contact with incoming air stream, a trailing edge182which is the last contact with outgoing airstream, an attachment point of fin to projectile body183, one or more pressure relief holes20a,bdefined by a diameter202aand a radial location201a,b, a tip chord184of the fin18(edge of fin18at the distal end of the fin18most outboard from centerline of projectile10) and a root chord185of fin18(edge of fin18located at the proximate end of the fin18at the surface of the projectile10).

Each of the pressure relief holes20are defined by the fin18and extend from a top surface186of the fin18to a bottom surface187of the fin18. The pressure relief holes20are sized and positioned on each fin to allow high pressure from one side of the fin due to canard and fin interaction to bleed to a low pressure area on the opposite side of the fin. The location of the pressure relief holes20target pressure differences due to canard roll control downwash while not affecting pressure differences due to projectile attitude. Incorporation of the pressure relief holes20to alleviate downwash effects allows the fin planform area to remain small enough to meet packaging and fire requirements for this type of munition and large enough to aerodynamically stabilize the airframe thereby enabling a more robust pitch and yaw autopilot design.

The inventors discovered the aerodynamic moments generated by the canard-fin interactions are created by localized pressure of the airstream pushing normal to the fin surface. When the pressure on the fins is integrated over the fin planform area, the resultant force is almost always located at a distance from the principle axes of projectile. This creates a force and a moment about the projectile center of gravity. For projectile control, the moments generated are of critical importance. By managing the pressure distribution with pressure relief holes20, particularly reducing the pressure differences in the localized areas where the canard-fin interactions are largest, the overall behavior of the projectile can be tailored to meet design specifications.

In the embodiment shown inFIG. 1andFIG. 2, each fin comprises a first pressure relief hole20aand a second pressure relief hole20b. For mortar rounds of the type and dimensions shown inFIG. 1andFIG. 2, it was discovered that the counter-roll moment produced on the fins is localized to a lower portion of the fins adjacent the root chord. Accordingly, pressure relief holes20were sized and dimensioned to minimize the fin surface area corresponding to this locations.

The first pressure relief hole20aand the second pressure relief hole20bare positioned such that their center is substantially aligned along a central longitudinal axis of the fin extending from the tip chord to the root chord. The pressure relief holes20have a diameter which is approximately 60% of the width of the fin at the location of the center of the pressure relief hole20. The approximately 60% width was chosen to satisfy structural requirements while providing holes with a relatively large and contiguous cross-sectional area for pressure relief.

The first pressure relief hole20aand the second pressure relief hole20bhave substantially the same diameter. In one embodiment, each of the first pressure relief hole20aand second pressure relief hole20bmay be chamfered thereby creating a chamfered circumferential edge. The outer circumference of the first pressure relief hole20aand the outer circumference of the second pressure relief hole20bare separated a distance of approximately ⅜ of the diameter of the pressure relief holes20.

The first pressure relief hole20aand the second pressure relief hole20bare positioned near the localized areas of counter-roll moment as described above. Accordingly, the first pressure relief hole20aand the second pressure relief hole20bare positioned substantially in a bottom portion of the fin at the proximate end of the fin near the root chord184. In the embodiment shown inFIG. 2, the first pressure relief hole20aand the second pressure relief hole20bare below a central lateral axis extending from the leading edge181to the trailing edge182.

Although two relatively large holes were found to be optimal for the particular embodiment shown inFIG. 1andFIG. 2, those skilled in the art will appreciate that other configurations may prove optimal for other embodiments, including but not limited to: more holes with smaller diameters, less holes with larger diameters, different locating patterns of the holes based on instantiation of the projectile design and differently shaped holes. The defining characteristic of any of these designs would be the specific targeted location of the holes to relieve the pressure differences caused by downwash from the canards during a roll command.

Testing was performed to compare the effect of the pressure relief holes20on roll control authority. First, wind tunnel testing was performed on fins with no holes to confirm that the fin experienced downwash effects. Computational fluid dynamics (CFD) analysis was then performed on the fins modeled without pressure relief holes and fins modeled with pressure relief holes20. The CFD analysis confirmed the findings from the wind tunnel hardware. The exact effects were isolated. CFD analysis was then performed on fins with pressure relief holes20. The inventors found that with the pressure relief holes20, approximately 40% of roll control authority was restored. Importantly, it was determined that projectile aerodynamic stability was maintained for the fins with the pressure relief holes20.