Aeronautics and astronautics – Missile stabilization or trajectory control – Externally mounted stabilizing appendage
Utility Patent
1997-03-03
2001-01-02
Tudor, Harold J. (Department: 3641)
Aeronautics and astronautics
Missile stabilization or trajectory control
Externally mounted stabilizing appendage
C244S003240
Utility Patent
active
06168111
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates in general to fins for stablizing a projectile, and in particular to fins that fold out after a projectile is launched from a gun tube.
Stability of projectiles can be generalized in two categories. The first type, gyroscopically stabilized, relies on spin to provide gyroscopic forces that maintain projectile stability. The second type, statically stabilized, depends on the lift of the fins or cone aft of the center of gravity (cg) to statically stabilize the projectile. Static stability occurs when the center of pressure is aft of the cg.
The mission of the projectile normally dictates the stability criteria. Generally, a cargo carrying artillery projectile is spin stabilized and an anti-tank round is often fin stabilized since cargo is not an issue. Yet, artillery projectiles cannot be spin stabilized if they do not have the necessary inertial characteristics.
If a projectile body is made of a lightweight material such as aluminum, the axial moment of inertia may be too small or, if the projectile is too long, the transverse moment of inertia may be too large and spin stabilization cannot be achieved. If spin stabilization cannot be achieved, then the projectile must be designed to be statically stable, usually with the aid of fins.
For a sub-caliber sabot launched projectile, such as a long rod penetrator, fins can be rigidly attached to the body. However, for a full-bore projectile where cargo is important, fins must be hinged so that they can be deployed after exit from the gun tube. The fold-out fin configuration of the present invention provides static stability through a unique fin packaging and deployment technique.
Numerous munitions with deployable fins exist but they often have undesirable aerodynamic characteristics such as high drag and roll instabilities.
Full-bore fin stabilized projectiles exist in the U.S. Arsenal, yet minor drawbacks are associated with each. The Copperhead projectile and Tow missile family have similar fin configurations. The fins in each are stowed within the cylindrical body and flip-out from within the body longitudinally to the axis of the projectile. This method provides for good stability when deployed, yet requires four long voids in the projectile body for stowing. These voids effectively make a cross pattern in the boattail section. This cross pattern reduces the cargo capacity and can cause some structural concerns depending on payload weight. Although this type of fin has been proven to be effective, it's minimal cargo space makes it undesirable.
Projectiles with wrap-around fins such as the 2.75″ rocket family have probably the most efficient fin packaging configuration. Yet, wrap-around fins can induce rolling moments and yawing moments. This behavior has been observed where the direction of roll changes at transonic speeds. Wind tunnel test results demonstrate transonic roll reversal. See Dahlke, C. W., Craft, J. C., “The Effect of Wrap-Around Fins on Aerodynamic Stability and Rolling Moment Variations,” Technical Report RD-73-17, U.S. Army Missile Command Technical Report RD-73-17, July 1973.
A comprehensive set of data for the wrap-around fins at Mach numbers 0.3 to 3.0 has been reported. See Humphery, J. A., Dahlke, C. W., “A Summary of Aerodynamic Characteristics for Wrap-Around Fins from Mach 0.3 to 3.0,” Technical Report RD-77-5, U.S. Army Missile Research and Development Command Technical Report RD-73-17, March 1977. Furthermore, and perhaps more important, the usual wrap-around fin has a rectangular shape with drag characteristics that are not optimal. In a scaled test with a similar fin configuration (fixed elliptical fin) to that of the invention, no adverse rolling moment was found and drag was considered low. See Kayser, L. D., “Aerodynamics of Fin-Stabilized Projectiles at Moderate Spin Rates,” BRL Memorandum Report No. BRL-MR-3965, U.S. Army Ballistic Research Laboratory, Aberdeen Proving Ground, Maryland, April 1992.
Probably the most similar existing design is the Navy's Harpoon missile. Similar to the present invention, this configuration has a set of four flat fold-out fins that rest on a square boattail. Unlike the present invention, the fins are square in shape and hinged with a spring for deployment. The Harpoon fins are square to accommodate for the square boattail. This square boattail is machined parallel to the principle axis of the missile, thereby providing an abrupt discontinuity or step. This step causes a pressure drop which increases the drag. The present invention provides a smoother transition at the boattail and the resulting fin is elliptical, which provides better drag characteristics. The present invention also utilizes a very efficient fin packing configuration. Non-conical boattails without fins have been examined and were found to have better drag and stability characteristics than conventional boattails. See Platou, A. S., “An Improved Projectile Boattail. Part III,” BRL Memorandum Report No. 2644, U.S. Army Ballistic Research Laboratory, Aberdeen Proving Ground, Maryland, July 1976, AD# B012781.
Some advantages of the present invention over previous designs include:
1. Good drag characteristics (low fin drag, improved boattail drag),
2. Efficient fin packaging,
3. No adverse rolling moment, and
4. Use of spin for fin deployment, i.e., no active deployment mechanisms needed.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a fold-out fin with a low drag configuration which avoids adverse aerodynamic behavior.
This and other objects of the invention are achieved by a projectile for launching from a gun tube, comprising a cylindrical projectile body having a longitudinal axis and a tapered non-conical boattail; at least two elliptical fins, each fin having a size and a shape defined by two parallel planes that intersect the projectile body at an angle parallel to the boattail, thereby insuring that the projectile with folded fins fits in the gun tube; and hinges for connecting the fins to the boattail.
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The Effect of Wrap-Around Fins on Aerodynamic Stability and Rolling Moment Variations, C.W. Dahike & J.C. Craft, Jul. 1973, Technical Report RD-73-17.
A Summary of Aerodynamic Characteristics for Wrap-Around Fins from Mach 0.3 to 3.0, Technical Report TD-77-5, James a Humphrey & Calvin W. Dahlke, Mar. 1, 1977.
An Improved Projectile Boattail, Part III, Memorandum Report No. 2644, Anders S. Platous, Jul. 1976.
Aerodynamics of Fin-Stabilized Projectiles at Moderate Spin Rates, Memorandum Report BRL MR-3965, Lyle D. Kayser, Apr. 1992.
Brown T. Gordon
Kayser Lyle D.
Clohan, Jr. Paul S.
Eshelman William E.
The United States of America as represented by the Secretary of
Tudor Harold J.
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