Mid-body obturator for a gun-launched projectile

An obturator is provided for a projectile launched from a gun barrel. The projectile has a mid-body annular groove that includes a shaped surface. The obturator includes an annular ring having an inner surface in contact with the shaped surface of the annular groove of the projectile. The annular ring further includes an outer surface. When the projectile is in the gun barrel, the outer surface of the annular ring contacts an inner surface of a bore of the gun barrel. The radial distance between the inner surface and the outer surface of the annular ring substantially equals or exceeds the radial distance between the shaped surface of the annular groove and the inner surface of the bore of the gun barrel at at least one point when the projectile is positioned in the barrel. This feature restricts a flow of charge gases from an aft end of the projectile to a forward end of the projectile when the projectile is launched from the gun barrel.

TECHNICAL FIELD OF THE INVENTION
 This invention relates generally to gun-launched projectiles and more
 particularly to a mid-body obturator for a gun-launched projectile.
 BACKGROUND OF THE INVENTION
 When launching projectiles out of large military guns or cannons, a typical
 loading technique is to first ram the projectile into the breach of the
 gun, and then to ram a propelling charge in a shell casing behind the
 projectile. The propelling charge is typically positioned in the breach by
 a shell casing rim that is similar to the rim on a bullet cartridge used
 with a handgun. This rim is larger than the diameter of the breach and is
 prevented from being inserted into the barrel of the gun.
 Projectiles launched from military guns are typically rear obturated. The
 aft end of the projectile has a protruding ring or flange of material
 called an obturator or a rotating band. The obturator has a diameter
 smaller than the diameter of the breach, but larger than the diameter of
 the bore of the gun barrel. The bore is the section of the barrel that
 typically contains a series of rifling grooves used to impart a spin on
 the projectile.
 During loading, the projectile is rammed into the breach in a manner
 similar to putting a bullet in a gun chamber. However, unlike a typical
 bullet, the projectile does not have a cartridge rim to stop it (only the
 separate propelling charge has a cartridge rim). Therefore, the aft end or
 rear obturator is used to stop the projectile once it has traveled an
 appropriate distance into the barrel. Because the rear obturator has a
 diameter larger than the bore diameter of the gun, the obturator is
 stopped during loading of the projectile in an area of the gun barrel
 where the inside diameter decreases from the breach diameter to the bore
 diameter. This area of inside diameter change is called the forcing cone.
 Because the obturator is located at the rear of the projectile, when the
 obturator stops at the forcing cone, most of the projectile is positioned
 in the bore of the barrel.
 When the propelling charge is ignited, the rear of the projectile is forced
 into the bore of the gun barrel. The obturator, which has a diameter
 larger than the bore of the gun, is forced to extrude into the rifling
 grooves. This extrusion helps to prevent the charge gases created by the
 ignition of the propelling charge from flowing past the projectile in the
 rifling grooves. By preventing the charge gases from blowing by the
 projectile, the obturator causes the charge gases to drive the projectile
 out of the gun at the optimal velocity. In addition, since the rifling
 grooves spiral down the barrel, the grooves impart a spin to the
 projectile to increase flight stability. It should be noted that the term
 "rotating band" is often used to denote a device that provides obturation
 (the obstruction of gas flow) as well as imparting a rotation to the
 projectile. The term "obturator" typically refers to a device that only
 performs the obturation function. However, for the purposes of this
 application, the term "obturator" will be used generically to refer to
 both rotating bands and obturators.
 Advanced projectiles ("smart" projectiles) are capable of being fired from
 the same guns that are used to fire the standard unguided projectiles
 described above. An example of an unguided projectile is a standard
 artillery shell, which is basically a large bullet. On the other hand,
 advanced projectiles have enhanced features such as electronic guidance
 and extended range rocket motors. For example, certain advanced
 projectiles are launched from a gun using a propelling charge, but then
 use a rocket motor and a guidance system to propel them to a selected
 target. These advanced projectiles must be designed to be loaded and fired
 in the same gun barrels that were designed to fire the standard unguided
 projectiles. However, advanced projectiles are often longer than standard
 projectiles due to their increased complexity. In addition, in order to
 increase the range of advanced projectiles, a relatively thin rocket motor
 wall is used. Because of the increased length and the thin rocket motor
 wall, if a standard rear obturator is used on such projectiles, the launch
 pressures created when the charge is ignited would buckle the aft portion
 of the advanced projectile.
 An obturator or related device must be used in order to stop the charge
 gases from blowing by the projectile. This function is important in the
 case of advanced projectiles due to the sensitivity of the guidance
 electronics. Any blow-by could potentially destroy the projectile's
 operability. Additionally, a brake is needed to stop the projectile when
 it is rammed into the gun. Traditionally, both of these functions have
 been performed by the rear obturator or rotating band, as described above.
 However, since the obturator cannot be located at the rear of the
 projectile on an advanced projectile, the standard rear obturator/rotating
 band design used with unguided projectiles must be replaced by one or more
 components that serve the functions of sealing the rifling grooves during
 firing.
 SUMMARY OF THE INVENTION
 Accordingly, a need has arisen for an obturator for use in conjunction with
 an advanced gun-launched projectile that functions to seal the rifling
 grooves of the gun during the launching of the projectile. The present
 invention provides a mid-body obturator for a gun-launched projectile that
 addresses this need.
 According to one embodiment of the present invention, an obturator is
 provided for a projectile launched from a gun barrel. The projectile has a
 mid-body annular groove that includes a shaped surface. The obturator
 includes an annular ring that has an inner surface that is in contact with
 the shaped surface of the annular groove of the projectile. The annular
 ring further includes an outer surface that contacts an inner surface of a
 bore of the gun barrel when the projectile is in the gun barrel. The
 radial distance between the inner surface and the outer surface of the
 annular ring substantially equals or exceeds the radial distance between
 the shaped surface of the annular groove and the inner surface of the bore
 of the gun barrel at at least one point when the projectile is positioned
 in the barrel. This feature restricts a flow of charge gases from an aft
 end of the projectile to a forward end of the projectile when the
 projectile is launched from the gun barrel.
 According to another embodiment of the invention, a projectile capable of
 being launched from a gun barrel includes a payload segment located toward
 a forward end of the projectile, and a propulsion segment coupled to the
 payload segment and located toward an aft end of the projectile. The
 projectile further includes an annular groove that has a shaped surface.
 The annular groove is located substantially at a mid-body location of the
 projectile. The projectile also includes an obturator that has an annular
 ring. An inner surface of the annular ring is in contact with the shaped
 surface of the annular groove of the projectile. In addition, when the
 projectile is loaded in the gun barrel, an outer surface of the annular
 ring contacts an inner surface of a bore of the gun barrel. When the
 projectile is loaded, the radial distance between the inner surface and
 the outer surface of the annular ring substantially equals or exceeds the
 radial distance between the shaped surface of the annular groove and the
 inner surface of the bore of the gun barrel at at least one point. This
 configuration restricts a flow of charge gases from an aft end of the
 projectile to a forward end of the projectile when the projectile is
 launched from the gun barrel.
 Embodiments of the invention provide numerous technical advantages. For
 example, in one embodiment of the invention, a mid-body obturator is
 provided that allows an advanced projectile to be launched from a gun or
 cannon that is normally used to fire standard unguided projectiles.
 Obturators incorporating teachings of the present invention operate to
 impede the flow of charge gases past the projectile in the gun barrel,
 even though the obturator may be positioned at a mid-body location on the
 projectile. Further technical advantages of the present invention include
 the use of tabs disposed along the outer surface of the obturator. The
 tabs are used to engage and fill the rifling grooves when the projectile
 is loaded. These tabs help to prevent the initial blow-by of charge gases
 through the grooves when the propelling charge is ignited.
 Other technical advantages are readily apparent to one skilled in the art
 from the following figures, descriptions, and claims.

DETAILED DESCRIPTION OF THE INVENTION
 Embodiments of the present invention and its advantages are best understood
 by referring to FIGS. 1 through 9B of the drawings, like numerals being
 used for like and corresponding parts of the various drawings.
 FIG. 1 illustrates a gun-launched projectile incorporating teachings of the
 present invention. The projectile 10 is an advanced or "smart" projectile
 that is fired from a gun that traditionally fires standard unguided
 projectiles. Examples of such guns are large naval and artillery guns.
 Projectile 10 includes a propulsion segment 12; typically a solid rocket
 motor. Once projectile 10 is fired from a gun, propulsion segment 12
 ignites to accelerate the projectile to the desired velocity. Also
 included as a part of projectile 10 is a payload segment 14. Payload
 segment 14 includes the non-propulsion systems of the projectile. For
 example, payload segment 14 typically includes a plurality of
 sub-munitions or some other explosive device or devices. Typically, the
 payload segment also includes an electronics package for controlling the
 guidance of the projectile 10. Projectile 10 has a tip 16 at its forward
 end and an aft closure 18 at the aft end. Further, the projectile includes
 a plurality of fins 20 used to guide and stabilize the projectile
 (although not explicitly shown, fins may also be disposed around payload
 segment 14). In addition, projectile 10 includes an obturator seat 22.
 Obturator seat 22 functions to position an obturator (not explicitly shown
 in FIG. 1). The function of the obturator and obturator seat 22 will
 described below.
 Due to the length and thin rocket motor walls of advanced projectiles, a
 traditional rear obturator, as used on shorter, unguided projectiles that
 are fired from the same type of gun, cannot be used. If projectile 10 was
 rear obturated (meaning that the obturator is positioned at or in close
 proximity to the aft end of the projectile), the forces placed on the
 projectile when launched from the gun would cause propulsion segment 12 to
 buckle. The structure of propulsion segment 12 cannot be augmented to
 overcome this problem because too much weight would be added to the
 projectile.
 However, if the obturator is moved near the middle of projectile 10 to a
 "mid-body" position, the launch forces applied to propulsion segment 12
 are reduced by approximately half. This is due to the fact that payload
 segment 14 (or any structure that is forward of the obturator) bears
 approximately half the load, while propulsion segment 12 (or any structure
 aft of the obturator) bears the other half. In addition, the forces that
 are applied to propulsion segment 12 are generally tensile when a
 "mid-body" obturator is used. When a rear obturator is used, the forces on
 propulsion segment 12 are generally compressive. Due to the reduction of
 launch forces and the fact that the tensile strength of propulsion segment
 12 is typically better than its compressive strength, a "mid-body"
 obturator is superior to a rear obturator for use with advanced
 projectiles such as projectile 10.
 For the reasons described above, obturator seat 22 is generally located
 near the middle of projectile 10. However, there is no strict requirement
 that the obturator be located at the exact center of projectile 10. All
 that is required is that the obturator be positioned at substantially a
 mid-body location to lower the launch forces applied to propulsion segment
 12. As will be discussed below, this generally means that the obturator,
 and thus obturator seat 22, is located at a point along projectile 10 that
 will be loaded into the bore of the gun barrel. For this reason, the
 obturator cannot have a larger outer diameter than the bore of the barrel.
 In order to further explain the configuration of the obturator and
 obturator seat 22, reference is now made to FIGS. 2 and 3. FIG. 2 is an
 illustration of a gun barrel for typically launching projectile 10.
 Included in the barrel 110 are three primary sections: a breach 120, a
 forcing cone 130, and a bore 140. Barrel 110 has three distinct inner
 surfaces corresponding to these sections. An inner surface 122 of breach
 120 tapers slightly inward from an aft end 114 to a forward end 124. An
 inner surface 142 of bore 140 is of a uniform bore diameter 144 throughout
 the length of the bore. The diameter of the breach at the forward end 124
 is larger than bore diameter 144. Thus, an inner surface 132 of forcing
 cone 130 forms a tapered cone that connects inner surface 122 of breach
 120 to inner surface 142 of bore 140.
 FIG. 3 illustrates a cross-section of bore 140 of FIG. 2, taken along line
 3--3. Machined within the bore 140 is a plurality of rifling grooves 146
 formed in inner surface 142. Rifling grooves 146 generally begin at the
 point where forcing cone 130 ends and where bore 140 begins. Each rifling
 groove spirals along bore 140 at a constant angle until reaching a forward
 end 116 of barrel 110. Rifling grooves 146 impart rotation to a projectile
 after the charge has been fired and the projectile travels along bore 140.
 Such rotation is needed to give unguided projectiles stability in flight.
 Referring now to FIGS. 2 and 3, when a rear obturated projectile is loaded
 into barrel 110, the projectile is first inserted into breach 120. The
 projectile has a diameter less than, but substantially equal to bore
 diameter 144. Therefore, the projectile will travel along barrel 110 and
 into bore 140 until the obturator reaches forcing cone 130. A typical rear
 obturator has an outside diameter that is smaller than the diameter of
 breach 120 at the forward end 124, but larger than bore diameter 144.
 Therefore, when the obturator enters forcing cone 130, the obturator will
 come into full contact with inner surface 132 at a point where the inside
 diameter of forcing cone 130 generally equals the outside diameter of the
 obturator. At this point, the rear obturator is prevented from moving
 forward, thus stopping the projectile. Therefore, the first function of
 the rear obturator is to act as a ramming brake to prevent the projectile
 from completely entering bore 140.
 Once the projectile has been stopped, a propelling charge is inserted into
 breach 120 behind the projectile. The projectile is then fired by igniting
 the propelling charge. A rear obturator is typically made of metal, such
 as copper or gilding metal. A rear obturator may also be fabricated from
 suitable non-metallic materials, such as thermosets or thermoplastics. The
 flow of charge gases created by the ignition of the propelling charge
 creates enough force to deform the rear obturator and force the aft end of
 the projectile into bore 140. As the obturator is forced into bore 140, it
 is extruded into rifling grooves 146. The obturator serves two other
 functions at this point. The first function is to impart a spin to the
 projectile by following the spiraling configuration of rifling grooves 146
 as the projectile travels along bore 140. The other function is to at
 least partially block the rifling grooves so that the charge gases are
 obstructed from flowing past the projectile.
 Referring now to FIGS. 1, 2 and 3 in combination, as described above, a
 rear obturator cannot be used with projectile 10. Instead, a "mid-body"
 obturator is utilized to minimize the charge gases from traveling through
 rifling grooves 146. However, when the projectile is loaded into barrel
 110, most of the projectile, including the obturator seat 22 is positioned
 in bore 140 to enable loading of the propelling charge in breach 120.
 Therefore, the obturator generally cannot have an outer diameter larger
 than bore diameter 144.
 Because the outer diameter of the mid-body obturator is smaller than bore
 diameter 144, and since the obturator is positioned in bore 140 before
 firing, the mid-body obturator may not be extruded into rifling grooves
 146 through the use of the forcing cone, as with a rear obturator. As
 discussed above, the function of stopping charge gas blow-by through the
 rifling grooves is important when using an advanced projectile. This is
 because such a projectile typically has an electronics package that can be
 easily damaged by the extreme heat and pressure of the charge gases. A
 traditional rear obturator design cannot be positioned mid-body on
 projectile 10.
 Referring now to FIGS. 4A-4C, an obturator 210 incorporating teachings of
 the present invention is illustrated in front, side, and cross-sectional
 views, respectively. Obturator 210 has a configuration to be assembled
 around an associated projectile in an obturator seat. The obturator has a
 main body 212 shaped as an annular ring having an outer surface 214 and in
 inner surface 216. A plurality of tabs 228, discussed below, are
 positioned around outer surface 214.
 Inner surface 216 has two distinct surfaces. The first such surface is a
 curved surface 218. Curved surface 218 forms an "ogive" shape toward an
 aft end 224 of obturator 210. The curved surface helps direct the charge
 gases to expand or "inflate" the obturator when the associated projectile
 is fired, as will be discussed in greater detail in conjunction with FIGS.
 9A and 9B. Inner surface 216 further includes a ramp surface 220. Ramp
 surface 220 is configured to contact a ramp of the associated obturator
 seat.
 As described above, the portion of the projectile 10 containing the
 obturator 210 is disposed in the bore of the barrel prior to firing. For
 this reason, the outer surface of obturator 210 has a diameter that is
 less than or generally equal to the bore diameter of the barrel. Obturator
 210 includes tabs 228 positioned around outer surface 214. The outer
 diameter of all the tabs 228 is generally greater than the bore diameter
 of the gun, and are configured to fit into the rifling grooves in the
 bore. The number of tabs 228 is generally equal to the number of rifling
 grooves. Because the tabs fit into the rifling grooves before firing,
 obturator 210 does not have to be extruded into the grooves like a
 traditional rear obturator. For this reason, the tabs operate to seal the
 grooves more quickly and completely than a traditional rear obturator.
 This reduces or substantially eliminates the amount of charge gases that
 reach the projectile's sensitive electronic equipment.
 In order to assure a tight seal, the tabs have a width and height
 approximately equal to the width and depth, respectively, of the
 associated rifling grooves. In addition, because the rifling grooves
 spiral around the bore of the gun barrel at a constant angle, each tab 228
 should be positioned on outer surface 214 at an angle 230 that is
 approximately equal to the spiral angle of the rifling grooves around the
 bore.
 In one embodiment of the obturator there is included features that assist
 in the loading of the projectile into the gun. For instance, tabs 228 have
 an inclined forward section 232 that helps to guide the tabs into the
 associated rifling grooves. In addition, since obturator 210 is typically
 fabricated from a flexible material, if the tabs are not aligned with the
 rifling grooves when obturator 210 initially enters the bore, the tabs and
 the entire aft end 224 of the obturator are compressed inward. As the
 projectile continues into the bore of the gun, the tabs "pop" into the
 grooves when subsequently aligned. The use of such "depressible" tabs
 allows the projectile to be loaded into the gun barrel without regard to
 the position of the tabs.
 The material or materials used to fabricate obturator 210 must meet certain
 requirements. First, the material must be able to withstand extreme
 temperatures. The gun barrel can reach temperatures of approximately eight
 hundred degrees Fahrenheit, and obturator 210 must be able to withstand
 this temperature while positioned in the barrel before firing. In
 addition, the projectile may experience below freezing temperatures during
 storage or when it is deployed in the field. Furthermore, when the
 propulsion charge is ignited, there is an extreme build-up of gas pressure
 against the obturator. Obturator 210 must be constructed of a material or
 materials that can withstand this pressure. Finally, as described below in
 conjunction with FIGS. 9A and 9B, the obturator preferably expands during
 firing to fill the rifling grooves and any space between the projectile
 and the bore of the gun. Such expansion requires that the obturator
 material elongate one hundred to two hundred percent in localized areas.
 The combination of extreme temperatures, high pressures, and the local
 elongation required of the material eliminates the use of many materials.
 In a particular embodiment of the present invention, obturator 210 is
 comprised of a combination of substances that form a "composite" material
 which meets the above requirements. The first substance used to fabricate
 this composite material is an elastomeric material, such as a
 perfuoroelstomer or silicone resin. These elastomeric materials exhibit
 the required elongation and temperature resistance, and do not become
 brittle or lose their elongation properties at cold temperatures. These
 materials can also handle the high temperature of the barrel for periods
 of time well in excess of what is needed for launch of the projectile.
 However, silicone and perfuoroelstomer cannot withstand the pressures
 created when the gun is launched. Therefore, these materials need to be
 reinforced. However, the reinforcing material must allow the elastomeric
 material to retain its ability to elongate.
 Reinforcing the silicon with short fibers will decrease the tear strength
 of the obturator. On the other hand, continuous fibers such as glass,
 carbon and aramid fibers alone may not have enough elongation to allow the
 obturator to function. Specialized fabrics may be used that are fabricated
 from continuous fibers, but that still have the elongation properties
 required of the obturator. Such fabrics include, but are not limited to,
 knitted textiles, continuous strand mats, and felt-type products of either
 glass or aramid fibers (sold under the trademark KEVLAR). These fabrics
 are commercially available, and exhibit the elongation and temperature
 properties required of the obturator. These fabrics alone do not have
 sufficient strength to withstand the launch pressures, nor are they able
 to form an adequate gas seal. However, when placed in combination with the
 elastomeric material, the composite material that is formed meets all of
 the strength, temperature and elongation requirements.
 This composite material may be fabricated using common methods of producing
 composite materials. Such methods include, but are not limited to,
 transfer molding of the elastomeric material onto a dry fiber pre-form,
 resin transfer molding of the elastomeric material onto a dry fiber
 pre-form, and a vacuum bag lay up using layers of the fabric material that
 are pre-impregnated with the elastomeric material (prepreg layers).
 It should be noted that other materials are available for the fabrication
 of obturator 210. Obturator 210 may be formed entirely from a metal, such
 as copper or gilding metal. Many metals meet the temperature, pressure,
 and elongation properties discussed above, and are available for use to
 construct obturator 210. However, it should be noted that fabricating tabs
 228 from metal may create jamming problems during loading of the
 projectile. The use of composite material typically does not create such
 problems. On the other hand, an all-composite obturator is not as strong
 as a metal obturator, and has a greater propensity to disintegrate
 prematurely in the gun barrel. A two-part obturator that includes an
 all-composite component and an additional metallic component may be used
 to improve the overall strength of the obturator. Such a configuration is
 described below.
 FIGS. 5A and 5B are schematic diagrams illustrating front and
 cross-sectional views, respectively, of a two-part obturator 310.
 Obturator 310 comprises a forward metallic portion 314 and an aft
 composite portion 312. Forward portion 314 and aft portion 312 may be
 referred to as forward annular ring and aft annular ring, respectively. In
 the illustrated embodiment, aft composite portion 312 comprises obturator
 210, described above, made from composite material. The use of aft
 composite portion 312 having tabs 228, ensures that the rifling grooves
 are sealed when obturator 310 is initially contacted by the charge gases.
 In addition, as described above, the tabs typically do not create loading
 problems when fabricated from a composite material. However, because the
 composite material of aft composite portion 312 is brittle compared to a
 metal, there is a possibility that the composite material will
 disintegrate before the projectile has traveled an adequate distance
 through the gun barrel. For this reason, forward metallic portion 314 is
 utilized. Forward portion 314 typically comprises copper, gilding metal,
 or any other suitable metal. As will be described below, the forward
 metallic portion is partially extruded into the rifling grooves during
 firing of the projectile in order to aid the aft portion in minimizing the
 blow-by of charge gases.
 Forward portion 314 is configured such that an aft surface 316 of the
 forward portion abuts and conforms with a forward surface 318 of aft
 portion 312. In addition, in the illustrated configuration, ramp surface
 220 of the aft portion is generally continuous with a ramp surface 320 of
 the forward portion. As with ramp surface 220, ramp surface 320 is
 configured to conform with the ramp of the obturator seat. The interaction
 of ramp surface 320 and the ramp will be discussed below in conjunction
 with FIGS. 9A and 9B. Furthermore, forward portion 314 includes an outer
 surface 322 that is generally continuous with outer surface 214 of aft
 portion 312. The forward portion and the aft portion are interconnected
 using an appropriate fastener or adhesive. In the alternative, both
 portions are not connected, but are assembled adjacent to one another in
 the obturator seat.
 It should be understood that the aft portion of obturator 310 may have
 alternate configurations. For example, although the aft portion (and
 obturator 210) have been illustrated and described as having an inside
 surface comprising only a curved surface 218 and a ramp surface 220, the
 inside surface in an alternate configuration includes a flat surface 219.
 Alternate configurations, including flat surface 219, are illustrated in
 FIGS. 6A-6C.
 Referring now to FIGS. 7A-7C, there is illustrated another obturator 410
 incorporating teachings of the present invention in front, side, and
 cross-sectional views, respectively. Obturator 410 is configured to be
 assembled around an associated projectile in the obturator seat. Obturator
 410 has a main body 412 shaped as an annular ring having an outer surface
 414 and an inner surface 416. The inner surface 416 includes two distinct
 surfaces, aft surface 418 and ramp surface 420. The aft surface starts at
 an aft end 424 of obturator 410 and tapers inwardly. Although aft surface
 418 is illustrated as a linearly tapering surface, it may also comprise a
 curved surface similar to curved surface 218 of obturator 210 (shown in
 FIG. 4C). The aft surface directs the charge gases created when the
 projectile is launched such that the gases expand or "inflate" the
 obturator when the projectile is fired. The inner surface further includes
 a ramp surface 420 that generally extends to a forward end 426 of the
 obturator. The ramp surface 420 is configured to contact a ramp of the
 associated obturator seat.
 Unlike obturators 210 and 310, obturator 410 does not include tabs that
 engage the rifling grooves of the gun barrel during loading. Therefore, in
 order to seal the rifling grooves, obturator 410 is typically made from a
 material that can be extruded by the launch forces into the rifling
 grooves (as with forward metallic portion 314 of obturator 310). This
 extrusion is accomplished by the "inflation" of obturator as it is moved
 up a ramp of the obturator seat during firing. Such inflation will be
 described below in conjunction with FIGS. 9A and 9B. In addition, the
 material from which obturator 410 is fabricated must withstand the
 pressure and temperature conditions found in the gun barrel, as described
 above. In order to meet these requirements, obturator 410 will typically
 be fabricated from a metal, such as copper or gilding metal.
 FIG. 8A is a schematic diagram of the projectile shown in FIG. 1 with parts
 broken away to illustrate obturator seat 22. Obturator seat 22 is a shaped
 annular groove that is formed into an outer surface 24 of projectile 10.
 As discussed above, the obturator seat 22 is located along the length of
 the projectile such that it is positioned in the bore of the gun when the
 projectile is loaded into the barrel for firing. Therefore, an obturator
 (excluding any tabs) positioned in seat 22 preferably does not extend past
 surface 24 of the projectile. Thus, the depth of seat 22 is sized to
 accommodate the thickness of the obturator. Likewise, the length of the
 obturator seat should be at least as long as the longitudinal dimension of
 the obturator. As will be discussed below in conjunction with FIGS. 9A and
 9B, seat 22 is preferably longer than the longitudinal dimension of the
 obturator.
 Obturator seat 22 has a curved surface 26 that forms an ogive shape at an
 aft end 40. Curved surface 26 extends from outer surface 24 to a ramp 30.
 When projectile 10 is fired from the gun barrel, the charge gases flow
 around the projectile on outer surface 24. For reasons discussed below in
 conjunction with FIGS. 9A and 9B, it is desirable that the charge gases
 flow into and not over seat 22. However, when a gas flowing along a
 cylinder encounters an abrupt change in the cylinder's surface, for
 example, a groove formed in the cylinder, the gas flow has a tendency to
 separate from the surface of the cylinder and flow over the groove.
 Due to the shape of the curved surface 26, the flow of gases follows the
 curved surface, and thus the flow is directed into seat 22. In a
 particular embodiment, curved surface 26 comprises a von Karman curve, but
 any curve or other configuration that minimizes flow separation may also
 be utilized. An example of another surface is a area of linearly
 decreasing diameter, similar to ramp 30, described below.
 Curved surface 26 terminates at the ramp 30. The ramp 30 has a linearly
 increasing diameter that forms a cone extending from the curved surface.
 In the illustrated embodiment, ramp 30 extends to a forward wall 32. The
 ramp shown in FIG. 8A has a generally smooth surface. In another
 embodiment, illustrated in FIG. 8B, the surface of the ramp has a series
 of serrations 34 that are inclined towards forward wall 32. These
 serrations allow the obturator to slide up ramp 30, but inhibit the
 obturator from sliding back down the ramp. Such serrations 34, or other
 methods of preventing the obturator from sliding down the ramp, are useful
 to counteract the force of friction applied on the obturator by the bore
 of the gun barrel as the projectile travels along the bore. Such
 retraction by the obturator down ramp 30 results in a degradation of the
 seal that is formed by the obturator.
 In another embodiment, the seat 22 includes a flat surface (not explicitly
 shown). In such configurations, the flat surface is an area of generally
 uniform diameter between curved surface 26 and ramp 30. A flat surface is
 included to conform with obturator configurations having a flat surface
 (such as flat surface 219, illustrated in FIGS. 6A-6C). The functions of
 curved surface 26, the bottom surface, ramp 30, and forward wall 32, and
 the interactions of these surfaces with the obturator disposed in seat 22,
 are discussed in conjunction with FIGS. 9A and 9B.
 Referring now to FIGS. 9A and 9B, obturator 310 of FIGS. 5A and 5B is shown
 positioned in obturator seat 22 of FIG. 8A. FIG. 9A shows the obturator
 310 in cross-section positioned in the obturator seat 22, and FIG. 9B
 illustrates the obturator in phantom lines positioned in the obturator
 seat. Obturator 310 is positioned in, but not affixed to, obturator seat
 22. Initially, the outer surface 214 of aft composite portion 312 is
 generally flush with the outer surface 24 of projectile 10. Tabs 228
 extend past outer surface 24 and into the rifling grooves of the gun
 barrel.
 When the propelling charge is ignited, the charge gases flow rapidly up
 into the bore of the gun barrel. In the bore of the gun, the charge gases
 flow around outer surface 24 and through the rifling grooves. When the
 charge gases reach the obturator seat and the obturator, the curved
 surface 26 directs the charge gases into the obturator seat. The charge
 gases then contact the aft portion 312 of the obturator and the obturator
 is pushed forward. Aft portion 312 is in contact with forward portion 314,
 and ramp surface 320 of forward portion 314 is pushed up ramp 30 until the
 forward portion contacts and stops against forward wall 32. As obturator
 310 moves up ramp 30, both portions 312, 314 are forced to expand or
 "inflate." In addition, the charge gases also contact curved surface 218
 of aft portion 312 and are directed inward, resulting in the further
 expansion of the aft portion 312.
 As the obturator expands, the tabs of aft composite portion 312 are forced
 into the rifling grooves, thereby preventing most, if not all, of the
 charge gases from passing the obturator. In addition, due to curved
 surfaces 26 and 218, aft portion 312 continues to expand outward as tabs
 228 are eroded in the rifling grooves. This feature ensures that a gas
 seal is maintained as the obturator experiences wear as it travels through
 the bore of the gun. Furthermore, the expansion of forward portion 314
 causes the metal comprising this portion to extrude into the rifling
 grooves. Such extrusion also minimizes the passing of the charge gases
 through the rifling grooves. In addition, if the composite material of aft
 portion 312 fails, the forward metal portion 314 will remain to at least
 partially seal the grooves.
 The constant outward pressure that is applied as the obturator slides up
 ramp 30 also enables the use of all-metal obturators, such as obturator
 410 illustrated in FIGS. 7A-7C, that do not include tabs. The main body of
 such obturators is extruded into the rifling grooves by this outward
 pressure, as with forward section 314 of obturator 310. However, without
 the presence of the tabs, the charge gases are initially allowed to pass
 by the obturator until the metal is sufficiently extruded into the rifling
 grooves. Although only a single configuration of obturator seat 22
 corresponding to obturator 310 is illustrated, it will be understood that
 the obturator seat may be modified to conform with different types of
 obturators without departing from the scope of the present invention.
 Furthermore, as stated above, the rifling grooves are typically used to
 impart a spin to a unguided projectile. This spin is usually imparted by
 extruding a rear obturator that is mounted to the projectile into the
 rifling grooves. The extruded obturator is spun as it travels through the
 spiral rifling grooves of the bore. Because the rear obturator is mounted
 to the projectile, this spin is imparted to the projectile. Similarly, the
 mid-body obturators of the present invention are also spun by the rifling
 grooves, either due to extrusion of the obturator into the grooves or due
 to the extension of tabs into the grooves. However, when such obturators
 are used with advanced projectiles, spinning is neither required nor
 desired. This is because advanced projectiles typically have fins and
 guidance systems that are used for stabilization.
 Therefore, an obturator, such as obturator 310, may be decoupled from
 projectile 10. Such decoupling is accomplished by placing a lubricant,
 such as a dry-film lubricant, between the obturator and the obturator
 seat. Because the obturator is not affixed to the projectile, the spin of
 the obturator as it moves in the rifling grooves is not significantly
 imparted to the projectile. Instead the obturator functions like a slip
 ring and, when a dry-film lubricant is used, imparts a spin on the
 projectile that is only approximately ten to fifteen percent of the rate
 at which the obturator is spinning.
 Although the present invention and its advantages have been described in
 detail, it should be understood that various changes, substitutions, and
 alterations can be made therein without departing from the spirit and
 scope of the present invention as defined by the appended claims.