Patent Publication Number: US-9410781-B1

Title: Fin-stabilized, muzzle-loaded mortar projectile with sabot

Description:
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 munitions and in particular to muzzle-loaded mortar projectiles. 
     Fin-stabilized, muzzle-loaded mortar projectiles may be fired from smooth bore or rifled tubes. Various means have been used with fin-stabilized projectiles to seal the propellant gas and thereby create the high pressure needed to propel the mortar projectile out of the mortar tube and down range. Obturators and grease grooves are some of the sealing means that have been used. 
     Some breech-loaded, smooth-bore projectiles, such as tank ammunition, use a sabot as the sealing or obturating device. The U.S. Army has used a 22 mm sub-caliber projectile with an 81 mm sabot (MI) as a training round. U.S. Pat. No. 3,430,572 issued to Hebert et al. on Mar. 4, 1969 discloses a disintegrating sabot for a fin-stabilized projectile. U.S. Pat. No. 4,318,344 issued to Price et al. on Mar. 9, 1982 discloses a spinning tubular projectile with a combustible sabot. U.S. Pat. No. 4,711,180 issued to Smolnik on Dec. 8, 1987 discloses a mortar training device with simulated propelling charges and a sub-caliber flight projectile. U.S. Pat. No. 6,779,463 issued to Mutascio et al. on Aug. 24, 2004 discloses a sabot-launched delivery apparatus for a non-lethal payload. 
     A need exists for a saboted, fin-stabilized, muzzle-loaded mortar round that is effective for warfare. 
     SUMMARY OF INVENTION 
     One aspect of the invention is a muzzle-loaded, fin-stabilized mortar round for launching from a mortar tube of a certain caliber. The round includes a projectile having an interior volume defined by a projectile wall. A payload is disposed in the interior volume. A tail boom is fixed to an aft portion of the projectile. A fin assembly is fixed to an aft portion of the tail boom. 
     A discarding sabot is disposed circumferentially around the projectile. The sabot includes a plurality of discrete sections arranged longitudinally one after another in abutting relationship and around the projectile. Each of the discrete sections is circumferentially divided into a plurality of discrete sabot increments. Each sabot increment includes a base portion mechanically connected to the projectile, two opposing side portions mechanically connected to circumferentially adjacent sabot increments, and at least one end portion mechanically connected to a longitudinally adjacent sabot increment. 
     In some embodiments of the mortar round, the projectile has an asymmetric shape. 
     In other embodiments, the mortar round includes a central longitudinal axis and the projectile is a sub-caliber projectile centered on the central longitudinal axis. The fin assembly may have a diameter at least as large as the caliber of the mortar tube. The discarding sabot may be centered on the central longitudinal axis. 
     The sub-caliber projectile may include a plurality of circumferential grooves formed therein. The base portion of each sabot increment may include a mating projection that is inserted in one of the plurality of circumferential grooves on the exterior surface of the sub-caliber projectile to thereby mechanically connect the base portion of the sabot increment to the sub-caliber projectile. 
     Rather than circumferential grooves, the exterior surface of the sub-caliber projectile may include a plurality of dimples formed therein. The base portion of each sabot increment may include mating dimples that engage some of the plurality of dimples on the exterior surface of the sub-caliber projectile to thereby mechanically connect the base portion of the sabot increment to the sub-caliber projectile. 
     One of the two opposing side portions of a sabot increment may include a trapezoidal projection and the other of the two opposing side portions may include a mating trapezoidal recess. Circumferentially adjacent sabot increments may be mechanically connected by inserting the trapezoidal projection of one sabot increment into the mating trapezoidal recess in a circumferentially adjacent sabot increment. 
     Non-parallel sides of the trapezoidal projection may each include a curved projection thereon. Non-parallel sides of the mating trapezoidal recess may each include a mating curved recess therein. The curved projection and the mating curved recess may be, for example, spherical surfaces. 
     Each sabot increment may have two opposing end portions. One opposing end portion may have a projecting ridge formed thereon and the other opposing end portion may have a mating groove formed therein. Longitudinally adjacent sabot increments may be mechanically connected by inserting the projecting ridge of one sabot increment into the mating groove in a longitudinally adjacent sabot increment. 
     The projecting ridge may include a plurality of depressions formed thereon and the mating groove may include a plurality of protuberances formed therein. The plurality of protuberances may be nested in respective ones of the plurality of depressions. 
     Another aspect of the invention is a muzzle-loaded, fin-stabilized mortar round for launching from a mortar tube having a certain caliber. The round includes a central longitudinal axis and a sub-caliber projectile. The sub-caliber projectile has an interior volume defined by a projectile wall and is centered on the central longitudinal axis. A payload is disposed in the interior volume. A tail boom is fixed to an aft portion of the sub-caliber projectile. A fin assembly is fixed to an aft portion of the tail boom. 
     A sabot is disposed circumferentially around the sub-caliber projectile and centered on the central longitudinal axis. The sabot defines an annular orifice centered on the central longitudinal axis. A plug may be inserted in an aft end of the orifice. Upon exit of the round from the mortar tube, air pressure forces the plug rearward out of the orifice. 
     In one embodiment, the annular orifice is an annular converging-diverging nozzle. In another embodiment, the annular orifice has a radially inward linear side and an opposing, radially outward, converging-diverging side. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily to scale, like or corresponding parts are denoted by like or corresponding reference numerals. 
         FIG. 1A  is a perspective view of one embodiment of a fin-stabilized, muzzle-loaded mortar round. 
         FIG. 1B  is a longitudinal sectional view of  FIG. 1A . 
         FIG. 2  is a schematic drawing of a mortar tube. 
         FIG. 3  is a side view of a sub-caliber projectile with dimples formed on its exterior surface. 
         FIG. 4  is a side view of a sub-caliber projectile with longitudinal grooves formed on its exterior surface. 
         FIG. 5  is an enlarged view of a portion of  FIG. 1B . 
         FIG. 6A  is a front view of the base portion of one embodiment of a sabot increment, showing dimples formed thereon. 
         FIG. 6B  is a right side view of  FIG. 6A . 
         FIG. 6C  is a left side view of  FIG. 6A . 
         FIG. 6D  is a top view of  FIG. 6A . 
         FIG. 6E  is a view of  FIG. 6D  rotated 90 degrees clockwise. 
         FIG. 6F  is a view of  FIG. 6D  rotated 90 degrees counterclockwise. 
         FIG. 6G  is an end view of  FIG. 6A . 
         FIG. 6H  is a view of  FIG. 6G  rotated 90 degrees counterclockwise. 
         FIG. 6I  is a view of  FIG. 6A  rotated 90 degrees counterclockwise. 
         FIG. 7A  is a perspective view of one embodiment of an asymmetrical, fin-stabilized, muzzle-loaded mortar projectile. 
         FIG. 7B-7E  are side, front end, aft end, and top views, respectively, of the projectile of  FIG. 7A . 
         FIG. 8A  is a perspective view of the projectile of  FIG. 7A  with a novel sabot. 
         FIGS. 8B-8E  are longitudinal sectional, front end, aft end, and top views, respectively, of the projectile of  FIG. 8A . 
         FIG. 9A  is a perspective view of an embodiment of a fin-stabilized, muzzle-loaded mortar round with a discarding sabot. 
         FIG. 9B  is a longitudinal sectional view of the round of  FIG. 9A . 
         FIG. 10A  is a perspective view of a sabot increment of the sabot of  FIG. 9A . 
         FIGS. 10B, 10C, and 10D  are side, end and top views, respectively, of the sabot increment of  FIG. 10A . 
         FIG. 11A  is a perspective view of an embodiment of a fin-stabilized, muzzle-loaded mortar round having a sabot containing an annular nozzle. 
         FIGS. 11B-11D  are longitudinal sectional, front end and aft end views, respectively of the round of  FIG. 11A . 
         FIG. 12A  is a perspective view of an embodiment of a fin-stabilized, muzzle-loaded mortar round having a sabot containing a variation of a nozzle. 
         FIGS. 12B-12E  are longitudinal sectional, front end, aft end and side views, respectively of the round of  FIG. 12A . 
     
    
    
     DETAILED DESCRIPTION 
     A fin-stabilized, muzzle-loaded mortar round includes a sabot. The sabot functions as an obturator for the mortar round. The mortar round includes a sub-caliber projectile that is propelled toward a desired target. The sub-caliber projectile may be longer than existing projectiles and have a smaller diameter than existing projectiles, thereby decreasing the drag on the sub-caliber projectile and increasing its ballistic coefficient. The sabot is made of a plurality of discrete, individual pieces or increments. The sabot quickly releases or disassembles into the individual pieces after the mortar round exits the mortar launch tube. The individual pieces have small momentum and velocity, thereby reducing the probability that the pieces will injure personnel or materiel. In addition, as the sabot breaks apart, the individual pieces impart little or no disturbance to the sub-caliber projectile. 
     The sabot increments are packaged with the mortar round. The mortar round is accelerated by gas pressure acting on the sabot. The sabot increments may be made of for example, metallics, composite, plastics or combustible materials. 
     As an example, an 81 mm diameter sub-caliber projectile may be fitted with a sabot sized for a standard 120 mm mortar tube. Because 81 mm is a standard diameter mortar projectile, the novel sub-caliber 81 mm projectile may be produced on existing 81 mm production equipment with little or no modification to the existing production equipment. The sub-caliber projectile may be longer than a standard 120 mm projectile to maintain the same mass as a standard 120 mm projectile. Or, the sub-caliber projectile may have a mass that is less than a standard 120 mm projectile, depending on the range desired. Higher muzzle velocities due to lower projectile mass may result in extended projectile range. Lower aerodynamic drag due to the smaller diameter sub-caliber projectile also results in extended range. 
     Another analogous example is a 60 mm diameter sub-caliber projectile fitted with a sabot sized for a standard 81 mm mortar tube. 
     The sub-caliber projectile has an interior volume for a payload, such as an explosive charge. The size of the interior volume is dependent on the wall thickness and length of the sub-caliber projectile. Thus, the wall thickness and length of the sub-caliber projectile may be varied to increase the lethal effectiveness of the projectile. In the case of an 81 mm sub-caliber projectile, the projectile fragments may not have the same velocity as fragments from a 120 mm projectile, but the probability of a hit from a sub-caliber fragment may be increased over the zone with highest kill probability. 
     In one embodiment, the novel mortar round includes an 81 mm sub-caliber projectile having a projectile wall that defines an interior volume therein. A payload, such as a high explosive, is disposed in the interior volume. A tail boom is fixed to an aft portion of the sub-caliber projectile. Propelling charges for a standard 120 mm mortar round may be mounted on the tail boom in a known manner. A 120 mm mortar fin assembly is fixed to an aft portion of the tail boom. A novel sabot made of a plurality of individual increments is fixed to the sub-caliber projectile. The sub-caliber projectile will have a range that is greater than the range of the standard 120 mm projectile. 
     In another embodiment, the novel mortar round includes a standard 81 mm projectile, such as an M821 projectile. The projectile includes a projectile wall that defines an interior volume therein. A tail boom is fixed to an aft portion of the standard 81 mm projectile. Propelling charges for a standard 81 mm mortar round may be mounted on the tail boom in a known manner. An 81 mm mortar fin assembly is fixed to an aft portion of the tail boom. A novel sabot made of a plurality of individual increments is fixed to the standard projectile. The sabot may be of a size for launching from a 120 mm mortar tube. The standard 81 mm projectile fitted with the novel sabot will have a range that is greater than the range of the standard 81 mm projectile without the sabot. 
     Compared to the logistical burden of the standard 120 mm mortar system, the logistical burden of using the novel mortar round with the sub-caliber projectile and the sabot will be the same or less. For example, a 120 mm high explosive mortar round may weigh about 31 pounds while an 81 mm high explosive round with a sabot may weigh about 11 pounds. Further, the use of the super-caliber fins with the sub-caliber projectile increases the aerodynamic stability and accuracy of the sub-caliber projectile. The increased accuracy reduces the ballistic circular error probability (CEP) and may reduce the number of rounds per kill, thereby further reducing the logistical burden. 
     The sub-caliber projectile may have a generally cylindrical overall shape. The sabot increments are fixed to the outer surface of the sub-caliber projectile. In addition to generally cylindrical shapes, the sabot increments may be fixed to non-cylindrical, uniquely shaped projectiles, thereby enabling the launch of those uniquely shaped projectiles from known mortar tubes. Examples of non-cylindrical projectiles include asymmetric lifting bodies, for instance, bodies with geometries similar to flying wing geometries. 
       FIG. 1A  is a perspective view of one embodiment of a fin-stabilized, muzzle-loaded mortar round  10  having a central longitudinal axis A.  FIG. 1B  is a longitudinal sectional view of  FIG. 1A .  FIG. 5  is an enlarged view of a portion of  FIG. 1B . Round  10  may be launched from a mortar tube  12  ( FIG. 2 ) having an inner diameter or caliber B. By way of example only, caliber B may be 120 mm or 81 mm. 
     Round  10  includes a sub-caliber projectile  14  having an interior volume  16  defined by a projectile wall  18 . Projectile  14  may be centered on axis A. Sub-caliber means that the caliber or diameter of projectile  14  is less than caliber B. For example, if caliber B is 120 mm, projectile  14  may be an 81 mm caliber projectile, or, if caliber B is 81 mm, projectile  14  may be a 60 mm caliber projectile. Caliber B may be other sizes, also. 
     A payload  20  is disposed in the interior volume  16 . Payload  20  may be, for example, high explosive material, smoke-producing material, etc. A tail boom  22  is fixed to an all portion of the sub-caliber projectile  14 . Propelling charges (not shown) may be disposed on tail boom  22  in a known manner. A fin assembly  24  is fixed to an aft portion of the tail boom  22 . The fin assembly  24  has an outer diameter at least as large as the caliber B of the mortar tube  12 . 
     A discarding sabot  26  is disposed circumferentially around the sub-caliber projectile  14  and centered on the central longitudinal axis A. Sabot  26  includes a plurality of discrete sections  28 ,  30 ,  32 ,  34 ,  36  arranged longitudinally one after another in abutting relationship and around the sub-caliber projectile  14 . The discrete sections  28 - 36  may be generally annular in shape. In  FIGS. 1A and 1B , additional discrete sections are shown between sections  34  and  36  but are not individually called out with a reference character. The number of discrete sections  28 - 36  in sabot  26  may vary. 
     The axial location of sabot  26  on projectile  14  may be varied to vary the chamber volume in mortar tube  12 . Varying the chamber volume will alter the ballistic performance of projectile  14 . 
     Each discrete section  28 - 36  is circumferentially divided into a respective plurality of discrete sabot increments  28   a ,  30   a ,  32   a ,  34   a ,  36   a . The number of sabot increments per section is at least two and may be up to twenty-four or more. In the embodiment of  FIGS. 1A-B , each discrete section  28 - 36  is circumferentially divided into twelve increments. The division of sabot  26  into discrete longitudinal sections  28 - 36  and into discrete increments  28   a - 36   a  in each section enables the sabot  26  to rapidly separate from the projectile  14  at muzzle exit. In addition, the relatively small size and mass of each increment  28   a - 36   a  greatly reduces the probability of the discarded increments  28   a - 36   a  causing harm to personnel or property. The small mass of each increment  28   a - 36   a  also minimizes or eliminates any disturbances that might be imparted to projectile  14  as the increments separate from the projectile at muzzle exit. Preferably, the sabot increments  28   a - 36   a  are made of a plastic material and may be formed by injection molding. 
     The aft most sabot section  28  may have an outer diameter about the same as the caliber B of the mortar tube to enable sealing of the propellant gases behind sabot  26 . The sabot sections  30 - 36  forward of section  28  may have smaller outer diameters than aft most section  28 . Aft most section  28  may optionally include an obturator groove  38  ( FIG. 1B ) for receiving an obturator (not shown). 
     Each sabot increment  28   a - 36   a  includes a respective base portion  28   b - 36   b  that is mechanically connected or engaged with the sub-caliber projectile  14 . The propelling force of the propellant gas behind sabot  26  is transferred to projectile  14  by the mechanical engagement between base portions  28   b - 36   b  and projectile  14 . Various types of mechanical engagement may be used. In the embodiment of  FIGS. 1A-B , a plurality of circumferential grooves  40  are formed in the exterior surface of projectile wall  18 . Projections  28   c - 36   c  on respective base portions  28   h - 36   b  of sabot increments  28   a - 36   a  engage respective grooves  40  in wall  18 . In addition, each increment  28   a - 36   a  may be sized to provide a snap or interference fit on projectile wall  18 . By increasing the number of sections  28 - 36 , the amount of propelling force transferred from sabot  26  to projectile  14  may be increased. 
     Another way to mechanically engage base portions of the sabot increments with projectile  14  is by forming dimples in the base portions of the sabot increments and forming mating or complementary dimples on the exterior surface of the sub-caliber projectile.  FIG. 3  is a side view of a sub-caliber projectile  42  having a central longitudinal axis C. Projectile  42  has dimples  44  formed on its exterior surface.  FIG. 6A  is a bottom view of one embodiment of a sabot increment  46  having a base portion  48  with dimples  50  formed there. Dimples  50  on sabot base portion  48  mechanically engage dimples  44  on projectile  42  and transfer propelling force from the sabot to the projectile  14 . Dimples  44 ,  50  may be similar in shape to dimples on golf balls. 
     A further way to mechanically engage base portions of the sabot increments with projectile  14  is by forming longitudinal grooves in the exterior surface of projectile  14  and forming mating or complementary projections on the base portions of the sabot increments.  FIG. 4  is a side view of a sub-caliber projectile  52  with longitudinal grooves  54  formed on its exterior surface. The base portions of the sabot increments have corresponding projections (not shown) that mate with the grooves  54 . 
     Additional features of the sabot increments will be described with reference to sabot increment  46  shown in detail in  FIGS. 6A-6I . Sabot increments  28   a - 36   a  have base portions with projections  28   c - 36   c  for engaging grooves  40 , while the base portion of sabot increment  46  is dimpled. However, the circumferential and longitudinal “increment to increment” interlocking features of sabot increment  46  correspond to, for example, the structure of sabot increments  28   a - 36   a.    
       FIG. 6A  is a front view of the base portion  48  of sabot increment  46 , showing dimples  50  formed thereon.  FIG. 6B  is a right side view of  FIG. 6A  and  FIG. 6C  is a left side view of  FIG. 6A .  FIGS. 6B and 6C  show the location of the central longitudinal axis C of the dimpled sub-caliber projectile  42  ( FIG. 4 ). Axis C is normal to the views in  FIGS. 6B and 6C . The dashed circle shown in  FIGS. 6B and 6C  illustrates the circumferential orientation of one increment  46 . 
       FIG. 6D  is a top view of  FIG. 6A .  FIG. 6E  is a view of  FIG. 6D  rotated 90 degrees clockwise.  FIG. 6F  is a view of  FIG. 6D  rotated 90 degrees counterclockwise.  FIG. 6G  is an end view of  FIG. 6A .  FIG. 6H  is a view of  FIG. 6G  rotated 90 degrees counterclockwise.  FIG. 6I  is a view of  FIG. 6A  rotated 90 degrees counterclockwise. 
     Sabot increment  46  includes two opposing side portions  56 ,  58 . The opposing side portions  56 ,  58  provide a mechanical connection between circumferentially adjacent sabot increments. Side portion  56  includes a trapezoidal projection  60  and side portion  58  includes a mating trapezoidal recess  62 . The opposing, non-parallel sides of the trapezoidal projection  60  each include a curved projection  64  thereon. The opposing, non-parallel sides of the mating trapezoidal recess  62  each include a mating curved recess  66  therein. In one embodiment, the curved projection  64  and the mating curved recess  66  are spherical surfaces. Circumferentially adjacent sabot increments  46  are mechanically connected by inserting the trapezoidal projection  60  of one sabot increment  46  into the mating trapezoidal recess  62  of a circumferentially adjacent sabot increment  46  and nesting the curved projections  64  in the curved recesses  66 . 
     Sabot increment  46  includes two opposing end portions  68 ,  70 . The opposing end portions  68 ,  70  provide a mechanical connection between longitudinally adjacent sabot increments. One opposing end portion  68  has projecting ridge  72  formed thereon. The other opposing end portion  70  has a mating groove  74  formed therein. The projecting ridge  72  includes a plurality of depressions  76  formed thereon. The mating groove  74  includes a plurality of protuberances  78  formed therein. Longitudinally adjacent sabot increments  46  are mechanically connected by inserting the projecting ridge  72  of one sabot increment  46  into the mating groove  74  in a longitudinally adjacent sabot increment  46 . In addition, the plurality of protuberances  78  in mating groove  74  are nested in respective ones of the plurality of depressions  76 . 
     As described above, the novel sabot includes a plurality of discrete sections arranged longitudinally in series. Each section is circumferentially divided into a plurality of discrete sabot increments. The mechanical connections between the sabot increments and the sub-caliber projectile, along with the mechanical connections between circumferentially adjacent and longitudinally adjacent sabot increments, insure the effective performance of the sabot in the mortar tube. Simultaneously, the features of the sabot insure, after muzzle exit, a quick discard of the sabot into small, non-lethal pieces that have a minimal, if any, effect on the ballistics of the sub-caliber projectile. 
       FIG. 7A  is a perspective view of one embodiment of an asymmetrical mortar projectile  80  having a projectile body  82 , a tail boom  84 , and a fin assembly  86 .  FIG. 7B-7E  are side, front end, aft end, and top views, respectively, of the projectile  80  of  FIG. 7A . 
       FIG. 8A  is a perspective view of the projectile  80  of  FIG. 7A  with a novel sabot  88  disposed thereon.  FIGS. 8B-8E  are longitudinal sectional, front end, aft end, and top views, respectively, of the projectile  80  and sabot  88  of  FIG. 8A . Sabot  88  is disposed circumferentially around projectile body  82 . Sabot  88  may be mechanically fixed to body  82  with one or more of the structures and methods described above with respect to projectile  10  and  FIGS. 1-6 . As described with respect to sabot  26 , sabot  88  may include a plurality of discrete sections (division lines between longitudinal sections are not shown in sabot  88 ) arranged longitudinally in series. Each section may be circumferentially divided into a plurality of discrete sabot increments (division lines between circumferential increments are not shown in sabot  88 ). The sabot increments of sabot  88  may include the mechanical features of the sabot increments of sabot  26  that enable the sabot increments to mechanically connect with circumferentially adjacent and longitudinally adjacent sabot increments. 
       FIG. 9A  is a perspective view of an embodiment of a fin-stabilized, muzzle-loaded mortar round  90  having a projectile body  91 , a tail boom  92 , a fin assembly  94  and a discarding sabot  96  disposed on the projectile body  91 .  FIG. 9B  is a longitudinal sectional view of the round  90  of  FIG. 9A . Tail boom  92  may have an outer diameter substantially the same as the caliber of projectile body  91 . 
     Round  90  (excluding sabot  96 ) may be, for example, a standard mortar round, such as an 81 mm M821 mortar round or a 60 mm M720 mortar round. Round  90  may be launched from a mortar tube larger than 81 mm, for example, a 120 mm mortar tube, by using sabot  96 . Sabot  96  may be circumferentially divided into a plurality of discrete sabot increments  96   a ,  96   b ,  96   c . The number of sabot increments may vary from at least two to as many as twenty-four. 
       FIG. 10A  is a perspective view of a sabot increment  96   a  of the sabot  96 .  FIGS. 10B, 10C, and 10D  are side, end and top views, respectively, of the sabot increment  96   a  of  FIG. 10A . Sabot increments  96   a ,  96   b ,  96   c  may be mechanically connected to projectile body  91  with one or more of the structures (not shown in  FIGS. 10A-D ) and methods described with respect to sabot  26 . Sabot increments  96   a ,  96   b ,  96   c  may be circumferentially mechanically connected to each other with the structure (not shown in  FIGS. 10A-D ) described with respect to sabot  26 . 
       FIG. 11A  is a perspective view of an embodiment of a fin-stabilized, muzzle-loaded mortar round  110  having a central longitudinal axis Y.  FIGS. 11B-11D  are longitudinal sectional, front end and aft end views, respectively of the round  110  of  FIG. 11A . Round  110  may be launched from mortar tube  12  ( FIG. 2 ) having an inner diameter or caliber B. By way of example only, caliber B may be 120 mm, 81 mm or 60 mm. 
     Round  110  includes a sub-caliber projectile  114  having an interior volume  116  defined by a projectile wall  118 . Projectile  114  may be centered on axis Y. A payload  120  may be disposed in the interior volume  116 . Payload  120  may be, for example, high explosive material, smoke-producing material, etc. A tail boom  122  is fixed to an aft portion of the sub-caliber projectile  114 . Propelling charges (not shown) may be disposed on tail boom  122  in a known manner. A fin assembly  124  is fixed to an aft portion of the tail boom  122 . 
     A sabot  126  is disposed circumferentially around the sub-caliber projectile  114  and centered on the central longitudinal axis Y. Sabot  126  is a monolithic structure that defines an interior, annular, converging-diverging nozzle  130 . Prior to launch of round  110 , a plug  132  is inserted in an aft end of the nozzle  130 . Plug  132  may also function as an obturator. Upon exit of the round  110  from the mortar tube  12 , air pressure forces the plug  132  rearward out of the nozzle  130 . The axial location of sabot  126  on projectile  114  depends on the effect (lift or drag) desired from the nozzle  130 . Thus, sabot  126  may be axially placed at the center of gravity of projectile  114 , or forward or aft of the center of gravity of projectile  114 . 
       FIG. 12A  is a perspective view of an embodiment of a fin-stabilized, muzzle-loaded mortar round  140  having a central longitudinal axis X.  FIGS. 12B-12E  are longitudinal sectional, front end, aft end and side views, respectively of the round  140  of  FIG. 12A . Round  140  may be launched from mortar tube  12  ( FIG. 2 ) having an inner diameter or caliber B. By way of example only, caliber B may be 120 mm, 81 mm or 60 mm. 
     Round  140  includes a sub-caliber projectile  142  having an interior volume  144  defined by a projectile wall  146 . Projectile  142  may be centered on axis X. A payload  148  may be disposed in the interior volume  144 . Payload  142  may be, for example, high explosive material, smoke-producing material, etc. A tail boom  150  is fixed to an aft portion of the sub-caliber projectile  142 . Propelling charges (not shown) may be disposed on tail boom  150  in a known manner. A fin assembly  152  is fixed to an aft portion of the tail boom  150 . 
     A sabot  156  is disposed circumferentially around the sub-caliber projectile  142  and centered on the central longitudinal axis X. Sabot  156  is a monolithic structure that defines an interior, annular, orifice  158 . Orifice  158  includes a radially interior, linear surface or side  160 . Linear side  160  may be defined by the projectile wall  146 . Opposite from the linear side  160  is a converging-diverging side  162 . Prior to launch of round  140 , a plug  164  is inserted in an aft end of the orifice  158 . Plug  164  may also function as an obturator. Upon exit of the round  140  from the mortar tube  12 , air pressure forces the plug  164  rearward out of the orifice  158 . The axial location of sabot  156  on projectile  142  depends on the effect desired from the orifice  158 . 
     As alternatives to nozzle  130  and orifice  158 , annular openings in sabots such as sabots  126 ,  156  may have other geometries as well. The annular openings may be used to maneuver the projectile, accelerate the projectile, or maintain the projectile&#39;s velocity using, for example, propulsion, scramjet or ramjet type orifices. 
     While the invention has been described with reference to certain embodiments, numerous changes, alterations and modifications to the described embodiments are possible without departing from the spirit and scope of the invention as defined in the appended claims, and equivalents thereof.