Abstract:
A light weight field howitzer includes a barrel which is supported by a cradle constructed from hollow members and which is pivotally mounted about a trunnion bearing secured to a chassis. The trunnion bearing lies on the barrel axis and is positioned beyond the limit of maximum recoil of the barrel. Front stabilisers and rear trail support legs are provided to spread the load of the howitzer and spades are rigidly secured to the chassis. The howitzer includes a single hydraulic accumulator arrangement constituting a combined recoil buffer and recuperator system. A barrel elevating means is provided comprising a geared manual means assisted by a precompressed gas system.

Description:
This application is a divisional of U.S. patent application Ser. No. 08/038,088, now U.S. Pat. No. 6,024,007, filed on Mar. 29, 1993; which is a continuation of U.S. patent application Ser. No. 07/456,818, now abandoned, filed on Dec. 13, 1989. 
    
    
     This invention relates to field howitzers and is concerned with the application, to field howitzers, of design techniques and philosophies not normally associated with the design of field artillery in order to produce such weapons having an absolute minimum of weight, yet still retaining all the other features required by such equipment, e.g range, reliability, accuracy, rate of fire, stability, robustness of construction etc. 
     The existence of rapid deployment forces is well known and it is desirable that the range of equipment available to these forces is as wide as reasonably possible. There is a need for the equipment available for these forces to include field artillery. 
     Newton&#39;s Third Law of Motion states that for every action, there is an equal and opposite reaction. Thus for field howitzers which can fire a heavy projectile over a great range, the recoil presents a particular problem. One means to minimise the recoil problem is to have heavy ordnance. However, a main purpose of the present invention is to minimise weight and it is an object of the invention to dissipate the recoil forces on light ordnance by the combination of:
         i) optimised recoil buffer efficiency,   ii) optimised muzzle brake efficiency, and   iii) a new design concept that takes the resultant recoil forces directly to spades via a damped, energy-absorbing means.
 
For a conventional field howitzer, which is intended to be air liftable, air dropable and moved around a battle field with comparative ease, a relatively light (though still robust) chassis is required. To increase stability and spread the recoil forces, one method is to deploy a pair of trail legs with ‘spades’ at their further ends; the purpose of the ‘spades is to dig into the ground and so absorb the recoil force. Despite their construction, such conventional field howitzers are far too heavy to be carried by the small or medium lift helicopters used near the actual battle zone.
       

     NATO is in the process of standardising ordnance and ammunition systems into a single calibre. There is thus a need for an ultra lightweight version of the standard 155 mm field howitzer which can be transported as a single unit by a battlefield helicopter. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a field howitzer which comprises:
         i) a howitzer barrel,   ii) a cradle supporting the barrel and having a rearward end,   iii) a chassis, and   iv) a trunnion support structure secured to the chassis and including a trunnion bearing about which the rearward end of the cradle is pivotally mounted, said trunnion bearing lying on the axis of the barrel and being positioned beyond the limit of maximum recoil of the barrel.       

     The trunnion bearing should be placed as low as possible consistent with the other requirements of a field howitzer (e.g training, towing, loading etc.) and the location of the trunnion bearing beyond the point of maximum recoil so that the barrel does not recoil through the trunnion bearing enables this to be achieved and also ensures that the howitzer exhibits a high degree of out-of-balance. 
     In order to enable the field howitzer to be as light in weight as possible, the chassis should be a lightweight chassis and weight saving design principles should be used in the construction of the other components of the howitzer. 
     This aspect of the invention combines features of both a field howitzer and a mortar and the minimum trunnion height greatly facilitates the transfer of the recoil forces to the ground. The weight saving design principles employed include the use of lightweight strong alloys, integral construction, etc. The single lightweight chassis should rest on the ground, as opposed to the conventional chassis which rests on a sole plate which is in contact with the ground. This feature is a radical change from previous design practice. 
     The position of the trunnion support structure on the chassis is basically the same as for other field howitzers. However, relative to conventional designs, the barrel is moved forward so that the whole of it, including the whole of the supporting cradle, whether in the ready-to-fire position, or the maximum recoil position, is always forward of the trunnion bearing. This leads to a intermediate and high degree of out-of-balance which acts to oppose the recoil moment, particularly when the gun is fired at low angles of elevation e.g., from −2.5° to +70° . 
     In a preferred embodiment, spades are secured directly to the chassis and the howitzer includes front stabilisers and rear trail support legs operable to spread the load over a large area of ground when not being fired, the latter also assisting to absorb recoil energy whilst resisting overturning and lateral forces. 
     Such spades, stabilisers and support legs may be incorporated in howitzers other than those of the type having a trunnion bearing lying on the barrel axis beyond the maximum recoil limit in accordance with the first aspect of the invention. 
     Accordingly, a second aspect of the present invention provides a field howitzer comprising:
         i) a chassis,   ii) spades rigidly secured to the howitzer chassis;   iii) front stabilisers operable to spread the load of the howitzer over a large area of ground when not being fired; and   iv) rear trail support legs operable to spread the load of the howitzer over a large area of ground and to assist with the absorbing of recoil energy while providing overturning and lateral stability.       

     Preferably the spades are of the ‘self-digging’ type so that they will be fully effective as the first round is discharged. The attachment of the spades directly to the rear of the chassis in conjunction with a low trunnion bearing height provides an essentially rigid means of restraining the gun during recoil. Front stabilisers and trail legs generally improve stability and hence the aiming of the gun, but also play a part in the absorption of recoil energy. The term ‘trail legs’ is a generally accepted term in this type of howitzer. Although it is not intended that the howitzer of the invention be towed by ‘trail legs’, the term is retained for descriptive consistency. 
     The spades may be secured directly to the chassis or they may be secured to the ends of the rear trail support legs provided that they are relatively short and inflexible 
     Preferably the spades are removable when the howitzer is in the mode for being towed/transported. 
     In a preferred embodiment, the rear trail support legs are hingedly mounted on the chassis and hydraulic dampers are provided at, or near, the attachment points of the rear trail legs to the chassis to assist with the absorbing of recoil energy. These dampers for the rear trail legs can automatically compensate for uneven ground and give protection against excessive recoil forces. 
     In a further embodiment, the howitzer barrel is mounted on the chassis so as to be displaceable from a first to a second position with respect to the chassis, as a consequence of recoil on firing and the howitzer includes a recoil buffer system to absorb the energy of recoil as the barrel is displaced on firing, and also a recuperator system to return the displaced barrel from the second position to the first position, said recoil buffer system and said recuperator system being combined and utilising a single hydraulic accumulator arrangement. 
     Such a combined recoil buffer/recuperator system can be utilised with howitzers which are other than of the type defined in accordance with the first and second aspects of the invention. 
     Accordingly, a third aspect of the present invention provides a field howitzer comprising:
         i) a chassis,   ii) a howitzer barrel mounted on the chassis so as to be displaceable from a first to a second position with respect to the chassis, as a consequence of recoil on firing,   iii) a recoil buffer system to absorb the energy of recoil as the barrel is displaced on firing, and   iv) a recuperator system to return the displaced barrel from the second position to the first position, said recoil buffer system and said recuperator system being combined and utilising a single hydraulic accumulator arrangement.       

     In a particularly preferred embodiment, the barrel is supported in a trunnion support structure by means of a cradle and the cradle is constructed from hollow members, the space inside said hollow members being used wholly, or in part, to provide the volume for the compressed inert gas forming part of said single hydraulic accumulator arrangement. 
     The hydraulic accumulator arrangement of the combined recoil buffer system and recuperator system serves as a ‘spring’ which absorbs some of the energy of the recoiling barrel. The energy absorbed is subsequently released in a controlled manner to run out the barrel to the firing position. Hydraulic accumulators operate against a given volume of compressed inert gas. The ‘spring constant’ is determined by the volume of gas and the amount by which this is reduced by the compression caused by the volume of hydraulic fluid displaced by the recoil. To provide a relatively uniform ‘spring constant’, a large volume of gas is required compared with the volume of fluid displaced. As it is desirable to allow the barrel to have as long a recoil as possible, a fairly large volume of hydraulic fluid needs to be displaced and hence as large a volume of gas as possible is required. As the weight of thick walled pressure-resistant gas cylinders would be excessive, the gas volume may be provided by using the bores of two of, say, the four hollow structural members which form the gun cradle. Interconnecting passages may be provided to allow the gas pressure to be equalised between said two members, if required. 
     In an embodiment, the howitzer includes an elevating means for pivoting the barrel about a horizontal axis, said elevating means comprising a geared manual means assisted by precompressed gas. 
     Such an elevating means can be incorporated in howitzers which are not constructed in accordance with the first, second and third aspects of the invention. 
     Accordingly, a fourth aspect of the present invention comprises a field howitzer comprising
         i) a chassis;   ii) a howitzer barrel supported in a cradle and mounted in a trunnion bearing on the chassis so as to be pivotable about a horizontal axis, and   iii) elevating means for pivoting the barrel about said axis, said elevating means comprising a geared manual means assisted by precompressed gas.       

     In a particularly preferred embodiment the howitzer barrel is mounted so as to be out-of-balance and the degree of assistance provided by the precompressed gas is sufficient to substantially counterbalance the barrel weight due to its positive out-of-balance. 
     Preferably the barrel weight is balanced by gas springs consisting of cylinders pressurised by an inert gas reservoir acting on pistons in the cylinders. In the case where the cradle is constructed from hollow members, the space inside the hollow members may be used, wholly or in part, to provide the volume for the gas. If some of the, say, four hollow members of the cradle are used for the combined recoil buffer recuperator system as above described, the remaining hollow members may be used for the gas for the elevating means. The gas connection between the hollow members and the cylinders of the gas springs may either be via flexible pressure-resistant tubes or via a bore down the axis of the piston rods of the pistons with the other ends of the rods secured to said hollow members. The actual elevation of the barrel is effected by means of a geared drive via a handwheel, but this would involve minimal physical effort because of the counterbalancing action. The gas springs may also incorporate hydraulic fluid, if required. 
     In a particularly preferred embodiment, the elevating means comprises a lead screw, essentially pivotally fixed at one end and along which a nut may be screwed, said nut being fixed relative to the cradle for the barrel of said howitzer but rotatable so that the resulting translational movement of said nut along said lead screw causes said cradle to move in a rotary direction about the trunnion bearing, thus elevating/depressing the barrel of the howitzer. Preferably the nut is readily rotated, e.g via a handwheel and gearing, and a reverse locking means is employed. 
     It is particularly preferred for the essentially pivotally fixed end of said lead screw to be provided with a flexible tunable mounting comprising;
         i) a spring means aligned parallel to the axis of said lead screw, and   ii) a damper;
 
wherein the spring constant, pre-load and resistance to motion provided by the damper are adjustable to give a tunable system. Preferably the spring comprises a series of spring washers and the damper is hydraulic.
       

     In an embodiment the howitzer barrel is mounted on the chassis by means of a training bearing so as to be pivotable about a vertical axis and said training bearing comprises (a) a small central locating bearing having inner and outer bearing surfaces one of which is fast with the chassis and the other of which is fast with a support for the barrel and (b) a separate large diameter thrust bearing formed as part of a concentric arc on the opposite side of said small central locating bearing to the barrel. 
     Such a training bearing may be incorporated in a howitzer which is not constructed in accordance with the first, second, third and fourth aspects of the invention. 
     Accordingly, a fifth aspect of the present invention provides a field howitzer comprising:
         i) a chassis,   ii) a howitzer barrel mounted on the chassis by means of a training bearing so as to be pivotable about a vertical axis, said training bearing comprising (a) a small central locating bearing having inner and outer bearing surfaces one of which is fast with the chassis and the other of which is fast with a support for the barrel and (b) a separate large diameter thrust bearing formed as part of a concentric arc on the opposite side of said small central locating bearing to the barrel.       

     Preferably the howitzer includes a training rack integral with a part of the thrust bearing arc. 
     In an embodiment, the howitzer barrel includes a muzzle brake and a hinged lunette attached to the barrel adjacent to the muzzle brake to enable the howitzer to be towed. 
     Such a muzzle brake and lunette may be incorporated in a howitzer which is not constructed in accordance with the first to the fifth aspects of the invention. 
     Accordingly, a sixth aspect of the present invention provides a field howitzer comprising:
         i) a chassis;   ii) a howitzer barrel mounted on the chassis,   iii) a muzzle brake on the barrel, and   iv) a hinged lunette attached to the barrel adjacent to the muzzle brake to enable the howitzer to be towed.       

     It is common for conventional designs of howitzer to be towed by the (rear) trail legs. The novelty in the sixth aspect of the invention is that the towing attachment is secured to the barrel, just behind the muzzle brake, and hinged forward to project beyond the muzzle brake to co-operate with the hook on the towing vehicle. In the case where the gun is out-of-balance, this will provide a net downward load on the towing hook, which is normal towing practice. 
     It is particularly preferred, in all aspects of the invention, for the howitzer to be constructed in a manner which enables it to be quickly and easily separated into two or more parts which can readily be reassembled. The advantage of a howitzer which can be separated into one or more component parts and easily reassembled is that transport problems are greatly reduced if two light sections have to be moved instead of one heavier one. In this context, transport may be by vehicle or helicopter on or near a battlefield or by aircraft, ship or road vehicle to or from the scene of conflict. Smaller sections of a howitzer may pack better in a ship or aircraft when many have to be transported. A further advantage is that damaged parts may be repaired by replacement of whole section. 
     For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a side elevation of a conventional 155 mm field howitzer in a ready-to-fire position; 
         FIG. 2  shows a side elevation of an ultra lightweight 155 mm field howitzer of the present invention in a ready-to-fire position; 
         FIG. 3  shows a side elevation of the howitzer of  FIG. 2  in the towed mode; 
         FIG. 4  shows a plan view of the howitzer of  FIG. 2  in the ready-to-fire mode; 
         FIG. 5  shows a cross section through the barrel and cradle of the howitzer of  FIGS. 2 to 4 ; 
         FIG. 6  shows schematically the action of the recoil buffer and recuperator of the howitzer of  FIGS. 2 to 5 ; 
         FIG. 7  shows a side elevation of the elevating mechanism of the howitzer of  FIGS. 2 to 6 ; 
         FIG. 8  shows a sectional plan view along line BB ( FIG. 9 ) of the training mechanism of the howitzer of  FIGS. 2 to 7 ; 
         FIG. 9  shows a sectional side elevation along line AA ( FIG. 8 ) of the training mechanism of the howitzer of  FIGS. 2 to 8 ; 
         FIG. 10  shows a side elevation of a rear trail leg and spade of another howitzer of the present invention; and 
         FIG. 11  is a simplified exploded view of the howitzer of  FIGS. 2 to 10 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention represents an innovative concept in the design of field howitzers. The main theme behind the design process is to produce an ultra lightweight version of the current standard NATO 155 mm ordnance. This design process has led to the adoption of a large number of innovative features, including the following features either singly or in any combination of two or more:
         1. greatly lowered trunnion bearing height.   2. location of the whole of the barrel, including full recoil length, forward of the trunnion bearing.   3. a single fabricated chassis, with a spherically or cylindrically convex lower surface to act as a combined chassis and sole plate.   4. self-digging spades attached directly to the chassis.   4a. self-digging spades attached at or near the ends of short, inelastic rear trail legs.   5. positive out-of-balance in all non-firing attitudes.   6. front stabilisers to counteract the out-of-balance when in normal ready-to-fire attitudes.   7. short light rear trail legs designed to resist only overturning effects rather than full recoil loading.   7a. short robust rear trail legs designed to resist overturning forces and transmit recoil forces via the spades into the ground.   8. hydraulic dampers in or near the rear trail leg and a chassis hinge to assist with transfer of recoil energy, yet protect the trail legs from damage due to excessive loadings.   9. a combined recoil and recuperator system.   10. the use of the hollow interiors of structural members forming the cradle to provide additional accumulator gas volume.   11. counterbalancing of the barrel using gas cylinders and a pressurised gas reservoir located in the hollow interiors of other structural members.   12. provision for towing by the muzzle of the barrel and using the out-of-balance to provide safer towing.   13. damped elevation gearing system.   14. resetting (rendering) device in the elevation system.   15. minimum size training bearing.   16. segmental arc training gear (rather than full gear ring) incorporating a pre-loaded thrust bearing arrangement.   17. extensive use of lightweight materials, such as titanium alloys, and aerospace technology.       

       FIG. 1  shows a current design of 155 mm field howitzer in a ready-to-fire position. The barrel  1  is horizontal and located in a trunnion carried on a substantial chassis  3 . The height of the trunnion is such that the barrel axis  1 A is a relatively large distance  2  from the ground. Two trail legs  4 , which are splayed out, and sole plate  5  give a stable 3-point support. A spade  6  near the end of each trail leg  4  is designed to ‘dig’ into the ground as the howitzer is fired and so provide the horizontal reaction  7  to the horizontal component of the recoil force. When firing at an elevated angle, the vertical components  8 A and  8 B of the recoil reaction are taken at the sole plate  5  and spades  6  via legs  4 , respectively. In order to withstand the horizontal reaction  7 , vertical component  8 B and turning component  8 T of the recoil forces, trail legs  4  are substantial box-section members. A further benefit of trail legs  4  is that their weight acts as a counterbalance to that of barrel  1  to bring the centre of gravity  9  above sole plate  5 . 
     Though the trail legs  4  are substantial box-section members, they still act as ‘springs’ when the gun is discharged. If the howitzer were to be discharged in the attitude shown in  FIG. 1 , there would be no vertical component in the recoil. Instead the recoil would consist of a horizontal force (balanced by reaction  7 ) and a turning moment  8 T (caused because the line of reaction  7  is off set from the barrel axis  1 A which is the line of action of the force). As the howitzer is fired, the horizontal component of the recoil forces the main body of the howitzer to move backwards. As spades  6  should not move, this component of the recoil would cause the two splayed out trail legs  4  to distort and absorb strain energy as they transferred the recoil energy to spades  6 . Because there is also a turning moment  8 T in the recoil, sole plate  5  may be lifted off the ground. As the energy of the recoil is dissipated by spades  6 , so the strain energy in trail legs  4  will be released causing the main body of the howitzer to move back to (and possibly overshoot) its original position. At the same time, the main body of the howitzer falls back onto the ground. Thus the prior art design, with spades  6  at the ends of trail legs  4  leads to a fairly violent motion of the howitzer under recoil. 
       FIG. 2  shows a side elevation of the ultra lightweight howitzer according to the invention. The design is based upon:
         i) a geometrically optimised weapon configuration;   ii) sensible use of available high strength lightweight materials; and   iii) the minimising of the recoil forces.       

     In  FIGS. 2 ,  3  and  4 , the same reference number is used for the same component as shown in  FIG. 1  but preceded by one hundred, e.g. 1 and 101. 
     The key features of the design will now be described either singly, or in related groups.
         1. Greatly lowered Trunnion Bearing Height.   2. Location of Barrel, including full recoil length, forward of trunnion bearing.   3. Single fabricated chassis.   4. Self-digging spades attached directly to chassis.   5. Positive out-of-balance.   17. Use of lightweight alloys and construction techniques.       

     The most readily apparent feature of the lightweight design is its low overall height as compared to the traditional design. The key factor in the design is the distance  102  of the axis  101 A of barrel  101  when in the horizontal position, which is about 650 mm above ground level, compared to over 1500 mm for distance  2  for the gun in  FIG. 1 . The next most apparent feature is that the trunnion bearing  113  of trunnion support structure  124  is located to the rear of the extreme recoil position of barrel  101  and lies on the axis  101 A. This makes the design a hybrid between that of a conventional field howitzer and a mortar. As shown by the centre of gravity  109 , there is positive out-of-balance. 
     It is a normal design criteria that structures should be stable under the whole range of operating conditions. However, it is a particular and novel feature of the present invention that a positive out-of-balance is provided. Because of the very low weight of this ordnance, it is essential that what weight there is, is used in the most effective manner in the most arduous mode of operation, i.e firing. Thus the design is such as to place the centre of gravity  109  as far forward of the trunnion  113  as possible, i.e. to create as much positive out-of-balance as practicable to counteract the overturning effect  108 T of the recoil. Though the result of this design philosophy is to require front stabilisers  110  to give stability in non-firing modes, the net advantages are considerable. Detailed studies of a range of options indicate that the embodiment shown offers the best compromise between weight-saving on the whole ordnance (i.e after allowing for the weight of the front stabilisers  110 ) and minimising the net recoil overturning moment  108 T. 
     The trunnion support structure  124  is carried by a platform/chassis  117 . Both these structures are fabricated from low weight, high strength alloys, in which metals such as titanium, magnesium and aluminium etc. are important constituents. Other high strength, low weight materials, e.g glass and carbon fibre reinforced plastic, may be used where appropriate. The design of the trunnion support  124  and platform/chassis  117  structures uses techniques not usually associated with artillery weapons to give robust lightweight components. 
     The underside of the platform/chassis  117  is convex so that it will rest naturally on all normal types of terrain to give a stable 3-point support with the front stabilisers  110 . (See points  6  and  7  later.) At the rear of the platform/chassis  117 , rear trail legs  104  are fitted via a hinged joint  104 A. Also incorporated in these hinges are self digging spades  106 . The method of hinging is such that the rearwards and downwards direction of the recoil forces causes spades  106  to lock against the rear of platform/chassis  117 , i.e. the spades are, in effect, fast with chassis  117  and not located at remote points connected by ‘resilient’ trail legs  4  ( FIG. 1 ). 
     Features  1 - 4  and  17  combine to give the following advantages:
         i) Greatly reduced mass of metal in the trunnion and chassis structures.   ii) Greatly reduced turning moments due to recoil forces.   iii) High out-of-balance which acts to oppose the turning moment  108 T due to the recoil.       

     These factors act synergistically because the reduced recoil moment requires less mechanical strength in the trunnion support structure  124 , allowing a greater choice of lightweight materials (Feature  17 ).
         5. Positive of out-of-balance.   6. Front stabilisers to counteract out-of-balance in all normal ready-to-fire attitudes.   7. Short, light rear trail legs to resist overturning.   8. Hydraulic dampers in or near trail leg—chassis hinge.       

     The front stabilisers  110  are used to counteract the out-of-balance  109  of the howitzer. Thus in the normal ready-to-fire mode, there is a stable, three-point support provided by chassis  117  and the two feet  111  at the ends of the front stabilisers  110 . The vertical reactions due to the howitzer&#39;s weight on the chassis  117  and on the front stabilisers  110  are indicated by arrows  108 B and  108 D. 
     The rear trail legs  104  are secured to the body by the composite hinges  104 A, which also secure self-digging spades  106 . Built in to the trail leg hinges  104 A are hydraulic dampers (not shown). The design of these dampers basically involves hydraulic fluid flowing through an orifice. Under a steady load, the fluid flows through at a constant rate; however, if the load is greatly increased, only a minimal increase in fluid flow occurs. 
     The recoil force may be considered as consisting of three components:
         a horizontal component,   a vertical component and   a turning moment  108 T.
 
Referring to  FIG. 2 , the horizontal component of the recoil is balanced by the horizontal reaction  107  of the two spades  106  in the ground. Though no vertical component is generated when the howitzer is fired horizontally, as shown in  FIG. 2 , the vertical component of the recoil force (when the barrel is elevated) is balanced by the vertical reaction  108 B from the ground via the convex base into chassis  117 . The turning moment  108 T is balanced by vertical reactions  108 C on the feet  112  at the end of the rear trail legs  104 , plus the out-of-balance  109 . Because the dampers (not shown) are incorporated into the hinges  104 A, the howitzer will tend to rotate clockwise slightly as the turning moment  108 T is dissipated; as soon as this has been done, the howitzer will rotate back onto its forward feet  111  under the effect of the out-of-balance  109 , possibly lifting rear feet  112  off the ground—the rear trail legs  104  will then slowly swing downwards under the control of the dampers (not shown) until feet  112  rest on the ground.
       

     No dampers are incorporated in the hinges  110 A for the front stabilisers  110  but these stabilisers can be locked in either the firing ( FIG. 2 ) or towing ( FIG. 3 ) modes. 
     Thus, a stable three-point support is provided in both ready-to-fire and recoil modes, i.e. 2× 108 D+ 108 B and 2× 108 C+ 108 B respectively. It will also be noted that spades  106  are hinged in such a way ( 104 A), that the horizontal and vertical components of the recoil act to ‘lock’ them in their operative position. Any rotation of the howitzer due to turning moment  108 T would probably occur about an axis roughly through the pair of hinges  104 A. Because dampers are used in hinges  104 A, their action will protect the rear trail legs  104  from excessive loading so that the scantlings of legs  104  may be minimised. 
     The importance of having spades  106  fast with chassis  117  should not be underestimated. The horizontal and vertical components of the recoil force are taken directly via the trunnion support structure  124  and the chassis  117  to ground as reactions  107  and  108 B, respectively. Thus, these recoil components pass through robust structures directly to the ground. This is in sharp contrast with the conventional field howitzer ( FIG. 1 ) where the horizontal component goes through long ‘resilient’ trail legs  4 . The release of the strain energy in these resilient trail legs  4  is like a second recoil and the combined effect is to make the howitzer move about violently. In contrast, each recoil on the ultra lightweight field howitzer of the present invention provides the spades  106  and chassis  117  with an ever more stable base accompanied by a small degree of rotation due to the effect of the turning moment. 
     Thus, on the ultra lightweight field howitzer of the present invention, the spades  106  provide the anchor at the structures  124  and  117  where the recoil forces are generated. On conventional field howitzers, the anchor is remote and is effectively connected by a ‘spring’. 
     To the casual observer, it may seem that the need to provide two front stabilisers is an additional weight penalty. However, this does not recognise the considerable advantages conferred by the out-of-balance, for example:
         A) out-of-balance acts to oppose the recoil turning moment  108 T.   B) the position of the trunnion bearing enables vertical and horizontal recoil components to go straight to ground and this allows;
           i) short light rear trail legs  104 .   ii) small light trunnion support structure  124 .   iii) small light chassis  117 .
 
Thus the net weight saving due to the above far exceeds that of front stabilisers 110.
   
           9. Combined recoil and recuperator system.   10. Use of hollow interiors of structural members for accumulator gas reservoirs.       

     11. Barrel weight counterbalanced using gas springs. 
       FIG. 5  shows a cross-section through a cradle which supports the barrel  101 . The cradle has a rearward end which is pivotally mounted about the trunnion bearing  113  (see  FIGS. 2 ,  3  +  4 ). The cradle consists of four hollow tubes  119  and  119 A located in position by cross-members  125  and  126 . Barrel  101  can move axially ( 101 A) within the cradle via lugs  127  which slide in cut outs  128  in members  126 . The internal volumes of hollow tubes  119  and  119 A are designated  129  and  129 A respectively. These volumes are cleaned and tested to the conditions laid down for pressure vessels. Cross connections (not shown) in cross-members  125  link the pairs of internal volumes  129  and  129 A respectively. Similar connections may be provided in cross members  126  if required. 
     When the howitzer is fired, there is a massive release of chemical energy which causes barrel  101  to move rapidly backwards from a first to a second position, i.e it recoils. The energy of the recoil is absorbed in several ways, of which the main ones are:
         i) by muzzle brake  118  ( FIG. 4 )   ii) in the recoil buffer and recuperator systems   iii) by spades  106  and trail legs  104 .       

     Muzzle brakes  118  are standard items on many gun barrels. They consist of a series of angled baffles, fast with the barrel, which deflect the exhaust gas rearwards and so exert a braking effect on the rearward motion of the barrel. Depending on the angle of the baffles and other factors, the magnitude and efficiency of the braking action may be varied. In this case the particular muzzle brake is chosen in such a way that, together with the design of recoil buffer and carriage geometry, the energy of the recoil is dissipated in the most acceptable manner. In this context, “carriage” covers the synergetic design of saddle (including trunnions), body, trail legs and spades. 
     Conventional recoil recoil systems use a recoil buffer and a recuperator on each side of the barrel to dissipate the recoil energy symmetrically, i.e. there is a total of four cylinders. In the current disclosure, the recoil buffer and recuperator ( FIG. 6 ) are combined into a single cylinder, so that there is only a total of two cylinders—one on each side of the barrel. This further contributes to the overall weight saving on the whole howitzer. 
     When the howitzer is fired, barrel  101  recoils to the right ( FIG. 6 ) and lugs  127 , via rods  134 , force pistons  136  into cylinders  177 . Inside cylinders  177  are perforated sleeves  174  so that the motion of pistons  136  causes hydraulic fluid in the central volume  173  of the cylinder to be forced, via perforated sleeves  174  into annuli  175  and thence, via pipes  185 ,  186  to accumulator  130 . The perforations in sleeves  174  are not uniform but decrease in number and/or size from left to right. Thus, as pistons  136  move to the right, the number (and sizes) of perforations through which hydraulic fluid can flow is reduced and, hence, the resistance to rearward movement of barrel  101  increases. Consequently, by varying the size and/or number of perforations, the recoil characteristics may be varied to suit particular requirements. Piston rods  134  pass through seals  135 . 
     Inside accumulator  130  is a floating piston  188  with hydraulic fluid  187  on the one side and inert gas  131  on the other. A pipe  189  connects accumulator  130  with two of the four tubular members  119  (or  119 A) so that the total volume of inert gas on the left of piston  188  is that in spaces  131  and  129  (or  129 A). During the recoil, essentially incompressible hydraulic fluid is forced from volume  173  via perforated sleeve  174  to annulus  175  and thence via pipes  185 ,  186  to space  187  so forcing piston  188  to the left and compressing inert gas  131 ,  129  (or  129 A). As the volume of inert gas  131  plus  129  (or  129 A) is large compared with that swept by pistons  136 , the pressure in accumulator  130  remains relatively constant. 
     When the howitzer is fired, pistons  136  are forced to the right raising the pressure in volume  173 . The flow of an incompressible fluid through an orifice is proportional to the square root of the pressure difference across it; thus if the pressure difference is doubled, the fluid flow will increase by only 41%. Thus the recoil buffer action is to exert a high and increasing braking effect on the rearward motion of barrel  101  progressively bringing it to a halt. In contrast, the recuperator action is to advance the barrel back to the firing position at a slow steady rate. This is done by using the relatively constant pressure difference between that of inert gas  131 ,  129  (or  129 A) and that in volume  173 . Despite the smaller pressure difference, the fluid flows through perforated sleeves  174  at an appropriate rate to move barrel  101  back to the firing position in time for the next shot. Inert gas  131 ,  129  (or  129 A) is precompressed to an appropriate pressure so that, under all conditions except when recoiling, barrel  101  is fully run out, irrespective of the angle of elevation. 
     The use of an hydraulic accumulator  130  on field howitzers is conventional but, because the additional gas volumes  129  (or  129 A) are used to supplement volume  131 , the overall size of accumulator  130  is reduced. This is a further weight saving. Also, due to the larger volume of pressurised inert gas  131 ,  129  (or  129 A), the recuperator characteristics are better. 
     As stated previously, the location of the centre of gravity  109  gives the howitzer a large out-of-balance. If a conventional elevating gear only were to be used, the effort required would either be very large or an excessively high ratio would have to be provided. In either case, the gearing would be heavy and cumbersome in use. In order to minimise this effort, elevating cylinders  114  filled with compressed inert gas, are used to provide a ‘counterbalancing’ effect. Here again the gas-spring principle is used with the volume  129 / 129 A of the other two of members  119 / 119 A, providing an increased gas volume. By appropriately pressurising the inert gas, the force exerted by the cylinders  114  may be adjusted to be approximately equal to the out-of-balance of barrel  101  and related equipment, e.g  119 ,  119 A,  130 , etc. (A slight degree of underbalance is preferred). Under these circumstances, the barrel  101  may be elevated via a lightweight geared rack (with an acceptable ratio) using a conventional handwheel as only a minimal effort would be required. 
     As the angle of elevation of the barrel  101  increases, so the total volume inside the cylinders  114  and members  119  (or  119 A) will increase thus lowering the pressure and degree of counterbalancing. However, this will be largely offset by the fact that the raising of the barrel  101  will tend to move the centre of gravity  109  to the right ( FIG. 2 ) so that the net out-of-balance will also decrease. 
     In the particular example herein described, the volumes  129  of the two upper hollow members  119  are used in conjunction with cylinders  114  and the volumes  129 A of the two lower members  119 A are used as part of the recoil buffer. This arrangement is chosen to give the best line of action for cylinders  114  on the barrel/cradle assembly. However, this arrangement may be varied in accordance with particular requirements. Similarly the 2-2 division of the internal volumes  129 / 129 A of members  119 / 119 A may be varied, e.g 3-1 or 4-0, depending on requirements. 
     Considerable weight savings accrue from the use of the internal volumes  129 / 129 A in conjunction with hydraulic accumulator  130 . If these volumes were not used, equivalent volumes of pressure-resistant cylinders would be required instead. Not only would this contribute a pure weight penalty to the whole howitzer, but it would also present some problems as to where physically to locate the cylinders. If the cylinders were located on the barrel, this would increase the out-of-balance and hence the elevating problems while location on the chassis/platform  117  would impede access to other components and may require a larger (and hence heavier) chassis to be used. 
     By using the internal volumes  129 / 129 A of members  119 / 119 A there is little or no extra weight penalty. The scantlings of members  119 / 119 A can be calculated by taking loadings and a suitable stress level for the material (including an appropriate factor of safety) and evaluating the desired metal thickness of the desired tube diameter. Then, from the range of standard thicknesses available in that diameter, the next thickness greater than that evaluated should be chosen, giving a further safety margin. In this case, the stress in the metal comes from the sum of the stress due to the loadings plus the stress due to the internal pressure. Since the stress due to internal pressure would probably be small compared to the loadings, e.g bending forces, it is quite probable that no increase in wall thickness would be required.
         11) Barrel weight counterbalanced using gas cylinders   13) Radical new concept of elevation gearing   14) Resetting (rendering) device in elevation systems       

     As mentioned before, the elevation gearing must be designed to accommodate the load due to the net weight of the barrel, cradle, etc. It must also be accurate enough for adjusting the barrel to a precisely determined angle, e.g seconds of arc. Clearly, if there were no gas cylinder counterbalancing action, the load on the gearing would be much greater requiring massive gear elements and/or a high ratio. While a high ratio permits accuracy of angular adjustment, it also involves many turns on the handwheel which can be time-consuming, especially in a battle situation. 
     The solution to this problem is to provide gas-spring counterbalancing to make the barrel, cradle, etc., effectively “weightless” while providing a light, highly accurate damped elevating means to give an optimised new concept to elevation gearing. 
     The counterbalancing means has been described hereinbefore. The elevating mechanism is shown in  FIG. 7 . Essentially, it consists of a lead screw  139  pivotally fixed at its right hand end to the trunnion support structure  124  and passing through main elevation gearbox  148  near its lefthand end. The main elevation gearbox  148  is fast with the cradle  119 ,  119 A (the non-moving part of the support for barrel  101 ), a resetter box  147  is pivotted at  147 A to the trunnion support structure  124  and the arrangement is such that the lead screw  139  is parallel to the axis  101 A of the barrel  101  and, preferably, vertically below it. The lead screw  139  passes through a planetary roller screw  149  in the gearbox  148  so that, as roller screw  149  rotates, the gearbox  148  moves along lead screw  139  in the direction shown by arrows  155 . However, as both the main elevation gearbox  148  and the resetter gearbox  147  are positively located, the result is to cause the barrel  101  and cradle  119 ,  119 A to be elevated (or depressed), i.e. angular motion occurs about the horizontal axis through the trunnion bearing  113  and about the pivot  147 A to maintain the axis of barrel  101 A parallel to that of lead screw  139 . 
     The means of elevation is from an elevation handwheel  115 , via bevel gears  156 , elevation hand/drive  116 , bevel gears  152 , input shaft  153  and bevel gears  150  to roller screw  149 . Lead screw  139  is located, and shaft  153  rotates, in bearings  154 . A reverse locking mechanism  151  operates on the shaft  153  to maintain the elevation angle once preset. 
     The right hand end of lead screw  139  is mounted by a flexible tunable arrangement to protect the elevating system from shock loadings after firing. Near the end of the lead screw  139  is a screw threaded portion  140  on which a thrust nut  142  and lock nut  141  are secured. Thrust nut  142  bears on a spring  143 , e.g. a series of spring washers, which is located at its other end by fixed thrust member  144 . The end of the lead screw  139  terminates in a piston (and/or orifice)  145  in cylinder  146  full of hydraulic fluid  146 A; this arrangement is, of course, a hydraulic damper. Thus, by adjusting the stiffness of spring  143  and the size of orifice  145  to vary the damping characteristics, the resetter gearbox  147  can be ‘tuned’ to damp out any movement in barrel  101  after firing and simultaneously protect the elevating system in general and lead screw  139  in particular. Frictional damping may be used in place of hydraulic damper  145 ,  146 ,  146 A. 
     Thus to fire the howitzer, the elevation is set via elevation handwheel  115 . After firing, the barrel  101  will recoil and the elevating mass will rock causing the lead screw  139  to move axially  155  with respect to the resetter gearbox  147 . This causes spring  143  to compress/relax until the action of damper  145 ,  146 ,  146 A stops the movement and spring  143  resumes its original length and the barrel elevation returns to that of pre-firing. 
     The discharge of a howitzer and its recoil are violent processes but, by carefully directing the recoil and allowing limited damped freedom of movement, these processes can be controlled by relatively lightweight members compared to where there is rigid mounting. Thus the example here of a lightweight damped elevation system coupled with the counterbalanced system offers a net weight saving over the conventional rigid gear arc, pinion and gearbox design. Also, the lighter system is more accurate, responsive and physically easier to operate.
         15. Minimum size training bearing   16. Segmented arc training gear       

     The lowered trunnion height and consequently reduced recoil overturning moment  108 T permits a reduction in the size of the trunnion support structure  124  on the chassis  117 . Because of the way in which the forces are transmitted from the trunnion bearing  113  to the ground, the traversing arrangement ( FIGS. 8 and 9 ) can be simplified to a small training bearing  158 ,  159  at the front, which acts as a fulcrum, and a thrust bearing fixed arc  160  at the rear. The actual training gear  163  is a small arc of a gear ring. This contrasts sharply with the massive ring bearings and full gear ring which have been used hitherto. The new design also provides a considerable saving of weight Like the elevating gear, the training gear is operated by a handwheel via gearing (not shown). Here again, because of the lightweight of the trainable mass, a smaller, lighter gearbox is used than for the conventional field howitzer. 
     The principal of the design is shown in  FIGS. 8  (plan) and  9  (elevation). The howitzer is trainable about vertical training axis  157  via a bearing, e.g with inner race  158  fast with chassis  117  and rotatable outer race  159  fast with the trunnion support structure  124 . Upper  161  and lower  162  thrust bearings are pre-loaded onto bearing arc  160 . These are shown as rollers, but any suitable type of bearing, or pad can be used. Rollers  162  support the positive out-of-balance and rollers  161  are loaded during recoil. The training gear arc  163  is machined onto the edge of the bearing arc  160  and a training gear pinion  164  driven by shaft  165  provides the training drive. The roller races  161  and  162  and the pinion  164  are all mounted on the trunnion support structure  124  though the actual mounting means are not shown to avoid confusing other detail. 
     5) Positive out-of-balance in all non-firing attitudes. 
     12) Provision for towing by muzzle of barrel 
     There are various methods by which the ultra lightweight howitzer can be converted into the towing mode. In one method, one or two men would support the end of the barrel  101  while lightweight wheels  123  are lowered hydraulically about pivot  122  by means of hydraulic cylinders  120  (see  FIG. 3 ). The men would thus be supporting the positive out-of-balance. The other men of the team would then hinge up front stabilisers  110  and trail legs  104  ( 104 B) and remove spades  106 ( 106 A) to be stowed on platform/chassis  117 . A towing bracket in the form of a lunette  137  is then deployed by swinging it forward about hinge  138  and hooking it on to the towing vehicle. (In  FIG. 2 , the wheels  123  etc. have been omitted so as not to obscure other detail). 
     The positive out-of-balance resulting from the location of the centre of gravity  109  places a downward load, via lunette  137 , onto the towing hook of the vehicle in conformity with safe towing practice. 
     The reduction of the barrel height  102  somewhat complicates the loading arrangements. However, one of the neatest solutions is to place the loading tray above the barrel  101 . The position of a shell  121  is shown ( FIG. 2 ), but not that of the loading tray or loading linkages, as these would obscure other detail.
         4a) Self digging spades attached at/near ends of rear trail legs       

     The problem with spades  6  ( FIG. 1 ) attached at the ends of long flexible trail legs  4  is that the legs  4  are elastic and absorb much energy in the recoil mode, to be released as a further subsequent reaction. The net effect is that the whole ordnance ‘bounces’ around violently. An alternative to locating the spades adjacent to chassis  117  as in  FIG. 2  is to secure them by bolts  106 B at the ends of short, inelastic legs  104 B (see  FIG. 10  where the spades are denoted by reference numeral  106 A) and to provide a hydraulic damper comprising cylinder  166  and piston  167  to allow the ordnance limited angular motion about hinge  104 A. The hydraulic damper comprises a cylinder  166  pivotally mounted by joint  169  on to chassis fixture  168  and a piston in the cylinder and connected to a piston rod  167  pivotally mounted by joint  171  on to leg fixture  170 . Symbol  172  indicates that the legs  104 B are not shown to their full length. 
     The advantage of using such a system is that, after recoil, the chassis  117  and spades  106 A will gently settle back onto the ground. The horizontal element  107  of the recoil would be spread over a larger area of ground than two spades  106  situated relatively close together. After several firings from the same spot, spades  106  would dig deeply into the ground stopping chassis  117  from settling down as firmly on the ground as desired. Spades  106 A eliminate this possibility. 
     A further advantage of spades  106 A at the end of trail legs  104 B is that they enable minor design changes to be made to the chassis  117  giving a further slight lowering of the trunnion bearing  113 . This gives a further level of improvement to many of the features described hereinbefore. 
     The present invention involves a large number of radical innovations to a standard item of field artillery. As will have been apparent, the whole raison d&#39;etre for the new design is to save weight and so produce an ultra lightweight field howitzer. Though this radical approach has led to the introduction of some additional components, e.g. front stabilisers  110 , the net affect is a new concept of ultra lightweight field howitzer. In addition, the new concept gives a much lower piece of artillery which is consequently much easier to conceal on a battlefield. 
     Throughout this description, reference has been made to the use of light and strong materials. As one of the foremost amongst this class of metals, titanium and its alloys are extensively used wherever the stress levels permit. Where structures can be designed on aerospace principles of combining lightness and strength, these will be used. These principles include where a given component can fulfil two or more duties. 
     Though the aim has been to produce a howitzer which, together with its crew and ammunition, can be carried by a single battlefield helicopter, and this aim has been achieved, smaller helicopters are also used on battlefields. The howitzer has thus been designed to separate quickly and easily into two or more parts so that, separately, the howitzer, crew and ammunition can also be carried by two smaller helicopters or land vehicles. Howitzers that can be readily separated into components and reassembled on a battlefield are much easier to transport in large numbers as the separate parts will pack better into the hold of a ship or aircraft than fully assembled items. An added feature of howitzers which can be separated into two or more major components is that a failure in one part can be rectified by replacement of the whole component allowing the faulty one to be returned to a workshop for subsequent repair. 
     There are two main occasions when it may be necessary to separate the howitzer into parts. The first is on the battlefield where speed would be essential. In this case ( FIG. 11 ), the howitzer would be separated into the “elevating mass” and “carriage” by the removal of the trunnion caps, etc. Each part would be light enough to be carried by a lorry or small battlefield helicopter. The second occasion could be when the howitzers are to be transported in large numbers from a base to a scene of possible conflict. Here, speed would not be as important as the density of packing. In this case, other parts such as stabilisers  110 , trail legs  104 B, wheels  123 , etc, may be removed. 
     A further feature is a lunette  137  ( FIG. 4 ) which may be hinged vertically downwards and locked to form a ‘leg’ to support the muzzle end of barrel  101 . This would greatly assist separation and reassembly of the howitzer&#39;s two main parts.