CA1220961A - Filament-wound venturi - Google Patents

Abstract

ABSTRACT
A venturi section or cone for a lightweight firearm such as a recoilless gun which is subjected to high transient pressures and temperatures on firing is formed from a resin impregnated multi-filament fibre material. The material in the form of an elongated tow is wound on a mandrel, cured, and then removed from the mandrel for subsequent operations such as machining or assembly. During manufacture, a hot gas erosion preventing layer is first formed on the mandrel and a plurality of layers of tow are helically wound thereon at a helix angle selected to provide adequate axial and hoop strengths for resisting axial thrust and hoop loads produced on firing. Lightweight recoilless guns using such venturi cones are lighter and cheaper to fabricate than conventional guns and will fire more rounds before excessive erosion causes them to be unusable.

Description

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This invention relates to a structural element in the form of a tubu]ar item made of a fibre reinforced composite material, More particularly, the invention relates to a venturi section or exit cone for a lightweight firearm and to ~,he method of making the same from a resin-treated multi-filament fibre material. The invention defined herein is related to that disclosed and claimed in commonly assigned Canadian Application Serial No. 454,284 filed May 15, ]984 for 7~ Carbon-Fibre Gun Barrel".
BACKGROUND OF THE INVENTION
As described in the U.S. Army Engineerins Design ~andbook on Recoilless Rifle Weapon Systems (AMCP 706,238), all recoilless rifles use a venturi behind the breech section thereof to vent propellant gases. This venturi is generally cone-shaped and is designed so that the'momentum of the escaping propellant gases will equal that imparted to the projectile, thus ensuring recoilless operation. The venturi must be designed to withstand hoop loads resulting Irom the static pressure of the gases, as well as an axial ~0- or ~hrust load which is a function of the cone divergence angle.
The majo,r in-service recoilless rifles (the U.S. 90 ~n and 106 mm Systems and the Swedish 84 mm Carl Gustaf) all use machilled steel venturis which are relatively heavv and costly to fabricate given their small wall thickness and three-dimensional configurations. For examPle, the venturi section of a typical 84 mm in-service recoilless gun, including the venturi cone and the breech adapter portion thereior weighs about 2.0 kg.
; A.ly improvement in that figure is a bonus to the infantryman who has to carry his gun over long distances.

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~ ntil recently the venturi design has not received as much attention as the barrel portion of the firearm; see for example the copending application referred to above. The barrel section is more important to the proper functioning of the firearm and there has been an absence of major problems with the venturi. However, significant weight savings in the barrel section through the use of a carbon-fibre/epoxy resin composite material (from 8.5 kg to 2.8 kg) have resulted in the venturi portion assuming a larger percentage of the overall weight ana it is there~ore advantageous to endea~our to red~ce the wei~ht of the venturi portion.
A prime consideration in redesigning the venturi portion was to ensure that there is sufficient gas erosion resistance in the critical breech area. It was therefore decided to retain the throat ring/breech adapter of the existin~ firearm and to concentrate on the cone portion of the venturi for weight reduction. Furthermore, the throat ring adapter is subjected to complicated loadings on firing an~ i~ was ~eemed desirable to continue with an accepted, suitable design.
The decision to use the existing throat ring adapter complicates, however, the problem of reducing venturi weight since in the existing venturi the adapter constitutes 38.2%
o~ the total weight. In a reduced-weight venturi it would represent a higher percentage of the total weight. In attempting to redesign the venturi cone the advantages achieved with filament wound barrels, as documented in the copending application referred to above, were deemed to be significant and conse~uentlv it was decided to applv the j~^c:

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knowledge gained in the barrel development work to the venturi situation. It was not possible, however, to prepare a venturi cone in the same manner as a barrel section because they are subjec~ed to different loadings and design requirements. For example, the transient thrust loads experienced by the venturi cone have a profound effect on the threaded connection between thè cone and the throat ring/breech adapter. Also, the cone portion is more prone to impact than is the barrel section and that fact must also be taken into consideration when choosing materials and winding angles.
SUM~A~Y OF THE INVENTION
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The invention described herein relates to the construction of the venturi cone of a recoilless weapon which is subjected to high transient internal pressures and temperatures on firing. A resin-treated multi-filament fibre material is wound on a suitably configured frustoconical mandrel as a tow in a plurality of superimposed layers. At least the innermost layer is formed of a material which is particularly suited to resisting hot gas erosion. The tow ~f material is wound on the mandrel at a helix angle which provides adequate axial and hoop strenths for resisting axial thrust and hoop loads produced on firing. The cone, after curing and removal from the mandrel is threaded onto ~he threaded portion of the throat ring/breech adapter to create a venturi section for use in a recoilless gun.
As indicated above the average helix angle ~ should meet the requirements for an adequate ratio of axial to hoop strenqths, as determined by the expression ~ = tan l(~axial~
~hoop ~
where ~a ial and ~hoop are the axial and hoop stresses produced _ ~Z;i~ 6~1;
in the ven~uri section by axial and hoop loads respectively.
Caleulations and tests have shown that a desirable average helix angle for the venturi cone is about 80 although the angle could vary from about 76.5 at the breech area to about ~0 at the exit area. Furthermore, for the purposes of re-dueing gas erosion as mueh as possible it is desirable that the innermost layer of the cone be hoop wound or consist of a resin-impre~nated fabrie of refraetory fibres. By the time this seetion has eroded other constraints would probably have rendered the gun ineffective.
Prototype cones have been constructed using earbon-fibrefepoxy resin eomposite materials and the weight of the eone,when eompared to existing systems has almost been halved. Further weight and/or cost savings might be gained through the use of other materials, sueh as Ke~vlar (q`rademark),iepoxy composites.
Other features and advantaqes will become apparent in the following deseription of the invention which is to be considered in eonjuetion with the drawings.
~RIEF DESCRIPTION OF THE DR:A~JINGS
Fi~ure 1 illustrates in perspeetive a light reeoilless gun (LR~) ~o ~hich the present invention is applicable.
Figure 2 shows the LRG of Figure 1 in a "breeeh open"
condition, ready for loading or unloading.
Figure 3 shows a mandrel which eould be used in produeing the venturi seetion for the rJRG of Figure 1.
Fi~ure 3a shows a longitudinal cross-section of a venturi seetion of the LRG of Figure 1.
Fisure ~ shows a typical winding machine for producing jre:

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a venturi cone for the LRG of Figure 1.
Fi~ure 5 shows a graph of ballistic pressure profiles within the LRG of Eigure 1 at various times during a firin~.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning to the drawings, Figure 1 shows a structural element in the form of a recoilless weapon (LRG) 10. A venturi or diffuser section 12 and a barrel or pressure section 14 are coupled togetller by a breech assembly 16. The breech assembly - 16 is totally conventional in formr and need not be further described here for an understanding of this invention. It is noted that the breech assembly 16 includes a two part base ring, whose parts 18 and 20 fixedly support the diffuser and pressure section 12 and 14. The parts 18 and 20 are pivotally inter-~onnected so as to provide for relative movement and enable loading of the barrel section 14. This is clearly seen by comparing Figure 1 in which the breech assembly 16 is closed, and Figure 2 in which the breech assembly is open. The recoil-less weapon 10 shown in Fiqures 1 and 2 includes a trigger and ~iring Inechanisin 22 su~ported from the ~ress~re section 14 by a support strap or ring 24~ These are also conventional in the recoilless weapons art and need not be further described hereO
It has been found, as indicated in commonly assigned Canadian Patent No. 1,167,676 issued May 22, 1984 that contrary to teachings of the prior art such as Canadlan Patent No. 582,160, a unitary barrel construction can be made from non-metallic fibre reinforced composite materials.
Moreover, such a barrel need not use metal liners. Reinforcing fibres such as graphite (carbon), boron, and beryllium, as well as glass have, in certain forms, greatly superior strength and jrc:

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stiffness properties as compared to bulk metals such as steel, titanlum or aluminum. It is known, of course, that allo~s of those me-tals are commonly used in makiny gun barrels. As mentioned earlier, conventional wisdom previously re~uired that barrels and venturi cones be made entirely of metal, or at least use a metal liner, overwrapped perhaps with reinforcin~
~lass fibre.
As pointed out in Canadian Patent No.
1,167,676 ~rrels acceptable for use in recoilless weapons can be made and used without the need for a liner, metallic or other-wise. Such a barrel is made in the form o~ a tubular structural element of a non-metallic fibre reinforced composite material and~ depending on the re~uirement of the element, it may have interior projections (rifling or the like) integrally formed therewith.
Furthermore, as pointed out in the copending application No. 454,284 referred to above, improvèments in the construction of the barrel section as described in Patent No. 1,167,676 can be achieved, which improvements are also ~eneficial to the construction of the venturi or diffuser cone 1~
In order to produce the desired venturi section it is necessary to first of all produce a mandrel on which the fibre-reinforced composite material will be wound and, after winding, cured to produce the desired element. Thus; in producin~ the mandrel~ the volume and thermal expansibility of the mandrel material must be taken into account since the final dimensions of -the mandrel are derived when both the mandrel and the composite material wound thereon are at curing temperatures.

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At room temperatures the dimenslons of the mandrel will normally be somewhat less then those at curing temperatures.
Figure 3 illustrates a typlcal mandrel which might be used to produce the venturi section or cone 12 shown in Figures 1 and 2. The mandrel 32 is provided with a generally frustoconical surface portion 34 which corresponds to the diffuser portion of the venturi. At the smaller diameter end there is a ~enerally cylindrical portion 36 which will form the forward portion which is to be threaded to the throat ring/breech 1~ adapter. The threads on the cone may be formed in two ways:
(a) they can be machined in the cone after curing and removal from the mandrel, in which case the surface of the portion 36 will be smooth; or (b) they can be formed directly during formation of the cone, in which case the surface of the portion 36 will be created with helical grooves therein corresponding to the desired thread form. To remove the final cured cone from the mandrel under (a) it need only be slid therealong and removed from the small diameter end. Thereafter the threads would be machined into the small cylindrical portion of the cone. To remove the final cured cone from the mandrel under (b) it need only be rotated so that it can be screwed off the groove portion 36 of the mandrel. If the threads formed in the cone are rough or unfinished it may be necessary to slightly touch them up with a suitable machining operationO
In either of the situations mentioned ahove, it may be necessary to coat the mandrel, or at least portions thereof with a suitable release agent to ensure easy removal of the cone from the mandrel.
The wall thickness of the venturi cone must be jrc:

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sufficiently yreat to withstand the static and dynamic forces placed thereon bv the exhaustin~ gases during firing. Furthex-more the venturi cone must be sufficiently rugged to avoid damage during handling and for this reason the analysis which determined the optimum windlng angle for the barrel section is not applicable to the venturi cone.
From Figure 5 it can be seen that on firing the maximu~ pressures occur in the breech area, as would be expected. Multiplying by appoximately 1.5 gives the design pressure profile, the maximum pressure in the breech area therefore being about 14,000 p.s.i. Although the pressures encountered in the cone portion of the venturi are much lower, it is stili necessary to design to the maximum pressures in the breech area since the venturi cone starts at the breech area, and at its threaded connection to the throat ring~breech adapter it will be subjected to pressures much higher than those encountered downstream.
Venturi stress analysis by strength-of-materials methods involves the calculation of a~ial and tangential stress factors (r,a and nt, respectively) as a function of se~tion radius (r), throat radius (rt), the reciprocal of nozzle slope ~), and the ratios of thrust (F/Ft) and pressures (P/PO). The effective stress factor fe is then computed usinq the relation fe= (na ~ nant + ~t ) with the required wall thickness being t Ft Po f y y being the yield strength of the wall material.
However, the actual wall thickness is usually con- -siderablv greater and, after computingan effective actual wall .r.- ~

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thickness t , the efEective stress is then p rt e fe t -Table 1 summarizes the required calculations for the nozzle throat, exit and two intermediate sections of an LRG with the chamber pressure being the peak design pressure of 14,000 p.s.i. It is seen that the effective stresses are less than 1/3 the yield stress of the material.
The optimum fibre winding angle at each section may be determined by taking the inverse tangent of ~t/na giving a range of 90 at the nozzle exit to 76.5 at the breech (~hroat) area. A fair compromise is to use a constant winding angleof 80 or less (if greater axial strength for resistance to impact is desired~.

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Calculation of Wall Stresses in LRG Venturi PO = 14,000 p.s.i. ~ = 114,000 p.s.i.

Exit Intermediate Position Throat . .
r 2.515 in 2.156 1.796 1.437 r~r~ 1.750 1.500 1.250 1.0 (r~rt) 2 3.063 in2 2.250 1.563 1.0 ~1 co co , , rt~r I O O O
1~ ~ 840~1~ 84.41~ 84.41 90 sin ~ 0.9952 0.9952 0.9952 1.0 F/Ft 1.2040 10065 1.063 1.0 1.5~3 ~.454 1.327 1.248 P~PO 0.0456 0.0761 0.235 0.630 ~e ~ ~ 0-049 0.176 0.255 n~ 0 0.0164 0.0707 0.128 ~t 0.0802 0.115 0.295 0.634 nt~na ~ 7.01 4.17 4.95 ~e a .0802 0.124 0.336 0.707 ~ 0.014 ins~ 0.022 0.059 0.125 tn 0-059 lns. 0.085 0.112 0.138*
W~a 1.023 1.039 1.062 1.096 W 1.022 1.037 1.059 1.091 te 055 0.0798 0.106 ~ 0.131 ~e 29,200 31,300 63,800 108,600 * obtained by linearly extrapolating v~nturi cone con~our; actual wall thickness is considerablv greater because of breech attachments.

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r - radius at given cross-section rl - radi.us at curvature of cone wall rt ~ radius at throat ~ - cone slope at cross-section F - thrust at cross-section Yt ~ thrust at throat - modified thrust factor ~a ~ axial stress factor ~t ~ tangential stress factor fe ~ equivalent stress factor tr ~ required wall thickness t - nominal wall thickness Wa ~ apparent wall ratio W - actual wall ratio te ~ effective wall thickness ~e ~ effective stress o~--sd~.

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Ilavi.ng prepared the mandrel 32 and determined the desired winding angle the next step involves the actual production of a venturi cone~ The mandrel 32 is placed in the bed of an automatic, preferably computer controlled, winding machine and a tow of the epoxy resin-treated fibre material is wound thereor..
Fiyure 4 shows a simplified typical winding machine 26 in which the mandrel 32 is mounted. ~he tow of winding material comes from a spool 27 and is fed over a guide roller 28, through die hole 29 in a guide member 30 to the mandrel. The guide roller 28 ahd guide member 30 are mounted on a carriage 31 thai moves along the bed of the machine 26 under the direction of a machine programme. Such programme can control the machine whereby ~he tow of composite material is wound on the mandrel at the desired constant tension, about 8 lbs., in the desired number or layers and at the desired angle. Further details of the machine need not be described, as such machines are commercially available and do not form a part of the present invention.
The tow o~ carbon fibre material may have been impreg-~ nated or coate~ with a suitable epoxy resin before bein~ wound on the spool 27, whereby no further operations on the tow are re~uired. Alternatively a non-treated tow of carbon fibre material may ~e provided on the spool 27 and, as the tow is wound from the spool, it may pass through the nip defined by a pair of kissing rollers (not shown) one of which rotates through an epoxy resin bath and thus transfers resin to the tow prior to winding on the mandrel.
As indicated previously it is desirable to include at least one layer~ being the innermost layer of the venturi cone, jrc:

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that improves the resistance to hot gas erosion. That layer could be a fabric of carbon fibres woven so as to have weft and warp filaments. Alternatively fabrics of silicon carbide or ceramic fibres may also be applied. It could be a tape of longitudinally extendinq carbon filaments embedded in an epoxy resin matrix and hoop wound on the mandrel as close to 90 D to the venturi a~is as possible. It could also include a plurality of layers of the tow hoop-wound on the mandrel at about 90 to the venturi axis.
After the desired number of layers have been wound on ~he mandrel, the mandrel is removed from the winding machine and placed in an oven for curing.
The curing times and temperatures will of course ~epend on the materials used. After curing, the venturi cone is removed from the mandrel in a manner as described previously and any machining that is required may be executed.
Figure 3a shows the venturi cone 12 with a thread section 44 at the smalle: end for ccnllection to the appropriate breech portion 18. Gas erosion and infiltration problems at 20` the breech area may be minimized by filling the threaded connection with epoxy resin.
In summary, the present invention is related to the construction of venturi sections for guns subjected to high internal pressures on firing. Specifically, the present invention utilizes a tow of fibres te.g. carbon) in the matrix of a low-viscosity, thermosetting epoxy resin, with the tow being wound on a mandrel in a particular configuration to obtain a venturi cone section having the desired properties of lightness and relatively long life. At least the innermost layer or lavers are provided primarilv to minimize hot gas c ~ ~ ~, . . .

erosion and the remaining layers are wound at an optimum helical angle relative to the venturi cone axis. The opti~um helix angle is in the vicinity of 80, although a range of about 70 to 90 is acceptable. Preferably, the hot gas erosion preventing material comprises resin-impregnated fabrics of carbon, silicon-carbide or ceramic ~ibre to resist the gas pressures and the temperatures e~perienced in the breech area, which temperatures can approach 30~K on firing, albeit for very short periods of time. LRG's using venturi sections produced in accordance with the present invention and barrel sections produced in accordance with Application No. 454,284 have been able to fire upwards of 30 rounds of ammunition before failing and such ~epresents a considerable improvement over existing ~eapons produced by conventional methods which fail after the filing of 5 to 10 rounds. In addi~ion to having a longer effective life, weapons usinq venturi sections produced in accordance with this invention are substantially lighter in weight than conventionally-produced weapons and they are also less expensive to produce.
Venturi sections prepared in accordance with this invention are much easier and less costly to fabricate than the erosion-resistant carbon-carbon cones used on several space roc~et vehicles. Given the short duration of hot gas flow through the venturi (about 3.5 m sec.) there is little ; heat build-up of the venturi, other than at the inner surface. Therefore, using an inner layer of fabric for erosion-protection and helical windings above for optimum strength gives the most effective venturi for the Sa/~ -13-:IL2;i~

LRG application.
While the foregoing has disclosed the basic features of the present invention it is clear that alternative configurations or changes could be effected by a person skilled in this art without departing from the spirit of the invention. ~hus, the protection to be afforded the invention is to be determined from the claims appended hereto.

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Claims (26)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A frustoconical venturi cone connectable to a breech adapter at the small diameter end thereof and for use in a lightweight weapon system subjected to high transient internal pressures and temperatures on firing, comprising a cured resin-treated multi-filament fibre material which has been provided in a plurality of super-imposed layers along the length of said cone whereby at least the innermost layer of said material is adapted to resist hot gas erosion on firing and the remaining layers comprise a tow of said material helically-wound at an helix angle selected to provide adequate axial and hoop strengths for resisting axial thrust and hoop loads produced on firing.

2. The venturi cone of claim 1 wherein said material comprises an epoxy resin-carbon fibre composite.

3 The venturi cone of claim 1 wherein said material comprises an epoxy resin-Kevlar (Trademark) composite.

4. The venturi cone of claim 1, 2 or 3 wherein at least said innermost layer comprises a fabric formed of refractory fibre material.

5. The venturi cone of claim 1, 2 or 3 wherein at least said innermost layer comprises a fabric formed of carbon fibre material.

6. The venturi cone of claim 1, 2 or 3 wherein at least said innermost layer comprises a fabric formed of silicon-carbide fibre material.

7. The venturi cone of claim 1, 2 or 3 wherein at least said innermost layer comprises a fabric formed of ceramic fibre material.

8. The venturi cone of claim 1, 2 or 3 wherein said innermost layer comprises a tape of longitudinally extending carbon filaments embedded in an epoxy resin matrix, said tape having been hoop-wound at approximately 90° to the venturi cone axis.

9. The venturi cone of claim 1, 2 or 3 wherein said helix angle lies in the range of 70° to 90° to the venturi cone axis.

10. The venturi cone of claim 1, 2 or 3 wherein said helix angle is approximately 76.5° adjacent the small diameter end of the venturi cone and is approximately 90° at the large diameter end.

11. The venturi cone of claim 1, 2 or 3 wherein said helix angle averages about 80° to the venturi cone axis.

12. The venturi cone of claim 1, 2 or 3 including a plurality of internal threads at the small diameter end for threaded connection to said breech adapter.

13. A method of producing a venturi cone for a lightweight weapon system subjected to high transient internal pressures and temperatures on firing, comprising the steps of:
(a) preparing a resin-treated multifilament fibre material;
(b) winding said material on a mandrel configured to the internal configuration of said venturi cone to create initially at least one layer of said material configured to resist hot gas erosion on firing, and subsequently, a plurality of layers of a tow of said material helically wound about the at least one layer at a helix angle selected to provide adequate axial and hoop strengths in the resulting cone for resisting axial thrust and hoop loads produced on firing;
(c) curing said superimposed layers of material; and (d) removing said cured venturi cone from said mandrel.

14. The method of claim 13 wherein said material comprises an epoxy resin-carbon fibre composite.

15. The method of claim 13 wherein said material comprises an epoxy resin-Kevlar (Trademark) composite.

16. The method of claim 13, 14 or 15 wherein said gas erosion resisting layer comprises a fabric formed of refractory fibre material.

17. The method of claim 13, 14 or 15 wherein said gas erosion resisting layer comprises a fabric formed of carbon fibre material.

18. The method of claim 13, 14 or 15 wherein said gas erosion resisting layer comprises a fabric formed of silicon-carbide material.

19. The method of claim 13, 14 or 15 wherein said gas erosion resisting layer comprises a fabric formed of ceramic fibre material.

20. The method of claim 13, 14 or 15 wherein said gas erosion resisting layer comprises a tape of longitudinally extending carbon filaments embedded in an epoxy matrix.

21. The method of claim 13, 14 or 15 wherein said helix angle is in the range of 70° to 90° to the axis of the venturi cone.

22. The method of claim 13, 14 or 15 wherein said helix angle ranges from about 76.5° adjacent the small diameter end thereof to about 90° adjacent the large diameter end thereof.

23. The method of claim 13, 14 or 15 wherein said helix angle averages about 80° to the venturi cone axis.

24. The method of claim 13, wherein step (a) includes the steps of winding a tow of multi-filament carbon-fibre material on said spool, positioning said spool adjacent a winding machine adapted to perform step (b), and passing said tow through epoxy-resin applying means as it is fed from said spool to said machine.

25. The method of claim 13 or 14 including the step of maintaining said tow under a tension of approximately 8 lbs.
as it is wound on said mandrel.

26. The method of claim 18, 14 or 15 including the step machining a plurality of internal threads in the inner surface of said venturi cone adjacent the small diameter end thereof for connection to a breech portion of said weapon system.