CONTRACT REPORT ARBRL-CR-00530

PROJECTILE FOUNDATION MOMENT GENERATION

PHASE 11to

Prepared by

Battelle, Pacific Northwest LaboratoryP.O0. Box 999

Richland, Washington 99352

August 1984

US ARMY ARMAMENT RESEARCH AND DEVE[QPMENI CENTER~ BALLISTIC RESEARCH LABORATORY

k o ABERDEEN PROVING GROUND, MARYLAND

4~proved for public release; distribution unlimited.

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Destroy this report when it is no longer needed.Do not. return it to the originator.

Additional copies of this report may be obtainedfrom the National Technical Information Service,U. S. Department of Commerce, Springfield, VirginiaZ2161.

The findings in this report are not to be construed as an officialDepartment of the Army position, unless so designated by otherauthorized documents.

The use. of trade names or manufacturers' names in this reportdoes not censtitute indorsement of any commercial product.

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I. REPORT NUMBER 12. GOVT ACCESSION NO,. RECIPIENT'$ CATALOG NUMBER

CONTRACT REPORT ARBRL-CR-00530 be73~~_______________A. TITLE (-'d 50111111.) S. TYPE OF REPORT & PERIOD COVERED

PROJECTILE FOUNDATION MOMENT GENERATION -PHASE II YEARLY REPORT FOR FY836. PERFORMING ORG. REPORT NUMBER

7 . AUTHOR(.) 6. CONTRACTOR GRA2T NUMBER(.)

E. M. PATTON D. R. ILLSL. R. SHOTWELL L. A. STROPE

S. PRPOUINGORGNIZAIONNAMEANDADDRSS 0. ROGRAM ELEMENT. PROJECT. TASK9. PRFOMIN ORGNIZTIO NAE AN ADRES toAREA A WORK UNIT NUMBERS

BATTELLE, PACIFIC NORTH14EST LABORATORYP. 0. Box 999 1L162618AH80Rirhland Waghinentnn OI~q9

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US Army Ballistic Research Laboratory I.NME uut18ATTN: DRXBR-OD-ST 13 UBROF PAGES

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IN. SUPPLEMENTARY NOTES

It. KEY WORDS (Coil~nuo w roveI-old* .50oor 0.440012 Upoiti by0. bok -. o)

Projectile Off-Axis Motion Rotating BandBalloting ObturatorYaw-In-Bore In-Bore DynamicsRotating Band Moment Gun Dynamics

In this second phase of the project, an analytical and experimentalstudy was undertaken to determine the restoring or foundation moment of anobturated projectile as it moves down the gun barrel. The first phase ofthe project showed that for a fixed projectile, this foundation moment wasvery large, and that for a moving projectile it would be less, but stilllarge. This phase of the project consisted of analysis and test of a

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projectile which is sliding down the gun barrel. An apparatus was:P constructed to input a measured angular deflection into the projectile's

travel and measure the amount of force which was required to inputthis measured deflection.

The same conclusions are reached about the foundation moment aswere reached in the first phase of this project, namely that it is large.Conclusions are also reached about the efficacy of the finite elementmethod for analysis of this problem and the importance of the propertiesof the nylon rotating or obturator band.

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TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS ............................................ 5

I. -INTRODUCTION ..................................................... 7

i1, FINITE ELEMENT ANALYSIS ................................ 4 ......... 9

Ill. TESTING CONCEPTS ................................................. 14

A. Test Concept - Sliding Versus Fixed Projectile ............ 14B. Project tle .Launching Concept ................................. 15

IV. PROJECTILE LAUNCHING SYSTEM...................................... 16

-A. Gun Barrel and Receiver Tank Pressure System ................. 16'B. Triggered Trap Door Mechanism ................................ 18C. Moment Inducing and Masisrement Device .......... ........ 20-

V. TEST RESULTS 'AND BAND DESIGN CHANGES .............................. 24

VI. RESULTS OF FINITE ELEMENT ANALYSIS AND COMPARISON TO TESTS ....... 29

VII. CONCLUSIONS ................................. .................. 32

VIll. PLAN 'FOR FUTURE WORK ............................................. 34

DISTRIBUTION LIST ................................................ 35

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I. INTRODUCTION

This study was undertaken as a continuation of a projectconducted by Battelle's Pacific Northwest Laboratory under thesponsorship of the U. S. Amy's Ballistic Research Laboratory.The purpose of the entire study is to develop methods to measurethe magnitude of the restoring or foundation moment generated bythe plastic obturator or rotating band when an obturated singlebore contact projectile cocks in the bore of a gun during firing.The stability of the projectile is directly affected by themagnitude of this moment. If its magnitude is large, theprojectile will potentially be more stable than if it is small. ':The overall intent of this project, then, is to provideinformation about single bore contact projectiles which can beused to design a stable, accurate, minimum weight round.

The first year's work showed that in fact this foundationmoment is very large. That portion of the study also showed,however, that the magnitude of the moment that can be measuredwith a suitable test apparatus is strongly dependent upon whetheror not the simulated projectile is in fixed or sliding contactwith its fixture. A projectile-like device whose plasticobturator band is fixed axially will provide a moment five timeslarger than a similar device which is allowed free axialmovement. The fixture used for all of the testing in the firstphase of the project kept the plastic band fixed firmly within adevice which was to simulate a short section of the gun barrel.Both the finite element results, and a confirmatory type handcalculation, showed that indeed there is a large differencebetween a sliding contact and a fixed contact.

In a real gun barrel, the projectile is moving axially at avery high speed. The purpose of this second phase of theproject, therefore, was to measure the foundation moment providedby a projectile moving within a simulated gun barrel. Thisprovided several problems which had to be resolved before theactual foundation moment could be measured. The first of thesewas providing a moving projectile whose motion was similar innature to a projectile in the bore of an actual gun, buttravelling at considerably less speed, and with less accelerationand violence. The second problem was how to input a measurableangular disturbance to a moving projectile, and measure the forceor moment required to provide that disturbance. Third, once thedata has been taken, how is it to be interpreted in light of thesolutions to the first two problems.

This report documents the work done in the past year to ,provide solutions to the aforementioned problems and providessome conclusions about the action of the obturatlon bands inservice rounds. A geometry was chosen initially for theprojectile which was similar to the simplified geometry used inthe first year's work. Preliminary finite element analyses wereperformed to determine the approximate magnitude of the expected --

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moment. Using this information, and a section of gun barrelprovided by BRL, a projectile launching and moment measuringsystem was designed and constructed for the moving projectiletests. Tests were performed using this system, and the data wereanalyzed to determine the magnitude of the foundation moment.This report, then, documents the analysis, testing, andconclusions reached as a result of this year's work.

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II. FINITE ELEMENT ANALYSIS

The finite element analysis performed in this year's workclosely parallels the analysis performed during the first phaseof the project. Again, the computer code used for all finiteelement computations was ANSYS(*). Both two and three dimensionalanalyses were performed using ANSYS, but on significantlydifferent geometries than those used in the first year's work.Figure 1 is a plot of the finite element mesh used for the twodimensional model. ANSYS supports a two dimensionalisoparametric axisymmetric element which can be loaded :non-axisymmetrically. This element is loaded with a Fourierseries loading about the axis of symmetry as was described inlast year's report (1). The variation in deflection was againinput as the first cosine term of the Fourier series multipliedby the required deflection (i.e. u = O.lcos 0). The twodimensional analysis should provide us with accurate results for Othe cases where the materials remain linear, but the element thatwe have to use does not support material non-linearity. To modelthe behavior of material non-linearities, we must go to a threedimensional analysis.

The large central portion of Figure I is the simulated borecontact area. The shaded portion of that region is the nylonwhich is to simulate the obturator or sealing band. The centerof the model runs along what is identified as the centerline inthe figure. The model was constrained in the radial andtangential directions along the outer edge of the nylon, therebysimulating the gun bore contact. One node at the center of theprojectile, centered under the simulated obturator, was alsofixed in the axial direction to prevent rigid body motions of themodel. The node identified as the displacement input node at theend of the projectile was given a specified displacement in theradial direction. This produced a reaction force at thedisplaced node which was used to calculate the foundation momentfor that value of induced displacement. Reaction forces andoverall bulk stresses were also obtained for the plasticobturator. From these values, it was determined that forrelatively small displacements, a portion of the nylon obturatormaterial could behave in a non-linear fashion. Also, it was notclear from the two dimensional results how much of the obturator 5band would be in contact with the gun bore during the angularexcursion of the projectile. Three dimensional analyses wereperformed last year in an attempt to determine the degree ofcircumferential refinement that was necessary to producereasonable results for this problem. It was determined that an -element size of 15 degrees circumferentially would produce -reasonable results, and not be excessively expensive in computertime.

• ANSYS is a proprietary engineering analysis program owned,marketed, and supported by Swanson Analysis Systems, Inc.

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Figure 2, then, is a plot of the finite element mesh used forthe three dimensional analysis for this project. This plot hasthe hidden element boundaries removed for clarity in itsptesentation. The large shaded section of the mesh is again thenylon simulated obturator band. The flare at the rear of themodel is the wobble inducing device, or angular deflectioninducing device. Note that the connection between the wobble

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FIXED RADIALLY

DISPLACEMENTIMPOSED ~*

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4I

Figure 1. Two Dimensional Finite Element Mesh

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inducing portion of this model and the large center section ismuch fatter than in the previous model. When this project wasfirst begun this year, we were going to use a steel projectile.This is because the test fixture used during the first year'swork had to be made out of steel in order to reduce the bendingof the moment inducing arm of the fixture. Originally it was notexpected that the foundation moment would be as large as it seemsto be, and the first fixture was made of aluminum and wasdesigned with a much thinner moment inducing arm. When it wasdiscovered that the foundation moment was indeed large, thefixture was re-designed with a much larger diameter steel momentinducing arm. This year, we expected the same magnitude for the"foundation moment and originally designed the projectile out ofsteel. The original design, however, was much too heavy, and wewere forced to re-design with aluminum. This forced us to make"the moment inducing arm (the connection between the flare and thelarge central portion) much thicker.

I -

DISPLACEMENT IMPOSED

FIXED RADIALLY

Figure 2. Three Dimensional Finite Element Mesh

............................-.. ',:5:.

At the beginning of the three dimensional analysis, weattempted to use a much more refined mesh than you see in Figure2. That mesh consisted of 684 elements using a maximum nodenumber of 1279. The solution time estimate for the elasticanalysis for this mesh was greater than 6000 seconds on theUNIVAC 1108 computer that was used on this project. This wouldhave cost on the order of $300 per run. Adding inelasticmaterial properties to this analysis would have increased thecost to well over $1000 per run. This was just too expensive forthe scope of the project. The mesh that is portrayed in Figure"2, then, consists of 432 elements with a maximum node number of774. The elastic analysis for this mesh cost approximately $50per run, or one sixth of the cost of the original mesh. Thisprovided an analysis at a reasonable cost, but it sacrificedaccuracy somewhat. This inaccuracy in the analysis will bedescribed later in this report, along with some conclusions aboutthe efficacy of using the finite element method for the solutionto this problem.

The sa!-s nylon properties were used initially for both thetwo and three dimensional models. The central portion of the twodimensional model, however, as stated, above, was steel. Thecentral portion of the three dimensional model was aluminum. The"Young's modulus used for the nylon was obtained from materialtests performed on the nylon material purchased for the firstyear's testing. This may or may not have been the actual valuethat should have been used in the analysis. We were required topurchase a different product form for this year's testing, as thediameter of the nylon band on the projectile had to besignificantly greater than last year's band. Last year, we wereable to purchase centrifugally cast Zytel 101 in the smalldiameter that we needed for those tests. The suppliers of Zytel101 do not make the spin cast nylon in as large as a 5 inch (13cm) diameter tube. We therefore were forced to buy extrudedtube. This product form should not be significantly differentthan the spin cast nylon, but may in fact be, as will bedescribed later in this report. We did not perform any materialcharacterization tests on this nylon, as we had extensivelytested the nylon from last year, and had a good idea of thebehavior of that material. For the analysis this year weinitially used the material properties from the tests done duringthe first year of the project. The material properties usedinitially in the finite element analyses, then, are as follows:

Material Young's Modulus (psi/mpa) Poisson's Ratio

2-D Steel 30xi06 (207,000) .3

10163-D Aluminum 10x10 (69,000) .33

Nylon 520,000 (3,400) .4

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The results of these anaiyss, some conclusions about theefficacy of the method for this application, and somere-analysis that was performed because of test results are allpresented later in this report. The reasons for this will becomeapparent In the next section of the report which details themajority of the work done during this year's phase of theproject, the testing using a mciving projectile.

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III. TESTING CONCEPTS

This portion of this document will detail the entire testprogram that was undertaken for this year's phase of the project.This includes the design concepts and designing of-he projectilelaunching mechanism, the construction of that device, theinstrumentation of that device, and the testing itself.A. Test Concept - Sliding Versus Fixed Projectile

As -was 'mentioned in the introduction to this report, it wasdiscovered in the first year of this project that the foundationmoment that could be impartsd by a moving projectile issignificantly less than that which can be imparted by aprojectile which is fixed axially. The testing portion of thisyear's work, then, was intended to measure just that phenomenon.It was also perceived that if a suitable device could beconstructed, the motion of a projectile could be measured and

. compared realistically to the motions expected in an actualround. This device would have to launch a projectile of asimilar weight and geometry to an actual round, at a

-* significantly reduced velocity, and violence, and with thecapability of retrieving the projectile easily (and hopefullyrelatively undamaged). For safety and cost considerations, wewere not able to use any pyrotechnics at all. The projectilewould have to be launched with a compressed gas, preferably air.To mimic tne a;tion of the projectile with actual propellantgasses would require some '-a of a triggering device which wouldallow the pressure to-change rapidly either at the front or therear of the projectile. In other words, we would either have topressurize the entire system, and release the pressure forward ofthe projectile very rapidly, or have some sort of device whichwould rapidly pressurize the rear of the projectile.

The pressure curve that a projectile sees during en actualfiring has a very short rise time to a peak pressure, and thenalmost a linear decline after the peak to a muzzle exit pressure.Getting the actual pressures that e projectile sees in firing agull was impractical in terms of the cost restrictions on theproject. As stated above, we could not use pyrotechnics, andthere is very little else that produces very high pressures veryquickly. We therefore we:e forced from the outset of theproject to use a relatively low pressure system. This requiredthat we use a relatively light weight peoJectile in order tobe able to accelerate the projectile with the low pressures thatwere required. The low pressures also, however, allowed us toperform the tests within the city limits without too muchconsideration for the noise that they would produce. Thesepressures also allowed us to catch the projectiie easily andwithin a few feet of its exit from the system.

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B. Projectile Launching Concept

- There were two basic requirements for the projectilelaunching system that had to be met before any design could beattempted. First, we needed some sort of a gun barrel-likedevice which was of sufficient length and interior diameter toaccommodate the projectile and some sort of wobble inducing andforce measuring mechanism. This gun barrel also had to be verysmooth on the interior, and not be overly costly. The second

4" requirement was a laboratory environment with power, compressedair or gas of some sort, and enough room either inside or outsidethe laboratory to accomodate the mechanism. The problem of the"gun barrel was solved by BRL personnel by donating a short"section of an actual barrel. The laboratory space was alreadyavailable in the form of Battelle's Mechanical DevelopmentLaboratory, This lab has a compressed air supply, power, andsufficient space both inside and outside to perform the tests.The lab also has bottled nitrogen, a welding apparatus, andsufficient machinery and tools to construct and alter a test -apparatus of this type.

Once these requirements were met, the problem of exactly howthe pressure was to be applied to the projectile was tackled."As stated above, there are two methods for applying thispressure, either by pressurizing the entire system and rapidlyreducing the pressure in front of the projectile, or by rapidlyapplying a pressure to the rear of the projectile. We opted forpressurizing the entire system, and releasing the pressurerapidly from the front of the projectile. This option was chosen 77.7

for several reasons. First, it was difficult to design anadequate receiver tank with a mechanism for releasing thepressure that would not interfere with the projectile itself.Essentially we would have had to have used a rupture disk at themouth of a pressurized tank. When the disk ruptured,the center of the disk would have impacted the rear of the"projectile and caused perturbations in the projectile that would"have made analyzing the data impossible. With the pressure beingreleased in the front of the projectile, we were able to design atriggered trap door mechanism that could be re-used for eachtest.

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IV. PROJECTILE LAUNCHING SYSTEM

This section of this report describes the entire projectilelaunching system in detail. The entire-system can be broken intointo three basic sub-systems; the gun barrel and receiver tank, .4.the triggered trap door, and the foundation moment inducing andmeasuring device. Figure 3 is a drawing of the entire system onits mounting with a center cutaway showing the projectile and itswobble inducing and moment measuring device.

AIR INLET -TO PRESSURIZE TRIGGER

AIR RECEIVER PROJECTILE /I •';;;-TANK / UN BARRE4L "•.-

ANWOBEO TI. RELEASEINDUCING DEVICE Di "i- '. OOR ! ?

Figure 3. Projectile Launching System

A. Gun Barrel and Receiver Tank Pressure System

The gun barrel and receiver tank pressure system consists offour main components: the gun barrel itself, the receiver tank,the mating flange system between the two above, and the airrelease mechanism. Thý last of these has been relegated to itsown section of this report. The first three will be described indetail hIere.

The gut, barrel itself is a section of an actual gun barrelwhich was supplied for this project by BRL. It is approximatelysix feet In length, with an interior diameter of 5.1 inches. Theexterior diameter varies from 6.9 to 7.8 inches, and there is aset of breech threads and a tack welded breech ring at the rearend of the barrel. The barrel looks as if it was originally muchlonger and has been cut off at what is now the muzzle end. Thematerial of the barrel is reported to be 4340 steel (it wasshipped to us as a section of 4340 steel tubing). This steel isfairly commonly used in gun barrels because of its strength,hardness, and resistance to plastic deformation. This also,however, makes the steel somewhat difficult to weld. Its •..hardness also makes it difficult to cut and drill. Nevertheless,

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we managed to design a flange for the end of the gun barrel which

mates to a similar flange on the receiver tank and to get theflange welded to the gun barrel. A detailed drcwing of this

flange is shown in Figure 4. The flange is a standard-6 inch -

pipe flange used in low pressure fluid applications. The gun

barrel side of the flange has been machined to provide a mating

surface for the breech end of the gun barrel. The inside

diameter of the flange has been machined with a ramp which ma-t..

on the gun barrel side to the exact inside diameter of tie gun ."-

barrel and provides a larger diameter on the other side for

loading of the projectile within the gun, in much the same manner

as the ramp at tCe end of the chamber in a real gun.

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BREECH GASKET SURFACE

Figure 4. Gun Barrel Mating Flange

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The receiver tank which mates with this flange is a 6 foot

section of six inch schedule 40 steel pipe with a hemisphericalpressure cap welded on one end and a standard flange on theother end which mates with the flange welded onto the gun barrel.The two halves of the flange can be bolted together and unboltedeasily, to facilitate loading of the projectile prior to a test.This receiver tank which is behind the projectile provides the 'S -impetus -when pressurized to accelerate the projectile across themoment inducing and measuring device, and thus,down the barrel tothe muzzle. With the triggered trap door mechanism in place, thegun barrel and receiver tank can be pressurized. This providesan equal pressure on both sides of the projectile. When the airrelease door is released, the air pressure in the gun barreldrops rapidly to atmospheric. The pressure on the front of theprojectile then is much lower than that at the rear of theprojectile, and the projectile accelerates down the gun bore. Thepressure drop in the front of the projectile mimiicks fairly wellthe pressure rise that occurs in an actual round when thepropellant is ignited. In this manner, we can simulate theactual firing conditions that a real projectile will see, withoutthe high pressures, accelerations, and violence of an actual test Lfiring.

As is shown in Figure 3, there is an air inlet system that is

used to pressurize the entire apparatus. This system consists of

quarter inch copper tubing which is bolted to both the gun barrelahead of the projectile and the receiver tank behind theprojectile with high pressure fittings designed for that purpose.

S There is a valve which allows air into the system from the*. laboratory air supply. From there, the air is free to pressurize

both chambers at an equal rate and to equalize the pressure bothahead and behind the projectile. The tubing is not large enough,however, to allow the rear receiver tank to de-pressurize

i rapidly. This allows the device to apply pressure to theprojectile as stated above. It also allows the personneloperating the device to stand clear of the entire system when thetrigger is pulled.

B. Triggered Trap Door Mechanism _

Figure 5 is a schematic of the air release door or triggered"trap door mechanism that is used to rapidly de-pressurize theportion of the gun barrel which is ahead of the projectile. A"mating flange is welded onto the end of the gun barrel. Thisflange contains a machined surface on one side to mate to thebarrel and a machined surface on the other side used as asealing surface for an o-ring gasket. The flange is-made of 3/4inch mild steel, and it contains a mounting for the triggermechanism and the bolts which hold the outer clamping plate. Amating plate with a machined slot for the o-ring gasket is placedagainst this flange. This plate is the actual gas seal for theend of the barrel and is fitted with the o-ring used to sealagainst the flange welded onto the barrel. It is also made of

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3/4 inch mild steel and is tapped in the center of the side awayfrom the muzzle for a centering bolt which is attached to theouter clamping plate. Another clamping plate (the outer clampingplate) is attached to the sealing plate and is used to tightenthe a-ring seal and provide a completely sealed system. Thelower end of this clamping plate is placed under two bolts whichare bedded into the muzzle flange and act as thq lower hinge forthe trap door mechanism. The upper end of the clamping plate isplaced under the trigger mechanism before being tightened. Thetriggering mechanism for the trap door is merely an L-shapedsteel bar which hooks over the top of the outer clamping plate.This steel bar is through bolted (and thus hinged) to the muzzleflange and has a long steel bar welded to the top for anactuator. As a safety precaution, and so as not to lose the trapdoor mechanism, a large chain is attached to the door whichcauses it to be deflected toward the ground, out of the way ofthe moving projectile. Once all bolts are tightened and thesystem is pressurized, the trigger mechanism can be pulled, theair release door opens, and the gun barrel ahead of theprojectile de-pressurizes very rapidly. The projectile is thusprovided with the impetus to accelerate down the gun barrel, pastthe moment inducing and measuring device.

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ME CLAMPINGoCNS O NG PIAE MUZZLE FLANGESEAL •

AIR•, R LEASELAE• :::-:

Figure 5. Air Release of Triggered Trap Door Mechanism

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"C. Moment Inducing and Measurement Device

Figure 6 shows a schematic of the moment inducing andmeasurement device. There are several components to this system,attached both to the projectile and to the gun barrel itself.The main idea behind the device, however, is that the rear of theprojectile is required to climb over a ramp-like device placed onthe bottom side of the gun barrel, thus inducing an angulardisturbance in its travel down the gun barrel. Beneath thisramp-like device is a load cell which measures the force that theprojectile exerts upon the ramp as it climbs over it. Themagnitude of the angular disturbance is measured carefully, as isthe force on the ramp which causes this disturbance. From these"two known quantities, we can measure the foundation moment.

LOAD WASHER MOUNTING FRAME

GUN WARREL

WOBGLE INDUCING RAMP PROJECILE

OBTURATOR S/ND

WASHER LOAD TRANSIERROD

Figure 6. Schematic of Moment Inducing and Measurement Device

The projectile used in the system is shown in Figure 7. Notethat the rear of the projectile is slightly different even thanthe three dimensional finite element model. From early resultsin the finite element analysis portion of this project, we found -that the foundation moment that we would be measuring wouldprobably be very large. A concern was raised at that time aboutthe ability of aluminum to withstand the contact stresses thatwould be imposed upon the rear of the projectile during thetesting. We also wanted to be able to impart differentmagnitudes of angular disturbance into the projectile, to get anidea of how the foundation moment changes with amount of angulardisturbance. Als-, it was easier to change the rear of theprojectile with each firing than to alter the height of the ramp.

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"We therefore came up with a faceted steel plate which is placed"over the end of the projectile before it is launched. The platehas eight different height facets, each mirrored on either sideof the plate. These pairs of facets on the plate are machined toallow a range of displacement of the rear of the projec'cile from0.066 inches (0.168 cm) to 0.243 inches (0.617 cm). The rear of ', •.the projectile with its faceted plate is required to go over anL-shaped ramp which is hooked over the end of the gun barrel andrests upon the force measurement actuator rod. Photographs of"the faceted plate and ramp are shown, on the following page asFigures 8 and 9. The faceted plate is made of high carbon steel,as is the ramp. The ramp is further hardened to Rockwell C 55 inorder to provide a very hard sliding surface for the faceted"plate.

Figure 7. Projectile as Used in Tests

Below the thickest portion of the ramp, going through the gunbarrel, is a 1/2 inch steel rod. This is the force measurement"rod, and it goes out of the bottom of the gun barrel, through agas seal, and against the top of a load washer. This load washeris placed within a very large and stiff frame which is boltedaround the outside of the gun barrel. The frame was designed tohave a deflection that was at least two orders of magnitude lessthan that which is imparted into the projectile. In this way, thedeflection of the measurement device itsilf could be discountedin our measurements. The thinnest portion of this frame is atthe lateral midline of the gun barrel. At this location, theframe is 1.5 inches by 2.0 inches. There are two sides to theframe, and thus,the stress area is 6 square inches. At a load of10,000 pounds, the maximum deflection that we could expectconservatively would be 0.0004 inches. This is the magnitude ofthe load that was initially expected for a 0.1 inch deflection ofthe rear of the projectile. The frame deflection, then, isnearly three orders of magnitude less than the measureddeflection and is not accounted for in the analysis of the data.

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Figure 8. Photograph of Faceted Projectile End Plate

Figure 9. Photograph of Moment inducicng Ramp

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Figure 10 is a detail of this frame. Note that the bottom of* the frame has a large open section at its center. The load

washer assembly is mounted into this open section and is held inplace by the force measurement actuator rod which protrudes fromthe gun barrel. The load washer is a Kristal -Model 9061 Fore,-Transducer which is a piezoelectric device used for measuringvery large loads. It has a maximum force measurable in the rangeof 50.000 pounds force (22,700 kg). The signal from the loadcell is conditioned through a Kristal Model 5001 ChargeAmplifier 'and displayed on a Nicolet 2090 Digital Oscilliscope.

*L There is also a laser velocity measurement system which is placedacross the path of the -projectile to measure the speed of the"projectile as it exits the muzzle. Originally the signal outputfrom the charge amplifier was routed to a Teac Model SR-50 14channel FM data recorder. The signal from the tape recorder wasanalyzed initially, and it was found that the rise time on thesignal exceeded the frequency response of the tape recorder. Theonly device available to us at the time to record the event wasthe Nicolet digital oscilloscope. After the first two tests, theNicolet was used exclusively to record the response of theKristal load washer.

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SPUT HERE FOR MOUNTING H-H

Figure 10. Load Washer Support Frame

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__ -. ;6,

V. TEST RESULTS AND BAND DESIGN CHANGES

The testing phase of this project was started in April of1983. Initially, we tested the air pressure system and the trapdoor mechanism. These tests also gave us confidence that thepressure system was going to function as designed. In thesetests, the system was dry fired, without the projectile. Wemounted the receiver tank to the gun barrel, closed and sealedthe trap door, and applied air pressure slowly to the system.The maximum air pressure available in the building was 120 psi(830 kpa), and we pressurized the system to that level. Oncethat pressure had been obtained and maintained for a sufficientlength of time, we actuated the trigger mechanism to release theair pressure. This gave us some idea of the recoil that we wouldexpect from the device and a conservative estimate of the noiselevel that the gun would produce. With no projectile in thebarrel, the entire system de-pressurized in these tests ratherthan just the gun barrel in front of the projectile. Thisproduced the loudest report that the system could produce atthat pressure. The noise from the device attenuated rapidly asit travelled away from the MDL building, and did not cause anoise problem. Also, the mounting held the assembly fixed wellenough that we could nut see any appreciative recoil from themechanism. The mounting was bolted to the concrete prior to thetests to alleviate any potential recoil problems.

The next tests that were performed included the projectile,but not the moment inducing ramp mechanism. The projectile wasloaded into the system, the system was again bolted together, andthe air release trap door was mounted and sealed. Again themaximum building air pressure was applied to the system (120 psior 830 kpa), and the triggering mechanism was actuated. Theprojectile shot out of the gun tube at what appeared to be 338feet per second (111 meters per second). Subsequent shots withthe projectile showed us that our laser velocity measurementdevice was being affected by the moisture in the air whichpreceeded the projectile. Essentially, the cloud of gas thatcame out of the gun ahead of the projectile was cutting the pathof the laser enough to trip the device. We moved the sand boxwhich catches the projectile farther from the muzzle and movedthe laser device farther from the muzzle. This corrected thelaser tripping problem. The projectile speeds that we measuredafter this problem was corrected were more in the range of 160 C

feet per second (52 mps) which is very close to the velocity thatwe originally expected the projectile to attain.

After the preliminary work of pressure testing the assembly,and firing the projectile without inducing a wobble, we wereready to fire the projectile with the wobble inducing device r.placed in the gun barrel. These tests were first performed withthe air intake to the system mounted on the gun barrel. Thisconfiguration caused the pressure to be slightly higher in thefront of the projectile during pressurization of the system than

24

I . .. • . . . . - • .- - .- • • - - -. . .: - . •. - . :- ,. .. • ., -

it was at the rear of the projectile. The projectile slidbackwards slightly during this pressurization. This forced theramp to move away from the end of -the gun barrel slightly. Onthe third test, the long portion of the ramp broke off at theposition shown in Figure 11. To cure this problem, we re-mountedthe air supply line onto the receiver tank so that it wouldalways be at a higher pressure than the gun barrel, forcing theprojectile forward onto the ramp, and not allowing the ramp tomove axially when the gun was fired. The second ramp that wasmade to replace the broken one has been in service since thattime.

POSITION OFBREAKAGE

Figure 11. Moment Inducing Ramp Showing Position of Breakage

The plate which rides up over the ramp and induces the wobbleinto the projectile has eight facets, as stated before. Thosefacets allowed us to input differing magnitudes of angulardisturbance into the projectile during its motion down the. gunbarrel. The magnitudes of those eight steps are shown in Table 1below. The values in the table are given both for the. angulardisturbance at each step (in degrees) and the actualdisplacement of the rear end of the sabot produced by that facetsliding over the ramp. The displacements range from less than 0.1inches to nearly 0.25 inches. This gave us a very broad range ofdisturbance in which we were interested, from a purely elastic"nylon behavior to an elastic-plastic behavior of the nylon bandmaterial.

"Table 1. Mainitudes of Disturbance for Each Facet

Step Angular Disturbance (deg) Displacement (in/cm)

1 0.75 0.066/0.172 1.1 0.093/0.24 "•z3 1.3 0.11/0.294 1.6 0.14/0.365 1.9 0.16/0.416 2.2 0.19/0.497 2.5 0.22/0.558 2.8 0.24/0.62

"...

Note that the amount of angular disturbance that we imposedupon the projectile is rather small. The, expected foundationmoment generated from this angular disturbance, however, is verylarge. In the case of facet 3, we fully expected a force ofnearly 10,000 pounds (4500 kg) to be exerted on the top of theramp when the projectile passed over it.

The first test results that we obtained are shown below inFigure 12. The forces that we measured initially with the systemwere much lower than those that the finite element analysis hadpredicted, The nylon band on the projectile in those early testswas machined to fit snugly into the bore of the gun barrel. Wedid not initially have any interference between the barrel andthe plastic band. When the projectile cocked in the bore,portions of the nylon band lost contact with the barrel and werenot providing any moment. In an actual gun firing, the obturatorband is forced into the barrel through the forcing cone, andthere is a large interference generated. We came to theconclusion that we needed at least.some interference between thenylon band and the gun barrel to simulate the actual firing of anobturated round. The reason that we had not provided thisinterference in the first place was the problem of loading theprojectile into the gun barrel. The design of the originalobturator band precluded any possibility of expanding the bandonce the projectile was in the bore. We therefore had to findsome method of pushing an oversize projectile into the gunbarrel.

9 P._ _,-= ~8 - / ?

7-.

-Gj 6

4C- • ,,

2 a

0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22

Projectile End Displacement (in)

Figure 12. First Test Results

26

%.~' %X .%%,.t.%.tts:\'-:&. Z Z N ~ ~ -:.~

The next series of tests that were run were performed with anobturator band with a fairly large interference. Initially, wetried to get a projectile with an interference of 0.040 inches(O.lcm) into the gun barrel. We devised a hydraulic jackingsystem which attaches to the flange on the rear of the gun barreland applies a force to the rear of the projectile to push it intothe gun barrel. We could not get the first projectile into thegun barrel with our hydraulic system, and we were forced tomachine off some of the outer diameter of the nylon band. Theband which we were finally able to load into the launching systemhad an interference of approximately 0.01 inches (0.025cm). Wewanted all parts of the nylon band to remain in contact with thegun barrel during firing so that we could compare the resultswith the finite element model. This necessitated using only thefirst facet on the projectile back plate. Any larger disturbancewould have caused some portion of the nylon to lose contact withthe gun barrel.

It required approximately 16,000 pounds force (7300 kg) topush the projectile into place in the gun barrel. During theloading of the projectile, we noticed that the hydraulic ramseemed to cause the projectile to overcome its static frictionand move suddenly. We would then have to apply a load to the rearof the projectile, and it would again pass a static frictionthreshold and move suddenly. We used this observation toadvantage when it came time to fire the projectile. It wasobvious that the building air supply would not provide a high

Z" enough pressure to overcome the static friction of the projectile *in the bore of the gun. It was also obvious that our air releasemechanism would probably not remain sealed at the pressure thatwould be required to fire the projectile. We therefore left theair release door off the muzzle end of the system and merelyapplied pressure to the rear of the projectile through thereceiver tank. This method worked very well. The projectilerequired 615 psi (4240 kpa) to overcome static friction and beginto move in the gun barrel. Once this pressure level was reached,and the projectile began to move, it accelerated very quicklydown the gun barrel, and exited at 273 feet per second (90 metersper second). This made us comfortable that the procedure wouldwork and that the data would not be adversely affected. Thismethod, however, uses up a nylon band on each shot. The nylon.begins to be deposited on the inside of the gun barrel about twofeet (0.6 meters) from the breech, leaving a very thin film ofnylon on the interior surface of the gun barrel. With thiserosion of the nylon band, the band diameter at shot exit is onlyslightly over the diameter of the muzzle. This procedure alsodoes not mimic the action of an actual firing as well as does thesystem with the trap in place, but was necessary in order tocollect meaningful data.

27

--..

Once we were convinced that the new technique would work, weFad three more plastic bands machined and we fired the systemthree more times. The measurements of those bands and pertinentprojectile ioadino informaLion ar: presented in Table 2 below.The data from the four firings is presented in Table 3. Notethat the second of those bands did not require as much force toseat in the breech, nor as much pressure to fire as the firstband, The projectile in this case in fact did not exit themuzzle during its initial travel down the gun barrel. The loaddata from this projectile is,therefore,suspect. The other threeprojectiles fired in a consistent manner, however, and the datafrom those tests is probably useable.

If we make the assumption that the average of the values inThble 3 minus the second entry gives a good indication of theactual foundation moment in an actual round, we can make anestimate as to the importance of this moment as a stabilizationmechanism for the round. For instance, let us determine how farback the center of gravity of the round would have to be to equalthe foundation moment in an actual case. The maximumacceleration that most tank rounds see in the gun is about 50,000g's. Let -.s take for an example the 120mm gun system and apostulated projectile weight of 16 pounds (7.3 kg). The averagefoundation moment that we measured was 36,700 in-lb (42,300

-* cm-kg) at a deflection of 0.066 inches (0.168 cm) or 0.76degrees. This means that the center of gravity of the roundwould have to be approximately 3.5 inches (8.9ai or 89rmm) orroughly two thirds of a caliber behind the center of theobturator band to equal the foundation moment provided by theobturator band. This is also at the maximum acceleration of theround. At any other time, the nylon bard would provide the samemagnitude of foupdation moment, but the lesser acceleration onthe round would lessen the action of the center of mass.

Table 2. Band Measurements and Pertinent Firing Data

Band Diameter Force to Load Pressure to Fire(in/cm) (lb/kg) (psi/kpa)

1 5.112/12.98 15,840/7200 615/42402 5.112/12.98 355/2450

* 3 5.112/12.98 16,200/7360 480/33104 5.111/12.98 9900/4500 510/3520

Table 3. Test Results With Band interference .

Band Force on Load Cell Foundation Moment- (lb/kg) (in-lb/cm-kg)

1 9200/4180 46,000/53,1002 5530/2510 27,650/31,9003 6660/3030 33,300/38, 5004 6200/2820 31,000/35,800

28

VI. RESULTS OF FINITE ELEMENT ANALYSIS AND COMPARISON TO TESTS

Initially, the finite element analyses were performed with theassumption that the nylon band material always came into contactwith the gun barrel. Both the two and three dimensional analyseswere performed in an effort to gage the magnitude of the largestforces that we would be measuring and to gain some confidencethat the finite element method might produce accurate enoughresults for design type calculations. The magnitude of thehighest forces that we would be measuring would drive the design P

of the projectile and load measuring system. More importantly,the suitability of the finite element method for predicting"foundation moment is a very important part of the overall aim of

-* the project - namely: to provide BRL with a method with which todesign an accurate, minimum weight projectile system with asingle gun bore contact.

At the first of this year's work on this project, some timewas spent in attempting to write a computer program that wouldprovide this design method. The outline for the program wasgenerated, and some FORTRAN code was written which wouldeventually have led to a complete system for modelling the motionof a single bore contact projectile in a smooth gun Lube. Theequations of motion of the system are fairly straighforward, asthe motion is basically axial, with perturbations only in twodirections. It soon became evident, however, that the onequantity that could not be easily estimated within the structureof this pro~ram was the foundation moment itself. That is, oncethis quantity is known, the rest is very simple. The only likelycandidate for a computer generated solution for the foundation

moment is the finite element method. This fact provided theimpetus for comparing the results of the tests with the finiteelement results and determining whether or not the method wouldprovide us with good estimates of the foundation moment.

With this in mind, let us look at Figure 13, the results ofthe linear elastic finite element analyses that were performedusing the models previously described in this report. Alsopresented in this figure are the first test results, the testrest-lts with the interference, and the results of an analysisthat takes into account the fact that some of the nylon is not incontact with the gun bore. This was done by relaxing theboundary constraints for several nodes on the outside of thenylon where it would otherwise contact the bore of the gun. Thenodes whose boundary constraints were relaxed were chosen on thebasis of the reaction forces at the nodes for the analysis with ".all outer nylon nodes fixed. Those nodes that exhibited iensilereaction forces in the first analyses had their radialconstraints relaxed in this analysis. Some adjustment wasrequired after the initial relaxation in order to get only thosenodes that were in tension relaxed, and no others. These

*: results, then. should match those cases where there is nointerference between the nylon band and the gun barrel, as in the

29

first series of tests. You can see in fact that the finiteelement results and the test results do not match well. The twodimensional results come fairly close to matching the testresults, but the three dimensional results are very high.

60=

50-

S 40

*C 30-

0030

0

0

0.06 00 . 012 0.14 0.16 0.18 0.2 0.22

Displacement at Projectile Rear (in)

Figure 13. Finite Element Results and Test Results

One of the things that is probably causing the differences* between test and analysis is that the finite 'element results are

for a linear elastic nylon material with the dynamic Young'sModulus used rather that the static value. In being linearlyelastic, the calculations do not take into account any inelastic

* ~deformation. The stresses in the outer elements of the nylon are *

above the quoted yield strength of the nyl1on even at therelatively small angular disturbances. Also, the event does nothappen quite as fast as the high strain rate trests wereperformed, and using the dynamic properties for the anlysis maynot be entirely correct. There is another factor influencing thereal situation that we have not taken into account in thesecalculations. This is the action of the propellant gasses on therear of the projectile. The propellant pressure at the rear ofthe obturator band in most of the high performance KE rounds thathave been designed is in the range of 50,000 pv~i at maximumlaunch pressure. This puts the band into a state of hydrostaticcompression. The behavior of the nylon band under these

30

%. • 4

conditions is not as yet known. There is yet another materialproperty question that has not been answered. The materialproperties used in the analysis were for material bought for thefirst year of the project. This material was centrifugally cast,and was purchased from a different supplier than the material forthis year's testing. The material used for the testing this yearwas extruded tube. The centrifugally cast material did not comain large enough diameters or wall thicknesses for the testprojectile this year. This material had a different "feel" andlook than last year's nylon, but it was assumed that the material .would behave in a similar fashion. That may or may not be a goodassumption.

There is also another possibility for discrepancy in theresults. The original finite element mesh designed for theanalysis this year was much finer than what is shown in Figure 3, .as was stated earlier in this report. The usual result of a toocoarse mesh is that the structure seems to be more stiff than itactually is. This could easily be the case in this'analysis. Ifone looks at the difference between the two and three dimensionalresults in this light, the mesh refinement difference becomesvery apparent. In the two dimensional analysis, the mesh can bethought of as infinitely fine in the circumferential direction.The three dimensional analysis, however, uses a circumferentialelement size of 15 degrees. This size was adequate for the smalldiameter test fixture used last year, but probably is not fine K:,:,enough for the large diameter projectile used in this year'stesting. If this is the case, and our mesh was indeed toocoarse, the results are then at least trending in the correctdirection. That Is, the coarser mesh is acting as if it werestiffer than the more refined mesh, and it provides a highernumber for the reaction force at the moment inducing end of theprojectile.

31

VII. CONCLUSIONS

There are several conclusions that can be drawn from theresults presented in this report. First, the foundation momentfor a moving projectile still seems to be very high. Thebest way to measure the relative importance of this quantity isto relate it to that moment produced by the center of gravitybeing behind the center of pivot for the round and the roundcocking-slightly in bore. To equal the righting or straighteningmoment produced by the nylon band in our experiments, the centerof mass would have to be roughly two thirds of a caliber behindthe center of pivot at the maximum acceleration that a common KEround sees in bore. This magnitude is very significant in lightof the desire on the part of BRL to produce accurate, lightweight projectiles whose performance exceeds current designtechnology. This conclusion supports the work done the firstyear on this project and the conclusions reached at that time.In any analysis of the motion of a single bore contact projectileduring firing the foundation moment should be taken into account,as it is not a small effect. It will, in fact, have apredominant influence on the ability of BRL to design a singlebore contact projectile which will provide needed accuracy.

The ability to evaluate the foundation moment for any designis also desirable in light of its importance in the motion of theround in the bore of the gun. This ability, is, in fact,manditory for a complete design of a single bore contactprojectile. The most promising method of providing thisevaluation is the finite element method. We have shown, in this"year's analysis and test, that the application of the finiteelement method to this problem is not a straightforward one. Itis, in fact, fraught with all of the difficulties that areinherent in attempting to model a real event with real materialsusing approximate methods. Sometimes the approximate methods arenot adequate to accurately model the physical event, or theapproximations that we make upon using those methods are notadequate to simulate a real event. That is the case in theanalysis work that has been done this year. Too little was known"about the inelastic or non-linear properties of the nylon thatwas used as obturator material. Too little was also known aboutthe amount of the nylon material that actually comes in contactwith the gun bore, and about whether or not the projectile enddisplacement that we thought we were measuring was in fact howmuch the projectile was displaced.

There are several things that can be done to alleviate mostof the inconsistencies that have been identified in this report.Of primary concern are the inelastic or non-linear nylonproperties. Any future finite element analysis will have to takeinto account those properties. This means that we will berequired to perform material characterization tests on the actualband material after we receive it from the supplier and beforewe attempt to model the projectile. Also of prime importance is

32

-.-~. : : . . - _ _ - •. -

to accurately measure the displacement that the projectile isseeing when it climbs over the ramp. Each projectile band wasmachined down to its final diameter by spinning the projectilewith band in a lathe. This should provide an outer band surfacewhich is as parallel to the centerline of the projectile aspossible. In any future testing, this will have to be a measuredquantity, and will have to be taken into account in the data.

33

_ -,

VIII. PLAN FOR FUTURE WORK

There are several questions which remain un-answered at theend of this phase of the project. Three of those questions canbe answered within the scope of the third and last phise of thework. First, in what way do the non-linear material propertiesof the nylon band have an effect upon the foundation moment.Second, and possibly most important, will the finite elementmethod be an effective tool for BRL to evaluate the foundationmoment for any particular projectile design. Third. what effectdoes changing the geometry of the band have on the foundationmoment. The first two years of this project have concentrated onbasically one band geometry. This geometry is a fairly simpleone to analyze, and to construct, and as such has been usedextensively. It is not, however, entirely characteristic of thecommon band designs used on the 120mm KE rounds.

The non-linear or inelastic properties of nylon at the strainrates and loads that exist in a gun bore are not well known, ashas been stated previously. One of the tasks in the third phaseof the project would be, then, to design and run materialcharacterization tests that will give us the correct propertiesto use in our finite element analysis. The nylon is essentiallyunder hydrostatic compression in the gun during firing. Thecocking of the projectile in bore adds another additional load tothe nylon which must be accounted for in our analysis. Thus,wewill need properties of the nylon which will describe theresponse of the material to those loads.

Once material characterization tests have been performed, afinite element model of the projectile being tested would have tobe constructed. This model would have to use a mesh which ismuch finer in element size than the mesh used for this year's

* work, and would have to take into account the inelasticproperties of the nylon band material.

When the finite element results finally match the testresults, and we are confident in the method, at least onedifferent band geometry would be tested and analyzed. lhis bandgeometry would be more characteristic of the geometry of thecurrent 120mm KE projectile. This would provide evidence of howthe foundation moment is affected by band geometry and allow BRLto make rational choices for that geometry in their designs.

34

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