connie hayes et al- the effects of fragment ricochet on munition lethality

8/3/2019 Connie Hayes et al- The Effects of Fragment Ricochet on Munition Lethality

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~AFATL-TR-75-102 .

I THE EFFECTS OF FRAGMENT RICOCHETON MUNITION LETHALITY

BOOZ, ALLEN & HAMILTON, INC.P. 0. BO 874SHALIMAR, FLORIDA 32579

%WWI DDCJULY 1975 DEC 15 WS-- F"} ~itFINAL REPORT: JANUARY 1975 - JUNE 1975

Distribution unlimited; approved for public release. ' :!.

AIR FORCE ARMAMENT LABORATORY-#,I, FOICE SYSTIMS COMMAND * UNITED STATES AIR IF O ICEI EGLIN All FORCE BASS, FLORIDA


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7. AUTHOTHACAN~6 PEATION --I- ADRS

BzAllenR 0. CONTlonc L REACTORI GRN HUBt~s4 haima1reeeFlorida 32579f5

#-ONROLKING OFF0d-AICEIONAME ANiD ADDRESS 10 RGAMEEET. ~2L1;jAAir Forcen rmamntLabrto rync.___________Egi i oreBsFlorida 325 42 39U?+

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Distribution unlimited; approvecd ECr public release.-

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Available in DDC.

19. KEY !NCRDS Continue on reverse side if ecesirary and identity by block number)Munition LethalityFragentRicochet Effects* Degradation Effects ProgramEAGLE Computer Program

20- AD ACT (Continue on reverse side If necessaoy and Identify by blockc number)-QThis report presents the results of a contractual effort to97- determine the effects of fragment ricochet on munition lethality intactical environments and to develop modifications to the lethalarea programs to account for fragment ricochet effects. TheDeraato Efet1-c-a ( )oeainl aawsaayeand used as the basis for the ricochet computer model.. Theriocheoeldeveloped wsincorporated into the computer porm

DDIJAN73173 EDITION OF INOV 65 S OBSOLETE UNCLASSIFIED/ . ~ U CL SSI FICAT ION OF THIS PAGE (".hnAArftlI~t7 /10,

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UNCLASSIFIEDECURITY CLASSIFICATION OF THIS PAGE(whon Date Entuerd)EAGLE, which was previously designed to predict the lethal area ofstatic DEP tests. The modified EAGLE computer program and itspredictions for the BLU-3/B are presented in this report. Thesepredictions show a correlation between experimental and predictedresults.-

UNCLASSIFIEDSECURITY CLASSIFICATION OF THIS PAGE(Ie.n DWet in!*?*0)


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to determine the effects of fragment ricochet on munitionin tactical environments and to develop modifica-to the lethal area progoams to account fo r fragmentricochet effects. The Degradation Effects Program (DEP)

operational data was analyzed and used as the basis fo r thecomputer model The ricochet model developed wasincorporated into the computer program, EAGLE, which waspreviously designed to predict the lethal area of static DEPThe modified EAGLE computer program and its predic--" tions for the BLU-3/B are presented in this report. Thesepredictions show a correlation between experimental and pre-

dicted results.

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(The reverse side of this page is [,lank)gIN


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PREFACE

This report documents the results of a study to deter-mine the effects of fragment ricochet on munition lethality.The study was performed during the period 10 January 1975through 30 June 1975 by Booz, 7llen, & Hamilton, Inc., P.O.Box 874, Shalimar, Florida, u-der Contract Number F08635-75-C-0055 with the Air Force Armament Laboratory, ArmamentDevelopment and Tent Center, Eglin Air Force Base, Florida.The progranm monitcr for the Armament Laboratory was Mr. JohnA. Collins (DLYV).

This technical report has been reviewed and is approvedfor publication.

IIt FOR THE COMMANDER:40C. COMPT 4Acting Chief, Weapon Systems Analysis Division

3(The reverse side of this page is blank)


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:1TABLE OF CONTENTSSectiLon Page

I. INTRODUCTION .. .................................. 7AII. DATA ANALYSIS. .. ................................ 8

.BLtJ-3/B. .. ............. .. ................ 11BL'u-26/B .. ................................. 23MK82 Bomb. ................................. 232.75-Inch Rocket. .. ....................... 23

III. PROPOSED CHANGES TO COMPUTER MODEL. .. ......... 24Other Ricochet Models .. ................... 31Results. ................................. 32

IV. CONCLUSIONS.. ................................... 36REFERENCES. .. ....................... . ...... 37

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LIST OF FIGURES

Figure Title Page1. Target Array fo r Degradation Effects

Program Static Tests ........ .... . . 92. Raw Data Form ............ ................ 103. Elevated Target Bundles With RicochetScreens fo r BLU-3/B Tests in OpenTerrain .... .......... ................... 134. Flow Diagram of the Ricochet Model inEAGLE's Main Program...... .............. ... 275. Flow Diagram of Subroutine PROB ............ 30

LIST OF TABLES

Table Title Page1. BLU-3/B Test Description ..... ........... ... 122. Summary of Ricochet Selection Criteria ....... 153. Summary of Ricochet Predictive Method

Retest Work in Open Terrain .... .......... ... 174. Comparison of BLU-3/B Ricochet Effectsin Various Environments ..... ........... ... 195. Environmental Lethality Comparisons ........ ... 216. Comparison of BLU-3/B Lethal AreaPredictions .......... .................. ... 33

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P SECTION IINTRODUCTION

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The Joint Chiefs of Staff recognized the need fo r opera-tional data based on munition perflormance rather than ontheoretical data. The Degradation Effects Program (DEP) wasset up to assess the influence of certain environments on theeffectiveness of munitions and weapons. Although the main- ieffort was to evaluate e;-.isting munitions, the objectives in-cluded developing the methodology and collecting data of asufficiently general nature to aid in the evaluation of newitems for future programs. DEP was essentially ended in 1972due to fu.Jing limitations. Much of the methodology for pre-dicting munition performance, particularly that dealing withfuze functioning, fragment ricochet, and fragment burial, wasleft in an incomplete state (Reference 1). The one area

vhiere major improvement could be made to the methodology with-in the constraints of limited resources is fragment ricocheteffects.The objective of this study was to determine the effectsof fragment -icochet on munition lethality. The raw testdata from the static arena tests conducted on the Air Forcemunitions were used as the basis fo r all analysis and modeling.The fragments penetrating the stationary targets were studiedand the actual fragment encounters which were due to ricochetwere identified. The effect of ricochet on lethal area wasstudied using the DEP experimental lethal area model. The DEP"predictive lethal area-model was modified to include the pre-dictive capability of fragment ricochet effects on lethal area.The frequency of occurrence of ricochet fo r the BLU-3/Bmunition in a number of operational environments is presented.The effect of ricochet on lethality computations and thesignificance of the effect are discussed.

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T.

SECTION IIDATA ANALYSIS

The raw data supplied by the Air Force Armament Laboratoryat Eglin Air Force Base, Florida, was a portion of the massive"-amount of operational test data generated by the DEP program.The program encompassed the dynamic, static, and fuze testingof 34 key munitions in widely varied environments. Of thesetest situations, only the static tests fo r the Air Force muni-tions were subjected to analysis. The Air Force munitions testdata were used to determine the frequency of occurrence ofricochet. All munitions were centered in a target array formedby standing 5- by 1- by 1-foot bundles in concentric rings(see Figure 1). The target bu.ndles were arranged eight to aring, with no target bundles providing shielding to others.The various tests were conducted with a large variation in theheight of burst. This included testing some munitions partial-ly buried. The weapon elevation angle was measured from thevertical axis with the weapon oriented nose down. Reference 2provides a detailed description of individual weapon test

The fragment path was described by the entrance positionon the front face of the bundle and the location of the frag-ment when the bundle was disassembled. In several cases, thefragment was lost, or it had penetrated the bundle and wasnoted as such. All measurements were made with respect to theforward, lower left-hand corner of the individual bundle. Aright-handed coordinate system was used with the X-axis hori-zontal to the face of the bundle and the Y-axis vertical tothe bundle.

The munition was described by the height of burst, ele-vation angle, and orientation. The radius of each of the tar-get bundle rings, the tilt and orientation of the bundles, andthe elevation of the individual target bundles was supplied todescribe the test arenas in the various environments.A typical form of data generated by static testing isshown in Figure 2 and is identified from left to right as:the munition shot number (8), the munition/environment identi-fication (232C), the target ring, the target bundle spiral,the X-, Y-, and Z-coordinates of the entry point in iriches,the X-, Y-, and Z-coordinates of the exit point in inches, andthe fragment weight in grams.411 8

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-11? 1 LINF '3? FGLIN TEST DATA APPENDIX C SHOT 88 232C I A 8.0022.25 0.,1 7.7523.00 7.,l0 1.0168 232C 1 A ?.2516.00 0.100 1.5016.5011.00 1.01205 232C i A 3.9029,5 n0o0 ?.563"1.5011.00 1.0?18 232C 3 A o.755?.50 0.00 6.0052*.50 3.001.02?8 232C t 13 5.026.25 0.110 7.0027.75 6.50 1.0166i 2?32C I E 7.7526.50, q.O9l 8.P027.00 .50 1.0158 232C t F 5.2523.90 0.00 5.25?5700 9.50 1.t19S 232C 1 E 3.5076.25 0.O0 3o5030.5012.00 1.0378 232C 2 E 7.7926.50 0.0010.0028.50 9.50r 1.0148 232C 5 F 4.5042.25 0.00 4.5043.50 5.50 1.0108 232C S E to.0051.00 0.0010.2553.0012.On 1.O138 232C E 3 5053.r,9 0.0n 3.2 554.50 5. 0 1.0153 232C I F 71q.sg 0.00 1.73? to011.00 1.018;232C 8 F 1.0038.715 0,00 1.0038,75 6,5n 1.0123 232C I G 4.7544.50 0.00 5.5043.S0 8.50 1.0378 232C 7 H 5.5f39,50 0,01 6.0010n.50 8.50 1.020

Figure 2. Raw Data Form

A FORTRAN program was written to compute the apparentmunition location for each fragment path and to select thefragments designated as ricochet by the various criteria.Data error was found to increase and sample size to decreasewith respect to increasing distance from the munition detona-tion point. The measurement error was the same for everybundle, but the error in projecting the fragment path back tothe munition location increased with distance. This circum-stance led to the elimination of data from all but the fourinner rings. The fragment path is traced back to its apparentorigin using a straight-line formula. The omission of allouter ring data leaves the remaining data with sufficientlyshort trajectories that ballistic effect can be assumed to benegligible. The mean and standard deviations of the apparentfragment origin are calculated fo r each ring, and all pointsincluded within one standard deviation of the mean on the Y-axis are used to compute a second set of means and standard. deviations. This procedure produced a more accurate estimateof the munition height.

The DEP fragment data format was designed for input tothe lethal area programs and any other use of the data re-quired special and tedious data handling procedures fc r opera-tion. To enable the data to be used for Various analyticalprocedures, it had to be t.'ansformed into a more flexibledesign.

The fragment information was stored in computer-basedfiles. The computer-based storage and retrieval was conductedusing the MARS VI (Multi-Access Retrieval System) data manage-ment system on the Control Data Corporation 6600 computeroperated by the Armament Development and Test Center at Eglin

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Air Force Base, Florida. The fragment data was organized topermit efficient data management, subsetting, sorting, and re-tr ieval by considering the various data requirements. Thepertinent fragment categories used to design the data bascwere:

* Fragment shot number. Target ring number.0 Target bundle identif ier .0 Fragment entrance X-coordinate.S Fragment entrance Y-coordinate.* Fragment entrance Z-coordinate.* Fragment terminal X-coordinate.* Fragment terminal Y-coordinate.

data Fragment teiminal Z-coordinate.S Fragment mass.The following paragraphs describe in detail the individualdata analysis of the various munitions studied.

S~ BLU-3/BIThe BLU-3/B is a munition composed of preformed spherical

fragments. Because of the homogeneity of th e fragment shapeand size and the relatively small range of inital fragmentvelocities (Reference 3), the BLU-3/B was selected fo r thor-ough analysis for ricochet effects. Table 1 is a summary cfth". arena and environment conditions fo r th e individual tests:further detail is provided in Reference 2.

Initial testing of th e BLU-3/B in -,en terrain indicateda higher fragment density reaching th e target bundles thanpredicted by the Joint Munitions Effectiveness Manual (JMEM)weapons characteristics description. It was thought that thiscould possibly be th e result of ricochet. A retest in theopen terrain arena was performed. Five of the shots of theretest were conducted with the munition and target bundlesraised 2 feet and ricochet stops installed (Reference 2) asshown in Figure 3.


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TABLE 1. BLU-3/B TEST DESCRIPTIONHeight of Munition

Test Number Burst Elevation AngleNumber Terrain of Shots (Feet) (Degrees)225/B High 5 0 30Canopy 5 0 605 15 30

5 15 605 25 305 25 60226/B Dense 5 0 30

Tangle 5 0 60S5 10 305 10 60

227/B Grass 5 1 305 1 60232/Ca Open, 5 0 30Eglin Sand 5 0 65

bI232/Cb Open, 5 0 30Eglin Sand 5 0 605c 2 60

233/C Temperate 5 0 30Forest 5 0 65

5 25 305 25 656 50 306 50 65235/C Water 5 0 30

5 0 60a Original tests on Eglin sand.bRetest on Eglin sand; double target bundles.c Retest on Eglin sand; double target bundles; ricochet

stops.

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Three main criteria were used to assess the occurrence ofricochet. The first criterion used was depth of penetration.All fragments with a computed height of burst below the arenafloor and a penetration of less than 7.5 inches into the tar-3et bundle were selected. This penetration depth correspondsto a minimum expected velocity of approximately 2,000 feet persecond (Reference 4). The second criterion selected all frag-ments with a height of burst computed to be less than onestandard deviation below the mean on the Y-axis. The thirdcriterion compared the apparent angle of incidence (vi) andthe angle of ricochet (Or) for each fragment. All fragmentswith an apparent ricochet angle less than or equal to theangle of incidence were picked as ricochet fragments. Thepoint of impact on the arena floor was computed from the frag-ment entry and exit points on the target bundle. The heightof the munition was assumed to be the mean plus one standarddeviation. This defined height of burst compensates fo r 67percent of the error in the fragment data, and thus makes themunition height a function of the error inherent in the data.

The results of the use of these criteria as ricochetselection models is summarized in Table 2 for three of thetest environments. All of the above criteria were consoli-dated fo r the fourth ricochet criterion. The results shownin Table 2 indicate a negligible enhancement due to ricochet.The fifth consideration combined the first and third ricochetcriteria. This ricochet medel has the advantage, unlike theprevious method, of not omitting a common form of ricochet,a fragm-nrt ricocheting with a very low angle into the targetbundle. The results show an increased enhancement from rico-chet in most cases.origination points for the individual fragment pathswere plotted in the X-Y plane at Z = 0, the munition detona-

tion point. With the weapon detonating on the ground, theerror in calibrating the fragment path at the target bundlesresultedn the location of a significant number of fragmentorigination points below the ground. This is further evi-dance that there could exist some low angle ricochet which isunaccounted fo r when using the second ricochet model. Rico-chet test results over Eglin sand arenas (summarized inleference 5) show that fragments have a 50-percent probabilityof ricocheting when the angle of incidence is less than 17degrees.

The third ricochet model defined an overly large numberof fragments as ricocheting fragments. It was decided thatthere were too many unknown factors concerning the arena sur-face to alwaya rely on the ricochet angle criteria.14


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On e additional method of screening fo r lethality enhance-ment due to ricochet is to compare test results with the pre-dicted number of hits given in a weapon characteristics de-scription of the munition. Although this information isapplicable to an open terrain environment, it can serve as agauge of the number of expected nonricocheting hits in otherenvironments.The purpose of the above analysis was to determine whichform of ricochet model would be suitable fo r use with the DEPstudy data. The results, as presented in Table 2, indicatedthat the depth of penetration model would be the most applica-ble.On e area of concern was thE omission of individual targetbundle elevations on some test da-i. To determine whether tar-get bundle height could be ignored in our test analysis, theopen terrain retest study was analyzed both with and withoutthe elevation heights included. The results of this compari-son are included in Table 2. In most cases, the same numberof ricocheting fragmeiits were predicted whether or not thetarget elevation data were included. The plots of the innertarget rings of both yielded almost identical graphs. It wasconcluded that for relatively flat test arenas the omission ofthe individual target elevations would not significantlyaffect analysis when the inner ring fragment data were used.Ricochet tests conducted by Cornell Aeronautical Labora-tory (Reference 6) were referred to in order to determinepossible ricochet behavior. Test results indicate that frag-ment velocity reduction due to ricochet is not as large asthat assumed by this study's initial analysis, as presented

in Table 2. Therefore, the cut-off depth of penetrationcriterion was raised to 9.0 inches. This value roughly cor-responds to a velocity of 2,400 feet per second (Reference 4).Compared to the top sp .ads that BLU-3/B fragments are capableof attaining, this can be considered a conservative estimate.The revised model was exercised using the open terrain retestdata. The results of this analysis were separated accordingto the different test conditions. A summary of the resultsis given in Table 3. Shot numbers 1 through 5 were with thearena set up with the munition elevated at a 30-degree angle.The munition was elevated at a 60-degree angle fo r shot num-bers 6 through 10. Shot numbers 11 through 15 had ricochetfences installed, and the munition had an elevation angle of60 degrees.One of the major differences observed was the marked re-ducticn in fragments collected for the test setup with ricochet

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fences as compared to similar tests without ricochet fences.The total fragment count went from 64 fragments collected with-out ricochet fences to 46 with ricochet fences-a decrease of28 percent. It was thought that this reduction in fragmentswas due to the omission of all ricocheting fragments by employ-ing the ricochet fences in the testing environment. However,when the ricochet criteria was applied to the retest data, theresults in Table 3 indicated a significant frequency of rico-chet occurrence with ricochet fences installed. A detailed in-vestigation indicated that the ricocheting fragments selectedwere fragments that had actually been lost or whose paths haddirected them close to the edge of the bundle, where trajectorycurvature within the bundle material becomes predominate. Th ericochet model was then applied to the retest data with allfragments that had been lost or that had penetrated the targetbundles omitted from the fragment data. The results of thisanalysis showed a major reduction in ricochet predicted forthe shots incorporating the ricochet stops, a decline from 41percent to 10 percent enhancement due to ricochet, while theSother test shots showed only mninor differences. This informa-tion substantiated two assumpticns. It proviCed proof that thereduction in fragment hits fo r the test incoiporating the rico-chet fences was due to the omission of ground ricocheting frag-ments. The inclusicn of the lost fragment data was shown notto noticeably affect the standard test results. In addition,it verified the conservative nature of the ground ricochetpredictions.

The ricc-het selection criterion used fo r all furtherBLU-3/B test analysis was a depth of penetration of less than9 inches and a computed origination point below ground level.The ricochet model was applied to all BLU-3/B test data.Table 4 is a coimiparison of the predicted ricochet effect forthe BLU-3/B in various environments. Because individual tar-get bundle heights were not supplied with the original datafo r test numbers 226, 227, and 233, the target elevation datawere omitted from all analysis. Since these arenas wererelatively flat, this omission did not affect the analysis,as suggested earlier.

Results indicate that ricochet effects are the most sig-nificant in a ground burst rituation. The only exceptions ob-served were fo r the temperate forest tests. For burst heightsabc-ve ground level, these tests produced very few fragments,.wb.-hould explain their unpredictable results. The differ-ence in enhanced fragment density due to ricochet in the vari-ous environments is primarily a function of the material andcondition of the arena floor.

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The computer program FOREST-III, developed at PicatinnyArsenal, was used to evaluate the DEP project static test re -sults in terms of lethal area. The original lethal area com-putations fo r the BLU-3/B and other Air Force munitions aregiven in Reference 7. FOREST-III was again used to computethe lethal area for the BLU-3/B munition with the input datamodified. Three runs were made fo r each test environment.

0 A run was made with all data intact like theoriginal analyses.* A run was made using direct hitting fragment datafragments (as previously defined)were omitted.o A run was made using ricochet hits only. Theequation fo r computing striking velocity (VS) fromdepth of penetration was changed as documented in

Reference 4. Equations (9) and (14) of Reference 4defined for s4pheres were:

Ie [321300 D(Ap)/M]-1VS = 4.25where a is a function of fragment size, D is thedepth of penetration in inches, Ap is the fragment'spresented area in square feet, and M is the mass inmilligrams.

Table 5 is a summary of the lethal area evaluations de-fined in this manner. In the cases where the munition detona-tion position was far off of the ground, the fragment samplesize was small and, consequently, any calculations with thisdata had a high degree of uncertainty. Results of the lethalarea evaluations showed that, with ricochet effects included,the average enhancement in lethal area was 25 percent. Thevariation is due to the different elevation angles of themunition, the height of burst, and the different environmentalconditions. The most significant results, as indicated inTable 5, show that ricochet is most predominant at low heightsof burst and can be expected to occur most frequently in anopen sand environment.

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BLU-26/B"The BLU-26/B is similar to the BLU-3/B in that it is amunition composed of preformed spherical fragments. However,the JMEM weapon characterization of it does not predict the

homogeneity of the fragment size like it did for the BLU-3/B.This may be due to fragment break-up and would therefore beS'I a function of environment. Consequently, a slightly largerrange of fragment velocities is predicted by the JMEM.

The major disadvantage in the analysis of the BLU-26/Bwas the lack of testing incorporating this munition. TheBLU-26/B was only tested in Type I snow (Reference 2). Theheight of burst used in the snow testing varied from surfacelevel to complete burial of the munition in snow. The frag-ments emitted from the buried munitions could not contributeto ricochet and only distorted the height of burst locationbecause of their tendency to follow nonlinear paths in thesnow. It was fo r these raasons that this munition was notconsidered for ricochet analysis.MK82 BOMB

The MK82, a 500-pound bomb, was tested in the highcanopy, dense tangle, open, temperate forest, and water en-vironments. The MK82 exhibited a wide range in fragmentsize, shape, and velocity. In addition, the bomb destroyedsome of the inner ring target bundles. Consequently, in manycases the most informative data on the MK82 fragment behaviorwas destroyed. This munition was not used for ricochet analy-sis.2.75-INCH ROCKET

The 2.75-inch rocket was tested in the high canopy, densetangle, grass, open, temperate forest, mud, and water arenaswith the M151 warhead, and in the high canopy, dense tangle,grass, open, and temperate forest environments with the MK-5warhead. Both warheads produced a very large variation infragment size and velocity. In all cases, the frequency dis-tribution of fragment weights and penetration capabilities wasbiased toward the smaller fragments. Differentiation amongthe direct fragments, broken-up fragments, and miscellaneousdebris in the large percentage of very small fragments was th emajor difficulty. In addition, the fragment shape was unknown.Because of the variability and deficiency of information, rico-chet analysis on the 2.75-inch rocket was not considered.

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SECTION IIIPROPOSED CHANGES TO COMPUTER MODEL

Munition lethality computations are generally performedusing the JMEM weapon characteristics data. These weaponcharacterizations are primarily based upon static fragmenta-tion tests conducted in a test arena designed to produce arayline descri-'ption of the fragment behavior without regardto any outside interference. It was illustrated in theprevious section that fragment ricochet contributes to amunition's lethality to various extents. It would be advan-tageous to be able to simulate fragment ricochet behavior incomputer lethality programs.

Fragment ricochet models describe in more detail theactual events that take place when a munition detonates in areal environment. Lethality computations from these modelsgive a more precise indication of the capability of variousmunitions in differing environments. To date, the Air Forcelethality computer programs do not have a ricochet model in-corporated into them. This is due to the complexity of thericochet parameters and the lack of knowledge of their rela-tions in differing environments.The predictive lethal area computer program (EAGLE) forstatically detonated munitions in a DEP style arena wasdeveloped by the Computer Programs and Evaluation Models (CPEM)Subgroup of the DEP Methodology and Evaluation Working Group(MEWG) (References 8 and 9). The EAGLE program assumes apoint fragment source at the center of th rena. The frag-ment density emanating from the point source and impinging

j upon a defined target is computed from the conical polar zonesdescribing the fragment distribution coming out from the pointsc-urce (an input to the program) and the distance of the tar-get from the point source. The target bundles, situated in aspiral fashion circumventing the point source, are describedas a six-point man and as a one-point man. The fragmentdensity directly striking the target bundles is defined ateach of the target points or at the single point. The densityI at the point is assumed to be applicable for the entire areaaround the point.

" ' The test arena environment is divided into layers whichdescribe the biomass density affecting fragment velocity. In

j addition, contributions to the biomass density by trees andlarge branches are considered to account for target shielding.24


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EAGLE limits its munition simulation to directly hittingS~fragments only. This computer model does not take into con-S~~sideration the effects of round-to-round variations on munitionS ~capability and fragment ricochet.

i ~In developing thle ricochet model, it was decided that theS~most convenient method for testing the effects of the ricochetSmodel was to incorporate it into the EAGLE program. SinceiEAGLE had been designed to compute the predicted lethality ofS~~a munition, insertion of a ricochet model would provide directi j comparison with published results (Reference 10) and wouldi" ~compensate fo r fragment ricochet in the munition lethality ~simulation.

i ~There are two major methods for developing a simulationS model. One method develops from the treatment of laboratory ~~testing data, which isnlates each individual phenomenon con-!i ~tributing to the resul,3. The second method makes use of em-: ~pirical data, allowing it to inherently express all variablesi without regard to what these variables actually may be. Be-cause of the availability of the data from the DEP program andS~the ricochet analysis applied to it by this study, the second i method was chosen.SWhen ricochet takes place, there are several occurrences:that have a major effect upon the fragment path. The frag-S.i ment's encounter with the ground is likely to cause fragmentS~burial or possibly fragment break-up. Upon ricocheting fromS~the ground, the fragment has a new velocity, angle of inci- ~~dence on a target, and weight. The DEP raw data was analyzedwith respect to these three parameters to develop a relation-S~~ship fo r each under each test setup,.

SSince the initial DEP ricochet analysis was performedS~primarily on the BLU-3/B, the BLU-3/B data was selected asSthe basis for the ricochet model. Only on rare occasions wasSthe BLU-3/B observed to break up. Thus, the assumption wasmade that the fragment weight remained constant throughoutany ricochet occurrence. This assumption reduces the ricochetS) ~parameters and simplifies the ricochet model considerably.

SThe re:;alts from the application of the ricochet criteria,S~degree to which ricochet reduced the fragment velocity in each ~environment. A critical ricochet angle was defined as theangle at which 50 percent of all ground hits would producericochet,.i

F2 EAL iisismnto imlto odrcl itn


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Analysis of the DEP data led to the development of abasic, heuristic ricochet model. Insertion of this mtodel intothe EAGLE computer program did not degraie the logic and com-putational capability already available in EAGLE. To insurecompatibility, the ricochet assumptions are similar to th eassumptions in the EAGLE computer program. The assumptionsimplied in th e ricochet model are:

0 Fragment weight and shape remain unchanged duringricochet.

* The fragment density impinging upon the center ofhe ground ricochet area directly in front of eachtarget bundle applies to the entire ground ricochetarea for that bundle.0 The probability of the fragments ricocheting fromi

the critical ricochet zone out of th e bundle path-line approximates the probability of th e fragmentsricocheting from outside th e critical ricochetzone into the bundle pathline.

* The ricocneting fragment travels through an environ-ment similar to the environment experienced by afragment directly hitting th e lowest point orn th etarget bundle.

"" The ricochet angle approximates th e ground inci-dence angle..Figures 4 and 5 are the flow diagrams of th e logic of thericochet model as it was incorporated into th e EAGLE computerprogram. The critical ground ricochet area directly in front

of each target bundle was computed. This area is bounded bythe line drawn between th e munition and each side of th ebundles on two sides and by the critical ricochet angle andthe base of the target bundle on the other two sides.

A representative fragment ricochet path is selected bytaking the average of th e critical ricochet angle and th eangle from th e munition to thp base of th e target. The pre-sented area of the critical ground ricochet area is then com-puted in th e plane normal to th e representative fragment rico-chet path.

The ricochet presented area is further reduced fo r thosecases where the fragment may ricochet over th e top of the tar-get bundle. The largest ricochet angle is assumed to be equalto the critical ricochet angle. This angle is projected onto

26i


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FROMEAGLEROGRAMl

PREP

EAGLE INPUT.INPu'T CRITICAL RICOCHET /\ANGLE. AN D RICOCHET VELOCIrY;DEGREDATION FACTOR /:

CONSIDER_ ATARGETRING

COMPUTECRITICAL RICOCHETI GROUND AREA

IN FRONT OFTARGE7 BUNDLE

IS- i SET INDICATOR,/ CRITICAL " NO TO BYPASS AL LO COMPUTATIONS

-. L= FOR THIS RING- YES ]

I COMPUTE ) MEANi- F CIICA ICOCHET

PA7H ANGLE

COMPUTEPRESENTED AREAOTCL RICOCHET.GROUND AREA

Figure 4. Flow Diagram of the Ricochet Model inEAGLE's Main Program

27

-~


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2i

-~DO ANYNO RICOCHET FRAGMENTSOVERSHOOT TARGET

RINGIYES

REDUCE RICOCHET PRESENTEDAREA TO EXCLUDEFRAGMENTS THAT OVERSHOT

RING

CONSIDER3 ATARGET BUNDLE

COMPUTE MUNITIONPOLAR ZONE ANGLESTRIKING MIDDI FOF

CRITICAL GROUND AREA

PROS

COMPUTE

RICOCHET HITSAND ADD TOEXPECI ED NUMiBERgOF DIRECT HITS

Figure 4. (Continued)

28


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5IsNO THIS L.AST3BUNDLEXE

NO


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I ENTERNE

SCOMPUTE'7EF RAGMENTS VELOCITY4TEDUCTION DUE TO /AR DRAG AND VEGETATION!

FRAGMENT DENSITYFOR0

DIRECT HITS

RICOCHETVELOCITYCOMPUTE

FRAGMENT DENSITY;a-DU E TO

RICOCHET

CRETUN

Figure 5. Flow Diagram of Subroutine PROB

30


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AA:1ithe target bundles in each ring. For those instances wherethe fragment projection overshoots the target, the ricochetpresented area is reduced until the reflection of the angleto the edge of the ricochet zone meets the top of the target.

The munition polar zone angle representative of theaverage ricochet path is computed as the arc cosine of thecross product of two vectors. The first vector is the ex-tension of the munition axis to the arena floor. The secondvector is the path of the fragment from the munition to themiddle of the ricochet zone in front of each target bundle.The resulting anqie is compared with the weapon descriptionfragment distribution to determine the polar zone responsiblefor contributing to ricochet for that particular target bundle.

The velocity degradation routines already available inthe EAGLE computer program are used fo r the ricochet velocitydegradation computation. The subprogram designed to computethe striking fragment density is entered for every point onthe six-point man target. At the time the fragment density iscomputed for the lowest point on the target, the ricochetdensity is computed. The velocity for the direct hitting frag-ments is obtained after they have undergone all velocity re-duction operations. For ricochet fragments, this value isthen multiplied by the velocity degradation factor to accountfo r the velocity loss due tc striking the ground and possiblefurrowing. The ricochet fragment density is computed in amanner similar to the computation of the direct fragment den-sity (Reference 9) using the ricochet velocity, the selectedricochet polar zone fragment number, and the solid angleproduced by the ricochet polar zone.

The number of ricochet hits for a particular target bundleis computed by multiplying the ricochet presented area timesthe ricochet fragment density. This value is added to the ex-pected number of direct hits to obtain a total expected numberof hits. EAGLE continues with its computation of lethal areagiven the total expected number of hits.OTHER RICOCHET MODELS

The Army Materiel Systems Analysis Agency (AMSAA)designed a ricochet model fo r use in the lethality computerprogram at AMSAA (Reference 5) . Unlike the ricochet modelpresented in this report, which was based on the DEP opera-tional data, the AMSAA model was based upon laboratory testdata designed to explore specific aspects of fragment

31


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- -W

ricochet. In addition, the AMSAA target representation has norelation to the DEP style arenas referred to in this report.Towards the end of the project, it was discovered that the

ricochet model had been incorporated into the EAGLE computerprogram by Picatinny Arsenal, Dover, New Jersey. A comparisonof the two models has not been made.RESULTS

Two computer models were used to determine the effective-of the ricochet model design. These were the DEP pre-dictive computer program EAGLE, and EAGLE modified to in-corporate the ricochet model as outlined above.The input to these two computer models was prepared to

approximate the conditions in the actual test environments.The munition elevation angle and height of burst were variedin an attempt to verify the placement of the munition in thevarious test environments. The only environments evaluatedwere high canopy, dense tangle, and open test arenas since theEAGLE input data to other test environments were not available.The input information to the two computer programs was similarto that used in the original analysis (Reference 10); however,the intent was not to reproduce the lethality figures inReference 10.Table 6 s a summary of the results produced from thesetwo computer models. The minimum and maximum lethal areas

computed were a function of munition elevation angle andheight of burst. The nonricochet results indicate that inall environments the highest lethality effects are preaictedfor ground level detonations. At low munition altitudes,munitions with a 60-degree elevation angle were the mostlethal and, at high altitudes, the 30-degree elevation angleis the predominately lethal one, as shown with the high canopy"data. The addition of the ricochet model did not affect theserelationships. The ricochet model produced greater lethalarea enhancement for the munition oriented at an angle of 30degrees.

The sandy surface of the open test arena seemed to pro-vide a better r2uochet medium than the earthen surface of thehigh canopy and dense tangle environments. The relationshipbetween the cohesive nature of soils and the critical ricochetangle, as detailed in Reference 5, was the srurce of th ecritical ricochet angle selection. Ricochet was permitted

32


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r- -rz

ou a'.c 0 -4 0'T ka c4~? LA C~h-4 -4 - N -

0 vj m 4 C% en r - m 'n 0

1-4 4 0 ~ ~ - '

u 00 en~ % co 0 m - -4

o CI3 r- 0 en D-4 -4 -4 C-

-441 I' -1, -- 0 '3 v' 0 co LAfn' r- -4 co 0 D co

&40CO- r 0) en LA '3 r- -4 LMW r, ' NO co CO 0. LA ..> 14

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9)4


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below an incidence angle of 17 degrees in the open terrain andbelow an incidence angle of 14 degrees in the high canopy anddense tangle. The actual composition of the soil in the lat-te r two environments was not known. This fact had littleeffect on the results since there is little difference be-tween the various ricochet angles. An analysis of the pene-tration characteristics indicated that the fragments selectedas ricochet hits in the open environment had higher velocitymeasurements than did the ricochet hits in either of the othertwo environments tested. This information was used to describethe effects of the different soils on ricochet. In fact, thevelocity vectors of the high canopy and dense tangle weresimilar, indicating similarity in soil content.The predictive lethal area results of Table 6 were com-pared with the computed lethality in Table 5. The predicted

results indicate higher values than expected from the com-puted lethalities in the environments where there was densevegetation. Consequently, an addition to lethality from rico-chet only compounds the problem. This cannot be considered asan accurate gauge of the ricochet model's credibility. Th eproblem fo r these environments must lie in the original dataor in EAGLE's representation of these environments. Reference10 documents severel of the suspected problem areas. Inparticular, the inability to properly represent the vegetativeenvironment and its effect on fragment trajectories could havestrongly affected the lethality results.

Comparison of the predictive lethalities in open ter inwith the computed lethalities of the retest work in open ter-rain indicates a closer agreement with the EAGLE computer pro-gram without the ricochet model. However, the data that wasobtained from a test arena with ricochet fences, a test con-dition that is closer to the EAGLE definition, falls far belowEAGLE's prediction.The original test data obtained from open terrain were

those that were initially suspected of incorporating ricochetfragments. In fact, fo r both the 30-degree and 60-degreemunition elevation situations, there is a direct correlationbetween the lethal area computed using only the selected non-ricocheting fragments and the EAGLE prcoram fo r the non-ricochet model. Furthermore, the computed lethal area usingall of the collected fragments falls within the range of re-sults derived from the predictive EAGLE computer program withthe ricochet model. Without the ricochet model, the predic-tive lethality would have been fa r below the computed values.

34


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In this last case, where environmental circumstances didnot present a description problem fo r the basic predictivecomputer model, the ricochet model was proven to be an accu-rate description of the munition lethality.The ricochet model is responsible fo r a slightly smallerand less divergent enhancement to the predictive lethal areathan the enhancement contributed by the application of thericochet criteria on the raw data. Thus, the ricochet modelfalls within a conservative description of ricochet effects.It is advantageous to have a conservative model until thephenomenon of ricochet can be more exactly defined.


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SECTION IVCONCLUSIONS

The in-depth analysis of selected munition tests fromthe DEP data clearly showed the presence of ricochet and pro-vided a sufficient basis to establish a ricochet criterionfrom which a predictive ricochet model could be derived. Thismodel incorporated in the EAGLE computer progrrm provided thecapability of producing comparative munition lethality data,which are presented in this report.

The ricochet model is an empirical model with capabili-ties and limitations similar to those implied in the DEP data.The effectiveness of this model can only be judged by directcomparison to observed test results. The application of cer-tain assumptions may preclude the use of this model with othermunitions. However, this study, as well as others, has shownthe effect of ricochet on munition lethality.

3c

-,,-_-..


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H REFERENCES1. Eusanio, L.A., Review of the Degradation Effects Program, AFATL-TR-74-124,Calspan Corporation for Air Force Amtme.nt ratory, Egin Air Force

Base, Florida, August 1974, Unclassified.2. Stone, J.W., and R.W. Conlan, Air Force Testing for the Degradation

Effects Pro~rm, Activities SuLary, 1966 to , 1969, AITC-TR-70-l98,SAMMent Development an Test Center, glin Air Force Base, Florida,December 1970, Unclassified.3. The title of this reference is available to qualified agencies uponrequest to AFATL (DLYV), Eglin Air Force Base, Florida 32542.4. Collins, J.A., Fiberboard Calibration for Determination of FragmentVelocities, AFATL-TR-73-193, Air Force Armament Laboratory, WeaponsEffect vision, Eglin Air Force Base, Florida, September 1973,Unclassified.5. The title of this reference is available to qualified agencies uponrequest to AFATL (DLYV), Eglin Air Force Base, Florida 32542.6. Lewandowski, G.M., Ricochet of Unstibilized and Stabilized ProjectilesOff Various Surfaces, CA L Report No. GM-Z338-G-5, Cornell AeronauticalLaboratory, Inc, Buffalo, New York, November 1970, Unclassified.f7. egdtion Effects Program, Analysis of Experimental Static Arra Datafor selected l Mnitions -n Various Uiviromnents, Report No. Z, AFATL-R

68-434, Air Force Armment Laboratory, Eglin Air Force Base, Florida,December 1968, Unclassified.8. A General Description of the Computer Prog= Used fo r the Analysisof LEP Static and l_,iamic Array TeSts, 61-J'fLCM-71-3, Methodology andEF0auation Workang Group Report No. 1, 1 February 1971, Unclassified.9. Th e title of this reference is available to qualified agencies uponrequest to AFATL (DLYV), Eglin Air Force Base, Florida 32542.10 . The title of this reference is available te qualified agencies uponrequest to AFATL (DLYV), Eglin Air Force Base, Florida 32542.MI

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tNI i IAL IISTRIBUTION-IIq USAF/\OO 1 USA Combat Dev Comd/SCS,N 1IIq USAI-iXOX l USA l-ngineer School Lil:r',ry 2

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