•i AD-785 520

AN IMPROVED PROJECTILE BOATTAIL

Anders S. Platou

Ballistic Research Laboratories

Prepared for:

Army Materiel Command

July 1974

DISTRIBUTED BY:

NatUrn Tochucal Infonua•w ;m-iceU. S. DEPARTMENT OF COMMERCE5285 Port Royal Road, Springfield Va. 22151

SECURITY CLASSIICA TION O F T MIS PA49 M bal Wt.6* SI " 11IT AX

REPORT DOWMENTATMO PAGE UA=h5T1C PONSBLMmrnuReotNo. 2395 11)Ef' 1 1

AL T1L[m~~6d S . Type OF REPORT A PERIOD COVERED

AN IMPRVED PROJECTILE BOATTAIL Fin&lS. PElrPoRtNNo One. REPORT NUMBER

7. ZT~te 41 COHTRxACT ORi*ANT MUMMER~.

Anders S. Platou

1.PERFORMING OGNZTNNAEAMC~S NUNS1OEC TAM

Ui. S. Army Ballistic Research Laboratories*Aberdeen Proving Ground, Maryland 21005 RlYT&E 1T161102A33D

11. CONTROLLING OFFICE MAUE AMD ADDRESS 12. REPORT DATE

July 19745001 Eisenhower Avenue 13. MNSUMR OF PAGES

I O~ NlU A N S A2 I Wlff 6..N* 81.4 O-u.*M 015..) 15. SECURITY CLASS. (of CU. ,.PWe)

UNCLASSIFPIED

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-S. suppL911ENTARY NOTES

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SB. KEY WORDS (COAME WI Fewer shia of aseaa md idmU by blaknu mb,,)Projectiles b., i IUC

Aeroballistics NATIONAL TECHNICALMagnus INFORMATION SERVICIF

Bodies of Revolution US Op-gield~ ,,A C'""151G Mosc ic StabilityWI ANTRACT (COWNW0 00.. 4098001 N a...NWWO -Uf5Uby bleek ) 2 /ca)

A series of projectile beattajis have show~n improved aerodynamic performanceoved ane itanoved gyconica satabili. Their Moagais havd dqalpn onrloteristiagover the itpndaed gyoconica soatabilt. Theie boattnus have eqal ohraloverisdragappear to be satisfactory so that the projectile should be dynamically stable.Also, these boattails increase the projectile wheel base considerably, therebydecreasing the balloting in the guna tube. The improved aerodynamic performancecould lead to longer reoges, longer projectiles or lover spin rates for future

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TABLE OF CONTENTS

Page

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

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

II. TEST FAC.tITIES ................. ....................... 8

III. WIND TUNNEL MODELS .................. .................... 8

IV. RANGE MODELS ................ ......................... 9 .

V. RESULTS ....................... .......................... 9

VI. THE PROJECTILE NOSE ........... ..................... ... 12

VII. CONCLUSIONS ............. ....................... ..... 13

REFERENCES .............. ......................... ... 35

LIST OF SYMBOLS ............... .................... ... 36

DISTRIBUTION LIST ..................................... . 39

A

Precediln page blank

31

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LIST OF ILLUSTRATIONS

Figure Page

1. The Cylindrical and Conical Boattail ...... ............ 14

2. The Square and Triangular Boattail ...... .......... ... 15

3. The Added Lifting Surfaces ...... ................. ... 16

4. A Canted Boattail ....... ......... ................... 17

S. The Cruciform Wedge Boattail Extended to Zero Base Area 18

6. The Cruciform Wedge Boattail Configuration Used for theWind Tunnel Model . ......... ......... . ,9

7. Drag of the S Caliber Army-Navy Spinner Rocket with VariousBoattails Obtained in the BRL Aerodynamics Range ..... 20

8. Drag of the S Caliber Army-Navy Spinner Rocket with VariousBoattails Obtained in the BRL Supersonic Wind TunnelNo. 1 ..... ......... ......... ......... ......... ... 21

9. Free Flight Base Pressure Data from Reference 4 ........ .. 22

10- The Norma' Force on the Army-Navy Spinner Rocket withVarious Boattails ..... ..... .................... ... 23

11. The Pitching Moment on the Army-Navy Spinner Rocket withVarious Boattails ....... ... .................... ... 24

12. The Center of Pressure on the Army-Navy Spinner Rocket withVarious Boattails ....... ..... .................. ... 25

13. Crossectional Areas of 7* Boattails ........... ........ 26

14. The Magnus Force on the Army-Navy Spinner Rocket withVarious boattails ....... ......... ....... ........... 27

IS. The Magnus Moment and Conter of Pressure on the Army-NavySpinner Rocket with Various Boattails . ......... 28

16. The Magnius Force and Moment Comparison of the Straight andCanted Boattails, M a .94, Pd/V - .36 .... .......... ... 29

17. The Damping in Pitch of the Army-Navy Spinner Rocket withVarious Boattails ....... ..... .................. ... 30

Preceding Paog blank

S. .. .. . . ,

LIST OF ILLUSTRATIONS (continued)

Figure Page

18. The SRC Projectile With and Without Bore Riders ...... 31

19, The Straight Triangular Nose ...... .............. ... 32

20. The Crossectional Area Distribution of the SRC andTriangular Noses ........ ... 33

21. The Canted Triangular Nose with a Canted TriangularBoattail. .............. ......... ......... ....... 34

6

-PIRM

k

I. INTRODUCTION

4 The purpose of a projectile boattail is to reduce drag, thereby in-creasing the range of th• projectile. The standard conical boattailsreduce the drag, compared to the cylindrical tail* (Figure 1); how-ever the conical boattail creates a "negative lift" which reduces the

4 projectile gyroscopic stability. The conical boattail also generateslarge Magnus moments at transonic speeds which can decrease the dynamicstability of the projectile. Gyroscopic and dynamic stability must be-raintained in order that the average angle of attack decreases as the

* projectile moves along its trajectory.

Recently it has been found that drag reductions can be attainedwithout creating the "negative lift" and reducing the gyroscopic sta-bility by using a new type of boattail. These boattails are formed bycutting the main projectile cylinder with planes inclined to the maincylinder axis by a shallow angle, such that fin like surfaces arecreated on the boattail. These boattails all have elements of themain projectile cylinder extending to the base and also have a muchbetter crossectional area-reduction gradient than the conical boattails.It is believed that these physical propert.es of the new boattails re-duce the viscous region in the vicinity of the boattail and improveits aerodynamic performance. Possible versions of these boattails are:

(1) A boattail formed using four cutting planes so that the basebecomes an inscribed square (Figure 2).

(2) A boattail formed using three cutting planes so that the basebecomes an inscribed triangle (Figure 2).

(3) Boattails formed similar to (1) or (21 but with the cuttingplane widths limited so that added lifting surfces are formed at thebase corners (Figure 3).

(4) Boattails formed similar to (1), (2), and (3) but with cuttingplanes canted at the gun twist angle (Figure 4).

(5) A boattail formed by eliminating all of the main body cylindervolume not included inside the volume of two orthogonal wedges (Figure5). This version can be extended to zero base area cr can be cut offat any station to form a cruciform base (Figure 6). The boattail lengthproviding minimum drag will depend on the location of flow separationand can only be determined experimentally.

*Previously known as square base configuration but changed here to

avoid confusion with the new veiion (1) boattail.

Patents applied for J .une 1972 --md 8 October Z973 for the boattaiZahapee mentioned in this report.

7

.I

All of these new boattails were originally conceived to minimizethe increase in Magnus forces and moments at transonic speeds occurringon the conical boattails. It was believed that the cylinder elementsextending to the base will form crude fins which would have Magnusforces acting opposite to those on the bodyl. Thus, the opposing forceswould minimize the resultant Magnus force and possibly the Magnus moment.Aerodynamic tests on these configurations were deemed advisable in orderto verify these beliefs and also to choose the configuration giving the

* best overall aerodynamic performance.

Aerodynamic tests on these boattails are being run over the trisonicMach number range, M = .5 to 4.0, to evaluate the aerodynamic forces andmoments acting on the projectile. Range and wind tunnel tests are beingused to complement each other so that good, reliable aerodynamic infor-

* mation will be available.

Also a triangular nose formed similar to the triangular boattail,but continued to a point,will be tried. The nose will be canted at thegu,. twist angle to prevent loss of spin and may provide a low drag withvery good wheel bearing.

II. TEST FACILITIES

The wind tunnel facilities used for the tests so far are:

(1) The NASA Ames Research Center 12 ft. subsonic wind tunnel,M = .5, .7 and .9, 4-1/4" model, Re/ft = 1.35 to 2.8 x 106, (Re/m r 4.23to 9.2 x 106).

(2) The Naval Ship Research and Development Center (NSRDC) 7 ft. x10 ft. transonic wind tunnel, M = .5, .7, .9, .94 and .98, 4-1/4" model,Re/ft = 2.65 to 4.0 x 106 (Re/m = 8.69 to 13.1 x 106).

(3) The Ballistic Research Laboratories (BRL) 1 ft. supersonicwind tunnel, M = 1.75 to 4.0, 2-1/4" model, Re/ft = 3.6 to 7.0 x 106(Re/m = 11.8 to 23.0 x 106).

III. WIND TUNNEL MODELS

These boattails are being tested using the Army-Navy Spinner Rocketnose and body with the complete configuration being both 5 and 7 cali-bers long. Later tests of some of the boattails may be run using alower drag nose.

.. A. S. Pl-•cgt• "magna C.a..ac.te...ticz of -i"ned and JV•'nfinne- dProjectiles," AIAA Journal Vol. 3, No. 2, January 1965, pp. 83-90.

8

Two sizes of models, 2-1/4" (5.715 cm) and 4-1/4" (10.795 cm) diam-eters, are required for the wind tunnel tests because of the variationof the tunnel sizes available for the different speed ranges. Themodels are designed according to the specifications descyibed in refer-ence 2. They consist of a central body mounted on ball bearings and astrain gage balance with various tails and noses attached to the centralbody. Variations in the lengths of the nc3es and tails make it possibleto test body lengths of 5, 6 or 7 calibers. The strain gage balancesused are numbers SB228B or C, SB4-1C and SB4-5C.

Tails for each boattail version, listed previously, are made usinga 7* cutting plane angle for both the 2-1/4" and 4-1/4" diameter models.Also, a straight cylindrical tail and a I caliber long 70 conical boat-tail are available for reference data (Figure 1). Each boattail versioncan be tested on the 2-1/4" diameter body with configuration lengths of5, 6 or 7 calibers; however, the 5 caliber, 4-1/4" model is limited tothe straight cylinder, the conical boattail and the square boattail.Six and seven caliber, 4-1/4" diameter models of all the boattail ver-sions can be tested.

IV. RANGE MODELS

A few configurations have been fired in the BRL aerodynamics rangeat transonic and low supersonic speeds and additional firings areplanned for the near future at all Mach numbers from .8 to 4.0. Themodels are 20am diameter and made mainly from aluminum with longitudi-nal base holes for adjusting the center of gravity. The models arelaunched using keyed pusher plates so that regular projectile rotatingbands are not required. The barrel rifling used for these tests havetwists of 1 revolution in 15 calibers or 1 revolution in 19 calibers.

V. RESULTS

So far very little data have been obtained on the 7 caliber config-urations since tunnel and range time have not been available. There-fore, only the 5 caliber data are presented here; however, the available7 caliber data do support the 5 caliber results.

The first tests at NASA Ames Research Center indicate that themeasured drag of these boattails is comparable to the drag of the 1caliber conical boattail. However, base pressure measuring difficulties

2. A. S. Ptatou, R. CoZburn, and J. S. Pedgonay, "The Design andThjnwnic Balancing of Sp~nning Models and a Testing Technique forObtaining Magnus Data in Wind Tunnels," Ballistic RcaearchLaboratories Memoranduw Report No. 2019, October 1969, AD 699803.

9|1

make it impossible to obtain estimated free flight drag values so thesedata are not presented here. Free flight data, obtained in the BRL aero-dynamics range at a later date, do show that the drag values are comparable at transonic speeds (Figure 7) to the conical boattail. Addi.tio:nj1free flight drag data on these boattails are required in order to deter.mine their performance over the complete speed range. Drag data atsupersonic speeds, from BRL Wind Tunnel No. 1, also indicates the tri-angular ant cruciform wedge boattails have lower drag values than the 1caliber conical boattail (Figures 8 and 9). No data are yet availableon the cruciform wedge boattail at subsonic and transonic speeds.

The subsonic tests at Ames also indicate a more rearward center ofpressure of the normal force and lower pitching moments than the conicalboattail (Figures 10, 11 and 12). The transonic tests at NSRDC and theBRL range confirm these results at higher Reynolds numbers and the testsin BRL Wind Tunnel No. I show the increased stability is also present atsupersonic speeds. At supersonic speeds the new boattails have highernormal forces and better stability characteristics than the cylindricaltail (Figures 10, 11 and 12). The range firings also show betterstability through the transonic speed range.

The increased normal force or lift developed on these new boattailsis contrary to the slender body theory for bodies of revolution and atthis point is not clearly understood. It is possible that the increasedlift is entirely due to the lifting surfaces on these boattails; however,it is also possible that the more gradual reduction of crossectionalarea (Figure 13) may reduce the viscous region and maintain a more uni-form Bernoulli head in the vicinity of the boattail. To gain insightinto this problem it is planned in the near future to obtain the liftand pitching moment on two additional configurations: (1) a boattailwith circular crossections and having the same area distribution as thecruciform wedge anu (2) a conical boattail having four in-caliber fins.It is expected that the first boattail will have more lift than theconical boattail, but less than the cylindrical tail while the

-- finned conical boattail will have more lift than the cylindricaltail, but less than the cruciform wedge.

The tests at Ames and NSRDC have almost eliminated the added lift-ing surface versions (Figure 3) since the drag increment due to thelarger base area is too high, while the added lift and stability areminimal. Tests, when time permits, may show that these lifting surfacesmay perform better at supersonic speeds.

The Magnus data on the new boattails are not yet as well determinedas the pitch data, because of the rore difficult testing techniques. Ingeneral, the Magnus characteristics are nonlinear in spin, angle ofattack, Reynolds number and Mach number and combined with turbulenceand poor flow in many tunnels it becomes very difficult and time con-suming to obtain accurate and meaningful data.

10

The Magnus results to date on the conical and the new hoattailsare as follows.

(1) The Magnus force on a cylindrical tail and a conical boat-tail is approximately the same at supersonic speeds, but differs con-siderably at transonic speeds. The conical boattail has much higherlMagnus forces at transonic speeds than the cylindrical tail (Figure 14and reference 3).

(2) At transonic speeds the Magnus force center of pressure on theS caliber cylindrical tail tested is located 55% of the length from thenose while at supersonic speeds the Magnus center of pressure is 70% ofthe length from the nose. The change in center of pressure locationwill change the Magnus moment sign of the standard projectile since itscenter of gravity is usually located near the 60% station. The result-ing negative Magnus moment at transonic speeds may cause dynamic pre-cessional instability of the projectile (Figure 15).

(3) The increased Magnus forces on the conical boattail at transonicspeeds hold the Magnus center of pressure rearward; however, large posi-tive Magnus moments at transonic speeds can be created by the largeMagnus force thereby causing a dynamic nutational instability.

(4) To date the Magnus tests and analysis of data on the new boat-tail shapes have not been completed but data which is available indicatesMagnus data comparable to that of the cylindrical and conical boattailsat supersonic speeds while at transonic speeds values close to zero havebeen obtained (Figures 14 and 15).

(5) The few Magnus tests run on the canted configurations indicatethat the canted configurations have higher Magnus forces and momentsthan the comparable straight configurations. This is most likely truefor the straight fins develop higher fin lift which creates higher rolldamping and also higher positive fin Magnus forces. This will reducethe overall Magnus iorce for it cancels more of the body Magnus force(Figure 16).

Aerodynamic damping measurements have been limited to range dataand so far indicate that the aerodynamic damping is independent of theconfiguration (Figure 17). Roll damping measurements have also beenrestricted to range data and show that the straight boattails have highroll damping. At transonic speeds the roll damping for the square boat-tail coefficient is -. 05 to -. 06, the triangular boattail value is -. 10to -.11, and the conical boattail value is -. 015. Canting the improveOboattails willj e.Mt controlling the roll damping.

3. G. I. T. Nielsen and A. S. Platou, "Effect of Boattail Configurationon the Magnus Cha-'acteristics of a ProjectiZe Shape at Subsonic andTykwonia McAh Nuwberu, " to be published as a Ballistic ResearchLaboratories Memorandum Report.

11

VI. THE PROJECTILE NOSE

The Army-Navy Spinner Rocket nose, which has been used for theseboattail tests, has a high drag and iz unsuitable for long range or lowdrag projectiles. Long, slender noses (Figure 18) have lower dragvalues, but have the disadvantage of shortening the projectile wheelbase. This causes large balloting in the gun barrel, which in turn canbe related to excessive barrel wear and large initial angles of attack.Bore riders fastened to the projectile nose alleviate the balloting, butincreases the projectile drag and nose lift (reference 4), therebypartially defeating the purpose of the long nose. Also, a long, slendernose projectile has a low payload volume and rearward center of gravityboth of which can be detrimental .o top performance of a projectilo.

A nose formed similar to the triangular boattail, but extended tozero crossectional area (Figure 19) may offer a good combination of allof these requirements. For instance, a 70 cutting plane angle makesthe nose 4.07 calibers long and is comparable to the SRC projectilenose (Figure 18 and reference 4). The nose contributes approximately2 calibers to the wheel base and if used with a 2 caliber triangularor cruciform wedge boattail the wheel base is 4 calibers long for a 6caliber long projectile. The crossectional area plot (Figure 20) ofthe SRC and triangular noses show larger volume and more forward centerof gravity for the triangular nose. Figure 20 also shows a more uni-form area transition to a cylindrical body which may lower the viýzousdrag at this point. The aerodynamic performance of the triangular noseis yet unknown, but there are plans for testing it in the near future.To decrease the expected high roll damping of the straight triangularnose the configuration to be tested will be spiraled at the gun twistangle (Figure 21) and this may strongly affect the Magnus character-istics as indicated by references 5 and 6. Also, since sharp pointednoses ar.- impractical for most military purposes the nose has beenblunted as seen in Figure 21.

4. K. Opalka, "Wind-TunneZ Test of a Spinning, Low-Drag Projectile WithCanted Bore Riders at Mach Numbers fýrom 1.75 to 2.5," BallisticResearch Laboratories Memorandum Report No. 2349, Jan 74. (AD #774804)

5. M. A. Sylvester and W. F. Braun, "The Influence of Helical Serrationsand Bullet Engraving on the Aerodynamic and Stability Proparties ofa Body of Revolutlon With Spin," Ballistic Research LaboratoriesReport No. 1514, November 1970, AD 719235.

6. M. A. SyIoester, "Wind Tunnel Tests of Square Base and BoattailArmy-Na•ty Spinner Projectiles With Smooth Surface and 20ran EquivalentEngrazing," to be published as a Ballistic Research LaboratoriesReport.

12

VII. CONCLUSIONS

The aerodynamic performance of these boattails appears to be supe-rior to that of the conical boattail. Further wind tunnel and rangetests are still required to compi~tely dete-.mirte their aerodynamic per-formance.

13

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REFERENCES

1. A. S. Platou, "Magnus Characteristics of Finned and NonfinnedProjectiles," AIAA Journal, Vol. 3, No. 1, January 1965, pp. 83-90.

2. A. S. Platou, R. Colburn and J. S. Pedgonay, 'nhe Design and DynamicBalancing of Spinning Models and a Testing Technique for ObtainingMagnus Data in Wind Tunnels," Ballistic Research Laboratories ReportNo. 2019, October 1969, AD 699803.

3. G. I. T. Nielsen and A. S. Platou, "Effect of Boattail Configurationon the Magnus Characteristics of a Projectile Shape at Subsonic andTransonic Mach Numbers," to be published as a Ballistic ResearchLaboratories Memorandum Report.

4. K. Opalka, "Wind-Tunnel Test of a Spinning, Low-Drag Projectile WithCanted Bore Riders at Mach Numbers from 1.75 to 2.5," BallisticResearch Laboratories Memorandum Report No. 2349, January 1974,AD 774804.

5. M. A. Sylvester and W. F. Braun, "The Influence of Helical Serrationsand Bullet Engraving on the Aerodynamic and Stability Properties ofa Body of Revolution With Spin," Ballistic Research LaboratoriesReport No. 1514, November 1970, AD 719235.

6. M. A. Sylvester, "Wind Tunnel Tests of Square Base and BoattailArmy-Navy Spinner Projectiles With Smooth Surface and 200M EquivalentEngraving," to be published as a Ballistic Research LaboratoriesS~Report.

- 7. C. H. Murphy and L. E. Schmidt, "The Effect of Length on the Aero-* •dynamic Characteristics of Bodies of Revolution in Supersonic

Flight," Ballistic Research Laboratories Report No. 876, August 1953,AD 23468.

8. E. S. Love, "Base Pressure at Supersonic Speeds on Two-DimensionalAirfoils and on Bodies of Revolution With and Without Fins HavingTurbulent Boundary Layers," National Advisory Committee for Aero-nautics TN 3819, January 1957.

4.

• 35

LIST OF SYMBOLS

CD Dra positive direction is aftp ~ V2 S

Pichng oetCm1 Moment center is .6 Z calibers from nose.p V2 S d Positive moment is due to positive normalforce ahead of the amomet center.

C d m a a-0

C Moment center is .6 it calibers from nose.1 zSdpp 2 Positive moment is due to positive M~agnus

V force ahead of moment center.

d CC3 at a 00

C C3 M.q

2

C Normal ForceCN~ positive direction is up

d CNCN~2 ac~

M~agnus Force positive direction is to right looking up-

CNdCI

CN at-P 8t 00

C base pressure coefficient

d main body diameter

36

A LIST OF SY MOLS (continued)

SMCp Magnus force center of pressure from moment center

NCp normal force center of pressure from moment center

i p body axial spin rate (positive is clockwise looking upstream)

PB base pressure

free stream pressure.P.

Sqt complex transverse angular velocity

S body area d2

V free stream velocity

angle of attack

P free stream air density

N3

37