Lessons learned on the application of vibration absorbers for enhanced cannon stabilization

10 Pages·1.964 MB·English

by  Kathe Eric.

131
Lessons learned on the application of
vibration absorbers for enhanced cannon
stabilization
EricKathe bump-course,andincreasesforsystemconfigurations
USArmyTank-AutomotiveandArmamentsCommand, that have utilized a shorter barrel [2]. This degrada-
ArmamentResearch,Development,andEngineering tion of accuracy is wholly absent for live-fire testing
Center,BenetLaboratoriesWatervlietArsenal,NY of the same gun system from a stationary mount [3]
12189-4050,USA wherethedevelopmentalsystemoutperformsitsinfield
E-mail: [email protected] counterpart,the120-mmM256.
Aneffectrelatingtogunperformanceaswellasac-
curacyistheyawstateofthebulletasitleavesthemuz-
This paper will summarize the successful application of zle. Inthiscontext,yawistheanglebetweenthemo-
muzzle-end vibrationabsorbers toreducecannon vibration. mentumvectorand the axial centerlineofthe projec-
Thistechnologyconstitutesaweaponsstabilizationapproach tile. Ingeneral,barrelvibrationactivitythatdecreases
thatfocusesonpassivemechanicalstructuralmodificationof
accuracy also tends to increase the yaw state of the
thecannon,ratherthanrelyinguponanexternalcontrollaw
bulletatshotexit. Increasedyawatimpactdecreases
toactivelycancelvibrations. Challengesencounteredduring
penetratorperformance[4]. This projectile condition
fieldtesting,non-idealbehavior,andperformanceevaluation
is damped out during flight by the fins of the round,
usingdigitalsignalprocessingwillbehighlighted.
andtheirassociateddragandrangelimitations. Thus,
theeffectivenessoftheweaponfornear-targetsmaybe
impaired,eventhoughprecisionaccuracymaynotbea
1. Introduction
considerationundersuchcircumstances.
For these reasons, the currentfocus of engineering
Structuralvibrationofgunsystems is becomingan
effortstoenhancetheaccuracyandperformanceofex-
increasinglyimportantissueinthetankcommunitydue
tendedlengthlaunchersis placeduponattenuationof
toincreasedexitvelocityrequirementsofroundsused
the receptance of the gun system to environmentally
todefeatanticipatedandcurrentthreatreactivearmor.
inducedbarrelvibrations,whilemaintainingthelength
Increasedvelocitydemandslongergunbarrelsandsub-
at levels that enable a muzzle velocity capableof de-
sequently increased reacceptance to environmentally featinganticipatedtargetarmor. Itisbelievedthatthis
inducedvibration. (Receptance is the ratio of deflec- approachwillachieveenhancedaccuracybyreducing
tionamplitudetoforceasafunctionoffrequency[1].) dispersioncausedbyvariationintheinitialconditions
Consideringthestrongdesiretoincreaseperformance of the gun structure at the commencementof launch;
while the weapon platformis outmaneuveringits tar- i.e.,shootoutofastraighterbarrel.
get,theneedtointroducemethodstoattenuatethere- Structuralcontrolofthebarrelduringthelaunchdy-
ceptanceofthegunsystemtoenvironmentalvibration namicswouldproveadauntingtaskduetotheparticu-
becomesclear. larlynarrowtimewindowduringwhichcorrectiveac-
Theincreasedsystemreceptancehasmanifestedit- tionmustbetaken–typicallylessthaneightmillisec-
self in decreased weapon accuracy during live-fire onds. Coupledwiththeneedforsignificantsensorin-
bump-course testing of a developmental extended formation,highactuationpowerlevels(duetothebrief
length120-mmtankcannon(theXM291)mountedto time window), and a severe operating environment;
a modified M1A1 tank conducted at Aberdeen Prov- theapplicationofsmartstructuretechnologytocontrol
ing Ground, Maryland. The accuracy is particularly launchdynamicsisnotviableforadevelopmentalsys-
sensitive to the traversing speed of the tank over the temthatistargetedfornear-termapplicationssuchasa
ShockandVibration8(2001)131–139
ISSN1070-9622/$8.002001,IOSPress.Allrightsreserved
132 E.Kathe/Lessonslearnedontheapplicationofvibrationabsorbersforenhancedcannonstabilization
potentialM1Atankseriesgunupgrade. Suchtechnol- alaboratorysetting[8]priortoconstructionofthetest
ogymayhoweverfindapplicationsfornextgeneration fixture.
systems. Detailsoftheabsorberintegrationwiththegunsys-
Forthesereasons,intermediatetechnologysolutions tem to enable function in a severe operating environ-
that are essentially compatible with the current M1A mentareavailableviaalimiteddistributionreport[9].
weaponplatformanditssubsystemshavebeensought. The primary function of the forward thermal shroud
showninFig.1istopreventthermalgradientswithin
thebarrelthatwouldotherwisecauseunacceptabledis-
2. Traditionalengineeringsolutions tortions (e.g., direct sunlight heating one side of the
barrel). The back of the forward shroud is affixed to
Severalengineeringapproachestoreducevibration theboreevacuatorusingacompressioncollarthaten-
receptanceofgunsystemshavebeenapplied,withdif-
ables a pivoting action; this prevents thermal distor-
feringadvantages.
tionsintheshroudfrombeingtransmittedtothebarrel.
1) Stiffeningthegunviathickerbarrelsraisesden- Theabsorberfixtureconsistedofasimplespringcol-
sity nearly in proportion to stiffness, especially larmountedto thefrontofthe shroud. Thisprovides
nearthemuzzleend,wherethecylinderisclose thespringactionofthevibrationabsorberbetweenthe
toathin-wallapproximation. barrel’s muzzle and the front of the forward shroud.
2) Cradle/mountextensioneffectivelydecreasesthe Thus,theinertiaoftheshroudasitpivotswasleveraged
cantileveredlengthof thegun, andhas success- to provide mass for the vibration absorber [6]. This
fullyincreasedmodalfrequencies[5]. However, approach was termed the dynamically tuned shroud
extendedcradlespresentsignificantweaponplat- (DTS).
formintegrationissues;e.g.,theydon’tfitinthe Forpragmaticreasons,thefreeamplitudeoftheab-
allottedspaceforaretrofitthatdoesnotrequirea sorberrelativetothebarrelwasseverelyrestrictedtoa
newturret. nominalvalueofplusorminusthreemillimeters. An
3) The application of composite overwraps to en-
energy absorbing rubber material was applied to the
hance the stiffness-to-density ratio has encoun-
innersurfaceofthevibrationabsorberfixturetoactas
teredchallengesduringfiring,manifestedasde-
asnubberandpreventmetal-to-metalimpact.
laminationof the composite from the gun steel.
Theproofofprinciplefixturedepictedutilizedoff-
This is not to rule out the future of composite
the-shelfhelicalspringstoprovidetheelasticrestoring
applications.
force of the absorber. The springs were interchange-
4) Activevibrationsuppressionviathefire control
ablewithspringsofdifferingstiffnesstoenabletesting
andweaponstabilizationsystem(thesystemthat
of variousabsorberparametersfor optimizingoverall
points the gun on target) has been considered.
systemperformance. Althoughshockabsorberscould
Although a promising candidate to enhance the
be integratedwith the design to controldampinglev-
performance,activecontrolapproachesareham-
els,significantfrictioninherentinthemechanismpre-
pered by the performancelimitations of the hy-
ventedfullcontrolofthisparameter. Thusthemecha-
draulicelevationandazimuthalactuators,sparse
nismconstitutedadampedvibrationabsorber. Provi-
sensoryinformation,andintegrationwiththeex-
sionswerealsoincludedtoaddweights(notshown)to
istingcontrolhardwareandsoftware.
adjustthemassoftheabsorber.
Avibrationabsorberoffersseveralclearadvantages.
3. Amuzzleendvibrationabsorber Theabsorberconstitutesapurelypassiveapproach,and
thusnosensor,actuator,orfirecontrolintegrationisre-
Amuzzleendvibrationabsorberhasbeenproposed quired. Theabsorberisfullycompatiblewithallother
asanintermediatetechnologyenhancementthatsuffers engineeringsolutions. Further, the absorberdoes not
virtuallynointegrationissueswiththecurrentweapon interferewiththeopticalpathusedbythecontinuous
platform[6]. Bothaconceptualschematicandanim- muzzlereferencesensor(CMRS).Theprincipaldisad-
age of the system utilized in the testing are shown in vantageofavibrationabsorberisthatamovingpartis
Fig.1. Suchanabsorberconceptwasnumericallymod- appliedtoaparticularlyharshdynamicenvironment–
eled[7]andsubsequentlyexperimentallyvalidatedin albeitasimplemovingpart.
E.Kathe/Lessonslearnedontheapplicationofvibrationabsorbersforenhancedcannonstabilization 133
Fig.1.Schematicdepictionofamuzzle-endvibrationabsorber(a)andanimageofthegunsystemtested(b).
Two types of bumps were used. Type A bumps
consistofasix-inch(0.15m)riseoverasix-foot(1.8m)
span,andtypeBconsistofathree-inch(0.075m)rise
over two feet (0.6 m). The geometry of the bumps
andthetimesequenceatwhichtheyareencounteredat
fifteenmilesperhour(6.7m/s)areshowninFig.3.
Thegunstabilizationsystemwasmaintainedinthe
active mode during the testing. This control system
maintainsthegun-pointingangle(asseenattheturret
andtrunnions)fixedrelativetoearthinbothelevation
Fig.2.ImageofthemodifiedM1A1tankreturningfromatest. and azimuth. Aside from live-fire testing, this is the
mostrealistic test availableto assess theperformance
4. Thetests ofthegunbarrelvibrationabsorber.
Three runs were made down the bump-course for
The vibrationabsorber was mountedto a modified
boththebase-lineandabsorbermodifiedgunsystems.
M1A1tankdepictedinFig.2. Thetankmodifications
A total of three absorber configurations were tested.
were extensiveand includedan XM291120-mmgun The effective spring rates for the vibration absorber
system, and XM91 bustle autoloader, enhanced feed collarwere23kN/m,48kN/m,and24kN/mforcon-
forwardandfeedbackstabilizationsystems,acontinu- figurations A, B, and C, respectively. All three used
ousmuzzlereferencesystem,extensivedatacollection, theinertiaoftheforwardthermalshroudas shownin
andtelemetricdatarelay. Fig. 1. The mass of the shroud is 30 Kg distributed
The XM291 gun system is 1.45 m longer than the nearlyevenlyalongitslengthof2.2m. Configuration
current5.3mlongM256. Simplyput,thisadditional Cincludedanadditional9Kgmassatthefrontcollarof
length provides greater working volume for launch. theforwardthermalshroud. Forthebase-linetesting,
This results in greater muzzle velocities with lower the vibration absorber mechanism was removed and
propellant temperatures and pressures than would be theremainderofthesystemwasrestoredtoitsoriginal
requiredbyanM256. BecausetheXM291islonger,it condition[9].
ismoresusceptibletoflexuralvibrations.
Testing was conducted on the RRC-9 stabilization
5. Theanalysis
course at Aberdeen Proving Ground, Maryland. The
courseconsistsofagravelroadwithseveraldozensteel
5.1. Datacollected
ramp bumps spaced along its length. The bumps are
allintroducedinpairs, oneforeachtrackofthetank, Datarecordedduringthetestingincludedameasure
suchthatonlypitchandheave(notroll)isintroduced ofverticalbarrelflexure,horizontalbarrelflexure,ver-
bythebumps. ticalpointingerror,andhorizontalpointingerror.
134 E.Kathe/Lessonslearnedontheapplicationofvibrationabsorbersforenhancedcannonstabilization
Fig.3.Exaggeratedplotofbump-courseprofileandcorrespondingtimeaxisforafifteenMPHcourserun.
The flexureis a measure of the differencebetween isnotinstantaneousandthedragofthebumpsis sig-
the pointing angle of the gun at the cradle and the nificant. Thereforedirect correlationanalysis, bump-
pointingangleofthe gunat the muzzle. Thevertical for-bumpisdifficult.
andazimuthalmountanglesaremeasuredviaresolvers
atthetrunnionsandturretring,respectively;whilethe
muzzleanglesaremeasuredopticallybyasystem(the 5.2. Root-mean-square(RMS)amplitude
CMRS) that bounces a source beam from the turret
off of a mirror mounted to the muzzle of the barrel
AllofthedatahasbeennormalizedbytheRMSam-
andmeasuresthedeflectionofthereturnedbeam. The
plitudeoftheverticalbendingangle. Thisvaluewould
deflectionofthereturnedbeamcorrelatestotheangular
providespecificaccuracyinformationonthegunsys-
deflectionofthemuzzle.
tem. This RMS flexure is on the order of one milli-
Thepointingerrorisameasureofthedifferencebe-
radian,representinganerrormagnitudethatcorrelates
tween the commandpointingdirection and the direc-
tion measured by the aforementionedresolvers at the toa targetwith a radiusof onemeterat a distance of
cradle. With the stabilization in the active mode, the onekilometer. ThenormalizedRMSvaluesarelisted
commandpointingdirectionis fixed relativeto earth. inTable1.
The semantics are a bit awkward as the pointing di- Clearly, the configuration of vibration absorber C,
rectionofa non-straightbarrelis somewhatofamis- wasthebestoverallperformer. ConfigurationsAandB
nomerandisnotlikelybestrepresentedbythedirection appeartohaveshiftedsomeenergyfromverticalbend-
measuredatthecradlealone. ingintohorizontalbending,althoughallthreereduced
Toensurethatonlyconsistentdatawas usedin the verticalbending. Theabsorbershadlittleaffectonthe
analysis,amethodidentifyingthebump-course–inthe pointingerrorRMSvalues.
measurement data – had to be developed. This need
wascausedbyalackofconsistencyinthestartandend
timesofthedatarecords. 5.3. Powerspectra
TheseriesofthirteentypeB bumpsencounteredat
84 m (121/2 seconds bump-course time) was found
Thepowerspectraforthebase-lineandvibrationab-
to providea distinct signaturein the horizontalbend-
sorberCwerecomputedusinga1024elementHanning
ingdataneartheten-secondlocationinthedatafiles,
window (2.2 seconds at the data sampling frequency
indicating truncation of the first seconds of the data.
of 463 Hz). An overlapof half the window size was
Thissignaturepointwasquantitativelyidentifiedusing
anarrowband-passfilterconstructedandimplemented usedtoreduceleakageeffectsfromthenon-stationary
in MATLAB (The Mathworks, Natick, MA) to iden- behavior of the bump-course. The power spectra for
tifythepeakthatisassertedtooccurat121/2seconds each of the three runs were subsequentlyaveragedto
bump-coursetime. Thedatasetswerethentruncatedto reducesensitivitytovariationintraversingthebump-
beginatthreesecondsandendattwenty-twoseconds course. TheresultsaredisplayedinFig.4.
bump-coursetime to providea consistent data set for
furtheranalysis[9].
Thisapproachdoesnotmake-upforuncontrollable
6. Discussion
variationinspeedandanoccasionallystraddledbump
whiletraversingthecourse. Thistestingisparticularly
challengingtothedriverwhenthebumpsareencoun- The vibrations of the gun system can roughly be
teredinaseries;thereactionoftheturbinepower-plant brokendownintothreefrequencyregimes:
E.Kathe/Lessonslearnedontheapplicationofvibrationabsorbersforenhancedcannonstabilization 135
Table1
NormalizedRMSperformanceoftheconfigurationstested.
Configuration Verticalbending Horizontalbending Verticalpointingerror Horizontalpointingerror
A 94.3% 61.4% 44.3% 36.5%
B 82.8% 52.2% 4.37% 41.2%
C 76.2% 43.4% 39.3% 35.9%
Base-line 100% 47.8% 41.0% 37.6%
6.1. Forcedvibrationnear2Hz comparesfavorablywiththehorizontalbendingpower
spectrainFig.4.
Although the entire tank is one structure, the hull The cause of the notch in the center of the power
uponitssuspensionmaybeconsideredaforcedexcita- spectraforthismodeisunknown.Itsexistenceforboth
tionplatformthatprovidesforceinputintothegunand theabsorberandbase-linecasesprecludesitfrombeing
mountstructure. The fundamentalmodes of the tank aconsequenceoftheabsorberitself. (Thisnotchwas
arelow(around1Hz)duetoitsneedforacompliant absent for absorber configurations A and B and was
suspensiontotraverseroughterrain. Vibrationenergy replacedwithapeakatthenotchwithapowerspectrum
resulting from traversing the bump-course is passed amplitudenearlythreetimeshigher[9]. Thisiswhere
through the tank suspension (which acts as low-pass thehorizontalRMSenergyinTable1islocated.)
filterforfrequenciesaboveitsfundamentalmodes)to Tobetterillustratethedisparityinsizebetweenthe
thegunsystem. gunsystemandturret,acomputer-aideddesignimage
Thehydraulicstabilizationsystemhasbeendesigned oftheturretandbarrelstructureisdepictedinFig.5.
tocontrolthepointingerrorasmeasuredatthecradle.
The performance of this feedback control system is 6.3. Fundamentalverticalmodenear19Hz
limitedbythefiniteactuationresponseavailable. The
disturbance and control energy in this low frequency
In the vertical orientation, the barrel is coupled to
rangeresultinaforcedverticalbendingmoderesponse,
theturretbythetwotrunnion-bearings,whichprovide
despite the lack of receptance of the gun to this low
forpivotingaction. In addition,the elevationmecha-
frequencyexcitation.
nismprovidescontrolactuationsomedistancebehind
thetrunnions(aboutahalfofameter). Thetrunnions
6.2. Fundamentalhorizontalmodenear7Hz preventvertical translation of the gun system relative
totheturretandhullwithanearlyrigidconstraint,and
In the horizontal orientation, the barrel is coupled littletorsionalresistance. Thecantileveredconfigura-
totherelativelymassiveturretbythetwohorizontally tion of the barrel is maintained by the vertical force
opposedtrunnionpins. Thesepinsmaybeconsidered inputoftheelevationsystem. Operatedbyhydraulics,
to mimic a verystiff torsionalspringthatcouples the andimplementedwithacontrolscheme,theconstraint
azimuthalangleof the barrel-at the trunnions-to the of the elevation system is not nearly rigid, but rather
turret. Inaddition,thetrunnionspreventlineardeflec- muchsofter.
tionrelativetotheturretinthehorizontalplane. Thus Thisforgivingservo-complianceisessentialtopre-
the trunnions provide two nearly rigid constraints to venttheforceinputfromthesuspensionfromreeking
thebarrel. Sincethemassandrotationalinertiaofthe havocwiththestabilizationandgunsystems; itisde-
turretaremuchhigherthanthatofthegunsystem,the sirabletodecouplethegunsystemfromthepitchand
boundaryconditionmaybeexpectedtoapproachthat heaveofthetank.
of a cantilevered barrel in flexure, with a rigid body Thecenterofmassofthegunsystemisnotcoinci-
modethatisnotdiscernibleintheflexuredata. dentwiththetrunnionsbutratheroverhangsthemby
The first two fundamental modes of a nearly rigid somedistance. Anopen-loopcontrolterm,basedupon
boundaryconditionhavebeenapproximatedforthegun a vertically oriented accelerometer fixed to the turret
system using beam finite elements. The modes have nearthetrunnionsisusedtocompensateinrealtimefor
been computed to be 7.24 Hz and 25.0 [10]. Instru- themomentcreatedbytheinertiaofthegunatthecen-
mentedimpact testing of the structure in a controlled terofmassandtheaccelerationatthetrunnions. This
laboratory setting has demonstrated the modes to be enablesthesoftfeedbackservo-controlparametersfor
7.25Hzand25.4Hz[8]. Thisfirstcantileveredmode theimbalancedsystem.
136 E.Kathe/Lessonslearnedontheapplicationofvibrationabsorbersforenhancedcannonstabilization
Fig.5. Imageofthecoupledbarrelandturretstructurethatvibrates
horizontallywithanopen-looprotationalrigidbodymodeaboutthe
turretring.
Fig.6. Imageofthebarrelstructurethatvibratesverticallywithan
open-looprotationalrigidbodymode.
is not clear. The second horizontal mode of the gun
systemis nearin frequency,yetnearlynodiscernible
energyis in its powerspectrumabovetwelveHz. At
welloversixtytonsinmass,thetankisaveryeffective
low-passfilterandshoulddropoffnominallyattwenty
decibelsperdecadefrom1Hzfundamental,aswould
anyothersecond-orderdampedsystem.
IthasbeensuggestedbyDr. MarkKregal,ARL,that
inthisfrequencyrange,theturretexhibitsvibrationas
a rigid body in a rocking motion relative to the hull
whosepivotisorthogonaltotheazimuthalaxis. This
couldbetheresultofclearancescausedbyout-of-plane
imperfectionsbetweenthematingsurfacesofthehull
and turret that bear the weight of the turret – as the
tanktraversesroughterrain,theturretmayrattle. Such
a structural coupling could cause a migration of this
vibrationenergytothegunsystem[11].
Another candidate for the high frequency source
would be the open-loopcontrol term used to counter
theoverhangingcenterofmassofthegunsystem. This
term,althougheffectiveatmaintainingtherigidbody
modeontarget,appliesapredictabledisturbancebend-
ing moment to the gun system and vertical load to
theturretandhullthroughthenon-complianttrunnion
Fig.4.Juxtaposedpowerspectra.
bearings. Thus there is the potential for an undesir-
Thefirsttwoflexiblemodesofapinnedfreebound- ablefeed-backloop,althoughthedisparityinmassbe-
aryconditionhavebeenevaluatedusingbeamfiniteel- tweenthetankandgunsystemshouldlessenthistype
ementsforthegunsystem. Theresultingmodeswere ofeffect.
computed to be 19.1 Hz and 32.4 Hz [10]. The first Ahigherorderopen-loopcontroltermcouldcounter
pinned-freemodecomparesfavorablywiththevertical anynegativeeffectsonperformancebyleveragingthe
bendingpowerspectrainFig.4. Anillustrationofthe inversedynamicsofagunsystemmodeltocompensate
vertical system is depicted in analogy with Fig. 5 in for the predictable disturbanceapplied by the current
Fig.6. open-loopterm. Thiswouldalsoprovideenhancement
The cause of the large amount of vibration energy tothelowfrequencydisturbance,regardlessofitsva-
thatisexcitingthefirstverticalmodeofthegunsystem lidityasacauseforthehighfrequencydisturbance.
E.Kathe/Lessonslearnedontheapplicationofvibrationabsorbersforenhancedcannonstabilization 137
6.4. Performanceinverticalbending 6.6. Performanceinverticalpointingerror
Theabsorberperformancewasmostpronouncedin Theeffectofthevibrationabsorberonverticalpoint-
vertical bending, currently believed to be the largest ingerrorwasazero-sum-gaininthetimedomain. As
contributortodegradedperformance. Theattenuation canbeseeninFig.4,theenergywasshiftedfromthe
oftheverticalbendingwasachievedwithoutthemigra- 19Hzmodetomuchlowerfrequencies,atraitshared
tionofenergytootherportionsoftheverticalbending by the two other absorbers configurations [9]. This
spectrum. However,theabsorberhadlittleaffectonthe provides significant advantage for any active control
low frequencyresponse that is asserted to be a direct schemeusingtheexistinghardware.
resultofthetanksuspension.
This lack of low frequency performance is to be 6.7. Performanceinhorizontalpointingerror
expected. A low frequency absorber would require
eitherexcessivemasstoloweritsnaturalfrequencyor The effect of the absorber on horizontal error was
an excessive amplitudeenvelopedue to the increased negligible. Thislackofhorizontalpointingsensitivity
displacementsthatwouldresultifsofterspringswere islikelycausedbythelargeinertiaoftheturretstructure
usedtoloweritsfrequency. that must be overcome. (Recall that in the vertical
mode,onlythegunispointed. Inthehorizontalmode,
the entire turret is pointed.) This error signal is not
6.5. Performanceinhorizontalbending
likely to be reduced by a simple lightweight passive
device, but neitheris it greatlyexacerbatedby longer
Of the three absorber configurationsanalyzed, two barrels.
made horizontal bending significantly worse. The
causeoftheincreaseinhorizontalbendingforthesetwo
caseshasbeenpostulatedtobethemigrationofvertical 7. Generalobservations
modeenergyintothehorizontalmode. Thereasonfor
theabsenceofthiseffectforabsorberconfigurationC
It became clear during the testing that snubber en-
isnotclear.
gagementwasacommonoccurrence. Theengagement
The shifting of this energy may be due to the se-
ofthesnubberresultedinatendencyfortheshroudto
vere amplitude restrictions placed upon the vibration
rotateaboutitscenterlineduringtesting. Thiswaspar-
absorber. Snubberengagementisacommonevent,and
ticularlyfrustratingas the topspringsof the absorber
it is likely to occur with the most momentum in the spring collar were preloaded in compression to hold
verticalorientation. Themomentumtransferwillgen- the static weight of the shroud above the barrel. As
erallyoccurofftheverticalaxiswithasignificantvec- theshroudwouldrotate,thepreloadandweightwould
torcomponentdeliveredintothehorizontalmode. For conspiretoholdtheshroudagainstthebarrel. Anef-
example, the vibration absorber ring may impact the fectivefieldrepairwastowrapthepivotingconnection
muzzlewithverticalmomentum,butstrikethemuzzle betweenthebackoftheforwardshroudandthecom-
atatwoo’clockpositionandthusreboundupandright, pression collar that held it against the bore evacuator
whileforcingthebarreldownandtotheleft. withasinglehoopwrapofducttape. Therepairoffered
Toattempttocounterthemigrationofenergyintothe little resistance to the intended low angle pivoting of
horizontalmode,futureeffortswilltunethehorizontal theshroudwhileitpreventedrotation.
andverticalmodeabsorbersseparatelyviaorientation The common occurrence of snubber engagement
dependent spring constants. An alternative would be would suggest that the effectiveness of the vibration
to constrain the absorber from relative motion in the absorberwasduelargelytoanimpactabsorberbehav-
horizontalmode. ior. Impactabsorbers,suchas theleadshotusedina
Regardless,modestincreasesinhorizontalbending dead-blow hammer, use inelastic collisions to reduce
energyareworthsignificantreductionsinverticalbend- energy.Credencetothiseffectmaybegainedbynoting
ing energy. This is true as the geometric error (Eu- it was the absorberwith the largestmass that hadthe
clideannorm)isdominatedbythelargerverticalerror. greatesteffect,despitethefactthatitsnaturalfrequency
It is also valid, becausemost targets tendto be much wasthefarthestfromtheverticalbendingmodethatit
widerthantheyaretall. attenuatedsowell.
138 E.Kathe/Lessonslearnedontheapplicationofvibrationabsorbersforenhancedcannonstabilization
Fig.7.Orbitplotsfromfiveroundfireofatest25mmM242gunatBene´t.
8. Futurework/preliminaryresults It must be emphasized that the performance gain
demonstratedwasachievedwithvirtuallynoimpacton
Althoughthisworkconcentratedonthedesignofa anyotherportionoftheweaponsystemandnorestric-
vibrationabsorberto reduce vibrationscaused by ex- tionsplaceduponotherfutureenhancementoptions.
ternal excitation, the concept may also be applied to Little direct correlation between the flexural state
ofthebarrelandtheultimateimpactonaccuracyand
attenuatebarrelflexurecausedbysuccessivefire. Pre-
performanceisavailable. However,muchindirectev-
liminary results of such a test firing are depicted in
idence indicates that shooting out of a vibrating bar-
Fig. 7. The tests demonstratedthat the vibrationam-
relisnotaseffectiveasshootingoutofaquietbarrel.
plitudecouldbeessentiallycutinhalfusinga simple
Strongevidenceforstaticcurvatureeffects(manufac-
vibration absorber. Evaluation of the effect on accu-
turingtolerance,thermalgradients,andgravitydroop)
racywasnotpossibleduetotheshorttrajectoryofthe
on accuracy has been published. (See for example
projectileflightwithintheBene´tGunDynamicsLabo-
Wilkerson[12],Bundy[13],andKatheetal.[14].)
ratory.
Acknowledgments
9. Conclusions
The author would like to acknowledge the sup-
This testing set out to demonstrate the proof-of- port of Roger Billington and Charles Cording of the
principlethat a simple muzzleendvibrationabsorber Program Manager – Tank Main Armament Systems,
couldsignificantlyenhancethestructuraldynamicsof (PM-TMAS), Picatinny Arsenal, New Jersey; techni-
a tank cannon. This has been demonstrated for ab- calcollaborationwithDrs. StevenWilkersonandMark
sorberconfigurationC,asshowninTable1andFig.4. Kregal,ArmyResearchLab,(ARL)andPeterMcCall,
Therefore, investment in a ruggedfixture to endurea ArmyTestCenter,(ATC),AberdeenProvingGrounds,
firingenvironmentiswarranted. Maryland,andProfessorAndrewLemnios,Rensselaer
E.Kathe/Lessonslearnedontheapplicationofvibrationabsorbersforenhancedcannonstabilization 139
PolytechnicInstitute, Troy, NY; and the assistance of main, in: Proceedings of the Eighth US Army Symposium
MichaelSoja,AlbertPflegl,anddedicationofEdmund onGunDynamics,G.AlbertPflegl,ed.,ARDECTechnical
Report ARCCB-SP-96032, Benet Laboratories, Watervliet,
Vanderwerken,ofBene´tLaboratories.
NY,May1996,pp.28.1–17.
[8] E.L.Kathe,DesignandValidationofaGunBarrelVibration
Absorber,Proceedingsofthe67thShockandVibrationSym-
posium: VolumeI,PublishedbytheShockandVibrationIn-
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