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IAC-09.A6.3.11
HONEYCOMB VS. FOAM: EVALUATING POTENTIAL UPGRADES TO ISSMODULE
SHIELDING
S. RvanUSRA Lunar and Planetary Institute (LPI), 3600 Bay Area
Blvd, Houston, TX, 77058, USA
shannon. j .ryan(a_)nasa. gov
E.L. ChristiansenNASA Johnson Space Center, 2101 NASA Pkwy,
Houston, TX, 77058, USA
eric.l.christiansen(c,.nasa.gov
ABSTRACT
A series of 19 hypervelocity impact tests have been performed on
ISS-representative structure walls to evaluate theeffect on
micrometeoroid and orbital debris (MMOD) protective capability
caused by replacing honeycombsandwich panel cores with metallic
open-cell foam. In the experiments, secondary impacts on individual
foamligaments were found to raise the thermal state of projectile
and bumper fragments, inducing break-up and melt atlower impact
velocities than the baseline honeycomb configuration. A ballistic
limit equation is derived for thefoam-modified configuration, and
in comparison with the honeycomb baseline a performance increase of
3-15% atnormal incidence was predicted. With increasin g impact
obliquity, the enhancement in protective capabilityprovided by the
modification is predicted to further increase. The reduction in
penetration and failure risk posed byMMOD impacts is achieved by
the foam-modified configuration without a significant decrease in
mechanical orthermal performance, and with no additional weight. As
such, it is considered a promising upgrade to MMODshielding on ISS
modules which incorporate honeycomb sandwich panels and are yet to
fly.
INTRODUCTION
The performance of a dual-wall protective spacecraftstructure
against the impact of micrometeoroid andorbital debris (MMOD)
particles is generallyconsidered to be degraded by the presence of
ahoneycomb core. For impacts which penetrate theshield outer wall
(bumper or front facesheet),projectile and bumper fragments
disperse radially asthey propagate through the shield
interior,distributing the load over an area of the shield rearwall
significantly larger than that of the originalprojectile diameter.
The presence of honeycomb cellwalls acts to restrict expansion,
effectivelychanneling the fragments within a limited number
ofhoneycomb cells for a more concentrated impactupon the rear
facesheet. However, missionrequirements often prevent the inclusion
of adedicated MMOD shielding structure, and as such,structural
panels (i.e. honeycomb sandwich panels)also commonly serve as the
protective system.
Metallic foams are a promising alternative tohoneycomb
structures as they offer comparablestructural and thermal
performance without thepresence of MMOD shielding-detrimental
channelingcells. In this paper, modifications to a double-layer
honeycomb sandwich panel shielding configurationrepresentative
of those used onboard the InternationalSpace Station (ISS) are
evaluated. The modificationsentail the substitution of aluminum
honeycomb foraluminum open-cell foams, while the total shieldweight
in maintained.
BACKGROUND
Honeycomb sandwich panels
Given their conunon application in space vehicleprimary
structures, the performance of honeycombwider impact of MMOD
particles at hypervelocityhas been investigated in a multitude of
studies. Jex etal. [1] and Sibeaud et al. [2] discussed that
thepresence of a honeycomb core enhanced the shieldingperformance
of a dual-wall structure at hypervelocity.They concluded that
secondary impacts betweenejecta fragments and cell walls
overcompensated forthe detrimental effect of channeling. A
morecommonly held view is that the presence of ahoneycomb core is
unfavorable to the shieldingperformance. Taylor et al. [3]
quantified thedegradation in performance through inclusion of
ascaling factor which acts to reduce the effective rearfacesheet
thickness by 50% in definition of the panel
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ballistic limit at hypen-elocities (i.e. molten and/orvaporized
ejecta). Ryan et al. [4] defined adegradation in shielding
performance due to thepresence of a honeycomb core equal to a
--46%reduction in shielding capability at normal impact,reducing
with increasing obliquity (e.g. for impact at60°, the degradation
in performance drops to --18%).Sennett and Lathrop [5] also
quantified the effect ofthe honeycomb core, stating that once the
panelthickness increases above two times the honeycombcell size, no
increase in shielding capability isachieved with an increase in
shield thickness whenfragments were either molten or vaporized. For
solidfragment ejecta, the effect was not nearly as severe.In Fig.
1, a comparison between the perfornance at 7km/s (normal impact)
predicted for a dual-wall shieldwith and without a honeycomb core.
For the Whippleshield configuration (i.e. no honeycomb core),
thenew non optimum (NNO) equation [6] is used. Itshould be noted
that the NNO equation (and hence,the Taylor and Sennett &
Lathrop approaches) mayprovide non-conservative predictions for
projectilediameter to shield spacing ratios (S/d t,) < 15.
UNNO
— — — Semmett & Lathrop----- Taylor— • — SRL
0 2 4 6 8 10
Spacing (No. of honeycomb cells)
Fig. 1: Predicted performance for a dual-wall shield withand
without a honeycomb core at hypervelocity.
Open-cell foams
Preliminary investigations of the hypervelocityimpact
performance of metal foam structures havedemonstrated their
potential, particularly incomparison with traditional structural
panels. In [7]alternative configurations for the ISS Columbusmodule
shielding were evaluated, one of whichincluded an open-cell
aluminum foam bLmiper. Thisconfiguration was found to provide
increasedprotection over the reference Columbus stuffed
Whipple shield at high velocities (>6 km/s) andnormal
incidence. For oblique angles, theperformance was comparable to the
referenceconfiguration at high velocities. For low velocitytesting,
the performance of the foam-bumperconfi guration was clearly worse
than the referenceshield, due to the inability of the foam bumper
toinduce projectile fragmentation. Although the foamconfi guration
provided a similar level of protectionoverall to the reference
stuffed Whipple shield, theauthors noted that the primary advantage
of themodified configuration are related to the extendedarea of the
pressure hull that can be protected (due toa concentration of mass
in the outer later), and toother design aspects such as a reduction
in non-ballistic mass (stiffeners, local reinforcements, etc.).
The shielding performance of sandwich panelstructures with
open-cell aluminum foam cores wasevaluated in [8] against that of
aluminum honeycombcore sandwich panels (Al HC SP). In Fig. 2
acomparison between damages induced by nominallyidentical impacts
are shown. It should be noted thatthe facesheets of the HC SP were
significantlythicker than those of the foam panel in order
toprovide comparable areal densities. In the figure, thefoam core
is shown to restrict fra gment radialexpansion to an equal or
greater degree than the HC_However, while fragments are expected to
bechanneled within the HC cells, the foam homogeneityshould ensure
that resistance to fragment cloudexpansion is equal in all
directions, therefore limitingthe degree of channeling. For these
impact
conditions, the performance of the foam panel isshown to be
clearly superior to that of the honeycombpanel.
TARGET DEFINITION
Double-laver honeycomb (DL-H)
The baseline tar get is constructed of two honeycombsandwich
panels, with two outer layers of stainlesssteel mesh and a
monolithic aluminum rear wall,shown in Fig. 3. Details of the
target components areprovided in Table 1. The total areal density
of theDL-H configuration is 1.57 g/=7^.
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_ACT TESTING AND RESULTS
A series of 19 hypervelocity impact tests wereperfornied on the
double-layer targets using the twostage light gas guns at NASA
JSC's White Sand'sTest Facility (WSTF). A summary of the
testconditions and results are presented in Table 3. Forthese
tests, failure was defined as the ejection ofmaterial within the
simulated pressure hull (i.e.perforation or detached spall of the
target rear wall).
Test Target Angle Diameter Velocity Result(deg) (can) (km/s)
1 8592 DL-F 0 0.877 6.76 Pass2 8593 DL-F 45 0.837 6.87 Pass3
8594 DL-F 60 1.114 66.9 Fail4 8595 DL F 0 0.717 3.29 Fail5 8599
DL-F 60 1.005 7.03 Fail6 8596 DL-F 0 0.637 3.67 Fail7 8597 DL F 45
0.662 3.68 Pass8 8598 DL-F 45 0.837 3.62 Pass9 9024 DL F 60 1.005
6.80 Pass10 9038 DL-F 60 1.115 6.69 Pass11 9064 DL-F 60 1.276 7.00
Fail12 7460 DL F 0 0.833 6.74 Pass13 7461 DL-F 45 0.873 6.89 Fail14
7458 DL-H 45 0.754 6.94 Pass15 7459 DL-H 45 0.650 6.88 Pass16 7504
DL-H 0 0.730 6.86 Pass17 7509 DL-H 0 0.754 6.93 Pass18 7510 DL-H 45
0.873 6.74 Fail19 7629 DL-H 0 0.833 6.91 Fail
Table 3. Impact test results.
In order to evaluate the effect of interchangingaluminum
honeycomb for open-cell aluminum foam,a direct comparison can be
made between impactdamages induced on both configurations
atnominally-identical impact conditions. In Fig. 6damages induced
in the DL-H and DL-F targets bythe impact of 0.833 cm diameter
projectiles at6.83±0.09 kin/s with normal incidence are
compared.Damage in the two mesh layers, and the entry hole onthe I"
sandwich panel are similar for bothconfigurations. The diameter of
rear facesheetmaterial peeled back from the I ` sandwich panel
exithole is also similar: however the extension of coredamage is
noticeably less in the foam. The throughhole in the 2nd panels is
shown to be significantlylarger for the DL-H confi guration than
the DL-Fshield (88x90 nun vs. 70x62 min), indicating that thedebris
cloud is more finely concentrated by the foamsandwich panel bumper
than the honeycombsandwich panel. The diameter of the through hole
inthe 2nd panels is similar to that of the core damage inthe first
sandwich panel for both configurations (--91vs. 84 1nm for DL-H;
--58 vs. _ 66 nun for DL-F).This suggests that the facesheets on
the 2 "d panel ofthe DL-H configuration have little effect on
the
expansion of the debris cloud (i.e. they have minimalre-focusing
effect).
The rear wall of the DL-H configuration isperforated, showing a
large through crack (80 mm inlength; 5 inln wide) and multiple
individual craters.Given the appearance of the through crack, it
isexpected that failure of the rear wall occurredthrough
penetration of individual solid fragmentswhich acted as crack
initiation sites that werepropagated during the impulsive load of
the fragmentcloud. The rear wall of the DL-F configuration
issignificantly deformed, yet there is no perforation ordetachment
of spalled material from the rear surface.The majority of deposits
on the rear wall are frommolten aluminum, visible as the bright
silver coatingin the target photograph. The rear wall shows
somecratering from impact of individual solid fragments,which form
small dimples on the rear side of thepanel. Under these impact
conditions, theperformance of the DL-F shield is clearly superior
tothat of the baseline DL-H shield.
Evaluation of shield performance
The effect of secondary projectile and bumperfragment impacts
upon individual foam cellligaments is expected to lead to
increasedfragmentation, melting and vaporization at lowervelocities
than for conventional shieldingconfigurations (e.g. Whipple shield,
honeycombsandwich panel). This mechanism is utilized in
themulti-shock shield, which was shown in [9][10] toprovide damage
features at 6.3 km/s representative ofthose seen at 10 km/s on
single bumper shields. Anapproximation of effective impact
velocities can bemade from projectile entropy (or internal ener
gy). In[11] Swift calculates required impact velocities formelt and
vaporization conditions based on theconcept of entropy trapping —
in which the entropyinjected into projectile and target materials
can becalculated from the Hugoniot and release isentrope.The
increase in entropy acts to raise the materialinternal energy (or
temperature); eventually reachingand exceedin g the material fusion
energy (melting)and vaporization energy.
The rear walls of the DL-H target in Fig. 6 shows adegree of
molten aluminum deposits, although thepredominant damage feature is
cratering about thecentral damage zone. Alternatively, the DL-F
targetshows significant molten aluminum over a largecentral area
with only a small number of finitecraters. Clearly, therefore,
secondary impacts on thefoam ligaments are effective in raising
fragmententropy.
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a*HrTF07a9
ow
4IT
SRI
F Y y^. ,.S
n^: h
.. ........i^
Fig. 6: Comparison of impact damages in the DL-H (left) and DL-F
(right) targets impacted by 0.833 cm diameter Al2017-T4spheres at
—6.9 lan/s with normal incidence (0'). From top to bottom: I"
sandwich panel (rear view), 2"a panel (rear view);rear wall (front
view).
5
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BALLISTIC LEVIIT EQUATIONS
To evaluate the effect of the shielding modificationsover the
complete range of expected in-orbit impactconditions, ballistic
limit curves can be used.Calculated using empirical ballistic limit
equations(BLEs), these curves demarcate between impactconditions
leading to pass or fail, and are used inmodern risk assessment
codes such as NASA'sBUMPER-II to determine mission risk
tomicrometeoroid and orbital debris (MMOD).
The DL-H configuration is representative of theentranced zone 11
shield onboard the FGB module ofthe ISS (Zarya) [12]. For FGB
shielding, a genericballistic limit equation was defined based on
theNNO Whipple shield equation [6]. In order to adjustthe equation
to suit the double-layer honeycombconfiguration, the bumper
thickness was estimatedusing the areal density of the I"
honeycombsandwich panel, and half the areal density of the
2"`1sandwich panel. The remaining 50% of the 2'dsandwich panel
areal density was added to thethickness of the shield rear wall,
and the equationconstants were empirically adjusted from test
data.The enhanced zone 11 FGB ballistic limit equation isdefined
(from [ 12]) as:
Hiah velocity: when V >_ 7/cos 0,
d,, = Cx (V COS 0)-2/3 pp-1/3
(1)
where V — Projectile velocity (km/s)VL — Low velocity regime
upper limit (k111/s)d,— Critical projectile diameter (cm)CH — High
velocity fit coefficient (-) = 4.6510 — Impact angle (deg)pP —
Projectile density (g/cm3)
Intermediate: when 3/cos 0 > V > 7/cos 0,
d,, =Chipp-1/3(V COS 0— VL)+...
(2)cl pp-9/19 (COS 8)-18
/19
(Vx — V COS B)
where VH — High velocity regime lower limit (kln/s)Ch; —
Inter.-high velocity fit coefficient (-) = 0.318C li — Utter.-low
velocity fit coefficient (-) = 0.203
Low velocity: when V 5 3/cos 0,
d = C, (COS 0) -31/11 V-12119pp -9/19
(3)
where CL — Low velocity fit coefficient (-) = 1.629
The diameter of the steel wire used in the enhancedzone 11
shield was 0.280 nun, less than that of theDL-H configuration
tested in this study (0.4064 nun).As such, the ballistic limit
equation constants must be
adjusted in order to fit the test data reported in Table3. The
low and high velocity coefficients, C L and CHrespectively, are y
calculated based on the arealdensities of the individual shield
components:
C = 3.11 t + (2AD._, + AD,, r + ADS z
O
)4w
2.8
CH = 3.52 + 3.OADwesh (5)
The intermediate fit coefficients are calculated as0.209 and
0.290 for C 11 and Chi respectively.
For the DL-F configuration, the areal densities of thespecific
shield components are also included in theequation fit
coefficients. For honeycomb sandwichpanels, the mass of the core is
generally ignored indetermining effective shield thicknesses (i.e.
treatedas non-ballistic mass). For foam core sandwichpanels,
however, the foam is an active shieldingcomponent. The ballistic
limit equation for the DL-Fconfiguration is defined as:
High velocity: when V >_ V H/cos 0,
de = C.x (V COS a)-a pp -113 (6)
where P — High velocity angle dependence constant (-) = 0.55
CH =3.0+2.4xADp^
Inter. velocity: when VL/cos 0 > V > VH/COS 0,
L) +d^(iV)—V
(VL).(V -VL) (7)de= dC(Vx L
Low velocity: when V:5 VL/cos 0,
de = CL (COs 9)—a
V -12119pp -9119 (8)
where a — LV angle dependence coefficient (-) = 1.75
C = 3(t., +(2AD..h + ADsPI + ADSP , )/2.8)
In Fig. 7 the ballistic limit curve of the modified DL-F shield
is plotted a gainst the baseline DL-Hconfiguration. For normal
impact, the modificationsresult in a small predicted improvement
over therange of applicable impact velocities. At 3 km/s theDL-F
target provides a 15% improvement in criticalprojectile diameter;
while at 7 km/s a 3% increase ispredicted. The larger low velocity
sizing constant(CL) leads to increasing performance gain
withincreasing impact obliquity, although there is a lackof test
data to support or disprove this extrapolation.
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UN.o
0.8
0.6
12 15
12 15
Velocity (km/s)
0.4 '
0 3 6 9 12 15
Velocity (km/s)
1.4
DL-H
DL-F1.2
Velocity (kin/s)
Fig. 7: Ballistic limit curves of the DL-H and DL-F shields.From
top to bottom: 0°, 45°, 60°.
DISCUSSION
Sandwich panels with open-cell metallic foam coresprovide
comparable mechanical and thermalperformance to those with metallic
honeycomb coresfor a minimal weight penalty. The homogenous
foamstructure avoids the MMOD shielding-detrimentalchanneling cells
of honeycomb panels, making thema promising alternative for
spacecraft primarystructures which are also required to provide
MMODprotection.
Destefanis et al. [13] reported on tests against a dualwall
configuration with a bumper of open cellaluminum foam. In the tests
a good deal of meltingwas observed at velocities as low as 2 km's,
withcomplete melting reported at velocities as low as 4knvs.
Similar enhanced fragmentation was reportedin [14] for nrm-sized
projectiles at normal impact. Inthis study, clear evidence of
melted deposits wasobserved on the target rear wall for test
#4(HITF08595), performed at 3.29 krn/s. For lowvelocity impacts at
oblique impact (e.g. test #6(HITF08596)) there was also clearly
observabledeposits of melted aluminum upon the shield rearwall.
Although the onset and degree of projectile andbumper melt is
clearly increased by the open-cellfoam bumpers, in all impact tests
performed there isevidence of solid fragment impacts upon the
targetrear walls. For oblique impacts, these solid fragmentcraters
are generally in-line with the projectilevelocity vector,
indicating that they are most likelyprojectile remnants.
In [14], the velocity regime transition limits of aballistic
limit equation for the foam bumper shieldingconfiguration were set
at 2.7 and 6.5 km'srespectively, in recognition of the
increasedfragmentation and melting provided by the
structure(compared to a traditional Whipple shield). However,due to
the evidence of individual solid fragmentimpacts upon the shield
rear wall for impactvelocities up to 6.76 km/s in this study, and
in theabsence of additional test data providing clearexperimental
justification; the transition velocitiesdefined in [6] for aluminum
Whipple shields and in[12] for the DL-H confi guration are
maintained in theballistic limit equation derived for the DL-F
shield.
Enhanced fragmentation and melting induced by thefoam
microstructure was found in [13] to beineffective against
projectiles in the cm-sized range atnormal incidence; and nun-sized
projectiles atoblique angles. The authors concluded that
secondaryimpacts were no longer able to induce fragmentationand
melting of the entire projectile at these impact
1.2
1
0.8
0.6.o
0.4
0.2
1.2
b 0.8
UN.o
0.6
-
conditions. In this study, however, there was nonoticeable
decrease in performance at obliquity, evenfor projectiles
considerably larger than 1 cm indiameter (e.g. test 411
(HITF09064)). The doublelayer of mesh on top of the I" sandwich
panel of theDL-F configuration is expected to break up
theprojectile prior to impact on the sandwich panelfacesheet.
Therefore, smaller projectile fragments arepropagated to impact
within the sandwich panel foamcore and the size-limitations of
secondaryfragmentation and melting discussed by Destefanis etal.
are not valid.
CONCLUSIONS AND StWIMARY
In this paper, the effect on shielding performanceachieved by
replacing metallic honeycomb cores formetallic open-cell foam cores
in a double sandwichpanel MMOD shielding confi guration
representativeof those used onboard the ISS was assessed. Abaseline
double-layer honeycomb (DL-H), andmodified double-layer foam (DL-F)
configurationwere subject to impact by projectiles athypervelocity,
from which ballistic limit equationswere derived. These equations
were based on theNNO IN'hipple shield [6] and general FGB
[12]equations, and included fit coefficients based on
arealdensities of individual shielding components. Atnormal
incidence the foam-modified shield wasfound to provide a 15%
improvement in criticalprojectile diameter at low velocity (i.e. 3
km/s) and a3% increase at high velocity (7 km/s). Withincreasing
impact obliquity the foam shieldperformance enhancement increases
at the low-shatter regime transition velocity, up to a
29%improvement in critical diameter at 60°. It should benoted that
the double-layer honeycomb equationconstants are defined for
consistency with theenhanced zone 11 shield described in [12], for
whichthere is no low velocity test data.
The presence of honeycomb cells is considered to bedetrimental
to the shielding performance of a dual-wall configuration due to
thecell walls acting torestrict the expansion of projectile and
bumper (orfront facesheet) fra gments — referred to aschanneling.
However, the thickness of thehoneycomb sandwich panels in the
double-layerconfiguration are less than twice the diameter of
eventhe smallest projectile used in the testing. Thus,dispersion of
the projectile and bumper fra gments isexpected to be uninterrupted
prior to impact upon thesandwich panel rear facesheet. As such,
theperformance enhancement gained by replacing thehoneycomb core
with open-cell foams is not expected
to result as a simple absence of through-thicknesschanneling
cells. Rather, secondary impacts ofprojectile and bumper fragments
upon individualfoam cell ligaments induced repeated
shocks,increasing fragment entropy and subsequentlyreducing
Vfailure strengths. Evidence of increasedprojectile fragmentation
and melting was shown forthe double-layer foam configuration
(compared to thedouble-layer honeycomb configuration).
Previousinvestigations on metallic open-cell foam bumpershave noted
a decrease in performance for obliqueimpact, and normal impact of
large cm-sizedprojectile due to an inability of the repeated
shockingprocedure to fragment the entire projectile at
theseconditions. However, the presence of the doublemesh outer
layers breaks up the projectile prior toimpact upon the I't
sandwich panel front facesheet,ensuring the propagation of smaller,
moremanageable impactors within the foam core.
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