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Byron W

Langley R

mber 2012

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form F

h Entry

W. Patterson a

Research Ce

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2-217763

Foam C

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and Louis J.

enter, Hamp

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pton, Virginia

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ultiation Mission 
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esearch Center

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h Entry

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Research Ce

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1-2199 

2

2-217763

Foam C

y Veh

and Louis J.

enter, Hamp

3

Crush T

hicle Im

 Glaab 

pton, Virginia

Testin

mpact A

a

ng for M

Attenu

ultiation Mission 
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iii

Contents

Contents....................................................................................................................................................................... iii

1. Abbreviations ............................................................................................................................................................1

2. Introduction ...............................................................................................................................................................2

3. Testing Method..........................................................................................................................................................4

3.1 Overview.............................................................................................................................................................4

3.2 Rohacell Foam ....................................................................................................................................................5

3.3 Hydraulic Loading Machine: ~0.17%/s and ~100%/s strain rates ......................................................................7

3.4Vertical Drop Tower: ~13,600%/s.......................................................................................................................8

3.5 Acceleration Analysis .......................................................................................................................................10

4. Data Analysis...........................................................................................................................................................11

4.1 Overview...........................................................................................................................................................11

4.2 Hydraulic Loading Machine..............................................................................................................................11

4.3 Vertical Drop Tower .........................................................................................................................................12

4.3.1 LabVIEWAnalysis Technique ....................................................................................................................12

4.3.2 Energy Balance...........................................................................................................................................15

4.3.3 Filter Techniques........................................................................................................................................16

4.3.4 Global Data Smoothing ..............................................................................................................................18

5. Results .....................................................................................................................................................................22

5.1 Foam Stress/Strain.............................................................................................................................................22

5.2 Acceleration Analysis .......................................................................................................................................27

6. Conclusions .............................................................................................................................................................28

References ...................................................................................................................................................................29

Acknowledgements .....................................................................................................................................................30

Appendix A – Current M-SAPE Database ..................................................................................................................31
1

Abstract

Multi-Mission Earth Entry Vehicles (MMEEVs) are blunt-body vehicles 

designed with the purpose of transporting payloads from outer space to the 

surface of the Earth. To achieve high-reliability and minimum weight, MMEEVs 

avoid use of limited-reliability systems, such as parachutes and retro-rockets, 

instead using built-in impact attenuators to absorb energy remaining at impact 

to meet landing loads requirements. The Multi-Mission Systems Analysis for 

Planetary Entry (M-SAPE) parametric design tool is used to facilitate the design 

of MMEEVs and develop the trade space. Testing was conducted to characterize 

the material properties of several candidate impact foam attenuators to enhance 

M-SAPE analysis. In the current effort, four different Rohacell foams are tested 

at three different, uniform, strain rates (~0.17, ~100, ~13,600%/s). The primary 

data analysis method uses a global data smoothing technique in the frequency

domain to remove noise and system natural frequencies. The results from the 

data indicate that the filter and smoothing technique are successful in 

identifying the foam crush event and removing aberrations. The effect of strain 

rate increases with increasing foam density. The 71-WF-HT foam may support 

Mars Sample Return requirements. Several recommendations to improve the 

drop tower test technique are identified. 

1. Abbreviations

a Acceleration (ft/sec2) ksi Thousand pounds per square inch

ag Acceleration in g units lbs Pound force

a(t) Acceleration of drop mass as a function 

of time mph Statute miles per hour

Ac Cross sectional area of payload sample m Drop mass

Ao Original area of foam sample ms Return sample mass

ASTM American Society for Testing and 

Materials m/s Meter per second

b(k) Sine series coefficients MMEEV Multi-Mission Earth Entry Vehicle

 

 Strain rate as a function of time MPa Mega Pascals

Δdi Displacement for current iteration M-SAPE Multi-Mission Systems Analysis for 

Planetary Entry

Δt Fixed time interval MSR Mars Sample Return

d(t) Crush displacement as a function of 

time n Filter order

D Diameter of payload sphere N Sample size

e Strain NASA National Aeronautics and Space 

Administration

ei Strain for current iteration Data smoothing filter

e(t) Strain as a function of time psi lbs per square inch

EDL Entry, Descent and Landing P(t) Crush load as a function of time

EEV Earth Entry Vehicle PMI Polymethacrylimide

ESA European Space Agency Ps Power spectral density
2

f(k) Frequency of Fourier series ρ Sample density

fco Cutoff frequency s Compressive strength of impact foam

ft Feet si Stress for current iteration

Fi Force for current iteration Crush stress as a function of time

FR Full Range slg/ft3Slugs per cubic foot

Fs Sample frequency SEM Scanning Electron Microscope

 Fourier transformation t Time

g Earth acceleration, (32.2 ft/sec 2) tend End of time period

g(i) Transformed periodic odd function 

signal ̃ 

Fourier transformation of the noise 

signal

 ̂ Smoothed periodic signal V(t) Velocity as a function of time

ho Original sample height v(t) Noise in the data signal

hd Drop height VI Virtual Instrument

Hz Hertz Vi Impact velocity

i Iteration index Wm Weight of drop mass

ISPT In-Space Propulsion Technology 

Program ̃ 

Fourier transformation of data signal

k Data smoothing index y(t) Data signal of interest

kg Kilogram z(t) Noisy continuous measured signal

kg/m3 Kilogram per cubic meter zs(i) Final smoothed measured signal

klbs Thousands of pounds %/s Percent per second, strain rate units

kN Kilo Newton

2. Introduction

Multi-Mission Earth Entry Vehicles (MMEEVs) are blunt-body vehicles designed with the purpose of 

transporting payloads from outer space to the surface of the Earth. MMEEVs are designed to serve as the 

last leg of missions that endeavor to gather samples from celestial bodies and return the samples to Earth 

for detailed analysis. NASA’s proposed Mars Sample Return (MSR) mission is one example of such a 

mission which aims to return 1.1 lbs (0.5 kg) of Martian samples [1]. NASA’s In-Space Propulsion 

Technologies (ISPT) Project, funded by the Science Mission Directorate, is continuing to conduct 

activities that will mature a class of vehicles in support of Earth entry, descent, and landing sample return 

mission phases. These vehicles serve as the base of future EEVs where key parameters such as the blunt 

body diameter or payload diameter can be scaled according to the specific mission requirements. 

Currently design assessment of MMEEVs up to two meters in diameter and a payload mass up to 30 kg is 

being conducted using the Multi-Mission Systems Analysis for Planetary Entry (M-SAPE) tool [2].

High reliability of sample containment is a requirement for transporting high-risk payloads. The MSR 

mission is an example of a high-risk payload due to the possibility living organisms in the sample which 

could contaminate Earth from an uncontrolled release due to the failure of the reentry system. The NASA 

and ESA Planetary Protection board have established a containment requirement calling for the 

probability of release of a Martian particle larger than 2.0 microns into the Earth’s biosphere to be less 

than 10-6[3][4][5].This small size and probability limit is more stringent than other robotic extraterrestrial 

missions. The demanding assured containment constraints further drive the design requirements of high 
3

risk missions, like MSR[6]. 

MMEEVs incorporate design characteristics that remove elements where high contingencies exist. 

Some of the high-risk contingencies excluded from the MMEEV design includes, but not limited to, 

parachutes, retro-rockets, and active guidance systems [3][6]. These systems are eliminated from the 

design space for multiple reasons including lack of flight heritage, operational complexities, and 

increased risk. To achieve high-reliability and minimum weight MMEEVs employ what is known as 

single-stage passive entry descent and landing (EDL) concepts. Single-stage passive EDL designs use 

built-in impact attenuators to absorb energy remaining at impact to meet landing loads requirements. 

The entire operation of the MMEEV after release is passive. The system is designed to hard land at a 

predetermined terrestrial landing location traveling at subsonic, terminal velocity. The structure of the 

MMEEV is therefore designed to minimize the impact that the returned sample experiences by 

incorporating impact attenuators around the sample canister. The NASA Langley concept for MSRMMEEV is depicted in Figure 1. For the current effort, the notable attribute of the MSR-EEV design is 

the inclusion of the impact sphere which encloses the extra-planetary sample. As documented by 

Mitcheltree et al, the purpose of the impact attenuator system is to limit the mechanical loads on the 

canister during landing. The sample canister accelerations during impact must not exceed 2500 g’s in 

order to maintain the integrity of the samples and the canister must not experience accelerations above 

3500 g’s so that the canister does not rupture [6]. Other research from Mitcheltree and Kellas delineates 

the maximum allowable deceleration as 400 g’s [8]. The disparity in these maxima demonstrates the 

varying level of payload maximum g-load requirements for the MSR project as the project requirements 

evolved. For the current work, the maximum allowable acceleration load for MSR is set at 2,500 g’s. 

However, discussion and analysis is also provided for the 400 g limit. The current effort derives the 

relationship between density of sample return and diameter of the impact sphere versus the maximum 

acceleration for the specific foams tested. These plots serve as necessary design tools for MMEEVs which 

are intended to be scalable for specific missions. One repeating design parameter in the background 

literature is the impact velocity of approximately 90 mph (~40 m/s) for the MSR-MMEEV. This 

constraint drives the design of the impact attenuator and consequently the purpose of this current research.

The objective of the present work is to perform analysis of data from a uniform strain rate foam crush 

test for the design of the MMEEV impact attenuator. The current effort endeavors to determine the initial 

linear slope of the stress-strain curve and the subsequent crush load for the foam samples. In the current 

Figure 1. NASA Langley MSR-MMEEV concept [7]
4

work, four Rohacell foams are tested in a hydraulic loading machine and a 14 feet vertical drop tower at 

three different strain rates. The data are analyzed to produce strain-stress relationships which are to be 

used to assess vehicle designs for multiple missions using the Multi-Mission Systems Analysis for 

Planetary Entry (M-SAPE) tool. The produced relationships are also useful for structural analysis 

programs, such as LS-DYNA, where the program requires stress curves at specific strain rates. The focus 

of this research is to produce the stress-strain curves for four different foams at strain rates representative 

of the entire impact event.

The foam testing focused on three different uniform strain rates which are selected to span the range of 

strain-rates during MMEEV impacts. As an MMEEV approaches its terrestrial landing, the vehicle 

components, namely the heat shield, vehicle structure, body foam, and the sphere canister, absorb all of 

the kinetic energy. The majority of the kinetic energy is designed to be absorbed by the body foam and 

the foam within the sphere canister. As the crushing of the MMEEV takes place, the distance between the 

nose and the sample decreases as the payload is decelerated. Therefore, the foam is experiencing different 

strain rates, a function of specimen height and velocity, as the MMEEV is dynamically crushed. The 

highest strain rate for the MSR-EEV vehicle is ~26,000 %/s, based on the baseline 6 inches (0.152 

meters) distance between the lower edge of the payload sphere to the rear side of the heat shield and a ~90 

mph (~40 m/s) impact speed. This current research expands the body of knowledge of Rohacell foams 

over a range of strain rates. The focus of this research is to produce the stress-strain curves for a family of 

candidate impact attenuator foams with various densities at strain rates representative of the entire impact

event.

3. Testing Method

3.1 Overview

The purpose of the current foam testing is to examine the response of four different foam materials 

over a range of strain rates. The target strain rates for this research effort are ~0.1, ~100, and ~10,000%/s. 

Testing at these exact strain rates is not important, but acquiring data at known strain rates that encompass 

the anticipated spectrum of conditions is critical. The largest strain rate has special significance because it 

is the same order of magnitude as the expected rate of the MMEEV at initial impact. The strain rate is 

dependent on two parameters, the impact velocity and the height of the specimen. Since the velocity will 

be the only changing variable in this test, it is necessary to use different testing techniques to obtain the 

different velocities. For the two lowest strain rates a hydraulic test machine is used. However, the highest 

strain rate required use of a 14 feet vertical drop tower. 

The testing of the four foams at three different strain rates was conducted over an extended timeframe 

due to resource availability and other priorities. The organization of the foam test effort establishes two 

initial phases. The first phase consists of the low speed hydraulic loading machine test conducted in 2010, 

which was completed at a strain rate of ~0.17%/s on the same four different Rohacell foam types. The 

second phase plan is further broken into four segments, two of which are pertinent to the current report. In 

the first segment, the goal is to determine if the foam in storage since the 2010 test series is still in 

adequate condition for the current testing. The initial finding from the spot-check effort in December 

2011 revealed that the four foams that were in storage for approximately one year are in adequate 

condition for subsequent testing. The second segment of the second phase is to use the hydraulic loading 

frame for ~100%/sec testing with the third segment employing the vertical drop tower to test the foams at 

the ~10,000%/s strain rate. One of the goals in the tentative fourth segment is to test the foams using the 
5

hydraulic loading frame at a strain rate of ~100%/s at elevated temperatures. Testing of the foams at the 

~100 and ~10,000%/s rates were conducted in January and February 2012 at room temperature. The 

analysis of the data from the 2010 and 2012 tests are the focus of the current effort. 

3.2 Rohacell Foam

Four different Rohacell foams, representing three different densities, are used in the current dynamic 

analysis. Table 1 provides the foam types selected, advertised density, compressive strength, shear 

strength, and the heat distortion temperature. There are two foams with the same 0.21 slg/ft3(110 kg/m3) 

density, however, the 110-XT-HT yields a higher distortion temperature. It is necessary to investigate a 

foam with a higher temperature tolerance because of the potential for high temperatures prevalent in 

MMEEV designs during the EDL phase. Rohacell, a closed-cell polymethacrylimide (PMI) foam, was 

chosen for the current effort because of its prevalent use in the space and aviation industry and its use for

structure impact attenuation. In the aerospace industry, Rohacell is used in Boeing’s Delta II, III, and IV 

rockets for noise attenuation and in the pressure bulkhead of Airbus’ A380 and A340[9]. Table 2 shows 

the number of samples and strain rate for each foam tested.

Table 1. Tested Rohacell foams

# Foam Density Compressive

Strength

Shear

Strength

Heat 

Distortion 

Temperature

kg/m3slgs/ft3 MPa ksi MPa ksi °C °F

1 71-WF-HT 75 0.15 1.7 0.25 1.3 0.19 200 392

2 110-WF-HT 110 0.21 3.6 0.52 2.4 0.35 200 392

3 110-XT-HT 110 0.21 3.6 0.52 2.4 0.35 240 464

4 200-WF-HT 205 0.40 9.0 1.3 5.0 0.73 190 374

Table 2. Number of samples tested for each strain rate

# Foam Number of Samples Tested

~0.17%/sec ~100%/sec ~13,600%/sec

1 71-WF-HT 8 10 8

2 110-WF-HT 8 10 10

3 110-XT-HT 8 10 10

4 200-WF-HT 8 10 10
6

The current state of the M-SAPE foam database is shown in Appendix A and the foams of investigation 

are highlighted. The Rohacell WF foam is used for three of the foams because the WF model is 

specifically designed for aerospace applications, and is the type used in the aforementioned aerospace 

examples. The XT model is used for its increased heat distortion temperature, as previously discussed. 

Figure 2 displays the foams in the M-SAPE database where the density is the independent variable and 

the compressive strength is the dependent variable. The selected foams span the range of compressive 

strengths in the foam database. The current database, displayed in Figure 2, is based on the advertised 

data likely acquired at very slow strain rates (~0.17%/sec). Even though the database already contains the 

selected foams, elements considered missing from the current database include dynamic test properties, 

such as different strain rates. 

The test specimens are manufactured from a foam sheet that is two inches (0.0508 meters) thick. The 

shape of the specimen is circular because of the ease of manufacturability and comparison to previous 

tests that were manufactured to the same shape. The diameter of the cylinders is 1.875 inches (0.0476 

meters). Variations in the dimensions of the specimen are present due to the manufacturing technique, 

albeit, the variations are ±2% for the diameter and less than ±1% for the length. Despite the larger 

variation range for the diameter, the error is not expected to impact the integrity of the testing results 

because the variation is orthogonal to the crush direction. However, these variations are accounted for in 

the drop tower test LabVIEW analysis tool by calculating the area of each specimen from multiple 

measurements using calipers. The sample manufacturing technique of the diameter and length are

displayed in Figure 3 and Figure 4, respectively. These figures explain the reason for the manufacturing 

variation, namely, the coarse hole-saw is the reason for the larger variation in the diameter. The finer 

band-saw blade yields more accurate dimensions, which is evident by the small length dimension 

variation.

Figure 2. M-SAPE foam compressive strength database.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

All foams

Selected foams

y = -0.24496 + 3.1177x R= 0.94591 

Compressive strength (Ksi)

Density (slg/ft3)
7

Figure 3. Specimen diameter manufacturing Figure 4. Specimen length manufacturing

3.3 Hydraulic Loading Machine: ~0.17%/s and ~100%/s strain rates

The first segment of the testing matrix looks at the two lowest strain rates, ~0.17 and ~100%/s, using a 

hydraulic loading machine/frame. The cylindrical samples were tested using a MTS Servo-hydraulic 

Floor-Standing Load Frame Model 370.10. One of the notable specifications of this system is the force 

capacity of 22.5 klbs (100 kN), which provides a compressive strength of 8.1 ksi (56 MPa) for the 

selected sample geometries, making the system’s capabilities more than sufficient to surpass the 

compressive strength of the 200-WF-HT foam. The machine also permits the operator to perform a varied 

rate of compression for different tests[10].

The data acquisition instrumentation for the MTS 370.10 used at the NASA Langley Aircraft Landing 

Dynamics Facility is the MTS 661.20H-03 (661.20F-03) Force Transducer [11]. The 661.20F-03 has a 

force capacity of 22.5 klbs (100 kN), a static overload capacity of 33.7 klbs (150 kN), and a 0.00197” 

(0.05 mm) deflection at 22.5 klbs (100 kN). The load cell has a 0.08% full range (FR) non-linearity error 

and 0.05% FR hysteresis error [12]. The combined non-linearity and hysteresis error present at full range, 

or force capacity, is only ±29.2 lbs (±130 N). This analysis indicates that the required maximum force, 

3.60 klbs (16 kN), for this effort, does not approach the 22.5 klbs (100 kN) force capacity of the MTS 

661.20H-03 force transducer. Therefore, the influence of hysteresis or non-linearity error is not expected 

to significantly compromise the integrity of the results. In addition, since the samples do not undergo a 

loading-unloading cycle, hysteresis error is further expected to be non-present. The results that are drawn 

from the force transducer software are the displacement of the servo-hydraulic crosshead and the force at 

each respective displacement location. 

The ~0.17%/sec test was performed following the ASTM D1621 Standard Test Method for 

Compressive Properties of Rigid Cellular Plastics. This standard dictates that the rate of compression be 

0.1 inches/min for each inch of specimen thickness, making the required velocity for the current effort 0.2 

inches/min. However, it is important to note that for the purposes of this effort, there is a requirement to 

have strain rates on the same order of magnitude as those anticipated for MMEEV initial impacts. ASTM 

D1621 specifies that compression of the specimen is to continue until the yield point or until the specimen 

has been compressed 13% of the original thickness, whichever occurs first[13].For the current effort, the 

specimens were compressed further to define the usable strain limit. As a result of this the samples were 

compressed to 20% of their initial length (i.e. 0.8 strain). The ASTM D1621 is used for the current effort 

so that the lowest strain rate can be compared to the data produced by the manufacturer of the foam, who
8

used the same compression test standard. The only rate that is used for comparison to the data produced 

by the manufacturer is the ~0.17%/s. 

A primary concern for the data acquisition from the hydraulic loading machine is the introduction of 

error. The foremost error component for the two slowest strain rates is human error. As dictated by 

ASTM D1621, the specimen needs to be placed between the compression platens with the specimen 

center-line aligned with the center-line of the compression platens. The intention of this requirement is to 

ensure that the load is evenly distributed over the entire loading surface of the specimen. Error in the 

testing could occur if the stipulation of the ASTM standard is not followed. Picture documentation 

indicates that the standard was followed and minimal error is introduced into the testing due to this type

of human error. Human error introduced due to operation of the machine during a test is not expected 

because once the rate of load is established by the operator, the machine automatically controls the rate 

during the entire test.

3.4Vertical Drop Tower: ~13,600%/s

The second segment of the testing matrix looks at the highest strain rate of approximately 

~13,600%/sec, using the 14 feet vertical drop tower. The high strain rate requires a high velocity for the 

duration of the crush stroke. Large servo-hydraulic machines are capable of producing the required 

velocity for the entire crush, however but were not available for this effort. A low-cost method to achieve 

the high-rate strain is possible by using the guided drop mass technique [14]. The schematic for the 

guided drop mass technique is shown in Figure 5 and the actual configuration just prior to impact is 

shown in Figure 6. The configuration consists of a drop mass plummeting in free fall along guide rods 

until it collides with the foam sample. A crushable foundation is necessary to absorb excess energy and 

protect sensors for this effort. Because of the need for a quasi-uniform strain rate over the entire foam 

sample crush stroke excess energy is required. By using the foundation, when the foam sample’s energy 

absorbing capacity is reached, the remaining kinetic energy is absorbed by the crushable foundation. 

Without the foundation, the highest usable impact velocity of the drop mass is limited by the amount of 

energy that the foam sample can absorb and the load capacity of the sensors at the end of the impact. The 

resulting lower energy impact would create a varying strain rate over the duration of the crush which is 

not acceptable. 

Ideally, the properties of the crushable foundation are chosen such that its stiffness and strength are 

significantly larger than that of the foam sample. Failure to choose a sufficient crushable foundation will 

result in damage to the vertical drop tower and accelerometers, and erroneous data. The foundation used 

in this effort, shown in Figure 6, is Dura-Core II 5052 Aluminum Honeycomb 6.5-3/8-0.0050 which has a 

compressive strength of 1.12ksi (7 MPa)[15]. This crushable foundation, chosen due to its material 

properties and availability, surpasses the compressive strength of all the foams tested. While the crushable 

foundation material has a lower crush strength than the 200-WF-HT foam, the area of the foam samples is 

only 11% of the foundation base. Since the rigid plate spreads the load over the entire crushable base, the 

load on the crushable base from the foam sample crushing event is 0.144 ksi (0.99 MPa).
9

Figure 5. Drop mass configuration[14] Figure 6. Actual drop mass configuration

With knowledge of the drop height, it is possible to predict the velocity that the drop mass will have at 

the point of impact. This technique allows for the principal investigator to easily manipulate the test so 

that any rate of displacement can be achieved. The relationship of the velocity of impact to the height of 

the drop mass is depicted in Eq. (1), where g is the acceleration of gravity and hd is the height of the drop 

mass from the rigid flat plate. This relationship is easily derived by realizing that the potential energy 

converts completely to kinetic energy during impact, other than frictional losses on the guide rods.

The drop tower used for this current effort has a maximum drop height of 14 feet resulting in a

maximum velocity of 360 inches/sec and maximum strain rate of 18,000%/s for a two inch (0.0508 m)

sample. The initial goal for this effort is to produce a strain rate approximately equal to ~10,000%/s and 

therefore an impact velocity of 200 inches/sec. The height to achieve this rate is 4.3 feet, however in order 

to compensate for the influence of frictional losses and greatly overload the foam sample to generate a 

quasi-steady strain rate, the nominal drop height is eight feet. This provides a strain rate of ~13,600%/sec. 

It is important to note that while the height is almost double the requirement, the impact velocity only 

increases by 1.41 times. As can be incited from Eq. (1), the theoretical impact velocity is not dependent 

on the weight of the cross bar (drop mass), but two different weights are used for the foam types. The 71 

and 110 series foam tests use a drop weight of 102.3 lbs (46.4 kg) and the 200 series uses a drop weight 

of 204.8 lbs (92.9 kg). The higher weight is used for the 200 series so that there is enough energy to 

ensure a uniform strain rate during the entire crush event. This is necessary because the compressive 

strength of the 200 series is more than double that of the nearest tested foam, as depicted in Table 1.

Another key component of the drop mass configuration is the instrumentation to record the crush event. 

In Figure 5 there are two accelerometers where one is on the drop mass and the other is on the rigid flat 

plate. For the actual configuration, the bottom accelerometer is placed in the crushable foundation on the 

bottom side of the rigid flat plate so that the drop mass does not damage the accelerometer. However, care 

must be taken so that the foundation does not crush to a point where the accelerometer is damaged. There 

are two configurations possible for the accelerometers. The first configuration is to have the 

accelerometer only on the drop mass such that the velocity and displacement can simply be found by 

 √ 

(1)

Foam Sample Accelerometer

Accelerometer

Flat Platen

Guide Rods

Crushable 

Foundation

V(t) Drop Mass

Rigid Flat Plate
10

performing integration of the acceleration data. In the second configuration, there are two accelerometers 

that document the acceleration during the entire crush event. The accuracy of the first configuration is 

dependent on the condition that the foundation does not move as the foam sample is crushed. If this 

condition cannot be satisfied, it is necessary to include the second accelerometer which records the 

movement of the rigid flat plate while the foam sample is crushed. The crush displacement of the foam 

sample is therefore found by taking the double integration of the relative acceleration between the drop 

mass and flat plate accelerometer. This method is the same as that documented by Kellas, who illustrates 

a technique to obtain a quasi-uniform strain rate using a vertical drop tower [14]. 

As noted, the current effort employs two accelerometers in order to record any movement of the 

crushable foundation of the rigid flat plate. The drop mass and the rigid plate accelerometer both use the 

Model 3801A-0200 accelerometer manufactured by Measurement Specialties. This accelerometer is 

designed for impact and structural testing and has an acceleration load of ±200 g. According to plots 

documented by Kellas, the expected acceleration load for a drop tower test is approximately 20 to 50 g, 

with spikes occurring much higher [14]. To guard against the acceleration spikes, the Measurement 

Specialties accelerometer includes a shock limit up to 10,000 g, thereby protecting the accelerometer. The 

accelerometer has a frequency response range of 0-1500 Hz where the sensitivity of the sensor will stay in 

a five percent error band. Another useful attribute of this accelerometer is its ability to be stud mounted, 

which allows for it to be easily attached to the drop mass and the underside of the rigid flat plate[16].

As with the hydraulic loading machine, there are concerns for error in the testing of the foam specimen. 

For the vertical drop tower test, the largest error is expected to be attributed to the LabVIEW analysis of 

the acceleration data, namely the location of the data markers; this is discussed in the Data Analysis 

section. In the testing of the foam specimen, some potential errors include incorrect placement of foam in 

the tower and interference of the cables for the accelerometers. It is unlikely that the former potential 

error is present due to the flat rigid plate and the large size of the drop mass. The latter error concern 

occurred in one instance of the testing and was noted. Consequently, the data was not included in the 

analysis and the error was avoided.

3.5 Acceleration Analysis

The major characteristic of MMEEVs is their ability to be scalable for the specific mission. As part of 

this foam crush testing, it is necessary to understand the reaction acceleration loads on the mission 

samples as the MMEEVs are scaled. Two of the major scalable constants for MMEEVs are the diameter 

of the containment sphere and the density of the returned samples. The containment sphere for the EEV 

for MSR [6] has a nominal diameter of 0.525 feet (0.16 meters), sample mass of 7.9 lbs (3.6 kg)and 

sample density of 3.30slg/ft3(1,701 kg/m3)The acceleration requirement of 2,500 g’s [6] is assumed for 

this analysis. In the current acceleration analysis, eleven scaled density values are employed ranging 

from half to double the nominal MSR value. Similarly, nineteen scaled sphere diameter values are 

explored ranging from 0.5 to 5.0 times the nominal MSR value.

The relationship for the acceleration loads analysis can be simply derived from Newton’s 2nd Law. 

Starting in Eq. (2), the force component is broken into the compressions strength, σ, the cross-sectional 

area, Ac, and is equal to the mass of the return sample, ms, and the acceleration load, a.

 (2)

The derivation for the acceleration loads analysis continues into Eq. (3), where the cross-sectional area 
11

of a sphere can be used. Additionally, the sample mass is equated to the sample density and sphere 

volume. As a result, a relationship can be established between impact foam stress and payload 

acceleration based on payload density, ρ, and sample diameter, D. 

 (

 

 

 

 

) (

 

 

 (

 

 

)

 

) 

(3)

Rearranging Eq. (3) and solving for the acceleration load, the equation for the acceleration loads 

analysis is complete in Eq. (4). An additional variable in the final version of Newton’s 2nd Law derivation 

is the division of the acceleration of gravity, g, which makes the entire equation non-dimensional and 

written as

 

 

 

 

 

(4)

The compressive strength in Eq. (4) represents the maximum compression static strength of each 

respective foam. To determine the maximum acceleration the maximum compressive strength should be 

used for the impact conditions considered. The sample density and the sphere diameter are the scalable 

constants from the nominal MSR values, as previously mentioned. 

4. Data Analysis

4.1 Overview

The analysis of the data from the hydraulic loading machine and the vertical drop tower underwent two 

different techniques because of the vastly different testing methods. In the ~0.17%/s and the ~100%/s 

tests, the produced data included the displacement of the crush load and the force at the respective 

displacement. For the ~13,600%/s test, the primary data includes the acceleration of the drop mass and 

the rigid flat plate during the entire crush event. 

4.2 Hydraulic Loading Machine

The analysis of the data from the hydraulic loading machine consists of transforming the force and 

displacement data from the machine’s software to a stress and strain curve. The i

th strain is calculated by 

dividing the ith displacement directly measured from the machine, Δdi, by the original height of the 

sample, ho, written as

 

 

 

(5)

In Eq. (6), the ith stress over the entire crush event is found by dividing the ith force from the hydraulic 

machine, Fi, by the original cross-sectional area of the cylindrical test specimen, Ao.

 

 

 

(6)
12

While the calculation of stress and strain is completed only using two equations, each dataset exhibited 

noise, thereby necessitating data manipulation. To smooth the data, the commercially available program, 

KaleidaGraph, was used. The software includes a built in function, called Smooth, which fits a function to 

the data by applying a Stineman function. The output of the Stineman function then has a geometric 

weight applied to the current point and ±10% of the data range [17]. The Smooth function in 

KaleidaGraph proved to be a sufficient tool in removing the noise in the data from the hydraulic loading 

machine. To further negate any irregular data behaviors, all of the stress-strain plots for each respective 

foam were averaged to arrive at the final plot.

4.3 Vertical Drop Tower

4.3.1 LabVIEW Analysis Technique

To analyze the data from the drop tower test, a different technique from the hydraulic loading machine 

is necessary because of the type of data available. The analysis of the data for the vertical drop tower is 

completed using the commercially available LabVIEW analysis software package, which is a system 

design platform for visual programming. A depiction of the virtual instrument (VI) created for this current 

effort is shown in Figure 7; elements of the VI are explained in detail below. The primary data from the 

vertical drop tower is the accelerometer data from two different sources, namely the drop mass and the 

rigid flat plate. For this current effort, the only accelerometer data that is used for analysis is the drop 

mass accelerometer. Initial analysis of the rigid plate accelerometer data indicated erroneous readings, 

due to the large acceleration loads. Neglecting the base accelerometer data primarily effects accurate 

determination of the maximum stroke strain.

Figure 7. LabVIEW vertical drop tower analysis tool

The incoming data from the accelerometer enters the VI as raw and therefore includes frequencies that 

are characteristic of mechanical vibrations and electrical noise. After the raw data is processed through 

the filter, which is explained in more detail in the following section, the next task is to identify the points 

of interest to determine the stress and strain of the foam crush event. Figure 8 displays the filtered

time/acceleration plot of a crush test for a 71-WF-HT sample. Several key events are evident in the figure 

that defines specific phases of the crush event. At approximately 1.6 seconds, the accelerometer reads 

zero, indicating that the drop mass is stationary and has not been dropped. At 1.9 seconds, the reading is -

1.0, indicative of a free fall. From 2.3 to 2.4 seconds, there are two key phases occurring. The first event, 

more visible in Figure 9, is the foam crush. This phase is characterized by a slight decrease from -1.0 g to 
13

about -1.1 g and a sudden increase to a plateau of about 5 to 10 g. After the plateau, there is another 

sudden increase, which is where the foundation is crushing. From 2.5 to 2.8 seconds, the accelerometer 

documents the cyclical motion of the drop mass as it rebounds (large positive g’s) and free falls again

(negative g’s). Eventually, this motion damps out and approaches an acceleration of zero. The time of 

these events and the magnitude of the acceleration loads differ from one foam test to the next.

Figure 8. Overall crush event of 71-WF-HT foam

Figure 9. Foam crush event of 71-WF-HT foam 

Included on Figure 8 and Figure 9 are line markers that are used by the VI to determine the locations of 

key occurrences and process the data. Table 3displays the general location of each marker and the 

purpose of each marker or group of markers. It is important to note that the VI locates these markers 

automatically, however, prior to processing the data the user must verify the locations, so as to ensure that 

the VI found the correct key points. A large amount of effort was placed in automating the marker 

locating system so that the marker locations were consistent for all of the data analyzed. Without the 

automation, the consistency of the marker locations is completely dependent on the user. The automation 

allows users who are not familiar with the problem and do not know what curve trends to locate to 

process data from the vertical drop tower with consistency. It is also believed that by automating the data 

process error attributed to humans is minimized, since the process is the same for all drop tower tests. 

-10

-5

0

5

10

15

20

25

30

1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9

Acceleration (g's)

Time (s)

0 1 6

-5

0

5

10

15

20

2.347 2.352 2.357 2.362

Acceleration (g's)

Time (s)

2 3 4 5
14

Table 3. Marker locations

Marker 

Number

Location Purpose

0 100 ms (few thousand points) behind marker #1 Data adjustment #1

1 Start of mass drop; accelerometer reads 0 g

2 5 ms (few hundred points) behind marker #3 Data adjustment #2

3 Start of -1.0 g to -1.1 g drop; accelerometer reads -1.0 g

4 Start of foam crush event; accelerometer reads 0 g Starting point for foam crush event data

5 End of foam crush event; accelerometer reads 40-50 g Ending point for foam crush event data

6 End of first rebound; accelerometer reads -1.0 g End point for Energy Balance Check #2 

and #3

After the data is initially filtered and the marker locations are found, the raw data and the initial filtered 

data are adjusted based on the locations of the markers. In the first data adjustment, all of the raw and 

initially filtered data is shifted upward by the average value between marker #0 and marker #1. This 

adjustment is implemented to negate any non-zero accelerometer readings at the onset of the testing. The 

second data adjustment shifts the raw and initially filtered data by the average value between marker #2 

and marker #3. The magnitude of the average value represents the weight of the drop mass minus any 

friction resistance produced by the guide rods. These two data shifts are consistent with the foam crush 

testing technique developed by Kellas [14].

Using the raw data after the data shifts, the velocity and displacement of the crush event are found. The 

crush velocity as a function of time, V(t), is found by taking the integration of the raw acceleration data 

from marker #4 to marker #5. Similarly, the crush displacement, d(t), is found by taking the double 

integration of the raw acceleration data. The impact velocity, Vi, is found by taking the integration of the 

raw acceleration data from marker #1 to marker #4 and locating the minimum value. This value is then 

multiplied by negative one to get the final impact velocity.

After the initially filtered data is processed through the global data smoothing tool, several mechanical 

properties can be determined. The crush load, P(t), is found by multiplying the smoothed acceleration 

response between marker #4 and marker #5, a(t), by the weight of the drop mass, Wm, as shown in Eq. (7).

 (7)

The crush stress, σ(t), is calculated by dividing the crush load, P(t), by the original cross-sectional area, 

Ao, and is written as

 

 

 

(8)

The strain, ε(t), is found by dividing the crush displacement between marker #4 and marker #5, d(t), by 

the sample’s original height, ho, and is found by

 

 

 

(9)
15

The final property that is calculated is the strain rate over the crush event. The strain rate, ∂ε/∂t, is 

calculated in Eq. (10) by dividing the crush velocity between marker #4 and marker #5, V(t), by the 

sample’s original height, ho. The strain rate is a necessary quantity to inspect because the purpose in this 

current effort is to produce stress-strain relationships at a fixed strain rate. The strain rate is considered to 

be uniform if the percentage difference between the rates at the 10% and 80% stroke locations is less than 

10%. In general, percentage change in strain rate between 10% and 80% strain was approximately 5%. 

 

 

 

 

(10)

4.3.2 Energy Balance

Since the analysis technique for the vertical drop tower relies on the accuracy of the accelerometers and 

the correct marker location identification, a number of energy balance checks are included in the drop 

tower test VI. The energy balance analysis serves as a check on the integration of the accelerometer data

and the analysis technique. 

The first energy balance check is shown in Eq.(11). This equation states that the potential energy of the 

drop mass must be greater than the kinetic energy at the point of foam impact. The potential energy is a 

function of the weight of the drop mass, Wm, and the drop height, hd. The kinetic energy is a function of 

the impact velocity, Vi, and the mass of the drop mass, m. 

 

 

 

 

 

(11)

The second energy balance check states that the total energy absorbed to foundation crush must be 

greater than the energy absorbed to foam crush, as shown in Eq. (12). The former quality is found by 

integrating the crush force from marker #4 to marker #6, P(t)4-6, over the displacement from the same 

markers, d(t)4-6. Similarly, the energy absorbed to foam crush is found by integrating the crush force from 

marker #4 to marker #5, P(t)4-5, over the displacement of the foam crush, d(t)4-5. This condition should be 

easily satisfied since the area under the acceleration curve from marker #4 to marker #6 is significantly 

larger than the area under the curve from marker #4 to marker #5. This check is considered as a method to

check against egregious errors.

∫ 

 ∫  

(12)

The third energy balance check uses calculations from the previous two checks. The third check states 

that the potential energy must be greater than or equal to the total energy absorbed to foundation crush. 

The check is written as

 ∫ 

 

(13)

Another simple check of the integration and marker locations is to calculate the maximum 

displacement from marker #4 to marker #5. The calculated maximum displacement must be less than the 

original height of the test specimen. These energy balance checks are useful in creating the VI, however, 

once the VI is established, the checks are satisfied without any data manipulation. 
16

4.3.3 Filter Techniques

Before any of the data can be processed, adjusted, and mechanical quantities found, the data must be 

placed through a filter so that the key markers can be identified. Figure 8 and Figure 9 display the result 

of the filtered data. The initially filtered data is processed forwards and backwards using a 3rd order lowpass Butterworth filter. It is necessary to process the data from both directions in order to negate any 

phase shift variations in the data. In using a low-pass Butterworth filter, there are two primary control 

parameters of the filter. The order of the filter was chosen based on heritage work. The second control 

parameter is the cutoff frequency of the low-pass filter. In the technique used by Kellas, the chosen cutoff 

frequency was 1000 Hz for the 71 and 110 type foams; the cutoff frequency was 500 Hz for the 200 type 

foam. In this current effort, analysis found that these cutoff frequencies were not sufficient to capture the 

initial crush slope of the foam. 

To determine the cutoff frequency for the initially filtered data, a set of cutoff frequencies was 

examined and a periodogram power spectral density was calculated for the entire raw acceleration data. 

Both of these techniques found the same ideal cutoff frequency in order to capture the initial slope. Figure 

10 displays the effect of filter cutoff frequency compared to the raw acceleration data. Figure 11 displays 

the impact cutoff frequency on the stress-strain curve. A total of nine frequencies were analyzed, 

however, only four are displayed for clarity. The plots show that the two lower cutoff frequencies have a 

small initial slope and minimal sinusoidal response over the crush stroke. At the highest frequency of 

4000 Hz, a steep initial slope is produced but other frequencies are introduced that increase the amplitude 

of the sinusoidal response. Note that the sinusoid response is evident in the raw data. It is hypothesized 

that the sinusoidal oscillatory content of the data may be due to either the honeycomb base and/or the 

rigid plate flexing during foam impact. Therefore, it was found that a cutoff frequency of 2300 Hz 

captured the initial slope sufficiently with reasonable sinusoidal response. This hypothesis is supported by 

the periodogram power spectral density analysis, as shown in Figure 12 and Figure 13. In the 

periodogram, which displays the frequencies of the acceleration signal, the density of the signal 

diminishes after 2300 Hz, indicating that the frequencies above have little significance and only introduce 

noise. This is evident in Figure 13, where there is a spike at 1500 Hz that diminishes as the frequency 

increases. The periodogram power spectral density analysis is found using Eq. (14), which is dependent 

on the discrete Fourier transformation of the acceleration, a(t), the sample size, N, and the sampling 

frequency, Fs. 

 

 | ( )|

 

 

(14)
17

Figure 10. Effect of filter cutoff frequency for the 110-

WF-HT foam

Figure 11. Effect of filter cutoff frequencyon stressstraincurve for 110-WF-HT foam 

Figure 12. 110-WF-HT spectral density Figure 13. 110-WF-HT high frequency spectral 

density 

While the increase in the cutoff frequency increases the initial slope and more effectively matches the 

lower strain rate results, a sinusoidal response is induced as shown in Figure 10. Figure 14 displays the

frequency of the stress-strain plot for each drop tower test for the 71 and 110 type foams; the data for this 

figure has only been processed using the Butterworth filter. The 200-WF-HT data is not included in this 

frequency analysis because of erroneous data, which is discussed in further detail in the Results section. 

All three foams exhibit a similar average frequency with a range between the three foams being 1460-

1473 Hz. The highest standard deviation is ±8.8 Hz. Even though the 2300 Hz cutoff frequency is needed 

for the initial slope, a frequency of approximately 1500 Hz is introduced, which is believed to be 

attributed to the structural frequency of the honeycomb foundation or rigid plate. This conclusion is 

supported by the periodogram power spectral density analysis of the rigid plate accelerometer (not 

presented), where a dominate 1500 Hz frequency is present. This frequency band must be removed since 

0

0.1

0.2

0.3

0.4

0.5

0.6

0 1000 2000 3000 4000

Power Spectrum Magnitude (dB/Hz)

Frequency (Hz)

0

0.002

0.004

0.006

0.008

0.01

1000 1500 2000 2500

Power Spectrum Magnitude (dB/Hz)

Frequency (Hz)

-10

0

10

20

30

40

0 1 2 3 4 5 6 7 8

Acceleration (g's)

Time (ms)

Raw Data

500 Hz

1000 Hz

2300 Hz

4000 Hz

0

200

400

600

800

1000

1200

1400

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Stress (psi)

Strain

500 Hz

1000 Hz

2300 Hz

4000 Hz
18

a sinusoidal motion as in Figure 10 is not considered to be a characteristic stress-strain curve. 

Figure 14. Frequency by foam 

4.3.4 Global Data Smoothing

The first effort in removing the 1500 Hz frequency regime consisted of creating a band stop 3rdorder 

Butterworth filter with a low cutoff frequency of 1200 Hz and a high cutoff frequency of 1600 Hz. This 

effort did not remove the sinusoidal motion of the plot and proved ineffective. The second attempt at 

removing the 1500 Hz consisted of creating a VI that performs a global smoothing of signals in the 

frequency domain. The data smoothing technique proved to be an effective technique in removing the 

1500 Hz frequency in the 71 and 110 foam types. The technique, with variations unique to this specific 

vertical drop tower problem, is based on a method produced by Klein and Morelli, is presented here[18].

This technique is displayed using data from this current vertical drop tower test.

Let z(t) be a noisy continuous measured signal, shown in Figure 15, that can be separated to the actual 

signal, y(t), and the noise, v(t), in the system such that

 (15)

The noisy signal is sampled N times, with a fixed time interval, Δt, over the time period 0≤ t ≤ tend, 

where tend=(N-1)Δt, such that the discrete time signal can be written as

 (16)

A periodic and odd signal, g(i), can be created from a transformation that removes the discontinuities in 

the signal by subtracting a linear trend from the original measured signal. This transformed signal is then 

reflected about the origin to remove slope discontinuities at the endpoints, resulting in g(-

N+1)=g(0)=g(N-1)=0, as depicted in Figure 16 and mathematically,

 *

 

 

+ 

(17)

71-WF-HT 110-WF-HT 110-XT-HT

1400

1450

1500

1550

1600

Frequency (Hz)

71-WF-HT

110-WF-HT

110-XT-HT
19

 (18)

Figure 15. Continuous time signal Figure 16. Continuous time signal with endpoint 

discontinuities removed 

Since g(i) is an odd function, it can be fitted with a Fourier sine series, to build the smoothed signal, 

 ̂ , written as

 ̂ ∑ ( (

 

 

))

 

 

 

(19)

The Fourier sine series coefficients for ̂ are computed in Eq. (20). It is important to note that the 

index i runs from 1 to (N-2), because g(0)=g(N-1)=0.

 

 

 

∑ ( (

 

 

))

 

 

 

(20)

The frequency of the series in the k index can be written as

 

 

 

(21)

The absolute value of the Fourier sine series coefficients, | |, can be plotted against the frequency 

to aid in the selection process of the cutoff frequency. As shown in Figure 17, the Fourier sine series 

coefficients indicate that there is a peak at approximately 1500 Hz, which damps out to zero at 

approximately 2300 Hz. After the coefficients first reach zero, there is noise in the data and the data 

cutoff is set at 2300 Hz. For low-pass smoothing, all of the sine coefficients above the cutoff frequency 

are set to zero and the frequencies below are set to one. This simple frequency-domain smoothing filter is 

shown in Figure 18.
20

Figure 17. Fourier sine series coefficients Figure 18. Smoothing filters 

While the simple filter allows the frequencies of interest to pass, this filter can be improved to buffer 

against visual errors in selecting the cutoff frequency. A method to smooth the selection of the cutoff 

frequency is to replace the simple filter with the Weiner filter, Φ(k), which forces a filter gain of 0.5 at the 

cutoff frequency and a third order smoothing from the cutoff frequency. The Weiner filter is defined in 

Eq. (22).

 

 ̃

 

 

 ̃

 ̃ 

 

(22)

The Fourier transformation of the signal, ̃ , can be written in Eq. (23), where fco is the cutoff 

frequency and n is the order of the filter, which is set to eight for this effort. The Fourier transformation of 

the noise, ̃ , is set to one, which is sufficient to create the Weiner filter depicted in Figure 18. 

 ̃ *(

 

 )

 

+

 

 

(23)

The final step in the global data smoothing technique is to restore the linear trend that was removed 

from the original signal in Eq. (17). The final smoothed time history of the signal is computed from 

 

 ̂  *

 

 

+ 

(24)

Figure 19 displays the continuous time signal with the discontinuities and noise removed from the 

signal, ̂ 

 and Figure 20 shows the final smoothed data,  , using the global smoothing of signals in 

the frequency domain technique. The amount of noise that is removed by this technique is evident when 

comparing Figure 19 to Figure 16.
21

The results from the global data smoothing technique produce the same results as that in Figure 10, 

where the only control was the Butterworth filter. However, a large difference with this technique is that 

the user has the ability to control the initial point of the raw signal, z(0), and the noise in the data can be 

extracted. Therefore, since a high cutoff frequency is necessary to capture the initial slope of the raw data, 

the data must be first processed using this frequency, as shown in Figure 20. After recreating the data with 

a high cutoff frequency, the next task is to smooth the rebuild data, essentially filtering the filter, with a 

lower cutoff frequency. In this technique, Eq. (15) through Eq. (24) are used again, however, the initial 

data, z(i), and the initial point, z(0), are different than the first iteration. For the second iteration, the initial 

data, z(i), is the rebuilt smooth data in Figure 20, and the initial point, z(0), is the first peak of the rebuilt 

smooth data. By forcing the filter to smooth the data from the first peak and lowering the cutoff frequency

to 1000 Hz, the initial slope matches the raw data and the sinusoidal nature of the acceleration data is 

removed, respectively. Using the previous Butterworth filter technique, achieving one of the constraints 

was met by an unacceptable response of the other. Figure 21 displays the result of the second iteration of 

the global data smoothing technique, where the input data is the result from the first iteration. An 

important characteristic of Figure 21 is the location of the initial point, which is located at approximately 

0.4 ms. The final smooth data compared to the raw data is shown in Figure 22. The final rebuilt signal 

demonstrates that by performing two iterations using the global smoothing technique, the initial slope 

matches the raw data and the characteristic 1500 Hz frequency is removed from the remainder of the 

crush stroke.

Figure 19. Continuous time signal with endpoint 

discontinuities and noise removed

Figure 20. Rebuilt smooth data from raw data
22

Figure 23 displays the stress-strain plots for the 110-WF-HT foam using the Butterworth filter with a 

cutoff frequency of 2300 Hz and using the global smoothing technique. By effectively rebuilding the

acceleration signal, the stress-strain plot for the foam data reflects a characteristic stress-strain plot, 

improving the effectiveness of the drop tower test technique.

Figure 23. Filter technique comparison for 110-WF-HT foam

5. Results

5.1 Foam Stress/Strain

The primary results from this current research effort are the stress-strain plots of the four tested 

Figure 21. Rebuilt smooth data, 2nd iteration Figure 22. Final rebuilt signal

0

100

200

300

400

500

600

700

800

900

1000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Stress (psi)

Strain

Butterworth Filter - CO 2300 Hz

Global Smoothing Technique
23

Rohacell foams. The plots for the four foams, by foam type, are shown in Figure 24 through Figure 27, 

respectively. The results from the 71-WF-HT foam testing indicate that as the strain rate decreases, there 

is a subsequent decrease in the maximum compressive strength and the plateau zone of the crush stroke. 

The compressive strength begins with 321 psi, decreases to 290 psi, and diminishes to 236 psi, from 

largest to smallest strain rate, respectively. For all of the foams, excluding the 200-WF-HT, the slight 

oscillatory motion for the ~13,600%/s data is deemed acceptable for these results, especially when 

compared to large sinusoidal motion of previous vertical drop tower analysis techniques as depicted in

Figure 23. In addition, for each respective foam, the initial slope matches as the strain rate changes. The 

110-WF-HT foam has a compressive strength of 546, 631, and 470 psi for the ~13,600, ~100, and 

~0.17%/s rates, respectively. The plastic deformation region for each of the three curves is relatively the 

same, with an approximate value of 450 psi. In Figure 26, the compressive strength of the 110-XT-HT 

foam is 475, 433, and 352 psi, for each respective strain rate. The order of the compressive strength by 

strain rate is different compared to the 110-WF-HT foam; the maximum compressive strength for 110-

WF-HT occurs at the ~100 %/s strain rate.

Figure 24. 71-WF-HT foam by strain rate Figure 25. 110-WF-HT foam by strain rate

0

50

100

150

200

250

300

350

400

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Stress (psi)

Strain

0.1 %/s

100 %/s

10000 %/s

0

100

200

300

400

500

600

700

800

900

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Stress (psi)

Strain

0.1 %/s

100 %/s

10000 %/s
24

Figure 26. 110-XT-HT foam by strain rate Figure 27. 200-WF-HT foam by strain rate

The final investigated foam is 200-WF-HT, where the compressive strengths are 2300, 1718, and 1110 

psi for the ~13,600, ~100, and ~0.17%/s strain rates, respectively. The two lower strain rates exhibit a 

similar trend in the crush plateau zone. The ~13,600 %/s strain rate curve is substantially different than 

the other strain rates, other than the initial peak. The plastic deformation region, approximately 0.1 to 0.6

strain, indicates an oscillatory motion of the foam. This trend occurs despite the global smoothing 

technique, which effectively removed the erroneous motion from the data of the other foams. This result 

has been attributed to the failure mode of the 200 type foam. As is visible in the high-speed video, the 

failure mode of the 200-WF-HT foam at the high strain rate is in the shear direction rather than the 

compressive; all the other foams fail in the compressive direction. By failing in the shear direction, the 

200-WF-HT foam exhibits an explosive failure, whereas the other foams collapse.

One hypothesis for the shear failure mode is the molecular structure of the 200 type foam. As 

documented by McIntyre and Anderton, the critical strain energy release rate, which is the slope of the 

strain energy released compared to the crack length, increases non-linearly as the density of the foam 

increases [19]. The authors concluded that this characteristic is attributed to the different physical 

molecular structure of the foam by density. They find that at the low densities, at or less than 110 kg/m3, 

the foams have a polyhedral type structure, characterized by thin cell walls extending to thicker interstitial 

points. At the high densities, larger than 110 kg/m3, the polymer shape changes to a spherical structure, 

characterized by a structure of isolated spheres. To compare the present work to background literature, all 

four foams were examined using a Scanning Electron Microscope (SEM), housed at NASA-LaRC. While 

the background work used microtome techniques to characterize the foam structures, the figures from the 

SEM analysis concur with background work. The 200 type foam exhibited a spherical polymer shape and 

the 71 and 110 foams have a polyhedral shape. The molecular structure of the 71-WF-HT foam is shown 

in Figure 28and the 200-WF-HT foam is shown in Figure 29. According to McIntyre and Anderton, the 

200 type foam is expected to have a higher critical strain energy release rate, making the failure of the 

foam more explosive in nature. The explosive response predicted by the higher release rate is validated by 

high-speed video, where the 200 type foam is the only one to demonstrate this trait. 

0

100

200

300

400

500

600

700

800

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Stress (psi)

Strain

0.1 %/s

100 %/s

10000 %/s

0

500

1000

1500

2000

2500

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Stress (psi)

Strain

0.1 %/s

100 %/s

10000 %/s
25

Testing this type of foam may require lower aspect ratios for the test samples than were tested. Note 

that the aspect ratio of the sample is defined as the sample length divided by its’ diameter. Vertical drop 

tower tests must be completed with foam samples with significantly larger diameters to verify this 

hypothesis

A summary of the maximum compressive strength for each foam test is displayed in Table 4 and Table 

5. The manufacturer static value, using the ASTM D1621 standard, should be approximately equal to the 

0.17%/s rate, which is true except for the 110-XT-HT and 200-WF-HT foams[13].

Table 4. Compressive strength of Rohacell foams (Imperial Units)

# Foam ~0.17%/s 

Stress (psi)

~100%/s Stress 

(psi)

~13,600%/s 

Stress (psi)

Manufacturer - 

(psi)

1 71-WF-HT 236.34 290.25 321.11 246.56

2 110-WF-HT 470.22 631.03 546.73 522.14

3 110-XT-HT 352.21 433.82 475.29 522.14

4 200-WF-HT 1110.48 1718.51 2300.39 1305.34

Table 5. Compressive strength of Rohacell foams (SI Units)

# Foam ~0.17%/s 

Stress (MPa)

~100%/s Stress 

(MPa)

~13,600%/s 

Stress (MPa)

Manufacturer - 

Static (MPa)

1 71-WF-HT 1.63 2.00 2.21 1.7

2 110-WF-HT 3.24 4.35 3.77 3.6

3 110-XT-HT 2.43 2.99 3.27 3.6

4 200-WF-HT 7.66 11.85 15.86 9.0

Another difference from the hydraulic machine tested foams to those tested by the vertical drop tower 

is the difference in the crush stroke. For all of the ~13,600%/s curves, the point where the crush event 

ends is shifted out by approximately 10%, as compared to the lower strain rates. The increase in the strain 

is due to the experimental setup. The displacement of the crush event is calculated by performing the 

double integration of the accelerometer data from the drop mass accelerometer. As previously mentioned, 

Figure 28. Polymer shape of 71-WF-HT(10x) Figure 29. Spherical shape of 110-WF-HT (15x)
26

the ideal method for determining the relative displacement of the crush event is to subtract the 

displacement from the rigid plate accelerometer from the displacement from the drop mass accelerometer. 

However, the data and displacement calculation from the rigid plate accelerometer proved to be erroneous 

and unpredictable from one drop tower test to another. This response is believed to be attributed to 

incorrect instrumentation choice. The rigid plate accelerometer experienced much of the data in the 200-

500 g range, which surpasses the range of the accelerometer used. To correct the plots and so that the high 

strain rate matches the other rates, it is necessary that the rigid plate accelerometer consistently measure 

the displacement of the crushable foundation from one drop tower test to another.

Figure 30 through Figure 33 displays the stress-strain plots of the four different foams for each strain 

rate and serves as a useful depiction of how the stress varies from one foam type to another. In Figure 30, 

the 71 and 110 type foams coalesce at a plateau zone from approximately 250 to 500 psi, whereas the 200 

type foam reaches a value greater than 1000 psi. The initial slope of the 200-WF-HT also differs, which is 

greater than the slope of the 71 and 110 type foams. At ~100%/s strain rate, as shown in Figure 31, the 

200 type foam has a peak at approximately 1,700 psi which drops down to an initial plateau at 1000 psi 

and increases substantially as the crush stroke increases from 20%. Like the ~0.17%/s strain rate, the 

initial slope of the 200 type foam is larger than the other three types. Figure 32 and Figure 33depict the 

Rohacell foams at a rate of ~13,600%/s, where the latter figure does not include the 200 foam. Just as for 

the other strain rates, the plateau zones for the 71 and 110 type foams are from 250 to 500 psi and the 

initial slope of 200-WF-HT is larger. A unique characteristic of this figure, however, is the sinusoidal 

motion of the 200 type foam. Based on the other strain rates, the 200-WF-HT foam is expected to plateau 

at approximately 1000 psi but the foam experiences a failure in the shear mode, causing an erratic

response. Figure 33 displays the foams analyzed that have similar trends and matching initial slopes for 

the ~13,600%/sec strain rate. Another visible characteristic of the ~13,600%/s data is the higher strain 

displacement, which is due to the inability to use the base plate accelerometer data, as previously noted. 

Figure 30. Rohacell foams at ~0.17%/s rate Figure 31. Rohacell foams at ~100%/s rate

0

500

1000

1500

2000

2500

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Stress (psi)

Strain

71-WF-HT

110-WF-HT

110-XT-HT

200-WF-HT

0

500

1000

1500

2000

2500

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Stress (psi)

Strain

71-WF-HT

110-WF-HT

110-XT-HT

200-WF-HT
27

Figure 32. Rohacell foams at ~13,600%/s rate Figure 33. Rohacell foams (less 200-WF-HT) at ~13,600%/s rate

5.2 Acceleration Analysis

The second investigation for this current effort is the acceleration analysis, which depicts the range of 

acceleration loads expected for MMEEV design considerations using a simplified analysis. The maximum 

stress from each of the four foams for all strain rates tested was used for this analysis. Figure 34 through 

Figure 37 show the payload acceleration curves for each foam type. In each of these plots are two black 

lines that represent the MSR baseline or requirement. The baseline diameter of the impact sphere is 0.525 

feet (0.16 meters) and the MSR acceleration requirement is 2,500 g’s. In addition, the baseline sample 

density for MSR payload is 3.3 slg/ft3(1,701 kg/m3). These plots serve as a useful tool for MMEEV 

design considerations because of the ease in observing the relationship between payload density and 

diameter with maximum g-loads for the foams tested. From Figure 34 through Figure 37 it can be seen 

that the 71-WF-HT, 110-WF-HT, and the 110-XT-HT could be expected to meet the 2,500 g-load 

requirements over a wide range of payload densities and diameters. However, for low-density payloads 

(i.e.; <3.3 slgs/ft3) and smaller payload diameters (i.e.; <0.5 ft), none of the foams tested would be 

expected to provide adequate performance. The more restrictive 400-g MSR requirement could not be 

met without increasing the payload diameter by a factor of 3, or the payload density by a factor of 2, for 

the 71-WF-HT foam. Significantly larger increases in payload diameter and density would be required 

for the other foams tested. While this analysis displays the baseline and requirement values for a MSR

mission, these values can differ depending on the particular mission of the MMEEV.

Overall, this acceleration analysis depicts the role each of the foams tested may have for MMEEV 

impact attenuation. The 71 and 110 type foams have application for missions where the samples can 

endure smaller acceleration loads. As the acceleration load limit is increased, the 200 type foam is usable 

in addition to the other foam types, which in turn introduces a wider range of acceptable sample densities. 

0

500

1000

1500

2000

2500

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Stress (psi)

Strain

71-WF-HT

110-WF-HT

110-XT-HT

200-WF-HT

0

100

200

300

400

500

600

700

800

900

1000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Stress (psi)

Strain

71-WF-HT

110-WF-HT

110-XT-HT
28

Figure 34. 71-WF-HT acceleration analysis Figure 35. 110-WF-HT acceleration analysis

Figure 36. 110-XT-HT acceleration analysis Figure 37. 200-WF-HT acceleration analysis

6. Conclusions

The current research effort supports the design of impact attenuators for MMEEV application by 

conducting dynamic foam testing with multiple uniform strain rates. In the current work, four different 

Rohacell foams are tested using two different testing techniques, a hydraulic test frame and a vertical drop 

tower. The latter technique permits the testing of a faster impact velocity, which in turn creates strain 

rates that are on the same order of magnitude as those predicted for MSR mission impacts. The 

uniqueness of this research is the varying strain rates which are ~0.17, ~100, and ~13,600%/s. The drop 

tower test technique necessitates careful data analysis because of the noise that is present in the 

accelerometer data. By implementing a global data smoothing technique in the frequency domain, it is 

possible to remove the structural frequency that may be introduced by the honeycomb foundation or the 

rigid plate. The stress-strain plots produced for the three strain rates are critical for accurate MMEEV MSAPE and LS-DYNA impact analysis. In addition to the foam analysis, an acceleration analysis technique 

is introduced for the four different foams. The generated plots delineate the maximum acceleration loads 

that the varying density scientific samples will experience based on the type of foam used and the 

diameter of the sample sphere canister. Note however, that the current data is based only on two inch

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0.00 0.50 1.00 1.50 2.00 2.50 3.00

Acceleration (g's)

Diameter (ft)

1.65

1.98

2.31

2.64

2.97

3.3

3.96

4.62

5.28

5.94

6.6

Density

(slg/ft3)

0

2000

4000

6000

8000

10000

12000

0.00 0.50 1.00 1.50 2.00 2.50 3.00

Acceleration (g's)

Diameter (ft)

1.65

1.98

2.31

2.64

2.97

3.3

3.96

4.62

5.28

5.94

6.6

Density

(slg/ft3)

0

1000

2000

3000

4000

5000

6000

7000

8000

0.00 0.50 1.00 1.50 2.00 2.50 3.00

Acceleration (g's)

Diameter (ft)

1.65

1.98

2.31

2.64

2.97

3.3

3.96

4.62

5.28

5.94

6.6

Density

(slg/ft3)

0

5000

10000

15000

20000

25000

30000

0.00 1.00 2.00 3.00

Acceleration (g's)

Diameter (ft)

1.65

1.98

2.31

2.64

2.97

3.3

3.96

4.62

5.28

5.94

6.6

Density

(slg/ft3)
29

diameter by two inch tall impact foam specimens. Subsequent analysis and testing will be needed to 

determine the integrated sample load. The product of this research are the stress-strain plots of four 

different foams at three different strain rates and a data analysis method that may improve the vertical 

drop tower technique. This work also finds that the high-density 200-WF-HT foam fails explosively, 

which is hypothesized to be attributed to a high aspect ratio of the test samples or the large critical strain 

energy release rate associated with a spherical molecular structure.

One of the most challenging aspects of foam testing was the data analysis from the vertical drop tower 

method. The data produced from the hydraulic test frame was able to be readily processed into the final 

form. The data from the vertical drop tower, however, presented challenges. For future foam testing using 

the vertical drop tower for high strain rates, a number of suggestions are presented. The honeycomb 

foundation and the rigid plate need to be chosen such that the natural structural frequency of the base and 

rigid plate is designed to be outside of the data frequencies of interest. The structural frequency of these 

elements is currently unknown and must be determined for future drop tower tests so that the frequency 

does not distort the accelerometer data. If this cannot be avoided, the VI developed in the current effort 

has the ability to mitigate the influence of this frequency. Another concern is the accuracy of the rigid 

plate accelerometer and its’ ability to measure high g-loads. The accuracy of this accelerometer is 

imperative in the development of the stress-strain plots and ensuring that the end of the crush event is 

properly characterized. For future work with the 200 type foam, it is suggested that the diameter of the 

samples increase so that the aspect ratio decreases; this may result in the mitigation of the explosive 

response for the ~13,600%/s test. Testing other aspect ratios of the other foam types is also recommended 

to enable estimation of the response for three dimensional structures.

References

[1] Robert Dillman and James Corliss, "Overview of the Mars Sample Return Earth Entry Vehicle," in 6th 

International Planetary Probe Workshop, Atlanta, GA, USA, 2008.

[2] Michelle M. Munk, Louis Glaab, Jamshid Samareh, and Parul Agrawal, "Multi-Mission Earth Entry Vehicle 

Development by NASA's In-Space Propulsion Technologies (ISPT) Project," in 9th International Planetary 

Probe Workshop, Toulouse, France, 2012.

[3] Robert Dillman, Bernard Laub, Sotiris Kellas, and Mark Schoenenberger, "Development and Test Plans for the 

MSR EEV ," in 2nd International Planetary Probe Workshop, Moffett Field, CA, USA, 2005, pp. 269-274.

[4] Catharine A. Conley and Gerhard Kminek, "Planetary Protection For Mars Sample Return," PPT June 17, 

2011.

[5] Gerhard Kminek, "Result of ESF Study Background and Draft Conclusions," in NASA Planetary Protection 

Subcommittee Meeting, Washington, D.C., May 1-2, 2012.

[6] R. Mitcheltree, S. Hughes, R. Dillman, and J. Teter, "An Earth Entry Vehicle For Returning Samples From 

Mars," in 2nd International Symposium on Atmospheric Reentry Vehicles and Systems, Arcachon, France, 

2001.

[7] Richard Mattingly, "Planetary Science Decadal Survey MSR Orbiter Mission (Including Mars Returned 

Sample Handling)," Pasadena, CA, March 2010.

[8] Robert A. Mitcheltree and Sotiris Kellas, "A Passive Earth-Entry Capsule for Mars Sample Return," in 

Proceedings of the International Symposium on Atmospheric Reentry Vehicles and Systems, Arcachon, France, 

March 1999.

[9] Evonik Industries. (2012, July) Rohacell WF.
30

[10] MTS, "MTS Landmark Testing Solutions," MTS Systems Corporation, Eden Prairie, MN, 2010.

[11] MTS, "Services and Accessories," MTS Systems Corporation, Eden Prairie, MN, 2012.

[12] MTS, "661.20 Force Transducer," MTS Systems Corporation, Eden Prairie, MN, 1996.

[13] ASTM International, "Standard Test Method for Compressive Properties of Rigid Cellular Plastics," ASTM 

International, West Conshohocken, PA, NASA Technical Standards 4/9972545004, 2010.

[14] Sotiris Kellas, "Quasi-Uniform High Speed Foam Crush Testing Using a Guided Drop Mass Impact," CR2004-213009.

[15] Alcore, "DURA-CORE II 5052 Aluminum Honeycomb," Edgewoord, MD, Product data Sheet June 2003.

[16] Measurement Specialities, "Model 3801A Accelerometer," Aliso Viejo, CA, 2010.

[17] Synergy, "The KaleidaGraph Guide to Curve Fitting," Product Information 2012.

[18] Vladislav Klein and Eugene A. Morelli, Aircraft System Identification: Theory and Practice, Joseph A. Schetz, 

Ed. Reston, Virginia, USA: American Institute of Aeronautics and Astronautics, 2006.

[19] A. McIntyre and G. E. Anderton, "Fracture Properties of Rigid Polyurethane Foam Over a Range of Densities," 

Polymer Journal, pp. 247-253, 1979.

[20] Science Mission Directorate, "NASA Procedural Requirements, NPR 8020.12D, Planetary Protection 

Provisions for Robotic Extraterrestrial Missions," NPR 8020.12D, April 20, 2011.

Acknowledgements

This work was completed with the help of multiple individuals. Nelson Seabolt (LaRC,D-503A) 

prepared the test specimens and Justin Littell (LaRC, D-322) operated the MTS machine and vertical drop 

tower. James Baughman (LaRC, D307) operated a Scanning Electron Microscope for initial foam 

analysis to investigate the molecular structure of all four foams. Juan Cruz (LaRC, D205) provided 

guidance in the validation of the initial results of the global smoothing technique. Robert Sykes (GSRP 

student) introduced the authors to the global smoothing technique, which was pivotal in the usefulness of 

the vertical drop tower results.
31

Appendix A – Current M-SAPE Database Name Density, kg/m

3

Compressive Strength, MPaElastic Modules, MPaStrain at Break, %Breaking StrengthMPaHeat Distortion Temperature, °CSpecific Strength, MPa/(kg/

m

3

)

200 WF 205 9 350 3.5 12.25 190 0.0439

110 WF 110 3.6 180 3 5.4 200 0.0327

110 RIST 110 3.6 180 3 5.4 200 0.0327

110 RIMA 110 3.6 180 7 12.6 210 0.0327

110 XT 110 3.6 180 4 7.2 240 0.0327

110 A 110 3 160 3 4.8 180 0.0273

110 IG/IG-F 110 3 160 3 4.8 180 0.0273

110 S 110 2.8 150 3.5 5.25 190 0.0255

71 WF 75 1.7 105 3 3.15 200 0.0227

71 RIST 75 1.7 105 3 3.15 200 0.0227

71 RIMA 75 1.7 105 7 7.35 210 0.0227

71 XT 75 1.7 105 4 4.2 240 0.0227

71 S 75 1.5 90 3.5 3.15 190 0.0200

71 A 75 1.5 92 3 2.76 180 0.0200

71 IG/IG-F 75 1.5 92 3 2.76 180 0.0200

71 HF 75 1.5 92 4.5 4.14 180 0.0200

51 A 52 0.9 70 3 2.1 180 0.0173

51 IG/IG-F 52 0.9 70 3 2.1 180 0.0173

51 HF 52 0.9 70 4 2.8 180 0.0173

51 WF 52 0.8 75 3 2.25 205 0.0154

51 RIST 52 0.8 75 3 2.25 205 0.0154

51 RIMA 52 0.8 75 7 5.25 210 0.0154

R82.80 80 1.1 62 18 2 0.0138

51 S 52 0.7 50 3.5 1.75 190 0.0135

R82.110 110 1.4 83 18 2.2 0.0127

31 A 32 0.4 36 3 1.08 180 0.0125

31 IG/IG-F 32 0.4 36 3 1.08 180 0.0125

31 HF 32 0.4 36 3.5 1.26 180 0.0125

AirEX R82.60 60 0.7 46 20 1.7 0.0117
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Technical Memorandum

 4. TITLE AND SUBTITLE

Uniform Foam Crush Testing for Multi-Mission Earth Entry Vehicle 

Impact Attenuation

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Patterson, Byron W.; Glaab, Louis, J.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

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Worked was performed by Byron Patterson under the Langley Aerospace Research Summer Scholar (LARSS) program.

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19a. NAME OF RESPONSIBLE PERSON

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14. ABSTRACT

Multi-Mission Earth Entry Vehicles (MMEEVs) are blunt-body vehicles designed with the purpose of transporting payloads from outer space to the surface of 

the Earth. To achieve high-reliability and minimum weight, MMEEVs avoid use of limited-reliability systems, such as parachutes and retro-rockets, instead 

using built-in impact attenuators to absorb energy remaining at impact to meet landing loads requirements. The Multi-Mission Systems Analysis for Planetary 

Entry (M-SAPE) parametric design tool is used to facilitate the design of MMEEVs and develop the trade space. Testing was conducted to characterize the 

material properties of several candidate impact foam attenuators to enhance M-SAPE analysis. In the current effort, four different Rohacell foams are tested at 

three different, uniform, strain rates (~0.17, ~100, ~13,600%/s). The primary data analysis method uses a global data smoothing technique in the frequency 

domain to remove noise and system natural frequencies. The results from the data indicate that the filter and smoothing technique are successful in identifying 

the foam crush event and removing aberrations. The effect of strain rate increases with increasing foam density. The 71-WF-HT foam may support Mars 

Sample Return requirements. Several recommendations to improve the drop tower test technique are identified. 

15. SUBJECT TERMS

Attenuators; Descent; Drop tower testing; Foam; Global data filtering; Landing; Mars sample return; Multi-Mission Earth 

Entry Vehicles

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