Mark E. McNelis and Thomas W. Goodnight

Glenn Research Center, Cleveland, Ohio

Michael A. Farkas

The Boeing Company, Huntsville, Alabama

CM-2 Environmental/Modal

Testing of Spacehab Racks

NASA/TM—2001-210677

February 2001
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Prepared for the

Seventh International Congress on Sound and Vibration

sponsored by the International Institute of Acoustics and Vibration

Garmisch-Partenkirchen, Germany, July 4–7, 2000

Mark E. McNelis and Thomas W. Goodnight

Glenn Research Center, Cleveland, Ohio

Michael A. Farkas

The Boeing Company, Huntsville, Alabama

CM-2 Environmental/Modal

Testing of Spacehab Racks

NASA/TM—2001-210677
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Available electronically at http://gltrs.grc.nasa.gov/GLTRS
NASA/TM—2001-210677 1 

CM-2 ENVIRONMENTAL/MODAL TESTING OF SPACEHAB RACKS

Mark E. McNelis and Thomas W. Goodnight 

National Aeronautics and Space Administration 

 Glenn Research Center 

Cleveland, Ohio 

Michael A. Farkas 

The Boeing Company 

Huntsville, Alabama

Abstract 

Combined environmental/modal vibration testing has been implemented at the NASA Glenn 

Research Center’s Structural Dynamics Laboratory. The benefits of combined vibration testing 

are that it facilitates test article modal characterization and vibration qualification testing. 

The Combustion Module-2 (CM-2) is a space experiment that launches on Shuttle mission 

STS-107 in the SPACEHAB Research Double Module. The CM-2 flight hardware is integrated 

into a SPACEHAB single and double rack. CM-2 rack level combined vibration testing was 

recently completed on a shaker table to characterize the structure’s modal response and verify the 

random vibration response. Control accelerometers and limit force gauges, located between the 

fixture and rack interface, were used to verify the input excitation. Results of the testing were 

used to verify the loads and environments for flight on the Shuttle. 

INTRODUCTION 

The Combustion Module–2 (CM-2) is a combustion science experiment consisting of eight 

packages that are integrated into SPACEHAB single and double racks (Figure 1). The CM-2 

hardware is a reflight of CM-1 hardware, which was designed and environmentally qualified for 

Spacelab and flew on Shuttle STS-83 (April 4, 1997) and STS-94 (July 1, 1997). The CM-2 

design loads and vibration environments for SPACEHAB are higher than CM-1, requiring 

requalification of the CM-2 hardware for mission assurance. 

 The NASA Glenn Research Center Structural Dynamics Laboratory (SDL) developed a 

cost and schedule saving approach to requalifying the CM-2 hardware by subjecting the 

integrated single and double racks to combined environmental/modal vibration testing. The 

objective of the rack level baseshake testing was to characterize the random vibration package 

response at 1⁄4 flight excitation, and scale the response to flight levels. The scaled flight level 

random vibration response is compared to the previous CM-1 qualification test history. The 

benefit of the low level test excitation (1⁄4 flight) is that it reduces the fatigue exposure of the 

CM-2 commercial, vibration sensitive electronic hardware. The scaled flight level package 

random vibration response was also used to verify random load factors used for CM-2 package 

design. 
NASA/TM—2001-210677 2 

BASESHAKE RACK LEVEL TESTING 

Combined environmental/modal rack level testing (References 1 and 2) of the CM-2 single and 

double racks was completed in the NASA Glenn Research Center SDL using a 35,000 poundforce vertical electrodynamic shaker and a 28,000 pound-force horizontal electrodynamic shaker 

and 96 channels of digital data acquisition. 

 The baseshake vibration tests were performed with the integrated rack attached to a rigid 

fixture, and supported by the shaker with a 72 inch expander head. The double rack test 

configuration is show in Figure 2. The L-shaped test fixture supporting the rack weighed 

1,360 pounds and was assembled from 6 inch x 6 inch x 1⁄2 inch box beams. The fundamental 

frequency of the bare fixture is 120 Hz. The test configured double rack weighed approximately 

1,120 pounds with all five packages integrated in the rack. A significant package mass in this 

rack is the Experiment Package (EP) weighing 600 pounds. The total weight of the test 

configuration including double rack, packages and test fixture was 2,480 pounds. Four control 

accelerometers and five force gauges (three-axis strain gauge type) located at the rack to fixture 

interface were used for the purpose of test control and limit response (Figure 3). The response 

accelerometer locations were defined based on pre-test finite element modal analysis of the 

integrated rack/fixture configuration. Rack test excitation included sinusoidal (excitation level: 

1

/8, 1⁄4, 1⁄2 g’s-peak, frequency range: 5-400 Hz) and random vibration (excitation level: 1⁄4 flight 

excitation with an overall of 0.75 Grms, frequency range: 20-2,000 Hz). Sinusoidal testing was 

conducted at several low level excitations to assess linearity of response. Linearity was 

reasonable for all three axes tested, providing confidence when scaling the 1⁄4 flight random 

vibration levels to flight levels. 

 Test control was excellent with respect to the 1⁄4 flight rack test specification (Figure 4). 

There was an inconsequential exceedance to the test specification from 900-1,600 Hz due to test 

configuration interaction with the shaker and expander head. 

ENVIRONMENTAL RANDOM VIBRATION TESTING 

A comparison of the EP Y-axis random vibration test specifications for the CM-1 and CM-2 

programs is shown in Figure 5. The package test specification is defined at the interface between 

the package and the rack mounting post. The 9.5 dB difference in the plateau of the EP random 

vibration test specifications requires package requalification for CM-2 mission assurance. 

Typical requalification is performed for each package individually using a rigid fixture. This 

approach is time intensive, costing the program schedule. The Structural Dynamics Laboratory 

instituted an integrated rack approach to testing the package hardware, thus saving the program 

schedule. Rack level testing was completed in one-half of the time of eight individual package 

tests. The advantage of rack level testing is it provides flight boundary conditions to the package. 

For individual package level testing, an artificial rigid fixture interface is used at the package 

interface. 

 The CM-2 package and rack test specifications were developed by the Boeing Company for 

SPACEHAB based on a reverberant ground acoustic test of the SPACEHAB Single Module. The 

acoustic test excitation was based on Shuttle launch acoustics measured in the cargo bay (140 dB 

overall SPL). The SPACEHAB Module was integrated with flight racks and package mass 

simulators designed to represent a variety of potential payload configurations. Accelerometer 
NASA/TM—2001-210677 3 

measurements were made at the rack-to-SPACEHAB Module interface and the rack-to-package 

interface. 

 NASA Glenn Research Center performed 1⁄4 flight rack level random vibration baseshake 

testing using excitation levels and accelerometer locations defined in the SPACEHAB ground 

acoustic test. The 1⁄4 flight response was scaled to flight levels. A comparison is made between 

the acoustic and baseshake interface response for the EP (Figure 6). The EP interface vibration 

response is much less in the baseshake test than in the acoustic test from 350-1,600 Hz. 

 A comparison is made between acoustic and baseshake test transfer functions calculated for 

the EP in the Y-axis test configuration (Figure 7). The transfer function is defined as the ratio of 

the Experiment Package interface response to the rack-to-module interface response. In the 

frequency range from 40-940 Hz, the baseshake transfer function has a value less than one. The 

transfer function calculated from the acoustic test is larger than from the baseshake test, except 

below 40 Hz where the CM-2 integrated rack fundamental frequency dominates. The 

consequence of baseshake rack level testing is that the inertial effects of the integrated rack with 

CM-2 flight packages provides mass attenuation, reducing the random vibration input to the EP 

package. 

 The benefit of 1⁄4 flight random vibration test is the reduced excitation levels minimized 

fatigue exposure to vibration sensitive, commercial electronic components mounted inside the 

CM-2 packages. The 1⁄4 flight excitation package responses were scaled to flight levels and 

compared to the previous CM-1 qualification test history. The environmental approach used to 

address mission assurance proved effective, without having to requalify CM-2 flight hardware. 

MODAL TESTING AND LOADS ANALYSIS 

Modal sinusoidal resonance survey testing of the CM-2 single and double racks was conducted to 

verify the CM-2 integrated rack finite element models. Pre-test finite element modal analysis was 

performed to define the rack test accelerometer locations. Post-test model correlation was 

performed to improve the finite element model prediction of the rack test mode shapes. 

 The CM-2 single and double rack finite element models are analytically integrated into the 

SPACEHAB Research Double Module and Shuttle for a coupled loads analysis of the launch 

configuration. The coupled loads analysis provides a set of transient load factors. The transient 

load factors are combined with package random vibration load factors to define a set of CM-2 

package design load factors. 

 Preliminary CM-2 design load factors were greater than CM-1 design loads. In order to 

reduce the CM-2 package design load to avoid any hardware redesign, the CM-2 program 

requested a design coupled loads analysis to specifically address Z-axis transient loads. 

 The CM-2 package random vibration load factors were recalculated based on the package 

interface random vibration response from the rack level testing. The random vibration loads were 

computed using the package natural frequencies and effective masses determined from a modal 

analysis of each package with flight boundary conditions. The effective mass is used as a 

weighting factor in the random vibration load calculation. For each natural frequency, a random 

vibration load is calculated using the Single Degree Of Freedom (SDOF) Frequency Response, 

assuming a dynamic magnification Q=10, and the package interface random vibration response 

measured from the rack level test. The load for each frequency is multiplied by the corresponding 
NASA/TM—2001-210677 4 

effective mass factor and then all of these products are summed (root-sum-squared) for each axis, 

for each package. 

 Using this methodology, the CM-2 design loads were lowered so that there were no 

exceedances to CM-1 design loads. This had a large impact on the project, saving the cost of 

redesign and maintaining the launch schedule.

CONCLUSIONS 

The application of combined environmental/modal testing for the CM-2 flight program was a 

cost effective way to reduce the design load factors and verify the package environments for 

mission assurance. The advantage of rack level testing is it provides flight boundary conditions to 

the package. Performing rack level testing instead of individual package level tests saved the 

program one half the testing time. Furthermore, the 1⁄4 flight rack tests reduced the fatigue 

exposure to the CM-2 commercial, vibration sensitive electronic hardware. Integrated rack level 

testing provided the inertial effect of mass attenuation reducing the package interface random 

vibration response. By reducing the CM-2 package loads and vibration environments, the CM-2 

program saved the cost of hardware requalification and redesign. 

REFERENCES 

1. “Combustion Module-1 (CM-2): SPACEHAB Double Rack with Protoflight Packages 

Sinusoidal Resonance Survey and 1⁄4 Flight Level Vibration Characterization Test Report,”

Structural Dynamics Laboratory Test Report SDL-TR 99-34, NASA Glenn Research Center, 

December 7, 1999. 

2. “Combustion Module-1 (CM-2): SPACEHAB Single Rack with Protoflight Packages 

Sinusoidal Resonance Survey and 1⁄4 Flight Level Vibration Characterization Test Report,”

Structural Dynamics Laboratory Test Report SDL-TR 99-29, NASA Glenn Research Center, 

December 30, 1999. 
NASA/TM—2001-210677 5 

 

FIGURE 1. CM-2 Single and Double Rack Configuration.

FSP 

VIP 

DPP 

VCRP

EVP 

EP 

DEPP 

PDP 

Z

X Y
NASA/TM—2001-210677 6 

FIGURE 2. CM-2 Double Rack Test Configuration. 

 

FIGURE 3. Rack-to-Fixture Interface Instrumentation. 

Interface Force Gauges and 

Control Accelerometers

Experiment

Package

Expander Head

Vertical Shaker

Fixture
NASA/TM—2001-210677 7 

FIGURE 4. CM-2 Double Rack Interface Control 

1⁄4 Flight Random Vibration Test Specification. 

FIGURE 5. CM-1 and CM-2 Experiment Package 

Flight Random Vibration Test Specifications (Y-Axis).

g p

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

10 100 1000 10000

Frequency (Hz)

PSD (G2/Hz)

Average Control [0.78 Grms] Test Spec [0.75 Grms]

1.0E-04

1.0E-03

1.0E-02

1.0E-01

10 100 1000 10000

Frequency (Hz)

PSD (G2/Hz)

CM-2 Spec [4.73 Grms] CM-1 Spec [1.86 Grms]

9.5 dB
NASA/TM—2001-210677 8 

FIGURE 6. Comparison of Acoustic and Baseshake Rack Testing 

Experiment Package Interface Flight Response (Y-Axis). 

FIGURE 7. Acoustic and Baseshake Transfer 

Functions for the Experiment Package (Y-Axis). 

p g( )

0.01

0.1

1

10

10 100 1000 10000

Frequency (Hz)

Transfer 

 Function 

Acoustic TF Baseshake TF

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

10 100 1000 10000

Frequency (Hz)

PSD (G2/Hz)

Acoustic Test [3.77 Grms] Baseshake Test [2.03 Grms]
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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CM-2 Environmental/Modal Testing of Spacehab Racks

Mark E. McNelis, Thomas W. Goodnight, and Michael A. Farkas

Modal testing; Vibration testing

Unclassified -Unlimited

Subject Category: 18 Distribution: Nonstandard

Prepared for the Seventh International Congress on Sound and Vibration sponsored by the International Institute of

Acoustics and Vibration, Garmisch-Partenkirchen, Germany, July 4–7, 2000. Mark E. McNelis and Thomas W. Goodnight,

NASA Glenn Research Center; and Michael A. Farkas, The Boeing Company, 499 Boeing Blvd., P.O. Box 240002,

Huntsville, Alabama 35824. Responsible person, Mark. E. McNelis, organization code 7735, 216–433–8395.

Combined environmental/modal vibration testing has been implemented at the NASA Glenn Research Center’s Structural

Dynamics Laboratory. The benefits of combined vibration testing are that it facilitates test article modal characterization

and vibration qualification testing. The Combustion Module-2 (CM-2) is a space experiment that launches on Shuttle

mission STS-107 in the SPACEHAB Research Double Module. The CM-2 flight hardware is integrated into a

SPACEHAB single and double rack. CM-2 rack level combined vibration testing was recently completed on a shaker table

to characterize the structure’s modal response and verify the random vibration response. Control accelerometers and limit

force gauges, located between the fixture and rack interface, were used to verify the input excitation. Results of the testing

were used to verify the loads and environments for flight on the Shuttle.