Trevor M. Jones, Jeffrey M. Larko, and Mark E. McNelis

Glenn Research Center, Cleveland, Ohio

Analysis of the STS–126 Flow Control Valve 

Structural-Acoustic Coupling Failure

NASA/TM—2010-216339

May 2010
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Trevor M. Jones, Jeffrey M. Larko, and Mark E. McNelis

Glenn Research Center, Cleveland, Ohio

Analysis of the STS–126 Flow Control Valve 

Structural-Acoustic Coupling Failure

NASA/TM—2010-216339

May 2010

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Spacecraft and Launch Vehicle Dynamic Environments Workshop

sponsored by The Aerospace Corporation

El Segundo, California, June 9–11, 2009
Acknowledgments

We would like to acknowledge our MSFC and industry counterparts in this FCV anomaly investigation: Clay Fulcher, MSFC, 

Jacobs Engineering—GH2 structural dynamic analysis; Bruce Laverde, MSFC, Jacobs Engineering—GH2 structural acoustic 

analysis; Matt Casiano, MSFC—GH2 CFD analysis; Jeff Rayburn, MSFC—stress analysis; and Paul Blelloch, ATA Engineering—

NX I–DEAS 5, PressMap, and IMAT software consultation. We would also like to acknowledge our GRC counterparts: Dr. 

Michael J. Barrett, GRC, chief engineer; Carol A. Quinn, GRC, project manager; Dr. Dexter Johnson, GRC, task point of contact, 

discipline lead engineer; and Scott B. Cutlip, GRC, CAD modeling and analysis.

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Trade names and trademarks are used in this report for identifi cation 

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Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
Analysis of the STS–126 Flow Control Valve 

Structural-Acoustic Coupling Failure 

Trevor M. Jones, Jeffrey M. Larko, and Mark E. McNelis 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

During the Space Transportation System mission STS–126, one of the main engine's flow control 

valves incurred an unexpected failure. A section of the valve broke off during liftoff. It is theorized that 

an acoustic mode of the flowing fuel, coupled with a structural mode of the valve, causing a high cycle 

fatigue failure. This report documents the analysis efforts conducted in an attempt to verify this theory. 

Hand calculations, computational fluid dynamics, and finite element methods are all implemented and 

analyses are performed using steady-state methods in addition to transient analysis methods. The 

conclusion of the analyses is that there is a critical acoustic mode that aligns with a structural mode of the 

valve. 

Introduction 

During the ascent phase of the Space Transportation System (STS) mission STS–126 an anomaly 

occurred. The gaseous hydrogen (GH2) flow control valve (FCV) of engine no. 2 of the main propulsion 

system appeared to switch from low towards a high flow position without being commanded to do so. 

Despite this anomaly the mission was successfully completed (Refs. 1 and 2). 

Upon completion of the STS–126 flight the FCV was removed and inspected. The FCV poppet head 

was damaged; approximately one quarter of the poppet head circumference was broken-off as showing in 

Figure 1. Optical and scanning electron microscope (SEM) examination of the fracture surface was 

performed. The fracture surface exhibited distinct “thumbnail” and “bench mark” features that are 

indicative of a fatigue crack. The fracture surface contained several distinct zones that might indicate that 

several different events or load cases caused the propagation of the crack. No evidence was found that 

would indicate any raw material defect or flaw. The fracture surface did not indicate that hydrogen 

embrittlement was a significant contributor to the failure. The conclusion of this examination is that the 

failure was caused by high cycle fatigue (Refs. 1 and 2). 

There are several important risks associated with this type of failure. If more than one FCV were to 

fail the orbiter External Tank (ET) might become over-pressurized. Another risk is internal damage to the 

Orbiter and ET hardware caused by the liberated FCV failure debris. If the liberated debris were to 

puncture a fuel line the result could be an explosion in the main engine compartment resulting in loss 

of vehicle. Another risk is under-pressurization of the ET due to blockage of the ET lines caused by 

the debris. 

This is the first failure of this type on a STS mission. The hypothesis is that an acoustic environment 

frequency coupled with a structural mode of the poppet causing a high cycle fatigue failure. The NASA 

Marshall Space Flight Center (MSFC) was assigned to perform acoustic and computational fluid dynamic 

(CFD) analysis as well as GH2 modal analysis in an effort to validate this hypothesis. The NASA Glenn 

Research Center (GRC) was tasked with verifying the structural dynamic and acoustic modeling and 

analysis of the GH2 flow control valve. GRC was to perform a similar analysis to MSFC using 

independent tools and methods to ensure the validity of the results. This report documents the work done 

by GRC in order to validate the work being done by MSFC. 

NASA/TM—2010-216339 1 
Background 

The Main Propulsion System (MPS) of the space shuttle consists of three main engines mounted on 

the rear of the orbiter. The engines are fueled with liquid hydrogen and liquid oxygen that is stored in the 

ET. The fuel is partially burned in a pre-chamber to produce hot gases at high pressure. The gas is then 

burned in the main combustion chamber and forced through the nozzles (see Fig. 2 for an engine 

schematic). The thrust rate can be controlled between approximately 65 to 109 percent thrust (Ref. 3). 

Figure 1.—STS–126 Damaged FCV Post Flight. 

Figure 2.—Main Propulsion System Pressurization System Schematic. 

NASA/TM—2010-216339 2 
Figure 3.—Orbiter MPS GH2 Flow Control Valve Section View. 

As part of the MPS, the FCV’s regulate GH2 pressurant to the ET LH2 tank during ascent. There is 

one FCV for each of the three main engines. The poppet of the FCV is actuated by a spring-loaded 

solenoid. The solenoid is either on or off. When the solenoid is on, or energized, the valve is in a low flow 

state. In the low flow state gas is still flowing through the valve. The flow is restricted so a relatively high 

static pressure exists on the poppet flange. When the solenoid is off, or de-energized, the valve is in a high 

flow state; a relatively large quantity of gas is able to flow through the valve. The H2 gas flows from the 

engine through annuluses on the FCV housing around the poppet flange and back to the ET. Figure 3 

shows a detailed schematic of the FCV assembly. 

Critical Frequency Calculation 

The first step taken in verifying the structural acoustic coupling is to determine the frequency range of 

interest using basic hand calculations. The critical frequency is the coupling frequency at which the 

acoustic mode most efficiently excites the structural response. The critical frequency is given by the 

following equation (Ref. 4): 

tc

c f

L

c 8.1

2

= (1) 

fc critical frequency 

t structure thickness 

cL structural longitudinal bending wavespeed

c speed of sound in a gaseous medium 

The structural thickness is simply the width of the FCV poppet fracture surface. This value is a 

constant. The structural longitudinal bending wavespeed is a function of the geometry and material 

NASA/TM—2010-216339 3 
properties. This value will remain constant. The structural longitudinal bending wavespeed is given by the 

following equation (Ref. 4): 

m

L

E

c

ρ = (2) 

E modulus of elasticity 

ρm density 

The speed of sound in a gaseous medium is a function of the density of the gas and is varied. The 

gaseous medium is GH2. The density of the GH2 is determined using CFD data provided by MSFC. This 

value varies depending on the flow condition; the density is different for the low verses the high flow 

configuration. The value varies depending on where the measurement is taken. The density of the GH2 

gas is computed on both the inside (c1) and immediately outside (c2) of the poppet flange as shown in 

Figure 4. 

This yields a total of four independent values for the speed of sound in the critical frequency 

calculation. The poppet material is 440 stainless steel and it is assumed to be homogeneous. The poppet 

flange is idealized as a beam in bending. Table 1 summarizes all of the input values and the resulting 

four values of critical frequency. Based on these results the frequency range of interest is approximately 

94.9 to 128.5 kHz. 

TABLE 1.—CRITICAL FREQUENCY RESULTS 

High flow Low flow 

Variable Value Units Variable Value Units 

E 2.9×107 lbf/in.2 E 2.9×107 lbf/in.2

ρm 0.276 lbf/in.3 ρm 0.276 lbf/in.3

cL 201,384 in./sec cL 201,384 in./sec 

t 0.075 in. t 0.075 in. 

c1-high 58,680 in./sec c1-high 59,246 in./sec 

c2-high 51,645 in./sec c2-high 50,914 in./sec 

f1-high 126,066 Hz f1-high 128,512 Hz 

f2-high 97,685 Hz f2-high 94,904 Hz 

Figure 4.—FCV Poppet CFD Results and Measurement Locations. 

NASA/TM—2010-216339 4 
Cavity Acoustic Modes 

In order to determine the existence of structural acoustic coupling it is important to understand the 

acoustic modes of the FCV cavity. This section investigates the acoustic modes of the FCV cavity using a 

combination of tools and methods. 

MSFC was primarily responsible for analyzing the acoustic modes of the FCV cavity. MSFC used 

several different methods to analyze the FCV cavity. Figure 4 is a graphical representation of MSFCs 

analysis results. The dashed lines running diagonal on the plot bound the bands where one would expect 

to find acoustic modes for the various families of acoustic mode type. These acoustic predictions were 

computed using a combination of hand calculations, MSC.Nastran, and Matlab. CFD solutions for the 

acoustic modes were also computed. High and low flow configurations at both 72 and 104.5 percent 

thrust were considered for both STS–119 and STS–126. This is a total of eight cases for the CFD 

analysis. The results for the eight cases are overlaid with the acoustic predictions on Figure 4. The relative 

size of the shapes indicates the relative energy levels present between the cases; a larger shape indicates a 

higher energy acoustic mode. The highest energy modes all seem to be clumped at 107.4 kHz. 107.4 kHz 

falls in the range for the critical frequency previously determined in the GRC hand calculations. 

Figure 4.—GH2 FCV Internal Cavity Acoustic Modes. 

NASA/TM—2010-216339 5 
FCV Structural Modes 

In order to determine the existence of structural acoustic coupling it is important to understand the 

structural modes of the poppet. This section investigates the structural modes of the poppet using both 

hand calculations based on tabulated idealized configurations as well as through the use of finite element 

(FE) methods. 

The first attempt at understanding the structural modes of the poppet was done using idealized hand 

calculations. The purpose of this calculation is to roughly determine the frequency of the poppet first 

bending mode. This information is used to increase confidence in the results from the finite element 

model (FEM). For this calculation the poppet is being idealized as a constant section annulus tube 

clamped at one end and free at the other. The equation for the natural frequency of this structure is given 

by the following equation (Ref. 5): 

m

EI

L f i

i 2

2

2π

λ = (3) 

( 44

4 − baI π = ) (4) 

fi natural frequency Annulus section 

λi function of boundary condition 

L length of beam 

E modulus of elasticity 

m mass per unit length of beam 

I area moment of inertia about neutral axis 

λ is a dimensionless parameter that is a function of our boundary condition; it is based on tabulated 

values. Only the first bending mode of the poppet is calculated here. The length of the beam is the 

distance from where the poppet is fixed, to the far face of the free end. The modulus of elasticity is the 

modulus of 440A stainless steel. The mass per unit length is the mass of the cantilevered portion of the 

beam divided by length of that portion. The area moment of inertia is based on the cross section 

dimensions of the poppet. Table 2 summarizes all the input values used and the resulting frequency for 

the poppet first bending mode. 

TABLE 2.—NATURAL 

FREQUENCY RESULTS 

Variable Value Units 

λ1 1.875 ------ 

L 0.77 in. 

E 2.9×107 lbf/in.2

m 2.1×10–4 lbf/g 

I 2.29×10–3 in.4

a 0.24 in. 

b 0.141 in. 

f1 16,795 Hz 

This calculation yields the first bending mode of the poppet at around 17 kHz. 17 kHz is high 

compared to what one might expect the first mode of a structure or part to be because the poppet is small 

and light weight; the cantilevered portion of the poppet is only 0.77 in. in length and weights about 1 oz. 

NASA/TM—2010-216339 6 
In order to more accurately predict the dynamic behavior of the poppet, a FEM of the part was created 

by MSFC. MSFC provided their model to GRC so that GRC could use it in our own analysis. The poppet 

FEM is shown in Figure 5. The model consists of 152,149 elements and 219,182 nodes. The 

MSC.Nastran element type is CTETRA; a four-sided solid element with four to ten grid points. Several 

preprocessor checks were used to verify the robustness of the FEM. These checks include free-free edge 

checks, duplicate grids, element quality, material property units and magnitude, and local coordinate 

system orientation (Refs. 3 and 4). The model was solved using MSC.Nastran 2008. To further validate 

the model, several PARAM and analytical diagnostics were performed including MAXRATIO (check for 

stiffness matrix ill-conditioning), GRDPNT (check of mass properties), GROUNDCHECK (strain energy 

check), free-free normal modes rigid body modes less than 1×10–4, and finally mode shape evaluation 

(Refs. 6 and 7). 

The next step was to determine the dynamic characteristics of the poppet using the checked-out 

FEM. Correct definition of the boundary condition is important for a correct modal analysis. The poppet 

was fixed in translation at every node on the blue surface shown in Figure 5. This mimics the way the 

actual poppet is constrained in the FCV assembly. Normal modes were solved using MSC.Nastran 2008 

solution 103 (Refs. 8 and 9). The resulting natural frequency values are given in Table 3. It should be 

noted that the first mode given by Nastran was 17,693 Hz. This is only about five percent different from 

the value computed using hand calculations. This comparison helps to verify the correctness of the FEM. 

The modal effective mass was also computed and is shown in Table 3. This information indicates what 

percent of the total mass of the model is moving or rotating in a given direction. The modal results file 

was imported into NX I–DEAS 5 for mode shape visualization. A select few mode shapes are shown in 

Table 3. Table 3 shows that there are several poppet modes which lie within the critical frequency range 

from 94.9 to 128.5 kHz. This is the range, calculated previously, at which a structural acoustic coupling is 

most likely to occur. 

Figure 5.—GH2 FCV Poppet FEM. 

NASA/TM—2010-216339 7 
TABLE 3.—FCV POPPET NORMAL MODES RESULTS 

Stress Prediction Methodology 

The next step in verifying the structural acoustic coupling is to perform forced and transient response 

analysis of the FCV. This analysis is performed using the same FEM utilized in determining the structural 

modes of the poppet. An overview of the stress prediction methodology used is shown in Figure 6. 

Figure 6 demonstrates the four independent paths taken in determining the cyclic stresses found on the 

poppet. The input for each of the analysis methods comes from computational fluid dynamics (CFD) 

results that were provided by MSFC. The CFD result used was the pressure distribution exerted on the 

poppet as a function of time. Pressures were also provided in the frequency domain as a function of 

frequency and phase. Two primary methods were used to determine the poppet stresses. The first method 

is a simpler steady-state method; the second method is a transient method performed in the time domain. 

Each of the two methods was performed independently by MSFC and GRC using different toolsets. The 

results are then compared. 

NASA/TM—2010-216339 8 
 

Figure 6.—Stress Prediction Methodology Flowchart. 

Steady State Methodology 

The first stress prediction method used is a steady-state method performed in the frequency domain 

(Fig. 6, left half). Figure 7 shows the inputs used in the simulated frequency response analysis. The CFD 

results were provided as pressures across the poppet face. To simplify the analysis the CFD data is 

decomposed into a discrete number of resolved forces that contained the appropriate frequency and phase 

information to approximate the complex pressure distributions given by the CFD model. This resolved 

forcing function is used as one of the inputs to the frequency response analysis. The resolved forces are 

applied to the poppet FEM that contains the poppet modal information. Because a flight failure occurred, 

a conservative value was chosen for damping. 0.25 percent critical damping was used for the analysis. 

0.25 percent represents damping inherent to the material only. The result from the frequency response 

analysis is a stress frequency response function (FRF). The FRF is a plot of stress verses frequency for the 

highest stress element in each of the three principle directions x, y, and z. MSFC used Matlab to calculate 

these FRF results. The modal information was taken from the MSC.Nastran output file and combined 

with the resolved forces in Matlab. GRC used NX I–DEAS 5 to calculate the FRF results. The results 

were then plotted in Matlab to make an easy comparison with MSFC. 

Steady State Results 

Results from the steady state simulation are in the form of stress FRF plots. Plotted is the highest 

stress element for each of the three principle directions. The steady state frequency response analysis was 

simulated using several different, increasingly complex, forcing functions. Figure 8 shows results for the 

case in which the CFD data is decomposed into a simple two point forcing function. Also shown on the 

plots is the location of the 107.4 kHz acoustic mode which was previously determined. Peaks on the plot 

indicate frequencies where structural modes are being excited by the input forcing function. A peak at 

around 107.4 kHz would indicate that there is a structural acoustic alignment; Figure 8 shows no such 

alignment. Figure 8 does indicate that the GRC and MSFC methodologies are both producing very similar 

results. Any differences in the plots are very minor and considered insignificant. The key information is 

the same for both analysis methods. 

NASA/TM—2010-216339 9 
Figure 7.—Steady-State Stress Prediction Methodology. 

Figure 8.—Steady-State Stress FRF Results for 2-Point Input. 

NASA/TM—2010-216339 10 
Once the analysis methodology had been developed it is possible to increase the complexity in order 

to more accurately simulate what was actually occurring. The simple 2-point forcing function input is a 

fairly crude model of the complex acoustic mode present in the FCV cavity. Figure 9 shows results for 

the case in which the CFD data has been decomposed into an 8 point forcing function. Once again the 

107.4 kHz acoustic mode is shown on the plot. With the increased fidelity input different peaks appear 

on the plot. This might indicate that the 2-point forcing function was insufficient in accurately 

representing the input force. Figure 9 shows an obvious peak at 110 kHz, which is nearly perfectly 

aligned with the 107.4 acoustic mode. 

The FRF plots change drastically from the 2-point to the 8-point forcing function. In order to validate 

the existence of the excited 110 kHz structural mode, the fidelity of the analysis is further increased. The 

CFD acoustic data is decomposed into a 288-point force applied to the poppet face. The 288-point forces 

are applied at varied distances in the radial direction on the poppet face in addition to being clocked 

around the face. Figure 10 shows results for the case where the CFD data has been decomposed into a 

288 point forcing function. As with the previous plots, the 107.4 kHz acoustic mode is shown on the plot. 

Once again there is an obvious peak at 110 kHz. Figures 9 and 10 share several other major peaks 

indicating that both the 8 and 288 point forcing functions are exciting similar structural modes. This 

increases confidence that the fidelity of the analysis is sufficient and that the 110 kHz excitation is real. 

Figure 9.—Steady-State Stress FRF Results for 8-Point Input. 

NASA/TM—2010-216339 11 
Figure 10.—Steady-State Stress FRF Results for 288-Point Input. 

Transient Methodology

The steady-state method is performed in the frequency domain. The second method used is a transient 

method carried out in the time domain (Fig. 6, right half). Figure 11 shows the inputs used in the transient 

response simulation. The CFD results are provided as pressures across the poppet face. The software 

PressMap is used to remap the CFD results onto the nodes of the FEM since the CFC and FEM meshes 

are dissimilar. The result of this mapping is a unique input force, which varies as a function of time, 

applied to every node on the poppet face. The time varying forces are then applied to the FEM to produce 

transient response results. These results plot stress verses time for the four highest stressed elements. The 

values of stress being plotted are the maximum principal stresses. MSFC used PressMap to remap the 

pressures and MSC.Nastran to perform the transient simulation. GRC used PressMap to solve for the 

remapped pressures and then NX I–DEAS 5 to run the transient simulation. The results from both parties 

are plotted in Matlab for comparison purposes. 

Transient Results

Results from the transient analysis are in the form of stress verses time plots. Plotted is the maximum 

principal stress for the four highest stressed elements. These elements are typically located in the poppet 

flange fillet. Figure 12 shows the stress state in the poppet for 0.15 msec of acoustic loading. Figure 12 

shows both results computed by GRC using NX I–DEAS 5 and MSFC using MSC.Nastran. There are 

slight differences in the plots which can perhaps be attributed to the different solution codes used during 

the solve. The overall trends of the two plots are the same. Both plots show a cyclic stress state in the 

poppet flange fillet that increases in amplitude. The plot has 11 distinct peaks over the last 0.1 msec of 

acoustic loading; this equates to a 110 kHz stress oscillation frequency. 

NASA/TM—2010-216339 12 
Figure 11.—Transient Stress Prediction Methodology. 

Figure 12.—Transient Stress Results. 

NASA/TM—2010-216339 13 
NASA/TM—2010-216339 14 

Conclusions 

The critical frequency calculation indicates that there is structural acoustic coupling from 

approximately 94.9 to 128.5 kHz. Both the acoustic and structural dynamic analyses indicate that there is 

a critical acoustic mode at 107.4 kHz that aligns with a structural mode at 110 kHz. During this coupling 

a relatively high stress state occurs in the poppet fracture region. Due to the coupling, high cycle fatigue is 

thought to be initiated during the high flow condition. This high cycle fatigue could initiate cracks in the 

poppet. During the low flow condition a high static pressure is applied to the poppet flange. This high 

static pressure could lead to further crack growth and eventual failure. As a result of this analysis 

establishing the structural acoustic coupling, NASA is proceeding to mitigate the risk by performing post 

flight FCV poppet inspections for all remaining flights. Additionally, GH2 FCVs are to be removed after 

every flight and refurbished. 

References 

1. Unknown, “STS–119 Flight Readiness Review,” Space Shuttle Program, Orbiter & GFE Project, 

Feb. 3, 2009. 

2. Unknown, “STS–119 Flight Readiness Review,” Space Shuttle Program, Orbiter & GFE Project, 

Feb. 20, 2009. 

3. Unknown, “MPS GH2 Flow Control Valve Poppet Failure Investigation Summary,” PRCB, Jan. 7, 

2010 

4. L.L. Beranek, “Noise and Vibration Control,” Chapter 11, 1971. 

5. R.D. Blevins, “Formulas For Natural Frequency And Mode Shape,” Krieger Publishing Company 

Malabar, FL, 1993. 

6. “Practical Dynamic Analysis in MSC/NASTRAN,” NASA 116, Section 2, 2004. 

7. T.P. Serafin, “Spacecraft Structures and Mechanisms: From Concept to Launch,” pp. 592–593, 2003. 

8. MSC Software Corporation, “MSC.Nastran 2005 Quick Reference Guide,” 2005. 

9. MSC Software Corporation, “NAS102—MSC.Nastran Dynamic Analysis Exercises Vol. 1”, Santa 

Ana, CA, 2004. 
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Analysis of the STS-126 Flow Control Valve Structural-Acoustic Coupling Failure 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

During the Space Transportation System mission STS-126, one of the main engine's flow control valves incurred an unexpected failure. A 

section of the valve broke off during liftoff. It is theorized that an acoustic mode of the flowing fuel, coupled with a structural mode of the 

valve, causing a high cycle fatigue failure. This report documents the analysis efforts conducted in an attempt to verify this theory. Hand 

calculations, computational fluid dynamics, and finite element methods are all implemented and analyses are performed using steady-state 

methods in addition to transient analysis methods. The conclusion of the analyses is that there is a critical acoustic mode that aligns with a 

structural mode of the valve. 

15. SUBJECT TERMS

Dynamics; Dynamic structural analysis; Dynamic response; Acoustics; Control valves; Space shuttles; Spacecraft models; Dynamic 

models; Acoustic coupling 

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