Matthew E. Melis

Glenn Research Center, Cleveland, Ohio

Finite Element Simulation of a Space Shuttle

Solid Rocket Booster Aft Skirt Splashdown

Using an Arbitrary Lagrangian-Eulerian

Approach

NASA/TM—2003-212093

January 2003

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Matthew E. Melis

Glenn Research Center, Cleveland, Ohio

Finite Element Simulation of a Space Shuttle

Solid Rocket Booster Aft Skirt Splashdown

Using an Arbitrary Lagrangian-Eulerian

Approach

NASA/TM—2003-212093

January 2003

National Aeronautics and

Space Administration

Glenn Research Center
Acknowledgments

The author would like to thank Mike Pereira and Kelley Carney, NASA Glenn Research Center, Dallas McFadden,

Richard Knockelman, and Pete McFadden, United States Alliance, and Khanh Bui, Livermore Software Technology

Corporation, for their contributions to this work, which greatly enhanced my efforts to characterize this problem.

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NASA/TM—2003-212093 1

Figure 1. Primary Components of Figure 2. Spashdown Orientation of SRB.

Space Shuttle Launch Configuration.

Aft Skirt

Space

Shuttle

Orbital

Vehicle

External

Tank Solid

Rocket

Boosters

Aft Skirt

Ocean Surface

Finite Element Simulation of a Space Shuttle Solid Rocket Booster Aft Skirt Splashdown

Using an Arbitrary Lagrangian-Eulerian Approach

Matthew E. Melis

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

ABSTRACT

Explicit finite element techniques employing an Arbitrary Lagrangian-Eulerian (ALE) methodology, within the transient dynamic code LS-DYNA, are used to predict splashdown loads on a proposed replacement/upgrade of the hydrazine tanks on the

thrust vector control system housed within the aft skirt of a Space Shuttle Solid Rocket Booster. Two preliminary studies are

performed prior to the full aft skirt analysis: An analysis of the proposed tank impacting water without supporting aft skirt

structure, and an analysis of space capsule water drop tests conducted at NASA’s Langley Research Center. Results from the

preliminary studies provide confidence that useful predictions can be made by applying the ALE methodology to a detailed

analysis of a 26-degree section of the skirt with proposed tank attached. Results for all three studies are presented and compared

to limited experimental data. The challenges of using the LS-DYNA ALE capability for this type of analysis are discussed.

INTRODUCTION

Reusing the Solid Rocket Boosters (SRB’s) on NASA’s Space Shuttle saves millions of dollars each launch. At the end of their

duty cycle, the boosters separate from the shuttle’s external tank, descend to earth via parachutes, and splashdown in the ocean

for recovery. Typical splashdown velocities of 80 feet per second create significant impact loads to the SRB structure, particularly to the aft skirt and its components. Figure 1 shows the SRBs integrated with the external tank and space shuttle orbiter

vehicle in launch configuration. Figure 2 identifies the aft skirt on the SRB and depicts the orientation of the booster as it

splashes down into the ocean.
NASA/TM—2003-212093 2

Figure 3. Aft Skirt in NASA Assembly Facility Awaiting Integration to SRB.

Figure 4. Thrust Vector Control System Inside Space Shuttle SRB.

Figure 3 shows a photograph of an actual aft skirt awaiting integration to an SRB (personnel in this figure provide a reference for

scale). Each SRB aft skirt houses a complex thrust vector control system (TVC), which mechanically directs the nozzle at the

end of the booster to steer the shuttle during its initial ascent. The TVC consists of two hydraulic gimbal servo-actuators; each

independently powered by its own auxiliary power unit (APU) and hydraulic system. Figure 4 is a photograph of the TVC

system inside the aft skirt with the actuators, APUs and hydrazine tanks identified. Hydrazine, currently used as the propellant

for the TVC, is an extremely hazardous material. Safety concerns, regarding this hazard, have provided motivation to propose a

new propellant system using helium in place of hydrazine.
NASA/TM—2003-212093 3

Experience with over 100 launches has enabled NASA to understand and minimize splashdown damage to current aft skirt and

TVC components. The proposed helium tanks, however, are approximately six times the volume of the hydrazine tanks and

would significantly change the exposure characteristics of water impact loads to the system should they be implemented. As a

consequence, a new detailed analysis of the water impact event with the new helium tanks would be required during the design

stage.

Only recently has the ALE capability in LS-DYNA1

, an explicit finite element code (ref. 1), become feasible for analyzing water

impact problems. The primary objective of this work is to establish the practicality of using such methods to model the SRB

splashdown event in a timely fashion. To demonstrate this, LS-DYNA is used to analyze several different types of water impact

problems. The analysis results, presented here, are in general agreement with the limited qualitative and quantitative experimental data available for comparison. This provides encouragement that the LS-DYNA ALE capability will be useful for characterizing complex water impact problems such as this one.

Modeling Approach and Analysis Results

The Arbitrary-Lagrangian-Eulerian method utilizes two mesh types in an analysis: The Lagrangian mesh, associated with the

typical finite element analysis, distorts as it responds to the loading and boundary conditions in the analysis. The Eulerian mesh,

conversely, remains fixed throughout the analysis and tracks material as it moves throughout the mesh. Combining these two

meshes together in a single analysis provides the ability to predict the interaction between fluid and structural elements. The ALE

approach was suited perfectly for this analysis with the SRB structure being modeled with the Lagrangian mesh, and the water

being modeled with the Eulerian mesh.

This effort was broken into three analysis tasks. The first two were performed to establish a workflow methodology for conducting the ALE analyses with LS-DYNA, as well as to develop a level of confidence that the LS-DYNA results were reasonable. All

of the analysis results presented here were run on a single processor Silicon Graphics Origin 2000 R10000 processor. The

results from the first two analysis tasks yielded reasonable predictions, as will be discussed, and provided justification to commit

the recourses to perform a full-scale aft skirt analysis.

One of the most significant issues in this effort was the determination of material properties and the respective constitutive

models to be used to characterize water for the analyses. LS-DYNA enables the user to choose from a variety of material models

for property definitions, however, in the absence of an advanced material model for water, the most rudimentary characterization

of water was made for these analyses using the LS-DYNA MAT_ELASTIC_FLUID definition. Three parameters were explicitly defined on this card: Water density, bulk modulus, and a tensor viscosity coefficient. Density and elastic modulus values,

taken from the literature, were 9.59e-5 lbm/in3

, 3.30e5 lb/in2, respectively. A tensor viscosity value of .05 was assigned based on

recommendations from the Livermore Software Technology Corporation, and a default value for cavitation pressure of 1.0E20

lb/in2 was used.

In each case, symmetric boundary conditions were utilized to reduce the problem size. These constraints were applied to the

ALE mesh in the same fashion as one would for a typical ALE structural analysis. In other words, nodes that lie on a plane of

symmetry were constrained to move only within that plane. Eight-node bricks were used for all of the Eulerian fluid meshes

which consisted of water and void. A void region must be defined to represent the air above the fluid free surface in order to

accommodate the splash effect upon impact. For ease of visualization, elements representing voids in the models are not shown

in the figures to follow. The Eulerian mesh sizes were somewhat driven by the Lagrangian meshes in these problems. Once the

structural mesh was developed, the fluid mesh was created with the same general element sizes in order to avoid contact problems that can occur with elements that have noticeable disparity in their sizes.

Initial velocities of the impacting structure for each analysis presented in this paper were set in the direction vertically towards

the horizontal free surface of the fluid mesh. No lateral initial velocities were assigned for any analysis.
NASA/TM—2003-212093 4

Figure 5. Mesh for Helium Tank Water Impact

Analysis.

Helium Tank Water Impact Analysis

The first analysis modeled a half section of the proposed helium tank

impacting a volume of water as an independent structure. The primary

purpose of this analysis was to establish that the ALE methodology was

working properly and to develop an idea as to what fidelity of mesh would

be necessary to get reasonable results. The mesh, shown in Figure 5,

consisted of 77184 elements and 82228 nodes. Symmetry was used to

model half of the problem. The tank, made of four-node shell elements,

was defined as elastic steel 0.5” thick. Elastic modulus, density, and

Poisson’s ratio were 28.0e6 lb/ in2

, 7.24e-4 lbm/in3, and 0.3 respectively.

An initial velocity of 960 inches/sec was assigned to the tank.

Results

Running the analysis to .1653 seconds took just over 38 cpu hours. The

predicted cavity formed in the water by the impacting tank was compared

to high-speed photographic images of a ball bearing being dropped into a

vessel filled with water. No scientific effort was made to establish scaling

or similarity parameters between the predicted and measured events but it

was noteworthy to see the striking resemblance of the two cavity formations. It was considered to be a positive outcome that the most basic

numerical representation of water used with a DYNA ALE analysis appeared to be capturing the general behavior of a water/structure impact.

Figure 6 shows a side-by-side comparison of the DYNA prediction with a

high-speed image.

Figure 6. Comparison of DYNA Analysis of Helium Tank Water Impact with High Speed Film

Image of Ball Bearing Impact in Water Bucket.

Reentry Capsule Water Impact

This second analysis was chosen for both the simplicity of the problem and the existence of experimental observations. It was

based on an experimental program conducted at NASA’s Langley Research Center in 1959,2

 to define the water landing characteristics of a space capsule similar to that used for the Mercury Program. The primary focus of the research was to establish peak

decelerations for various splashdown orientations of the capsule for both full and 1/12-scale test articles.
NASA/TM—2003-212093 5

Figure 7. Mesh for Reentry Capsule

Water Impact Analysis.

These experiments were conducted using several different initial conditions; however,

only one case was selected for DYNA analysis: A full-scale capsule impacting a body of

water at 360 inches per second. The full-scale capsule was 10.5 feet in height, 7.0 feet

in diameter at the base, and weighed 2150 lb. Figure 7 depicts the capsule and fluid

mesh created for this analysis. As with the previous problem, symmetry was used to

reduce the problem size by half. This model consisted of 84836 elements and 90066

nodes. Primary interest was in predicting the capsule deceleration; hence, the capsule

was modeled from four-node shells as a magnesium rigid body with an accelerometer

element added to it. Elastic modulus and density for the capsule were 6.5e6 lb/ in2

,

1.674e-4 lbm/in3 respectively.

Results

This analysis was run out to .2145 seconds taking just 13.5 cpu hours to complete. As

with the previous analysis, this problem was well behaved throughout the run. Attention

to the results of this analysis was focused on the capsules Z-acceleration (direction transverse to the water surface) predictions. Details, provided in reference 2, for the accelerometer that measured the downward acceleration on the full scale test, stated that it

could measure up to 100 g, had a natural frequency of 640 Hz, and was damped to 65%

of critical damping. The recording equipment, which read the data, did so at 600 Hz. Figure 8 shows an acceleration plot as a

function of time, which compares data from the Langley full-scale capsule drop tests to both raw and filtered accelerometer

predictions from this analysis. Curve B is the raw unfiltered analysis data, curve A is the raw data filtered with a Butterworth filter

at 50 Hz and curve C has been plotted from the observed data in reference 2.

Figure 8. Comparison of Predicted and Measured Acceleration Data for

Langley Capsule Drop Tests.

Clearly, the data from the capsule drop tests had been filtered in some fashion, however, no discussion was provided in reference

2 as to how it was done. It was reasoned that by applying a filter to the DYNA predictions and increasing its frequency cutoff

incrementally until all of the higher frequency oscillations not seen in the observed data were removed, that a comparison of the

experiment and the predictions could be made. The Butterworth filter using 50-60 Hz limits yielded curves that were very close

to the Langley experimental data providing a relatively high degree of confidence in the applicability of the DYNA ALE methodology to this class of problems.

B

C

A
NASA/TM—2003-212093 6

Full Scale Aft Skirt Analysis

Results of the helium tank and reentry capsule analyses provided confidence that it would be worthwhile to pursue a full scale aft

skirt analysis. A mesh was constructed of a 26-degree section of an SRB aft skirt with the proposed helium tank integrated into

the structure. The mesh for the aft skirt analysis, seen in Figures 9a and 9b, contained 164537 nodes and 161133 elements. The

majority of the structural model was made up of shell elements, which lay on the geometric centerlines of the plate structure they

were representing. In some areas, plates were welded or bolted together on the skirt to add more strength to the overall structure.

These instances were modeled by connecting the joined panels with very stiff beam elements whose lengths were determined by

the distance between the centerlines of those panels. Over 8000 beams were used in this model. Contact between fluid and

structure was limited to the aft skirt outer skin, the three main circumferential stiffening rings, and the helium tank. The initial

velocity of the skirt was 960 inches per second. The aft skirt was modeled entirely as an elastic material using aluminum for the

material properties. Density, elastic modulus, and Poisson’s ratio, were 2.656e-4 lbm/in3

, 10.5e6 lb/in2, and .33 respectively.

Figure 9. Mesh for Full Scale SRB Aft Skirt Water Impact Analysis.

(a) (b)

Results

Running this analysis to .09 seconds took 510 cpu hours to complete. The analysis was well behaved up to .03 seconds at which

point a notable increase in hourglass energy was observed indicating that remeshing portions of the mesh might be necessary.

Figure 10 shows five instances from the analysis. Note that the model predicts the secondary water impacts on the middle and

upper channel sections of the skirt. Post impact inspection of the aft skirt has indicated the existence of these secondary water

impacts. Figure 11 is a graph comparing measured and predicted accelerometer data. The measured data was taken from an

onboard data acquisition system (DAS) on the left hand booster from the STS-106 mission.3

 This data was provided electronically from SRB engineers at Kennedy Space Center. No filtering was applied to the DYNA predictions and it is unknown what,

if any, filtering was applied to the measured data. With the exception of the initial spike in the DYNA data, it can be argued that

there is general agreement between analysis and prediction. The sampling rate of the DAS data was substantially less than that

of the analysis and in order to make a stronger case for agreement between results, a higher DAS sampling rate would be desired.

Expertise with data filtering techniques of accelerometer data is a necessity in order to make quality assessments as to the validity

of analysis predictions for problems of this nature, particularly when comparing measured and computed data. Future work on

this problem will employ a filtering technique in order to make better engineering judgments from the data.
NASA/TM—2003-212093 7

SUMMARY

Using LS-DYNA with its ALE capability to characterize a Space Shuttle Solid Rocket Booster splashdown event with reasonable

results has been demonstrated suggesting that this analysis technique could yield valuable insight to water impact problems.

Future work is anticipated on analyzing the latter part of the SRB splashdown event in which the booster falls on its side and the

forward skirt at the top of the booster is potentially subjected to severe water impact loads.

Several issues were brought to light during this project; 1) the water model used in this analysis was a very rudimentary one as no

verified material models were available for our use in this problem. It would be of significant value should a more robust

characterization of water be developed for this class of problems. 2) The computational demands of a problem of this nature are

immense as demonstrated by the full aft skirt analysis. In order to keep the analysis turnaround times to a reasonable level, the

models in this effort were held to under 200,000 elements, however it ultimately would be desired that we increase the resolution

Time = 0.02 seconds Time = 0.035 seconds Time = 0.045 seconds

Time = 0.055 seconds Time = 0.07 seconds

Figure 10. Five Animation Images at Various Time Steps from Full Aft Skirt Analysis.

Figure 11. Comparison of Predicted and Measured Acceleration Data

for Full Aft Skirt Splashdown.
NASA/TM—2003-212093 8

of our ALE mesh by a factor of 8 to 10 in order to obtain a better solution. Further investigation is in order to develop a parallel

solution procedure enabling a model of higher resolution. 3) Early considerations in setting up the meshes for these analyses

included the use of non-reflecting boundary conditions on the ALE mesh. Some problems were created in using these and it was

decided to apply basic planar constraints to the planes of symmetry to simplify the problem. It is possible that a better-conditioned analysis might be obtained using the non-reflecting constraints and this issue should be considered in future analyses of

this type. 4) Data filtering (numerically removing non-pertinent frequency data from the experimental and computational results)

is imperative for correct interpretation of such highly transient results. Without a strong fundamental understanding of filtering

techniques, the quality of engineering decisions made from impact analyses is reduced.

REFERENCES

1. LS-DYNA Keyword Users Manual, Livermore Software Technology Corporation, Livermore, CA, March 2001,

Version 960.

2. McGehee, J.R. and Hathaway, M.E. (1959). “ Water-Landing Characteristics of a Reentry Capsule” NASA

Memo 5-32-59L NASA Langley Research Center, Langley Field, VA.

3. REFERENCE TO REPRESENT DATA E-MAILED FROM ENGINEERS AT KSC.

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14

Finite Element Simulation of a Space Shuttle Solid Rocket Booster Aft Skirt

Splashdown Using an Arbitrary Lagrangian-Eulerian Approach

Matthew E. Melis

Explicit finite element; Fluid structure; Arbitrary Lagrangian-Eulerian; ALE; Splashdown

water impact; Solid rocket booster; SRB

Unclassified -Unlimited

Subject Categories: 15, 39, and 34 Distribution: Nonstandard

Responsible person, Matthew E. Melis, organization code 5930, 216–433–3322.

Explicit finite element techniques employing an Arbitrary Lagrangian-Eulerian (ALE) methodology, within the transient

dynamic code LS-DYNA, are used to predict splashdown loads on a proposed replacement/upgrade of the hydrazine

tanks on the thrust vector control system housed within the aft skirt of a Space Shuttle Solid Rocket Booster. Two

preliminary studies are performed prior to the full aft skirt analysis: An analysis of the proposed tank impacting water

without supporting aft skirt structure, and an analysis of space capsule water drop tests conducted at NASA's Langley

Research Center. Results from the preliminary studies provide confidence that useful predictions can be made by

applying the ALE methodology to a detailed analysis of a 26-degree section of the skirt with proposed tank attached.

Results for all three studies are presented and compared to limited experimental data. The challenges of using the LSDYNA ALE capability for this type of analysis are discussed.