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i

NASA/TM—2011–217452

Ares I Scale Model Acoustic 

Test Overpressure Results

M.J. Casiano, D.A. Alvord, and D.M. McDaniels

Marshall Space Flight Center, Huntsville, Alabama

December 2011

National Aeronautics and

Space Administration

Marshall Space Flight Center • Huntsville, Alabama 35812
ii

Available from:

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076 –1320

443–757–5802

This report is also available in electronic form at

<https://www2.sti.nasa.gov/login/wt/>
iii

TABLE OF CONTENTS

1. INTRODUCTION ......................................................................................................................... 1

1.1 Ares I Scale Model Acoustic Test Ignition Overpressure Series Configuration 

and Objectives ......................................................................................................................... 1

1.2 Overpressure Background ....................................................................................................... 3

2. GENERAL DESCRIPTION AND DATA CHARACTERIZATION ............................................ 6

2.1 Ignition Transient Characterization ......................................................................................... 6

2.2 Scaling Methodology .............................................................................................................. 12

2.3 Overpressure Suppression ....................................................................................................... 12

2.4 Environment Characteristics ................................................................................................... 15

2.5 Scaled Environment ................................................................................................................ 19

3. OBSERVATIONS AND RECOMMENDATIONS ........................................................................ 23

REFERENCES ................................................................................................................................... 24
iv

LIST OF FIGURES

 1. ASMAT vertical firing of Vert03 at MSFC TS 116 on November 18, 2010 ...................... 2

 2. Vehicle in hold-down configuration on the LM .................................................................. 3

 3. Pretest photograph of ASMAT Vert02 at MSFC TS116 

on November 11, 2010 ....................................................................................................... 3

 4. ASAMAT below-deck IOP SS (exhaust duct)..................................................................... 4

 5. STS-1 overpressure measurement KSRPF034A with 50-Hz low-pass filter 

(STS-001 SRB IOP top of MLP) ........................................................................................ 5

 6. Space shuttle configuration with water bags in MLP secondary exhaust duct ................... 5

 7. Pressure contour from an unsteady CFD transient simulation for STS-1 .......................... 6

 8. Illustration of the Ares I and ASMAT ignition transient timing ......................................... 7

 9. Abrupt change in chamber pressure: (a) Chamber pressure 

and (b) chamber pressure rise rate....................................................................................... 8

10. Waveform event timing and identification: (a) DOP, (b) SOP, and (d) IOP ....................... 9

11. Measurement locations for data in figure 10 ...................................................................... 10

12. IOP and DOP wave progression in changing instrument frame (ASMAT data) ................ 11

13. Snapshot during motor start of (a) test IOP 1, (b) test IOP 2, 

and (c) test IOP 3 ................................................................................................................ 13

14. ASMAT below-deck IOP SS (main flame deflector with crest water) ............................... 14

15. ASMAT IOP suppression (water bags) ............................................................................... 14

16. Knockdown factor: Dry compared to IOP SS/water bags .................................................. 15

17. Knockdown factor: IOP SS to IOP SS/water bags ............................................................. 15

18. Vehicle maximum ASMAT peak-to-peak overpressure amplitude scaled 

to Ares I .............................................................................................................................. 16
v

LIST OF FIGURES (Continued)

19. Maximum ASMAT peak-to-peak overpressure environment amplitude 

along the tower scaled to Ares I ......................................................................................... 17

20 Maximum ASMAT peak-to-peak overpressure amplitude below the deck 

scaled to Ares I .................................................................................................................. 20

21. ASMAT frequency spectrum scaled to Ares I, amplitude not scaled 

(energy spectral density plot: Test-P8019-004, sensor-IOP_M05H) ................................. 19

22. ASMAT test IOP 3 (a) instantaneous pressure contour, (b) vehicle pressure profiles, 

and (c) instrument time histories at time t = 47 ms ........................................................... 20

23. Dry comparison: Data at the top of the TSM location for STS-1 and Ares I 

(scaled from ASMAT test IOP 3) ....................................................................................... 21

24. Wet comparison: Data at the top of the TSM location for STS-2, Ares I-X, 

and Ares I (scaled from ASMAT test IOP 1) ..................................................................... 21
vi
vii

LIST OF TABLES

1. Test nomenclature ................................................................................................................ 2

2. Range of ASMAT to Ares I overpressure scale factors for each IOP test ............................ 12

3. ASMAT IOP SS and water bag configuration ...................................................................... 14

4. Worst-case scaled Ares I peak-to-peak values ..................................................................... 17
viii

LIST OF ACRONYMS

ASMAT Ares I Scale Model Acoustic Test

CFD computational fluid dynamics

DOP duct overpressure

ESD energy spectral density

IOP ignition overpressure

IOP 1 ignition overpressure test 1

IOP 2 ignition overpressure test 2

IOP 3 ignition overpressure test 3

IOP SS ignition overpressure suppression system

KSC Kennedy Space Center

LM launch mount

ML mobile launcher

MLP mobile launch platform

MSFC Marshall Space Flight Center

NEP nozzle exit plane

RATO rocket assisted takeoff

RCS reaction control system

RSRM reusable solid rocket motor

RSRMV-DM1 five-segment reusable solid rocket motor demonstration motor number 1

SOP source overpressure

SRB solid rocket booster
ix

LIST OF ACRONYMS (Continued)

SRM solid rocket motor

STS Space Transportation System

TM Technical Memorandum

TS116 test stand 116

TSM tail service mast

Vert01 first vertical firing configuration

Vert02 second vertical firing configuration

Vert03 third vertical firing configuration

VSS vehicle stabilization system
x
1

TECHNICAL MEMORANDUM

ARES I SCALE MODEL ACOUSTIC TEST OVERPRESSURE RESULTS

1. INTRODUCTION

A summary of the overpressure environment from the 5% Ares I Scale Model Acoustic Test 

(ASMAT) and the implications to the full-scale Ares I are presented in this Technical Memorandum (TM). These include the scaled environment that would be used for assessing the full-scale 

Ares I configuration, observations, and team recommendations. The ignition transient is first characterized and described, the overpressure suppression system configuration is then examined, and 

the final environment characteristics are detailed. Preliminary results were presented in reference 1.

ASMAT was a development test that was performed on a 5% scale model of the Ares I 

vehicle mounted on the mobile launcher/launch pad to obtain data that will be used in analytical 

models to update the predicted Ares I lift-off acoustic and ignition overpressure environments. 

ASMAT was performed at the Marshall Space Flight Center (MSFC) East Test Area Test 

Stand 116 (TS116).

Several transient events occur during the startup of the solid rocket motor (SRM) for each 

test in the ASMAT ignition overpressure (IOP) test series. Each event produces a complex transient 

signal and requires systematically assessing each instrument’s waveform individually. The events 

discussed are the source overpressure (SOP), IOP, and duct overpressure (DOP).

The ASMAT IOP test series consists of three motors that were fired in the vertical configuration as shown in figure 1. The first vertical firing configuration is identified as Vert01, which 

is a configuration with below-deck water from the IOP suppression system (IOP SS) and includes 

water bags. The second vertical firing configuration is identified as Vert02, which is a configuration 

with water from the IOP SS and does not include water bags. The third vertical firing configuration 

is identified as Vert03, which is a dry configuration with no water from the IOP SS and does not 

include water bags.1 The horizontal firing is not discussed in detail in this TM but is identified as 

horizontal. Table 1 can be used as a quick reference for the test identification nomenclature.

1.1 Ares I Scale Model Acoustic Test Ignition Overpressure Series Configuration and Objectives

The ASMAT IOP test series consisted of Vert01, Vert02, and Vert03 firings. The Vert01 

test used an IOP SS and water bags. The Vert02 test used an IOP SS only with no water bags. The 

Vert03 test did not contain any IOP suppression. The IOP SS developed for Vert01 and Vert02 

stems from the space shuttle IOP SS, which comprises three primary components: Launch mount 

(LM) duct water, mobile launcher (ML) duct water, and crest water. Water is injected at 146 gpm in 

both the LM and the ML exhaust ducts in the ASMAT configuration. The crest water subsystem 

feeds water at a total flow rate of 873 gpm into both sides of the flame trench in the ASMAT 

configuration. 
2

Figure 1. ASMAT vertical firing of Vert03 at MSFC TS 116

on November 18, 2010.

Table 1. Test nomenclature.

Nomenclature 1 Nomenclature 2 Designation Description Date

Horizontal P8019-001 No IOP SS or water bags 7/30/2010

IOP 1 Vert01 P8019-002 IOP SS with water bags 11/5/2010

IOP 2 Vert02 P8019-003 IOP SS without water bags 11/10/2010

IOP 3 Vert03 P8019-004 No IOP SS and no water bags (dry) 11/18/2010

The vehicle model was held down in the launch configuration (figs. 2 and 3) with the nozzle exit plane located 0.539 in below the top deck of the LM in all three ASMAT tests. For the 

ASMAT case with water bags, the base of the water bags was located 1.3 in below the LM top deck 

with a water fill depth of 0.8 in. Since water was injected into the ASMAT exhaust duct through 

horizontal slits to better represent the full-scale water spray patterns, dummy splash plates were 

welded 2.1 in below the top deck of the LM for geometric similarity (see fig. 4).2 The instrumentation suite comprises 78 unsteady pressure sensors with 31 sensors on the vehicle, 14 on the tower, 

10 on the mobile launcher, 14 within the exhaust duct, and 9 within the flame trench. Two pressure 

measurement devices were located inside the igniter assembly and four strain gages were mounted 

on the rocket assisted takeoff (RATO) motor case used to estimate the transient chamber pressure.3 Other instrumentation details can be found in reference 4.

The primary objectives for the ASMAT IOP test series were to validate and improve upon 

the Ares I overpressure environments and to determine the effect of the IOP SS and water bags 

on the Ares I overpressure environment. The secondary objective was to obtain overpressure data 

for computational fluid dynamics (CFD) validation. Several other details and characteristics were 

addressed and reviewed including general IOP waveform characteristics, knockdown factors for 
3

the water bags and IOP SS, comparisons to the shuttle and Ares I-X environment, aft skirt/thermal 

curtain environments, and the locations and amplitudes of the highest environments.

Nozzle Exit Plane

Figure 2. Vehicle in hold-down configuration on the LM.

Figure 3. Pretest photograph of ASMAT Vert02 at MSFC TS116 

on November 11, 2010.

1.2 Overpressure Background

The overpressure (IOP and DOP) is a transient environment at lift-off resulting from fluid 

compression of the accelerating plume gas, subsequent rarefaction, and subsequent propagation 

from the exhaust trench and duct holes. These high-amplitude unsteady fluid-dynamic perturbations can adversely affect the vehicle and surrounding structure.
4

Launch Mount Dummy

Splash Plates

Launch Mount

Suppression Slot

Duckbill Mobile

Launcher Nozzles

Figure 4. ASAMAT below-deck IOP SS (exhaust duct).

There were several overpressure related issues for the launch of the first space shuttle flight, 

Space Transportation System 1 (STS-1). Accelerations measured at the wing, body flap, vertical 

tail, and crew cabin exceeded the 3s liftoff design environment.5 The forward reaction control system (RCS) oxidizer tank aft Z strut was deformed due to accelerations exceeding the design limits 

that were caused by the solid rocket booster (SRB) overpressure.5,6 Excessive environments were 

found on the orbiter flap with a pressure pulse that peaked at 2 psi, causing a 6-g acceleration.5

The STS-1 IOP waveform also excited the orbiter wing symmetric bending mode at approximately 

6 Hz, causing a 12-g acceleration.5 A full-scale STS-1 overpressure waveform from measurement 

KSPF034A (low-pass filtered at 50 Hz) just 20.9 in below the mobile launch platform (MLP) 

deck-0, inside of the top of the primary exhaust duct, is shown in figure 5.7 A solution to these 

exceedances and damage was developed in 4 months. The below-deck water in the MLP was 

diverted to spray directly into the SRM plume inside the exhaust ducts. A limp-wall barrier was 

placed at the top of the exhaust ducts in the space shuttle configuration. The barrier, shown in 

figure 6, consisted of water bags that contained 12 in of standing water. The important lesson 

from STS-1 was that inadequate suppression of the overpressure led to vehicle damage, delays, and 

unsafe flight.
5

Time (s)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

4

3

2

1

0

–1

–2

–3

–4

Overpressure (psi)

Figure 5. STS-1 overpressure measurement KSRPF034A with 50-Hz low-pass filter 

(STS-001 SRB IOP top of MLP).

Figure 6. Space shuttle configuration with water bags in MLP secondary exhaust duct.
6

2. GENERAL DESCRIPTION AND DATA CHARACTERIZATION

2.1 Ignition Transient Characterization

In general, an igniter starts the burning process of an SRM. Subsequently, there is an 

abrupt change in pressure inside the motor from its original ambient pressure to its final steady 

state pressure. This abrupt change in pressure corresponds to an abrupt change in flow through 

the nozzle. Somewhere outside the nozzle, the fluid compression due to the accelerating plume gas 

causes an increase in pressure at a location within the plume. This location is called the apparent 

source. Pressure waves propagate from the apparent source within the flame trench as the SOP, out 

the flame trench as the DOP, and up through the exhaust duct as the IOP.

Some very useful guidance for understanding overpressure is also given through CFD 

analysis. Figure 7 shows a full-scale CFD transient simulation for STS-1 in the form of a pressure 

contour plot at an instant in time during the source overpressure development (J.S. West, Personal 

Communication, 2008). The apparent pressure source is below the pad between the exhaust duct 

and the deflector.

SOP, Apparent Source

Figure 7. Pressure contour from an unsteady CFD transient simulation for STS-1.
7

Figure 8 shows a cartoon with a series of snapshots describing instants during the transient 

events for Ares I or ASMAT. The following progression of steps occurs: 

(1) The fire command is sent and the initiator ignites the propellant (fig. 8(a)).

(2) The igniter pulse exits the nozzle (fig. 8(b)).

(3) The igniter pulse begins to propagate up the vehicle as the plume gas accelerates out 

of the nozzle (fig. 8(c)).

(4) The SOP develops first as the apparent source and propagates through the flame 

trench and exhaust duct (fig. 8(d)).

(5) The IOP propagates out the exhaust duct and the DOP propagates out the flame 

trench exits (fig. 8(e)).

(6) The IOP propagates up the vehicle and the DOP impinges the vehicle primarily from 

the tower side of the ML (fig. 8(f)).

T0: Ignition command sent.

T3: SOP develops.

T1: Igniter pulse develops near nozzle.

T4: IOP and DOP propagate

out exhaust duct and trench.

T2: Igniter pulse propagates up the vehicle.

T5: IOP and DOP impinge

along the vehicle.

(a) (b) (c)

(d) (e) (f)

Figure 8. Illustration of the Ares I and ASMAT ignition transient timing.
8

The overpressure waveform behaves proportionally to the chamber pressure rise rate. 

Figure 9 shows the abrupt change in chamber pressure over the short duration for each test in the 

IOP test series and the horizontal test.

1,500

1,000

500

0.01

Horizontal

Vert01

Vert02

Vert03

0.015 0.02 0.025 0.03 0.035

Time (s)

Pressure (psia)

0.04 0.045 0.05 0.055 0.06

0 (a)

(b)

20 × 104

15

10

5

0

0.01

Horizontal

Vert01

Vert02

Vert03

0.015 0.02 0.025 0.03 0.035

Time (s)

Rise Rate (psi/s)

0.04 0.045 0.05 0.055 0.06

–5

Figure 9. Abrupt change in chamber pressure: (a) Chamber pressure 

and (b) chamber pressure rise rate.

A few measurements are chosen that show the ignition transient events. The time histories 

shown in figure 10 are filtered to represent the same scaled frequency content that would be present 

in the Ares I vehicle up to 100 Hz.3,8 Note that the time history scales are not the same. The igniter 

pulse contains nonnegligible spectra out to a very high frequency so the filtering significantly reduces their amplitude. The purpose of figure 10 is primarily for identification and characterization of 

the overpressure events. Figure 11 indicates the instrument locations for the measurements shown 

in figure 10.

The earliest occurrence of the SOP waveform was evident near the bottom of the exhaust 

duct. The instruments here also exhibited some of the highest overpressure amplitudes. The SOP 

waveform, shown in figure 10(b), propagated through the trench and finally reached the exit as the 

DOP waveform, shown in figure 10(a). Figure 10(c) shows the SOP waveform once it exited the 

exhaust duct. It is here that the waveform became the IOP waveform.

The colored windows in figure 10 show the extent of each event. The primary variable that 

determined the timing, location, and distribution of the overpressure source was the chamber pressure rise rate. This was used to select the window. It was then slid in time based on the amount of 

time needed for the wave to propagate to each measurement. A logical set of consistent rules was 

then used to identify each of the overpressure events at the multitude of measurement locations. 

A detailed discussion on the physical mechanisms and timing window selection is reported in 

reference 3.
9

IOP

Window

Time (s)

(c) 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

2

1

0

–1

–2

DOP

Window

SOP

Window

Igniter

Window

Igniter

Window

Igniter

Window

*Example for Test IOP 3

(psi)

Duct Overpressure

(Duct Exit)

Source Overpressure

(Bottom of Exhaust Duct)

Ignition Overpressure

(Top of Exhaust Duct)

(b)

8

7

6

5

4

3

2

1

0

–1

–2

–3

(psi)

(a)

1.5

1

0

–1

–2

–3

(psi)

Figure 10. Waveform event timing and identification: (a) DOP, 

 (b) SOP, and (d) IOP.
10

Figure 11. Measurement locations for data in figure 10.

Figure 12 shows an example of the IOP and DOP waveforms in a series of ASMAT time 

history plots that helped provide verification of the selected events. It shows the IOP wave progressing upwards and away from the exhaust duct and the DOP wave progressing away from the duct 

exit. It can be tracked by recognizing that the measurements shown in the subplots are at separate 

heights but are also along the vehicle in figure 12(a), the south-side tower in figure 12(b), and the 

north-side tower in figure 12(c). The illustrations at the left of each subplot show these locations; 

it should also be noted that the measurement heights are approximately the same when compared 

between each illustration.

Considering that a pressure wave emanates from the top of the exhaust duct and travels in 

the direction from the vehicle to the north-side tower, the IOP should appear to move forward in 

time through each frame from the vehicle to the north-side tower. The DOP should appear to move 

backward in time through each frame from the vehicle to the north-side tower, since the wave emanates from the trench exit and propagates in the direction from the north-side tower to the vehicle. 

The black curve should be the most exaggerated in its progression because the waveforms travel 

nearly horizontally near the top of the ML. The colored dashed lines help emphasize these points 
11

by connecting the peaks of each waveform. The slopes are not parallel because the distance traveled by the wave is different for each measurement.

Time (s)

0.03 0.035 0.04 0.045 0.05 0.055 0.06

IOP 006_202H Time History

Not to Scale

North

006_202

072_180

112_180

Thrust Plate

Thrust Plate

Thrust Plate

Thrust Plate IOP DOP

IOP 072_180H Time History

IOP 112_180H Time History

*Test IOP 1

*Test IOP 1

*Test IOP 1

(a)

0.1

0.09

0.07

0.05

0.03

0.01

–0.01

–0.03

–0.05

(psi)

IOP T03H Time History

IOP DOP

IOP T09H Time History

IOP T12H Time History

(b)

0.11

0.1

0.05

0

–0.05

–0.06

(psi)

IOP T01H Time History

IOP

DOP IOP T07H Time History

IOP T10H Time History

(c)

0.1

0.09

0.07

0.05

0.03

0.01

–0.01

–0.03

–0.05

(psi)

Not to Scale

North

T12

T9

T03

Thrust Plate

Thrust Plate

Thrust Plate

Thrust Plate

Not to Scale

North

T10

T07

T01

Thrust Plate

Thrust Plate

Thrust Plate

Thrust Plate

Figure 12. IOP and DOP wave progression in changing instrument frame (ASMAT data).
12

2.2 Scaling Methodology

The scaling methodology is detailed in references 8 and 9. The primary scaling factors are 

based on the differences in geometry and the motor performance parameters. Table 2 shows the 

resulting scale factors for each test. The RATO performance parameters used were estimated using 

strain gage and igniter pressure measurements (i.e., steady state chamber pressure and peak chamber pressure rise rate).3,10 The full-scale Ares I motor performance parameters were obtained from 

five-segment reusable SRM (RSRM) demonstration motor number 1 (RSRMV-DM1) data and the 

upper and lower bounds of the approved RSRMV MODEL5V ballistic dispersion curves. These 

curves are based on heritage flight data and account for historical motor variability in the RSRM. 

The horizontal motor scale factors and the details on the approach used to determine the scale 

factors in table 2 are discussed in references 3 and 8.

Table 2. Range of ASMAT to Ares I overpressure scale factors for each IOP test.

ASMAT to Ares I Overpressure Scale Factors

Scale Factor IOP 1 IOP 2 IOP 3

Lower bound (n/a) 1.43 1.35 1.46

RSRMV DM-1 (n/a) 1.46 1.38 1.49

Upper bound (n/a) 2.39 2.25 2.43

Frequency scaling (Hz) 1,365 1,452 1,345

2.3 Overpressure Suppression

The details of the overpressure suppression system are documented in the ASMAT IOP 

Series Test Report.2 Figure 13 shows a snapshot during the RATO motor start. Steam from the 

IOP SS is observed on test 1 and test 2. Table 3 shows the design flow conditions for the ASMAT 

IOP test series. The water bag design is detailed in reference 11. Figures 4, 14, and 15 show some 

views of the below-deck IOP SS.

ASMAT knockdown factors were estimated from the ASMAT data. Knockdown factors 

are values that represent ratios of the overpressure peak amplitude in a dry configuration to the 

overpressure peak amplitude in a suppressed configuration. ASMAT amplitude and timing information was characterized for every measurement and every overpressure event for each test. 

Figures 16 and 17 show knockdown factor plots comparing IOP test 3 (IOP 3) with IOP test 1 

(IOP 1) and IOP test 2 (IOP 2) with IOP 1, respectively. Statistical information is labeled on each 

plot. This information includes the mean knockdown factor, the standard deviation, and the coefficient of variation (which is a measure of dispersion). The distance axis is also scaled to full-scale 

distance.

The mean knockdown factor due to the water bags and IOP SS is 7.03. Comparison of the 

IOP SS configuration with the IOP SS and water bag configuration indicated the effectiveness of 

the water bags alone. The mean knockdown factor in the vicinity of the aft skirt was 1.52; however, 

this value is not indicative of the knockdown factor of water bags without the IOP SS. Subscale 
13

shuttle 6.4% model results indicate that the effect of the water bags was different with and without 

the IOP SS. The IOP SS and water bags show excellent suppression throughout. The water bags are 

effective near the exhaust duct and aft skirt and not effective away from this area. The knockdown 

factors exceed those used in the shuttle configuration and will be shown to bring the environment 

to within the Ares I data book environment.

Vert01

t=21.8 ms

(a)

(b)

(c)

Vert02

t=22.3 ms

Vert03

t=19.7 ms

Figure 13. Snapshot during motor start of (a) test IOP 1, (b) test IOP 2, 

and (c) test IOP 3.
14

Figure 14. ASMAT below-deck IOP SS (main flame deflector with crest water).

Figure 15. ASMAT IOP suppression (water bags).

Table 3. ASMAT IOP SS and water bag configuration.

IOP 1 IOP 2 IOP 3

Water bags Yes No No

Main flame deflector water north (gpm) 640 640 N/A

Main flame deflector water south (gpm) 233 233 N/A

Exhaust duct water (gpm) 291 291 N/A
15

Knockdown Factor (–)

Full Scale Distance (ft)

 10

9

8

7

6

5

4

3

2

1

0

0 50 100 150 200 250 300

Tower and Vehicle Data

μ = 7.03

σ = 0.60

cv = 8.5% Effectiveness of Water Bags and IOP SS:

Excellent Suppression Throughout

IOP 3 to IOP 1 Knockdown Factors

Figure 16. Knockdown factor: Dry compared to IOP SS/water bags.

IOP 2 to IOP 1 Knockdown Factors

0 50 100 150 200 250 300

Effectiveness of Water Bags in an IOP SS Environment:

Primary Reduction is Near the Aft Skirt

Tower and Vehicle Data

Knockdown Factor (–)

Full Scale Distance (ft)

6

5

4

3

2

1

0

=1.52

=0.56

cv =37.0%

μ

σ

Figure 17. Knockdown factor: IOP SS compared to IOP SS/water bags.

2.4 Environment Characteristics

The ASMAT overpressure environment was scaled to the full-scale Ares I configuration. 

To characterize the differences between the tests, the scaled ASMAT environments (scaled to Ares 

I environments) were compared. The maximum measured overpressure (which includes both the 

IOP and DOP waves) is selected for comparison purposes. This maximum measured overpressure 
16

waveform would impinge on the vehicle and surrounding structure. Using the previously mentioned 

scaling method and the appropriately filtered data (i.e., low-pass, 100-Hz, full-scale equivalent), the 

maximum overpressure from each ASMAT IOP test was scaled to Ares I conditions.8 The overpressure environment is compared from each ASMAT IOP test was scaled to Ares I conditions.8

The overpressure environment is compared for each test along the vehicle and separately along the 

tower. The worst-case (scaled upper bound), full-scale, peak-to-peak overpressure amplitude 

is shown in figures 18 and 19. The distance axis is also scaled to full-scale distance. 

On the first stage (fig. 18), the overpressure at the top of the exhaust duct near the nozzle 

exit plane (NEP), the thermal skirt and curtain, and the aft skirt were carefully examined. The data 

again show that the water bags are most effective at the aft end. Table 4 provides a list of pressure 

values that are the worst-case scaled Ares I peak-to-peak values (scaled from ASMAT to Ares I). 

IOP 1 (Upper Bound)

IOP 2 (Upper Bound)

IOP 3 (Upper Bound)

Elevation up the Vehicle From the Mobile Launcher Top Deck (ft)

0 50 100 150 200 250 300

Pressure (psi)

Full Scale Distance (ft)

6

5

4

3

2

1

0

Figure 18. Vehicle maximum ASMAT peak-to-peak overpressure 

amplitude scaled to Ares I.
17

IOP 1 (Upper Bound)

IOP 2 (Upper Bound)

IOP 3 (Upper Bound)

Full Scale Elevation Above the Mobile Launcher (ft)

0 50 100 150 200 250 300

Pressure (psi)

3.5

3

2.5

2

1.5

1

0.5

0

Figure 19. Maximum ASMAT peak-to-peak overpressure environment 

amplitude along the tower scaled to Ares I.

Table 4. Worst-case scaled Ares I peak-to-peak values.

Top of 

Exhaust Duct

(psi)

Thermal 

Curtain

(psi)

Aft Skirt

(psi)

IOP SS + waterbags 0.7 0.7 0.2

IOP SS 1.7 1.0 0.4

No suppresion 5.0 2.0 1.6

The overpressure data on the tower shown in figure 20 are again the worst-case scaled 

Ares I peak-to-peak values. A pressure measurement at the vehicle stabilization system (VSS) 

(measurement KSAPE047A during the Ares I-X launch) is compared to the predicted Ares I data 

(worst-case scaled from ASMAT). The Ares I-X VSS is 156 ft above the MLP and the instrument 

measures 0.2 psi. There is not an instrument in the scaled ASMAT tower location; however, the 

data along the tower can be interpolated from figure 20. Because the decay of finite waves and 

acoustic waves follow a power law, an estimate is based on a power curve fit to the tower data. The 

interpolated amplitude at 156 ft is 0.2 psi for the comparable configuration with IOP SS and water 

bags. This provides a good comparison at a particular location (at the VSS) between the amplitude 

of the Ares I-X overpressure (0.2 psi) and the overpressure for the analogous ASMAT configuration scaled to Ares I (0.2 psi). It is an analogous configuration in that the Ares I-X configuration 

uses the below-deck IOP SS and space shuttle water bags. The other predicted Ares I configurations give 0.2 psi (IOP SS only) and 1.2 psi (no IOP SS and no water bags).
18

The maximum overpressure environment was also characterized. As expected, the maximum 

amplitudes occurred below the deck, specifically at the bottom opening of the exhaust duct near 

the apparent source. This is shown in figure 20. The blue diamonds are data scaled from IOP 1, the 

red squares are data scaled from IOP 2, and the green triangles are data scaled from IOP 3.The 

distance axis is also scaled to full-scale distance. The bottom opening of the exhaust duct is 

expected to be near the overpressure source as shown in figure 7. Here, the predicted Ares I peakto-peak overpressure amplitude is in the range of 13 to 18 psi for all three suppression configurations. In fact, the maximum amplitudes with IOP SS are higher than the cases without the IOP SS 

near the source location. This is expected because the water takes up much of the volume underneath the deck during the formation of the overpressure events. However the maximum amplitudes 

away from the source for the case without the IOP SS (IOP 3) remain consistently higher than the 

cases with IOP SS due to the attenuating features of the IOP SS.

The overpressure wave that exits the side from underneath the deck has the frequency content shown in figure 21. Although this spectrum is specific to one instrument, it gives a sense of 

the relative amplitude as a function of frequency for the Ares I prediction. The pressure amplitude 

was not scaled to Ares I (only the frequency); however, the purpose of the energy spectral density 

(ESD) plot is to get a sense of the Ares I frequency content. The dominant frequency bandwidth is 

clearly in the 0- to 10-Hz range peaking at approximately 4 Hz.

IOP 1 (Upper Bound)

IOP 2 (Upper Bound)

IOP 3 (Upper Bound)

Inside Exhaust Hole

(near the source)

Full Scale Below Deck Distance From the Overpressure Source (ft)

0 50 100 150 200

Pressure (psi)

20

16

12

8

4

0

Figure 20. Maximum ASMAT peak-to-peak overpressure amplitude below the deck 

scaled to Ares I.
19

Test: IOP 3

Instrument: M05

Magnitude: ASMAT Amplitude

Time Window (ASMAT Data): 0 ms–64 ms

Recored Sample Rate (ASMAT Data): 256 ksps

Block Size (ASMAT Data): 16,384 Samples

Frequency Scaling Factor (ASMAT to Ares I Predictied): 13.43x

Scaled Requency Resolution (Ares I): 1.2 Hz

Full Scale Ares I Frequency (Hz)

0 10 20 30 40 50 60 70 80 90 100

ASMAT Amplitude ((psi2-s)/Hz)

10–5

10–6

10–7

10–8

10–9

10–10

Figure 21. ASMAT frequency spectrum scaled to Ares I, amplitude not scaled 

(energy spectral density plot: Test-P8019-004, sensor-IOP_M05H.

Figure 22 is a single frame from a data visualization video interpolating the vehicle pressure 

measurements across the surface of the vehicle. The full animation is available on Zhora, the secure 

MSFC Fluid Dynamics ER42 data server. It shows several subplots for ASMAT IOP 3. The subplot on the left is a pressure contour plot at an instant in time occurring at 47 ms. The middle subplot shows the ASMAT vehicle pressure profiles along orientations of 0°, 90°, 180°, and 270°. The 

final column of subplots on the right contains instrument time histories at locations shown with 

the marker of the associated color in the contour plot. The contour plot and the vehicle profile plot 

show a differential pressure between the forward and aft end of the vehicle. For this case with no 

IOP SS or water bags, the differential pressure is approximately equivalent to a scaled Ares I value 

of 2 psi. The instrument time histories on the right show the progression of the overpressure wave 

up the vehicle.

2.5 Scaled Environment

Figures 23 and 24 provide a concise summary of the ASMAT IOP test series environment 

results. The ASMAT data are scaled to the Ares I configuration and plotted versus STS-1 data for 

the case with no IOP SS or water bags (fig. 23) and is plotted versus STS-2 data and Ares I-X data 

for the case with IOP SS and water bags (fig. 24). These comparisons provide the best possible oneto-one comparison for overpressure with and without water suppression. Additionally, the location 

at the top of the tail service mast (TSM), which is a consistent location for all the MLPs for shuttle 

and Ares I-X, was chosen for comparison. This measurement does not exist for the ASMAT or 

Ares I configuration because there is no TSM. However, the ASMAT data were scaled and fitted 

appropriately based on the nearest instrumentation. The timing of the overpressure event cannot 

be easily scaled from the ASMAT test data partly because of differences in motor igniter burning 
20

and timing. However, the waveform peak amplitude time is adjusted based on the Ares I Broadwell 

and Tsu predictions.12,13 The envelope from the previous predictions, shown with the dashed lines, 

is correctly estimated for the location of the TSM measurement.

Circumferential Location (°) Pressure (psi)

0 45 90 135 180 225 270 315 360 –0.5 0 0.5

ASMAT Vehicle Elevation (in)ASMAT Vehicle Elevation (in)

Pressure (psi)

150

100

50

0

150

100

50

0

Time Relative to t0 (s)

ASMAT Pressure Field Instrument Time Histories

Vehicle Pressure Profiles

0°, 90°, 180°, and 270°

2

1.5

1

0.5

0

–0.5

2

1.5

1

0.5

0

–0.5

2

1.5

1

0.5

0

–0.5

 0 0.05 0.1

 0 0.05 0.1

 0 0.05 0.1

 0 0.05 0.1

2

1.5

1

0.5

0

–0.5

t

0 0.047

0.5

0.4

0.3

0.2

0.1

0

–0.1

–0.2

–0.3

–0.4

–0.5

(a) (b) (c)

Figure 22. ASMAT test IOP 3 (a) instantaneous pressure contour, (b) vehicle pressure profiles, 

and (c) instrument time histories at time t = 47 ms.
21

Time (s)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

STS-1 (Dry)

Ares I Scaled From ASMAT (Dry)

Dry Environment

Predicted Range

Current Data Book

Environment (Wet)

Top of the TSM Location

Equivalent Full-Scale 100-Hz LP Filter

Pressure (psi)

3

2

1

0

–1

–2

–3

Figure 23. Dry comparison: Data at the top of the TSM location for STS-1 and Ares I 

(scaled from ASMAT test IOP 3).

Time (s)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

STS-2 (Wet)

Ares I-X (Wet)

Ares I Scaled From ASMAT (Wet)

Dry Environment

Predicted Range

Current Data Book

Environment (Wet)

Top of the TSM Location

Equivalent Full-Scale 100-Hz LP Filter

Pressure (psi)

3

2

1

0

–1

–2

–3

Figure 24. Wet comparison: Data at the top of the TSM location for STS-2, Ares I-X, 

and Ares I (scaled from ASMAT test IOP 1).
22

Figures 23 and 24 show several features of interest. First, the dry ASMAT-scaled Ares I prediction is slightly higher than the STS-1 flight data but lower than the dry Broadwell and Tsu Ares 

I environment. The ASMAT-scaled Ares I prediction is a prediction that can be used to remove 

much of the conservatism in the Broadwell and Tsu model predictions.12 This is shown in figure 23 

by comparing the blue Ares I prediction to the green dashed lines (which is the range of the previous dry predictions). It also shows that the dry Ares I prediction (scaled from ASMAT) is higher 

than the orange dashed lines or the current data book environment, which considers IOP water 

suppression and water bags.13 Future work for vehicle design would have to include a new ASMAT 

IOP environment evaluated by an additional loads cycle. Finally, the wet (IOP SS and water bags) 

Ares I prediction (scaled from ASMAT) (fig. 24) meets the environment specified in the data book. 

The wet environment is also consistent with the wet environment from STS-2 and the Ares I-X 

flight data. The predicted Ares I overpressure environment with the IOP SS and water bag configuration is within the family of data from Ares I-X and space shuttle. 
23

3. OBSERVATIONS AND RECOMMENDATIONS

The key observations can be taken from figures 16, 17, 23, and 24. The IOP SS and water 

bags successfully knocked down the overpressure events. Figures 16 and 17 detail the effectiveness 

of the IOP SS and water bags. The combined knockdown factor was 7.03 along the majority of the 

vehicle and tower. Water bags appear to be effective in knocking down the overpressure near the aft 

end of the vehicle. The IOP SS is effective over the entire pad, tower, and vehicle. Figures 23 and 

24 show that the dry Ares I prediction (scaled from ASMAT) is higher than the current data book 

environment. Also, the wet Ares I prediction (scaled from ASMAT) meets the environment specified in the data book.

The Broadwell and Tsu prediction with water suppression has a higher amplitude than the 

ASMAT-scaled Ares I prediction with water; therefore, it is most likely that the vehicle design would 

also withstand the new ASMAT-scaled water-suppressed Ares I prediction. However, future work would 

include a new ASMAT-scaled Ares I overpressure environment evaluated under a subsequent-loads 

cycle. This environment would consider more representative frequency and amplitude content of the 

overpressure transient. Review of the first stage, upper stage, and crew module component requirements 

and design limits should be reviewed with respect to the new environment. The recommendation for 

Ares I is to keep the space shuttle heritage IOP SS (below-deck IOP water in the LM and ML and also 

the crest water on the main flame deflector) and water bags.
24

REFERENCES

1. Alvord, D.A.; Casiano, M.J.; and McDaniels, D.M.: “Ares I Scale Model Acoustic Test: Ignition Overpressure Characterization and Analysis Presentation to KSC-LXD10,” ESTSGFY11-00502, Marshall Space Flight Center Internal Memo, Huntsville, AL, 2011.

2. Houston, J.D.; Alvord, D.A.; and Vargas, M.B.: “ASMAT IOP Series Test Report,” ESTSGFY11-00327, Marshall Space Flight Center Internal Memo, Huntsville, AL, 2011.

3. Casiano, M.J.; Alvord, D.A.; and McDaniels, D.M.: “ASMAT IOP Series Analysis Report,” 

Marshall Space Flight Center, Huntsville, AL, to be published.

4. Casiano, M.J.: “ASMAT IOP Instrumentation and Configuration Assessments,” ER42 (11-

021), NASA Marshall Space Flight Center Internal Memo, Huntsville, AL, 2011.

5. Dougherty, N.S.: “Shuttle SRB Ignition Overpressure (IOP)—STS-1 and STS-2 and Subsequent Data,” ERC/EV-33-06-01, NASA Marshall Space Flight Center Internal Memo, Huntsville, AL, 2006.

6. Bejmuk, B.I.: “Space Shuttle Integration—Lessons Learned,” Boeing Presentation, 2005, <http://search-www.boeing.com/search?q=Space+Shuttle+Integr

ation—Lessons+Learned&site=www_boeing&btnG=Search&client=www_

boeing&proxystylesheet=www_boeing&output=xml_no_dtd&btnG.x=0&btnG.y=0&sort=dat

e%3AD%3AL%3Ad1&entqr=0&oe=UTF-8&ie=UTF-8&ud=1>, Accessed April 2011.

7. Casiano, M.J.: “Ignition Overpressure Benchmarking and Parameters and Launch Acoustics 

Parameters,” ER42 (07-39), NASA Marshall Space Flight Center Internal Memo, Huntsville, 

AL, 2007.

8. Alvord, D.A.: “Ares I Scale Model Acoustic Test: Ignition Overpressure Scaling Analysis,” 

ESTSG-FY11-00001, NASA Marshall Space Flight Center Internal Memo, Huntsville, AL, 

2010.

9. Casiano, M.J.: “Ignition Overpressure Scaling Methodologies Summarized,” ER42 (08-006), 

NASA Marshall Space Flight Center Internal Memo, Huntsville, AL, 2007.

10. Alvord, D.A.: “Ares I Scale Model Acoustic Test: Chamber Pressure Rise Rate for the RocketAssisted Take Off Solid Rocket Motor,” ESTSG-FY10-02462, NASA Marshall Space Flight 

Center Internal Memo, Huntsville, AL, 2010.
25

11. Alvord, D.A.: “Ares I Scale Model Acoustic Test Water Bag Design Considerations and Development,” ESTSG-FY10-00074, NASA Marshall Space Flight Center Internal Memo, Huntsville, AL, 2009.

12. Broadwell, J.E.; and Tsu, C.N.: “Transient Pressures Caused by Rocket Start and Shutdown 

in Ducted Launcehers,” J. Spacecraft Rockets, Vol 4, No. 10, pp. 1323–1328, 1967.

13. “Ares I Acoustic Environments Data Book,” CxP 72164, Marshall Space Flight Center, 

Huntsville, AL, 2009.
26

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Ares I Scale Model Acoustic Test Overpressure Results

M.J. Casiano, D.A. Alvord, and D.M. McDaniels

George C. Marshall Space Flight Center

Huntsville, AL 35812

National Aeronautics and Space Administration

Washington, DC 20546–0001

Unclassified-Publicly Available

Subject Category 71

Availability: NASA CASI (443–757–5802)

Prepared by the Fluid Dynamics Branch (ER42) in the Propulsion, Structural, Thermal, and Fluid 

Analysis Division, Engineering Directorate, Marshall Space Flight Center, Huntsville, AL

M–1328

Technical Memorandum

NASA/TM—2011–217452

ignition overpressure, ASMAT, acoustics, scaled environment

01-12-2011

UU 36

NASA

U U U

A summary of the overpressure environment from the 5% Ares I Scale Model Acoustic Test (ASMAT) and 

the implications to the full-scale Ares I are presented in this Technical Memorandum. These include the scaled 

environment that would be used for assessing the full-scale Ares I configuration, observations, and team recommendations. The ignition transient is first chracterized and described, the overpressure suppression system 

configuration is then examined, and the final environment characteristics are detailed. The recommendation for 

Ares I is to keep the space shuttle heritage ignition overpressure (IOP) suppression system (below-deck IOP 

water in the launch mount and mobile launcher and also the crest water on the main flame deflector) and the 

water bags. 

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