Jeffrey T. Csank and Thomas J. Stueber

Glenn Research Center, Cleveland, Ohio

A Turbine Based Combined Cycle Engine Inlet 

Model and Mode Transition Simulation 

Based on HiTECC Tool

NASA/TM—2012-217714

September 2012

AIAA–2012–4149
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Jeffrey T. Csank and Thomas J. Stueber

Glenn Research Center, Cleveland, Ohio

A Turbine Based Combined Cycle Engine Inlet 

Model and Mode Transition Simulation 

Based on HiTECC Tool

NASA/TM—2012-217714

September 2012

AIAA–2012–4149

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

48th Joint Propulsion Conference and Exhibit

cosponsored by the AIAA, ASME, SAE, and ASEE

Atlanta, Georgia, July 30–August 1, 2012
Acknowledgments

The authors would like to thank Dr. James L. Pittman, the NASA Fundamental Aeronautics Program 

Hypersonic Project Manager for funding this work.

Available from

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Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identification 

only. Their usage does not constitute an official endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

This work was sponsored by the Fundamental Aeronautics Program 

at the NASA Glenn Research Center.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-217714 1

A Turbine Based Combined Cycle Engine Inlet Model and Mode 

Transition Simulation Based on HiTECC Tool

Jeffrey T. Csank and Thomas J. Stueber

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

An inlet system is being tested to evaluate methodologies for a turbine based combined cycle 

propulsion system to perform a controlled inlet mode transition. Prior to wind tunnel based hardware 

testing of controlled mode transitions, simulation models are used to test, debug, and validate potential 

control algorithms. One candidate simulation package for this purpose is the High Mach Transient Engine 

Cycle Code (HiTECC). The HiTECC simulation package models the inlet system, propulsion systems, 

thermal energy, geometry, nozzle, and fuel systems. This paper discusses the modification and redesign of 

the simulation package and control system to represent the NASA large-scale inlet model for Combined 

Cycle Engine mode transition studies, mounted in NASA Glenn’s 10- by 10-Foot Supersonic Wind 

Tunnel. This model will be used for designing and testing candidate control algorithms before 

implementation.

Nomenclature

AIP Aerodynamic Interface Plane

AoA Angle of Attack

CCE Combined Cycle Engine

CCE-LIMX Combined Cycle Engine Large scale Inlet for Mode transition eXperiments.

CDM Control Design Models

CP Cold Pipe

DMSJ Dual-Mode Scram Jet

FAP Fundamental Aeronautics Program

GN&C Guidance, Navigation, and Control

GRC Glenn Research Center

HiTECC High Mach Transient Engine Cycle Code

HSFP High-Speed Flow Path

LAPIN LArge Perturbation INlet simulation

LIMX Large scale Inlet for Mode transition eXperiments

LSFP Low-Speed Flow Path

NASA National Aeronautics and Space Administration

PLA Power Lever Angle

RALV Reusable Airbreathing Launch Vehicle

RBCC Rocket Based Combined Cycle 

TBCC Turbine Based Combined Cycle

TSTO Two-Stage-To-Orbit

10×10 SWT 10- by 10-Foot Supersonic Wind Tunnel

1.0 Introduction

The National Aeronautics and Space Administration (NASA) Fundamental Aeronautics Program 

(FAP) Hypersonic Project is concentrating on the development of tools and technologies to design and 
NASA/TM—2012-217714 2

control reusable airbreathing launch vehicles (RALV). RALVs are intended to provide hypersonic flight 

through the Earth’s atmosphere and create safe, routine, airline-type access to space. The RALV concept 

currently under study focuses on a two-stage-to-orbit (TSTO) vehicle that contains two separate sub 

vehicles each with its own propulsion system. The propulsion system for the first sub vehicle is 

responsible for horizontal take-off and acceleration of both vehicles to a hypersonic staging point. At the 

staging point, the two sub vehicles separate and each moves forward autonomously. The propulsion 

system being considered in this work is a Combined Cycle Engine (CCE) concept for the first sub 

vehicle, which incorporates an airbreathing turbine-based engine for take-off and acceleration to 

supersonic speeds, and a dual-mode scramjet (DMSJ) combustor for further acceleration to hypersonic 

speed. Together, these two stages are referred to as a Turbine Based Combined Cycle (TBCC) propulsion 

system. This TBCC propulsion system offers a split-flow inlet to provide airflow to an over-under engine 

configuration consisting of a turbine engine in the upper low-speed flow path (LSFP) and a DMSJ 

combustor in the lower high-speed flow path (HSFP). One of the challenges being addressed in this work 

is the act of transitioning primary thrust production from the LSFP turbine engine to the HSFP DMSJ, 

referred to as an inlet mode transition, while maintaining acceptable stability margin and performance.

To further understand the technology, obtain data to validate simulation models, and demonstrate a 

controlled mode transition, a large scale CCE inlet system has been designed, built, and mounted in the 

NASA Glenn Research Center (GRC) 10- by10-Foot Supersonic Wind Tunnel (10×10 SWT). A diagram of 

the CCE Large-scale Inlet for Mode transition eXperiments (CCE-LIMX) is shown mounted on a strut in 

the 10×10 SWT in Figure 1. The CCE-LIMX inlet system as illustrated in Figure 1 includes both a LSFP 

and a HSFP; the wind tunnel air flows from left to right. The variable geometry features of the CCE-LIMX, 

highlighted in the Figure 1 illustration, include a rotatable splitter, rotatable high-speed cowl, variable ramp, 

and overboard bypass doors. The following are other prominent features of the CCE-LIMX: the pivot pin 

located on the strut to control angle of attack (AoA), the pre-compression forebody plate, and the high-speed 

isolator. The distribution of compressed air coming off of the pre-compression forebody plate into the LSFP 

and HSFP is determined by the position of the splitter. The splitter can be rotated clockwise, as illustrated in 

Figure 1, towards the LSFP to close off, or counterclockwise, as illustrated in Figure1, towards the LSFP for 

full open. At LSFP full open, most of the compressed air off of the pre-compression forebody plate is 

directed into the LSFP. The upper surface of the splitter serves as a cowl for the LSFP, and the lower surface 

serves as a compression ramp for the HSFP. The CCE-LIMX also enables changing the cross sectional area 

of the LSFP throat by raising or lowering the variable ramp, which is significant since the cross sectional 

area of the LSFP is scheduled to decrease as vehicle speed increases. The air flow into the HSFP is also 

regulated by the rotation angle of the high-speed cowl. 

Figure 1.—Diagram of the CCE-LIMX installed in GRC’s 10×10 SWT. This figure illustrates the CCE-LIMX configured 

for inlet characterization and system identification experiments, with the variable geometry features highlighted.
NASA/TM—2012-217714 3

The CCE-LIMX is being evaluated in several phases to develop a unique TBCC database to assess 

and validate design and analysis tools and address operability, integration, and interaction issues for this 

class of advanced propulsion system (Ref. 1). For the first three phases of experiments, cold pipes with 

variable conical mass-flow plugs are mounted at the aerodynamic interface plane (AIP) of the LSFP and 

the HSFP. These cold pipe and plug assemblies will be used to create back pressure in the LSFP and 

HSFP similar to what is expected from turbine and DMSJ engines (Refs. 2, 3, 4, and 5).

The work in this report focuses on the LSFP. The LSFP mass-flow plug can be moved toward cold pipe

close-off to create a subsonic volume that extends from the cold pipe exit up into the LSFP. The interface 

between the supersonic flow and the leading edge of the subsonic volume is known as the terminal shock or 

normal shock. The terminal shock is where the airflow nonlinearly changes from supersonic to subsonic.

Careful placement of the LSFP mass-flow plug can position the terminal shock aft of the minimum cross 

sectional area—this is known as the started condition. Figure 2 shows a diagram of the LSFP in the started 

condition. The minimum cross sectional area, also known as the aerodynamic throat, is displayed in Figure 

2, while the volume downstream of the terminal shock is known as the diffuser. A well-controlled inlet will 

keep the normal shock within a tolerance window about a design point to meet performance specifications 

and stability margins. Inlet performance (air flow quality such as distortion and pressure ratio) is optimized 

with the normal shock located at the aerodynamic throat. However, this is a marginally stable position, 

meaning typical inlet disturbances can move the normal shock upstream of the throat and cause it to be 

ejected from the inlet—this is known as the unstarted condition. The unstarted condition results in a large 

and rapid reduction in air mass-flow to and pressure at the engine-inlet interface, and thus results in a thrust 

loss along with increased drag. Therefore, inlet operating points are specified to include stability margins 

that are set to prevent diffuser noise and potential disturbances from unstarting the inlet.

The purpose of the CCE-LIMX experiments is to develop tools and procedures that can be used to 

investigate and develop mode transition methods for a TBCC inlet system (Ref. 6). The series of 

experiments with the CCE-LIMX are designed to support the development of these tools and procedures 

and demonstrate their effectiveness. A successful mode transition in a TBCC is one that transitions from the 

turbine engine in the LSFP to the DMSJ in the HSFP while maintaining specified terminal shock position 

stability margins. 

Figure 2.—Diagram showing the LSFP in a started condition, defined as the shock located 

aft of the throat.
NASA/TM—2012-217714 4

Potential inlet disturbances can be classified into the following three frequency ranges: high, 

moderately high, and low. A conventional turbine engine flow-path is designed with a diffuser volume 

that is capable of dampening high frequency disturbances, and a passive stability bleed system to reject 

moderately high frequency disturbances. Active control systems to regulate diffuser pressure by opening 

and closing diffuser bypass doors are needed to reject low frequency disturbances (Ref. 7). To aid the 

offline design of the active controllers, computational control design models (CDMs) are required. A

CDM is a simplified version of either the physics-based simulation model or the physical system. The 

early CCE-LIMX test phases are designed to investigate the dynamic response of the diffuser (Ref. 8),

and develop a CDM from the system to validate the CDM determined from physics-based computational 

models. The validated models will then be used for controls design, and the physics-based computational 

models will be used to demonstrate controlled mode transition in simulation prior to testing with the 

CCE-LIMX. The data contained in this paper are simulation results and have not been validated or 

compared against test data obtained from CCE-LIMX.

The Hypersonics propulsion controls team at GRC is developing a suite of tools and procedures, 

including simulation models. The tools and procedures are being designed to streamline experiment 

dynamic data analysis, inlet mode transition controls design, and controls evaluation (Ref. 9 and 10). One 

such tool is the High Mach Transient Engine Combined Cycle (HiTECC)(Ref. 10) code, developed by 

SPIRITECH Inc., which contains tools and models to simulate a complete TBCC propulsion system. The 

work reported on in this paper used the HiTECC simulation. HiTECC (Ref. 11) is a CCE simulation 

package featuring turbine and DMSJ propulsion system models, nozzle models, a heat transfer model, 

fuel flow models, hydraulics models, etc. The HiTECC tools were designed to be generic and modular to 

allow propulsion system changes to consider alternate inlet designs, DMSJs, or other turbine engine 

models. The HiTECC tools consist of the following four building blocks: a propulsion system (Ref. 12), a 

thermal management and fuel system model (Ref. 13), a hydraulics and kinematics model (Ref. 14), and a 

control system. 

This paper focuses on the capabilities and the use of HiTECC as a control design tool for the CCE 

type propulsion systems such as one that may use an inlet system similar to the CCE-LIMX. Section 2.0

of this paper contains more information on HiTECC and modifications to the HiTECC simulation to 

match the CCE-LIMX design geometry. Section 3.0 proposes and shows HiTECC mode transition 

simulation results with simulated turbine and DMSJ engines. Section 4.0 of this paper discusses adding 

cold pipes and mass-flow plugs to HiTECC similar to the CCE-LIMX inlet as mounted in the wind 

tunnel. This paper concludes with a summary of the work.

2.0 HiTECC Modifications for the CCE-LIMX Inlet

The inlet system defined by the HiTECC propulsion system simulation is not representative of the 

actual CCE-LIMX geometry because the detailed mechanical design information was not available at the 

time the simulation was developed. However, since HiTECC was designed as a propulsion system

simulation toolbox that employs the MathWorks, Inc. MATLAB software as a programming platform, the 

geometry, actuators, and control systems can be easily modified. This flexibility made the HiTECC 

simulation software readily modifiable to suitably represent the CCE-LIMX. 

The HiTECC propulsion system includes an inlet geometry that can be defined using SimMechanics, 

a subset of Simscape (MathWorks, Inc.). The SimMechanics software accepts mechanical design 

documents created from a computer aided design (CAD) software package and converts them into a block 

diagram model suitably formatted for Simulink software using the MathWorks, Inc. Link software

(Ref. 15). Installing SimMechanics Link into Pro/ENGINEER enables the Pro/ENGINEER software to 

export an Extensible Markup Language (XML) file based on the CAD drawings. The XML file can then 

be reformatted using the importxml function from the SimMechanics toolbox to conform to Simulink 

software—instructions and details are available from the MathWorks, Inc., website. This process was 

used to define the correct CCE-LIMX geometry for the HiTECC tools to simulate. The XML file can also 

be imported into an existing Simulink simulation.
NASA/TM—2012-217714 5

After entering the new CCE-LIMX geometry into HiTECC, the actuator set points and control 

systems needed to be modified as well. The original actuator set points defined the variable geometry 

alignment of the CCE-LIMX actuators for a flight mission that includes acceleration from low supersonic, 

through inlet mode transition event, and high supersonic Mach numbers. The variable geometry operating 

points were designed to provide the propulsion systems—turbine engine and DMSJ—with the required 

airflow throughout missions that include an inlet mode transition event. The original actuator controllers 

were designed to keep the actuators on point throughout the mission. The actuator control systems for the 

HiTECC simulation were unstable once the new geometry was installed. Therefore, to simulate inlet 

mode transition using the modified HiTECC, new operating points were defined and new actuator 

controllers were designed to keep the inlet system on schedule.

Comparisons of the modified HiTECC inlet geometry to the geometry of the actual CCE-LIMX are 

shown in Figure 3. The three left column traces in Figure 3 are actual hardware to simulation geometry 

comparisons at the Mach numbers of 4.0, 3.1, and 2.5. The three right column traces in Figure 3 illustrate 

the percent error between the actual hardware and simulated inlet geometries. Figure 3 indicates that the 

normalized area percent error between the modified HiTECC inlet model and the CCE-LIMX is within 

±2 percent.

Figure 3.—Plots comparing the HiTECC geometry to the CCE-LIMX geometry at 

Mach numbers 4.0, 3.1, and 2.5. The left column compares the two geometries 

while the right column shows the percent difference in area between the HiTECC 

simulation and the CCE-LIMX hardware. The first row is at Mach 4.0, the second 

row is Mach 3.1, and the third row is at Mach 2.5. The area comparisons are only 

made for the LSFP that starts at the tip of the splitter (117 in.) and ends at the AIP 

(230 in.). 

100 150 200

-30

-20

-10

X Position, in

Y Position, in

Mach 4.0

100 150 200

-2

0

2

X Position, in

Area Error, %

Mach 4.0

100 150 200

-30

-20

-10

X Position, in

Y Position, in

Mach 3.1

100 150 200

-2

0

2

X Position, in

Area Error, %

Mach 3.1

100 150 200

-30

-20

-10

X Position, in

Y Position, in

Mach 2.5

100 150 200

-2

0

2

X Position, in

Area Error, %

Mach 2.5

CCE-LIMX HiTECC
NASA/TM—2012-217714 6

Figure 4.—HiTECC inlet showing the LSFP subsonic diffuser divided into three volumes.

Each subsonic subvolume includes pressure and temperature states. Between each 

subsonic subvolume is a mass-flow rate state. The upstream boundary of the subsonic 

volume is defined by the normal shock position. This figure is not drawn to scale.

The HiTECC simulation includes a LSFP diffuser that is divided into three subsonic volumes, 

denoted Volume 15, Volume 17, and Volume 19, shown in Figure 4. The volume of Volume 15 is 

dependent on both the ramp position and normal shock position, which can change, while the volumes of 

Volume 17 and Volume 19 are defined by fixed geometry and remain constant. Each of these volumes 

includes pressure and temperature states. Furthermore, there are two flow states, W16 and W18. The 

subsonic flow from Volume 15 to Volume 17 is W16, and W18 is the subsonic flow from Volume 17 to 

Volume 19. The two flow states should be equal in steady state but will be different during transients. 

There is an additional state representing the normal shock position. 

The HiTECC simulation is initiated with the simulation control integrators, or states, preset to steadystate operating values. If the error in the initial conditions is too large, the value of the integrator can ramp 

uncontrollably and crash the simulation. The initial conditions pertain to a given operating condition, or 

flight condition. The approach taken to establish the initial conditions for this simulation was to run the 

system from a previously known steady-state operating condition to a new operating condition. To use 

this approach, the original HiTECC geometry and control systems were implemented in parallel with the 

new updated geometry and control systems. The control system modifications included updating the 

actuator setpoints, modifying the inputs to the actuator setpoint generators, and retuning the controllers to 

meet the required bandwidth requirements. The parallel operation enabled each set of controllers to 

receive the same feedback signals. A time dependent switch was used to pass controller commands (u) to 

the inlet—let u1 be signals from the original controllers and u2 be signals from the new controllers. The 

control signal for the inlet can be written as:



















>

≤ ≤ −  +



 



 − −

<

=

, 5

, 5 15

10

( 5)

10

( 5) 1

, 5

2

1 2

1

u t

t

t

u

t

u

u t

u
NASA/TM—2012-217714 7

From t = 0 s when the simulation started to 5 s, the original geometry and control system operated the 

inlet model. From 5 to 15 s, the control commands to the inlet from both the original system and the new 

system were combined to form a transitional command. Finally, the new geometry and control system 

were used exclusively as inputs to the inlet beyond 15 s. This is a slow transition from one design to the 

other, but is capable of determining suitable initial conditions for the simulation by allowing the system 

values to settle at the end of the simulation. With the geometry updated for the CCE-LIMX inlet and 

initial conditions identified, the modified HiTECC simulation can be used to simulate a full mission 

profile that includes a mode transition event.

3.0 Mode Transition

During inlet mode transition, thrust production switches from 100 percent of the total vehicle thrust 

coming from the gas turbine engine, to a combination of thrust from both engines, to finally 100 percent

thrust production from the DMSJ. This event occurs within a Mach number window that includes both 

turbine Mach number range and the DMSJ Mach number range. For this work, the gas turbine engine is 

assumed to be operable up to a speed of Mach 4.0 and the lower end of the DMSJ operability range is 

Mach 3.0, as was assumed in Reference 16, thus defining the inlet mode transition window. For this 

work, mode transition initiates at Mach 3.75, which was chosen to allow for a potential slight decrease in 

vehicle speed without losing operability of the DMSJ. Likewise, a slight increase in vehicle speed is 

allowable while mode transitioning without endangering the turbine engine.

The modified HiTECC software is used for simulating inlet mode transition as a vehicle flies a 

mission that includes accelerating from below Mach 3 to above Mach 4 The mission flight profile

(vehicle altitude and Mach number) for this work is shown in the top plot of Figure 5. As illustrated in 

this plot, the vehicle Mach number and altitude are assumed to remain constant throughout the mode 

transition event. After mode transition, the vehicle will accelerate and increase altitude. The 

corresponding turbine engine and DMSJ power lever angles are shown in the bottom plot of Figure 5.

Once the vehicle reaches Mach 3.75, which occurs at t = 1 s, the afterburner will begin to shut down. The 

afterburner operates from a power lever angle (PLA) of 100 to 150. As the afterburner shuts down, the 

DMSJ increases thrust production. The actual mode transition, defined as the process of closing off flow 

through the LSFP, will begin once the afterburner is shut down at a PLA of 100 percent, and mode 

transition is completed when the fuel command to the turbine is shut down at a PLA of 10 percent.

For the simulated mode transition event, the thrust produced by each engine—the turbine engine and 

DMSJ—is shown in the top plot of Figure 6. The combined total thrust is shown in the bottom plot of 

Figure 6. During mode transition the propulsion system would ideally provide constant thrust to the 

vehicle. The bottom plot includes an assumed upper and lower thrust limit necessary to maintain the 

designated flight profile illustrated in Figure 5. In the top plot of Figure 6, the rapid thrust loss from the 

turbine engine at approximately 7 s is due to the engine turbine shutting down and the simulation lacking 

a windmilling model to accurately represent the transition from idle operation to complete shutdown, 

which would show a more gradual loss in thrust. This also leads to a momentary increase of total thrust 

from 7 to 7.25 s in the bottom plot of Figure 6, due to the rapid change in airflow for the LSFP to the 

HSFP, also due to the lack of a windmilling model. 

Another consideration during the mode transition is the LSFP diffuser pressure and the location of the 

normal shock. The objective is to avoid an inlet unstart. For the simulation runs, the inlet will be 

considered unstarted if the diffuser pressure increases to a level such that the terminal shock reaches the 

aerodynamic throat, which for this inlet is at approximately 160 in. Figure 7 shows the LSFP diffuser 

pressure ratio, which is a ratio of the diffuser total pressure divided by the free-stream total pressure, and 

the location of the normal shock during the mode transition period. During mode transition, the pressure 

ratio is expected to decrease as the turbine spools down. Likewise, the normal shock is expected to move 

upstream. 
NASA/TM—2012-217714 8

Figure 5.—A potential mode transition flight profile (top plot) and corresponding 

PLA commands for the turbine engine and DMSJ as determined using the

modified HiTECC simulation. The mode transition is defined as the event of 

rotating the splitter towards low-speed flow path close off, thus redirecting all 

captured airflow to the DMSJ. The splitter will not rotate until the afterburner is 

turned off.

Figure 6.—Thrust produced by the turbine engine and DMSJ (top plot) and the 

total thrust produced by the TBCC (bottom plot). The rapid loss of the turbine 

thrust at approximately 7 s is due to the simulated turbine engine shutting 

down without a windmilling model representative of a turbine spooling down. 

In the bottom plot, hypothetical upper and lower bounds of thrust to maintain 

the flight profile are plotted in red.

0 2 4 6 8 10 12

0

2

4

6

8

10

12

Time, s

Altitude (10,000 ft)

Mach Number

0 2 4 6 8 10 12

0

50

100

150

Time, s

Power Lever Angle, %

Turbine

DMSJ

0 2 4 6 8 10 12

0

2000

4000

6000

Time, s

Thrust, lbf

Turbine

DMSJ

0 2 4 6 8 10 12

0

2000

4000

6000

Time, s

Total Thrust, lbf

Mode

Transition

Afterburner

On

Upper Bound

Lower Bound
NASA/TM—2012-217714 9

Figure 7.—The LSFP pressure ratio (top plot) and calculated shock position 

(bottom plot) through the mode transition event from 3 to 7 s. At t = 7 s, the 

LSFP is considered closed off and the normal shock does not exist as evident 

by the absence of data in the second plot. In this simulation, supersonic flow is 

free to flow through the LSFP to the bypass gates.

4.0 HiTECC Wind Tunnel Model

The CCE-LIMX as mounted in GRC’s 10×10 SWT consists of the inlet and two cold pipes with

mass-flow plugs in lieu of engines, as shown in Figure 1. The cold pipe and mass-flow plug systems will

be used to create back pressure for the inlet, simulating the effect of the engines. The previously updated 

HiTECC simulation was modified to model the CCE-LIMX as mounted inside the 10×10 SWT. This

model, referred to as the HiTECC wind tunnel model, can serve as a computational test bed for test 

procedures and control algorithms. In addition, the HiTECC wind tunnel model can be tuned to match the 

data obtained from the wind tunnel experiments. This simulation does not include engine specific 

components such as the gas turbine engine, DMSJ, nozzle, fuel/thermal management systems, and 

associated control systems. Added to the HiTECC wind tunnel model is a simulated cold pipe (CP) and 

mass-flow plug system. This feature includes a linear actuator arm for inserting a mass-flow plug into the

cold pipe to reduce the exit area of the cold pipe and increase back pressure in the inlet. A diagram of the 

cold pipe with a mass-flow plug is shown in Figure 8, and the relationship between the linear arm and 

cold pipe exit area is shown in Figure 9. The HiTECC model also is modified to include the additional 

volume of air, which is added to the system by the cold pipe.

0 2 4 6 8 10 12

0

0.2

0.4

0.6

0.8

Time, s

Pressure Ratio

0 2 4 6 8 10 12

182

184

186

188

190

Time, s

Shock Position, in
NASA/TM—2012-217714 10

Figure 8.—Illustration of the cold pipe and mass-flow plug system. As the massflow plug is moved towards close-off, from 0 percent full stroke towards 100

percent full stroke, the exit cross sectional area of the cold pipe decreases.

Figure 9.—Relationship between mass-flow plug position and cold pipe exit cross 

sectional area

Using the HiTECC wind tunnel model, a simulated mode transition event can be conducted at Mach 

3.75. Simulation results comparing the inlet model with just a mass-flow plug added (No CP) and the 

system with the additional air volume of the cold pipe (CP), are shown in Figure 10. Figure 10 illustrates 

that the addition of the cold pipe volume to the simulation decreases the dynamic response of the inlet.

For these simulations, throttle commands are replaced by a mass-flow plug position command. During 

mode transition, as the turbine is throttled down, the decreasing speed of the turbine engine will increase 

the diffuser back pressure. The diffuser back pressure can be similarly increased by moving the mass-flow 

plug in towards cold pipe close off, which simulates the decreasing speed of the turbine engine. At the 

1.5 s point, the splitter is located to match the captured air flow requirements of an engine at idle.

0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

50

60

70

80

90

100

Plug Position, %Full Stroke

Cold Pipe Area, %Cold Pipe Area
NASA/TM—2012-217714 11

Figure 10.—LSFP Mass-flow plug and splitter positioning during simulated mode 

transition using the HiTECC wind tunnel model. The mass-flow plug position and 

splitter commands are shown in the top row. The bottom row illustrates the normal 

shock position and the pressure ratio during the 0.5 to 1.5 s mode transition period.

The propulsion system performance can be improved by operating the inlet with a higher pressure 

ratio, placing the normal shock closer to the aerodynamic throat. However, this reduces stability margin.

The stability margin is based on a probability of an inlet unstart given potential disturbances in the inlet 

flow field. Therefore, to operate the inlet at a higher pressure ratio without decreasing stability margin 

requires a method to also decrease the probability of a potential disturbance that will push the terminal 

shock upstream of the aerodynamic throat at the higher pressure ratio. One proposed method is to employ 

an active control system to regulate the pressure ratio by controlling the opening and closing of bypass 

doors in the diffuser section, thereby controlling the position of the normal shock. Opening the bypass 

doors will decrease pressure ratio and the terminal shock position will move downstream; closing the 

bypass doors will increase pressure ratio and move the terminal shock position upstream and closer to the 

throat. Results of a simulation of the effect of opening the diffuser bypass doors are shown in Figure 11, 

for both the original system and the system with the additional cold pipe volume added. When the bypass 

door opens, the shock position location moves further from the throat, and the pressure ratio decreases.

An active control system can be designed to automatically regulate the bypass doors based on the desired 

and current pressure ratio or current shock position. This simulation model will be used to develop CDMs 

suitable for controls studies and for testing candidate active inlet control systems prior to testing the 

controllers on actual CCE-LIMX hardware. Figure 11 again illustrates that the cold pipe volume slows the 

response of the system.

0 1 2 3

-14

-12

-10

-8

Time, s

Splitter, deg

0 1 2 3

3.2

3.3

3.4

3.5

Time, s

LSFP Mass Flow Plug, in

0 1 2 3

165

170

175

Time, s

Shock Position, in

0 1 2 3

0.2

0.4

0.6

0.8

Time, s

Pressure Ratio

No CP

CP
NASA/TM—2012-217714 12

Figure 11.—The same mode transition schedule as illustrated in Figure 10

except a bypass door is actuated to show a change the diffuser pressure 

ratio and shock position during the mode transition period. For this set of 

simulations, the estimated shock position stays downstream from the 160 in.

station.

5.0 Summary

In this paper, the High Mach Transient Engine Cycle Code (HiTECC) simulation package, created by 

SPIRITECH, Inc., has been modified to create two new models, both representative of the combined 

cycle engine large scale inlet for mode transition experiments (CCE-LIMX). The first model was updated 

to reflect the actual geometry of the CCE-LIMX, which included updating the actuator setpoints and 

controllers, and creating new initial conditions for the simulation. A mission profile demonstrating a 

successful full mode transition was simulated. This model will be used to evaluate the performance 

increase of a propulsion system with an active shock position control system employed, and the effect of 

the mode transition on the thrust produced by the propulsion system. The second model was designed to 

represent the CCE-LIMX as it is tested in the NASA Glenn Research Center Supersonic Wind Tunnel. To 

create the HiTECC wind tunnel model, the propulsion systems were replaced with cold pipes and massflow plugs, and the nozzle, fuel/thermal management systems, and associated control systems were 

removed. This model will be compared to and validated with test data. The validated model will be used 

to develop control design models suitable for controls studies and for testing candidate active inlet 

controllers prior to testing the controllers on actual CCE-LIMX hardware.

References

1. Saunders, J.D., Stueber, T.J., Thomas, S.R., Suder, K.L., Weir, L.J., and Sanders, B.W., “Testing of 

the NASA Hypersonic Project’s Combined Cycle Engine Large Scale Inlet Mode Transition 

Experiment (CCE LIMX),” Joint Army-Navy-NASA-Air force (JANNAF), 32nd APS Joint 

Subcommittee Meeting, Arlington, VA, April 2011.

2. Musial, N.T., “Comparison of the Effect of a Turbojet Engine and Three Cold-Flow Configurations 

on the Stability of a Full-Scale Supersonic Inlet,” NACA RM E56K23, January 1957.

0 0.5 1 1.5 2 2.5

-14

-12

-10

-8

Time, s

Splitter, deg

0 0.5 1 1.5 2 2.5

0

2

4

6

Time, s

Bypass Gate, sq in

0 0.5 1 1.5 2 2.5

165

170

175

180

Time, s

Shock Position, in

0 0.5 1 1.5 2 2.5

0.4

0.5

0.6

0.7

Time, s

Pressure Ratio

No CP

CP
NASA/TM—2012-217714 13

3. Bowditch, D.N. and Wilcox, F.A., “Dynamic Response of a Supersonic Diffuser to Bypass and Spike 

Oscillation,” NASA TM X–10, August 1959.

4. Wasserbauer, J.F., “Dynamic Response of a Mach 2.5 Axisymmetric Inlet with Engine or Cold Pipe 

Supersonic Internal Area Contraction,” NASA TN D–5338, July 1969.

5. Baumbick, R.J., Wallbagen, R.E., Neiner, G.H., and Battertou, P.G., “Dynamic Response of Mach 

2.5 Axisymmetric Inlet with 40 Percent Supersonic Internal Area Contraction,” NASA TM X–2833, 

July 1973.

6. Sanders, B.W., and Weir, L.J., “Aerodynamic Design of a Dual-Flow Mach 7 Hypersonic Inlet 

System for a Turbine-Based Combined-Cycle Hypersonic Propulsion System,” NASA/CR—2008-

215214, June 2008.

7. Sanders, B.W. and Mitche, G.A., “Throat-Bypass Bleed Systems for Increasing the Stable Airflow 

Range of a Mach 2.5 Axisymmetric Inlet with 40-Percent Internal Contraction,” NASA TM X–2779, 

May 1973.

8. Stueber, T.J., et al., “Dynamic Testing of the NASA Hypersonic Project Combined Cycle Engine 

Testbed for Mode Transition Experiments,” NASA/TM—2011-217133, November 2011. 

9. Soloway, D.I., Ouzts, P.J., Wolpert, D.H., Moerder, D.D., and Benivides, J.V., “The Role of 

Guidance, Navigation, and Control in Hypersonic Vehicle Multidisciplinary Design and 

Optimization,” 16th AIAA/DLR/DGLR International Space Planes and Hypersonic Systems and 

Technology Conference, AIAA–2009–7329, 2009.

10. Stueber, T.J., Vrnak, D.R., Le, D.K., and Outzs, P.J., “Control Activity in Support of NASA Turbine 

Based Combined Cycle (TBCC) Research,” Joint Army-Navy-NASA-Air-Force (JANNAF) 

Interagency Propulsion Committee, NASA/TM—2010-216109, La Jolla, CA, December 7–11, 2009.

11. Haid, D.A., and Gamble, E.J., “Integrated Turbine-Based Combined Cycle Dynamic Simulation 

Model,” 58th Joint Army-Navy-NASA-Air-Force (JANNAF) Interagency Propulsion, Arlington, VA, 

April 18–22, 2011.

12. Gamble, E.J., Haid, D., D’Alessandro, S., and DeFrancesco, R., “Dual-Mode Scramjet Performance 

Model for TBCC Simulation,” 45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and 

Exhibit, AIAA–2009–5298, Denver, CO, August 2–5, 2009. 

13. Gamble, E.J. and Haid, D., “Thermal Management and Fuel System Model for TBCC Dynamic 

Simulation,” AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA–2010–6642, 

Nashville, TN, July 25–28, 2010.

14. Gamble, E.J. and Haid, D., “Hydraulic and Kinematic System Model for TBCC Dynamic 

Simulation,” 46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA–2010–

6641, Nashville, TN, July 25–28, 2010.

15. http://www.MathWorks.com/help/toolbox/physmod/smlink/ug/brrdnfv.html, September 28, 2011.

16. Outzs, P.J., “Mode Transition Design Considerations for an Airbreathing Combined-Cycle 

Hypersonic Vehicle,” 15th AIAA International Space Planes and Hypersonic Systems and 

Technologies Conference, AIAA–2008–2621, Dayton, OH, 28 April–1 May 2008.
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A Turbine Based Combined Cycle Engine Inlet Model and Mode Transition Simulation 

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Csank, Jeffrey, T.; Stueber, Thomas, J. 

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WBS 599489.02.07.03.07.11.02 

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14. ABSTRACT

An inlet system is being tested to evaluate methodologies for a turbine based combined cycle propulsion system to perform a controlled inlet 

mode transition. Prior to wind tunnel based hardware testing of controlled mode transitions, simulation models are used to test, debug, and 

validate potential control algorithms. One candidate simulation package for this purpose is the High Mach Transient Engine Cycle Code 

(HiTECC). The HiTECC simulation package models the inlet system, propulsion systems, thermal energy, geometry, nozzle, and fuel

systems. This paper discusses the modification and redesign of the simulation package and control system to represent the NASA large-scale 

inlet model for Combined Cycle Engine mode transition studies, mounted in NASA Glenn’s 10- by 10-Foot Supersonic Wind Tunnel. This 

model will be used for designing and testing candidate control algorithms before implementation. 

15. SUBJECT TERMS

Engine controls; Hypersonics 

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