T.J. Stueber, R. Thomas, D.R. Vrnak, B.V. Nguyen, J.M. Nappier, J.A. Downey, E. Wong, and P. Raitano

Glenn Research Center, Cleveland, Ohio

Dynamic Testing of the NASA Hypersonic 

Project Combined Cycle Engine Testbed 

for Mode Transition Experiments

NASA/TM—2011-217133

November 2011
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T.J. Stueber, R. Thomas, D.R. Vrnak, B.V. Nguyen, J.M. Nappier, J.A. Downey, E. Wong, and P. Raitano

Glenn Research Center, Cleveland, Ohio

Dynamic Testing of the NASA Hypersonic 

Project Combined Cycle Engine Testbed 

for Mode Transition Experiments

NASA/TM—2011-217133

November 2011

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

58th Joint Army-Navy-NASA-Air Force (JANNAF) Propulsion Meeting 

sponsored by the JANNAF Interagency Propulsion Committee

Arlington, Virginia, April 18–22, 2011
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Trade names and trademarks are used in this report for identification 

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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—2011-217133 1 

Dynamic Testing of the NASA Hypersonic Project Combined Cycle 

Engine Testbed for Mode Transition Experiments

T.J. Stueber, R. Thomas, D.R. Vrnak, B.V. Nguyen, 

J.M. Nappier, J.A. Downey, E. Wong, and P. Raitano 

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135 

Abstract

NASA is interested in developing technology that leads to more routine, safe, and affordable access to 

space. Access to space using airbreathing propulsion systems has potential to meet these objectives based 

on airbreathing access to space system studies. To this end, the NASA Fundamental Aeronautics Program 

Hypersonic Project is conducting fundamental research on a turbine based combined cycle (TBCC) 

propulsion system. The TBCC being studied considers a dual flow-path inlet system. One flow-path 

includes variable geometry to regulate airflow to a turbine engine cycle. The turbine cycle provides 

propulsion from take-off to supersonic flight. The second flow-path supports a dual-mode scramjet 

(DMSJ) cycle which would be initiated at supersonic speed to further accelerate the vehicle to hypersonic 

speed. For a TBCC propulsion system to accelerate a vehicle from supersonic to hypersonic speed, a 

critical enabling technology is the ability to safely and effectively transition from the turbine to the 

DMSJ—referred to as inlet mode transition. To experimentally test inlet mode transition methods, a 

combined cycle engine (CCE) large-scale inlet testbed was designed with two flow paths—a low-speed

flow-path (LSFP) sized for a turbine cycle and a high-speed flow-path (HSFP) designed for a DMSJ. This 

testbed system is identified as the CCE large-scale inlet for mode transition experiments (CCE-LIMX).

The test plan for the CCE-LIMX in the NASA Glenn Research Center 10- by 10-ft Supersonic Wind 

Tunnel is segmented into multiple phases. The first phase is a matrix of inlet characterization (IC) tests to 

evaluate the inlet performance and establish the mode transition schedule. The second phase is a matrix of 

dynamic system identification experiments designed to support closed-loop control development at mode 

transition schedule operating points for the CCE-LIMX. The third phase includes a direct demonstration 

of controlled mode transition using a closed loop control system developed with the data obtained from 

the first two phases. Plans for a fourth phase include mode transition experiments with a turbine engine.

This paper, focusing on the first two phases of experiments, presents developed operational and analysis 

tools for streamlined testing and data reduction procedures.

Nomenclature

Acronyms and Abbreviations

AAS airbreathing access to space

A/D analog-to-digital converter 

ARMD Aeronautics Research Mission Directorate

CAD computer aided design

CCE combined cycle engine

CDMs controller design models

COTS commercial off-the-shelf

D/A digital-to-analog converter

DMSJ dual-mode scramjet

FAP Fundamental Aeronautics Program 

GN&C Guidance Navigation and Control 
NASA/TM—2011-217133 2 

GRC NASA Glenn Research Center

GUI graphical user interface

HSFP high-speed flow-path 

IC inlet characterization

IMX inlet for mode transition experiments

LIMX large-scale inlet for mode transition experiments

LSB least significant bit 

LSFP low-speed flow-path 

NASA National Aeronautics and Space Administration

PC personal computer 

RALV reusable airbreathing launch vehicles 

RBCC rocket based combined cycle

rms root-mean-squared

SBC single-board-computer 

S/N signal to noise ratio

SPDT single-pole double throw

10x10 SWT Supersonic Wind Tunnel

SysID system identification

TBCC turbine based combined cycle

TSTO two stage to orbit 

V Volts

Symbols

Amin minimum stopband attenuation 

fr corner frequency of interest (Hz)

fi frequency of interest

fs minimum sampling rate frequency

Mp normalized peak 

p(qe) probability density function

q quantization level 

qe quantization error 

Step T point in time when step increase in signal is applied

T time (sec)

Tp time to first peak (sec)

Ts settling time (sec)

Tsi transient stability index

Vfs full scale voltage level of A/D

Δ small incremental change

σ standard deviation

ξ damping ratio

Introduction

The Hypersonic Project of the NASA Aeronautics Research Mission Directorate (ARMD) 

Fundamental Aeronautics Program (FAP) foundational research is towards technology development to 

enable the capability of future vehicles to operate across a broad range of Mach numbers, including the 

hypersonic flight regime (Ref. 1). One hypersonic application calls for hypersonic research on reusable 

airbreathing launch vehicles (RALV) to enable sustained hypersonic flight through Earth’s atmosphere 

(Ref. 2). This will in turn enable routine, airline-type access to space. The RALV mission focuses on two 

stage to orbit (TSTO) (Ref. 3) systems using airbreathing combined cycle engine (CCE) (Ref. 4) 

propulsion technology. Two prominent airbreathing CCE propulsion schemes are the turbine based 
NASA/TM—2011-217133 3 

combined cycle (TBCC) and the rocket based combined cycle (RBCC) systems (Ref. 5). This work 

supports investigations that consider using a TBCC propulsion system for the first stage of an RALV 

concept TSTO system.

The TBCC propulsion system for the current envisioned vehicle (Ref. 6) combines a turbine (turbojet 

or turbofan) engine flow-path with the flow-path of a dual-mode ramjet and scramjet (DMSJ) combustor. 

A schematic of such a vehicle is illustrated in Figure 1 with the DMSJ flow-path located under the turbine 

flow-path in an over/under configuration. For takeoff, and through the low supersonic flight regime, the 

turbine flow-path provides thrust to accelerate the vehicle and the DMSJ flow-path is cold-flowed (no 

fuel applied to the DMSJ). Continued acceleration through the supersonic regime leads to escalating 

temperatures that limit the operability of the turbine system. Therefore, to continue further acceleration to 

the hypersonic staging point, the DMSJ is started while the gas turbine is shut down and cocooned. This 

requires the propulsion system to transition from the turbine flow-path to the DMSJ flow-path (Ref. 7). 

The transition, diverting intake airflow from one flow-path to the other, is referred to as inlet mode 

transition (Ref. 8). Passing the inlet mode transition window is a critical event for enabling a TSTO 

RALV system to reach the hypersonic staging speed (Ref. 9)

A TBCC TSTO system will employ an inlet system that is conducive to enabling mode transition and 

performs well prior, during and after the inlet mode transition event. The inlet system is designed to 

provide an engine (turbine or DMSJ combustor) with air at specified levels of pressure and velocity. At 

flight speeds above Mach 2, an inlet having a mixture of internal and external compression allows optimal 

propulsion system performance by supplying the engine with airflow at a high pressure level while 

maintaining minimal drag and low distortion at the engine face. To meet optimal performance for this 

type of inlet, the terminal shock must be kept at the inlet throat. However, external airflow transients such 

as atmospheric turbulence and internal airflow changes such as a reduction in engine airflow demand can 

cause the terminal shock to move off of its design location. If the terminal shock moves forward of the 

throat where it is unstable, it will be expelled ahead of the inlet cowling. This shock expulsion, referred to 

as inlet unstart, causes a large rapid reduction in mass flow and pressure to the engine, and thus a large 

thrust loss along with increased drag. The closer the terminal shock is to the throat, the smaller the 

disturbance necessary to unstart the inlet. It is desirable for the inlet to have a sufficiently large stable 

margin to absorb such transients without unstarting. Increasing this margin, operating the inlet supercritical, will lower pressure and increase distortion at the inlet diffuser exit. Since any loss in inlet 

performance is reflected directly as a loss in propulsion-system thrust and efficiency, super-critical 

operation should be avoided. 

An acceptable balance between system stability margin and performance can be provided through 

inlet design and by employing active controls. A conventional turbine engine flow-path is designed with a 

diffuser volume to dampen very high frequency disturbances, a passive stability bleed system to reject 

moderately high frequency disturbances, and an active control system to reject low frequency 

disturbances (Ref. 10).

Two events of concern for a DMSJ flow-path are unstart and blowout. An unstart condition for the 

DMSJ flow-path is one where the leading edge of the shock train is forward of the isolator. This type of 

inlet unstart will greatly reduce the air mass flow to the combustor, thus reducing thrust. Equally 

undesirable is the blowout event where the combustor loses its flame and it must be restarted.

Figure 1.—Conceptual dual, over/under, flow-path propulsion system design integrated into a hypersonic vehicle.
NASA/TM—2011-217133 4 

A concern for a TBCC TSTO propulsion system during inlet mode transition is that a turbine flowpath unstart may unstart DMSJ flow-path or blowout the DMSJ combustor. Therefore, control research 

on this type of system is necessary to maintain adequate turbine flow-path performance prior and during 

mode transition and to minimize the probability of adversely affecting thrust production from the DMSJ 

combustor during and after mode transition. 

To experimentally study the inlet mode transition event, a new hypersonic split-flow inlet was 

designed to provide flow to an over-under propulsion system with a turbine engine and a DMSJ for 

airbreathing propulsion from takeoff to Mach 7 (Ref. 11). A computer aided design (CAD) rendition of 

this inlet is illustrated in Figure 2. This inlet system has the following nine variable actuators: ramp, 

splitter, high-speed cowl, low-speed mass flow plug, high-speed mass flow plug and four overboard 

bypass gates. The low-speed flow-path (LSFP) for a turbine includes the top surface of the splitter as a 

variable cowl, the variable ramp, four bypass gates at the aft end of the diffuser, and a mass flow plug and 

cold pipe assembly to back pressure the LSFP. The high-speed flow-path (HSFP) for the DMSJ includes 

the variable high-speed cowl, the bottom surface of the splitter as a variable ramp, an isolator, and a mass 

flow plug and cold pipe assembly to back pressure the HSFP. The design of the inlet was based on a 

small-scale concept inlet mode transition experiment (IMX) model (Ref. 12). In comparison to the smallscale model, the CCE large scale model (CCE-LIMX) is larger and more complex to address the 

objectives of mode transition experiments (Ref. 13). A near term goal for the CCE-LIMX is to 

demonstrate that smooth inlet mode transition is possible, between the two flow-paths, with a split-flow 

inlet design. To meet this goal, the inlet will be rigorously tested to determine an open loop schedule for 

mode transition that maintains clean airflow to the turbine as it spools down (Ref. 14). These inlet mode 

transition schedules do not address dynamic disturbances in the LSFP that can unstart the inlet. Therefore, 

experiments will be conducted to enable control system studies that will focus on controller designs to 

reject disturbances that can unstart the inlet prior to and during mode transition. The NASA Hypersonic 

Guidance Navigation and Control (GN&C) discipline team will be supporting the testing of this inlet 

system with CCE system mode transition controls design.

To experimentally investigate and demonstrate smooth inlet mode transition with the CCE-LIMX, the 

following four test phases are planned to span a 3-yr period within the NASA Glenn 10- by 10-ft Supersonic 

Wind Tunnel (10x10 SWT) (Ref. 15) environment: Phase I) inlet performance and operability

characterization (IC); Phase II) inlet dynamic system identification (SysID); Phase III) demonstration of 

mode transition control strategies; and Phase IV) mode transition testing and demonstrations incorporating

Figure 2.—A computer automated design rendition of the combined cycle engine large scale inlet for mode transition 

experiments. The facility labels are black, stationary features of the inlet have red labels, variable inlet features 

have blue labels, and the bleed ducting is orange.
NASA/TM—2011-217133 5 

the WJ38 turbine engine and integrated turbine nozzle (Ref. 14). Phase I testing will conclude with a 

demonstration of steady-state scheduled mode transition and the results passed on to the GN&C team 

from Phase I will be the mode transition schedule (Ref. 16). Phase II will provide the GN&C team a 

database suitable for designing control design models for this inlet system (Ref. 16). Phases III and IV

will be opportunities for the GN&C team to test and demonstrate inlet mode transition controller designs.

This paper focuses on activities the GN&C discipline team is engaged in to support the first two phases of 

experiments.

The GN&C team activities have led to the buildup and development of hardware and software tools 

that streamline experimental processes, data acquisition, and documentation. These tools are necessary to 

minimize downtime between tests while the wind tunnel air is flowing, reduce the risk of operator error, 

automate mode transition schedules, and insure that experimental data is appropriately documented. The 

hardware and software tools have been collected and assembled into a single unit identified as the SysID 

Rack. The SysID Rack hardware and software development activities are presented in this paper.

This paper continues with descriptions of the Phase I and Phase II experiments from the perspective 

of the GN&C team with an interest in demonstrating scheduled mode transition and designing disturbance 

rejecting controls. Next, an overview of the hardware and software tools developed to support the GN&C 

activities are presented. Next, software tools developed to reduce the Phase II data are presented. To 

explain sampling rate decisions, an appendix has been attached to the end of this document that describes 

the selection process for the anti-aliasing filters and the sampling rate. 

Discussion

Phase I Experiments

The primary Phase I experiment objective is the determination of a mode transition schedule. A mode 

transition schedule is an equation set used to determine positions for all actuators based on the position of 

a select lead actuator. Any of the nine CCE-LIMX actuators can be selected as the lead actuator, although 

only one at a time. The remaining eight actuators are deemed followers. Therefore, a mode transition 

schedule will consist of a set of eight unique actuator follower equations that express the positions of their 

respective actuators as functions of the lead actuator position. The actuator follower equations can be a 

constant, a linear, or an nth order polynomial function of the lead actuator position. To satisfactorily 

demonstrate inlet mode transition, all follower actuators must move in concert with the lead actuator as 

defined by the inlet mode transition equations. Because of the splitter, high-speed cowl, and ramp ranges 

of motion, a potential for actuator travel paths to intersect exists. Therefore, care must be taken to prevent 

collisions while demonstrating inlet mode transition. 

Phase I test results will reveal distinct positions for each actuator at select positions of a lead actuator.

The collection of these positions represents the inlet mode transition operating points. The inlet mode 

transition equations are then determined with curve fits through the operating points of each follower 

actuator with respect to the position of a lead actuator. Therefore, each inlet mode transition schedule will 

reveal (9 potential lead actuators) * (8 follower actuators) = 72 curve fit equations. The coefficients of 

these curve fit equations will be recorded. Potentially, the Phase I data analysis activity may reveal 

multiple candidate inlet mode transition schedules—multiple sets of coefficients for the 72 equations. 

Once candidate inlet mode transition schedules have been determined, the curve fit process 

completed, and curve fit equation coefficients logged, a scheduled inlet mode transition can be 

demonstrated with the following five steps: 

1. Select a candidate inlet mode transition schedule to be demonstrated. 

2. Select an actuator to serve as the lead actuator. 

3. Select a new position for the lead actuator. 

4. Check movement paths of all actuators for potential collision scenarios. 

5. Instruct the lead actuator to commence movement and the other actuators to synchronously follow the 

movement of the lead actuator. 
NASA/TM—2011-217133 6 

Upon completion of a movement, another movement can be initiated, a new lead actuator can be 

selected, or another logged mode transition schedule can be tested. Multiple movements with the same 

schedule and lead actuator can be conducted by repeatedly performing the third through fifth steps. A new 

lead actuator may be selected by repeating the process starting at the second step. To test another one of 

the logged candidate inlet mode transition schedules, the five steps listed above are repeated. 

For Phase I experiments, the GN&C discipline team has the following three objectives to prepare for 

Phase II experiments and to support the primary Phase I objective: First, to obtain sufficient data to 

determine signal noise levels under experimental conditions. This information will be useful for setting up 

Phase II experiments. Second, identify discrete operating points that can be sequenced to provide a 

piecewise continuous inlet mode transition schedule. Third, to provide the CCE-LIMX research team a 

tool that will implement inlet mode transition schedules designed for the CCE-LIMX hardware in the 

10x10 SWT. To meet the first objective, select sensor signals need to be monitored and recorded with 

hardware that includes analog-to-digital (A/D) converters. All signals applied to the A/D converters are 

measured and saved for post processing data reduction with noise analysis software. Objectives two and 

three will lead to inlet mode transition demonstrations, which is the primary objective for this program.

The second objective identifies the discrete operating points to which SysID experiments will be 

conducted in Phase II. The third objective requires the capability of the SysID Rack to select and perform

the five steps for demonstrating the scheduled inlet mode transition. To add flexibility to the experiment 

process, the SysID Rack also included the following four user friendly capabilities: readily make changes 

to the inlet mode transition schedules, easily switch to and execute a different inlet mode transition

schedule, easily select or change a lead actuator, and graphically illustrate simulated actuator movements 

as a check for potential collision scenarios. Providing a resource to meet the requirements for this third 

objective is a challenging GN&C team deliverable. Hardware and software development activities to meet 

GN&C Phase I objectives are included below. 

Phase II Experiments

For Phase II experiments, the GN&C discipline team has the following three objectives: First, is to 

identify the inlet disturbance frequency range that is not protected by either the diffuser volume or the 

passive stability bleed system. Second, is to identify high-speed feedback pressure sensor locations that 

will be suitable for supporting an active disturbance rejection controller. Third, is to create empirical 

controller design models (CDMs) suitable for supporting control research.

The inlet mode transition schedules, revealed from Phase I IC experiments, are collections of steadystate operating points. These operating points are configurations, which are experimentally proven to be 

safe and acceptable, where the inlet can slowly transition to and hold. A safe and acceptable operating 

point is a selected configuration where the inlet is started, has acceptable performance, and has acceptably 

clean airflow at the turbine aerodynamic interface plane.

The Phase II experiments are designed to apply stimulating signals and acquire data for populating a 

database suitable for developing dynamic CDMs (Ref. 16). The following four types of stimulating 

signals will be used (Ref. 16): step, single sine pulse, staircase, and sinusoidal sweep. Step response data 

will determine signal-to-noise ratio (S/N) and the data will be used to select CDMs. Single sine pulse data

will reveal the disturbance frequency range that needs to be addressed by an active control (Ref. 10).

Staircase response data will be used to check for hysteresis. Sinusoidal sweep data will be reduced to 

CDMs (Ref. 17). The CDMs will then be used to aid designing an active disturbance rejection control 

system using bypass gate valve actuators and high-speed pressure sensor feedback. The control actuators 

will be four bypass gate valves positioned 90° apart at the aft end of the diffuser. A select group of highspeed pressure sensors will provide LSFP diffuser feedback to the control system. The Phase II 

experiments will primarily consist of SysID tests where various perturbation signals will be applied to the 

CCE-LIMX bypass gate valve actuators. These experiments will be conducted while operating the inlet at 

the select inlet mode transition operating points identified from the Phase I tests. Another outcome of the 
NASA/TM—2011-217133 7 

Phase II experiments will be the identification of the optimal high-speed pressure sensor locations that 

will provide feedback to the controller. 

The SysID Rack requirements, for performing the Phase II experiments, include the capability to do 

the following: 

• Position the CCE-LIMX hardware to any of the operating positions identified from the Phase I IC 

tests.

• Apply step, staircase, sine-pulse, and sine-sweep signals to the bypass gates.

• Sense and log signals from high-speed pressure sensors.

• Sense and log actuator control and position feedback signals.

Providing a resource to meet the challenging Phase II experiment objectives are a focus for the hardware 

and software activities described below.

SysID Rack Overview

The SysID Rack hardware and software is designed to be a tool that performs the procedures required 

for demonstrating scheduled mode transitions in Phase I and conducting SysID experiments with the 

CCE-LIMX in Phase II. To use this tool to perform these experiments entails hardware buildup and 

software development. The hardware applies and senses signals on digital to analog (D/A) and A/D 

channels that interface the SysID Rack with CCE-LIMX actuator, actuator feedback signals, and highspeed pressure sensors. The software controls the signals applied to the D/A channels based on operator 

input and feedback signals. The software also supports experiment documentation. 

The SysID Rack controls and positions the CCE-LIMX actuators by applying signals to setpoint input 

ports of proportional controllers for hydraulic cylinders. The setpoint signals represent desired hydraulic 

cylinder positions. The signals applied to the proportional controller feedback nodes, representing a 

cylinder piston position, corresponds to the current CCE-LIMX actuator position. Therefore, 

repositioning the CCE-LIMX actuators involves adjusting the setpoint values applied to the cylinder’s 

controller units with the SysID Rack D/A channels.

Precautions were taken to prepare for the event of a SysID Rack system lockup or failure. The 

concern is that if this event happens, errant commands from the SysID Rack may be transmitted to the 

CCE-LIMX actuator controllers that will reposition the actuators to an unsafe configuration while the 

wind tunnel air is flowing. To relieve this risk scenario, a Dspace (dSPACE GmbH) system is employed 

to receive and pass the nine control signals from the SysID Rack to the actuator controllers. In addition to 

these nine signals, a tenth “status” analog signal from the SysID Rack that is programmed to maintain 5 V 

is also monitored by the Dspace system. In the event of a SysID Rack lockup or failure, the status signal 

will drop to 0 V. Upon detection of the step change from 5 to 0 V on the status signal, the Dspace system 

will respond by maintaining previous output signals on its D/A channels. This course of events will cease 

movement of the CCE-LIMX actuators, as opposed to leading to potential unsafe step changes in the inlet 

geometry.

SysID Rack Hardware

The SysID Rack includes the following components: real-time operating system Target Computer, 

A/D boards, D/A boards, user interface Host Laptop, Target Computer to Host Laptop communication, 

and a real-time Dspace system. Not included within the SysID Rack is also an 8th order anti-aliasing 

Bessel function filter. Determining the Bessel function parameters and the sampling rate used for these 

experiments is reviewed in the Appendix. 

The Target Computer periodically, 2.5 kHz, applies signals to the D/A channels based on instructions 

received from the Host Laptop and reads signals from the A/D channels. To meet the Target Computer 

requirements, a rack-mount computer was procured. The Target Computer is a rugged 4U personal 
NASA/TM—2011-217133 8 

computer (PC) compatible enclosure that includes the following: An Intel Core (Intel Corporation) dualcore Xeon (Intel Corporation) Processor based single-board-computer (SBC); an A/D card with 64 

channels, 16-bit, 500k samples/Sec; and two D/A cards with 8 channels each, 16-bit, 750K samples/sec.

The Target Computer employs an xPC TargetBox (The MathWorks, Inc.) configuration to run the 

MATLAB (The MathWorks, Inc.) real-time xPC Target (The MathWorks, Inc.) operating system. The 

xPC Target system enables code written with MATLAB and Simulink (The MathWorks, Inc.) software to 

run as executable applications on the Target Computer. The A/D board is wired to receive signals from 20 

high-speed pressure sensors, nine actuator position feedback signals, nine actuator output control signals 

from the Dspace system, and nine actuator output control signals applied to the D/A board. The D/A 

board channels apply nine actuator control signals to the Dspace system. Additionally, the signal from a 

tenth D/A channel is transmitted to the Dspace system as a SysID Rack status line. The Dspace system 

employed for these experiments can receive and transmit signals within either the ± 5 or ± 10 V range.

The Host Laptop is used to develop a Target Model that can be downloaded to the real-time xPC 

Target system, develop a Host Model that can serve as a user interface to the Target Model, and serve as a 

user interface point for an operator to set control parameters and receive feedback. To serve these 

purposes, a Host Laptop computer system was procured. The Host Laptop is a dual-core Intel Xeon 

Processor, 2.33 GHz system that runs the Windows Vista Ultimate (Microsoft Corporation) operating 

system. Other commercial off-the-shelf (COTS) software loaded onto the Host Laptop to support this 

project include: Microsoft Excel (Microsoft Corporation), MATLAB, Simulink, Real-Time Workshop 

(The MathWorks, Inc.), xPC Target, and a C compiler. The Host Laptop communicates with the Target 

Computer via an Ethernet crossover cable.

The Host Model and Target Model are described in the following section.

SysID Rack Software 

The SysID Rack software was designed to enhance research capabilities by automating the 

experiment process without constricting research flexibility. Automating the experiment process 

streamlines experiment procedures and data acquisition while minimizing the probability of operator 

error. The SysID Rack software operating in conjunction with the SysID Rack hardware, functions as a 

tool that supports the performance of select CCE-LIMX experiments by providing the operator with the 

capability to perform the following four tasks: 

1. Configure the experiments witha. CCE-LIMX geometry specifications.

b. Experiment type.

c. Experiment parameters.

2. Start or stop experiments—automated or manual.

3. Perform general housekeeping functions, such asa. Guiding the operator through task sequences. 

b. Checking experiment parameter information for proper entry. 

c. Providing operator warnings for potential hardware crash scenarios.

d. Delivering status feedback for the CCE-LIMX.

4. Perform data transfer and save operations. 

The SysID Rack software graphical user interfaces (GUIs) were designed to facilitate operator 

communication with the Target Model. The capabilities of the GUIs are as follows: 

• Receive operator instructions to reconfigure an experiment.

• Initiate data acquisition.

• Start an experiment. 

• Stop an experiment.
NASA/TM—2011-217133 9 

• Stop data acquisition.

• Coordinate data transfer.

• Display general housekeeping status.

Code design and development employs the following convention: 

• The operator enters and reads data in engineering units. 

• D/A boards accept signals in units of volts.

• A/D boards read signals in volt units.

• Database includes all data saved in the same units that they enter or leave the SysID Rack-

○ Setup variables have engineering units.

○ Input and output variables have voltage units. 

Therefore, calibration coefficients are needed to translate between the engineering units and voltage

values. The calibration coefficients define polynomials for translating engineering units to voltage or 

voltage units to engineering units. The coefficients for these calibration polynomials are defined pretest 

by exercising the CCE-LIMX hardware, recording comparisons between measured actuator positions 

against D/A setpoint voltages (engineering units to volts curve), and recording measured actuator 

positions against A/D feedback measured voltages (volts to engineering units curve). The product of the 

calibration exercises are setpoint and feedback calibration coefficients for each actuator. Similarly, 

calibration coefficients are also saved for translating the high-speed feedback pressure sensor signal 

millivolt units to pressure. The calibrations coefficients are conveniently saved in a customized document 

that is populated using Microsoft Excel spreadsheet software.

Customized SysID Rack software code has been composed, to work with the SysID Rack Hardware, 

to meet the Phase I and Phase II objectives. Further software development will be needed to meet Phase 

III and IV objectives. Code for SysID Rack software is grouped into two pairs of Host Models and Target 

Models. One model pair is designed to support the Phase I scheduled inlet mode transition demonstration. 

The other pair is designed to support the Phase II SysID dynamic experiments. Both pairs include code to 

enable sampling and recording all signals applied to the A/D boards in real time within each sampling 

period. The recorded data becomes the database for identifying CDMs. Both pairs also employ GUIs to 

guide the operator through multi-step procedures by employing color changing and button enabling or 

disabling graphic enhancements.

The Host Models are programmed using MATLAB Simulink software and run within a MATLAB 

environment on the Host Laptop. These models are designed to run on a PC with the Microsoft Vista 

operating system to perform the following tasks: 

• Receive experiment setup and control information via GUIs from an operator. 

• Transmit experimental configuration and control information to the Target Model. 

• Translate user input values to the GUI from engineering units to Target Model required voltage 

values. 

• Translate all Target Model feedback voltage values to engineering units.

• Periodically display signals from the Target Model. 

• Check for potential hardware collisions. 

• Streamline testing procedures. 

The Target Models are also programmed using MATLAB Simulink software; however, these models 

are uploaded to the Target Computer from the Host Laptop. These Target Models are designed to run in 

real-time on the xPC Target computer and continuously perform the following tasks: 

• Read experiment configuration information from the Host. 
NASA/TM—2011-217133 10

• Calculate values to be applied to D/A channels. 

• Update all D/A channels. 

• Read and log all A/D channels. 

• Download information to the Host. 

Target Model experiment configuration information received from the Host includes calibration 

information, start or stop logging feedback A/D signals flag, and initiate or stop an experiment sequence

flag. Since the Target Model is running on the Target Computer, all acquired data must be transferred 

from the Target Computer to the Host Laptop for post processing. Therefore, a flag to initiate this data 

transfer is included in the experiment configuration information. 

The subsequent sections will present details about the Target Model software and the Host Model 

software, respectively, that will support both Phase I and Phase II experiments. 

SysID Rack Target Model Software

The Target Model code to support Phase I scheduled mode transition demonstrations and Phase II 

SysID experiments are very similar. Both Target Models receive instruction from the Host Model 

pertaining to D/A signal generation. Both also sense and log all A/D channels using MATLAB software 

filescopes. The filescopes are data storage devices with memory allocated in the xPC Target when the 

model is compiled. During data saving operations, if incoming information exceeds allocated memory, 

then the oldest data gets written over. Both Target Models receive instruction words from the Host Model 

indicating that it is time to apply new signals to the D/A channels, start or stop saving A/D channels, and 

to transfer saved data logged on the Target Computer to the Host Laptop memory. The significant 

difference between the two Target Models is the amount of information that each receives from the Host 

Model. For Phase I mode transition demonstrations, the Target Model receives the next setpoint values to 

be applied to the hydraulic cylinder controllers. Whereas, the Phase II Target Model receives an 

instruction packet that includes information for applying step, staircase, sine-pulse, or sine-sweep signal 

traces to the D/A channels. The reason for having two separate Target Models is to insure real-time 

capability by minimizing the size of the Target Model length of code—Phase I does not need SysID

signal generation capability and Phase II does not need scheduled mode transition demonstration 

capability.

The Target Model for Phase I will receive a signal from the Host Model to start saving A/D channel 

signals into filescopes. Next, the Target Model will receive a new set of nine signals to apply to the D/A 

channels. Upon receipt of an instruction to update all D/A channels, new hydraulic cylinder controller 

setpoint signals will be applied to the D/A channels. These setpoint signals are applied with a rate limiting 

ramp movement profile. The actuator feedback signals are monitored and periodically transmitted to the 

Host Model for display on the GUI. Upon receipt of an instruction to stop recording A/D signals, the 

filescopes will stop accepting new data. Finally, the Target Model will transfer all filescope data to the 

Host Model upon receipt of the instruction to do so. 

The Target Model for Phase II will receive an experiment information packet from the Host model 

that describes the following parameters for a specific SysID experiment: actuator(s) selection, signal type, 

amplitude, frequency, and number of cycles. Next, the Target Model will receive a signal from the Host 

Model to start saving A/D channel signals into the filescopes. Upon receipt of an instruction to initiate an 

experiment, the Target Model will apply scheduled signals to the D/A channels based on the experiment 

information packet. After all scheduled signals for an experiment have been applied; the last signal is 

maintained on the D/A channel. An interrupt signal from the Host Model will cut short the predetermined 

schedule of signals for the SysID experiments. Upon receipt of an instruction to stop recording A/D 

signals, the filescopes will stop accepting new data. Finally, the Target Model will transfer all filescope 

data to the Host Model upon receipt of the instruction to do so. 

Besides the four fundamental SysID experiment types for Phase II, the experiment information packet 

may contain instructions to move all or some of the hardware actuators, that is, to reconfigure the CCE-
NASA/TM—2011-217133 11

LIMX. The filescopes can be employed while the CCE-LIMX is reconfigured by having the Target 

Model start them prior to having the Host transmit the instruction to initiate an experiment. For this last 

experiment instruction case, the D/A channels are rate limited to maintain a ramp movement type of 

profile. 

SysID Rack Host Model Software 

The Host Model for Phase I, including its accompanying GUI, was developed specifically to support 

mode transition schedule following experiments. Similarly, the Host Model for Phase II, including its 

accompanying GUI, was developed to support the SysID experiments. In order for either Host Model to 

satisfactorily perform their respective tasks, they must be initialized with calibration coefficients, actuator 

range-of-motion limits, and maximum acceptable ramp rates-of-motion for each actuator while 

reconfiguring the inlet geometry. This information is saved in a document, which is populated using 

Microsoft Excel software, and automatically accessed upon initial startup of the Host Models and their 

respective GUIs.

The custom designed Mode Transition GUI, illustrated in Figure 3, allows an operator to interact with 

the Phase I Host Model, which in turn provides the ability to configure and direct the operation of the 

associated Target Model. In this manner, Phase I Mode Transition tests can be fully managed and 

controlled. By operating the GUI’s input fields, selection elements and control buttons, an operator can 

select a mode transition schedule, select a lead actuator, specify a new position to which the lead actuator 

will move, and initiate moving all actuators in concert with the lead actuator. Upon initial startup of the 

Phase I Host Model, the configuration information is read and the state of the Mode Transition GUI is as 

illustrated in Figure 3. The elements of the GUI are grouped according to function. The left half of the 

GUI window is comprised of groups of input and selection components and the right half is dedicated to 

groups of data monitoring and data collecting components. The typical sequence of operation for the 

Mode Transition GUI is as follows: 

1. Use the file selection tool in the “Actuator Transition Profiles” group to select a desired mode 

transition schedule. 

2. Identify the lead actuator by manipulating the radio buttons within the “Select Lead Actuator” group. 

3. Check for hardware collision scenarios.

4. Move the selected lead actuator to a new position using the fields within the “Set Desired Setpoint for 

Actuator” group. 

Since multiple schedules may be available, a specific schedule is selected by identifying a document, 

by filename, which includes the desired mode transition polynomial coefficients. The Mode Transition 

GUI will not allow the operator to further operate this GUI until a specific mode transition schedule has 

been selected. The selected filename is illustrated in the text display field within this group box. Since 

only one actuator can be selected to lead, the Mode Transition GUI will only allow one radio button to be 

selected at a time. Because of the CCE-LIMX mechanical design, it is possible to schedule actuator 

crashes. To minimize this probability, the Mode Transition GUI provides the availability of the Visual 

Tool component that displays a graphic schematic representation of the CCE-LIMX. As the operator 

enters a new value for the lead actuator position, the Visual Tool will dynamically calculate and illustrate 

the final position of all actuators upon application of a newly chosen lead actuator controller setpoint.

This graphic makes available the opportunity for visual inspection of the final hardware configuration and 

to reveal potential hardware crashes. If no hardware crashes are revealed and the new configuration is 

desired, pressing the “Update Setpoint” button will instruct the Target computer to start applying new 

setpoint signals to the controllers. As these new setpoint signals are applied, their values can be monitored

in the “Setpoint Values Out” column of the “Actuator Monitoring Display” group. This column consists 
NASA/TM—2011-217133 12

Figure 3.—Mode transition GUI as presented to operator upon initiation of the Phase I Host Model. 

of continuously updating numeric display fields—one field for each actuator. Alongside this column is a 

similar column of continuously updating numeric fields labeled “Feedback Values In” that display the 

monitored values of the feedback signals from each actuator. A third corresponding column labeled 

“Tolerance (Adjustable)” consists of numeric input fields that allow the operator to view and set the 

allowable difference between each actuator’s setpoint signal and the corresponding feedback signal. In the 

event that a tolerance is exceeded, the associated field will warn the operator of this condition by 

changing to a red display color. Default tolerance values are loaded during initialization. However, these 

default tolerances can be individually adjusted for any actuator by entering new values into the 

corresponding input field. The Mode Transition GUI was designed to safely lead the operator through the 

sequence of steps involved in managing a mode transition by displaying the mode transition filename, 

activating and deactivating fields as appropriate, posting lead actuator travel limits, and illustrating the 

final position for all actuators after selecting the “Update Setpoint” button. 

Another GUI was developed to support the Phase II SysID experiments. This GUI is the CCE_GUI 

illustrated in Figure 4 and is part of the Phase II Host Model. The fields on the CCE_GUI are used to help 

the operator perform the following tasks with the SysID Rack:

• Reconfigure the CCE-LIMX geometry. 

• Prescribe and initiate SysID experiments. 

• Start and stop data acquisition. 

• Electronically document an experiment. 
NASA/TM—2011-217133 13

Figure 4.—SysID experiment GUI as presented to operator upon initiation of the Phase II Host Model.

The CCE_GUI receives new desired CCE-LIMX geometries by reading values entered into the 

“Default Position” group fields. The CCE-LIMX can be reconfigured, as described by the “Default 

Position” values, by selecting the “Move all to default pos” button. This button initiates the process that 

applies a stream of setpoint values (V) to the CCE-LIMX actuator hydraulic cylinder controllers that will 

promote a ramping movement of all actuators to a configuration as described by the “Default Position” 

fields. 

All SysID experiments involve one of the following types of perturbation tests: step, staircase, sine 

pulse, or sine sweep. The check boxes and numeric entry fields within the SysID experiment groups 

identify the perturbation type and the actuator(s) that will be perturbed. The step size and amplitude fields 

define the perturbation amplitudes, in either degrees or inches, with respect to the default positions. The 

time fields have units of seconds and the frequency fields have units of Hertz. When conducting a step, 

staircase, sine pulse, or sine sweep test, the signals applied to the actuator controller setpoints are not rate 

limited—a step test will apply a step change in setpoint value. 

The buttons and display fields within the “Spreadsheet Files” group are for conveniently identifying 

documents that are suitably formatted for reading and editing with Microsoft Excel software. The 

document identified using the “Rack Test Cond” button includes information pertaining to the experiment 

environment. This document will be manually populated at test time using Microsoft Excel software and 

includes much information that will be applied to the documentation file. The document identified using 

the “Test data out file” button will be populated with information that electronically documents the 

experiments. When viewing the contents of the test documentation file using Microsoft Excel software, 

each experiment will be displayed in cells corresponding to a unique row number—the unique row 

number is identified under the “next row to write” label. Each column in the documentation file will have 

information pertaining to a specific test parameter. 

The five buttons located in the lower middle of the CCE_GUI are used to start or stop data 

acquisition, initiate an experiment, document an experiment, or clear the CCE_GUI. This GUI also 

incorporates an internal algorithm to check for potential hardware crashes. Finally, the fields within the 

“Test Input File” group are for reading documents that contain information for populating the CCE_GUI. 
NASA/TM—2011-217133 14

Each test phase will span multiple evenings (runs)—up to five runs per week. For each test run, Phase 

I or Phase II, the CCE-LIMX will be configured to minimize drag while the 10x10 SWT is brought to a 

desired free-stream operating condition using facility control. After the wind tunnel has settled and all 

instruments indicate flow within and around the inlet have stabilized to a steady-state condition, control of 

the CCE-LIMX geometry can be transferred from 10x10 SWT facility to the SysID Rack. Up to this 

point, the signals being applied to the actuator hydraulic cylinder controller setpoint nodes are coming 

from the SWT facility control panel. Until this path is redirected from the SWT facility control panel to 

the SysID Rack, through single-pole double throw (SPDT) relay contacts, all signals from the SysID Rack 

D/A channels are open and will not affect the CCE-LIMX hardware. Transferring control to the SysID 

Rack will occur while the air is flowing supersonic in the wind tunnel; therefore, care must be taken to be 

sure the outgoing setpoint values from the SysID Rack match the current setpoint values applied to the 

actuator controllers. A mismatch will result in a geometry step change at the onset of control transfer. To 

minimize this mismatch risk, feedback signals that reveal current actuator positions will be manually 

compared with SysID rack D/A signals—both sets of signals are electronically translated and displayed in 

engineering units for ease of operator inspection. Furthermore, an indicator will change from green (safe 

to transfer control) to red (unsafe to transfer control) if a mismatch between the values are beyond a safe 

range. Therefore, upon settling at a tunnel operating condition and all indicators are green, hydraulic 

actuator controller setpoint signal control can be transferred from the 10x10 SWT facility controllers to 

the SysID Rack.

10x10 SWT Test Plans

For the Phase I testing, the SysID Rack will periodically be used to sample steady-state signal noise

levels on all A/D channels and to log dynamic sensor data during CCE-LIMX geometry transitions. 

While performing these tasks, the SPDT relay will be configured so that only the facility controllers can

apply signals to the actuator hydraulic cylinder controller setpoint nodes—all SysID Rack D/A channels 

are open. Therefore, these tasks are started and stopped independent of and without interruption to the 

Phase I testing schedule. 

The SysID Rack will also be available to facilitate demonstrating mode transition schedules. When a 

mode transition schedule demonstration is desired, control authority to the CCE-LIMX hardware can be 

transferred from the 10x10 SWT facility controllers to the SysID Rack following set procedures. After the 

SysID Rack assumes control, the Mode Transition GUI will streamline the process for a safe 

demonstration. In the event that a mode transition schedule puts the CCE-LIMX geometry into an 

undesirable configuration, such as one that promotes violent shaking of the hardware, control authority 

can quickly be returned to the facility controller with the SPDT relay. The results, due to a certain step 

change in applied setpoint signals, from this corrective action are acceptable. 

Another possible recourse is to instruct the CCE-LIMX actuators to reconfigure to a Safe 

configuration. This action is instigated by issuing a 10x10 SWT facility command, termed “Safe the 

inlet,” that will switch the controller setpoint signal sources to a set of preset voltage signals. The Safe 

signals are preset and hardwired to a switch that will result in all actuators moving to a configuration that 

was previously tested and deemed safe. Therefore, signal flow priority to the actuator controller setpoints 

by facility permission is: Safe hardwire, facility controller, SysID Rack.

For the Phase II testing, up to three candidate mode transition schedules will be identified for SysID

study. One schedule will be determined to be the Main and will be subject to the most attention. The other 

schedules will be alternates and the SysID experiments will not focus on these as much as the Main. The 

following SysID experiments are planned for and about each inlet operating point identified along the 

mode transition schedules: step, staircase, sine sweep, and transient stability index (Tsi). 

The CCE-LIMX in the 10x10 SWT facility will have the following configuration for each of the 

SysID experiments. For the LSFP, bypass gates located at the aft end of the diffuser will be used to create 

small pressure perturbations in the diffuser. For the HSFP, the cold-pipe mass flow plug will be used to 

create small pressure perturbations in the isolator. For each candidate mode transition schedule, seven 
NASA/TM—2011-217133 15

splitter positions will be chosen that nonlinearly span the range from operating point to LSFP cut-off. An 

example set of splitter positions would be to define an operating point such that a 10° rotation of the 

splitter will close off the LSFP, then the seven splitter positions, that are offsets from the operating point, 

would be 0°, 1°, 2°, 4°, 6°, 8°, and 10°. Each splitter position on the Main mode transition schedule will 

be subject to five SysID experiments. Experiments will be conducted at each splitter position with the

ramp on the operating point, the ramp raised a small Δ° off the operating point, and the ramp lowered a 

small Δ° off the operating point. Furthermore, with the ramp located on a Main operating point, SysID

experiments will be conducted with the splitter rotated open and closed by a small angle off of the 

operating point. For the HSFP, SysID experiments will be conducted with the splitter on a Main point and 

the high-speed cowl located on a Main point, rotated slightly open, and rotated slightly closed. Therefore, 

each splitter position on the Main schedule will be subject to eight SysID experiments. 

For the alternate mode transition schedule candidates, SysID experiments will only be conducted on 

the operating points. Therefore, each splitter position on each alternate schedule will be subject to only

two SysID experiments. To conservatively reduce the number of experiments to be conducted, the time 

consuming measurements to obtain Tsi data will be limited to only the LSFP and four of the seven splitter 

positions. 

Data Reduction Tools 

Experiment parameter settings, stimulating signal amplitudes and durations, will be determined based 

on the CCE-LIMX system performance during testing. The system performance will be analyzed first 

with the step test experiment. The step test will reveal information to define the system S/N and settling 

time. This information will be used to set the step size and hold times for the staircase test. To minimize 

errors while reducing the test data and to streamline the process so as to maximize useful test time, 

several custom data reduction tools have been developed. They are designed for rapid data reduction to 

move forward with the experiments. The following data reduction tools were developed using MATLAB 

software to support the SysID experiments: Steady-State Noise Analysis, Step Signal Analysis, Staircase 

Signal Analysis, and Sin Sweep Analysis. Each of the tools are GUI applications developed to streamline 

the following data analysis procedures: documentation file selection, experiment data selection, perform 

various data analysis routines, and plot analysis results.

The Steady-State Noise Analysis tool is a GUI driven software tool. The GUI for this tool, 

SS_NoiseGUI, is illustrated in Figure 5 as it appears when initially called. The following are the four 

general steps for using the SS_NoiseGUI: 

1. Select a documentation file. 

2. Select an experiment.

3. Define the range of interest.

4. Plot the results. 

The documentation file is a custom formatted file that is populated whenever the experimenter selects the 

“Save Data To Host” button on the SysID experiment control Host Model GUI (CCE_GUI), see Figure 4. 

This documentation file includes calibration coefficients for all actuator feedback signals, calibration 

coefficients for all pressure tap sensors, calibration coefficients for all actuator setpoint signals, free 

stream conditions at time of experiment, test type, test parameter settings, bleed settings, and a few static 

pressure tap measurements. Each documentation file can hold data that documents multiple experiments. 

A wind tunnel test run, that spans several hours, can all be documented within the same documentation 

file. For ease of identifying a desired documentation file, the naming convention used includes the test run 

date. 
NASA/TM—2011-217133 16

Figure 5.—Steady-state noise analysis GUI.

After selecting the documentation file, the number in the field immediately to the right of the “Excel 

doc Files” button will indicate the number of experiments documented in that file. The next field 

immediately to the right will be highlighted in yellow upon successful selection of a documentation file, 

and allows the operator to choose which experiment will be analyzed with the tool. The “Open File 

button” will then become available to read the data files (saved in voltage units) that were saved from the

Target Model file scopes and translate the data to engineering units using the calibration coefficients 

retrieved from the documentation file. The time to perform this last step is dependent on the size of the 

data files. 

After opening the data files, figures can be created to illustrate the saved data with respect to time.

Curve traces for each signal sampled and saved with the SysID Rack can be plotted in these figures. To 

open figures for all signals, select the “Plot Data” push button. The figure placed on top will correspond 

with the signal name identified immediately to the right of the “U” and “D” buttons. However, to select a 

specific curve trace for plotting, first use the “U” (Up) and “D” (Down) buttons to scroll through the list 

of signal names. Then place a check mark in the square field above the signal name, next to the label 

“Plot This”, to have that data illustrated in a figure. Using the “U” and “D” buttons along with the “Plot 

This” check box, multiple figures can be created illustrating select curve traces with respect to time.

The numerical input fields with the blue highlight background require operator input to clarify data 

analysis boundary conditions. This GUI has a low time and a high time field that defines the evaluation 

period. These fields need information from the operator to properly assess the data—by default, the 

evaluation considers all data. Finally, the fields to the left of the Max, Min, and Mag labels are software

populated with data that pertains to the period of the identified data set—Ramp for the example illustrated 

in Figure 5. The Mag value is the difference between the Max and Min. 

The Step Signal Analysis tool is also a GUI driven software tool. The GUI for this tool, 

Step_NoiseGUI, is illustrated in Figure 6 as it appears when initially called. Working with this GUI, the 

initial steps to recall saved experiment data is similar to the steps identified above for the SS_NoiseGUI.

Operator attention is required to complete the following fields highlighted in blue: Data analysis period

(Period), step time (Step T), settling time (Ts), and time to first peak (Tp). Each of these values have units 

of seconds. The four blue fields below the Period label identify the data analysis periods. The “Low” row 

defines a period before the step point and the “High” row defines a period after the step point. The Step 

time value should coincide with the time the step signal was applied to the actuator. The Settling Time 

value pertains to when the signal has settled after the step. Finally, the value for Time to first peak is the 

time point of maximum signal immediately following the step. The Step time, the Settling Time and Time 

to first peak can be determined through inspection of the signal traces in the figures. With these userprovided information, the software will reduce the data to populate the remaining fields: delT, Peak, fr, 
NASA/TM—2011-217133 17

Figure 6.—Step signal data reduction GUI.

S/N, and ave S/N. The value applied to the “delT” field is simply, 2x the difference between the settling 

time (Ts) and the step time (Step T). This span will be applied to the staircase experiments. The value 

applied to the “Peak” field is the signal value at the Tp mark. The value applied to the “fr” field is the 

corner frequency of interest for the sinusoidal sweep experiments. This frequency is calculated based on 

the Tp value entered and the step response normalized peak (Mp) measurement. The value of Mp is the 

peak measurement normalized with respect to the settling value; where, the settling value is the average 

signal in the High Period window. To compute a corner frequency, first calculate a damping ratio (ξ) 

based on Mp and then calculate the corner frequency using ξ and Tp as described in the following two 

equations (Ref. 18):

( ) 0

1

ln 1

2 = ξ−

ξ +

π

M p − (1) 

Tp

fr

π

ξ−π = 21 2

(2) 

The sinusoidal frequency sweep experiment must include the fr value determined in Equation (2) and the 

minimum frequency which will be less than the reciprocal of Ts. The value applied to the field adjacent to 

the S/N label is the calculated signal-to-noise ratio for the step test. This calculation is the ratio of the trim 

value for the high period to half the noise magnitude for the high period. This parameter is feedback on 

the acceptability of the applied step amplitude. It is desired to have a S/N ratio greater than 10.0. The 

illustration in Figure 7 is that of a populated Step_NoiseGUI and an example of a data trace for the step 

response of a simulated Ramp actuator. Using the View zoom feature, the values for Step T and Tp can be 

determined. 

The Staircase_Analysis_GUI is a GUI developed to support the Staircase Analysis tool. This GUI is

used to reduce staircase experiment data to figures that illustrate the staircase response. An example of 

this GUI is illustrated in Figure 8, along with a staircase response trace. Although there is no information 

to retrieve from analysis of the Staircase response pertinent to discerning sinusoidal experiment setup 

parameters, this GUI is useful to assure the operator that the staircase data was successfully captured and

will be available for post processing analysis, and that the system appears linear within two amplitude 

steps. 
NASA/TM—2011-217133 18

Figure 7.—This figure illustrates an example of a populated Step_NoiseGUI and one step response plot—

the Ramp actuator as indicated in the GUI. The rectangular red dash traces indicate the periods 

specified in the GUI. The green band highlights the range of data considered steady-state. The trace in 

this illustration is to aid explanation only and not based on hardware performance.

Figure 8.—This figure illustrates an example of a populated Staircase_Analysis_GUI and one of its plots.

As indicated in the GUI, this plot graphically displays the data for the staircase response of the Ramp 

actuator. The red dash traces indicate the periods specified in the GUI. The trace in this illustration is 

to aid explanation only and not based on actual or expected hardware performance.
NASA/TM—2011-217133 19

After completing the sinusoidal sweep perturbation SysID experiments, the data can be reduced to 

linear models with the aid of the Sin Sweep Analysis tool. The tool is also a GUI driven tool that is 

designed to select and format collected data suitable for analysis using the MATLAB System 

Identification Toolbox (The MathWorks, Inc.) software. An example of this GUI is illustrated in Figure 9. 

This GUI will determine linear models between sinusoidal sweep perturbed Gate control signals and 

Ramp position feedback signals. The linear models will be compared to acquired step response signals.

The code behind the GUI will create a Fit matrix for operator review based on an iterative process of 

testing different linear models. The Fit matrix will identify which linear model is representative of the 

process at a given operating point. The following four lines of MATLAB software code are used to 

reduce the data files to linear models. 

IddStep = iddata (StepOut, StepIn, Ts);

IddSin = iddata (SinOut, SinIn, Ts);

arxSYS = arx (IddSin,′ na′, poles, ′nb′, zeros, ′focus′, ′simulation′) 

compare (IddStep, arxSYS);

Let StepIn be the unbiased step perturbation to the DS_Gate_1 actuator and let StepOut be the unbiased 

Ramp position sensor feedback signal. Similarly, let SinIn be the unbiased sinusoidal sweep perturbation 

to the DS_Gate_1 actuator and let SinOut be the unbiased Ramp position sensor feedback signal that 

responded to the perturbation. Finally, let Ts be the sampling period. The number of Poles and Zeros for 

this tool will be an integer within the range of one to six. 

Figure 9.—Sin Sweep Analysis GUI used 

to reduce data files resulting from 

sinusoidal sweep experiments and 

comparing the results to saved step 

response data.
NASA/TM—2011-217133 20

Conclusions

This paper presented the objectives of the Phase I and Phase II wind tunnel experiments with the 

combined cycle engine large scale inlet for mode transition (CCE-LIMX). The principle take-away from 

the Phase I testing is an open-loop mode transition schedule. The mode transition schedule will include 

multiple operating points to which SysID experiments will be conducted in Phase II testing. The Phase II 

experiments will conduct further investigations into the operating points for dynamic analysis. The results 

from these two experiments will be a piece-wise linear model of the inlet diffuser as the system undergoes 

a mode transition. This paper also gave an overview of the tools developed to support the experiments in 

an effort to streamline the experiment process, facilitate data acquisition and documentation, and 

minimize potential for error. The work presented in this paper documents the project’s readiness to 

perform Phase I and Phase II testing efficiently and effectively.
NASA/TM—2011-217133 21

Appendix—Sampling Rate Selection and Anti-Aliasing Filter Design

Since the combined cycle engine, large scale inlet for mode transition study (CCE-LIMX) system 

identification (SysID) Experiment will use digital equipment to sample and save analog signals from 

high-speed dynamic pressure taps, appropriate analog filtering and discrete sampling rates need to be 

determined to satisfactorily acquire data. The analog filter will serve as an anti-aliasing filter to minimize 

fold-over frequencies that may irreparably corrupt the digitized signals of interest. For the SysID 

experiments, four bypass doors are located just upstream from the inlet aerodynamic interface plane (AIP) 

between the inlet and the turbine engine. Since each of these doors will have their own hydraulic cylinder 

actuator and controller, these bypass doors can be controlled independently or combinations can be 

operated synchronously. 

The purpose for the bypass doors is two-fold. First, these doors will be available as control actuators 

to attenuate low frequency disturbances forthcoming from the engine. Second, these bypass doors will be 

able to simulate low frequency disturbances to the inlet low-speed diffuser originating from the engine. 

Since the passive inlet bleed system is designed to attenuate high frequency disturbances, an active 

controller will be designed to use the bypass valves to mitigate low frequency disturbances. The cut-off 

frequency separating disturbance control responsibility between the passive bleed system and the active 

bypass valve control system is hardware dependant. Based on experience, this cut-off frequency is 

expected to be higher than 20 Hz and lower than 80 Hz (Ref. 10). To adequately test the responsiveness 

of the disturbance attenuation system, these bypass doors can be controlled to create disturbances at the 

AIP. To this end, the bypass doors are designed to operate at frequencies ranging from 0 to 100 Hz. For 

the SysID experiments, these doors will oscillate over a linear frequency sweep from 0 to 100 Hz. 

Therefore, the high-speed dynamic pressure taps will pick-up frequencies within this range. Since the 

SysID experiments are conducted with a cold pipe and mass flow plug assembly in place of a turbine 

engine, the only disturbances in the LSFP diffuser will be from the bypass gates. 

The remainder of this appendix reviews the approach for determining an adequate cut-off frequency 

for a Bessel function anti-aliasing filter and it continues with the approach for determining an appropriate 

data sampling rate.

Ideally, the anti-aliasing filter will pass, with a gain of 1.0, all frequencies less than 100 Hz and 

attenuate all signals with frequencies greater than 100 Hz to values less than what can be read by the A/D

least significant bit (LSB). This ideal filter would allow for an A/D sampling at 100 Hz. However, lowpass filters such as what would be employed for an anti-aliasing filter, have a frequency roll-off as 

opposed to an abrupt frequency cut-off. This section addresses the following three tasks for defining the 

anti-aliasing filter: 

1. Define a minimum stopband attenuation, Amin, 

2. Define a minimum sampling rate frequency, fs, and 

3. Identify the level of aliasing error in the passband with respect to the signal and the calculated Amin

and fs. 

The anti-aliasing filter and the A/D sampling rate hardware available employ 16-bit, 8th order Bessel 

functions. The anti-aliasing filter should attenuate the signals with frequencies in the stopband to less than 

the root-mean-square (rms) quantization noise level for the A/D. A 16-bit linear A/D has a quantization 

level, q, as defined in Equation (A.1) where Vfs is the full scale voltage level of the A/D, and B = 16 (bits). 

B

fs

B

fs VV

q 212 ≈ − = (A.1) 

The quantization error (qe) that will not be detectable or removable after digitization by the linear A/D, is

half of the quantization level (q/2). This discussion assumes the quantization errors for each sample are 

random and uniformly distributed within ±q/2 with zero mean. Therefore, the quantization noise power is 

the variance as described in Equation (A.2) (Ref. 19).
NASA/TM—2011-217133 22

( ) ee

q

q e dqqpq ∫− =σ 2

2

2 2 (A.2) 

Over an interval from –q/2 to q/2, the probability density function (p(qe)) of a continuous uniform 

distribution is as defined in Equation (A.3) (Ref. 20).

( ) 







 << − = = − −

 0 else

22 for 11

22

q q q

qp q e qq e (A.3) 

Therefore, the variance equation simplifies by applying (A.3) to (A.2) as described in (A.4) and solving to 

arrive at Equation (A.5). 

e

q

q e dqq

q ∫− =σ 2

2

2 2 1 (A.4) 

( ) ( )

1233

1 2

3

2

3

2 2 q

q

qq

=

















−=σ

−

(A.5) 

The rms level for a Gaussian probability density function is the standard deviation σ. Therefore, set the 

desired stopband signal level to the rms quantization noise level, σ, as defined in Equation (A.6). 

32

2 q =σ=σ (A.6) 

For the SysID experiments, the sensed signal traces will be sinusoidal. Therefore, let the maximum rms 

passband signal be as defined in Equation (A.7). 

2

fs

rms

V

V = (A.7) 

Rearranging (A.1), the full scale voltage can be represented as described in Equation (A.8). 

B

fs = qV 2 (A.8) 

Substitute Equation (A.8) into Equation (A.7) results in the Equation (A.9) expression for maximum rms

pass-band signal in terms of quantization level and number of bits in the A/D. 

2

2B

rms

q V = (A.9) 

The ratio of maximum rms passband signal to the desired stopband signal level is defined in 

Equation (A.10). 

62 32

2

2

32

2

2

B

B

B

q

q

q

q

= = 





 







(A.10) 

The minimum attenuation in the stopband for the anti-aliasing filter is determined with Equation (A.11). 
NASA/TM—2011-217133 23

20log ( ) 62 104. dB11 at 16 bits Amin −= 10 −= B = B (A.11) 

The 8th order Bessel function analog filters will apply a –48 dB/octave roll-off. This specification can be 

translated to db/decade understanding that one octave is a factor of 2 and given the relationships described 

in Equations (A.12) and (A.13). 

( ) octave

decades log10 = .02 301 (A.12) 

( ) ( ) decade 160dB

decade

octaves

301.0

1

octave − 48 dB = (A.13) 

Next, the anti-aliasing Bessel filter is designed and the required A/D sampling rate is determined. Let the 

frequency of interest (fi) for data acquisition be twice the highest signal frequency expected (fi = 200 Hz). 

Since the frequency of interest is much higher than the highest signal frequency expected, a Bessel filter 

designed for –3 dB attenuation at fi should be acceptable. The Bessel filter for this application, designed 

with a –3 dB attenuation at 206 Hz, will fall below the desired stopband attenuation of –104 dB at 

approximately 1.8 kHz. Therefore, the minimum sampling rate necessary is 2 kHz (fi + 1.8k Hz)—foldover frequency of 1 kHz. The Bode plot illustrated in Figure 10 is an 8th order Bessel function filter with a 

cut-off frequency at 206 Hz. 

The Bode plot illustrated in Figure 10 was generated by applying the following two commands to the 

MATLAB software where 400 is a group delay that essentially places the cut-off frequency: 

• [b8, a8] = besself (8, 2*π*400); 

• Bode(b8, a8, ‘r’); 

To promote further attenuation of the aliased signals, a sampling rate of 2.5 kHz was chosen for this 

application. 

Figure 10.—Bode plot for an 8th order Bessel Function filter with a cut-off frequency of 206 Hz.
NASA/TM—2011-217133 24

References

1. Auslender, A.H., Suder, K.L., and Thomas, S.R., “An Overview of the NASA FAP Hypersonic 

Project Airbreathing Propulsion Research,” AIAA–2009–7277, 16th AIAA/DLR/DGLR 

International Space Planes and Hypersonic Systems and Technologies Conference, Bremen, 

Germany (Oct. 2009). 

2. Aeronautics Science and Technology Subcommittee Committee on Technology National Science 

and Technology Council, National Plan for Aeronautics Research and Development and Related 

Infrastructure, (Dec. 2007) 

3. Hank, J.M., Franke, M.E., and Eklund, D.R., “TSTO Reusable Launch Vehicles Using Airbreathing 

Propulsion,” AIAA–2006–4962, 42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & 

Exhibit, Sacramento, CA (Jul. 2006). 

4. McDaniel, J.C., Chelliah, H., Goyne, C.P., Edwards, J.R., Givi, P., and Cutler, A.D., “US National 

Center For Hypersonic Combined Cycle Propulsion: An Overview,” AIAA–2009–7280, 16th

AIAA/DLR/DGLR International Space Planes and Hypersonic Systems and Technologies 

Conference, Bremen, Germany (Oct. 2009). 

5. Culver, G., “Probabilistic Performance Comparison Of RBCC- and TBCC-Based Reusable Launch 

Vehicles With Enhancing Technologies,” AIAA–2003–5035, 39th AIAA/ASME/SAE/ASEE Joint 

Propulsion Conference and Exhibit, Huntsville, AL (Jul. 2003). 

6. McClinton, C.R., Hunt, J.L., Ricketts, R.H., Reukauf, P., and Peddie, C.L., “Airbreathing 

Hypersonic Technology Vision Vehicles and Development Dreams,” AIAA–99–4978, 9th

International Space Planes and Hypersonic Systems Technologies Conference, Norfolk, VA 

(Nov. 1999). 

7. Ouzts, P.J., “Mode Transition Design Considerations for an Airbreathing Combined-Cycle 

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

Technologies Conference, Dayton, OH (May 2008). 

8. Slater, J.W. and Saunders, J.D., “CFD Simulation of Hypersonic TBCC Inlet Mode Transition,” 

AIAA–2009–7349, 16th AIAA/DLR/DGLR International Space Planes and Hypersonic Systems 

and Technologies Conference, Bremen, Germany (Oct. 2009). 

9. Kaminski, P., Decadal Survey of Civil Aeronautics: Foundation for the Future, Statement before 

the Subcommittee on Space and Aeronautics Committee on Science U.S. House of Representatives 

(July 2006). 

10. Sanders, B.W. and Mitchel, 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). 

11. 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 (Jun. 2008). 

12. Saunders, J.D., Slater, J.W., Dippold, V., Lee, J., Sanders, B.W., and Weir, L.J., “Inlet Mode 

Transition Screening Test for a Turbine-Based Combined-Cycle Propulsion System,” 2008 

JANNAF Propulsion Meeting, Boston, MA (May 2008). 

13. Saunders, J.D., Slater, J.W., Dippold, V., Weir, L.J., and Sanders, B.W., Inlet Research Status for a 

Large-Scale Turbine-Based Combined Cycle Engine Testbed, 2009 JANNAF Propulsion Meeting, 

La Jolla, CA (Dec. 2009). 

14. Suder, K.L. and Thomas, S.R., “An Overview of the NASA Hypersonic Project’s Combined Cycle 

Engine Mode Transition Research Experiment,” 2009 JANNAF Propulsion Meeting, La Jolla, CA

(Dec. 2009). 

15. Soeder, R.H., Roeder, J.W., Linne, A.A., and Panke, J., “User Manual For NASA Glenn 10- By 10-

Foot Supersonic Wind Tunnel,” NASA TM—2004-212697 (May 2004). 

16. Stueber, T.J., Vrnak, D.R., Le, D.K., and Ouzts, P.J., “Control Activity in Support of NASA 

Turbine Based Combined Cycle (TBCC) Research, NASA/TM—2010-216109 (Mar. 2010). 
NASA/TM—2011-217133 25

17. Kein, v. and Morelli, E.A., Aircraft System Identification Theory and Practice, AIAA Education 

Series, ISBN 1-56347-832-3 (2006) 

18. Sinha, N.K. and Kuszta, B., Modeling and Identification of Dynamic Systems, Van Nostrand 

Reinhold Company, ISBN: 0-442-28162-5, LOC: 82-16123 (1983). 

19. Ifeachor, E.C. and Jervis, B.W., Digital Signal Processing, Addison-Wesley Publishing Company, 

(1996). 

20. Weisstein, E.W., CRC Concise Encyclopedia of Mathematics, Chapman & Hall/CRC (2003). 
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4. TITLE AND SUBTITLE

Dynamic Testing of the NASA Hypersonic Project Combined Cycle Engine Testbed for 

Mode Transition Experiments 

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6. AUTHOR(S)

Stueber, T., J.; Thomas, R.; Vrnak, D., R.; Nguyen, B., V.; Nappier, J., M.; Downey, J., A.; 

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WBS AERX22010D 

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14. ABSTRACT

NASA is interested in developing technology that leads to more routine, safe, and affordable access to space. Access to space using airbreathing propulsion systems has 

potential to meet these objectives based on Airbreathing Access to Space (AAS) system studies. To this end, the NASA Fundamental Aeronautics Program (FAP) Hypersonic 

Project is conducting fundamental research on a Turbine Based Combined Cycle (TBCC) propulsion system. The TBCC being studied considers a dual flow-path inlet system. 

One flow-path includes variable geometry to regulate airflow to a turbine engine cycle. The turbine cycle provides propulsion from take-off to supersonic flight. The second 

flow-path supports a dual-mode scramjet (DMSJ) cycle which would be initiated at supersonic speed to further accelerate the vehicle to hypersonic speed. For a TBCC 

propulsion system to accelerate a vehicle from supersonic to hypersonic speed, a critical enabling technology is the ability to safely and effectively transition from the turbine 

to the DMSJ-referred to as mode transition. To experimentally test methods of mode transition, a Combined Cycle Engine (CCE) Large-scale Inlet testbed was designed with 

two flow paths-a low speed flow-path sized for a turbine cycle and a high speed flow-path designed for a DMSJ. This testbed system is identified as the CCE Large-Scale Inlet 

for Mode Transition studies (CCE-LIMX). The test plan for the CCE-LIMX in the NASA Glenn Research Center (GRC) 10- by 10-ft Supersonic Wind Tunnel (10x10 SWT) is 

segmented into multiple phases. The first phase is a matrix of inlet characterization (IC) tests to evaluate the inlet performance and establish the mode transition schedule. The 

second phase is a matrix of dynamic system identification (SysID) experiments designed to support closed-loop control development at mode transition schedule operating 

points for the CCE-LIMX. The third phase includes a direct demonstration of controlled mode transition using a closed loop control system developed with the data obtained 

from the first two phases. Plans for a fourth phase include mode transition experiments with a turbine engine. This paper, focusing on the first two phases of experiments, 

presents developed operational and analysis tools for streamlined testing and data reduction procedures.

15. SUBJECT TERMS

Engine inlets; Supersonic inlets; Hybrid propulsion 

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