CPIA Pub655 Combustion Stability 1997 .pdf

7 CPlA PUBLICATION 655

JANUARY 1997

Reproduction is not authorized except by specific permission.

GUIDELINES FOR

COMBUSTION STABILITY

SPECIFICATIONS AND

VERIFICATION PROCEDURES

FOR

LIQUID PROPELLANT ROCKET ENGINES

1946 1996

50 Years o f

CHEMICAL PROPULSION INFORMATION AGENCY

THE JOHNS UNIVERSITY

WHITING SCHOOL OF ENGINEERING COLUMBIA, MARYLAND 21044-3200

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TITLE AND SUBTITLE I

Guidelines for Combustion Stability Specifications and Verification Procedures for Liquid Propellant Rocket Engines

6.

Klem, Mark D. and Fry, Ronald (Editors)

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The Johns Hopkins University Chemical Propulsion Information Agency 10630 Little Patuxent Parkway, Suite 202 Columbia, MD 21 044-3200

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Phillips Laboratory 8725 John J. Rd. (OLAC) PL/RK Suite 0944 Edwards AFB, CA 93524-7048 Ft. Belvoir, VA 22060-621 8

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Report, Jan 93-Jan 97

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Publication 655

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13. ABSTRACT (Maximum200 words)

Guidelines are described and documented for the writing of specifications and verification procedures to dynamic combustion stability in liquid rocket engines.

14. SUBJECT TERMS 15. NUMBER OF PAGES

Combustion chambers Liquid propellant rocket engines Combustion stability Specifications Dynamic stability Test methods

Verification

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NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89) Prescribed by ANSI

CPlA PUBLICATION 655

JANUARY 1997

Reproduction is not authorized except by specific permission.

GUIDELINES FOR

COMBUSTION STABILITY

SPECIFICATIONS AND

VERIFICATION PROCEDURES

FOR

LIQUID PROPELLANT ROCKET ENGINES

1946 1996

50 Years of Service

CHEMICAL PROPULSION INFORMATION AGENCY

THE JOHNS UNIVERSITY

WHITING SCHOOL OF ENGINEERING COLUMBIA, MARYLAND 21044-3200

DISTRIBUTION STATEMENT: Approved for public release; distribution is unlimited.

CPlA IS a DTIC-sponsored DoD Information Analysis Center operating under contract

The Chemical Propulsion Information Agency (CPIA) is a Information Analysis Center operated by The Johns Hopkins University, Whiting School of Engineering, under Office of Naval Research Contract SP0700-97-D-4004. The applicable Instruction is 3200.12-R-2, "Centers for Analysis of Scientific and Technical Information." The mailing address is The Johns Hopkins University, Attn: Scty Off - CPIA, 10630 Little Patuxent Parkway, Suite 202, Columbia, Maryland, 21 044-3200. The CPlA Technical point of contact is Ronald S. Fry,

(410) 992-9951.

The CPlA also provides technical and administrative support to the Joint Army-Navy-NASA-Air Force (JANNAF) Interagency Propulsion Committee.

The Government Administrative Manager for CPlA is the Defense Technical Information Center, 8725 John J. Road, Suite 0944, Ft. Belvoir, VA 22060-6218. The Government Technical Manager (Contracting Officer's Technical Representative) is Dr. Robert C. Corley, (OLAC) Phillips Laboratory, Edwards AFB, CA 93524-7048.

PREFACE

These Guidelines represent a revision of the "Combustion Stability Specifications and Verification Procedures," originally prepared in September 1971 (CPIA 218 ) and revised in 1973 (CPIA 247).

This revision was initiated in 1993 at the request of the JANNAF Liquid Rocket Engine Combustion Instability Panel after a survey in 1992 of Industry and Government organizations indicated that a revision was needed and desirable and all the organizations would participate in a revision.

The following individuals were involved in the revision. They are noted on the basis of the contributions made t o the document:

Jerry Pieper Eric Hurlbert

Aerojet

NASA JSC

John Hutt NASA MSFC Robert Jensen George B. Cox Kevin Breisacher

Rocketdyne Pratt & Whitney NASA LeRC

Carol Dexter NASA MSFC Frank Stoddard TRW Robert Braendlein Robert Glass

Marquardt

AF-SMC

Jim Clark Allen Frankel Jay Levine Brantly Adams

Pratt Whitney Marquardt AF-Phillips Lab NASA Stennis

The following organization:

individuals reviewed and approved the final document as representing their lndustrv Government Jerry Pieper Allen Frankel Jim Clark Robert Jensen Frank Stoddard

- Aerojet

- Marquardt

- Pratt Whitney

Rocketdyne

- TRW

Air Force Jay Levine

NASA

Eric Hurlbert Kevin Breisacher

Phillips Lab

JSC

LeRC

John Hutt MSFC

The assistance of all these individuals is greatly appreciated as it was necessary t o have all the organizations agree that these guidelines represent a reasonable approach t o achieving stability in Liquid Rocket Engines'.

Richard J. Priem (Chairman and Organizer) iii

GUIDELINES FOR

COMBUSTION STABILITY SPECIFICATIONS AND

VERIFICATION PROCEDURES

FOR

LIQUID PROPELLANT ROCKET ENGINES

CONTENTS

PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

INTRODUCTION

DYNAMIC STABILITY

VERIFICATION PROCEDURES

A. ARTIFICIAL DISTURBANCE DEVICES

B. INSTRUMENTATION

STABILITY EVALUATION TESTING

A. COMBUSTION CHAMBER DEVELOPMENT

B. ENGINE DEVELOPMENT AND QUALIFICATION . . . . . . . . . . . . . . . . . . . . . 1 1

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1

INITIAL DISTRIBUTION

V

A

I. INTRODUCTION

The purpose of this report is to provide guidelines for the writing of combustion stability specifications and the planning of stability verification procedures for the development of liquid propellant rocket engines. The guidelines are written quite generally, t o be applicable t o a wide variety of engine systems. A stability specification for a particular engine should be based on the characteristics and requirements pertinent t o that engine. It is hoped that sufficient guidance is given in this report to facilitate the development of realistic stability specifications and verification procedures.

Guidelines are presented for engine stability verification using three different test procedures t o demonstrate stability. "Dynamically Stable by Damping" refers t o stability demonstration, with a minimum number of specified tests, of an engine system that returns t o normal operation after the combustion process has been artificially perturbed. "Dynamically Stable by Low Amplitude" refers t o engine systems that have demonstrated stability through testing within acceptable standards with artificial disturbances, but the disturbances have not generated sufficient amplitude of oscillations t o determine damping rates as required for "Dynamically Stable by Damping" requirements. with No Spontaneous Instabilities" refers t o engine systems that are not artificially disturbed, for whatever reasons, but have extensive, within acceptable standards, testing without observations of an instability.

Included in this consideration are organized periodic oscillations generally known as "intermediate", and "high" frequency instability, as well as the random fluctuations termed "rough combustion". Oscillatory and rough combustion are regarded as engine malfunctions. A stability specification will prescribe the allowable response of the engine t o these malfunctions and the procedure for experimentally verifying its behavior in an acceptable design. The complete engine specification should contain, under the reliability requirements, provisions for dealing with malfunctions, such as limits on the allowable number of malfunctions.

Combustion instability typically is manifested by violent combustion chamber pressure oscillations. Therefore, the amplitude, frequency, and duration of such pressure oscillations are used in the criteria for specifying and verifying the stability of an engine. Other forms of criteria can be used criteria based on vibration measurements) if they can be correlated t o chamber pressure oscillations or follow the guidelines described under Section B, Instrumentation.

However, criteria based on chamber pressure measurements is preferred.

The purpose in requiring a rocket engine t o be dynamically stable is to ensure that catastrophic loss of the engine vehicle will not occur as a result of combustion instability and t o demonstrate this with minimum cost, testing and time. Dynamic stability should be considered in the initial design t o minimize the overall development cost. Achieving "Stability by No Spontaneous Instabilities" requires extensive testing and should be considered only when it is impossible or very difficult t o artificially disturb the engine. Stability testing should begin early in the development program and should be extensive enough t o provide high confidence in the stability of the evolving engine design. Extensive stability testing should not be deferred until the end of the development program when the discovery of unacceptable stability characteristics would cause major cost and schedule impacts.

To ensure the development of a stable engine a t minimum cost, stability requirements should be included in the initial engine model specifications. In addition, stability demonstration requirements for development, qualification, and production should be specified a t the outset of the development program. To be of maximum benefit, these specifications should be based on the engine characteristics and mission. A detailed stability history should be maintained throughout the entire development program. Potential injector designs should be evaluated wi th respect t o stability characteristics as soon as they become available for testing. Costs can be reduced by combining stability evaluation with component and engine performance testing.

Subject t o overall development considerations, the designs with the greatest stability margin should be selected for further development.

I Once a design is selected, testing should be conducted t o demonstrate that the specified stability requirements of the engine are being met. Qualification and subsequent stability testing should be minimal and conducted only to provide confidence that some unknown production factors have not degraded the stability behavior of the production engines below that required by the engine specification. Engine-to-engine variability requires that several engines be tested t o establish a confidence level for the design. Engines with different accumulations of test time should be tested t o determine whether engine stability deteriorates with time.

DYNAMIC STABILITY

Combustion stability is defined for an engine system that has demonstrated sufficient stability margin t o ensure that a catastrophic loss of the engine will not occur as a result of combustion instability. This capability can be demonstrated by any of the following three methods:

(1) Demonstration of combustion stability damping (via testing as defined in Table I Section A . l "Test Procedure for Dynamically Stable by Damping") by sufficiently disturbing the combustion process of the engine system with artificial disturbances and observing through adequate measurement that the engines return t o normal operation within a specified time interval without any adverse operating effects. Engines demonstrating this capability are defined as "Dynamically Stable by Damping". This is the preferred method for demonstrating stability.

(2) Sufficient attempts (as defined in Table Sect A.2 "Test Procedure for Dynamically Stable by Low Amplitude") at disturbing the combustion process with artificial disturbance devices without achieving sufficient overpressure or organized oscillations t o determine the time to return to normal operation are defined as "Dynamically Stable by Low Amplitude". The overpressure should be determined by adequate measurements (defined in the Instrumentation Section of this guideline) and the engine must always maintain stable combustion characteristics. Engines demonstrating this capability are considered as equivalent t o "Dynamically Stable by Damping" engines as far as demonstration of adequate combustion stability margin.

Successful completion of sufficient testing within acceptable standards (as defined in Table Sect. A.3 "Test Procedure Stable with No Spontaneous Instabilities") while maintaining stable combustion are defined as with No Spontaneous Instabilities". The should be tested over an expanded range of operating conditions (defined in Table and have no detection of oscillations for periods longer than required for dynamic stability or encounter damage associated with combustion instability injector face erosion). Detection of oscillations should be determined with adequate measurements as defined in the instrumentation section Adequate stability margin is demonstrated through expanded testing over an intentionally wider operating box. The preferred "Dynamic Stability" demonstration with these engines may be precluded because of engine size limitations, cost constraints, or other design limitations. In general, "normal operation" of an engine is considered t o be satisfactory (from the standpoint of combustion instability) if there are no sustained chamber pressure oscillations, in any combustor of the engine, with peak-to-peak amplitudes that exceed 10% of the mean combustion chamber pressure. However, in certain cases, engine or vehicle system characteristics may require an amplitude limit of 5% or even 3% in order t o prevent damage or malfunctioning of the system.

It is generally useful t o distinguish rough operation from unstable operation. In unstable operation, chamber pressure oscillations occur at one or more discrete frequencies with amplitudes beyond the allowable limit 10% peak-to-peak) for a duration greater than a prescribed time. Rough operation is evidenced by chamber pressure variations that are outside the allowable amplitude and duration limits but which are not characterized by large amplitude discrete frequencies large amplitude discrete frequencies are more than three times the background frequency amplitudes).

An engine can be considered t o be "Dynamically Stable by Damping" when combustion oscillations generated through artificial disturbances exceed the amplitude limit specified for normal operation and such combustion oscillations damp t o within allowable levels within the following time interval:

(1 where, is measured in milliseconds, is the frequency (hertz) of the oscillation component with the largest amplitude as determined by a frequency-amplitude analysis of the high frequency transducer signals.

This criteria is valid for all size engines and all frequency levels. The criteria is based on experience for the allowable time, with an acceptable margin, than an oscillation can be present without the engine encountering damage that would prevent it from fulfilling its intended mission. Obviously, if an engine damped within this time but encountered damage due t o the oscillations then it could not be considered stable by any criteria.

Combustion oscillations from artificial disturbances are best determined from chamber pressure measurements using unfiltered signals from high frequency response transducers. As noted in the Instrumentation section of this guideline, combustion related accelerations and/or propellant injector dome pressures may be used if high frequency chamber pressure measurements are not available.

The high frequency transducers used shall be capable of responding t o frequencies up t o oscillations corresponding t o the 3T mode. As a practical maximum, the instrumentation response shall be flat up to the 3T mode. However, if not practical, the response as a minimum should be flat t o the or 10 Khz, whichever is largest. Frequency analysis of the signal shall also be conducted t o the largest of these frequencies. Oscillations that exceed the amplitude limit but do not exhibit any measurable predominant frequency peaks greater than three times the background as determined by the amplitude-frequency analysis) are considered t o be excessive combustion roughness and should damp t o within the acceptable limit in no more than milliseconds.

VERIFICATION PROCEDURES

It is generally accepted that the "PREFERRED METHOD FOR VERIFYING STABILITY" is by artificially disturbing the combustion process during engine operation and observing the damp times of "THE RESULTING CHAMBER PRESSURE OSCILLATIONS". Artificial disturbances are normally achieved by detonating explosive charges within the combustion chamber or in an auxiliary chamber connected t o the combustion chamber by a short duct (as in a pulse gun).

If these, or other, types of artificial disturbances do not produce oscillations of sufficient amplitude t o determine the damp rate the engine is considered "Dynamically Stable by Low Amplitude" after completing sufficient tests (as described in Table with adequate transducers (as described in the Instrumentation section, B, of this Guideline) t o assure that the engine always produced normal operation including periods with attempted combustion disturbances.

Engines that can not be artificially disturbed (size, cost, etc. limitations) can demonstrate stability via "Stable with No Spontaneous Instabilities" by successfully completing sufficient tests (as described in Table over an expanded range of operating conditions. These tests should be made with adequate high frequency transducers (as described in the Instrumentation section of this guideline) t o assure that over this range of operating conditions the engine has never encountered an instability. Also the engines should not have exhibited any adverse effects associated with combustion instability injector face erosion problems, increased heat transfer t o local areas or periods of high vibration levels). Adequate stability margin demonstrated by this type of testing provides a high confidence that such an engine will not encounter an instability during normal operation.

Development testing of a combustion chamber should be conducted with a dynamically similar injector, manifold and feed system as proposed for the final system, but may be conducted with either a pressurized-tank or pump feed system. The need for a dynamically similar feed system requires that the vehicle feed system be defined before stability testing or additional testing may be needed t o demonstrate that with the final feed system the engine is still stable. A heat sink workhorse combustion chamber may be used, provided that it has the same internal configuration as that proposed for the final system and the range in propellant inlet ( to the injector) temperature and operating conditions are the same as those of the final engine system.

"Internal configuration" includes the size and shape of the injector, chamber, and nozzle convergent section, Final evaluation of dynamic stability should include the flight wall conditions if film or transpiration cooling is used or i f the flight design has walls with appreciable acoustic admittance.

Engine development testing should be conducted with the proper feed system in order t o evaluate the effects of feed system interactions. Qualification and/or acceptance testing should be conducted with the complete engine system t o demonstrate that dynamic stability has been achieved.

Once "Dynamic Stability by Damping or Low Amplitude or Stable with No Spontaneous Instabilities" has been demonstrated further stability testing is not required. However, a stability test failure where the engine encounters an instability and fails t o damp in the acceptable time indicates unacceptable stability margin and design changes with subsequent retesting of the new design is recommended. A statistically designed test program t o demonstrate stability margin can minimize the testing required. To ensure that hardware variability will not cause stability problems it is recommended that a t least t w o injector units and/or t w o complete engines be included in the statistically designed test program t o demonstrate stability.

A. ARTIFICIAL DISTURBANCE DEVICES

Currently the t w o most effective techniques for artificially triggering instability in rocket combustors are nondirection explosive charges (bombs) mounted inside the combustion chamber and detonated during the test run and pulse guns that direct a shock wave into the combustion chamber. Other techniques are available t o induce an instability, including hydrogen injection temperature ramping. Disturbance techniques can be experimented with early in the development program, but one technique should be selected as the baseline for a particular combustor. The baseline disturbance technique should be selected with a view toward using it in the final engine system. For example, explosive charges may be preferred for combustors with regenerative cooling t o avoid inserting devices through the chamber wall, whereas the pulse gun may be preferred for combustors driving turbines t o preclude bomb shrapnel going through the turbine.

Tests should be conducted with disturbance devices sufficient to excite an oscillation in the engine. Generally if an oscillation can be excited in an engine the amplitude and frequency characteristics are relatively insensitive t o the size and location of the disturbance (within reasonable limits) with that engine. Figure 1 is provided t o assist in selecting a disturbance size.

Figure 1 shows the range of disturbances that have been successfully used t o excite oscillations in engines with various diameters. Also shown is a "recommended" range in bomb size (Grains of RDX) as a function of combustion chamber diameter. Selecting disturbances within this range should provide a high probability of exciting an oscillation. If an oscillation is excited by a disturbance it is not necessary to change the disturbance size or location t o determine the engines sensitivity to artificial disturbances.

Location of the artificial disturbances is normally determined by physical restrictions within the engine design. Generally the disturbance is located axially as close to the injector face as physically possible. The radial position is generally determined by the injector face elements.

The artificial disturbance should be located t o minimize its impact on the liquid streams of propellant exiting the injector face. The disturbance device should be near the chamber wall t o provide a disturbance similar t o that which exists with transverse waves where the amplitude is maximum at the chamber wall.

If an oscillation is not excited (insufficient amplitude or no evidence of an oscillation after the disturbance) using bomb disturbances within the recommended range shown in Figure 1, then Table (Test Conditions for Dynamic Stability with Low Amplitude) provides a range of disturbances and test conditions that should be examined before the engine can be declared "Dynamically Stable with Low Amplitude", as described in Section A.2.

B. INSTRUMENTATION

All combustion stability characterization should be based on accurate high response instrumentation. Transducer selection and location principles are discussed in detail in References 1 and 2. Since combustion instability is associated with oscillations in gas pressure within the combustion chamber the preferred method for detecting the oscillations is to directly measure chamber pressure with flush mounted high response pressure transducers. In some applications regeneratively cooled chambers) this may not be possible and other techniques for measuring the oscillations may be required. Alternate techniques include helium bleed transducers, transducers in the acoustic absorber, transducers in the propellant injector manifolds, and accelerometers mounted on the combustion chamber. For flight combustion pressure transducers should be installed at positions 1 and 2 as shown in Figure 2 see Figure 2 of Reference 1). Positions 1 and 2 should be located axially approximately one inch downstream of any anticipated baffle tip. If baffles are not used, positions 1 and 2 may be located on the injector face near the outer periphery or on the chamber wall approximately one inch downstream of the injector face.

It is recognized that in most cases it may be impossible t o install flush-mounted transducers through the walls of the chamber cooled chambers). In these instances, a helium bleed transducer, such as the one described in Reference 1 can be used. If a helium bleed transducer is used, its response should be compared with flush mounted transducers tested with heat sink, workhorse chambers, or corrected for amplitude response via analysis of the response of the helium bleed system. If the combustor has an acoustic absorber, pressure measurements can be made inside the acoustic cavity compartments. The amplitude of the oscillations measured should be corrected for the admittance of the cavity via analysis or comparisons with results from heat sink chambers.

If high frequency chamber pressure measurements can not be made via any of the techniques described above, then alternate approaches are permissible. These include transducers in the fuel and oxidizer injector manifolds accelerometers mounted on the combustion chamber.

If transducers in the manifold have been used during stability testing with heat sink chambers, a correlation between the amplitudes of manifold pressure t o chamber pressure at various frequencies should be made. This correlation can be used t o determine equivalent amplitudes of chamber pressure oscillations from manifold pressure transducer measurements t o determine damp times. If heat sink chamber testing has not been performed and a correlation between manifold and chamber pressure oscillations is not available, the allowable oscillations are determined by performing a frequency analysis of the manifold pressure signals. Amplitudes at frequencies around the calculated resonant frequencies of the chamber should be less than 3 times the background amplitudes around the resonant frequencies or 1.5 times the steady state amplitudes (when the engine is known to be stable and not sustaining damage).

0 100

N

E 60

3 4 6 8 10 20 30

Engine Diameter in inches

Figure 1. Recommended Bomb Size

An instability is considered present whenever the amplitude exceeds this level (3 times the background or 1.5 times the steady state) and the damp time is determined as the interval between when the oscillation exceeds this level until it returns t o this level. If an artificial disturbance does not produce oscillations greater than 3 times the background (or 1.5 times the steady state value) then the test can be considered for the criteria of "Dynamically Stable by Low Amplitude".

Another alternate t o directly measuring chamber pressure oscillations is the use of accelerometers on the engine. If accelerometers are used, selection for is very important and will depend on the engine design. The preferred method is t o locate accelerometers on a component injector dome) that will be sensitive t o the vibrations and is the same in flight chambers as used in heat sink tests. Then the accelerometer amplitudes can be correlated at the same frequency with chamber pressure oscillations obtained from high frequency transducers mounted on heat sink workhorse chambers. This correlation can then be used t o determine allowable accelerometer amplitudes and damp times for flight chamber testing.

If equivalent chamber pressure amplitudes can not be determined from accelerometer measurements correlated with chamber pressure measurements, then allowable accelerometer signals should determined by the same procedure as described above for propellant injector manifold transducers. If unusual hardware damage or operation occurs as a result of the response to artificial disturbances, regardless of the observed amplitude and decay time, the engine should be classified as demonstrating unacceptable combustion stability margin.

Conversely, an engine that does not produce sufficient amplitude t o measure a damp time and has no unusual hardware damage or operation as a result of the engines response t o artificial disturbances could be considered t o have sufficient stability margin t o be "Dynamically Stable via Low Amplitude" providing it has successfully completed evaluation testing given in Table

The output of high response transducers can be very sensitive t o the mechanical vibration of the combustor during a firing. Therefore, proper attention should be given t o the transducer mounting arrangement, so that erroneous readings and/or transducer damage do not result (Reference 2).

For heat sink, workhorse chambers, transducers should be installed at positions 1 and 2 (Figure

2) as in the case of flight chambers. However, t o follow very complex wave phenomena adequately, more than the minimum number of transducer locations is required. For transverse waves, recommended locations for 5 transducers are shown in Figure 2. These additional transducers are recommended along the length of the chamber (Positions 4 and 5 of Figure 2).

Accelerometers should be located at positions that will be sensitive t o the expected oscillations and will have similar oscillation characteristics in the flight engine.

High frequency transducers when installed in the feed system are recommended in both the fuel and oxidizer injector inlet manifolds and positioned as close as possible t o be in line with chamber positions 1 and 2.

The disturbance level produced by the artificial disturbance is very difficult, if not impossible, to determine accurately from pressure measurements at the chamber boundaries. Generally, the measured pressure amplitude is the composite result of the disturbance natural decay with distance, its combustion enhancement or damping, and reflections from solid boundaries such as chamber walls and baffle surfaces. For practical purposes, in view of this limitation, the maximum initial amplitude at the chamber wall resulting from a disturbance device should be taken as the disturbance level. NOTE: THIS DISTURBANCE LEVEL IS RELATED TO THE LEVEL

OF DISTURBANCE PRODUCED BY THE ARTIFICIAL DEVICE BUT IS NOT A N ACTUAL

MEASUREMENT OF THE DISTURBANCE AT ITS SOURCE.

The disturbance level should be monitored by a minimum of t w o high-frequency-response pressure transducers located in the chamber wall or on the injector face as previously discussed.

One transducer is located near the source of the artificial disturbance (generally degrees from the bomb). The second position is located directly across-chamber from the bomb and t

Radial Cross Section

Axial Cross Section

Figure 2. Transducer Placement in a Typical Combustor measures the propagation (or refection) of the wave. Where one bomb near a wall is used with the minimum 2-transducer instrumentation arrangement, they should be placed as indicated in Figure 2. These positions ,provide information close t o the explosive bomb (transducer and across the chamber (transducer 2). In the case of "moderate" combustion enhancement, transducer 2 usually will provide the greater output for the first wave t o reach the transducers and as the .wave grows. With large combustion enhancement both transducers may have equal amplitudes. With moderate combustion damping transducer 2 will have the lower output and with large combustion damping the disturbance will be damped before it reaches either transducer (as could be the case for "Dynamically Stability with Low Amplitude") With transducer locations above the minimum number the general principle of locations near the bomb and across the chamber from the bomb should be maintained. The minimum number of pressure transducer locations recommended in monitoring combustion instability may not be sufficient for monitoring disturbance level when more than one disturbance device is used in a given test. In that case, more transducer locations should be selected.

STABILITY EVALUATION TESTING

The stability test program for a particular engine should be designed for that engine and be consistent with its mission and the scope .of its overall development .program. However, all stability test programs should have certain common features that:

1. Minimize the probability of encountering damage t o the engine system due t o combustion instability with a "Reasonable" expenditure of time, testing, cost and hardware.

2. Demonstrate that the engine is stable over the entire operating envelope of the engine and expected duty cycle.

3. Determines engine stability during the early phases of the development of a combustion chamber.

4. Verifies that the engine maintains stability within the expected range of variation due t o manufacturing.

A. COMBUSTION CHAMBER DEVELOPMENT

Following are recommended test procedures t o be followed for demonstrating each of the three types of dynamic stability as defined in section Test procedures for one type has t o be completed for the engine t o be classified as "DYNAMICALLY STABLE". If an engine is not bombed then test procedure for "Stable with No Spontaneous Instabilities" would be followed. If an engine is bombed the initial bomb tests would determine which test procedure t o follow. If the initial bomb tests produced sufficient amplitude t o determine damping rates then procedure A . l for "Dynamically Stable by Damping" would be followed. If the initial bomb test amplitudes were low then procedure A.2 for "Dynamically Stable with Low Amplitude" would be followed.

1 . Test Procedure for "Dynamically Stable by Damping".

Dynamic stability of "An" injector-combustion chamber design should be accomplished by testing the combustion chamber with artificial disturbances over its entire expected operating range of chamber pressure, mixture ratio and propellant temperature as shown in Table I . Normally the nominal or design operating point and the extremes of the operating conditions would be evaluated. For example, an engine with a conventional operating box consisting of a nominal propellant temperature and a +/- range on mixture ratio and chamber pressure may be evaluated at a minimum of five operating conditions (Nominal O/F and and the four operating corners for the extreme range of O/F and expected in the engine). If the start transient of the engine is long 10 times the damp time for the 1T instability mode) it is recommended that a bomb test be conducted between 50 and 75 of full thrust. This provides sufficient time t o measure a damp time and determine if the engine will recover before it reaches full thrust.

A n artificial disturbance can be selected from within the range shown in Figure 1. Location of the disturbance should be "near" the injector face and chamber wall consistent with minimum interference with liquid propellant streams and near the maximum combustion zone. If the disturbance selected measured pressure oscillations sufficient t o determine that the damp time is within the requirements of the specification, additional disturbances do not have t o be tested. For new concepts or injector-combustion-chamber designs without a stability technology base or with different propellants it would be advisable to perform additional tests with different disturbances, locations and broader operating conditions.

A design is unacceptable from a combustion stability standpoint if sustained instability is encountered in any test within the expected operating conditions of the engine. In this case, design modification t o provide additional damping and/or reduced combustion response or a change in the required operating conditions (to not include the unstable operating conditions) would be necessary. Dynamic stability evaluation of the changed design and/or operating conditions would then be required. Test conditions that produce the longest damp time should be noted as these are considered t o have the smallest stability margin and would be selected as test conditions for Engine Development and Qualification Test Conditions.

2. Test Procedure for "Dynamically Stable by Low Amplitude"

If the disturbance techniques initially selected for demonstrating stability do not produce measured pressure oscillations suitable for determining damp times then additional tests have t o be performed t o demonstrate "Dynamic Stability by Low Amplitude" as shown in Table These tests should be performed at the operating conditions that produced the largest oscillations with disturbances within "reasonable limits" for the size and location. If the start transient of the engine is long times the damp time for the instability mode) it is recommended that a bomb test be conducted between 50 and 75 of full thrust. This provides sufficient time t o measure a disturbance and determine if the engine can not be excited before it reaches full thrust.

It is recommended that bombs corresponding t o the maximum range, minimum range and 2 times maximum, as determined from Figure 1 (Bomb size vs. chamber diameter) be used and the maximum size bomb be tested in at least t w o positions. Injector-chamber designs that complete these tests without producing oscillations from which damp times can be determined are considered "Dynamically Stable by Low Amplitude". Test conditions that produced the largest oscillations (or disturbances) should be noted as they would be selected as test conditions for Engine Development and Qualification Test Conditions. Testing with different sizes and location with various operating conditions is recommended t o ensure that the condition with the smallest stability margin is determined. To satisfy stability requirements, it is very important that the high frequency pressure measuring system has adequately demonstrated the capability of measuring the expected disturbances and/or oscillations for the injector-combustion-chamber system.

3. Test Procedure for Stable with No Spontaneous Instabilities.

For combustion chambers that can not be tested with artificial disturbances, injector-chamber development tests should be conducted t o satisfy "Stable with No Spontaneous Instabilities" as given via Table 111. Note that the testing recommended in test series 1 8 of Table is intended t o evaluate and demonstrate safe engine operation over an operating box that exceeds that required for the engine duty cycle or qualification testing. This extended operating box testing will verify stability margin of the engine in the absence of margin demonstration using artificial disturbances. The extent t o which the operating box is extended must be defined on the basis of adding sufficient confidence in the design stability margin and limitations of the hardware and/or test facility. Typically the variation beyond the required operating box may be from 10% t o 100% of the operating box. Only after the design has completed the required number of tests at the conditions given in Table can the design be considered "Stable with No Spontaneous Instabilities". It is recommended that a statistically designed test program be used t o demonstrate "Stable with No Spontaneous Instabilities" t o maximize the confidence of the stability margin with the minimum tests. Until completing this requirement the probability of encountering an instability is always determined by expecting the next test t o be unstable.

B. ENGINE DEVELOPMENT AND QUALIFICATION

Adequate dynamic stability margin for the engine system is demonstrated during the engine development and/or qualification program by performing a minimum of t w o tests with artificial disturbance devices at test conditions corresponding to the longest damp time and maximum amplitude as determined from combustion chamber testing. If combustion chamber stability verification testing has not been performed, then all tests required for the combustion chamber should be performed with the engine system. Additional tests may be required when the engine system conditions differ from those verified for the combustion chamber, because of start or shutdown transient differences, the propellant temperature has changed, or differences resulting from engine throttling or operation.

Engine systems that are not subjected to artificial disturbances, for whatever reason, should be tested over an expanded range of conditions as given in Table (Test Conditions for Stable with No Spontaneous Instabilities) to be considered Stable with No Spontaneous Instabilities. In this case a minimum of t w o engine systems should successfully complete this testing during engine qualification t o demonstrate adequate statistical stability margin.

REFERENCES

1. "Selection of Instrumentation for Analyzing Combustion Instability in Liquid Propellant Rocket Engines," prepared by the Committee on Instrumentation and Test Data of the Working Group on Liquid Propellant Combustion Instability, Interagency Chemical Rocket Propulsion Group (ICRPG) CPlA Publication No. 148, July 1967.

2. "Special Considerations for Combustion Instability Instrumentation and Data Representation" prepared by the Committee on Instrumentation and Test Data of the Working Group on Liquid Propellant Combustion Instability, Interagency Chemical Rocket Propulsion Group (ICRPG), CPlA Publication No. 170, June 1968.

TABLE I

RECOMMENDED "MINIMUM TESTING" For "Dynamically Stable by Damping"

Test TEST CONDITIONS Bomb No. 0 Prop. Size Location

1 Nom Nom Nom Min Rqd.

2 Min Min 3 Max 4 Min Max 5 Max 6 (Max. Damp. Min I f

7 (Max. Damp. Max 8+* Start Transient

Note: Max. Damp, refers t o test conditions previously tested that produced the largest measured damp time.

Test is performed if the start transient is significantly longer 10 times) than the allowable damp time of the mode of instability. Bomb should be excited between 50 to 75 of full thrust.

TABLE

RECOMMENDED "MINIMUM TESTING" For "Dynamically Stable with Low Amplitude"

Test TEST CONDITIONS b No. O/F ProD. TemrJ Size Location 1 Nom Nom Nom Min Rqd.

2 Max Rqd.

3 2 x Max * * 4 (Max. Ampl.)

5 Min Min (Max. Ampl. Size 6 Max 7 Min Max 8 Max 9 (Max. Ampl. Min 10 (Max. Ampl. Max 1 1 Start Transient

Note: Max. Ampl. refers to test conditions previously tested that produced the largest measured disturbance amplitude.

Larger bomb size test if engine is large enough to accommodate larger bomb.

Test 1 is performed if the start transient is significantly longer 10 times) than the allowable damp time of the instability mode. Bomb should be excited between 50 to 75 of full thrust.

TABLE

RECOMMENDED "MINIMUM" TESTING FOR

"Stable with No Spontaneous Instabilities"

Series No. TEST CONDITIONS

Tests 10 Nom

O/F Nom

ProDellant TemD Nom

Duration Start

2 Max Max Min

Full Duty Start

4 -2 Min 5 2 Max Min 6 2 Min 7 2 Max Max Max 8 2 Min 9 2 Max Min 1 1

Min Max + Max +

3 Max +

3 Max + Max- Max +

3 Max +

18 3

Total Test Conditions Specified = 52 Total Tests with No Instabilities 100

Note: Max+ indicates the parameter exceeds the maximum as required for duty cycle or qualification testing.

Min - indicates the parameter is less than the minimum as required for duty cycle or qualification testing.

2 0 0 1 - 0 196

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Figure