Military Engine Acquisition: A Primer and Cost Estimating Methodology - Brossura

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ISBN 10: 0833032828 ISBN 13: 9780833032829

Casa editrice: Rand Corp, 2003

Riassunto

Adds new observations to the cost-estimating database from ealier RAND studies and presents a method for forecasting the development cost, development time, and production cost fo future military engine programs.

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Military Jet Engine Acquisition

Technology Basics and Cost-Estimating MethodologyBy Obaid Younossi Mark V. Arena Richard M. Moore Mark A. Lorell Joanna Mason John C. Graser

RAND Corporation

Copyright © 2003 RAND Corporation
All right reserved.
ISBN: 978-0-8330-3282-9

Contents

Preface...........................................................................................iiiFigures...........................................................................................ixTables............................................................................................xiSummary...........................................................................................xiiiAcknowledgments...................................................................................xviiAcronyms..........................................................................................xixChapter One INTRODUCTION.........................................................................1Study Background and Purpose......................................................................1Updating of Previous Study Methods................................................................2The Organization and Content of This Report.......................................................2Part I: Engine Basics and Performance ParametersChapter Two JET ENGINE BASICS, METRICS, AND TECHNOLOGICAL TRENDS.................................9Jet Engine Basics.................................................................................9Jet Engine Parameters.............................................................................14Approaches to Jet Engine Development..............................................................22Summary...........................................................................................23Chapter Three TRENDS IN TECHNOLOGICAL INNOVATION.................................................25Programs and Initiatives..........................................................................25Component and Related Technical Advancements......................................................28Low Observables...................................................................................28Integrally Bladed Rotors..........................................................................29Alternatives to Engine Lubrication Systems: Air Bearings or Magnetic Bearings.....................30Thrust-Vectoring Nozzles for High-Performance Tactical Aircraft...................................31Fluidic Nozzles for Afterburning Thrust-Vectoring Engines.........................................32Integral Starter-Generators and Electric Actuators................................................32Prognostics and Engine Health Management..........................................................33Advanced Fuels....................................................................................34Cooled Cooling Air................................................................................35Advanced Materials................................................................................35Ceramics and Ceramic Matrix Composites............................................................36Intermetallics....................................................................................36Summary...........................................................................................37Part II: Data Analysis and Cost-Estimating TechniquesChapter Four AN OVERVIEW OF COST-ESTIMATING METHODS..............................................41Bottom-Up Method..................................................................................41Estimating by Analogy.............................................................................42Estimating by Parametric Method...................................................................42Summary...........................................................................................45Chapter Five ESTIMATING PARAMETERS AND GATHERING DATA............................................47Estimating Parameters.............................................................................48Performance and Physical Parameters...............................................................48Technical Risk and Design Maturity Parameters.....................................................48Additional Measures of Technical Risk and Maturity................................................52Criteria for Including Parameters.................................................................55Data Gathering....................................................................................56Extent of Data....................................................................................57Data Verification Process.........................................................................59Chapter Six STATISTICAL ANALYSIS.................................................................63Development Cost..................................................................................64Development Time..................................................................................75Production Cost...................................................................................76Normalizing the Data..............................................................................76Production Cost CER...............................................................................79Applying the Results: A Notional Example..........................................................81Summary...........................................................................................84Chapter Seven CONCLUSIONS........................................................................85AppendixA. AN EXAMINATION OF THE TIME OF ARRIVAL METRIC...................................................87B. AN OVERVIEW OF MILITARY JET ENGINE HISTORY.....................................................97C. AIRCRAFT TURBINE ENGINE DEVELOPMENT............................................................121D. MODERN TACTICAL JET ENGINES....................................................................137Bibliography......................................................................................147

Chapter One

INTRODUCTION

STUDY BACKGROUND AND PURPOSE

Realistic cost estimates for military aircraft play an important role in developing sound budgets and in contributing to an effective acquisition policy. RAND has a long tradition of developing cost-estimation techniques and has published a number of widely read reports on the topic. As design approaches and manufacturing processes and materials used in engine production change and new information on aircraft engine technology becomes available, these cost-estimation techniques should be updated. This report presents the results of a RAND research project to develop a methodology for estimating military engine costs.

This work is part of an ongoing RAND research project on military aircraft costs. Three earlier publications stemming from this project are relevant to the discussion in this report. One of those three reports, Cook and Graser (2001), is on the effect of lean manufacturing on airframe costs, Another report, Lorell and Graser (2001), analyzes the effect of acquisition reform on military aircraft costs. The third report, Younossi, Kennedy, and Graser (2001), addresses the effect of advanced materials and manufacturing methods on airframe costs.

UPDATING OF PREVIOUS STUDY METHODS

The methodology for estimating aircraft engine costs has traditionally been based on historical cost data on various aircraft engines; typically, the data are on development and production costs and aircraft quantities produced by engine type. These costs are used as the dependent variables in statistical regression analyses. Explanatory variables or estimating parameters typically include such factors as engine turbine inlet temperature, airflow, thrust-to-weight ratio, and some technology and maturity proxies. The products of the regression analysis are equations that are referred to as "cost-estimating relationships" (CERs).

The most recent RAND studies that used this approach were Nelson (1977) and Birkler, Garfinkle, and Marks (1982). This study updates the 1977 and 1982 studies in three ways:

1. We use a more recent set of cost data provided by the Naval Air Systems Command (NAVAIR) to capture the effect of technological evolution that has occurred over the past two decades. Changes in technology that have occurred over the past five decades are summarized in Table 1.1.

2. We segregate the turbofan engine cost data from the turbojet and turboshaft cost data. This approach provides a more homogenous population for the parametric cost analysis.

3. We treat each engine model (or "dash number") as a separate observation, unlike the earlier studies, which did not explicitly address how to treat a family of engine types.

THE ORGANIZATION AND CONTENT OF THIS REPORT

This report is divided into two parts: "Engine Basics and Performance Parameters" in Chapter Two and Chapter Three, and "Data Analysis and Cost-Estimating Techniques" in Chapters Four through Six. In Chapter Seven, we present our overall conclusions.

Chapter Two presents an introductory discussion of jet engine basics and engine performance parameters that affect costs. The government and industry engine acquisition and engineering communities use a variety of parameters to assess and compare the quality and performance of jet engines and their components. Some parameters describe the physical characteristics of an engine (such as weight, length, and material composition) whereas others describe the performance of an engine (such as thrust) and other performance and design characteristics of individual components (such as combustor efficiency and maximum fuel-to-air ratio). Chapter Three describes emerging engine technologies and industry and government initiatives that may influence the costs of the future engines.

The first two chapters provide background information for a general audience or for cost analysts who are unfamiliar with the basics of engine technologies. Also, an understanding of these concepts should enable program managers and cost analysts to employ the cost-estimating relationships described in the second part of this report and facilitate discussions on jet engines and what affects their costs.

Readers who do not need the basic information presented in Chapters Two and Three and are nterested primarily in our cost analysis can begin at Chapter Four, which presents an overview of our principal cost-estimating methods-analogy, bottom-up, and parametric. Chapter Five discusses technical estimating parameters, the data used in our analysis, and the data normalization process. Chapter Six presents a statistical analysis of historical turbofan engine cost data and the resulting parametric-cost and schedule-estimating relationships (i.e., the equations that result from our regression analysis). Chapter 6 concludes by integrating these estimating methods into a notional example for projecting the costs of all future military engines. Chapter Seven presents our conclusions, and the appendices provide substantial historical background on the development of military jet engines.

Chapter Two

JET ENGINE BASICS, METRICS, AND TECHNOLOGICAL TRENDS

This chapter provides a basic overview of jet engine technologies and the metrics used to compare them. This background information on engine components and performance parameters should be useful in interpreting the engine data and cost-estimating relationships presented in Chapters Five and Six. In addition, some related emerging technologies and cost-reduction initiatives are also described in the next chapter to illuminate some factors that may influence the costs of future jet engines.

JET ENGINE BASICS

Jet engines operate on what thermodynamicists know as the Brayton cycle. The Brayton cycle consists of three distinct stages: compression (raising the pressure of the air entering an engine), heating (raising the temperature of the air to increase its energy greatly), and expansion (allowing the pressure of the flowing air and fuel combustion products to drop in order to extract energy and accelerate the flow). While variations in hardware design and complexity exist, these three stages are normally achieved in jet engines by using the following processes:

The pressure of the air entering an engine is raised as the air is initially slowed by the engine's inlet and as it flows through the engine's compressor. Next, heating occurs in a combustor, where fuel is burned with the high-pressure air. Finally, expansion occurs as energy is extracted from the exhaust gases by a turbine. These gases accelerate through the engine's nozzle to produce thrust. The turbine extracts power from high-pressure and high-temperature combustion products (much like a windmill extracts energy from wind) to drive (turn) the rotating compressor. A small percentage of the turbine's power is also drawn off to run auxiliary systems, such as the oil pump, fuel pump, hydraulic pump, and alternator.

A jet engine produces thrust by making a net change in the velocity of the air that is moving through the engine. In the words of Sir Isaac Newton, for every action there is an equal and opposite reaction. As the engine "pushes" on the air to accelerate it, the air pushes back on the engine, providing thrust for the aircraft. This effect is illustrated by the basic thrust equation:

Thrust = mdot * (Vout - Vin)

where, mdot is the rate at which air moves through the engine (kilograms [kg]/second), Vout (meters/second) is the velocity of the flow leaving the exhaust nozzle (i.e., the flow's velocity relative to the nozzle), and Vin is the velocity of the air as it approaches the engine (which is also the aircraft's true airspeed).

A turbojet is a basic jet engine that integrates the five primary components mentioned earlier (inlet, compressor, combustor, turbine, and nozzle). Some turbojets include a second combustor after the turbine, called an afterburner (or augmentor). The afterburner adds energy to the turbine discharge flow to maximize the thrust from the engine. The afterburner is usually engaged only when the maximum thrust is required because the fuel efficiency of a jet engine drops by a factor of three or four when the afterburner is at its maximum setting. Most early jet engines were turbojets. However, with some exceptions, such as some small and relatively inexpensive turbojets designed for one-time-use missile applications, modern jet engines have evolved into more-complicated devices called turbofan engines.

A turbofan engine is more complex and more efficient than a turbojet. A turbofan adds a second compressor, called a fan, a low-pressure turbine to drive the fan, and an annular-shaped bypass duct that allows part of the fan's discharge air to flow around the high-pressure compressor, combustor, and both turbines. The fan compresses air, much like the high-pressure compressor, and some of the air leaving the fan enters the high-pressure compressor, while the remainder flows through the bypass duct. This bypass air is eventually accelerated through a nozzle to produce thrust.

Figure 2.1 is a cutaway drawing of a Pratt & Whitney (P&W) F100-220 afterburning turbofan. The fan, high-pressure compressor, combustor, high-pressure turbine, low-pressure turbine, bypass duct, after-burner, and nozzle are labeled. (The inlet is not shown because each tactical aircraft would have a different inlet design.) The combination of high-pressure compressor, combustor, and high-pressure turbine is known as an engine's core.

In afterburning turbofans, the portion of the fan's air that passes through the bypass duct is remixed with the core's combustion products in the afterburner, before the mixture is accelerated through the nozzle. When maximum or near maximum thrust is necessary, the afterburner injects additional fuel into these flows as they are mixing, and then burns this air-fuel mixture before it reaches the nozzle. Due to fuel efficiency (flight duration and range) considerations, the afterburner is used only for takeoff and when maximum acceleration is needed for a short period of time. In fact, the F-22's afterburning turbofan (Pratt & Whitney F119-100) is powerful enough to allow this aircraft to supercruise (fly supersonically without afterburning).

Turbofans are the only engines on military fighter aircraft that are equipped with afterburners. Most of the engines flying on modern commercial airliners and similar wide-body and military aircraft are high-bypass-ratio (BPR) turbofans and do not use afterburners. The BPR is the ratio of the bypass airflow rate to the core airflow rate.

Therefore, a high-BPR turbofan engine has a relatively large diameter fan, which handles much more air than the high-pressure compressor it precedes. These high-BPR turbofans are significantly more fuel-efficient than turbojets or low-BPR turbofans. This increased efficiency makes the added size and complexity of a large fan and corresponding low-pressure turbine cost effective for many applications. On the other hand, high-BPR turbofans have large diameters and relatively low thrust-to-weight ratios, requiring large nacelles on wings or large ducts through fuselages. This is incompatible with aircraft designed for supersonic flight due to the high drag and weight implications. Instead, fighter engines are typically designed with low BPRs (typically 0.3 to 0.8) to strike a balance between engine efficiency, diameter, and weight.

Turboprop and turboshaft engines also operate on variations of the Brayton cycle. These engines have cores similar to turbojet and turbofan cores. In addition, they typically have a low-pressure turbine that extracts most of the remaining available energy from the combustion products after they leave the core. This low-pressure turbine turns a shaft, which is not connected to a fan or compressor. Instead, this shaft is used to drive a propeller (turboprop) or a helicopter rotor (turboshaft). Intuitively, it may be helpful to think of a turboprop as a turbofan with an extraordinarily large bypass ratio but without a nacelle around the propeller to form the bypass duct. At times, the visible presence of a propeller or rotor leads some to incorrectly assume that these aircraft are powered by internal combustion engines like early propeller-driven aircraft, rather than by these forms of jet engines.

Like the turbofan or turbojet, these engines have a nozzle downstream of the low-pressure turbine, and the flow exiting this nozzle typically produces some thrust. However, the low-pressure turbine extracts so much of the flow's energy before it reaches the nozzle that the main propulsive effect is achieved by the propeller or helicopter rotor, rather than by the flow exiting this nozzle. Virtually all turboprop and turboshaft engines employ highly efficient gearboxes to reduce the power shaft's rotational speed to an RPM appropriate for the propeller, rotor, and other engine components.

JET ENGINE PARAMETERS

Several parameters have been defined and are used widely to characterize the quality and performance of jet engines. In many cases, these parameters also have the greatest affect on engine cost. The most common of these metrics are defined in this section.

Thrust from turbofans and turbojets is measured in pounds or Newtons (N). Maximum thrust is the highest level of thrust available from an engine. This level is achieved by positioning the throttle at maximum afterburner (if so equipped), by injecting water into the engine's airflow to increase thrust for takeoff on some turbojets and turbofans, or by setting the throttle at a temporary "overspeed" maximum RPM, which may have a time or altitude restriction associated with it. Many engines do not use any of these augmentation techniques.

Military thrust is conventionally defined as the highest level of thrust produced by the engine without using these augmentation capabilities (e.g., with the afterburner turned off).

Shaft horsepower (SHP)(measured in horsepower, kilowatts (kw), and other units of power) is the "capability" metric for turboprop and turboshaft engines, analogous to a turbojet's or turbofan's thrust. The power transferred by a shaft is proportional to the product of the shaft's torque (foot-pounds, Newton-meters, and such) times its rate of rotation (revolutions per minute, radians per second, and such).

Specific fuel consumption (SFC) is the conventional fuel efficiency metric for jet engines. This metric assumes different forms. The two most common forms are described next.

For turbojets and turbofans, SFC is often referred to as the thrust specific fuel consumption (TSFC) and is the ratio of the fuel flow rate to the thrust. Clearly, low values of TSFC are good. Measured in pounds of fuel per hour/pounds of thrust, which is usually shortened to 1/hour. In Systeme Internationale (SI) units, SFC is measured in units of kilograms of fuel per second/kiloNewtons of thrust). When only one value of TSFC is reported for an engine, it is often the TSFC corresponding to the military thrust level, rather than the maximum (augmented) thrust level.

Figure 2.2 illustrates the military thrust SFC advantage offered by turbofans compared with turbojets. The very low SFC engines indicated at the bottom of the figure are all high-BPR turbofans.

For turboshafts and turboprops, the most common form of this metric is the power specific fuel consumption, which is frequently written simply as SFC. This form of SFC is the ratio of the engine's fuel flow rate to the shaft horsepower from the engine. (The units of this metric are written as 1/length but are most often reported as "pounds of fuel per hour per horsepower" and when reported in SI units "kilogram per hour per kilowatt.") Again, low values of SFC reflect an efficient engine

(Continues...)

Excerpted from Military Jet Engine Acquisitionby Obaid Younossi Mark V. Arena Richard M. Moore Mark A. Lorell Joanna Mason John C. Graser Copyright © 2003 by RAND Corporation. Excerpted by permission.
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Dettagli dell'edizione

• Editore: Rand Corp
• Data di pubblicazione: 2003
• Lingua: Inglese
• ISBN 10: 0833032828
• ISBN 13: 9780833032829
• Rilegatura: Copertina flessibile
• Numero di pagine: 174
• Redattore: Younossi Obaid
• Contatto del produttore: non disponibile
• Persona responsabile: non disponibile