Common Solutions to Commercial and Military Propulsion Requirements
Total Page：16
File Type：pdf, Size：1020Kb
24TH INTERNATIONAL CONGRESS OF THE AERONAUTICAL SCIENCES COMMON SOLUTIONS TO COMMERCIAL AND MILITARY PROPULSION REQUIREMENTS Stephen John Bradbrook Rolls-Royce plc Keywords: Gas Turbine, Unmanned Air Vehicle. Abstract The propulsion requirements for commercial This analysis enables the identification of transports and military combat aircraft differ those key requirements which drive the basic considerably, so much so that it is usual for gas thermodynamic cycle and architecture of the turbine engines to be developed independently engine and hence enables understanding of the for each application with little or no possibilities for commonality between engines. commonality of parts. As military budgets tighten and with the need in the commercial Examples of engine families with common world to remain competitive on price, there is architectural features are presented to illustrate increasing pressure to search for greater a typical approach to increasing commonality. commonality as a means of reducing These examples highlight the following issues: • development costs. design compromises • potential savings in development cost This paper reports on studies to identify a • other benefits and issues future family of gas turbine engines designed to satisfy a wide range of applications with the Through increased commonality it is maximum commonality. estimated that the development cost for a family of engines to meet three differing applications The propulsion requirements are examined could be reduced by over 25%-30% relative to for a range of applications including the the development of three independent engines. following: • commercial transports, e.g. regional jets A future engine family will benefit from the • military manned aircraft, e.g. fighters, introduction of new technologies, for example, primary and advanced trainers in the fields of aerothermal and mechanical design, environment (emissions and noise), low- • military unmanned aircraft, e.g. high altitude surveillance, strike and multi- observables, materials, controls, monitoring role systems and manufacturing. The paper These requirements cover the following examines technologies which support the attributes: concept of increased commonality. • performance, e.g. thrust, power off-take, inlet mass flow and fuel consumption • mass • environment, e.g. emissions, noise • low observables, e.g. infrared radiation, radar cross section • reliability and maintainability • cost, e.g. development, unit, in-service 1 S J BRADBROOK this analysis it will be shown how the requirements drive towards differing propulsion 1 Introduction system attributes and hence differing In the early days of gas turbine propulsion, architectures. In addition, the analysis will engines such as the Rolls-Royce Avon were indicate where there is scope for commonality used in both high performance military fighters, and hence shared technologies and hardware. In e.g. the Hawker Hunter, and in commercial particular, the paper will explore the concept of transports such as the Sud Aviation Caravelle, a common core approach to engine development with only relatively minor changes between and the technical difficulties this imposes. Mks. As time has progressed the differing requirements for commercial transports and Finally, the paper explores technologies combat aircraft have resulted in different which can be shared between types of engines engines for each of these classes of applications. and highlights those which support the common The benefits in terms of performance and core philosophy. operating cost, which have come from optimizing the engine design to the application, 2 Commonality have outweighed the extra cost associated with When discussing ‘common solutions’ it is engine development. This is particularly the helpful to define what is meant by commonality. case when a large number of units is involved in a particular programme. For example, the Fig 1 illustrates steps of increasing commonality Eurofighter engine programme was planned starting with a minimum of a common ‘house against an anticipated production of in excess of style’ and leading to common part numbers. In 1500 engines. the ‘Common Core’ approach, described in this paper, the aim is to have common part numbers In the future, it is possible that military within the core however in some cases a requirements will call for lower numbers of common architecture with common forgings more specialized vehicles such as: stealthy may be a more likely outcome. subsonic strike Unmanned Combat Air Vehicles (UCAVs), High Altitude Long Endurance (HALE) surveillance Unmanned Air Vehicles 3 Propulsion System Requirements (UAVs), and tactical reconnaissance UAVs. It is In this paper the propulsion system is therefore likely to become increasingly more defined as the basic engine, exhaust and difficult to generate an acceptable business case associated systems, e.g. controls, electrical for developing an all-new military engine with a power system. large step change in technology level. Fig 2 illustrates the considerable An alternative approach is to develop differences between the basic engines in a technologies and hardware that can be used in commercial subsonic transport (low specific more than one engine type suitable for differing thrust) and a military supersonic fighter (high applications, e.g. a strike UCAV and a specific thrust). Although these engine have commercial regional transport. In this way the similar levels of Sea Level Static (SLS) they overall development cost for both engines is have very different architectures which result in reduced, thereby making the business case more different levels of Specific Fuel Consumption attractive. (SFC) and mass. These differences are driven by the differing requirements of the each of the This paper investigates the differing applications as summarized in the following propulsion requirements for a number a sections. commercial and military applications. Through 2 COMMON SOLUTIONS TO COMMERCIAL AND MILITARY PROPULSION REQUIREMENTS Steps What is common? Pros and cons As ‘Step 3' plus Step 4 Part nunmbers As ‘Step 2 plus Step 3 Forgings Manufacturing tooling As ‘Step 1' plus Engine architecture Step 2 Turbo machinery aerodynamics & technology (same scale) Manufacturing facilities "House style" Turbo machinery Increasing commonality Increasing Step 1 aerodynamics & technology design compromises due to (different scale) savings cost unit Increased Lower development risk and cost risk Lower development Materials cost and unit mass increased and/or due to greater manufacturing volume due to greater manufacturing Greater potential for reduced performance, performance, reduced for potential Greater Company name tag Step 0 Fig 1 What is commonality? Effect of specific thrust Low specific thrust High specific thrust Specific thrust High Low SLS Thrust (lbf) 13500 14700 SLS SFC (lb/hr/lbf) 0.8 (approx) 0.4 (approx) Mass (lb) 1700 3600 Airflow (lb/s) 165 435 Fig 2 Comparison of commercial transport engine (low specific thrust) and military fighter engine (high specific thrust) 3 S J BRADBROOK and intake flow distortion, typical of a highly 3.1 Propulsion requirements vs aircraft agile military fighter. requirements NOx = Nitrous oxides, a pollutant Fig 3 identifies four types of commercial generated during combustion. transport aircraft and six types of military aircraft which are typical of those which require ‘Cyclic usage’ refers to the number of gas turbine jet propulsion. In addition, a number major throttle changes required during a mission of propulsion requirements are identified which which has an impact on the rate that Low Cycle are divided into the categories of: performance, Fatigue (LCF) life of various engine environment, low observables, reliability and components is used up. maintainability and cost. RCS = Radar Cross Section The figure indicates which requirements are typically the strongest and weakest drivers IR = Infrared Radiation for each of the applications through the use a High (H), Medium (M) and Low (L) symbol It can be seen that all the commercial where H indicates the greatest influence, applications are typically driven by low SFC, relatively speaking. environmental requirements and low in-service costs, via long life and low maintenance costs . It is noted that the absolute level of thrust, As the commercial applications tend towards although an important parameter when smaller vehicles with shorter range then the specifying propulsion systems for the various emphasis for low SFC is generally replaced by applications, is not included. The subject of greater emphasis on low purchase cost. thrust and its effect on engine scale is discussed separately in later sections of this paper. The military situation is generally more complicated. The majority of the military It should be emphasized that this is a applications have little or no requirement to generalized subjective assessment used to meet environmental standards. The relative illustrate the overall trends in propulsion emphasis on SFC and thrust/weight ratio varies requirements and, therefore, may vary where between applications. For example, a manned individual aircraft are concerned. supersonic fighter requires high maneuverability for air-to-air combat and therefore there is Notes: considerable emphasis on high thrust/weight. In contrast, a HALE UAV is required to have very “High T1 operation” refers to high air inlet long endurance and little maneuverability and total temperature generally encountered at