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Review and Evaluation of the Air Force Hypersonic Technology Program Glossary1 advanced concept technology demonstrations: U.S. Department of Defense programs that demonstrate the use of technology in a weapon system concept; the demonstration establishes a residual, usable military capability. adjoint analysis: a method of computation based on the reciprocal relation between a system of ordinary linear differential equations and the adjoint system; the matrix of the adjoint system is the conjugate transpose of the matrix of the original system. aerothermodynamic environment: the flight environment in which a high-speed vehicle’s aerodynamic properties are coupled with the thermodynamic properties, including heat transfer, of the constituent gases through which the vehicle is moving. avionics: electronic systems, subsystems, and components used in aeronautical or astronautical systems; usually refers to the flight computer and associated sensors and control devices used in the flight control system of an air or space vehicle. Castor IVB class booster: a launch vehicle capable of boosting an experimental test vehicle weighing 3,000 to 5,000 pounds to hypersonic speeds. catalyst: a solid substance, usually placed on the passage walls of a propulsion system, that causes the rapid endothermic decomposition of hydrocarbon fuels into desired compounds prior to combustion. circular error probable: the radius of a circle centered on the target, within which approximately half of a large number of shots at the target will fall. combined-cycle propulsion: a system capable of more than one mode of propulsion (e.g., the turboramjet, which provides turbojet propulsion at low speeds and ramjet propulsion at higher speeds). combustor: the portion of a propulsion device in which fuel is injected, mixed with air, and burned. concept exploration and definition: the earliest phase of the U.S. Department of Defense’s process for acquiring weapon systems, during which alternative concepts are evaluated and the most promising concept is determined. control effectors: devices used in a feedback control system to cause changes in the variable being controlled; in an air vehicle, effectors typically are the surfaces (e.g., fins, ailerons, and rudders) that can be moved to apply forces to the vehicle and cause it to change its trajectory. cracking (of fuel): the process of heating a hydrocarbon fuel in the presence of a catalyst, which causes one or more of the hydrocarbon bonds to break resulting in the production of lighter hydrocarbons. dehydrogenation (of fuel): the process of heating a hydrocarbon fuel in the presence of a catalyst, which frees hydrogen atoms. demonstration and validation: an early phase of the U.S. Department of Defense’s process for acquiring weapon systems, during which a system is designed and its critical technologies are demonstrated in early prototypes. dual-mode scramjet: a propulsion device that can operate with either subsonic combustion (i.e., as a ramjet) or supersonic combustion (i.e., as a scramjet). dynamic pressure: an aerodynamic quantity used as the reference value for lift and drag forces on a vehicle (i.e., the pressure of air on a flight vehicle); it is defined as one-half of density times speed squared. endothermic fuel (and endothermics): fuels that undergo chemical transformations when heated in the presence of a catalyst; of interest for hypersonic vehicles because of their large heat sink capacity. engineering and manufacturing development: a major, and expensive, phase of the U.S. Department of Defense’s process for acquiring weapon systems, during which the design is matured, manufacturing and production processes are validated, and the system is tested and evaluated. 1 The definitions in the glossary are simplified to assist readers who do not use these terms regularly to understand the text.
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Review and Evaluation of the Air Force Hypersonic Technology Program flame-holding mechanism: the device or process in an engine that allows combustion to occur continuously after ignition; usually contains a small recirculation region in the engine where the residence time of a fuel-air mixture is sufficient for combustion to occur. flow field: the distributed portion of the flow around a vehicle. free-jet wind tunnel: a special purpose wind tunnel for testing integrated airframe-engine components (e.g., airframe forebody, air intake engine, exhaust system, and airframe aftbody); allows the entire test article to be immersed in an unconfined jet of air (or other test gases) flowing from an appropriately sized nozzle at the desired simulated Mach number. hydrocarbon-based propulsion (hydrocarbon-fueled): air-breathing propulsion system that uses hydrocarbon fuels. hypersonic speeds: generally defined as speeds greater than five times the speed of sound (Mach 5); the more slender a vehicle is, the higher the Mach number at which hypersonic effects are in evidence; inside the engine, the term hypersonic refers to stagnation temperatures at which chemical reactions become important and simple models of gas behavior break down. initial operational capability: the point at which a newly acquired weapon system is ready to perform a military mission. initial operational test and evaluation: tests conducted during the engineering and manufacturing development phase of the U.S. Department of Defense’s acquisition process to ensure operational suitability. inlet contraction: the geometric degree to which the air stream captured by the engine is compressed. inlet relaminarization: under some flight conditions, the cooling of the boundary layer by the walls of the air inlet causes the turbulent boundary layer to return to a laminar state, which greatly increases its displacement thickness; this can also happen in the nozzle, although the cause is primarily rapid acceleration rather than cooling of the wall. launch offset: ability of a vehicle to acquire an orbital plane different from the plane of the launch site. launch window: exactly defined period within which the earth and other bodies (in orbit or interplanetary) are in the proper relationship for vehicle launch and optimum interception. liquid hydrocarbon: carbon-hydrogen compound with sufficiently low molecular weight that it is liquid at ambient temperatures but sufficiently high molecular weight that it is not a gas under the same conditions. Mach number: the ratio of the speed of a body to the speed of sound in the fluid through which the body is moving. milestone dates: the major decision points in the U.S. Department of Defense’s process for acquiring weapon systems; for example, Milestone 0 is the approval to begin concept exploration and definition; Milestone I is the approval to begin demonstration and validation; Milestone II is the approval to begin engineering and manufacturing development; and Milestone III is the approval to begin production and deployment. mission need: a deficiency in current capabilities or an opportunity to provide new capabilities (or enhance existing capabilities) through new technologies. multistage space launch: orbit-capable launch vehicles consisting of two or more stages, each having a propulsion system. nozzle recombination loss: losses of efficiency that result when the expansion is too fast for the chemistry to remain in equilibrium. operational requirements: a system capability or characteristic required to accomplish approved mission needs; usually performance parameters, but may also be derived from cost and schedule; for each parameter, an objective and threshold value must also be established. piloting the combustion (or combustor) process: a technique in which the combustion process of the flowing gases is supported by providing an adjacent region of high temperature, low velocity, combustion products (e.g., in a dual-combustion scramjet, a small fraction of the air is decelerated to achieve combustion, the products of which then promote the burning of, or “pilot,” the larger supersonic stream). production and deployment: a phase of the U.S. Department of Defense’s process for acquiring weapon systems, during which a system is produced, fielded, and supported, and its performance is monitored. pseudo-air: a mixture of gases that has the same volumetric concentration of oxygen as standard air (i.e., 21 molar percent), but the quantities of the other 79 percent of gases, such as nitrogen, carbon dioxide, carbon monoxide, water vapor, hydrogen, nitric oxide, and hydroxyl, are substantially different from the quantities present in standard air. pyrolyzed fuel: fuel that is chemically decomposed by heating in the presence of a catalyst, i.e., dehydrogenation or cracking. ramjet: a propulsion device that replaces the mechanical compressor of the turbojet engine with the natural compression of the incoming “ram” air; combustion occurs at subsonic speeds, and the device can operate effectively in the Mach 1 to Mach 6 range. refractory metals: metals having extremely high melting points (in the broad sense, metals having melting points above the range exhibited by iron, cobalt, and nickel); oxidation-resistant coatings are typically required for high-temperature use. scramjet: a supersonic combustion ramjet; above about Mach 5 or 6, it becomes impractical to decelerate the air flowing into the engine to subsonic speeds prior to combustion, and supersonic combustion is required to generate thrust.
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Review and Evaluation of the Air Force Hypersonic Technology Program specific impulse: a basic propulsion performance parameter measured in seconds (also, thrust per unit weight of fuel flow per second, which is measured in seconds); the higher the specific impulse, the better the performance of the stored propellant, which is generally only fuel for airbreathing engines and fuel plus oxidizer for rockets. speed of sound: in air at sea level and at a temperature of 59°F, sound travels approximately 1,116 feet per second; as the temperature diminishes, the speed of sound decreases (e.g., at the temperature associated with an altitude of 100,000 feet, the speed of sound is approximately 990 feet per second). subsonic: less than the speed of sound (i.e., < Mach 1). supersonic speeds: generally considered between approximately Mach 1 and Mach 5. supersonic diffuser: the contracting duct or compressing part of an engine inlet. system design integration: the complex, multidisciplinary engineering task that includes selecting technologies, performing design trade-off studies, and generating a balanced system design that meets requirements and can be implemented. technology maturity levels: the degree to which a technology has been developed, ranging from level one for basic principles observed and reported, to level six for system or subsystem model demonstration in a relevant environment, to level nine for an actual system that has been flight-proven in mission operations. transitions: changes from one scramjet operating mode to another without disrupting the overall process (e.g., the change from subsonic combustion to supersonic combustion while maintaining continuous thrust output). wooden round: a term indicating that a weapon can be stored for long periods of time with little checking or maintenance and can be employed on short notice when taken out of storage.
Representative terms from entire chapter: