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Liquid rocket engine operational factors can be described in terms of extremes: temperatures ranging from that of liquid hydrogen (-250°C) to circa 3300°C hot gases1, enormous thermal shock (3800ºC/sec), large temperature differentials between contiguous components, reactive propellants, extreme acoustic environments, high rotational speeds for turbomachinery and extreme power densities. These factors place great demands on materials selection, and each must be dealt with while maintaining an engine of the lightest possible weight. Combustion chambers and nozzles must be cooled to prevent them from melting due to the heat of combustion. Cooling methods include regenerative, dump, film, transpiration, and ablative, or some combination of these. Commonly used for cooling is a double-walled design through which fuel (and sometimes oxidizer) is circulated to extract the heat. For directional control of the vehicle, gimbaling of the engine or vanes in the exhaust stream may be used; either of these requires actuators. Sensors are required at key locations to monitor the engine performance and make appropriate propellant flow changes or allow for rapid shut down if an anomaly is detected. All of these elements must come together in an efficient, light-weight package which operates smoothly, efficiently, and reliably. The materials of construction are key elements to achieving the goals of performance, reliability, and relatively lightweight. Materials have been vital from the very beginning of rocketry science. Dr Robert Goddard1 used an eclectic combination of materials – aluminium tubing, ceramic-lined aluminium combustion chambers, ceramic coated tool steel nozzles, asbestos thermal protection, pump housings made of aluminium, brass and steel impellers, aluminium alloy turbines, etc. Material usage progressed to increasingly higher-strength and higher-temperature alloys, as pressures and temperatures increased to achieve higher performance. More recently, non-metallic materials such as ceramics, ceramic matrix composites, and even polymetric composites have been considered for specific applications. Environmental factors are a major concern in the selection of materials for liquid rocket engines. Most of the propellants are reactive fluids. Oxygen, hydrogen, nitrogen tetraoxide, hydrazine and red fuming nitric acid – all of which have potentially detrimental effects on materials. Their effects vary considerably from material to material, but at one time or another, each has caused problems. The specific environmental effect of propellant must be taken into account in the material selection and design process. One of the challenges is to develop suitable screening tests to evaluate the effect of a propellant, under expected operational conditions, on the materials of choice. Efforts also have been made to develop materials that are tailored to the operational environment. Considerable work has been expended in developing materials such as hydrogen-embrittlement-resistant and oxygen-ignition-resistant alloys. Development of new materials for rocket engine application requires a lengthy development process. This involves advancing the material technology readiness from understanding the fundamental material properties, to developing mechanical properties over the anticipated operating range, developing manufacturing processes and establishing a design database. A considerable financial investment is required to develop, certify, and incorporate new materials into components for a flight system. The material selection for a space propulsion engine is determined by five general factors1: the size of the engine; the engine duty cycle (expendable or reusable); the propellants; the turbine drive cycle; and the stage at which the engine will be used (booster or upper stage). Large engines today almost always require extensive use of metals. Small engines (in the range of a few kilograms to a few thousand kilograms thrust) can be made from ablatives, high-temperature alloys, or high-emissivity alloys. In general, the higher the combustion chamber pressure in a rocket engine, the greater the material issues for two main reasons. First, higher pressures mean that makes material strength, particularly specific strength (strength-to-weight ratio) becomes very important. At high operating pressures, material thickness and component weight can become unwieldy unless materials of suitable specific strength can be employed. Second, high strength materials tend to be more complex and difficult to fabricate. With some notable exceptions, high-strength materials also tend to have toughness and ductility issues2. They also tend to be more susceptible to environmental effects. Hence, careful screening and thorough material characterization are necessary. For material selection, the engineer must first know and understand the duty cycle of the particular