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at sea level the skirt is generally short and highly angled, at least in comparison to a skirt designed for operations in space, which are longer and more gradually shaped. This means that a rocket engine that spends any significant amount of time climbing through the atmosphere cannot be optimally shaped; as it climbs the ambient pressure changes, so the exact shape and length of the skirt would have to change in order to maintain the proper pressure. Rocket designers have to select the sweet spot that is most appropriate to their needs, realizing that this will reduce thrust by as much as 30% at other altitudes.
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Generally simple in concept, the expanding nozzle is considerably more complex to build than it might seem. Engine bells must be cooled to avoid damage from the hot rocket exhaust, and this has presented problems in expanding nozzle designs. The cooling is normally accomplished by running either the
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the engine skirt is shaped to gradually flare out from the small-diameter exit from the combustion chamber, growing larger further from the chamber. The basic idea is to lower the pressure of the exhaust by expanding it in the nozzle, until it reaches ambient air pressure at the exit. For operations
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fueled engines) through tubing in the bell. With the bell moving, plumbing carrying the coolant to the bell has to be flexible and this increases complexity to the extent that the advantages of the design are often considered too costly. In the case of liquid hydrogen, the fluid also has the
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The expanding nozzle addresses this to a degree by including two skirts on a single engine, one inside the other. The first skirt, attached directly to the
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design that used staged combustion and generated about 250,000 lbf (1,100 kN) thrust. An enlarged version of the XLR-129 was proposed for the
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disadvantage of being highly reactive chemically, making a variety of common flexible materials unsuitable for use in this role.
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that, unlike traditional designs, maintains its efficiency at a wide range of altitudes. It is a member of the class of
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The XLR-129 featured an expanding nozzle. The extension is almost cylindrical compared to the inner section.
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The first engine design to include an expanding nozzle appears to be the
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boost-glide aircraft design that was entered as part of the
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nozzle extensions needing no coolant plumbing at all.
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For the aforementioned reasons modern designs (e. g.
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310:Categories
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260:References
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96:July 2022
248:See also
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