Expanding nozzle
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The expanding nozzle is a type of
In the traditional bell nozzle 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 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.
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 combustion chamber, is designed for use at lower altitudes and is short and squat. The second, sitting outside the first, fits over the lower altitude bell to extend it into a longer and narrower (measured in terms of length) bell used for higher altitudes. At liftoff the outer bell is pulled up from the inner bell, out of the way of the exhaust. As the spacecraft climbs, the outer bell is pushed back down over the inner bell to increase the thrust efficiency. Thus an expanding nozzle can have two sweet spots, which can lead to a major improvement in overall performance.
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 oxidizer or fuel (in the case of
For the aforementioned reasons modern designs (e. g.
The first engine design to include an expanding nozzle appears to be the
See also
References
- ^ "Pratt & Whitney GTF™ Engine MRO Network Expanding to Eight Engine Centers by 2020". www.epicos.com. Retrieved 2022-06-08.