Basic Nozzle Structure
The basic structure of both external and submerged nozzles (fig. 1) is subjected to internal pressure loads and flight loads. The internal pressure load is divided into an axial ejection (blowout) load and an opposing axial thrust load; the flight loads include aerodynamic loads, inertial loads, and vibration loads. In addition to these loads, the submerged structure of a submerged nozzle is subjected to chamber pressure loads. If the nozzle is used with an attached TVC system, the nozzle structure must support the attached system and react the localized loading produced by TVC.
The configuration of the structural components depends primarily on the most critical design requirement imposed. The governing design requirement generally will fall into one or more of the following four categories:
(1) Strength limitations. — The configuration is determined by the ability of the component to withstand the imposed stresses without exceeding the material design strength.
(2) Deflection limitations. — The configuration is designed to limit a particular displacement to a predetermined critical value in order to limit strain in the liner and insulator components supported by the structure.
(3) Stability limitations. — The configuration is designed to prevent buckling.
(4) Economic limitations. — Program expense limitations prohibit the use of the optimum design.
To a varying extent, economic limitations enter into all designs. For example, in the exit shell of a small nozzle for a low-pressure motor, a shell thickness of 0.001 in. may satisfy stress, deflection, and buckling; but the expense of fabricating so thin a shell likely would be prohibitive even if fabrication were technically feasible.
The first step in design of a nozzle structure is to determine the loads and subsequent load combinations that will be applied. The primary sources of internal loads on a nozzle structure are the internal pressure of the gas and the tendency of the liner materials to expand when heated. Thermal loads generated by such expansion often are the major factor in structural design. These loads produce a complex state of stress in the structural shell, the insulator, and the erosion-resistant liner. Pressure distributions are determined with a high level of confidence by reasonably-well-defined laws for compressible flow; however, the thermal loading is predicated on material property data that can be reliable or uncertain, the validity depending on the particular material. Other internal loads that must be considered when they exist include the higher-than-normal pressure distribution that can exist in the nozzle exit section during aft-end ignition and the high-frequency flow oscillation sometimes occurring in high-area-ratio nozzles during ignition transient. In some cases, dynamic excitation of the exit-cone section can induce excessive loading and therefore must be considered. During motor tailoff, the internal pressure on the exit of an overexpanded nozzle may be lower than atmospheric pressure, and the exit cone may collapse.
Additional possible sources of loads are listed below:
(1) Operational external loads
(a) TVC system
• Asymmetrical internal pressure distribution
• Mechanical support and attachment
(b) Flight trajectory environment
• Dynamic pressure
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