SolidPropellant Vehicles
Solid-propellant vehicles have a compact structural design and have been modeled by both the lumped-parameter and continuous-model methods. In solid-propellant vehicles, the propellant is usually bonded to the case wall. Simulation of the
Lumped masses Shroud
Pay load
Tank wall and dome
Propellant
Engine Aft skirt
Lumped masses Shroud
Pay load
Tank wall and dome
Propellant
Propellant
Tank wall and dome and interstage
Propellant
- Tank wall and dome
Propellant
Tank wall and dome Propellant
Tank wall and dome, interstage and engine
Tank wall and dome and interstage
Tank wall and dome
Propellant
Tank wall and dome
Dashpot
Idealized longitudinal spring
Lumped mass
Idealized longitudinal spring
Figure 6
Simplified longitudinal dynamic model of space vehicle held to launch stand longitudinal elastic properties of this composite structure is difficult, in part because of the effects of dynamic interaction between the propellant and the case wall. Because of its structural properties, the propellant contributes little to the stiffness of the composite structure. However, because of its viscoelasticity, the propellant does provide considerable damping.
Lumped-parameter models with spring-mass analogies of solid propellants require special treatment to account for the effective mass and shear stiffness of the solid-propellant core and the stiff outer casing. For example, reference 14 discusses a model in which springs, masses, and dampers of the propellant segments were chosen so that they would have the same frequency and damping as the computed first shear mode of the propellant grain. This approach not only requires separate calculations of the dynamic behavior of the propellant, but also careful integration of the propellant mass into the model. Calculated responses using this approach have compared closely with experimental data, and as a result this technique is often used for analyzing solid-propellant vehicles.
The continuous-model approach is based on the assumption that the solid-propellant segments can be represented by a continuous-beam model with mass, stiffness, and viscoelastic properties uniformly distributed along its length. The dynamic characteristics of such a model, including the stress-strain relationships for solid-propellant materials, are discussed in reference 15. Once a continuous model of the solid-propellant segments has been obtained, the remainder, of the vehicle structure, modeled by a lumped-parameter analogy, can be interconnected with the solid propellant by the technique described in reference 16. This combination of approaches is straightforward and poses no difficulty in synthesizing a model.
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