A numerical simulation for prediction of thermal ablation in composite rocket motor casing

Thermal protection systems (TPS) are used in space applications to protect structures failing from burning and/or excessive temperatures. In this work, a finite element simulation is performed to analyze the behavior of a composite rocket motor casing during the expansion of combustion gases inside the motor. A two-dimensional axisymmetric model of a rocket motor casing provided with an insulating liner is modeled in a finite element software ANSYS. Variable equivalent heat flux at the inside faces of the liner, due to radiation and convection of gases, is estimated and applied as a boundary condition. The reduction of heat load with time due to latent heat of fusion and the resistance offered by char that exists above the pyrolysis front is also considered. At the same time, the material properties of the portion of the liner exposed to its melting point temperature are regulated to offer negligible resistance to move the boundary load on to the pyrolysis front at every instant. A transient analysis is carried out with appropriate mesh quality and time steps for 10 s. Ablation, charring, and unaffected regions are identified and the required insulation liner thickness is recommended. Extension of the procedure to model a similar motor with any other cylindrical length is discussed.

Prediction of thermal ablation in rocket nozzle using CFD and FEA

To protect the structural part of the rocket nozzle, an insulation liner is provided at its inner surface. Charring ablators are used for this purpose. The thickness of the insulation liner is one of the major design considerations of the nozzle. In the present analysis, an attempt has been made to predict thermal erosion (ablation) in the insulation liner through numerical studied CFD and FEA. The problem is modeled in ANSYS software. Fluid flow analysis is performed using the fluent module that works on the finite volume method, and the transient thermal module is used for the thermal analysis that works on the finite element method. Appropriate mesh convergence, residual convergence, and time step convergence exercises are made and the numerical results are verified with the analytical solution wherever possible. The possibility of reduction of thermal load due to the presence of char in ablator liner is considered and the thermal resistance in the region exposed to melting point temperature is altered to permit the propagation of thermal loads to the current pyrolysis front at any instant. The concept of this work is useful in the prediction of thermal ablation in rocket nozzles and in selecting the required thickness of the insulation layer.

DISBOND DETECTION TECHNIQUE FOR LINER/PROPELLANT INTERFACE USING ULTRASONIC RESONANCE AND LAMB WAVES

Adhesive interface tests using ultrasonic waves are far superior to other nondestructive tests for detecting the disbond interface. However, a multilayered structure consisting of a steel case, rubber insulation, liner, and propellant poses many difficulties for analyzing ultrasonic waves because of the superposition of the reflected waves and the large differences in the acoustic impedances of the various materials. Therefore, ultrasonic tests for detecting the disbond interface of multilayered structures have been applied in very limited areas between the steel case and rubber insulation using an automatic system. The existing ultrasonic test cannot detect the disbond interface between the rubber and propellant of a multilayered structure because most of the ultrasonic waves are absorbed in the rubber material, which has low acoustic impedance. This problem could be overcome by amplifying the ultrasonic waves using the ultrasonic resonance method. The Lamb waves were used to evaluate the instability of the ultrasonic waves caused by the contact condition on the surface of the multilayered structure. In this paper, a new technique to detect the disbond interface between the liner and propellant using the property of ultrasonic resonance and Lamb waves is discussed in detail.