A Comparison of Finite Element Model Transition Methods for Launch Vehicle Flex Simulation

For large launch vehicles, it is necessary to simulate the structural dynamics response in flight. Due to the fuel depletion, in practice, a finite set of Finite Element Models (FEMs) for various propellant levels are created and used for time domain flight simulation due to computational constraints. This creates the problem of transition between two FEMs. To gain insight into the problem, a kinetic constraint posed by two different finite element models at the transition time is derived. Based on this result, this paper analyses and compares three existing transition methods using a free-free beam example. First, results from these methods are compared to the response of the beam with continuous varying mass, so the true performance of each method can be evaluated. FEMs including the same and different set of modes are also simulated. Both the initial conditions and the forced response are considered. As the outcome of this simulation study, the recent method proposed in a previous paper is shown to be the best candidate for FEM transitions for simulation purposes by accounting for the kinetic consistency following the transition in addition to the kinematic continuity at the transition time in other methods.