Analysis of aerodynamic loading on a rocket and design

Abstract

The University of Queensland's Centre for Hypersonics are investigating the viability of a reusable three-stage, rocket-scramjet-rocket based access to space system, aimed at reducing the cost to orbit for small satellites (Preller & Smart, 2015). The final stage of this system is a conventional liquid fuelled rocket, designed to carry the satellite into its final orbit after being released from the second stage scramjet (Preller & Smart, 2015). Previously, the third stage vehicle was designed for exo-atmospheric operations, but new trajectories place the separation point well within the atmosphere, exposing the vehicle to high dynamic pressures (up to 50 kPa), at large angles of attack (10 degrees) (Forbes-Spyratos, Kearney, Smart, & Jahn, 2017;Preller & Smart, 2015). The large angle of attack on the vehicle resulted in lift dominating the aerodynamic forces, as opposed to in a conventional rocket where drag dominates. As such, it was hypothesised that conventional rocket design literature may not be applicable for the design of the third stage vehicle (Benson, 2014). The purpose of this thesis was to evaluate this hypothesis, to determine if rocket based design and optimisation literature could be adapted to a lift dominated, rocket like structure, in particular, the third stage vehicle.Phase One of this thesis involved reviewing available rocket design and optimisation literature and adapting it to develop a parametric CAD model of the third stage vehicle. The CAD model consisted of two main assemblies, an external aeroshell and the internal structure. Due to the atmospheric release point of the vehicle, the protective aeroshell needed to resist all aerodynamic loads, making it a critical component of the current design which needed to be validated.Phase Two then involved undertaking finite element analysis on the third stage to evaluate the applicability of using conventional rocket optimisation literature in the design of the third stage vehicle. To achieve this goal, static and buckling simulations of the third stage aeroshell were undertaken in ANSYS for freestream dynamic pressures ranging from 30 to 50 kPa (to simulate different release altitudes). For the dynamic pressures tested the aeroshell was statically 3.6 to 6.0 times stronger than necessary. Buckling was also not a critical failure mode. It was concluded that the restrictions imposed on the stringer and backing thicknesses of the aeroshell, due to the literature optimisation techniques implemented, resulted in the vehicle being heavily overdesigned for the applied loads. This suggested that the adaption of conventional rocket optimisation methods for the vehicle in this thesis was not the most effective method to optimise the load bearing structure. The final optimised configuration of the aeroshell weighed 281 kg, but to further reduce the safety factors and vehicle mass, evidence suggested that the backing and stringer thicknesses of the third stage vehicle needed to be reduced.ii