It’s no secret that rockets are extremely powerful. Launch thrusts are as high as 5,100,000 lbf (Falcon Heavy by SpaceX) for today’s rockets and 13,900,000 lbf (BFR by SpaceX) for rockets under development. With such massive forces at launch, it should be no shock that the engines, accelerations, and air resistance also create vibration loads great enough to shake critical components apart if they’re not designed properly. In order to design spacecraft to survive launch loads, engineers perform a dynamic analysis.
How Engineers Design Components To Withstand Dynamic Launch Loads
In order to prevent vibration loads from damaging critical components, engineers perform a dynamic analysis to ensure each component is designed to withstand launch loads while also minimizing excess material to reduce launch weight.
To perform a dynamic analysis of components, 3D finite element models of each component is generated. Once finite element models have been generated, each component first goes through a series of analysis cases including a dynamic analysis. The dynamic analysis is a multi-part analysis including a modal analysis, random vibration analysis, shock analysis, and load combination analysis.
In order to determine the resonant or modal frequencies and mode shapes of a component a modal analysis is performed. The resonant or modal frequencies of a components are the frequency at which the response amplitude are a relative maximum. When a component or structure vibrates at its resonant frequency it experiences large oscillations due to the storage of vibrational energy which can cause significant stress or even failure. A mode shape is the specific pattern of vibration executed by a mechanical system at a specific frequency. Engineers use the results of a modal analysis to analyze spacecraft for vibrations experienced during launch including random and shock vibrations.
During launch, broad band random vibrations are produced by a combination of engines firing, structural response, and aerodynamic turbulence. These vibrations are non-deterministic meaning that they can’t be precisely predicted. Through using statistics, random vibrations are analyzed for a range of potentialities.
A shock load is is used to describe a sudden force exerted on a structure such as a hammering action or a falling object hitting the ground. Spacecraft experience shock during mechanical actions such as release mechanisms for stage and satellite separation as well as deploying mechanisms such as unfolding solar arrays.
With the modal analysis completed, a dynamic analysis is done on the spacecraft in order to ensure that the vehicle can withstand loads from random vibrations and shock. This analysis is performed by using the mode shapes and resonant frequencies found as a result from the modal analysis.
Combined Load Cases
A combined load case is then generated which includes loads from shock, random vibrations and ongoing static loads. The combined load case results predicts the stress the structure experiences which is plotted as a stress contour.
Designing For Dynamic Loads
Through use of the stress contour plot generated during the dynamic analysis process, engineers compare the peak stress of a structure to the structure material’s properties to ensure that the allowable stress of a material is not exceeded. At this point it’s also common to check for locations that have excessive material which increases weight of a spacecraft adding significant unnecessary cost to a launch. With these results engineers will add and remove material as well as perform a redesign on sections in the finite element model to give a spacecraft more optimized geometries. With the updated design, engineers will then run another analysis on the new design to ensure that it meets performance specifications and that the excess material has been removed.
ASR Engineering provides mechanical engineering analysis & design services for spacecraft and other structures. Contact us today for a free quote!