Study on Actuator Types Within TREETOPS: A Multibody Dynamical Simulation*

Multibody dynamical simulations are used widely at NASA for vehicle design, vehicle certification, and flight dynamics. Marshall Space Flight Center (MSFC) has designed a multibody flight dynamics tool chain called CLVTOPS. This tool chain consists of high fidelity simulations, geometric algorithms, advanced data analytics, and post flight telemetry to demonstrate positive clearance between separating bodies. CLVTOPS is currently being used for analysis of the Space Launch System (SLS) and Mars Ascent Vehicle (MAV). At the foundation of this tool chain another simulation tool, TREETOPS, models forces by using a function generator that feeds the force magnitude into an actuator. TREETOPS has multiple built in force actuators that can apply forces in different ways. The Liftoff/Separation Dynamics team at MSFC has primarily used jet type actuators, also known as J type, to model forces for spring separation events. J type exhibits a single force vector onto a node on a body. To get an equal and opposite reaction, one must apply two J type actuators of equal and opposite forces. However, J type struggles to accurately model dynamics in certain situations, such as events with longer separation times or high initial tipoff rates. This study focused on the hydraulic type actuator, also known as H type. H type actuators couple two nodes and exhibit equal and opposite forces on the line of action between them. Due to this, H type exhibits lateral forces in response to misalignment angles between nodes, unlike J type which remains unaffected by misalignment angles. Multiple studies were conducted comparing J type and H type actuators. H type actuators were found to be a viable method of modeling forces for spring separation systems, capturing dynamics that J type does not, such as slightly higher body rates. However, an axial distance between nodes must be implemented to avoid exaggerated lateral forces. J type and H type will be used as a lower/upper bounding analysis for body rates in future analyses of the SLS. There are many spring driven separation systems that are analyzed using CLVTOPS – implementing this bounding analysis will add further security to mission requirement verification done by the Liftoff/Separation Dynamics team at MSFC.