Structural Assessment of NEP Vehicles For Human Mars Missions

Abstract

NASA’s Space Technology Mission Directorate is studying Nuclear Electric Propulsion (NEP) as an attractive option for in-space propulsion for exploration missions to Mars and beyond. NEP offers virtually unlimited energy density and a specific impulse (Isp) roughly twelve times that of the highest performing traditional chemical systems. One drawback of NEP, relative to high-thrust options such as Nuclear Thermal Propulsion, is that NEP generates relatively low thrust during electric propulsion (EP) operation. For round-trip human missions to Mars, this low thrust capability is not an issue for the long multi-month transfers between Earth and Mars, but is an issue for getting in and out of the Earth and Mars gravity wells. One solution to this issue is to utilize a high-thrust chemical propulsion system to perform these gravity well maneuvers. This hybrid approach of chemical propulsion for gravity well maneuvers and NEP for maneuvers between gravity wells is currently under examination by both NASA and Aerojet Rocketdyne (AR). In support of NASA, AR has performed an architecture trade study which includes: multiple mission Concept of Operations (CONOPS) and multiple launch vehicle options with a goal of minimizing the number of launches, NEP/Chem vehicle mass, and cost. The AR NEP/Chem configurations utilize a liquid oxygen (LOX) / liquid methane (CH4) chemical stage with two 25K Lbf thrust engines, two nuclear reactors, radiators, EP thrusters, and xenon propellant tanks. The NEP/Chem vehicle would be assembled in Low Earth Orbit (LEO) before transferring to Lunar Distance High Earth Orbit (LDHEO) where it will rendezvous with the Deep Space Habitat. The backbone of the NEP/Chem vehicle is a composite truss structure. The focus of this paper is the design and analysis of the composite truss structure under launch and flight loads. The launch analysis uses the launch truss configuration, applies the NASA Space Launch System (SLS) launch loads, and reviews the critical fundamental modes, deflections, and stresses. The flight analysis considers all the CONOPS and applies the loads from the NEP/Chem vehicle chemical stage engines. The analysis determines the stresses, modal frequencies, and deflections resulting from the flight loads. The AR structural analysis methodology was used to assess all launch and flight loads. Also a dynamic transient analysis was performed to consider dynamic amplification from the chemical engines due to start-up and shut-down transients. The structural analysis was performed using a simplified Finite Element Model (FEM) from CAD model (Creo) and analyzed using Finite Element Analysis (FEA) Software (Creo Simulate). In conclusion this paper will identify what the design driving loads (both launch and flight) are on the NEP/Chem vehicle primary composite truss structure and optimizing for minimum mass.