Gigawatt-Class Terrestrial Legged Vehicles: A Nuclear-Thermal, Pulse-Power, and Electrohydraulic Systems Architecture

This paper develops a first-principles systems architecture for gigawatt-class terrestrial legged vehicles operating in an extreme mobility regime defined by high-speed translation, rapid directional changes, and transient hover-assisted maneuvering. In this regime, systems must simultaneously satisfy multi-gigajoule onboard energy storage, gigawatt-scale transient power delivery, megawatt-scale continuous recharge, and meganeuton-scale ground interaction forces---requirements that exceed the capability envelope of any single existing technology. We show that no physically realizable subsystem can simultaneously achieve high energy density, high power density, and high response bandwidth, establishing an inherent energy--power--time incompatibility. This constraint implies that monolithic powertrain architectures are fundamentally insufficient. As a consequence, we propose a layered energy architecture in which these requirements are distributed across distinct subsystems: (i) nuclear thermal power cores for continuous high-density energy generation, (ii) pulse-power buffering layers for transient gigawatt-scale events, and (iii) distributed electrohydrostatic actuation modules for high-bandwidth force generation and terrain interaction. Advanced nuclear thermal propulsion concepts---including supercritical-flow nuclear thermal reactors (SF-NTR) and acoustically enhanced heat transfer mechanisms---are reinterpreted as compact, high-pressure thermal power sources suitable for terrestrial integration. A quantitative mobility envelope is derived for a representative 70-ton tetrapod platform, yielding requirements of 40--60 MW continuous power, 350--500 MW sustained maneuver power, and 0.8--1.0 GW transient burst power. To partially address the absence of formal system closure, a minimal multi-timescale dynamical model is introduced, capturing the coupled evolution of energy storage, thermal state, and mechanical output. This formulation demonstrates that maneuver authority is intrinsically constrained by energy and thermal states, rather than being an independent control variable. The resulting architecture defines a new class of terrestrial systems in which energy, power, and force are explicitly decoupled and coordinated across hierarchical time scales. While the framework remains conceptual and lacks empirical validation, it establishes a physically consistent operating regime, identifies fundamental architectural constraints, and delineates the key theoretical and technological challenges required to advance toward realizable systems.