Constraint-First Engineering as Epistemic Method: A Solar Flash-Boiler Rankine Engine for Vacuum

Abstract This paper is written as a demonstration of a constraint-first epistemic method: a design is treated as real only to the extent that its limiting constraints are explicitly identified and shown to dominate the behavior of the system. To illustrate this method, I analyze a compact solar thermal engine intended for vacuum environments: a closed-loop flash-boiler Rankine cycle driving a microturbine generator and reset by radiative condensation. The concept is mechanically straightforward–sunlight heats a sealed boiler, fluid flashes through a pressure drop, vapor drives a turbine, and exhaust is condensed by radiator panels and returned to the boiler. What determines whether this is viable is not the elegance of the cycle diagram, but three coupled constraints: radiator heat rejection capacity, condenser saturation pressure (backpressure), and two-phase vapor quality stability at the turbine inlet. Under these constraints, performance is governed less by “efficiency” in the abstract and more by the ability of a finite radiator to hold the condenser at a temperature that enables meaningful pressure ratio without collapsing flow stability. This paper therefore functions both as a technical proposal and as a methodological proof: the system becomes legible only when its constraints are treated as first-class objects. ⸻ 1. Introduction (Method Framing) Most “concept engines” fail for a predictable reason: they are described as if the environment behaves like a classroom. In vacuum, the environment does not care about diagrams. There is no air, no convection, no “heat just goes away.” If waste heat cannot be rejected radiatively, the system saturates, pressure rises, the turbine loses expansion ratio, and output collapses. This is the first rule of the method: A system is real only within its constraints. If the constraints aren’t named, the system isn’t specified. Vacuum power generation therefore forces a hard epistemic shift: the central limiting resource is not fuel, but the ability to dump entropy via radiation. With stable solar input available, the feasibility question becomes: - Can the condenser hold a low enough temperature to set a low enough saturation pressure? - Can the radiator reject enough heat at that temperature? - Can the expansion stage remain stable and non-destructive under two-phase flow? This paper answers by proposing a solar flash-boiler turbine and then showing that its viability is dominated by these constraints.