Abstract
Water is a practical candidate for small-spacecraft propulsion because it is dense, non-toxic, inexpensive, and relevant to future in-space resource use. It is not, however, one propulsion technology. Water may be heated and expanded in a resistojet, electrolyzed into H2/O2, used in hydrolytic gas generation, or fed to electric thrusters. This paper proposes an Evidence-Weighted Mission Architecture Suitability Index (EW-MASI) to compare these families under mission-specific priorities. The framework combines source quality, fixed 1-5 architecture scores, evidence-confidence factors, mission weights, score ranges, and rank-stability checks. Using a targeted 2011-February 2026 evidence base, the results show that electrothermal water resistojets are the strongest near-term choice for conservative CubeSat orbit correction, deorbit support, precision RCS, and demonstrated deep-space CubeSat correction. Electrolysis-driven systems are promising for stored-energy impulsive maneuvers but remain integration-limited. Hydrolytic concepts require stronger repeatability and residue-control evidence. Water-fed electric propulsion is most suitable for long-duration low-thrust missions. EW-MASI is intended as a transparent screening tool, not a substitute for thruster design, qualification, or mission optimization.