Date of Award
2026
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Mechanical and Aerospace Engineering
Committee Chair
Robert Frederick
Committee Member
Gang Wang
Committee Member
Judith Schneider
Committee Member
Jeffery Williams
Research Advisor
L. Dale Thomas
Subject(s)
Rocket engines--Design and construction, Rocket engines--Reliability, Additive manufacturing, Systems engineering
Abstract
Rocket engine development has historically relied on test–fail–fix (TFF) methodologies to uncover structural and fatigue deficiencies under representative operating conditions. While effective, TFF is inherently costly and schedule-intensive, often revealing life-limiting behavior late in the development lifecycle. The increasing use of additive manufacturing (AM) amplifies this challenge by introducing material property scatter and process-induced variability. Without explicit early treatment of uncertainty, these effects can increase reliance on physical testing and drive additional TFF cycles. This work presents a reliability-driven life-consistent design-bounding framework intended as an early lifecycle decision-support tool to assess and mitigate TFF risk. Although material variability is inherently aleatory, it is treated epistemically as bounded worst-case behavior through conservative knockdown factors derived from available data. Only known epistemic uncertainties, such as measurement and modeling noise, are propagated probabilistically. Probability is therefore used solely to establish safe and interpretable limit states, enabling material behavior to be decoupled from uncertainty in a manner not achievable with traditional deterministic Factor of Safety (FoS) approaches. The resulting limit states define credible elastic operating bounds that are mapped through calibrated Basquin-Coffin-Manson (BCM) strain-life fatigue models validated with experimental data to quantify remaining strain capacity at a specified target life. A strain amplification metric (SAF) and total strain allowables are derived for laser powder bed fusion (LPBF) and laser powder directed energy deposition (LP-DED) Inconel 718 and Inconel 625, LP-DED NASA HR-1, and LPBF GRCop-42 at both room temperature and elevated temperature conditions. The methodology is validated against SSME-era Inconel 718 data and probabilistic studies, demonstrating consistency with heritage material behavior. The SAF’s and strain-based allowables provide actionable inputs to drive higher-fidelity finite element analyses, enabling fail-fix iterations to be conducted within a digital realm rather than during physical testing. The mathematical underpinnings are implemented within a Model-Based Systems Engineering (MBSE) framework in a forward-looking manner, establishing an executable and expandable foundation for integration with performance models, reliability assessments, and test planning as designs mature. The approach is demonstrated for both RS-25 and Nuclear Thermal Propulsion applications, showing architecture-agnostic applicability. This methodology does not replace TFF, but transforms it by minimizing test-induced failure incidence through informed digital exploration of the design space prior to testing.
Recommended Citation
Lakshmipuram Raghu, Shreyas, "Utility of model-based and probabilistic approaches for rocket engine affordability : exemplified on the RS-25 engine" (2026). Dissertations. 484.
https://louis.uah.edu/uah-dissertations/484