Thermodynamics of bubble-through liquid-core nuclear thermal rockets

Author

• Dakota John Santana

Date of Award

2026

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Aerospace Systems Engineering

Committee Chair

Keith Hollingsworth

Committee Member

Jason Cassibry

Committee Member

Robert Frederick

Committee Member

George Nelson

Committee Member

Babak Shotorban

Research Advisor

Keith Hollingsworth

Subject(s)

Space vehicles--Propulsion systems, Nuclear propulsion, Bubbles--Thermodynamics, Liquid-core nuclear thermal rocket (LNTR)

Abstract

Liquid-Core Nuclear Thermal Rockets (LNTR) were originally investigated in the 1960s. Exploratory efforts have thus far been relegated to preliminary models that illustrate proof-of- concept or identify theoretical limits. Collectively they would report ranges for 𝐼𝑠𝑝 between 1000 −2000 𝑠 and thrust-to-weights between 0.5 −6. This work expands a model for the Bubbler class of LNTRs into an engineering model suitable for design exploration and system model integration. Bubblers use centrifugal force to trap molten fissile fuel while bubbling propellant radially inward to cool structural members and achieve temperatures >4500 𝐾. The Bubbler model detailed herein, dubbed Fornax, expands the fidelity of earlier models by incorporating a more comprehensive bubble velocity model, bubble-induced convection, vaporization, updated material databases, real rocket nozzle modeling, melt plane locator, and chamber temperature optimization. Fornax reveals the existence of temperature optimizing bubble radii (5 −50 𝜇𝑚, calculatable as a function of fluid properties and centrifugal acceleration) and fuel layer thickness (inner-to-outer radius near 2 3 ⁄ ) whereas previous efforts sought to maximize these parameters. Additional iii parameters are investigated to develop reduced-order power scaling laws for optimized chamber temperature operations. Computational Fluid Dynamics (CFD) investigations into bubble heat transfer mechanics are conducted to inform Fornax due to a lack of literature under relevant conditions. The results show that the high thermal gradients in Bubblers (1000𝑠 𝐾𝑐𝑚 ⁄ ) produce a thermal response more dependent on wake behavior than is evident in their pure step input response. The wake entrains downstream fluid, effectively increasing the thermal mass of the bubble. As a result, the thermal settling time increases by 2 orders of magnitude under explored conditions. This effect appears significantly weaker for spherical bubbles with less defined wakes. A modified lumped- capacitance model is used to represent the response to both initial temperature differences and a liquid phase temperature gradient. Overall, Fornax predicts an upper 𝐼𝑠𝑝 limit of 1300 𝑠, limited by 𝐼𝑠𝑝 deterioration due to vaporization. If fuel vapor can be extracted from the exhaust, it may be thermodynamically feasible to obtain an 𝐼𝑠𝑝 of 1500 𝑠 limited by the need to balance containment wall cooling needs with the consequences of increasing void fraction.

Recommended Citation

Santana, Dakota John, "Thermodynamics of bubble-through liquid-core nuclear thermal rockets" (2026). Dissertations. 509.
https://louis.uah.edu/uah-dissertations/509