Date of Award

Summer 7-25-2023

Access Type

Thesis - Open Access

Degree Name

Master of Science in Aerospace Engineering

Department

Aerospace Engineering

Committee Chair

Mark Ricklick

Committee Advisor

Mark Ricklick

First Committee Member

Lakshman Narayanaswami

Second Committee Member

Sandra Boetcher

College Dean

James Gregory

Abstract

Rocket Based Combined Cycle (RBCC) engines have been theorized as a possible means of powering launch vehicles and high-speed atmospheric vehicles. By incorporating aspects of both air-breathing and rocket propulsion, RBCC engines promise up to a 230 % increase in specific impulse over traditional chemical rocket propulsion by entraining a secondary flow of atmospheric air and mixing it with the exhaust of a rocket motor. Students within the Embry-Riddle Future Space Explorers and Developers Society (ERFSEDS) identified a
problem of excessive heating and structural failure of the mixing duct during launch and transonic flight of a student-built flight test vehicle. In order to be a feasible means of propulsion, adequate cooling of the mixing duct is necessary to achieve long burn times. Research revealed a lack of studies on the thermal trends and cooling of RBCC engine mixing ducts in the non-classified literature. This study utilized computational fluid dynamics (CFD) to analyze the flow mixing behavior and resulting thermal trends on the walls of the mixing duct. The two parameters investigated were the effects of varying the mixing duct length to diameter (L/D) ratio as well as the effects of different phases of flight on the thermal trends within the mixing duct. Launch, transonic, supersonic, and high-altitude cases were all selected from a reference trajectory. Additionally, the interaction between the entrained secondary flow of air and the mixing duct walls was of interest to see if the secondary flow had a natural capacity to film cool the mixing duct walls. Thermal trends within the mixing duct were characterized through the analysis of wall temperatures, wall heat transfer coefficient, and film cooling effectiveness. Additionally, contour plots were used to qualitatively evaluate internal flow behaviors. After modelling was completed, it was discovered that increasing mixing duct L/D to values over 7 resulted in greater flow mixing, but heightened thermal stresses on the mixing duct. Following analysis the four different phases of flight, it was determined that the high-altitude phase of flight posed the highest thermal stresses on the mixing duct with 79.6 % higher peak wall temperatures and 67.5 % higher heat transfer coefficients compared to the supersonic case, which posed the lowest thermal stresses. This study indicates that RBCC engine designs should avoid long mixing ducts and flight conditions with highly under-expanded primary flows that promote harsh thermal conditions.

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