Date of Award

2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Committee Chair

Robert A. Frederick, Jr.

Committee Member

James J. Swain

Committee Member

K. Gabriel Xu

Committee Member

George J. Nelson

Committee Member

James K. Baird

Subject(s)

Solid propellant rockets--Combustion., Rockets (Aeronautics)--Fuel--Research., Sulfur., Fluoropolymers.

Abstract

Widening the flammability limits of solid fuels used in ramjet engines broadens the operational range of altitudes and speeds and enhances the possibility of autoigni- tion from incoming air alone. This experimental research studies how to decrease the solid fuel lower flammability limit while maintaining or improving other key thermal and mechanical properties. Candidate fuels are formulated using epoxy cured liquid polysulfide doped with sulfur and a perfluoropolyether. Sulfur doped formulations are hypothesized to produce greater mass fractions of low ignition temperature species to widen flammability limits, but degrade low-temperature mechanical properties. Perfluoropolyether is added to the mixtures to preserve mechanical properties, but is hypothesized to narrow the flammability limits. A simplex axial mixture design identifies ten fuel formulations that systemati- cally vary the three fuel components over a 10 % range. Flash point temperature and pyrolysis-gas chromatography mass spectroscopy analysis are used to examine fuel flammability limits. An array of laboratory methods assesses the effects of varying the fuel components on thermal and mechanical characteristics. The results show that both sulfur and perfluoropolyether act to decrease the lower flammability limit when added to epoxy cured liquid polysulfide fuel. Low ig- nition temperature species including hydrogen sulfide are desorbed from sulfur doped formulations. The mole fraction of a diluent species decreases with the addition of either sulfur or perfluoropolyether. Sulfur addition up to 6.66 % increases the number of crosslinking bonds, resulting in fuel materials with increased Shore hardness, glass transition temperature, and peak reaction rate while lowering the heat and temper- ature of decomposition. The Shore hardness and glass transition temperature only decrease in blends of all three components. When combined, the Scheffe models for each response identify an optimal mixture which is predicted to broaden flammability limits while demonstrating acceptable values for all of the key fuel property responses. Using epoxy cured polysulfide polymer in this work contributes to the body of knowledge on the use of these materials in solid fuels for ramjets. The addition of sulfur and perfluoropolyether to this polymer system followed by the formulation study and optimization are all novel contributions to the state-of-the-art.

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