Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel High density solid fuel ramjet fuel based on 1,6-hexanediol diglycidyl ether and methyltetrahydrophthalic anhydride Brian McDonald a, ⁎ , Jeremy Rice a , Brian Hayes b , Chris Marshall a , Larry Pledger a , Doug Myers a , Dan Jones c a U.S. Army CCDC Aviation and Missile Center, Redstone Arsenal, AL 35898, United States b Applied Poleramic, Inc., 6166 Egret Court, Benicia, CA 94510, United States c Department of Mechanical and Aerospace Engineering, University of Alabama, Huntsville, AL, United States ARTICLEINFO Keywords: Solid fuel ramjet Polymer combustion Epoxide Total impulse Regression rate ABSTRACT A solid fuel ramjet fuel (SFRJ) is developed to have high material density and net impulse density. Theoretical performance and thermochemical analyses are performed on a wide range of polymers as potential fuel can- didates leading to an emphasis on epoxies. The compositions of these epoxy thermoset polymers are selected based on previous performance analyses identifying polyethers as preferred materials. Accordingly, a selection of epoxy resins and curatives are independently theoretically evaluated for their fuel value in the SFRJ application. A material is made from the combination of the highest performing materials which are 1,6-hexanediol digly- cidyl ether and methyltetrahydrophthalic anhydride as the epoxy constituent and curative respectively. The material is further characterized by conducting thermogravimetric and differential scanning calorimeter ana- lyses, bomb calorimetry, pyrolyzer gas chromatography mass spectroscopy, and direct connect fuel grain testing. An examination of the collected data suggests that this material represents an excellent candidate for SFRJ applications. The test data shows that the material has a relatively low temperature for the onset of decom- position and decomposes into a predominance of gaseous species that have autoignition temperatures lower than the peak decomposition temperature. The fuel grain testing demonstrates stable combustion and acceptable regression rates. 1. Introduction The solid fuel ramjet (SFRJ) propulsion cycle offers the potential to extend range over conventional solid and liquid propulsion cycles given that the oxidizer is not stored onboard allowing for all propellant vo- lume to be utilized as fuel [1]. In extended range applications, such as a cruise condition, the thrust level needs only to match or slightly exceed the axial drag. Thus, for these applications, total impulse exceeds thrust magnitude in design and fuel development importance. In terms of intensive qualities, impulse density is of greater importance than spe- cific impulse, but, the two quantities need not be mutually exclusive nor are they unrelated. Since impulse density is the product of the material density and the specific impulse, an increase in specific impulse will also increase the impulse density. However, the impulse density in- creases linearly with the material density while the specific impulse increases proportionally to the square root of the flame temperature or heat release. Therefore, increasing the fuel density has the most direct effect on impulse density and ultimately the vehicle total impulse. SFRJ fuel development has typically focused on maximizing the energy density of the fuel grain through the use of metal additives leading to formulations that utilize high mass fractions of boron ranging up to 40–60% [2–7]. Boron has one of the highest energy density values of all metal fuel additive candidates and has been the subject of ex- tensive investigation for many years, but has also shown tendencies of low combustion efficiency as a result of the oxide coating that forms on the boron particle. Nieder, et al. [7] conducted a thorough experi- mentally based survey of combustion aides for boron based fuels in combination with hydroxyl terminated polybutadiene (HTPB). Like- wise, Natan, et al. and Hedman, et al. [8–10] have explored boron combustion in SFRJ applications both experimentally and analytically. Aluminum and magnesium have also found use in SFRJ fuels with the former having excellent material density and heat of combustion, while the latter is easily ignitable [11]. Pang, et al. [12] have looked at other high density metals, such as titanium, zirconium, and hydrides of each in HTPB based fuel formulations, thus applicable to SFRJ performance. The combustion chamber of a SFRJ operates at lower pressures and https://doi.org/10.1016/j.fuel.2020.118354 Received 29 April 2020; Received in revised form 6 June 2020; Accepted 8 June 2020 ⁎ Corresponding author. E-mail address: brian.a.mcdonald16.civ@mail.mil (B. McDonald). Fuel 278 (2020) 118354 0016-2361/ Published by Elsevier Ltd. T