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Are All Rockets the Same?
Published in Travis S. Taylor, Introduction to Rocket Science and Engineering, 2017
The grain of the solid rocket motor is an interesting mixture of materials that are practically the consistency of a rubber elastomer. In fact, the grain is a mixture of fuel, oxidizer, catalyst, some elastomer binder compound, plasticizer, curing agents, and, in some cases, other additives. The additives and binding materials may vary from manufacturer to manufacturer, but the most common fuel used is an elastomer binder and fuel combination. The two most common are hydroxyl-terminated polybutadiene (HTPB) and polybutadiene acrylonitrile (PBAN). HTPB is a clear viscous polymer belonging to the class known as polyols and is commonly used in the manufacture of polyurethane. PBAN is a copolymer and is less toxic during the curing process.
Tailoring Binder Melting Temperature to Study the Binder Melt Layer Flow in Ammonium Perchlorate Composite Propellants
Published in Combustion Science and Technology, 2021
A. R. Demko, B. Lormand, Z. Doorenbos
The lower melting temperature of the hydroxyl-terminated polybutadiene (HTPB) propellants potentially create unique melt layer flow dynamics that will impact the combustion behavior of the propellant. Previous melt layer investigations revolved around lowering the melt temperature to investigate the surface plateau propellants. While working on a one-dimensional model for composite propellant combustion using a granular diffusion flame model, Steinz and coworkers proposed several instrumental deductions on the influence of the melt layer on the burning mechanism among multiple other significant observations (Steinz, Stang, Summerfield 1969). One key observation was the notion of the molten binder melt layer having an influence on the plateau by slowing the combustion based on binder readiness to melt. Molten binder would flow over the surface of the ammonium perchlorate (AP) crystal and impede the decomposition of the oxidizer. Further studies showed that the binders that readily melt, such as polyurethane and polystyrene, would present the most dramatic melt layer behavior. However, binders that are relatively more difficult to melt, such as polybutadiene acrylic acid and polybutadiene acrylonitrile acrylic acid, also produce such melt layer behaviors but to a more limited degree (Beckstead, Boggs, Derr 1970; Chaiken and Anderson 1960; Cohen, Fleming, Derr 1974; Derr and Boggs 1970; Ide 2002; Varney and Strahle 1971). The use of the various binders was an effort to tailor multiple physical parameters to study the melt layer flow properties. The thickness of the melt layer was also shown to change depending on the curative used to cast the propellant for the various binders previously mentioned. Several groups have studied the melting behavior of hydrocarbon binders using a hot stage microscope (Beckstead, Boggs, Derr 1970; Cohen, Fleming, Derr 1974; Demko 2013; Derr and Boggs 1970; Ide 2002; Varney and Strahle 1971). A significant finding of the studies was that the use of specific additives can lower the melt temperature of the binder, thus allowing the binder melt to be thicker and flow better (Chakravarthy, Price, Sigman 1995; Ide 2002).