Solid rocket motor solid propellant

Solid propellant is a solid mixture composed of oxidant, fuel (combustible agent) and other additives, which can be divided into single-base propellant, double-base propellant, composite propellant, modified double-base propellant and so on according to the properties of formula components. According to the uniformity of texture, it can be divided into homogeneous propellant (such as single-base propellant and double-base propellant) and heterogeneous propellant (such as composite propellant and modified double-base propellant); According to energy levels, propellants can be divided into high energy, medium energy and low energy. Propellants with specific impulse greater than 2450 nS/kg (that is, 250 seconds) are high energy, those with specific impulse between 2255 nS/kg (that is, 230 seconds) and 2450 nS/kg are medium energy, and those with specific impulse less than 2255 nS/kg are low energy. According to the characteristic signal, it can be divided into smoky, slightly smoky and smokeless propellants. The classification of solid propellant is simple. The basic propellant consists of a single compound (such as nitrocellulose, abbreviated as nitrocellulose). Its molecular structure contains combustible agent and oxidant, which are dissolved in volatile solvent, swollen, plasticized and extruded, and then the solvent is removed. Modern solid motors no longer use single-base propellant because of its low energy level. The theoretical specific impulse of composite propellant is 225 ~ 265 seconds (2200~2600 nS/kg), and its density is 1.65~ 1.80 g/cm3. The hazard level is 1.3. The composite propellant uses separated combustible and oxidant materials, uses liquid polymer binder as fuel, and adds crystalline oxidant solid filler and other additives to melt and solidify into multiphase objects. In order to improve energy and density, some powdery light metal materials can be added as combustible agents, such as aluminum powder (al). Composite propellant is usually named after the chemical name of binder. Oxidizer usually accounts for 60~90% of the total weight of propellant. Many inorganic chemicals can be used as oxidants, such as perchlorates (potassium perchlorate, ammonium perchlorate and lithium perchlorate) and nitrates (ammonium nitrate, potassium nitrate and sodium nitrate). At present, the most widely used is ammonium perchlorate (AP) with high oxygen content. Polymers are used as both flammable agents and adhesives, and commonly used are polysulfide rubber, polyurethane (PU), polybutadiene-acrylonitrile (PBAN), carboxyl-terminated polybutadiene (CTPB), hydroxyl-terminated polybutadiene (HTPB), hydroxyl-terminated polyether (HTPE) and polyvinyl chloride. Other additives generally include: a combustion rate regulator that regulates the combustion rate; Combustion stabilizer for improving combustion performance; Plasticizer with better mechanical properties than basic adhesive; Stabilizers to reduce mechanical sensitivity; Anti-aging agent for improving storage performance; Diluent, wetting agent, curing agent and curing catalyst to improve process performance. Except thermoplastic polyethylene propellant, it is generally made by casting process, which is simple and suitable for making grain of various sizes. Composite propellant has good comprehensive performance, wide temperature range, high energy and good mechanical properties, and is widely used in all kinds of solid rocket engines, especially large rocket engines. 1942, the United States developed asphalt potassium perchlorate composite propellant, and the first generation of composite propellant polysulfide rubber propellant appeared in the late 1940s. PBAN and HTPB propellants are commonly used now. PBAN propellant is used for Militia 3 and space shuttle solid booster, HTPB propellant is used for the first and second stages of Peacekeeper MX, CTPB propellant is used for French M4, and CTPB composite propellant is also used for China Julang-1. Modified double-base propellant includes composite modified double-base propellant (CMDB) and cross-linked modified double-base propellant (XLDB for short). The theoretical specific impulse is 260 ~ 270 seconds (2550~2646 N seconds /kg), and the density is 1.75~ 1.80 g/cm3. The hazard level is 1. 1. On the basis of double-base propellant, the proportion of nitrocellulose and nitroglycerin as basic components is greatly reduced, and high-energy solid components are increased, including oxidant (ammonium perchlorate AP, [RDX] or [HMX]) and combustible agent (aluminum powder, etc.). ). Nitrocellulose (nitrogen content is about 12%) is plasticized with nitroglycerin, or nitrocellulose and nitroglycerin are used as binders, nitroglycerin is also used as plasticizer, and some additives are added. After mixing, the composite modified double-base propellant (CMDB) was made by extrusion molding or casting molding. On the basis of CMDB formula, polymer compounds are added as cross-linking agents, and their active groups react with the residual (unesterified) hydroxyl groups on nitrocellulose to form prepolymers, and chemical bonds are formed between the macromolecular main chains of the prepolymers, which cross-link into a network structure. As a binder, the prepolymer can greatly improve the mechanical properties of the propellant, which is called cross-linked modified double-base propellant (XLDB). The main crosslinking agents are isocyanate (such as hexamethylene diisocyanate HDI and toluene diisocyanate TDI), polyester (such as polyglycolide PGA), polyurethane (such as polyethylene glycol PEG), hydroxyl-terminated polybutadiene and acrylate. The energy level of modified double-base propellant is higher than that of composite propellant, and it is widely used in various strategic and tactical missiles. All three-stage engines of American Trident C4 submarine-launched strategic missile use XLDB propellant, which is called XLDB-70. In its formula, the solid filler reaches 70% (including 43%HMX/8% AP/ 19% Al), and the theoretical specific impulse is 2646 N s/kg.