Soviet Solid Rockets - (raketa toplivniy, solid fuel rocket)

A solid propellant is basically a mixture of fuel and oxidizer which burn together, with no other outside substance injected into the combustion chamber. During the Cold War, the United States had a significant lead over the Soviet Union in the development and deployment of missiles using large solid rocket motors. The United States deployed the solid propellant Polaris SLBM, while Moscow deployed a variety of liquid fueled SLBMs.

Most American liquid-fueled ICBMs quickly passed from the scene, while liquid propellant ICBMs have remained a mainstay of Moscow's forces. The Americans deployed a thousand solid fueled Minuteman ICBMs starting in 1962, while the Soviet counterpart, the RT-2/SS-13, was deployed in only limited numbers starting in 1969. Early Soviet ABM interceptors used liquid propellants, while American ABM interceptors used solid propellants from the start.

J.D.Hunley of NASA Dryden Flight Research Center wrote that the history of solid-propellant rocketry is "particularly difficult to write for a variety of reasons, including the technical complexity of the subject and the resultant division of labor among rocket engineers into a variety of disciplines and subdisciplines. Other reasons include ... the fact that most people with technical expertise in rocketry know only a part of the history of their subject, and many of them disagree regarding technical details or matters of interpretation such as the origins of a particular technology or its relative importance." The history of Soviet solid propellants is particularly difficult to write, for all these reasons, along with the well known challenges of Soviet historiography generally.

Solid fuel strategic missiles are attractive for several reasons. Since they are already fueled, they require little maintenance, and can be fired on short notice. The are smaller than their liquid propellant counterparts, and can be deployed in a smaller submarine at sea, or a smaller silo on land. They can also be deployed on mobile launchers, moved about on trains or road-mobiel vehicles, complicating enemy counterforce targetting. Anti-missile interceptors with solid propellants can have vastly faster acceleartion rates than their liquid propellant analogs.

In a solid rocket, the fuel and oxidizer are mixed together into a solid propellant which is packed into a solid cylinder. A hole through the cylinder serves as a combustion chamber. When the mixture is ignited, combustion takes place on the surface of the propellant. A flame front is generated which burns into the mixture. The combustion produces great amounts of exhaust gas at high temperature and pressure. The amount of exhaust gas that is produced depends on the area of the flame front and engine designers use a variety of hole shapes to control the change in thrust for a particular engine. The hot exhaust gas is passed through a nozzle which accelerates the flow. The amount of thrust produced by the rocket depends on the design of the nozzle. The pointed star profile develops a relatively constant thrust which decreases slowly to zero as the last of the fuel is consumed.

A solid fuel’s geometry determines the area and contours of its exposed surfaces, and thus its burn pattern. There are two main types of solid fuel blocks used in the space industry. These are cylindrical blocks, with combustion at a front, or surface, and cylindrical blocks with internal combustion. Since the combustion of the block progresses from its free surface, as this surface grows, geometrical considerations determine whether the thrust increases, decreases or stays constant.

Most modern solid-propellant grains belong to one of two classes, double-base or composite. The double-base propellant is a mixture of two very energetic compounds, either one of which alone would make a rocket propellant. Both substances are monopropellants which burn without any added oxidizer. The nitrocellulose provides physical strength to the grain, while nitroglycerin is a high-performance and fast-burning propellant. Such double-base grains are generally formed by mixing the two constituents and additives, then pressing or extruding the puttylike mixture into the proper shape to fit the motor case.

A composite grain is so named because it is formed of a mixture of two or more unlike compounds into a composite material with the burning properties and strength characteristics desired. None of these constituent compounds would make a good propellant by itself; instead, one is usually the fuel component, another the oxidizer. The most modern of the composite propellants use a rubbery polymer (in fact, a synthetic rubber such as polybutadiene or polysulfide) which acts as the fuel and as a binder for the crumbly oxidizer powder. The oxidizer is generally a finely ground nitrate or perchlorate crystal, as, for example, potassium nitrate (KN03) or ammonium perchlorate (NH4Cl04). The composite mixture can be mixed and poured like cake batter, cast into molds or into the motor case itself, and made to set (cure) like hard rubber or concrete. The cured propellant is rubbery and grainy with a texture similar to that of a typewriter eraser. The American solid rocket fuel called hydroxyl-terminated polybutadiene (HTPB) is a hard rubbery material, with a consistency like a pencil eraser, that binds together the fuel and oxidizer.

Composite propellants often contain an additional fuel constituent in the form of a light-metal powder. Ten to twenty percent by weight of aluminum or beryllium powder added to a polymer-based grain has the effect of smoothing the burning (combustion) process and increasing the energy release of the propellant. This added energy in the hot gases produced in combustion appears as added specific impulse of the rocket propulsion system.