| A solid rocket or a solid fuel rocket is a | | | | different purposes. Slow, long burning |
| rocket with a motor that uses solid | | | | rockets have a cylinder shaped grain, burning |
| propellants (fuel/oxidizer). The earliest | | | | from one end to the other. Most grains, |
| rockets were solid fueled, powered by | | | | however, are cast with a hollow core, burning |
| gunpowder, used by the Chinese in warfare as | | | | from the inside out (and outside in, if not |
| early as the 13th century. All rockets used | | | | case bonded), as well as from the ends. |
| some form of solid or powdered propellant up | | | | |
| until the 20th century, when liquid rockets | | | | The thrust profile over time can be |
| and hybrid rockets offered more efficient and | | | | controlled by grain geometry. For example, a |
| controllable alternatives. Solid rockets are | | | | star shaped core will have greater initial |
| still used today in model rockets, and on | | | | thrust because of the additional surface |
| larger applications for their simplicity and | | | | area. As the star points are burned up, the |
| reliability. | | | | surface area and thrust are reduced. |
| | | | |
| Basic concepts | | | | Casing |
| | | | |
| A simple solid rocket motor consists of a | | | | The casing may be constructed from a range of |
| casing, nozzle, grain (propellant charge), | | | | materials. Cardboard is used for model |
| and igniter. | | | | engines. Steel is used for the space shuttle |
| | | | boosters. Filament wound graphite epoxy |
| The grain behaves like a solid mass, burning | | | | casings are used for high performance motors. |
| in a predictable fashion and producing | | | | |
| exhaust gases. The nozzle dimensions are | | | | Nozzle |
| calculated to maintain a design chamber | | | | |
| pressure, while producing thrust from the | | | | A Convergent Divergent design accelerates the |
| exhaust gases. | | | | exhaust gas out of the nozzle to produce |
| | | | thrust. |
| Once ignited, a simple solid rocket motor | | | | |
| cannot be shut off, because it contains all | | | | Some designs include directional control of |
| the ingredients necessary for combustion | | | | the exhaust. This can be accomplished by |
| within the chamber that they are burned in. | | | | gimballing the nozzle, as in the Space |
| More advanced solid rocket motors can not | | | | Shuttle SRBs, by the use of jet vanes in the |
| only be throttled but can be extinguished and | | | | exhaust similar to those used in the V2 |
| then re-ignited by controlling the nozzle | | | | rocket, and by liquid injection thrust |
| geometry or through the use of vent ports. | | | | vectoring (LITV). |
| | | | |
| Modern designs may also include a steerable | | | | An early Minuteman first stage used a single |
| nozzle for guidance, avionics, recovery | | | | motor with four gimballed nozzles to provide |
| hardware (parachutes), self-destruct | | | | pitch, yaw, and roll control. |
| mechanisms, APUs, controllable tactical | | | | |
| motors, controllable divert and attitude | | | | LITV consists of injecting a liquid into the |
| control motors and thermal management | | | | exhaust stream after the nozzle throat. The |
| materials. | | | | liquid then vaporizes, and in most cases |
| | | | chemically reacts, adding mass flow to one |
| Design | | | | side of the exhaust stream and thus providing |
| | | | a control moment. For example, the Titan IIIC |
| Design begins with the total impulse | | | | solid boosters injected nitrogen tetroxide |
| required, this determines the fuel/oxidizer | | | | for LITV; the tanks can be seen on the sides |
| mass. Grain geometry and chemistry are then | | | | of the rocket between the main center stage |
| chosen to satisfy the required motor | | | | and the boosters 1. |
| characteristics. | | | | |
| | | | Performance |
| The following are chosen or solved | | | | |
| simultaneously. The results are exact | | | | Solid fuel rocket motors have a typical |
| dimensions for grain, nozzle and case | | | | specific impulse of 285 seconds (2.6 kN·s |
| geometries; | | | | kg). This compares to ~330 seconds (3.2 |
| | | | kN·s/kg) for kerosene/Lox and ~450 seconds |
| The grain burns at a predictable rate, given | | | | (4.4 kN·s/kg) for liquid hydrogen/Lox |
| its surface area and chamber pressure. | | | | bipropellant engines 1. |
| | | | |
| The chamber pressure is determined by the | | | | Solid rockets have a long history as the |
| nozzle orifice diameter and grain burn rate. | | | | final boost stage for satellites. This is |
| | | | related to their simplicity, reliability, |
| Allowable chamber pressure is a function of | | | | compactness and reasonably high mass |
| casing design. | | | | fraction. |
| | | | |
| The length of burn time is determined by the | | | | Solids can also provide high thrust for |
| grain 'web thickness'. | | | | relatively low cost. For this reason, solids |
| | | | have been used as initial stages in rockets |
| The grain may be bonded to the casing, or | | | | (the classic example being the Space |
| not. Case bonded motors are much more | | | | Shuttle), whilst reserving high specific |
| difficult to design, since deformation of | | | | impulse engines, especially less massive |
| both the case and grain, under operating | | | | hydrogen fuelled engines for higher stages. |
| conditions, must be compatible. | | | | |
| | | | But the ability of solid rockets to remain in |
| Common modes of failure in solid rocket | | | | storage for long periods, and then reliably |
| motors include fracture of the grain, failure | | | | launch at a moments notice, makes them the |
| of case bonding, and air pockets in the | | | | design of choice for a very many military |
| grain. All of these produce an instantaneous | | | | applications. |
| increase in burn surface area and a | | | | |
| corresponding increase in exhaust gas and | | | | Amateur rocketry |
| pressure, which may potentially induce | | | | |
| rupture of the casing. | | | | Solid fuel rockets can be bought for use in |
| | | | model rocketry; they are normally small |
| Another failure mode is casing seal design. | | | | cylinders of fuel with an integral nozzle and |
| Seals are required in casings that have to be | | | | a small charge that is set off when the fuel |
| opened to load the grain. Once a seal fails, | | | | is exhausted. This charge can be used to |
| hot gas will erode the escape path and result | | | | ignite a second stage, trigger a camera, or |
| in failure. This was the cause of the Space | | | | deploy a parachute. |
| Shuttle Challenger disaster. | | | | |
| | | | Designing solid rocket motors is particularly |
| Grain | | | | interesting to amateur rocketry enthusiasts. |
| | | | The design is simple, materials are |
| Solid fuel grains are usually molded from a | | | | inexpensive and constructions techniques are |
| thermoset elastomer, fuel, oxidizer and | | | | safe. |
| catalyst. HTPB and PBAN are typical | | | | |
| elastomers which double as fuel. Ammonium | | | | Early amateur motors were gunpowder. Later, |
| perchlorate is the most common oxidizer. This | | | | zinc/sulfur formulations were popular. |
| fuel mixture is known as Ammonium perchlorate | | | | |
| composite propellant (APCP). | | | | Typical amateur formulations in use today |
| | | | are; sugar (sucrose, dextrose, and sorbitol |
| The exhaust from a solid rocket motor | | | | are all common)/potassium nitrate, HTPB (a |
| contains hydrochloric acid and aluminium | | | | rubber like epoxy)/magnesium/ammonium |
| sulfate. These a have negative effect on the | | | | nitrate, and HTPB or PBAN/aluminum/ammonium |
| environment. Furthermore, for military use, | | | | perchlorate (APCP). Most formulations also |
| the smoke trail and the infrared radiation | | | | include burn rate modifiers and other |
| from the hot particles make it possible to | | | | additives, and also possibly additives |
| detect the launch from space. These problems | | | | designed to create special effects, such as |
| lead to the research in smokeless grain which | | | | colored flames, thick smoke, or sparks. |
| contains nitrogen-containing organic | | | | |
| molecules. | | | | Amateur rocket builders are very active in |
| | | | hybrid motor research. |
| The grain is cast in different forms for | | | | |