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A simple solid rocket motor consists of a casing, nozzle, grain (propellant charge), and igniter.
The grain behaves like a solid mass, burning in a predictable fashion and producing exhaust gases. The nozzle dimensions are calculated to maintain a design chamber pressure, while producing thrust from the exhaust gases.
Once ignited, a solid rocket motor cannot be shut off.
Modern designs may also include; steerable nozzle for guidance, avionics, recovery hardware (parachutes), self destruct mechanisms, apu's, and thermal management materials.
Design begins with the total impulse required, this determines the fuel/oxidizer mass. Grain geometry and chemistry are then chosen to satisfy the required motor characteristics.
The following are chosen or solved simultaneously. The results are exact dimensions for grain, nozzle and case geometries;
The grain may be bonded to the casing, or not. Case bonded motors are much more difficult to design, since deformation of both the case and grain, under operating conditions, must be compatible.
Common modes of failure in solid rocket motors are; fracture of the grain, failure of case bonding, and air pockets in the grain. All of these produce an instantaneous increase in burn surface area, and a corresponding increase in exhaust gas and pressure, and rupture of the casing.
Another failure mode is casing seal design. Seals are required in casings that have to be opened to load the grain. Once a seal fails, hot gas will erode the escape path and result in failure. This was the cause of the Space Shuttle Challenger disaster.
Solid fuel grains are usually molded from a thermoset elastomer (which doubles as fuel), additional fuel, oxidizer, and catalyst. HTPB is commonly used for this purpose.
Ammonium perchlorate is the most common oxidizer used today.The fuel is cast in different forms for different purposes. Slow, long burning rockets have a cylinder shaped grain, burning from one end to the other. Most grains, however, are cast with a hollow cross section, burning from the inside out (and outside in, if not case bonded), as well as from the ends.
The thrust profile over time can be controlled by grain geometry. For example, a star shaped hole down the center of the grain will have greater initial thrust because of the additional surface area. As the star points are burned up, the surface area and thrust are reduced.
The casing may be constructed from a range of materials. Cardboard is used for model engines. Steel is used for the space shuttle boosters. Filament wound graphite epoxy casings are used for high performance motors.
A Convergent Divergent design accelerates the exhaust gas out of the nozzle to produce thrust.
Sophisticated solid rocket motors use steerable nozzles for rocket control.
Solid fuel rocket motors have a typical specific impulseThe specific impulse of a rocket (commonly abbreviated I is the impulse (change in momentum) per unit mass of its fuel. It is a measure of how much push can be obtained from a fixed mass of fuel. Essentially it is simply the exhaust velocity. A rocket mus of 250s. This compares to 350s for keroseneKerosene or paraffin is a colorless flammable hydrocarbon liquid. It is obtained from the fractional distillation of petroleum at 150°C and 275°C (the to range). At one time it was widely used in kerosene lamps but it is now mainly used as a fuel for jet/ LoxLiquid oxygen (also LOx LOX or Lox in the aerospace industry) is the liquid form of oxygen. It has a pale blue colour and is strongly paramagnetic. Liquid oxygen has a density of 1140 kg/m³ and is moderately cryogenic (Freezing Point: -219 deg C. Boiling and 450s for liquid hydrogen/Lox. For this reason solids are generally used as initial stages in a rocket, with better performing liquid engines reserved for final stages. However, the venerable Star line motors manufactured by Thiokol have a long history as the final boost stage for satellites. This is due to their simplicity, compactness and high mass fractionIn aerospace engineering, for any given target orbit, the mass fraction of a rocket is an important measure of its efficiency. The mass fraction is one minus the total amount of mass delivered to orbit, divided by the mass of the fully-fueled vehicle prio.
The ability of solid rockets to remain in storage for long periods, and then reliably launch at a moments notice, makes them the design of choice for military applications.