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solid, one of the three commonly recognized states in which matter occurs, i.e., that state, as distinguished from liquid and gas, in which a substance has both a definite shape and a definite volume. Solids resemble liquids in having a definite volume, but differ from both liquids and gases in having a definite shape. The molecules of a solid, like those of a liquid, are very close together, but whereas the molecules of a liquid are free to move around, those of a solid have less thermal energy and are held fixed in their places by intermolecular forces. Their only movement is a vibration about a fixed position. A solid changes to a liquid when its temperature is raised to its melting point. A definite quantity of heat (called the heat of fusion) is needed to change each gram of the substance from solid to liquid. Some substances change directly from solid to gas without passing through the liquid state (see sublimation), but most change from solid to liquid before becoming gaseous. Solids are of various types. Metals, their alloys, some nonmetals, and ionic chemical compounds are crystalline in form. Some solids, e.g., chalk and clay, have no regular structure and are called amorphous. Substances such as pitch and resin are called semisolids; these are actually very viscid liquids, but their flow or change of shape is so slow at ordinary temperatures as to be scarcely discernible by the human eye (see viscosity). Properties in which solids differ from one another include density, hardness, malleability, ductility, elasticity, brittleness, and tensile strength.

A solid rocket or a solid-fuel rocket is a rocket with a motor that uses solid propellants (fuel/oxidizer). The earliest rockets were solid fueled, powered by gunpowder, used by the Chinese and Arabs in warfare as early as the 13th century. All rockets used some form of solid or powdered propellant up until the 20th century, when liquid rockets and hybrid rockets offered more efficient and controllable alternatives. Solid rockets are still used today in model rockets, and on larger applications for their simplicity and reliability. Since solid fuel rockets can remain in storage for long periods -- and then reliably launch on short notice -- they have been frequently used in military applications such as missiles. Solid fuel rockets are unusual as primary propulsion in modern space exploration, but are commonly used as booster rockets.

Basic concepts

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 simple solid rocket motor cannot be shut off, because it contains all the ingredients necessary for combustion within the chamber that they are burned in. More advanced solid rocket motors can not only be throttled but can be extinguished and then re-ignited by controlling the nozzle geometry or through the use of vent ports. Also, pulsed rocket motors which burn in segments and which can be ignited upon command are available.

Modern designs may also include a steerable nozzle for guidance, avionics, recovery hardware (parachutes), self-destruct mechanisms, APUs, controllable tactical motors, controllable divert and attitude control motors 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 burns at a predictable rate, given its surface area and chamber pressure.
  • The chamber pressure is determined by the nozzle orifice diameter and grain burn rate.
  • Allowable chamber pressure is a function of casing design.
  • The length of burn time is determined by the grain 'web thickness'.

The grain may be bonded to the casing, or not. Case-bonded motors are much more difficult to design, since the deformation, under operating conditions, of the case and the grain must be compatible.

Common modes of failure in solid rocket motors include 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, which may potentially induce 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, fuel, oxidizer and catalyst. HTPB and PBAN are typical elastomers which double as fuel. Ammonium perchlorate is the most common oxidizer. This fuel mixture is known as Ammonium perchlorate composite propellant (APCP).

The exhaust from a solid rocket motor contains hydrochloric acid and aluminium oxide. These have a negative effect on the environment. Furthermore, for military use, the smoke trail and the infrared radiation from the hot particles make it possible to detect the launch from space. These problems led to the research in smokeless grain which contains nitrogen-containing organic molecules.

The grain 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 core, 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 core 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.

Some solid rocket motors, such as the ones used in the AGM-114 Hellfire missile, utilize a "rod and tube" grain design.


The casing may be constructed from a range of materials. Cardboard is used for model motors. 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.

Some designs include directional control of the exhaust. This can be accomplished by gimballing the nozzle, as in the Space Shuttle SRBs, by the use of jet vanes in the exhaust similar to those used in the V2 rocket, or by liquid injection thrust vectoring (LITV).

An early Minuteman first stage used a single motor with four gimballed nozzles to provide pitch, yaw, and roll control.

LITV consists of injecting a liquid into the exhaust stream after the nozzle throat. The liquid then vaporizes, and in most cases chemically reacts, adding mass flow to one side of the exhaust stream and thus providing a control moment. For example, the Titan IIIC solid boosters injected nitrogen tetroxide for LITV; the tanks can be seen on the sides of the rocket between the main center stage and the boosters .


Solid fuel rocket motors have a typical specific impulse of 285 seconds (2.6 kN·s/kg). This compares to ~330 seconds (3.2 kN·s/kg) for kerosene/Lox and ~450 seconds (4.4 kN·s/kg) for liquid hydrogen/Lox bipropellant engines .

Solid rockets have a long history as the final boost stage for satellites. This is related to their simplicity, reliability, compactness and reasonably high mass fraction .

Solids can also provide high thrust for relatively low cost. For this reason, solids have been used as initial stages in rockets (the classic example being the Space Shuttle), whilst reserving high specific impulse engines, especially less massive hydrogen fuelled engines for higher stages.

But the ability of solid rockets to remain in storage for long periods, and then reliably launch at a moment's notice, makes them the design of choice for many military applications.

Amateur rocketry

Solid fuel rockets can be bought for use in model rocketry; they are normally small cylinders of fuel with an integral nozzle and a small charge that is set off when the fuel is exhausted. This charge can be used to ignite a second stage, trigger a camera, or deploy a parachute.

Designing solid rocket motors is particularly interesting to amateur rocketry enthusiasts. The design is simple, materials are inexpensive and constructions techniques are safe.

Early amateur motors were gunpowder. Later, zinc/sulfur formulations were popular.

Typical amateur formulations in use today are: sugar (sucrose, dextrose, and sorbitol are all common)/potassium nitrate, HTPB (a rubber like epoxy)/magnesium/ammonium nitrate, and HTPB or PBAN/aluminum/ammonium perchlorate (APCP). Most formulations also include burn rate modifiers and other additives, and also possibly additives designed to create special effects, such as colored flames, thick smoke, or sparks.

Amateur rocket builders are very active in hybrid motor research.

Advanced research

  • Environmentally sensitive fuel formulations
  • Ramjets with solid fuel
  • Variable thrust designs based on variable nozzle geometry.
  • hybrid rockets that use solid fuel and throttleable liquid or gaseous oxidizer

See also


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