The Space Shuttle Columbia is initially launched using solid-fuel boosters

Solid rockets are rockets with a motor that uses solid propellants (fuel/oxidizer). The Chinese invented solid rockets and were using them in warfare by the 13th century. All rockets used some form of solid or powdered propellant up until the 20th century. Solid rockets are considered to be safe and reliable due to the long engineering history and simple design.

Basic Concepts

Solid Rocket Motor

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

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 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.

Grain

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.

Casing

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.

Nozzle

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.

Performance

Solid fuel rocket motors have a typical specific impulse of ~250s. This compares to ~350s for kerosene/Lox 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 fraction.

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.

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/potassium nitrate, sorbitol/potassium nitrate, HTPB (a rubber like epoxy)/aluminum or magnesium/ammonium nitrate.

Ammonium perchlorate is rarely used by amateurs because it is dangerous to handle and offers only a small performance advantage over safer oxidizers.

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 throttalble liquid or gaseous oxidizer

Related Articles

• intercontinental ballistic missile
• Jetex engine
• Skyrocket
• Spacecraft propulsion

External Links

• Robert A. Braeunig rocket propulsion page
• Astronautix Composite Solid Propellants
• Ariane 5 SRB
• Amateur High Power Rocketry Association
• Nakka-Rocketry (Design Calculations and Propellent Formulations)