Solid-Propellant Rocket Engine

a jet engine that operates on solid fuel. In a solid-propellant rocket engine, all the propellant is placed in the combustion chamber as a single charge. The engine is usually operated continuously until the propellant is completely expended.

Solid-propellant rocket engines were the first rocket engines to find practical applications. Rockets with solid-propellant engines (powder rockets) have been known for approximately 1,000 years and were used as signal flares, fireworks, and weapons. Descriptions of “fire arrows” that were prototypes of powder rockets are found in tenth-century Chinese and Hindu manuscripts. These weapons consisted of a conventional arrow to which a powder-filled bamboo tube was attached. Onisim Mikhailov’s Rules, written during the first half of the 17th century, describes the first Russian rockets—cannonballs with a channel to hold a powder charge. In 1799, Indian forces used military rockets as weapons in the fight against the British colonialists, and in 1807 the British used similar rockets in the war against Denmark, during the siege of Copenhagen. Originally, black powder was used as a rocket propellant. Smokeless powder was developed at the end of the 19th century. It had the advantages of superior combustion stability and higher efficiency. Later, new and highly efficient types of solid propellants were developed. As a result, military rockets are now being built for operation over a wide range of distances, including those covered by intercontinental ballistic missiles. Solid-propellant rocket engines are currently used primarily in missiles and in spacecraft (as retro-rockets and engines for the first stage in launch vehicles).

A solid-propellant rocket engine consists of a housing—the combustion chamber—in which the total propellant charge is placed, and a jet nozzle. The housing is usually made of steel, but fiberglass may also be used. The section of the jet nozzle that receives the greatest thermal loads is made of graphite, refractory metals, or alloys of refractory metals. The supercritical section of the nozzle is made of steel, plastic, or graphite.

Solid propellant is usually poured into the housing in a semiviscous, flowing state. After hardening, the propellant grips the walls tightly, thus protecting them from hot gases. Sometimes, as with the engines of unguided rockets, the propellant loaded into the combustion chamber consists of granules or blocks of compressed grains of powder. Combustion is initiated by an igniter, which may be a part of the engine itself or a separate, self-contained device, for example, a special launch engine. The simplest igniter consists of a suspended fabric bag or metal container filled with black powder. The charge is fired by an electric fuse or by a squib with an explosive cylinder.

The thrust of a solid-propellant rocket engine can be adjusted by increasing or decreasing the combustion surface of the charge or the critical cross-sectional area of the jet or by injecting a liquid, such as water, into the combustion chamber. The direction of the thrust can be adjusted by means of jet vanes, cylindrical deflectors, auxiliary control engines, or oscillating nozzles on the main engines. In order to ensure the required velocity of the rocket at the end of the powered portion of flight, combustion of the propellant is cut off by quickly lowering the pressure in the combustion chamber, by deflecting the reactive jet, or by other means.

The thrust of solid-propellant rocket engines ranges from a few hundredths of a newton for microthrusters to 10–15 meganewtons for powerful engines installed in launch vehicles. An experimental, solid-propellant rocket engine developed in the USA generates approximately 16 meganewtons of thrust. The specific impulse of the best solid-propellant rocket engines can be as high as 2.5–3 kilonewton-seconds per kg.

Solid-propellant rocket engines are characterized by high reliability (99.6–99.9 percent), a long storage life in launching condition, the availability of significant thrust for a very short combustion time, safety in handling resulting from the absence of toxic substances, and a high propellant density (1.5–2 g/cm³). The disadvantages of solid-propellant rocket engines include high weight of the rocket’s rigid structures (because of the high pressures in the combustion chamber), the sensitivity of most propellant types to shock and temperature changes, difficulties in transporting charged engines, difficulties in adjusting the thrust vector, and a specific impulse that is low in comparison with those of liquid-propellant rocket engines.