Home - Search - Browse - Alphabetic Index: 0- 1- 2- 3- 4- 5- 6- 7- 8- 9
A- B- C- D- E- F- G- H- I- J- K- L- M- N- O- P- Q- R- S- T- U- V- W- X- Y- Z

Solid Propellant Isp

Solid Propellant Isp
Credit: Aerojet

Solid propellants have the fuel and oxidizer embedded in a rubbery matrix. They were developed to a high degree of perfection in the United States in the 1950's and 1960's. In Russia, development was slower, due to a lack of technical leadership in the area and rail handling problems

The disadvantages of solid propellants in space applications include:

Advantages of solid rocket motors, many of which make them ideal for military applications:

Brief History of Solid Propellant Development

Solid propellant rockets, using black powder as the propellant, were introduced by the Chinese in the early 13th century. The next significant event occurred in the late 17th and 18th centuries when the development of nitro-cellulose, nitroglycerine, cordite, and dynamite resulted in their consideration as a rocket propellant. Immediately before World War I, the French used nitro-cellulose as a propellant for artillery rockets.

In 1936, Dr. Theodore von Karman, and his associates at Caltech began a program that resulted in the first composite propellants using an organic matrix (asphalt) and an inorganic oxidizer (potassium perchlorate). Their work also covered the beginnings of understanding the associated interior ballistics, combustion, ignition, and related structural-materials issues. This was the start of modern solid propellant rocketry. Composite propellants virtually replaced double base propellants (based on mixtures of nitro-cellulose and nitroglycerine) in most applications.

Following World War II many companies and agencies began propellant development programs involving a wide variety of oxidizers, fuels (binders) and processing methods. In this era, improvements in performance (quantified as specific impulse) were largely achieved by increasing oxidizer loading. Most of the binders were supplied by the rapidly expanding plastics industry.

The ever increasing number of potential missile programs resulted in growing pressure to provide other propellants that had improvements in terms of: performance, structural properties (strength, stability, impact resistance) thermal characteristics (temperature range, cycling), processing, cost, safety, quality, and reliability. In the early 1950s, Atlantic Research invented the use of up to 15 percent powdered aluminum to replace a like amount of oxidizer - giving a performance gain of about 15 percent. Propellant researchers began to understand the complete chemistry of solid propellants, and the need for molecular chain extensions and cross linking of the binders became apparent. The invention of bonding agents (as part of the fuel) greatly improved not only the mechanical properties, but also the resistance to ageing, humidity, and temperature cycling.

Two mainstream composite propellant-binder families emerged (Polyurethane and Polybutadiene), but these were accompanied by a large number of variations and evolutionary products. In addition, there were numerous associated-alternative formulations and concepts tailored to specific missile program requirements. Included among them were: Nitro-polymers, Fluorine based propellants, Beryllium additives, etc. At the same time double base propellants (based on mixtures of nitro-cellulose and nitroglycerine) continued to evolve and compete. When double base propellants were used to replace conventional binders this resulted in the highest values of specific impulse ever attained.

Aerojet initially concentrated on Polyurethane (PU), and Thiokol favored Polybutadiene (PB). Thiokol's work included PBAA, a copolymer of Butadiene and Acrylic Acid. This was replaced by PBAN, a terpolymer including Acrylic Acid and Acrylonitrile. Aerojet also conducted considerable development effort in this area, and PBAN was used in Aerojet's 260-inch space booster.

Several other companies also worked in these and other related areas. For example Phillips Petroleum with Rocketdyne developed Carboxyl Terminated Polybutadiene (CTPB) using both a Lithium initiated polymerization, and a free radical type. These propellants were widely used, but were later overtaken by Hydroxyl Terminated Butadiene (HTBD). By the 1990's Aerojet favored HTBD and formulations thereof including double base binders.

In addition to the binder evolution, there was a variety of oxidizers to choose from: ammonium and potassium nitrates, perchlorates, and picrates. Perchlorates were generally favored, but later environmental concerns were expressed at the amount of chlorine compounds (mainly hydrochloric acid) emitted into the atmosphere. One possible solution was the use of a hybrid (liquid and solid) system with a PBAN or similar grain and liquid oxygen as the oxidizer. This also provided a substantial cost saving, and allowed thrust variation and control features that were otherwise not easily achieved.

Paralleling the propellant formulation was development in the design of the propellant grain shape. In most asphalt rockets, the propellant was simply cast into the cylindrical motor chambers (or in some cases into a thin metal jacket which was then inserted into the chamber). Burning occurred only on the exposed aft end of the propellant, resulting in a constant level of thrust. The Aeroplex and other free-standing, rigid cylindrical grains (burning on the inner diameter and outer diameter.) also produced a constant thrust-time curve, because the increase in internal burning surface area just matched the decreasing external surface area.

Case-bonded propellants called for a different configuration of the burning surface. The outside of the propellant was bonded to the chamber and protected it from the hot gases. A simple cylindrical perforation down the center of the grain would produce a steadily increasing pressure and thrust from very low at start to very high at completion of burning. The solution was to use a central star shaped perforation, which could produce an essentially flat thrust-time curve. The perforation was accomplished by casting the propellant around a core of the desired shape, which was removed after the propellant was completely cured. The tapered rays of the star provided an initial large burning surface, which decreased as the points burned away. Variations in the core geometry allowed a wide range of thrust-time characteristics, to match overall missile requirements.

Additional variations could be achieved by longitudinal variations in the core size and shape, as well as by casting layers of propellant having different characteristics. This latter concept was used for many tactical missiles requiring a boost-sustain thrust curve. For years, grain design was performed by manual geometric manipulation, but computer aided design greatly simplified the task.

The earliest production process for asphalt propellant was actually to hand-stir the ground oxidizer into the heated asphalt. Quality control and consistency were highly questionable, and the safety aspects were in hindsight, terrifying. The immediate solution was to use commercial bread dough mixers in steadily increasing size and robustness. For the more viscous propellant families, much more sturdy mixers were adapted from the tire industry. In addition, the commercially available oxidizers required grinding to achieve the desired fine grain sizes and grain size distribution.

Following fatal accidents in both propellant mixing (asphalt) and oxidizer grinding (potassium perchlorate), production processes were improved to include remote operation, modern instrumentation and control, and a host of other subsystems which significantly improved safety, versatility, and consistency.

Progressive Development of Large Solid Rocket Motors

In the United States:

Black Powder Rockets

Solid propellants of the composite type contain separate fuel (or reducer, chemically) and oxidizer (in a separate compound) intimately mixed. While generally not considered as composite, black powder was in fact the oldest composite propellant. Before 1940 black powder, in common use, was nearly synonymous with the words 'rocket motor'.

Black powder technically should not be called gunpowder because its use in rockets preceded that in guns. The ingredients are charcoal, sulfur, and saltpeter (potassium nitrate). These three ingredients were known in China for many centuries, however, before they were combined into black powder. Charcoal was known from the earliest times, and sulfur and saltpeter at least since the sixth century AD, and probably as far back as the first century BC That the saltpeter is definitely of Chinese origin is indicated by the names given to this material by the Arabs, who called it "Chinese snow", and the Persians, who called it "salt from China".

By 1045, just twenty-one years before William the Conqueror invaded Saxon England, the Chinese were well acquainted with black powder. The Wu-ching Tsung-yao (Complete Compendium of Military Classics) published that year, contained many references to the subject.

In black powder, saltpeter (potassium nitrate- KNO3) is the oxidizer, while sulfur (S), and charcoal (mainly carbon- C) are the fuel. But, depending on the percentage of each ingredient, sulfur may also act as an oxidizer for potassium in the reaction: 2KNO3 + S + 3C = K2S + N2 + 3CO2.

Some early black powder formulae: