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Solid rocket propellants differ from liquid propellants in that the oxidiser and fuel are embedded or bound together in a solid compound that is cast into the rocket motor casing. They began with black powder rockets in medieval times, progressed through double base propellants in the early 1900's, and finally achieved high performance as composite propellants from the 1940's. Composite motors were developed to a high degree of perfection in the United States in the 1950's and 1960's. In Russia, due to a lack of technical leadership and rail handling problems, serious use of composite propellants did not begin until the 1960's, and then primarily for military rockets. The detailed chemistry and development of solid propellants is provided by Andre Bedard in the following separate articles:

The following summarises the development of solid rocket propellants very briefly.

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, nitro-glycerine, 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 oxidiser (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 nitro-glycerine) in most applications.

Following World War II many companies and agencies began propellant development programs involving a wide variety of oxidisers, fuels (binders) and processing methods. In this era, improvements in performance (quantified as specific impulse) were largely achieved by increasing oxidiser 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 aluminium to replace a like amount of oxidiser - 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 nitro-glycerine) 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 favoured 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" space booster.

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

In addition to the binder evolution, there was a variety of oxidisers to choose from: ammonium and potassium nitrates, perchlorates, and picrates. Perchlorates were generally favoured, 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 oxidiser. 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 centre 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 oxidiser 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 oxidisers required grinding to achieve the desired fine grain sizes and grain size distribution.

Following fatal accidents in both propellant mixing (asphalt) and oxidiser 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.

The disadvantages of solid propellants in space applications include:

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

Progressive Development of Large Solid Rocket Motors

In the United States:

Fuel: Solid. Fuel Density: 1.35 g/cc.

Solid propellants have the fuel and oxidiser 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 include:

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

Engines Using Solid

Eng-engineslink Thrust(vac)-kgf Thrust(vac)-kN Isp-sec Isp (sea level)-sec Designed for Status
15D305 1 0.01     First Stages Out of Production
Star 5A 17   250   Upper Stages In Production
NOTS-4 72 0.70 250 204 Upper Stages Out of Production
Star 5C/CB 199   266   Upper Stages In Production
Star 5CB 203 2.00     Upper Stages In Production
Star 5C 208       Upper Stages Out of Production
NOTS-3 231 2.26 250 204 Upper Stages Out of Production
Star 6B 256   273   Upper Stages In Production
Star 10 342   251   Upper Stages In Production
Star 13 387 3.80 273   Upper Stages Out of Production
PRD-22 400 3.92     First Stages Out of Production
Martlet 4-3 550 5.39 300 210 Upper Stages Study 1962
Star 12 567   252   Upper Stages In Production
Star 13A 599   287   Upper Stages In Production
Sergeant 680 6.66 235 214 Upper Stages Out of Production
Star 12A 739   270   Upper Stages In Production
Star 13B 775   286   Upper Stages Out of Production
MIHT-4 1,000 9.80 295   Upper Stages In Production
Star 17 1,116 19.60 286 220 Upper Stages In Production
GCRC 1,179 11.60 230 210 Upper Stages Out of Production
X-248 1,270 12.40 256 233 Upper Stages Out of Production
X-248A 1,406 13.80 255 232 Upper Stages Out of Production
Kartukov LL 1,500 14.70     First Stages Developed 1946-48
Star 17A 1,633   287   Upper Stages In Production
Star 24 1,891 20.00 283   Upper Stages In Production
Mage 1 1,978 19.40 295 220 Upper Stages Out of Production
Martlet 4-2 2,100 20.60 300 210 Upper Stages Study 1962
Star 24C 2,189   282   Upper Stages In Production
X-258 2,268 22.20 266 242 Upper Stages Out of Production
Star 20B 2,495   289   Upper Stages In Production
SPRD-99 2,500 24.50     First Stages Out of Production
FW4-D 2,549 25.00 287 250 Upper Stages Out of Production
Star 27 2,726 27.00 288   Upper Stages In Production
SLV-4 2,736 26.80 283 60 Upper Stages In Production
Star 30BP 2,753 27.00 292   Upper Stages In Production
Star 20 2,767   287   Upper Stages In Production
FW-4S,TEM640 2,800 27.40 280 255 Upper Stages Out of Production
GF-02 2,957 29.00 230 200 Upper Stages Out of Production
P6 3,000 29.40 211 211 Upper Stages Out of Production
Iris 3,000 29.40 291 115 Upper Stages Out of Production
Black Arrow-3 3,000 29.40 278 245 Upper Stages Out of Production
Star 30C 3,329 1,647.00 287   Upper Stages In Production
Star 26 3,402 39.10 271 220 Upper Stages In Production
Pegasus-3 3,525 34.60 287   Upper Stages In Production
Star 26B 3,531   272   Upper Stages In Production
Star 26C 3,570   272   Upper Stages In Production
Star 30E 3,608 1,780.00 291   Upper Stages In Production
Star 37XFP 3,878 31.50 290   Upper Stages In Production
Star 37 4,441 43.50 260 220 Upper Stages Out of Production
Mage 2 4,638 45.50 293   Upper Stages Out of Production
Star 37FM 4,819 47.90 290   Upper Stages In Production
Star 37X 5,216 51.10 296 230 Upper Stages Out of Production
M-V-4 5,300 52.00 298   Upper Stages In Production
RSA-3-3 5,300 51.00 292   Upper Stages Out of Production
NOTS-1 5,441 53.40 204 204 First Stages Out of Production
X-254 6,169 60.50 256 233 Upper Stages Out of Production
Star 48 6,848 67.20 287   Upper Stages Out of Production
Martlet 4-1 6,900 67.70 300 210 First Stages Study 1962
Star 48A s 7,863   283   Upper Stages In Production
Star 48B s 7,863   286   Upper Stages In Production
UM-129A 7,900 77.50 291 220 Upper Stages In Production
H-1-3 7,900 77.00 291 220 Upper Stages Out of Production
SRM-2 7,996 78.40 304 200 Upper Stages In Production
Star 48B 8,044 66.00 286   Upper Stages In Production
X-259A 8,239 80.80 295   Upper Stages Out of Production
Star 31 8,391 80.00 294   Upper Stages In Production
Star 63D 8,641   283   Upper Stages In Production
SLV-3 9,249 90.70 277 190 Upper Stages In Production
X-259 9,493 93.10 293 233 Upper Stages Out of Production
Star 63F 10,669   297   Upper Stages In Production
Star 63 10,931 107.20 282   Upper Stages In Production
Pegasus-2 12,053 118.20 290   Upper Stages In Production
M-3B-J 13,469 132.10 294   Upper Stages Developed 1995-
M-3B-Mu 13,472 132.10 294   Upper Stages In Production
Pegasus XL-2 15,653 153.50 290 240 Upper Stages In Production
Kartukov Soyuz T - TM SAS 17k 17,500 171.00     First Stages In Production
EPKM 18,000   292   Upper Stages Hardware
P4 18,000 176.50 273 240 Upper Stages Out of Production
Kartukov P-5 18,300 179.00     First Stages Out of Production
SRM-1 18,508 181.50 296 115 Upper Stages In Production
Star 75 20,511 242.80 288   Upper Stages In Production
M56A-1 23,300 228.50 297 270 Upper Stages Out of Production
MIHT-3 25,000 245.20 280 220 Upper Stages In Production
TX-354-3 26,402 258.90 262 232 First Stages In Production
SLV-2 27,227 267.00 267 216 Upper Stages In Production
M33-20-4 29,164 286.00 247 232 First Stages Out of Production
SPRD-30 30,000 294.00     First Stages Out of Production
Kartukov P-35 30,000 294.00     First Stages Out of Production
M34 30,000 294.20 301   Upper Stages In Production
SB-735 33,430 327.80 263 238 First Stages In Production
PSLV-3 33,519 328.70 291 160 Upper Stages In Production
S-44 33,900 332.40 282   Upper Stages In Production
PRD-15 40,000 392.00     First Stages Out of Production
SPRD-15 41,000 402.00     First Stages Out of Production
Castor 4 41,524 407.20 261 228 First Stages Out of Production
Castor 4BXL 43,746 429.00 267   Upper Stages In Production
Castor 4B 43,910 430.60 281 220 Upper Stages In Production
Algol 3A 47,387 464.70 259 226 First Stages In Production
Algol 1 48,022 470.90 236 214 First Stages Out of Production
Algol 3 48,121 471.90 284 238 First Stages In Production
Castor 4A 48,774 478.30 266 237 First Stages In Production
Pegasus-1 49,447 484.90 285 180 First Stages In Production
MIHT-2 50,000 490.30 280 220 Upper Stages In Production
GEM 40 50,905 499.20 274 245 First Stages In Production
RSA-3-1 51,000 500.00 273 230 First Stages Out of Production
SLV-1 51,251 502.60 253 229 First Stages In Production
RSA-3-2 53,000 519.00 284 220 Upper Stages Developed -1995
M-23-Mu 53,433 524.00 285 220 Upper Stages In Production
M-23-J 53,515 524.80 282   Upper Stages Developed 1995-
Algol 2 57,537 564.20 255 232 First Stages Out of Production
Pegasus XL-1 60,062 589.00 293 180 First Stages In Production
Castor 4AXL 61,164 599.80 269   Upper Stages In Production
GEM 46 62,000 608.10 274 242 First Stages In Production
RSA-4-2 69,000 676.00 275 220 Upper Stages Out of Production
SPB 7.35 70,360 690.00 263 240 First Stages Out of Production
P9.5 70,360 690.00 263 240 First Stages In Production
Kartukov Soyuz T - TM SAS 73k 73,000 715.00     First Stages In Production
LK-1 79,000   272 250 Lower Stages Development
PRD-52 80,000 784.00     First Stages Out of Production
Kartukov Soyuz SAS 80,100 785.00     First Stages Out of Production
M55/TX-55/Tu-122 80,700 792.00 262 237 First Stages Out of Production
GEM 60 86,830 851.50 275 245 First Stages In Production
MIHT-1 100,000 980.60 263 238 First Stages In Production
M24 126,984 1,245.30 288 203 Upper Stages In Production
M-13 128,731 1,262.40 263 238 First Stages In Production
RSA-4-1 155,000 1,520.00 263 238 First Stages Out of Production
H-2-0 157,036 1,540.00 273 237 First Stages In Production
H-2/J-1-1 158,730 1,556.60 273 248 First Stages In Production
Castor 120 168,000 1,650.00 280 229 First Stages In Production
S-40TM 212,500 2,083.90 272 204 Upper Stages In Production
Peackeeper-1 224,796 2,204.40 282 250 First Stages In Production
Peacekeeper 1 224,796 2,204.50 282 250 First Stages In Production
SRB-A 230,000   280   First Stages In Development
S-43 309,000 3,030.20 265 225 First Stages In Production
S-43TM 327,000 3,206.70 276 170 Upper Stages In Production
M14 385,488 3,780.30 276 246 First Stages In Production
PSLV-1 495,590 4,860.00 264 237 First Stages In Production
UA1205 596,474 5,849.30 263 238 First Stages Out of Production
UA1206 634,977 6,226.90 265 240 First Stages Out of Production
P230 660,000 6,472.30 286 259 First Stages In Production
UA1207 725,732 7,116.90 272 245 First Stages In Production
USRM 770,975 7,560.50 286 259 First Stages In Production
UA-156 910,044 8,924.30 263 238 First Stages Developed to 1966
AJ-260-1/3 1,030,455 10,105.00 275   First Stages Design concept 1960's
200 inch solid, segment x 4 1,134,000 11,120.00 285   Upper Stages Study, NASA, 1960
AJ-260X 1/3 1,136,300 11,143.00 263 238 First Stages Design concept 1960's
SRB 1,174,713 11,519.80 269 237 First Stages In Production
Redesigned SRM 1,174,736 11,520.00 269   First Stages In Production
Thiokol 156 1,503,716 14,746.10 263 238 First Stages Developed to 1966
Hercules 1,587,302 15,565.80 286 259 First Stages Developed 1995-
AJ-260-2 1,804,460 17,695.30 263 238 First Stages Developed to 1966
200 inch solid, segment x 6 2,857,000 28,017.00 263 238 First Stages Out of Production
AJ-260X 3,608,918 35,390.70 263 238 First Stages Developed to 1966
280 inch solid 4,712,000 46,208.00 265 238 First Stages Study 1963
300 inch solid 6,485,000 63,595.00 263 234 First Stages Study 1963
325 in solid 7,041,000 69,047.00 263 238 First Stages Study General Dynamics 1963

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Last update 3 May 2001.
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