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{{Short description|Rocket with a motor that uses solid propellants}}
{{more citations needed|date=March 2017}}
{{Use American English|date=September 2016}}
[[File:Space Shuttle Columbia launching.jpg|thumb|300px|The [[Space Shuttle]] was launched with the help of two solid-fuel boosters known as [[Space Shuttle Solid Rocket Booster|SRBs]]]]

A '''solid-propellant rocket''' or '''solid rocket''' is a [[rocket]] with a [[rocket engine]] that uses [[Rocket propellant#Solid chemical propellants|solid propellants]] ([[fuel]]/[[oxidizer]]). The earliest rockets were solid-fuel rockets powered by [[gunpowder]]. The inception of gunpowder rockets in warfare can be credited to the ancient Chinese, and in the 13th century, the Mongols played a pivotal role in facilitating their westward adoption.<ref>chapters 1–2, ''Blazing the trail: the early history of spacecraft and rocketry'', [[Mike Gruntman]], AIAA, 2004, {{ISBN|1-56347-705-X}}.</ref>

All rockets used some form of solid or powdered [[propellant]] until the 20th century, when [[liquid-propellant rocket]]s offered more efficient and controllable alternatives. Because of their simplicity and reliability, solid rockets are still used today in military armaments worldwide, [[model rocket]]s, [[solid rocket booster]]s and on larger applications.

Since solid-fuel rockets can remain in storage for an extended period without much propellant degradation, and since they almost always launch reliably, they have been frequently used in military applications such as [[missile]]s. The lower performance of solid propellants (as compared to liquids) does not favor their use as primary propulsion in modern medium-to-large launch vehicles customarily used for commercial satellites and major space probes. Solids are, however, frequently used as strap-on boosters to increase payload capacity or as spin-stabilized add-on upper stages when higher-than-normal velocities are required. Solid rockets are used as light launch vehicles for [[low Earth orbit]] (LEO) payloads under 2 tons or escape payloads up to {{convert|1100|lb|kg|order=flip}}.<ref>{{Cite web|title=LADEE - Lunar Atmosphere Dust and Environment Explorer|url=http://www.nasa.gov/mission_pages/ladee/main/index.html|last=Culler|first=Jessica|date=2015-06-16|website=NASA|access-date=2020-06-02}}</ref><ref>{{cite web|url=http://www.space-travel.com/reports/LockMart_And_ATK_Athena_Launch_Vehicles_Selected_As_A_NASA_Launch_Services_Provider_999.html|title=LockMart And ATK Athena Launch Vehicles Selected As A NASA Launch Services Provider|website=www.space-travel.com}}</ref>

==Basic concepts==
[[File:Solid-Fuel Rocket Diagram.svg|thumb|A simplified diagram of a solid-fuel rocket.{{ordered list
| list_style=margin:0;
| item_style=list-style-position:inside;
| A solid [[Rocket propellant#Solid chemical propellants|fuel-oxidizer mixture]] (propellant) is packed into the rocket, with a cylindrical hole in the middle.
| An [[Pyrotechnic initiator|igniter]] combusts the surface of the propellant.
| The cylindrical hole in the propellant acts as a [[combustion chamber]].
| The hot exhaust is choked at the throat, which, among other things, dictates the amount of thrust produced.
| Exhaust exits the rocket.
}}]]
A simple solid [[rocket motor]] consists of a casing, [[De Laval nozzle|nozzle]], grain ([[propellant charge]]), and [[igniter]].

The solid grain mass [[combustion|burns]] in a predictable fashion to produce exhaust gases, the flow of which is described by [[Taylor–Culick flow]]. The [[propelling nozzle|nozzle]] dimensions are calculated to maintain a design [[Rocket engine#Combustion chamber|chamber]] pressure, while producing [[thrust]] from the exhaust gases.

Once ignited, a simple solid rocket motor cannot be shut off, as it contains all the ingredients necessary for combustion within the chamber in which they are burned. More advanced solid rocket motors can be [[throttle]]d, or extinguished<ref>{{cite web|url=https://patents.google.com/patent/US8336287B1/en|title=Solid propellant rocket motor having self-extinguishing propellant grain and systems therefrom}}</ref> and re-ignited, by control of the nozzle geometry or through the use of vent ports. Further, [[pulsed rocket motors]] that burn in segments, and that can be ignited upon command are available.

Modern designs may also include a steerable nozzle for guidance, [[avionics]], recovery hardware ([[parachute]]s), [[self-destruct]] mechanisms, [[Auxiliary power unit|APUs]], controllable tactical motors, controllable divert and [[Spacecraft attitude control|attitude control]] motors, and thermal management materials.
{{clear}}

==History==
[[File:RIAN archive 303890 A battery of Katyusha during the 1941-1945 Great Patriotic War.jpg|thumb|A battery of [[Katyusha rocket launcher]]s fires at German forces during the [[Battle of Stalingrad]], 6 October 1942]]
[[File:Aerojet260.png|thumb|Aerojet 260 motor test, 25 September 1965]]
The medieval [[Song dynasty]] Chinese invented a very primitive form of solid-propellant rocket.<ref>{{Cite book |title=Space Science in China |last= Hu |first=Wen-Rui |year=1997 |isbn= 978-9056990237 |publication-date=August 20, 1997 |page=15|publisher= CRC Press }}</ref> Illustrations and descriptions in the 14th century Chinese military treatise ''[[Huolongjing]]'' by the Ming dynasty military writer and philosopher [[Jiao Yu]] confirm that the Chinese in 1232 used proto solid propellant rockets then known as "[[fire arrows]]" to drive back the Mongols during the [[Mongol siege of Kaifeng]].<ref name="Greatrix 2012 1">{{Cite book |title=Powered Flight: The Engineering of Aerospace Propulsion |last=Greatrix |first= David R.  |publisher=Springer |year=2012 |isbn=978-1447124849 |pages=1}}</ref><ref name="Nielsen 1997 2–4">{{Cite book |title=Blast Off!: Rocketry for Elementary and Middle School Students P |last= Nielsen  |first= Leona |publisher= Libraries Unlimited  |year=1997 |isbn= 978-1563084386 |pages=2–4}}</ref> Each arrow took a primitive form of a simple, solid-propellant rocket tube that was filled with gunpowder. One open end allowed the gas to escape and was attached to a long stick that acted as a guidance system for flight direction control.<ref name="Nielsen 1997 2–4"/><ref name="Greatrix 2012 1"/>

The first rockets with tubes of cast iron were used by the [[Kingdom of Mysore]] under [[Hyder Ali]] and [[Tipu Sultan]] in the 1750s. These rockets had a reach of targets up to a mile and a half away. These were extremely effective in the [[Second Anglo-Mysore War]] that ended in a humiliating defeat for the [[British East India Company]]. Word of the success of the Mysore rockets against the British triggered research in England, France, Ireland and elsewhere. When the British finally conquered the fort of [[Siege of Seringapatam (1799)|Srirangapatana]] in 1799, hundreds of rockets were shipped off to the [[Royal Arsenal]] near London to be reverse-engineered. This led to the first industrial manufacture of military rockets with the [[Congreve rocket]] in 1804.<ref name="Bowdoin 2004">{{Cite book |title=Rockets and Missiles: The Life Story of a Technology|last= Van Riper  |first= Bowdoin |publisher= The Johns Hopkins University Press |year=2004 |isbn= 978-0801887925 |pages=14–15}}</ref>

In 1921 the [[Soviet]] research and development laboratory [[Gas Dynamics Laboratory]] began developing solid-propellant rockets, which resulted in the first launch in 1928, that flew for approximately 1,300 metres.<ref name="RSB_GDL">{{cite web |last1=Zak |first1=Anatoly |title=Gas Dynamics Laboratory |url=http://www.russianspaceweb.com/gdl.html |website=Russian Space Web |access-date=29 May 2022}}</ref> These rockets were used in 1931 for the world's first successful use of rockets to [[JATO|assist take-off of aircraft]].<ref name="Glushko">{{cite book |last1=Glushko |first1=Valentin |title=Developments of Rocketry and Space Technology in the USSR |date=1 January 1973 |publisher=Novosti Press Pub. House |page=7 |url=https://www.amazon.com/Development-rocketry-space-technology-USSR/dp/B0006CHI4I}}</ref>  The research continued from 1933 by the [[Reactive Scientific Research Institute]] (RNII) with the development of the [[RS-82 (rocket family)|RS-82 and RS-132 rockets]], including designing several variations for ground-to-air, ground-to-ground, air-to-ground and air-to-air combat.<ref name="W&WRRP">{{cite web |title=Russian Rocket Projectiles – WWII |url=https://weaponsandwarfare.com/2018/11/19/russian-rocket-projectiles-wwii/ |website=Weapons and Warfare |date=18 November 2018 |access-date=29 May 2022}}</ref> The earliest known use by the [[Soviet Air Force]] of aircraft-launched unguided [[Air-to-air rocket|anti-aircraft rockets]] in combat against heavier-than-air aircraft took place in [[1939 in aviation|August 1939]], during the [[Battle of Khalkhin Gol]].<ref name="W&WRRP" />  In June 1938, the RNII began developing a multiple rocket launcher based on the RS-132 rocket.<ref>{{cite book| author = Akimov V.N., Koroteev A.S., Gafarov A.A. | chapter = The weapon of victory - "Katyusha"| url = https://elibrary.ru/item.asp?id=19633690& | title = Research Center named after M.V. Keldysh. 1933-2003 : 70 years at the forefront of rocket and space technology (in Russian). |location= М |year= 2003 |work=Mechanical Engineering  | pages = 92–101| isbn = 5-217-03205-7}}</ref> In August 1939, the completed product was the [[Katyusha rocket launcher|BM-13 / Katyusha rocket launcher]]. Towards the end of 1938 the first significant large scale testing of the rocket launchers took place, 233 rockets of various types were used. A salvo of rockets could completely straddle a target at a range of {{convert|5,500|m|mi}}. By the end of [[World War II]] total production of rocket launchers reached about 10,000.<ref name=Zaloga>{{cite book | last = Zaloga | first = Steven J | author-link = Steven Zaloga | author2 = James Grandsen | title = Soviet Tanks and Combat Vehicles of World War Two | url = https://archive.org/details/soviettankscomba00zalo | url-access = limited | publisher = Arms and Armour Press | year = 1984 | location = London | pages = 150–153 | isbn = 0-85368-606-8}}</ref> with 12 million rockets of the RS type produced for the Soviet armed forces.<ref name="RSW_RNII">{{cite web |last1=Zak |first1=Anatoly |title=History of the Rocket Research Institute, RNII |url=http://www.russianspaceweb.com/rnii.html |website=Russian Spaceweb |access-date=18 June 2022}}</ref>

In the [[United States]] modern castable composite solid rocket motors were invented by the American aerospace engineer [[Jack Parsons (rocket engineer)|Jack Parsons]] at [[Caltech]] in 1942 when he replaced double base propellant with roofing [[Bitumen|asphalt]] and [[potassium perchlorate]]. This made possible slow-burning rocket motors of adequate size and with sufficient shelf-life for jet-assisted take off applications. [[Charles Bartley]], employed at JPL (Caltech), substituted curable [[synthetic rubber]] for the gooey asphalt, creating a flexible but geometrically stable load-bearing propellant grain that bonded securely to the motor casing. This made possible much larger solid rocket motors. Atlantic Research Corporation significantly boosted composite propellant I<sub>sp</sub> in 1954 by increasing the amount of powdered aluminium in the propellant to as much as 20%.<ref>{{cite book |author= M. D. Black |title= The Evolution of Rocket Technology |page= 39 |publisher= Native Planter, SLC |date= 2012 }} payloadz.com under ''ebook/History'' {{dead link|date= July 2014}}</ref>

Solid-propellant rocket technology got its largest boost in technical innovation, size and capability with the various mid-20th century government initiatives to develop increasingly capable military missiles. After initial designs of [[ballistic missile]] military technology designed with [[liquid-propellant rocket]]s in the 1940s and 1950s, both the [[Soviet Union]] and the [[Federal government of the United States|United States]] embarked on major initiatives to develop solid-propellant [[Tactical ballistic missile|local]], [[Medium-range ballistic missile|regional]], and [[Intercontinental ballistic missile|intercontinental]] ballistic missiles, including solid-propellant missiles that could be launched from [[Air-launched ballistic missile|air]] or [[Submarine-launched ballistic missile|sea]]. Many [[List of missiles by country|other governments]] also developed these military technologies over the next 50 years.

By the later 1980s and continuing to 2020, these government-developed highly-capable solid rocket technologies have been applied to [[orbital spaceflight]] by many<!-- possibly all of the governments of countries that have developed orbital spaceflight technology, which is 6 or 8 now; but would need to confirm --> [[nation state|government-directed programs]], most often as [[solid rocket booster|booster rockets]] to add extra thrust during the early ascent of their primarily liquid rocket [[launch vehicle]]s.  Some designs have had solid rocket upper stages as well.  Examples flying in the 2010s include the European [[Ariane 5]], US [[Atlas V]] and [[Space Shuttle Solid Rocket Booster|Space Shuttle]], and Japan's [[H-II]].

The largest solid rocket motors ever built were Aerojet's three {{convert|260|in|m|adj=on|sigfig=3|order=flip|sp=us}} monolithic solid motors cast in Florida.<ref>{{Cite journal|url = https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20000033816.pdf|title = The 260 - The Largest Solid Rocket Motor Ever Tested|date = June 1999|access-date = July 24, 2014|website = nasa.gov}}</ref> Motors 260 SL-1 and SL-2 were {{convert|261|in|m|sigfig=3|order=flip|sp=us}} in diameter, {{convert|80|ft|8|in|m|order=flip|sp=us}} long, weighed {{convert|1,858,300|lb|kg|order=flip}}, and had a maximum thrust of {{convert|3.5|e6lbf|MN lbf|order=out|abbr=on}}. Burn duration was two minutes. The nozzle throat was large enough to walk through standing up. The motor was capable of serving as a 1-to-1 replacement for the 8-engine [[Saturn I]] liquid-propellant first stage but was never used as such. Motor 260 SL-3 was of similar length and weight but had a maximum thrust of {{convert|5.4|e6lbf|MN lbf|order=out|abbr=on}} and a shorter duration.

==Design==
Design begins with the total [[Impulse (physics)|impulse]] required, which determines the [[fuel]] and [[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.{{citation needed|date=June 2020}}<ref>{{Cite book |last1=Kosanke |first1=K. L. |url=https://books.google.com/books?id=AK8zAwAAQBAJ&dq=the+grain+burns+at+a+predictable+rate%2C+given+its+surface+area+and+chamber+pressure&pg=PA888 |title=Encyclopedic Dictionary of Pyrotechnics: (and Related Subjects) |last2=Sturman |first2=Barry T. |last3=Winokur |first3=Robert M. |last4=Kosanke |first4=B. J. |date=October 2012 |publisher=Journal of Pyrotechnics |isbn=978-1-889526-21-8 |language=en}}</ref>
* The chamber pressure is determined by the nozzle throat 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".{{clarify|date=June 2020}}

The grain may or may not be bonded to the casing. Case-bonded motors are more difficult to design, since the deformation of the case and the grain under flight 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 production rate and pressure, which may rupture the casing.

Another failure mode is casing [[seal (mechanical)|seal]] failure. 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 [[STS-51-L|Space Shuttle ''Challenger'' disaster]].

==Grain geometry==
Solid rocket fuel [[deflagrates]] from the surface of exposed propellant in the combustion chamber. In this fashion, the geometry of the propellant inside the rocket motor plays an important role in the overall motor performance. As the surface of the propellant burns, the shape evolves (a subject of study in internal ballistics), most often changing the propellant surface area exposed to the combustion gases. Since,the propellant volume is equal to the [[cross sectional area]] <math>A_s</math> times the fuel length, the [[volumetric]] propellant consumption rate is the cross section area times the linear burn rate <math>\dot{b}</math>, and the [[instantaneous]] [[mass flow rate]] of combustion gases generated is equal to the volumetric rate times the fuel density <math>\rho</math>:

:<math>\dot{m} = \rho \cdot A_s \cdot \dot{b}</math>

Several geometric configurations are often used depending on the application and desired [[thrust curve]]:

<gallery>
Image:circ ex.jpg|Circular bore simulation
Image:cslot ex.jpg|C-slot simulation
Image:moon ex.jpg|Moon burner simulation
Image:fino ex.jpg|5-point finocyl simulation
</gallery>

* Circular bore: if in [[BATES]] configuration, produces progressive-regressive thrust curve.
* End burner: propellant burns from one axial end to other producing steady long burn, though has thermal difficulties, center of gravity (CG) shift.
* C-slot: propellant with large wedge cut out of side (along axial direction), producing fairly long regressive thrust, though has thermal difficulties and asymmetric CG characteristics.
* Moon burner: off-center circular bore produces progressive-regressive long burn, though has slight asymmetric CG characteristics
* Finocyl: usually a 5- or 6-legged star-like shape that can produce very level thrust, with a bit quicker burn than circular bore due to increased surface area.

==Casing==
The casing may be constructed from a range of materials. Cardboard is used for small [[black powder]] model motors, whereas aluminium is used for larger composite-fuel hobby motors. Steel was used for the [[Space Shuttle Solid Rocket Booster|space shuttle boosters]]. Filament-wound [[Graphite-Epoxy Motor|graphite epoxy casings]] are used for high-performance motors.

The casing must be designed to withstand the pressure and resulting stresses of the rocket motor, possibly at elevated temperature. For design, the casing is considered a [[pressure vessel]].

To protect the casing from corrosive hot gases, a sacrificial thermal liner on the inside of the casing is often implemented, which [[Ablation|ablates]] to prolong the life of the motor casing.

==Nozzle==
{{Main|Rocket engine nozzle}}
A [[De Laval nozzle|convergent-divergent]] design accelerates the exhaust gas out of the nozzle to produce thrust. The nozzle must be constructed from a material that can withstand the heat of the combustion gas flow. Often, heat-resistant carbon-based materials are used, such as amorphous [[graphite]] or [[reinforced carbon–carbon]].

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 as in the [[V-2]] rocket, or by liquid injection thrust vectoring (LITV).

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 III]]C 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.<ref name="S">{{cite book | author=Sutton, George P. | title=Rocket Propulsion Elements | edition=7th | publisher=Wiley-Interscience | date=2000 | isbn=0-471-32642-9}}</ref>

An early [[LGM-30 Minuteman|Minuteman]] first stage used a single motor with four [[gimbal]]led nozzles to provide pitch, yaw, and roll control.

==Performance==
[[File:STS-130 exhaust cloud engulfs Launch Pad 39A.jpg|thumb|right|An [[exhaust gas|exhaust]] cloud engulfs [[Launch Pad 39A]] at [[NASA|NASA's]] [[Kennedy Space Center]] as the {{OV|105}} lifts off.]]
A typical, well-designed [[ammonium perchlorate composite propellant]] (APCP) first-stage motor may have a vacuum [[specific impulse]] (''I''{{sub|sp}}) as high as {{convert|285.6|isp}} (Titan IVB SRMU).<ref>{{cite web
 |url=http://www.ltas-vis.ulg.ac.be/cmsms/uploads/File/DataSheetSolidATK.pdf
 |title=ATK Space Propulsion Products Catalog, p.30
 |date=May 2008
 |publisher=Alliant Techsystems (ATK)
 |access-date=8 Dec 2015
 |archive-url=https://web.archive.org/web/20180730082316/http://www.ltas-vis.ulg.ac.be/cmsms/uploads/File/DataSheetSolidATK.pdf
 |archive-date=30 July 2018
 |url-status=dead
}}</ref> This compares to {{cvt|339.3|isp}} for RP1/LOX (RD-180)<ref>http://www.pw.utc.com/Products/Pratt+%26+Whitney+Rocketdyne/Propulsion+Solutions/Space{{dead link|date=November 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> and {{cvt|452.3|isp}} for LH<sub>2</sub>/LOX (Block II [[RS-25]])<ref>{{cite web |url=http://www.pw.utc.com/Products/Pratt+%26+Whitney+Rocketdyne |title=Pratt & Whitney Rocketdyne |access-date=2014-01-07 |url-status=dead |archive-url=https://web.archive.org/web/20110426223005/http://www.pw.utc.com/Products/Pratt+%2526+Whitney+Rocketdyne |archive-date=2011-04-26 }}</ref> bipropellant engines. Upper stage specific impulses are somewhat greater: as much as {{cvt|303.8|isp}} for APCP (Orbus 6E),<ref name="spaceandtech.com">{{cite web |url=http://www.spaceandtech.com/spacedata/elvs/titan4b_specs.shtml |title=Titan IVB - Specifications |access-date=2014-02-09 |url-status=usurped |archive-url=https://web.archive.org/web/20130719221015/http://www.spaceandtech.com/spacedata/elvs/titan4b_specs.shtml |archive-date=2013-07-19 }}</ref> {{cvt|359|isp}} for RP1/LOX (RD-0124)<ref>http://www.russianspaceweb.com/engines/rd0124.htm {{Dead link|date=February 2022}}</ref> and {{cvt|465.5|isp}} for LH<sub>2</sub>/LOX (RL10B-2).<ref>{{cite web |url=http://www.pw.utc.com/StaticFiles/Pratt%20.../Products/.../pwr_rl10b-2.pdf |title=RL10B-2 brochure|publisher=Pratt & Whitney Rocketdyne|year=2009 |access-date=2018-08-25 |url-status=dead |archive-url=https://web.archive.org/web/20120326211303/http://www.pw.utc.com/products/pwr/assets/pwr_rl10b-2.pdf |archive-date=2012-03-26 }}</ref> 

Propellant fractions are usually somewhat higher for (non-segmented) solid propellant first stages than for upper stages. The {{convert|117,000|lb|kg|order=flip|adj=on}} Castor 120 first stage has a propellant mass fraction of 92.23% while the {{convert|31,000|lb|kg|order=flip|adj=on}} Castor 30 upper stage developed for Orbital Science's Taurus II COTS (Commercial Off The Shelf) (International Space Station resupply) launch vehicle has a 91.3% propellant fraction with 2.9% graphite epoxy motor casing, 2.4% nozzle, igniter and thrust vector actuator, and 3.4% non-motor hardware including such things as payload mount, interstage adapter, cable raceway, instrumentation, etc. Castor 120 and Castor 30 are {{convert|93|and|92|in|m|2|order=flip|sp=us}} in diameter, respectively, and serve as stages on the Athena IC and IIC commercial launch vehicles. A four-stage Athena II using Castor 120s as both first and second stages became the first commercially developed launch vehicle to launch a lunar probe (''[[Lunar Prospector]]'') in 1998.

Solid rockets can provide high thrust for relatively low cost. For this reason, solids have been used as initial stages in rockets (for example the [[Space Shuttle]]), while reserving high specific impulse engines, especially less massive hydrogen-fueled engines, for higher stages. In addition, solid rockets have a long history as the final boost stage for satellites due to their simplicity, reliability, compactness and reasonably high [[Propellant mass fraction|mass fraction]].<ref>[http://www.astronautix.com/props/solid.htm Solid<!-- Bot generated title -->] {{webarchive|url=https://web.archive.org/web/20020105224630/http://www.astronautix.com/props/solid.htm |date=2002-01-05 }}</ref> A spin-stabilized solid rocket motor is sometimes added when extra velocity is required, such as for a mission to a comet or the outer solar system, because a spinner does not require a guidance system (on the newly added stage). Thiokol's extensive family of mostly titanium-cased ''Star'' space motors has been widely used, especially on Delta launch vehicles and as spin-stabilized upper stages to launch satellites from the cargo bay of the Space Shuttle. ''Star'' motors have propellant fractions as high as 94.6% but add-on structures and equipment reduce the operating mass fraction by 2% or more.

Higher performing solid rocket propellants are used in large strategic missiles (as opposed to commercial launch vehicles). [[HMX]], C<sub>4</sub>H<sub>8</sub>N<sub>4</sub>(NO<sub>2</sub>)<sub>4</sub>, a nitramine with greater energy than ammonium perchlorate, was used in the propellant of the Peacekeeper ICBM and is the main ingredient in NEPE-75 propellant used in the Trident II D-5 Fleet Ballistic Missile.<ref>{{cite web|url=http://www.globalsecurity.org/wmd/systems/d-5-features.htm|title=Trident II D-5 Fleet Ballistic Missile FBM / SLBM - United States|first=John|last=Pike|website=www.globalsecurity.org}}</ref> It is because of explosive hazard that the higher energy military solid propellants containing HMX are not used in commercial launch vehicles except when the LV is an adapted ballistic missile already containing HMX propellant (Minotaur IV and V based on the retired Peacekeeper ICBMs).<ref>''Minotaur IV User's Guide, Release 1.0'', Orbital Sciences Corp., January 2005, p. 4</ref> The Naval Air Weapons Station at China Lake, California, developed a new compound, C<sub>6</sub>H<sub>6</sub>N<sub>6</sub>(NO<sub>2</sub>)<sub>6</sub>, called simply [[CL-20]] (China Lake compound <sup>#</sup>20). Compared to HMX, CL-20 has 14% more energy per mass, 20% more energy per volume, and a higher oxygen-to-fuel ratio.<ref name="navair.navy.mil">http://www.navair.navy.mil/techTrans/index.cfm?map=local.ccms.view.aB&doc=crada.13{{dead link|date= October 2016}}</ref> One of the motivations for development of these very high [[energy density]] military solid propellants is to achieve mid-course exo-atmospheric ABM capability from missiles small enough to fit in existing ship-based below-deck vertical launch tubes and air-mobile truck-mounted launch tubes. CL-20 propellant compliant with Congress' 2004 insensitive munitions (IM) law has been demonstrated and may, as its cost comes down, be suitable for use in commercial launch vehicles, with a very significant increase in performance compared with the currently favored APCP solid propellants. With a specific impulse of 309 s already demonstrated by Peacekeeper's second stage using HMX propellant, the higher energy of CL-20 propellant can be expected to increase specific impulse to around 320 s in similar ICBM or launch vehicle upper stage applications, without the explosive hazard of HMX.<ref>M. D. Black, ''The Evolution of ROCKET TECHNOLOGY'', pp. 92-94, Native Planter, SLC, 2012, payloadz.com under ''ebook/History''</ref>

An attractive attribute for military use is the ability for solid rocket propellant to remain loaded in the rocket for long durations and then be reliably launched at a moment's notice.

==Propellant families==
===Black powder (gunpowder) propellant===
[[Gunpowder|Black powder]] (gunpowder) is composed of [[charcoal]] (fuel), [[potassium nitrate]] (oxidizer), and [[sulfur]] (fuel and catalyst). It is one of the oldest [[pyrotechnic]] compositions with application to rocketry. In modern times, black powder finds use in low-power model rockets (such as [[Estes Industries|Estes]] and Quest rockets),<ref>{{cite web|url=https://www.apogeerockets.com/Rocket_Motors/Estes_Motors|title=Model Rocketry Resources and Components|access-date=16 Aug 2017}}</ref><ref>{{cite web|url=http://www.asp-rocketry.com/ecommerce/Quest-Black-Powder-Model-Rocket-Engines.cfm?cat_id=88|title=Quest Black Powder Model Rocket Engines|access-date=16 Aug 2017|archive-url=https://web.archive.org/web/20170816190952/http://www.asp-rocketry.com/ecommerce/Quest-Black-Powder-Model-Rocket-Engines.cfm?cat_id=88|archive-date=16 August 2017|url-status=dead}}</ref> as it is cheap and fairly easy to produce. The fuel grain is typically a mixture of pressed fine powder (into a solid, hard slug), with a burn rate that is highly dependent upon exact composition and operating conditions. The [[specific impulse]] of black powder is low, around {{convert|80|isp |abbr=on}}. The grain is sensitive to fracture and, therefore, catastrophic failure. Black powder does not typically find use in motors above {{convert|40|N|lbf|abbr=off}} thrust.

===Zinc–sulfur (ZS) propellants===
Composed of powdered [[zinc]] metal and powdered sulfur (oxidizer), ZS or "micrograin" is another pressed propellant that does not find any practical application outside specialized amateur rocketry circles due to its poor performance (as most ZS burns outside the combustion chamber) and fast linear burn rates on the order of 2&nbsp;m/s. ZS is most often employed as a novelty propellant as the rocket accelerates extremely quickly leaving a spectacular large orange fireball behind it.

==="Candy" propellants===
In general, [[rocket candy]] propellants are an oxidizer (typically potassium nitrate) and a sugar fuel (typically [[dextrose]], [[sorbitol]], or [[sucrose]]) that are cast into shape by gently melting the propellant constituents together and pouring or packing the [[amorphous]] [[colloid]] into a mold. Candy propellants generate a low-medium specific impulse of roughly {{convert|130|isp|abbr=on}} and, thus, are used primarily by amateur and experimental rocketeers.

===Double-base (DB) propellants===
DB propellants are composed of two [[monopropellant]] fuel components where one typically acts as a high-energy (yet unstable) monopropellant and the other acts as a lower-energy stabilizing (and gelling) monopropellant. In typical circumstances, [[nitroglycerin]] is dissolved in a [[nitrocellulose]] gel and solidified with additives. DB propellants are implemented in applications where minimal smoke is required yet a medium-high I<sub>sp</sub> of roughly {{convert|235|isp|abbr=on}} is required. The addition of metal fuels (such as [[aluminium]]) can increase performance to around {{convert|250|isp|abbr=on}}, though [[metal oxide]] [[nucleation]] in the exhaust can turn the smoke opaque.

===Composite propellants===
A powdered oxidizer and powdered metal fuel are intimately mixed and immobilized with a rubbery binder (that also acts as a fuel). Composite propellants are often either [[ammonium nitrate|ammonium-nitrate]]-based (ANCP) or [[ammonium perchlorate|ammonium-perchlorate]]-based (APCP). Ammonium nitrate composite propellant often uses [[magnesium]] and/or [[aluminium]] as fuel and delivers medium performance (I<sub>sp</sub> of about {{convert|210|isp|abbr=on}}) whereas [[ammonium perchlorate composite propellant]] often uses aluminium fuel and delivers high performance: vacuum I<sub>sp</sub> up to {{convert|296|isp|abbr=on}} with a single-piece nozzle or {{convert|304|isp|abbr=on}} with a high-area-ratio telescoping nozzle.<ref name="spaceandtech.com"/> Aluminium is used as fuel because it has a reasonable specific energy density, a high volumetric energy density, and is difficult to ignite accidentally. Composite propellants are cast, and retain their shape after the rubber binder, such as [[Hydroxyl-terminated polybutadiene]] (HTPB), [[cross-links]] (solidifies) with the aid of a curative additive. Because of its high performance, moderate ease of manufacturing, and moderate cost, APCP finds widespread use in space, military, and amateur rockets, whereas cheaper and less efficient ANCP finds use in amateur rocketry and [[gas generator]]s. [[Ammonium dinitramide]], NH<sub>4</sub>N(NO<sub>2</sub>)<sub>2</sub>, is being considered as a 1-to-1 chlorine-free substitute for ammonium perchlorate in composite propellants. Unlike ammonium nitrate, ADN can be substituted for AP without a loss in motor performance.

Polyurethane-bound aluminium-APCP solid fuel was used in the submarine-launched [[Polaris missile]]s.<ref>{{Cite web|url=https://fas.org/nuke/guide/usa/slbm/a-1.htm|title = Polaris A1 - United States Nuclear Forces}}</ref> APCP used in the [[Space Shuttle Solid Rocket Boosters|space shuttle Solid Rocket Boosters]] consisted of ammonium perchlorate (oxidizer, 69.6% by weight), aluminium (fuel, 16%), iron oxide (a catalyst, 0.4%), polybutadiene [[acrylonitrile]] (PBAN) polymer (a non-urethane rubber binder that held the mixture together and acted as secondary fuel, 12.04%), and an epoxy [[Curing (chemistry)|curing]] agent (1.96%).<ref name="sts-newsref-srb">{{cite web | url = http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html | title = Shuttle Solid Rocket Boosters | publisher = NASA | access-date = 2015-10-02 | archive-date = 2019-04-30 | archive-url = https://web.archive.org/web/20190430095734/https://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html | url-status = dead }}</ref><ref name="returntoflight-system-SRB">{{cite web | url = http://www.nasa.gov/returntoflight/system/system_SRB.html | title = Solid Rocket Boosters | publisher = NASA | access-date = 2015-10-02 | archive-date = 2013-04-06 | archive-url = https://web.archive.org/web/20130406193019/http://www.nasa.gov/returntoflight/system/system_SRB.html | url-status = dead }}</ref> It developed a specific impulse of 242 seconds (2.37&nbsp;km/s) at sea level or 268 seconds (2.63&nbsp;km/s) in a vacuum. The 2005-2009 [[Constellation Program]] was to use a similar PBAN-bound APCP.<ref>{{cite news |title=NASA Tests Engine With an Uncertain Future |url=https://www.nytimes.com/2010/08/31/science/space/31rocket.html?hpw |work=[[New York Times]] |date=August 30, 2010 |access-date=2010-08-31 | first=Kenneth | last=Chang}}</ref>

In 2009, a group succeeded in creating a propellant of [[water]] and nanoaluminium ([[ALICE (propellant)|ALICE]]).

===High-energy composite (HEC) propellants===
Typical HEC propellants start with a standard composite propellant mixture (such as APCP) and add a high-energy explosive to the mix. This extra component usually is in the form of small crystals of [[RDX]] or [[HMX]], both of which have higher energy than ammonium perchlorate. Despite a modest increase in specific impulse, implementation is limited due to the increased hazards of the high-explosive additives.

===Composite modified double base propellants===
Composite modified double base propellants start with a nitrocellulose/nitroglycerin double base propellant as a binder and add solids (typically [[ammonium perchlorate]] (AP) and powdered [[aluminium]]) normally used in composite propellants. The ammonium perchlorate makes up the oxygen deficit introduced by using [[nitrocellulose]], improving the overall specific impulse. The aluminium improves specific impulse as well as combustion stability. High performing propellants such as [[NEPE-75]] used to fuel the [[UGM-133 Trident II#Design|Trident II]] D-5 [[SLBM]] replace most of the AP with [[polyethylene glycol]]-bound [[HMX]], further increasing specific impulse. The mixing of composite and double base propellant ingredients has become so common as to blur the functional definition of double base propellants.

===Minimum-signature (''smokeless'') propellants===
One of the most active areas of solid propellant research is the development of high-energy, minimum-signature propellant using C<sub>6</sub>H<sub>6</sub>N<sub>6</sub>(NO<sub>2</sub>)<sub>6</sub> [[Hexanitrohexaazaisowurtzitane|CL-20 nitroamine]] ([[Naval Air Weapons Station China Lake|China Lake]] compound #20), which has 14% higher energy per mass and 20% higher energy density than HMX. The new propellant has been successfully developed and tested in tactical rocket motors. The propellant is non-polluting: acid-free, solid particulates-free, and lead-free. It is also smokeless and has only a faint shock diamond pattern that is visible in the otherwise transparent exhaust. Without the bright flame and dense smoke trail produced by the burning of aluminized propellants, these smokeless propellants all but eliminate the risk of giving away the positions from which the missiles are fired. The new CL-20 propellant is shock-insensitive (hazard class 1.3) as opposed to current HMX smokeless propellants which are highly detonable (hazard class 1.1). CL-20 is considered a major breakthrough in solid rocket propellant technology but has yet to see widespread use because costs remain high.<ref name="navair.navy.mil"/>

===Electric solid propellants===
Electric solid propellants (ESPs) are a family of high performance [[plastisol]] solid propellants that can be ignited and throttled by the application of electric current. Unlike conventional rocket motor propellants that are difficult to control and extinguish, ESPs can be ignited reliably at precise intervals and durations. It requires no moving parts and the propellant is insensitive to flames or electrical sparks.<ref>{{cite book|chapter-url=http://arc.aiaa.org/doi/abs/10.2514/6.2013-4168|chapter=Electrical Solid Propellants: A Safe, Micro to Macro Propulsion Technology|first1=Wayne N.|last1=Sawka|first2=Michael|last2=McPherson|title=49th AIAA/ASME/SAE/ASEE Joint Propulsion Conference|date=12 July 2013|publisher=American Institute of Aeronautics and Astronautics|doi=10.2514/6.2013-4168|isbn=978-1-62410-222-6}}</ref>

==Hobby and amateur rocketry==
Solid propellant rocket motors can be bought for use in [[model rocket]]ry; they are normally small cylinders of black powder fuel with an integral [[nozzle]] and optionally a small charge that is set off when the propellant is exhausted after a time delay. This charge can be used to trigger a [[camera]], or deploy a [[parachute]]. Without this charge and delay, the motor may ignite a second [[multistage rocket|stage]] (black powder only).

In mid- and [[high-power rocketry]], commercially made APCP motors are widely used. They can be designed as either single-use or reloadables. These motors are available in impulse ranges from "A" (1.26&nbsp;Ns– 2.50&nbsp;Ns) to "O" (20.48&nbsp;kNs – 40.96&nbsp;kNs), from several manufacturers. They are manufactured in standardized diameters and varying lengths depending on required impulse. Standard motor diameters are 13, 18, 24, 29, 38, 54, 75, 98, and 150 millimeters. Different propellant formulations are available to produce different thrust profiles, as well as special effects such as colored flames, smoke trails, or large quantities of sparks (produced by adding [[titanium]] sponge to the mix).

==Use==
===Sounding rockets===
Almost all [[sounding rocket]]s use solid motors.

* [[Astrobee]]
* [[Black Brant (rocket)|Black Brant]]
* [[S-310]], [[S-520]]
* [[Skylark (rocket)]]
* [[Terrier-Orion]], [[Terrier-Malemute]]
* [[VSB-30]]

===Missiles===
Due to reliability, ease of storage and handling, solid rockets are used on missiles and ICBMs.

* Air-to-air missiles: [[AIM-9 Sidewinder]], [[AIM-54 Phoenix]], [[Fakour-90]], [[Meteor (missile)|Meteor]], [[R-33 (missile)|R-33]], [[PL-15]]
* Ballistic missiles: [[Jericho (missile)|Jericho]], [[Sejjil]], [[9K720 Iskander]], [[Fateh-110]], [[ATACMS]], [[DF-21]]
* ICBMs: [[LGM-30 Minuteman]], [[UGM-133 Trident II]], [[LGM-118 Peacekeeper]], [[RT-2PM Topol]], [[DF-41]], [[M51 SLBM]]

===Orbital rockets===
Solid rockets are suitable for launching small payloads to orbital velocities, especially if three or more stages are used. Many of these are based on repurposed ICBMs.
{{colbegin}}
* [[Scout (rocket family)|Scout]]
* [[Athena (rocket family)|Athena]]
* [[Mu (rocket family)|Mu]]
* [[Pegasus (rocket)|Pegasus]]
* [[Taurus (rocket)|Taurus]]
* [[Minotaur (rocket family)|Minotaur]]
* [[Start-1]]
* [[PSLV]] - alternating solid and liquid stages
* [[Small Satellite Launch Vehicle|SSLV]] - all lower 3 stages are solid stages
* [[Solid Fuel Ducted Ramjet|SFDR]]
* [[Shavit 2|Shavit]]
* [[Vega (rocket)|Vega]]
* [[Long March 11]]
* [[Omega (rocket)|OmegA]] (project was cancelled in 2020, rocket unflown)
* [[Qaem 100]]
{{colend}}
Larger liquid-fueled orbital rockets often use solid rocket boosters to gain enough initial thrust to launch the fully fueled rocket.
{{Main|Solid rocket booster}}
{{colbegin}}
* [[Delta II]]
* [[Titan IV]]
* [[Space Shuttle]]
* [[Space Launch System]]
* [[Ariane 5]]
* [[Atlas II]]
* [[Atlas V]] (optionally 1-5 boosters)
* [[Delta IV]] (optionally 2 or 4 boosters)
* [[H-IIA]], [[H-IIB]]
* [[Polar Satellite Launch Vehicle|PSLV]] - optional solid boosters to lift heavier payloads
* [[Geosynchronous Satellite Launch Vehicle Mk III|GSLV Mk III]]{{colend}}
Solid fuel is also used for some upper stages, particularly the [[Star 37]] (sometimes referred to as the "Burner" upper stage) and the [[Star 48]] (sometimes referred to as the "[[Payload Assist Module]]", or PAM), both manufactured originally by [[Thiokol]], and today by [[Northrop Grumman]]. They are used to lift large payloads to intended orbits (such as the [[Global Positioning System]] satellites), or smaller payloads to interplanetary—or even interstellar—trajectories. Another solid-fuel upper stage, used by the [[Space Shuttle]] and the [[Titan IV]], was the [[Boeing]]-manufactured [[Inertial Upper Stage]] (IUS).
* ''[[Pioneer 10]]'' and ''[[Pioneer 11]]'' were both sent out of the solar system by Star 37E upper stages from [[Atlas-Centaur]] rockets.
* ''[[Voyager 1]]'' and ''[[Voyager 2]]'' were both sent out of the solar system by Star 37E upper stages from [[Titan IIIE]] rockets.
* ''[[Magellan (spacecraft)|Magellan]]'' was sent to Venus on an IUS after being deployed from [[Space Shuttle Atlantis|Space Shuttle ''Atlantis'']] on [[STS-30]].
* ''[[Galileo (spacecraft)|Galileo]]'' was sent to Jupiter on an IUS after being deployed from [[Space Shuttle Atlantis|Space Shuttle ''Atlantis'']] on [[STS-34]].
* ''[[Ulysses (spacecraft)|Ulysses]]'' was sent to Jupiter on an IUS and a Star 48 PAM after being deployed from [[Space Shuttle Discovery|Space Shuttle ''Discovery'']] on [[STS-41]]. It then was placed in a polar orbit around the Sun following a [[gravity assist]] around Jupiter.
* ''[[New Horizons]]'' was sent out of the solar system on a Star 48 PAM from an [[Atlas V]] rocket.

Some rockets, like the [[Antares (rocket)|Antares]] (manufactured by Northrop Grumman), have mandatory solid-fuel upper stages. The Antares rocket uses the [[Northrop Grumman]]-manufactured [[Castor (rocket stage)|Castor 30]] as an upper stage.

==Advanced research==
* Environmentally sensitive fuel formulations such as [[ALICE (propellant)|ALICE propellant]]
* [[Ramjet]]s with solid fuel
* Variable thrust designs based on variable nozzle geometry
* [[Hybrid rocket]]s that use solid fuel and throttleable liquid or gaseous oxidizer

==See also==
{{colbegin}}
* [[Comparison of solid-fuelled orbital launch systems]]
* [[Comparison of orbital launch systems]]
* [[Comparison of orbital launcher families]]
* [[List of canceled launch vehicle designs]]
* [[List of missiles]]
* [[List of orbital launch systems]]
* [[List of sounding rockets]]
* [[List of military rockets]]
* [[Fireworks]]
* [[Pyrotechnic composition]]
* [[Intercontinental ballistic missile]]
* [[Jetex]]
* [[Space Shuttle Solid Rocket Booster]]
* [[Crawford burner]]
* [[Nano-thermite]]
* [[Skyrocket]]
{{colend}}

==References==
{{Reflist|30em}}

==Further reading==
* {{cite book  |editor=A. Davenas | year=1992 | title=Solid Rocket Propulsion Technology | publisher=Pergamon | isbn=978-0080409993}}

==External links==
* [http://www.braeunig.us/space/propuls.htm Robert A. Braeunig rocket propulsion page]
* [https://web.archive.org/web/20020806073439/http://astronautix.com/articles/comlants.htm Astronautix Composite Solid Propellants ]
* [http://www.esa.int/SPECIALS/Launchers_Access_to_Space/ASEDYQI4HNC_0.html Ariane 5 SRB]
* [http://www.tripoli.org/ Amateur High Power Rocketry Association]
* [http://www.nakka-rocketry.net/ Nakka-Rocketry (Design Calculations and Propellant Formulations)]
* [https://web.archive.org/web/20141017181140/http://balloons.space.edu/ndra/nickle.html 5 cent sugar rocket]
* [http://www.allstar.fiu.edu/aero/rocket2.htm Practical Rocketry] {{Webarchive|url=https://web.archive.org/web/20060618142816/http://allstar.fiu.edu/aero/rocket2.htm |date=2006-06-18 }}
* [https://web.archive.org/web/20061004175705/http://quest.nasa.gov/space/teachers/rockets/rocketry.html NASA Practical Rocketry]

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