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much larger solid rocket motors atlantic research corporation significantly boosted composite propellant i sp in 1954 by increasing the amount of powdered aluminium in the propellant to as much as 20 15 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 rockets in the 1940s and 1950s both the soviet union and the united states embarked on major initiatives to develop solid propellant local regional and intercontinental ballistic missiles including solid propellant missiles that could be launched from air or sea many 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 government directed programs most often as booster rockets to add extra thrust during the early ascent of their primarily liquid rocket launch vehicles 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 and japan s h ii the largest solid rocket motors ever built were aerojet s three 6 60 meter 260 in monolithic solid motors cast in florida 16 motors 260 sl 1 and sl 2 were 6 63 meters 261 in in diameter 24 59 meters 80 ft 8 in long weighed 842 900 kilograms 1 858 300 lb and had a maximum thrust of 16 mn 3 500 000 lbf 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 24 mn 5 400 000 lbf and a shorter duration design edit design begins with the total 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 17 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 clarification needed 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 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 space shuttle challenger disaster grain geometry edit 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 a s times the fuel length the volumetric propellant consumption rate is the cross sectional area times the linear burn rate b ̇ and the instantaneous mass flow rate of combustion gases generated is equal to the volumetric rate times the fuel density ρ m ρ b a s displaystyle dot m rho dot b a_ s several geometric configurations are often used depending on the application and desired thrust curve circular bore simulation c slot simulation moon burner simulation 5 point finocyl simulation circular bore if in bates configuration produces a progressive regressive increasing and then decreasing over time thrust curve moon burner an off center circular bore produces a progressive regressive long burn which has slight asymmetric cg characteristics end burner propellant burns from one axial end to other producing a steady long burn which has thermal difficulties and shifts the center of gravity cg over time c slot propellant with large wedge cut out of the side along axial direction producing fairly long regressive decreasing over time thrust though it has thermal difficulties and 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 edit 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 boosters filament wound 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 ablates to prolong the life of the motor casing nozzle edit main article rocket engine nozzle a 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 dinitrogen tetroxide for litv the tanks can be seen on the sides of the rocket between the main center stage and the boosters 18 an early minuteman first stage used a single motor with four gimballed nozzles to provide pitch yaw and roll control performance edit an exhaust cloud engulfs launch pad 39a at nasa s kennedy space center as the space shuttle endeavour lifts off a typical well designed ammonium perchlorate composite propellant apcp first stage motor may have a vacuum specific impulse i sp as high as 285 6 seconds 2 801 km s titan ivb srmu 19 this compares to 339 3 s 3 327 km s for rp1 lox rd 180 20 and 452 3 s 4 436 km s for lh 2 lox block ii rs 25 21 bipropellant engines upper stage specific impulses are somewhat greater as much as 303 8 s 2 979 km s for apcp orbus 6e 22 359 s 3 52 km s for rp1 lox rd 0124 23 and 465 5 s 4 565 km s for lh 2 lox rl10b 2 24 propellant fractions are usually somewhat higher for non segmented solid propellant first stages than for upper stages the 53 000 kilogram 117 000 lb castor 120 first stage has a propellant mass fraction of 92 23 while the 14 000 kilogram 31 000 lb 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 2 36 and 2 34 meters 93 and 92 in 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 mass fraction 25 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 4 h 8 n 4 no 2 4 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 26 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 27 the naval air weapons station at china lake california developed a new compound c 6 h 6 n 6 no 2 6 called simply cl 20 china lake compound 20 compared to hmx cl 20 has 14 more energy per mass 20 more energy per volume and a higher oxygen to fuel ratio 28 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 29 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 edit black powder gunpowder propellant edit 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 and quest rockets 30 31 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 80 s 0 78 km s the grain is sensitive to fracture and therefore catastrophic failure black powder does not typically find use in motors above 40 newtons 9 0 pounds force thrust zinc sulfur zs propellants edit 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 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 double base db propellants edit 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 sp of roughly 235 s 2 30 km s is required the addition of metal fuels such as aluminium can increase performance to around 250 s 2 5 km s though metal oxide nucleation in the exhaust can turn the smoke opaque composite propellants edit 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 based ancp or ammonium perchlorate based apcp ammonium nitrate composite propellant often uses magnesium and or aluminium as fuel and delivers medium performance i sp of about 210 s 2 1 km s whereas ammonium perchlorate composite propellant often uses aluminium fuel and delivers high performance vacuum i sp up to 296 s 2 90 km s with a single piece nozzle or 304 s 2 98 km s with a high area ratio telescoping nozzle 22 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 generators ammonium dinitramide nh 4 n no 2 2 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 missiles 32 apcp used in th...
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