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Ammonium perchlorate composite propellant (APCP) is asolid rocket propellant. It differs from many traditional solidrocket propellants such asblack powder orzinc-sulfur, not only in chemical composition and overall performance but also by beingcast into shape, as opposed topowder pressing as with black powder. This provides manufacturing regularity and repeatability, which are necessary requirements for use in the aerospace industry.
Ammonium perchlorate composite propellant (APCP) is typically for aerospace rocket propulsion where simplicity and reliability are desired andspecific impulses (depending on the composition and operatingpressure) of 180–260 s (1.8–2.5 km/s) are adequate. Because of these performance attributes, APCP has been used in theSpace Shuttle Solid Rocket Boosters, aircraftejection seats, and specialty space exploration applications such as NASA'sMars Exploration Rover descent stageretrorockets. In addition, thehigh-power rocketry community regularly uses APCP in the form of commercially available propellant "reloads", as well as single-use motors. Experienced experimental and amateur rocketeers also often work with APCP, processing the APCP themselves.
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Ammonium perchlorate (AP) composite propellant is a composite propellant, meaning that it has both fuel and oxidizer combined into a homogeneous mixture, in this case with a rubberybinder as part of the fuel. The propellant is most often composed of ammonium perchlorate, anelastomer binder such ashydroxyl-terminated polybutadiene (HTPB) orpolybutadiene acrylic acid acrylonitrile prepolymer (PBAN), powdered metal (typicallyaluminium), and variousburn ratecatalysts. In addition,curing additives induce elastomer bindercross-linking to solidify the propellant before use. The perchlorate serves as theoxidizer, while the binder and aluminium serve as thefuel. Burn rate catalysts determine how quickly the mixture burns. The resulting cured propellant is fairlyelastic (rubbery), which also helps limit fracturing during accumulated damage (such as shipping, installing, cutting) and highacceleration applications such as rocketry. This includes theSpace Shuttle missions, in which APCP was used for the two SRBs.
The composition of APCP can vary significantly depending on the application, intended burn characteristics, and constraints such asnozzle thermal limitations orspecific impulse (Isp). Rough mass proportions (in high-performance configurations) tend to be about 70/15/15 AP/HTPB/Al, though fairly high performance "low-smoke" can have compositions of roughly 80/18/2 AP/HTPB/Al. While metal fuel is not required in APCP, most formulations include at least a few percent as a combustion stabilizer, propellantopacifier (to limit excessiveinfrared propellant preheating), and increase the temperature of the combustion gases (increasing Isp).
Though increasing the ratio of metal-fuel to oxidizer up to thestoichiometric point increases the combustion temperature, the presence of an increasing molar fraction of metal oxides, particularlyaluminium oxide (Al2O3)precipitating from the gaseous solution creates globules of solids or liquids that slow down the flow velocity as the mean molecular mass of the flow increases. In addition, the chemical composition of the gases changes, varying the effectiveheat capacity of the gas. Because of these phenomena, there exists an optimal non-stoichiometric composition for maximizing Isp of roughly 16% by mass, assuming the combustion reaction goes to completion inside thecombustion chamber.
The combustion time of the aluminium particles in the hot combustion gas varies depending on aluminium particle size and shape. In small APCP motors with high aluminium content, the residence time of the combustion gases does not allow for full combustion of the aluminium, causing a substantial fraction of it to burn outside the combustion chamber and reducing performance. This effect is often mitigated by reducing aluminium particle size, inducing turbulence (extending characteristic path length and residence time), and/or reducing the aluminium content to increase net oxidizing potential, ensuring more complete aluminium combustion. Aluminium combustion inside the motor is the rate-limiting pathway since the liquid-aluminium droplets (even still liquid at temperatures 3,000 K (2,730 °C; 4,940 °F)) limit the reaction to a heterogeneous globule interface, making the surface area to volume ratio an important factor in determining the combustion residence time and required combustion chamber size/length.
The propellant particle size distribution has a profound impact on APCP rocket motor performance. Smaller AP and Al particles lead to highercombustion efficiency but also lead to increased linear burn rate. The burn rate is heavily dependent on mean AP particle size as the AP absorbs heat to decompose into a gas before it can oxidize the fuel components. This process may be a rate-limiting step in the overall combustion rate of APCP. The phenomenon can be explained by considering the heat-flux-to-mass ratio: As the particle radius increases the volume (and, therefore, mass and heat capacity) increases as the cube of the radius. However, the surface area increases as the square of the radius, which is roughly proportional to the heat flux into the particle. Therefore, a particle's rate of temperature rise is maximized when the particle size is minimized.
Common APCP formulations call for 30–400 μm AP particles (often spherical), as well as 2–50 μm Al particles (often spherical). Because of the size discrepancy between the AP and Al, Al will often take an interstitial position in a pseudo-lattice of AP particles.
APCPdeflagrates 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. Themass flux (kg/s) [and therefore pressure] of combustion gases generated is a function of theinstantaneoussurface area (m2), propellantdensity (kg/m3), and linearburn rate (m/s):
Several geometric configurations are often used depending on the application and desiredthrust curve:
While the surface area can be easily tailored by careful geometric design of the propellant, theburn rate is dependent on several subtle factors:
In summary, however, most formulations have a burn rate between 1–3 mm/s atSTP and 6–12 mm/s at 68 atm. The burn characteristics (such as linear burn rate) are often determined prior to rocket motor firing using astrand burner test. This test allows the APCP manufacturer to characterize the burn rate as a function of pressure. Empirically, APCP adheres fairly well to the following power-function model:
It is worth noting that typically for APCP,n is 0.3–0.5 indicating that APCP is sub-critically pressure sensitive. That is, if surface area were maintained constant during a burn the combustion reaction would not run away to (theoretically) infinite as the pressure would reach an internal equilibrium. This isn't to say that APCP cannot cause anexplosion, just that it will not detonate. Thus, any explosion would be caused by the pressure surpassing the burst pressure of the container (rocket motor).
Commercial APCP rocket engines usually come in the form ofreloadable motor systems (RMS) and fully assembled single-use rocket motors. For RMS, the APCP "grains" (cylinders of propellant) are loaded into the reusable motor casing along with a sequence of insulator disks ando-rings and a (graphite or glass-filledphenolic resin) nozzle. The motor casing and closures are typically bought separately from the motor manufacturer and are often precision-machined from aluminium. The assembled RMS contains both reusable (typically metal) and disposable components.
The major APCP suppliers for hobby use are:
To achieve different visual effects and flight characteristics, hobby APCP suppliers offer a variety of different characteristic propellant types. These can range from fast-burning with little smoke and blue flame to classic white smoke and white flame. In addition,colored formulations are available to display reds, greens, blues, and even black smoke.
In the medium- and high-power rocket applications, APCP has largely replacedblack powder as a rocket propellant. Compacted black powder slugs become prone to fracture in larger applications, which can result incatastrophic failure in rocket vehicles. APCP's elastic material properties make it less vulnerable to fracture from accidental shock or high-acceleration flights. Due to these attributes, widespread adoption of APCP and related propellant types in the hobby has significantly enhanced the safety of rocketry.
The exhaust from APCP solid rocket motors contains mostlywater,carbon dioxide,hydrogen chloride, and ametal oxide (typicallyaluminium oxide). The hydrogen chloride can easily dissolve in water and create corrosivehydrochloric acid. The environmental fate of hydrogen chloride is not well documented. The hydrochloric acid component of APCP exhaust leads to the condensation of atmospheric moisture in the plume and this enhances the visible signature of the contrail. This visible signature, among other reasons, led to research in cleaner burning propellants with no visible signatures. Minimum signature propellants contain primarily nitrogen-rich organic molecules (e.g.,ammonium dinitramide) and depending on their oxidizer source can be hotter burning than APCP composite propellants.
In the United States, APCP for hobby use is regulated indirectly by two non-government agencies: theNational Association of Rocketry (NAR), and theTripoli Rocketry Association (TRA). Both agencies set forth rules regarding theimpulse classification of rocket motors and the level ofcertification required by rocketeers in order to purchase certain impulse (size) motors. The NAR and TRA require motor manufacturers to certify their motors for distribution to vendors and ultimately hobbyists. The vendor is charged with the responsibility (by the NAR and TRA) to check hobbyists for high-power rocket certification before a sale can be made. The amount of APCP that can be purchased (in the form of a rocket motor reload) correlates to the impulse classification, and therefore the quantity of APCP purchasable by a hobbyist (in any single reload kit) is regulated by the NAR and TRA.
The overarching legality concerning the implementation of APCP in rocket motors is outlined in NFPA 1125. Use of APCP outside hobby use is regulated by state and municipal fire codes. On March 16, 2009, it was ruled that APCP is not an explosive and that manufacture and use of APCP no longer requires a license or permit from theATF.[5]