Absolute radio-frequency channel number

2014-01-14T17:08:40Z

31.221.65.50: /* Example ARFCN for TETRA */

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{{citation style|date=April 2013}}
{{more footnotes|date=September 2010}}
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'''Ammonium perchlorate composite propellant''' ('''APCP''') is a modern [[solid rocket]] [[Rocket propellant|propellant]] used in both manned and unmanned rocket vehicles. It differs from many traditional solid rocket propellants such as [[black powder]] or [[Solid-fuel_rocket#Zinc-Sulfur_(ZS)_Propellants|zinc-sulfur]], not only in chemical composition and overall performance, but also by the nature of how it is processed. APCP is [[Casting|cast]] into shape, as opposed to [[Powder metallurgy#Powder pressing|powder pressing]] as with black powder. This allows for manufacturing regularity and repeatability, which are necessary requirements for use in the aerospace industry.

== Uses ==
Ammonium perchlorate composite propellant is typically used in aerospace propulsion applications, where simplicity and reliability are desired and [[specific impulse]]s (depending on the composition and operating [[pressure]]) of 180–260 seconds are adequate. Because of these performance attributes, APCP is regularly implemented in booster applications such as in the [[Space Shuttle Solid Rocket Booster]]s, aircraft [[ejection seat]]s, and specialty space exploration applications such as NASA's [[Mars Exploration Rover]] descent stage [[retrorocket]]s. In addition, the [[high power rocket]]ry 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.

== Composition ==

=== Overview ===
Ammonium perchlorate composite propellant is a composite propellant, meaning that it has both fuel and oxidizer mixed with a rubbery [[binder (material)|binder]], all combined into a homogeneous mixture. The propellant is most often composed of [[ammonium perchlorate]] (AP), an [[elastomer]] binder such as [[hydroxyl-terminated polybutadiene]] (HTPB) or [[Polybutadiene acrylonitrile|polybutadiene acrylic acid acrylonitrile prepolymer]] (PBAN), small amounts of powdered metal, typically [[aluminum]] (Al), and various [[burn rate (chemistry)|burn rate]] [[catalyst]]s. In addition, [[curing (chemistry)|curing]] additives induce [[elastomer]] binder [[cross-link]]ing to solidify the propellant before use. The AP serves as the [[oxidizer]], while the binder and aluminum serve as the [[fuel]]. Burn rate catalysts determine how quickly the mixture burns. The resulting cured propellant is fairly [[:wikt:elastic|elastic]] (rubbery), which also helps limit fracturing during accumulated damage (such as shipping, installing, cutting) and high [[acceleration]] applications such as hobby or military rocketry.

The composition of APCP can vary significantly depending on the application, intended burn characteristics, and constraints such as [[nozzle]] thermal limitations or [[specific 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, propellant [[opacifier]] (to limit excessive [[infrared]] propellant preheating), and increase the temperature of the combustion gases (increasing Isp).

=== Common species ===
Oxidizers:
* [[Ammonium perchlorate]] as the primary oxidizer
* [[Metal-oxide]] [[catalysts]] as [[thermite]] oxidizers

High energy fuels:
* [[Aluminium]] (high performance, most common)
* [[Magnesium]] (medium performance)
* [[Zinc]] (low performance)

Low energy fuels acting as binders:
* [[HTPB]]
* [[CTPB]]
* [[Polybutadiene acrylonitrile|PBAN]]

=== Special considerations ===
Though increasing the ratio of metal fuel to oxidizer up to the [[stoichiometric]] point increases the combustion temperature, the presence of an increasing molar fraction of metal oxides, particularly [[aluminum 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 change, varying the effective [[heat 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 the [[combustion chamber]].

The combustion time of the aluminum particles in the hot combustion gas varies depending on aluminum particle size and shape. In small APCP motors with high aluminum content, the residence time of the combustion gases does not allow for full combustion of the aluminum and thus a substantial fraction of the aluminum is burned outside the combustion chamber, leading to decreased performance. This effect is often mitigated by reducing aluminum particle size, inducing turbulence (and therefore a long characteristic path length and residence time), and/or by reducing the aluminum content to ensure a combustion environment with a higher net oxidizing potential, ensuring more complete aluminum combustion. Aluminum combustion inside the motor is the rate-limiting pathway since the liquid-aluminum droplets (even still liquid at temperatures 3000 K) limit the reaction to a heterogeneous globule interface, making the surface area to volume ratio and important factor in determining the combustion residence time and required combustion chamber size/length.

=== Particle size ===
The propellant particle size distribution has a profound impact on APCP rocket motor performance. Smaller AP and Al particles lead to higher combustion 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) increase 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.

== Characteristics ==

===Geometric===
APCP [[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. The [[mass flux]] (kg/s) [and therefore pressure] of combustion gases generated is a function of the [[instantaneous]] [[surface area]] <math>A_s</math> (m2), propellant [[density]] <math>\rho</math> (kg/m3), and linear [[Burn rate (chemistry)|burn rate]] <math>b_r</math> (m/s):

<math>\dot{m} = \rho \cdot A_s \cdot b_r</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, 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.

===Burn rate===
While the surface area can be easily tailored by careful geometric design of the propellant, the [[burn rate (chemistry)|burn rate]] is dependent on several subtle factors:

* Propellant chemical composition.
* AP, Al, additive particle sizes.
* Combustion pressure.
* [[Heat transfer]] characteristics.
* Erosive burning (high velocity flow moving past the propellant).
* Initial temperature of propellant.

In summary, however, most formulations have a burn rate between 1–3 mm/s at [[Standard conditions for temperature and pressure|STP]] 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 a [[strand 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:

<math>b_r = a \cdot p^n</math>

It is worth noting that typically for APCP, 0.3<n<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 runaway to (theoretically) infinite as the pressure would reach an internal equilibrium. This isn't to say that APCP cannot cause an [[explosion]], but rather that the explosion would be caused by the pressure surpassing the burst pressure of the container (rocket motor).

== Model/High Power Rocketry applications ==
[[Image:Rocket.jpg|thumb|A high-power rocket launch using an APCP motor.]]
Commercial APCP rocket engines usually come in the form of [[model rocket#Reloadable_motors|re-loadable motor systems]] (RMS) and fully-assembled single use rocket motors. For RMS, the APCP "[[solid rocket#Grain_geometry|grains]]" (cylinders of propellant) are loaded into the reusable motor casing along with a sequence of insulator disks and [[o-ring]]s and a ([[graphite]] or glass-filled [[phenolic resin]]) nozzle. The motor casing and closures are typically bought separately from the motor manufacturer and are often precision machined aluminum. The assembled RMS contains both reusable (typically metal) and disposable components.

The major APCP suppliers for hobby use are:
* [[Aerotech Consumer Aerospace]]
* Animal Motor Works
* Cesaroni Technology
* Kosdon (by Aerotech)
* Loki Research

To achieve different visual effects and flight characteristics, hobby APCP suppliers offer a variety of different characteristic propellant types. These can range in from fast burning with little smoke and blue flame to classic white smoke and white flame. In addition, [[pyrotechnic colorant|colored formulations]] are available to display reds, greens, blues, and even flameless black smoke.

In medium and high power rocket applications, APCP has largely replaced [[black powder]] as a rocket propellant. Compacted black powder slugs become prone to fracture in larger applications, which can result in [[catastrophic failure]] in rocket vehicles. APCP's elastic material properties makes 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.

== Environmental and other concerns ==
The exhaust from APCP solid rocket motors contain mostly [[water]], [[carbon dioxide]], [[hydrogen chloride]], and a [[metal oxide]] (typically [[aluminium oxide]]). The hydrogen chloride can easily dissociate into water and create corrosive [[hydrochloric acid]], damaging launch equipment and biasing the [[pH]] of local water and rainfall (See [[acid rain]]). Furthermore, for military use, the smoke trail and the [[infrared radiation]] from the hot particles make it possible to detect the launch from space. These problems led to the research in smokeless grain which contains nitrogen-containing organic molecules (e.g. [[ammonium dinitramide]]).

== Regulation and legality ==
In the United States, APCP for hobby use is regulated indirectly by two non-government agencies: the [[National Association of Rocketry]] (NAR), and the [[Tripoli Rocketry Association]] (TRA). Both agencies set forth rules regarding the [[Amateur rocket motor classification|impulse classification of rocket motors]] and the level of [[certification]] required by rocketeers in order to purchase certain impulse (size) motors. The NAR and TRA require motor manufactures 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 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 the [[Bureau of Alcohol, Tobacco, Firearms and Explosives|ATF]].<ref>{{citation|url=http://www.tripoli.org/LinkClick.aspx?fileticket=B%2bnHQF5CiZk%3d&tabid=113|title= Judge Reggie Walton's ruling of 03/16/2009 on motions for summary judgment, Civil Action No. 00-0273 (RBW), US District Court for the District of Columbia}}</ref>

== Footnotes ==
{{Reflist}}

== References ==
* ''Rocket Propulsion Elements.'' Sutton, George P.
* [http://nakka-rocketry.net/propel.html Amateur Experimental Solid Propellants] by Richard Nakka
* [http://nakka-rocketry.net/burnrate.html Solid Propellant Burn Rate] by Richard Nakka
* [http://www.space-energetics.com/Public_Release/solidsV6.pdf Intro to Solid Propulsion] by Graham Orr
* [http://www.tripoli.org/News/Internal/BATFELawSuit/tabid/113/Default.aspx BATFE Lawsuit Documents, 2002–Present, Tripoli Rocketry Association]

[[Category:Rocketry]]
[[Category:Model rocketry]]
[[Category:Rocket propellants]]
[[Category:Solid fuels]]

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Absolute radio-frequency channel number