why is AP called a 'composite' propellant?

I was just wondering why AP is often called a composite propellant? Doesn't that just mean it's made from 2 or more substances? In that
case, isn't BP also a 'composite' propellant?
impakt
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Since this is usenet, I did not bother to actually look this up. I will guess.
Could it be because one component (the fuel or one component of the fuel since it's just the rubbery part and not the aluminum or other metals/soot) is actually cured with the other components (oxidizer) mixed inside. Once cured it is not possible to get them apart. With BP motors, everything may be pressed into a grain, but it could be crushed up and powdered and you could seperate components using physical or chemical means.
Am I wrong?
Anybody....
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On 29 Nov 2005 06:14:52 -0800, "shreadvector"

I think you're pretty close, Shread...
composite
A complex material, such as wood or fiberglass, in which two or more distinct, structurally complementary substances, especially metals, ceramics, glasses, and polymers, combine to produce structural or functional properties not present in any individual component.
mixture
Chemistry. A composition of two or more substances that are not chemically combined with each other and are capable of being separated.
tah
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snipped-for-privacy@weinerboy.org wrote:

This should be in the FAQ.
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Jerry Irvine, Box 1242, Claremont, California 91711 USA
Opinion, the whole thing. <mail to: snipped-for-privacy@gte.net>
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BAH! It's cuz it sounds cool
wrote:

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Fred,
You are close. Both types of motors could be theoretically seperated into individual components, as there are no chemical reactions taking place (save for the epoxy curing in the composite.)

loved that Star Trek episode where Kirk mixes up some black powder to kill that lizard man creature ... very nice to see chemistry used in a practical manner.
The above mixture is ground up to meet required densities. The more fine the ground meal is, the more surface area is exposed at the molecular level, and therefore the greater the amount of energy is released in the blast. This is referred to as "corning". The greater the corning, the greater the burn rate as more surface area is available for burning.
BP motors must have a lot of pressure applied to them while they dry so that fissures and air bubbles are eliminated from within the grain. Fissures and bubbles lead to spikes in the burn rates. Once the rate reaches a certain value, a shock wave will develop that will break down the lattice structure of the propellant, and then allow heat too most of the grain, that then causes the fuel and oxidizer to react within the entire grain, then over pressurizing the casing, and boom ...
Composite motor chemicals are held in what is called a BINDER. The binder can be plastic or rubber in nature. The grain is cast (poured) into a mold, and allowed to cure.
Composites come in a variety of formulations. The Shuttle SRB motors are 70% NH4CLO4 + 16% Al + 12% Polymer + 2% curing agent (epoxy).
Once the binder dries, it restrains the burn rate so that there is no resulting shock wave (explosion) through the propellant as the binder only allows the exposed surface area of the propellant to react with the heat.
Composites really are safer than black powder motors, but because of its "ease" to make, BP was the established and championed doctrine of model rocket motor manufacturing.
There are several good propellant books on the market. If you plan to make your own motors, I would suggest you go the composite route as it is more bang for the buck.
You could just order some 'model airplane parts' and get your motors that way too ...
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On 29 Nov 2005 06:14:52 -0800, "shreadvector"

ok, so you are saying the difference is that AP motors are bound at the mollecular level while BP is only bound by the material being compressed?
Sounds like a good enough distinction for me. I expected to find the answer to this in one of my rocketry books. I didnt have much luck with google either. I thought the distinction would be common knowledge in the rocketry community.
Cheers.
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I do not know if the stuff mixed inside the rubbery fuel is chemically bonded at the molecular level when the rubbery martix is cured. A propellant expert might be able to answer that. I'm simply supplying the usenet "guess answer" which I hope is a usenet "semi-educated guess answer".
-Fred Shecter
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shreadvector wrote:

I'm not a motor maker(yet) but IIRC, HTPB/binder + metal/fuel + AP/oxidiser = composite propellent. The fuel and oxidiser are suspended in the binder. So no, no chemical bonding, just physical.
As always, I'm open for correction.
Ted Novak TRA#5512 IEAS#75
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wrote:

Well, I know that with some explosives that use AP and a plasticiser, they also use a 'linker'. I imagine the 'linker' bonds the AP to the fuel in some way, but this is a bit beyond the scope of the original question I think.
Paul Perth Rocket Club Australia
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Hiya Fred,
I don't know if it is chemically bonded ... I don't think so. I think the matrix is just a "3-D net" to hold the reactants in place. I would think the motor looses efficiency if you have to over come breaking of bonds between the matrix and reactants. Plus, when the matrix cures, I don't believe there is any electron sharing (chemical reaction) between the matrix and the reactants.
There may be electrostatic attraction/repulsion forces going on, but I don't think any of the alkaline (metal) reactants are getting an electron(s) from the non-alkaline substances.
If there were the case, I would suspect that composite solids would be able to conduct current and that would be a VERY bad thing ... but I am not a motor engineer, I only understand the underlying physics and the application there of.
Moose, who we both know (you far better than I), was holding chunks of Delta SRB fragments in the post explosion photo from years back. Maybe he still has a few pieces and could just place a volt/ohm meter across a piece and find out. The same could be done with an Aerotech slug. Would be interesting to find out if solid propellants can conduct current, as it might be a way of pre-launch heating of the propellant in order to increase motor performance; also it might be a way of getting that 'edge' in NAR/TRA competition.
I don't think there is any rule restricting the pre-heating of solid motors in order to increase their performance ... is there?
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Heh,
On second thought, its probably NOT a good idea to go around heating your solids unless you have the proper equipment ... don't want a bunch of people getting their hands or faces burned while they are sitting there trying to pre-heat their motors. Plus, you could have lightning strikes or insulation problems that could prematurely ignite your motors while you are packing the chute, or placing a $500 million payload on your booster ... thats not good either unless you have this stuff all worked out well in advance.
For the hobby user, just walk around with the motor in your pocket prior to launch. This way you can impress people with your bulge, and your body heat in no way would come close to igniting the propellant, but would impart a SLIGHT amount of energy into the motor. Enough to edge out the other guy's flight? Who knows ...
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snipped-for-privacy@juno.com wrote:

I was at Pittcon in the early '70s when ENERJET motors had just been introduced. Larry Brown of Centuri tried to launch a few at the demo launch on a bitter cold day only to find out they wouldn't ignite in sub - freezing temps. The solution - the armpit brigade! He grabbed a bunch of kids and said stick a motor under your shirt in your armpit to warm it up - it worked!
Dale Greene SPAAR 503
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On 30 Nov 2005 06:35:39 -0800, "shreadvector"

Wasn't the first APCP motor just AP and tar with no real chemical bonding, or vulcanizing of the binder?
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Well, I've seen Asphalt/AP rockets fly, but have not built the motors. Obviously Asphalt is melted and then mixed with oxidizer and then poured and allowed to solidify. Is the solidification a form of crystallization with the oxidizer particles trapped inside the crystal? Again, this is not a powder pressed together that can become a powder again with simple crushing, like BP.
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shreadvector wrote:

Fred,
I think the components are mixed together and no chemical reaction occurs in the process. It is like the igniter kits that can be bought. Mix the powders together with the lacquer and dip. No chemical reaction occurs until it is ignited.
Kurt Savegnago
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One could say the binder is the "composite",the curing is a chemical reaction making it one -composite- material,containing other materials to aid in combustion.
--
Jim Yanik
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The Cambridge Dictionary of Space Technology doesn't discuss black powder.
But their "solid propellant" entry says that "there are two main types of solid propellant: the homogeneous or double-base propellants (nitrocellulose plasticized with nitroglycerin plus stabilizing products) which are limited in power and not widely used in space applications; and the heterogeneous or composite type."
Note the use of homogeneous and heterogeneous.
Ley claims that Goddard (or maybe Sander) did the first experiments with smokeless powder propellants (i.e. homogeneous or double-base). If my scan is correct, Ley then claims that composite propellants grew out of the RATO development efforts in WWII.
Both were developed to overcome the engineering issues surrounding the use of black powder in military weapons.
Anyway, I think that black powder has been around for so long that it is in a class by itself. Technically, I believe, it too could be called a "composite" as it is just a mixture and not a chemical molecule of its own.
I've been enjoying my re-reading of Willy Ley's "Rockets, Missiles, and Men in Space." It goes through a lot of the history of propellant development. I recommend it.     Happy holidays,     Will
Impakt wrote:

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Impakt wrote:

I looked this up in a dictionary:
composite: consisting of separate interconnected parts
That's what an AP composite propellant grain is.
Roland
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Yes, I can sum it up (finally): BP is just pressed together.
APCP is "glued together".
Just like composite structures are glued together.
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