Since this is usenet, I did not bother to actually look this up. I will
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
Am I wrong?
I think you're pretty close, Shread...
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.
Chemistry. A composition of two or more substances that are not
chemically combined with each other and are capable of being
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
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
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
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 ...
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
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.
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
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.
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.
Perth Rocket Club Australia
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?
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 ...
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!
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.
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.
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
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.
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