Model Forum / General / Rockets / November 2005

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

--

Tod A. Hilty
Hilty Information Systems

Do not look in the direction of the flash...
Curl up in a ball as you hit the ground...

CAUTION: The Mass of This Product Contains the Energy Equivalent
of 85 Million Tons of TNT per Net Ounce of Weight

Please replace weinerboy dot org with adelphia dot net for reply.

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 ...