COMPOSITE MOTORS
Composite vs. Black Powder
Composite motors are the motors we think of when we think "High Power Rocketry". Black Powder (BP) motors become, more and more, temperature sensitive, the longer and wider the fuel slug is, hence cracks can appear in the slug. As black powder has no burn modulation techniques, it inherently burns/ignites more rapidly than composites. A larger surface area from a crack can over-pressurize a nozzle in microseconds. The specific impulses of black powder are very low compared to composite propellants so more and more energy is wasted in lifting itself, pound for pound, relative to composite propellant motors. Composite motors are safer, requiring a high, consistent heat source to ignite, whereas BP can be lit with a spark.
Composite Ingredients
Composite Propellant is made of, primarily, two major materials, mixed wet, and then cured into a solid, albeit, rubbery form via an "epoxy style" process. Other items are used for effects and mass flow regulation. The major composite motor components are:
1. Oxidizer. Commercially, Ammonium Perchlorate (AP) in granular form between 90 to normally 200 micron size is used, although some experimentalists use other oxidizer materials such as Ammonium Nitrate (AN). AN propellants traditionally have a lower specific impulse (energy per pound) but this, of course relies on finer tuning of the other components.
2. Fuel-Binder. This can be one material or two but both aspects are important. The fuel , of course does the work, and the binder is the item that makes the mass a solid.
3. Effects. Various metals are usually added to allow a multitude of flame and burning characteristics. Normally, composite motors burn without smoke, but additives are normally added to enhance tracking or even to modify the burn to a slower impulse for some vehicles.
4. Miscellaneous. Items such as a blackening agent to darken the
propellant to inhibit IR transmission and other ingredients to deaden noise and
other characteristics of a solid fuel burn are also important. Composite
Motor Types There are two primary types of approved composite rocket motors, the single
use and the reloadable. They are as follows: Single Use This motor is assembled at the manufacturer in either
a plastic, fiberglass or aluminum tube (called a case). Propellant is
either poured or inserted into the case and then the ends are permanently
attached These ends consist of a nozzle on the aft end and forward plug
that may include either a delay stack with an ejection well or just a closed
plug. This motor is always thrown away after use. Reloadable motor These types of motors come in two closure
systems, threaded and bushing/snap-ring closures. The Aerotech
Reloadable Motor System (RMS™) is an example of a threaded system. The forward
pressure closure/delay well and the aft nozzle are retained by threads on the
inside of the case body. The bushing/snap ring system pioneered by Dr.
Frank Kosdon, among others, is a system where the forward bushing plug and the
aft nozzle bushing are retained through snap rings inserted into internal
grooves in the case at the proper length case for that motor. These
bushing plugs can slide, within the case, but at pressurization, are forced to,
and retained by, the snap rings. The essential propellant
components, the grains, are loaded the same, however. Different length cases are
provided, for all reloadable motors, in standard diameters, normally measured in
millimeters, and various lengths, measured either by length or, even more
common, by maximum thrust (Newton Seconds) per case. Motor Thrust Characteristics All motors come in various average thrusts regulated by three primary
characteristics Formulation is already described above and is one third of
the motor thrust equation. Surface burning is the another important
factor. Composites can be formed, cored or sawn to give a different amount
of surface to the flame area during motor firing. These are known as grain
patterns and are traditionally down the center of the grain and can be a simple,
consistent diameter, center hole (Bates grain), a star pattern or a pattern of
multiple lobes. Some professional motors vary this pattern throughout
the grain column to provide a high pressure boost phase, and a slower burning
sustainer phase to the motor, but these are seldom seen in Hobby Rocket motors.
The one exception to the traditional core burning motors is the "C-slots".
These are solid grains with slots cut down one side. They typically
provide a much lighter thrust over a longer period of time in relation to a
"core burner" due to the restricted surface area for gas production.
Finally, the length of the grain is the third factor. As Composites burn from
the inside out, double the length and face of each grain (in a Bates grain),
roughly double the gas producing area. This is
analogous to a four vs. eight cylinder engine, more cylinders (grains) = more
power added. There is a limit to this length versus core diameter,
though. As the propellant gas requires an adequately sized exit point, if
one is not available in a relatively short distance, and of the proper diameter,
it'll make it's own. This is the reason for the seal ring in 29 and 38 mm
RMS motors, it provides a head end restriction to force the gas back down the
core. Summary Composite motors are very safe, when handled properly, and can provide good
power in a low weight, case system. They come in a multitude of thrust
ranges from one to thousand's of pounds of thrust and provide a good amount of
"whoosh" for your buck.