BLACK POWDER (BP) MOTORS*
Black Powder Motor Components
Black powder motors consist of four primary components, all of them non reusable. The case is, almost always, wound paper. Paper has good coefficent of expansion at standard storage temperatures when loaded with black powder, an item important when you don't want your grain to seperate from the liner. It also ablates well with the bare flame present in the end burn BP motor. Clay is used for the nozzle, due to its low cost and its low ablative properties with BP's consistent pressure and burning properties. A delay/smoke column similar to composite motor delays is placed at the top of the grain. The final item is the loose black powder ejection charge, which is sealed with either a thin layer of clay or a paper cap.
Black Powder Motor Manufacture
Black powder motors
are manufactured with specialized hydraulic pressing machines which have precise
controls to press black powder into the casings. Black Powder has a
fine line between compression and self ignition, pressures up to 900 to 1000 psi.
are typical pressures for pressing but a slight increase in this pressure can
lead to combustion. The
rear section of the motor then gets a clay nozzle, the forward end of the motor
gets the delay and smoke generator charge followed by the ejection charge and
charge cap. Grain Types There are two types of black powder
motor configurations. These are "end burners", grains that burn from the
end face of the grain and "port burners" grains that had a small drilled or
molded core hole in the end of the grain, usually around 1/4 to 1/2 inch deep.
This is not to be confused with the standard end burner port (less than 1/8"
deep hole used to enhance ignition). End burner BP motors are always "flat
curve" motors exhibiting a true "top hat" curve with a slight peak at ignition
(from that small port). Port burners had a large spike at the start of the
burn that gradually regressed to the same level as as a standard end burner, but
of course it would lead to a shorter burn. Port Burners (B14 or B8 for
example) are not being manufactured by the known BP motor manufacturers at this
time. Thrust Classes Black Powder cannot be easily
modulated or enhanced by additives as easily as composite propellants, other
than "effects" additives such as titanium or other metals for spark effects.
This means the only way to produce different thrust motors is to change the
diameter of the grain face (average thrust) or lengthen the grain (total
thrust).
Cons on BP motors The two primary disadvantages to using
black powder motors are black powders low ISP (ability to lift its own weight)
and storage sensitivity. Black powder has a low power ratio, the bigger
the motor, the larger the case to contain it, which equals more weight,
requiring a longer, heavier rocket to fly it in. There are currently no
commercial motors being built any higher than the E thrust class although in the
past BP motors up to H class were made. Sensitivity to storage is
the other problem with BP motors. The larger the grain there is a problem
with temperature cycling on the grain. The pressed black powder
grain exhibits the same tendencies of a large crystal, any temperature changes
can cause fractures in that "crystal" and any cracks present during the thrust
cycle could cause a large pressure spike possibly causing failure of the nozzle,
The larger the grain the larger the possible cracks. Summary Black Powder Motors are easy motors to use and very stable especially when
new. They can be cheaper than composite motors and are more easily
available as well in a lot more locations than higher power composite
propellant. They are good motors for contest and sport use due to the
standard thrust characteristics from lot to lot.