In the “B” power class and below, current US-made black powder model rocket motors are non-competitive at World Spacemodeling Championships and no one (including the US team) uses them. Conversely, US ammonium perchlorate composite model rocket motors in the C and above power class are world class and everyone else wishes they had them!
US motors in the 1/2A through B power classes are made by hydraulically ramming black powder into relatively thick-wall and heavy paper casings. Black powder has a “specific impulse” (energy density) of about 80 seconds. Just for comparison purposes, the ammonium perchlorate composite propellants used in higher power class motors in the US have about three times this specific impulse — they get much more total impulse per unit of mass and volume of their propellant. US model rocket motors are made for the mass consumer market, so their manufacturing technique with black powder supports mass production and delivers good-enough accuracy and repeatability of performance. They typically deliver about 10 percent less total impulse than the maximum allowed for their power class, both by design and because (in 13mm A motors) that is all the propellant that will fit into the fixed casing size. And they come in a limited number of delay times suited for consumer rockets, not high-performance competition rockets.
A number of European nations have small companies that hand-make very small 1/2A through B motors specifically for FAI competition. These nations that do this include (to our knowledge) Serbia (“Ultra” brand), the Czech Republic (“Delta” brand, which may not still be producing), Poland (“EMI” or “PSn” brands, which may not still be producing), Germany (“MRK” brand), Bulgaria (“Jambol Jet” brand), the Ukraine, and Russia. All of these companies are very small, and do not have the resources or interest to get into the export or mass-market business that might make their motors available and certifiable in the US. Not all of the motors they make are reliable enough to pass US certification standards. Their motors use a pressed-powder propellant that is reported to include ammonium perchlorate and some form of rubber polymer and delivers about twice the specific impulse of black powder. Their total impulse is exactly at the upper limit for their power class, and their delay times are more accurate and are available in far more values than US motors. They are also much smaller and lighter (both before ignition and after burnout) than US black powder 13mm motors, or even than the now-defunct Apogee micro-motors (which also used black powder). This is a big advantage in duration events where the burnt casing is part of the recovery mass; the increased total impulse and diversity of delay times are a big advantage in all events.
Here is a comparison between the smallest typical US and one good (but not the smallest or lightest)European “A” power class motors:
All of these motor availability issues are a handicap for the US Team at a WSMC, particularly for first-time fliers in events where the models are not a symmetric tube plus fins configuration during boost (e.g. S4A), or where staging is required (S1 and S5) and understanding the unique characteristics of the European motors is important to flight success. Teammates with prior WSMC experience always help first-time teammates understand and adapt their designs to the unique characteristics of the European motors.
|130825_Euro_Champ_Motor_Testing||May 28, 2014, 3:05 am||205 KB|
|EMI Polish Rocket Motor Catalog||Other||May 28, 2014, 3:05 am||467 KB|
|MRK A-2-2||PNG||May 28, 2014, 3:05 am||52 KB|
|MRK German FAI Motors||Other||May 28, 2014, 3:05 am||46 KB|
|Polish A2 Motor Data||May 28, 2014, 3:05 am||351 KB|
|Ultra A-2-3||PNG||May 28, 2014, 3:05 am||52 KB|
|Ultra Motor Specs||Other||May 28, 2014, 3:05 am||1 MB|
|Ultra Motor Thrust Curves||Other||May 28, 2014, 3:05 am||841 KB|