this enhances the visible signature of the contrail. This visible signature, among other reasons, led to research in cleaner burning propellants with no visible signatures. Minimum signature propellants contain primarily nitrogen-rich organic molecules (e.g., ammonium dinitramide) and depending on their oxidizer source can be hotter burning than APCP composite propellants. Regulation and legality In the United States, APCP for hobby use is regulated indirectly by two non-government agencies: the National Association of Rocketry (NAR), and the Tripoli Rocketry Association (TRA). Both agencies set forth rules regarding the impulse classification of rocket motors and the level of certification required by rocketeers in order to purchase certain impulse (size) motors. The NAR and TRA require motor manufacturers to certify their motors for distribution to vendors and ultimately hobbyists. The vendor is charged with the responsibility (by the NAR and TRA) to check hobbyists for high-power rocket certification before a sale can be made. The amount of APCP that can be purchased (in the form of a rocket motor reload) correlates to the impulse classification, and therefore the quantity of APCP purchasable by a hobbyist (in any single reload kit) is regulated by the NAR and TRA. The overarching legality concerning the implementation of APCP in rocket motors is outlined in NFPA 1125. Use of APCP outside hobby use is regulated by state and municipal fire codes. On March 16, 2009, it was ruled that APCP is not an explosive and that manufacture and use of APCP no longer requires a license or permit from the ATF. Footnotes References Rocket Propulsion Elements. Sutton, George P. Amateur Experimental Solid Propellants by Richard Nakka Solid Propellant Burn Rate by Richard Nakka Intro to Solid Propulsion by Graham Orr, Harvey Mudd College Experimental Engineering BATFE Lawsuit Documents, 2002–Present, Tripoli Rocketry Association Rocketry Model rocketry Rocket propellants Solid fuels