re big amateur rockets actually they are legal i not familiar with the ad you are speaking of but knowing popular science it is probably on the fringe however you may be speaking of public missle inc which is a legitimate company that has been around for a while due to advances in composite fuels engines are now available for model rockets using similar composites to srb fuel roughly times more powerful than black powder motors they are even available in a reloadable form i e aluminum casing end casings o rings the engines range from d all the way to m in common manufacture n and o i ve heard of used at special occasions to be a model rocket however the rocket can t contain any metal structural parts amongst other requirements i ve never heard of a model rocket doing i have heard of foot flights these require faa waivers of course there are a few large national launches ldrs fireballs at which you can see many k sized engine flights actually using a g engine constitutes the area of high power rocketry which is seperate from normal model rocketry purchase of engines like i have been describing require membership in the national association of rocketry the tripoli rocketry assoc or you have to be part of an educational institute or company involved in rocketry amatuer rocketry is another area i m not really familiar with this but it is an area where metal parts are allowed along with liquid fuels and what not i don t know what kind of regulations are involved but i m sure they are numerous high power rocketry is very exciting if you are interested or have more questions there is a newsgroup rec model rockets
