Why does nobody use Liquid Propellant rocket motors? If someone were to develop one that was "pop in/out" like our AP and Hybrid motors are, could it be certified by the NAR and used? It just seems odd. I feel like constructing a little mini liquid rocket wouldn't be terribly hard. Of course, you'd have to supply the fuel, which is probably not the safest thing to have hanging about in your basement. These things can be averted, however. A tank of oxygen and a bottle of kerosene are sitting in my garage... Is there anything that explicitly denies Liquid propellant motors from being certified?