Hey guys! I'm currently performing a research project for one of my engineering classes on hybrid rocket engine design, particularly in regards to homemade designs. While researching, I've found examples and demonstrations of engines made out of acrylic or rubber but I was interested if anyone on here knew of any other materials being used as the solid propellant? I also found that most people used either compressed air, oxygen, or nitrogen as their oxidizer which I understand because those are the most suitable oxidizing gases due to availability and cost. However, if any of you have any knowledge of any others being used, feel free to leave a comment below. Lastly, I am also researching how normal people are making their engines safe to use and be around. I've found examples of people extending strings from the throttle so they can activate them from a distance, as well as having an emergency cut off switch that can end the engine at any given moment. If you have any examples of features others that may benefit safety, I would greatly appreciate it! Thanks!