Ok, lets get back to basics.
There is no such thing as a safe rocket motor. A rocket motor contains very energetic materials, which when combusted as per design, yield thrust. But there is no guarantee that it will work as per design.
I am one of the people who does some work on experimental hybrid rocket motors. My "little" test motor has a safe standoff distance of 1500', IIRC. If something goes seriously wrong you don't want to be nearby. Rocket motors are NOT forgiving of design or operational errors, and even when done right, still might CATO or do other bad things. Those videos you see of people starting motors with strings, or turning a valve on a tank by hand, are videos by people who really don't have a clue what they are doing. They are serious accidents waiting to happen. DO NOT EMULATE THEM!
By the way you asked your question, I'm going to assume you have no background in chemistry, probably not much yet in physics or engineering? I'm guessing you are a student. Please do not take this the wrong way, but you need a lot more knowledge and study, plus ideally some experience, before trying to design/make your own hybrid rocket motor.
Rocket motors are fascinating!
My recommendation if you are really interested is to join the NAR or TRIPOLI, and go through the certification process through level 2. At that point you can start to be able to fly research motors, provided you are an adult. IMHO, if you are not an adult, do not work on research motors. Research motors are ones that you engineer and produce, rather than purchasing commercially.
Now if you are simply researching what others have done, and are not going to work on producing a motor, then that is fine. But you will find that lots of the details you might want to get are not as available here as you might think. Rocket motor and propellant/oxidizer/fuel design is restricted to the research section of these forums. That subforum requires US citizenship and level 2 certification to get access. The reasons are (1) we don't want kids and adults killing or seriously injuring themselves or others, or destroying property, (2) we don't want the reputation of having helped someone do #1, and (3) there are ITAR and other regulations in the US concerning rocket technology.
For instance I've put enough info into a thread of mine in the research section for a dedicated person to essentially reproduce my first EX hybrid rocket motor. I spent quite a while designing before starting to machine the parts, and the machining took me longer than it should have. Others here have more experience than I, but again, they are not publishing their designs out in the open. It would be nice if ITAR didn't exist, etc, but that's not the case. Sorry.
Keep in mind that in the end, the only difference between idiot, amateur, and professional rocket motors, is the extent of the knowledge base and the size of the budget. There are some small professional motors. There are some decent sized amateur ones. And then there are the things the idiots do. Please don't be one of those!
Gerald
PS - Generally a hybrid rocket motor does not use a solid propellant, but does use a solid fuel. A propellant has fuel and oxidizer both present in the mix. A fuel is simply something combustible. That's why hybrids are safer before you load the oxidizer. The motor can't go boom if the oxidizer is not present. The motor only contains a flammable solid. Most anything that will burn will work. Some things of course work much better than others.
