[Type] Hybrid rocket [Description] Separate oxidiser/fuel; typically the oxidiser is liquid and kept in a tank and the fuel is solid. [Advantages] Quite simple, solid fuel is essentially inert without oxidiser, safer; cracks do not escalate, throttleable and easy to switch off. [Disadvantages] 
Some oxidisers are monopropellants, can explode in own right; mechanical failure of solid propellant can block nozzle (very rare with rubberised propellant), central hole widens over burn and negatively affects mixture ratio.