It seems intuitive enough to me. Solid fuel is basically "pre-mixed". That's why this Proton rocket didn't blow up until it slammed into the ground, even thou

It seems intuitive enough to me. Solid fuel is basically "pre-mixed". That's why this Proton rocket didn't blow up until it slammed into the ground, even though it was rapidly falling apart and completely consumed in fire. The speed at which it burns is basically the extent to which that fuel and oxidizer is mixed. Consider a bucket of kerosene. Gasoline doesn't have it's own oxidizer, it uses the air for that. If you light the bucket on fire then it will burn slowly for quite a while, but if you then grab the bucket and fling it into the air then the fuel and oxidizer will mix and you'll have a rather decent fireball on your hands. If you mix the fuel with an oxidizer first (say, ammonium nitrate....^), then you are going to have a bad situation the moment it catches; no need to shake it up. The other major sort of engine, a hybrid engine, is even safer than either solid or liquid rockets. This is because the fuel and the oxidizer are separated and unmixed like in liquid fuel rockets but additionally the fuel is not going to be able to rapidly mix with the oxidizer if it hits the ground or something (since it is a giant chunk of basically rubber or paraffin or whatever.) ^ ...and now I'm on a watchlist.