What is hypergolic rocket fuel?



We all know how rocket science is relatively complicated. Actually, not entirely. Hypergolic fuels are pretty simple. A Hypergolic fuel mixture is usually used in rockets, and consists of 2 main materials, a fuel and an oxidizer. The fuel is usually a form of Hydrazine, and the oxidizer is usually Dinitrogen Tetroxide. The Hydrazine and Dinitrogen Tetroxide ignite spontaneously upon contact, which is why it is considered Hypergolic.
Hypergolic fuels are indeed less complicated than what can be called "usual" rocket fuel, like a mixture of RP-1 (rocket grade kerosene), and LOX (liquid oxygen). The LOX helps ignite your fuel, which in this case is RP-1/Kerosene. You also need an outside ignition source to ignite the LOX + Kerosene mixture, as they do not instantly combust. With Hypergolic fuels, you don't need an outside ignition source, at all. They ignite themselves, an outside ignition source like ignition fluid or spark plugs would be quite useless.
SpaceX's Crew Dragon capsule uses a Hypergolic fuel mixture of Hydrazine + Dinitrogen Tetroxide for its abort system and Reaction Control System (RCS), because it acts fast, which is what an Abort System and RCS Need. Now you are asking, since Hypergolic fuels have all these great benefits, why not use them? Well, here's your answer. The best Hypergolic fuel mixture for efficiency and power that we can currently get, is, as mentioned before, Hydrazine + Dinitrogen Tetroxide. This mixture is extremely toxic and carcinogenic, meaning, if a little bit of it leaks or drips, it's not a good idea to be anywhere near it, like, anywhere. We also already do use this hypergolic fuel mixture for rockets, for example, the Russian Proton Rocket and as we mentioned before the SpaceX Crew Dragon Capsule. Maybe we will find a better Hypergolic fuel mixture in the future that is not toxic and completely safe, but for now, this is the best one we have.