I would suggest that the chemistry parts of rocket science are some of the easier parts (how complicated can burning kerosene and LO2 be?), but I suspect I would piss off a few rocket scientists. ;)
Read Ignition!. It is, in fact, fairly difficult to find a combination of fuels that ignite exactly in the manner one wants them to, in a reasonable range of temperatures, and can be stored without decomposing or eating the storage container.
Of course, today, in large part thanks to a large number of people who died horrible deaths investigating this during the time period documented in Ignition!, it's not hard at all. But it was sure hard until the 1960s.
Apparently, it's not the complexity that kills you. It's the unexpected explosions (who knew titanium could be explosive?), the deadly poisons, and the times where you discover that the oxidizer tanks for all the rockets you manufactured seven years ago are leaking fuming nitric acid, and you haven't changed the design in seven years. The solution for that problem was truly astonishing.
