Comment by oskarkk

It's usually kerosene + O2, not H2O2. But H2 + O2 is another classic liquid rocket fuel that has higher specific energy (energy per mass), but lower energy density (energy per volume) than kerosene, and is used mostly as a 2nd stage fuel (where mass is most important). Its lower density and the need to keep it very cool mean heavier tanks, and it's also hard to work with due to being such a small molecule that likes to leak, which causes development costs to be higher.