[bmuon]

While it might be true that the temperatures of rocket fuel burning are enough to create plasma and some reactions that lead to mass turning into energy, I think Jabavu is right in that the energy comes from the chemical reaction, not from E=mc2.

[plopilop]

Well, yes, but it's the same thing, really. It's what my last paragraph above is about.

E=mc² is not about nuclear reactions, it's about every reaction. A chemical reaction of combustion transforms some fuel molecules into new molecules. If you compute the mass of the products of the combustion and compare it to the mass of the fuel before burning, you will notice a sliiiiight difference (probably negligible, barely detectable). The disappearing mass has been converted to kinetic energy (and heat, but who cares).