Two potential sources for engine complications with methyl-hydrazine/nitrogen tetroxide rocket engines were examined. The products formed during a cold pre-reaction before the actual ignition of the rocket engine and their later behavior were investigated with Car-Parrinello molecular dynamics simulations. It was found that methyldiazene is the main product of the cold pre-reaction. As a side product, especially in the presence of an excess of methylhydrazine, dimethyltetrazanes are formed. These may be responsible for violent side reactions. We also examined possible plume deposits, which are mainly formed in pulsed mode operation, for their influence upon the operation of a rocket engine. We synthesized methylhydrazinium and dimethylhydrazinium nitrate as well as methylhydrazinium azide and determined their crystal structure and sensitivity.