In the licence screen type:

NCHSSS - no chassis

FSTFRWRD - fast mode

to use more than one code enter one code, exit, start a race, quit, go back, enter the next code, and repeat untill you have all the codes you want. Using this you can have flying drivers with rocket engines.

Enter BUILD mode and create or edit a driver. Select Make A License from the in-game menu and name the driver FLYSKYHGH. This will also unlock the rocket car.