Some tip-jets move their rotors by running jet engines on each end of the rotor. Obviously there must be some ways to supply fuel to those engines through a rotating body. How is this accomplished?
There are pipes inside the blades which connect the engines to a rotary seal inside the main rotor head assembly. That seal allows fuel to be pumped into the blades from a port outside the main rotor head. Such seals are used in a variety of mechanical engineering applications where a fluid, usually lubricating oil, has to be provided to a rotating piece of machinery.
One example which you can probably find a cross-section of on the web is the method of using oil under pressure to change the blade pitch of a constant-speed propeller.