Hi.
Rotring Rapidograph's have a single-use ink reservoir. They can be re-filled using a pipette with a long, thin tube but it can get messy. I prefer Rotring Isograph pens, they are similar to Rapido's but with a reservoir that is designed to be re-filled.
Just make sure that you use the "MF" ink, ment for plastic drafting film. Even then the adhesion is not allways the best, I've found that a homeopatic amount (add with tip of a pin to ink reservoir) of dishwashing liquid is enough to kill the surface tension. L