If Rotring Rapidograph is your pen of choice, then so is this ink. And as far as ink goes, this is surprisingly waterproof and strongly black without being likely to clog the thin tips of technical pens unless left to dry out. For anyone who has worked with ink, it is amazing how one can manage these qualities while maintaining such a good and strong black.
Can one get third party inks for Rapidograph? I've not looked into that, but I wouldn't trust them to have the unique qualities that make them perfect for my technical pens. I'd only use the original, knowing that Rotring have more than half a century behind them of perfecting both pens and ink together.