An unmanned Falcon rocket streaked through the early morning darkness of Florida local time, carrying 2267kg of supplies for the International Space Station.
Besides the docking ring, the Dragon cargo ship contains a DNA decoder for high-flying genetic research.
It is SpaceX's second shot at delivering a new-style docking port for Nasa.
The last one went up in smoke over the Atlantic last year, a rocket accident casualty.
Nasa needs this new docking setup before Americans can fly up in crew capsules set to debut next year.
The capsules would dock to this ring and another due to fly in a year.
SpaceX, meanwhile, aimed to land its leftover booster back at Cape Canaveral.
@SpaceX tweeted: "First stage landing confirmed at LZ-1. Second stage and Dragon continuing to orbit."