From Kerbal Space Program Wiki
Orbiter One is a peculiar rocket that flies into space with one booster. This is a low tier spacecraft, as would be one that is built in early career mode. It only uses parts from the first 3 levels of the Tech tree.
|“|| We're not sure if the engineers named this craft for its series, or for the number of boosters, and our pilots call it "The Unfinished". Whatever the reason for the name, it's an interesting craft to fly.