The N-I or N-1 was a derivative of the American Delta rocket, produced under licence in Japan. It used a Thor-ELT first stage, a Mitsubishi Heavy Industries-designed LE-3 engine was used as a second stage, and three Castor SRMs. Seven were launched between 1975 and 1982, before it was replaced by the N-II. Six of the seven launches were successful, however on the fifth flight, there was recontact between the satellite and the third stage, which caused the satellite to fail.
On 29 February 1976, the second N-I conducted the only orbital launch to occur on a leap day (as of year 2008).