The Delta-K was an American rocket stage, developed by McDonnell Douglas and Aerojet. It was first used on 27 August 1989 as the second stage for the Delta 4000 series. It continued to serve as the second stage for subsequent variants of the Delta rocket. It was propelled by a single AJ10-118K rocket engine, fueled by Aerozine 50 and dinitrogen tetroxide, which are hypergolic. As of 25 May 2008, 138 have been launched, and excluding one which was destroyed by the explosion of a lower stage, none have failed.