The Delta 4000-series and 5000-series were developed in the aftermath of the Challenger disaster and consisted of a combination of 3000-era and Delta II-era components. The first stage had the MB-3 main engine and Extended Long Tank of the 3000-series and mounted upgraded Castor 4A motors. The new Delta K second stage was also included. A total of three were launched in 1989 and 1990, carrying two operational payloads.