Delta 1604 was a three-stage Delta 1000-series configuration with the MB-3-powered extended long-tank first stage, six Castor II strap-on motors, the 1.65-metre AJ10-118F second-stage and fairing arrangement, and a TE-M-364-4 solid third stage. Two vehicles successfully launched Explorer 47 and Explorer 50 into highly elliptical magnetospheric orbits in 1972 and 1973.