Several variants existed, differentiated by a four digit numerical code.

Delta 1000 was developed by McDonnell Douglas company (now — Boeing) in 1972.

Four, six or nine Castor-2 solid rocket boosters were attached to increase thrust at lift-off.

The Japanese N-II and H-I launch vehicles used the same Extended Long Tank Thor first stage.

For the second stage, the N-II used the Delta-F while the H-I had the Japanese LE-5 engine which used liquid hydrogen and oxygen.