Remember the Air Force’s moon shot concept that Charley Wilson tossed? It returned as the Thor Able launch vehicle. Three moon shot attempts were made but none were successful. However, a fourth Thor Able launched the first weather satellite—Tiros 1.
And the Thor booster went on to become the workhorse of the unmanned space program, getting ever more powerful upper stages including the Agena A and the Agena B. The Thor itself evolved, getting a more powerful motor. (The original was the H1 with 150,000 lbs thrust. Eventually its motor would have 207,000 lbs thrust.)
The launch vehicle itself finally morphed into a new design—the Delta—with three solid boosters. This increased its payload capacity. But the venerable old rocket was not done yet. In 1987, the Air Force awarded what was then McDonald-Douglas Aircraft (now Boeing) a contract to produce the Delta II. Gone was the distinctive tapered fuselage, giving the new booster more propellant capacity. The three solid boosters were replaced with nine.
NASA liked the design as well. Delta II became a significant part of the space fleet. Boeing did not rest on Delta’s laurels, however. It introduced Delta III, a total redesign of the vehicle. Delta III was 13 feet in diameter, compared to eight feet for previous Deltas. The first stage motor was replaced with a new one generating almost 250,000 lbs of thrust. The strap-on solids were larger and more powerful.
And the second stage modernized Agena was replaced with an uprated Centaur LH2/LOX stage.
Unfortunately, III did not fare well. Only three were flown, with no successes.
So Boeing completely redesigned the package again. The basic fuselage structure was retained, but the propellant and motor were changed. The RP 1 and LOX were replaced with LOX and LH2. A new motor, the RS-68, designed by Pratt and Whitney, powered Delta IV, with an average 700,000 lbs of thrust.
Boeing developed a series of five launch vehicles (LVs) around this basic rocket, all with a modernized Centaur second stage. A medium launch vehicle used just the basic Delta IV. Three 'medium plus' LVs used the Delta IV with strap-on solid boosters.
A heavy LV uses two strap-on Delta-IVs to create an LV that can lift 50,000 lbs to LEO.