Saturn V Saturn INT-21

The premise was simple enough to be appealing: for heavy payloads bound for Earth orbit, the Saturn V's third stage is dead weight. The INT-21 kept the S-IC and the S-II and dropped the S-IVB, bringing height down to 98.5 m without payload and liftoff mass to 2,916,080 kg. In exchange it offered 115,900 kg to low Earth orbit, a figure no other American launcher would come close to for decades. Since the instrument unit normally rode above the third stage, the study relocated it to the top of the S-II.

At 17:30 UTC on 14 May 1973 a two-stage Saturn V placed the Skylab station in orbit from the Kennedy Space Center. That vehicle, SA-513, had originally been assigned to a cancelled lunar mission and flew without an S-IVB because its payload was, quite literally, an S-IVB: the orbital workshop had been built from one of those stages and carried its own instrument unit, so the rocket's did not have to move. It was the final Saturn V launch.

The configuration was meant for more work than it got. The Apollo Applications Program lined it up for further missions and, specifically, for lofting Skylab B and other American space stations. The budget cuts of the early 1970s ended all of it. Of the second workshop only the backup article survives, on display at the National Air and Space Museum, while the third stage of the vehicle that launched Skylab was replaced by the station itself.

Study and flight article are worth telling apart. The Saturn INT-21 as documented on paper — instrument unit relocated and all — was never built. What flew was a closely related variant, the two-stage Saturn V of the Skylab mission, which is why the designation is commonly applied to that configuration. The figures recorded here come from the study; the date is that of the single flight actually made.