In the 1970s and early 1980s, NASA and the DoD evaluated four different existing or proposed upper stages for use with the space shuttle:
- The Centaur hydrolox stage (specifically, the Centaur G/G' versions thereof, with tankage enlarged and widened to fill the shuttle's payload bay), originally developed as an upper stage for the Atlas, but later adapted to several other launchers.
- The Interim Orbital Transfer Vehicle (IOTV), another cryogenic upper stage (like the Centaur), but nowhere near production in any form (unlike the Centaur).
- The Transtage, a hypergolic (Aerozine 50/dinitrogen tetroxide) upper stage originally developed for the various variants of the Titan III.
- The Inertial** (originally Interim) **Upper Stage (IUS), a small solid-fuelled stage (actually two stages) consisting basically of two bog-standard kickmotors stacked on top of one another.
At the time,1 the Centaur was chosen over the other three contenders, as the IUS and (to a lesser extent) the Transtage weren't powerful enough for some of the missions under consideration, while the IOTV would require considerable development and offer nothing not already provided by the Centaur:
... NASA debated the actual design of the tug and whether it would be a modification of an existing upper stage or an entirely new rocket. Four proposals were evaluated including the Air Force’s Inertial Upper Stage (IUS), a redesigned wide-body Centaur, Transtage, and the Interim Orbital Transfer Vehicle (IOTV).
Although all four of these stages could lift Shuttle payloads into geosynchronous orbit, each did so with very different levels of capability. A more powerful upper stage rocket permits the launch of heavier, more complex spacecraft that help scientists conduct more sophisticated research. Of all the options, Centaur was the most powerful, enabling spacecraft of up to 13,000 pounds to be sent into orbit. The IOTV, like Centaur, was a cryogenic rocket and was designed to give a similar performance. The next most powerful was the Transtage, which was a storable propellant system capable of lifting spacecraft of up to 8,000 pounds into orbit, followed by the IUS, which had a 5,000-pound lifting capability.
The Department of Defense (DOD) gave the following assessment of each of these rockets. They concluded that the IUS would be able to satisfy most defense needs and, with modifications, could also take on a limited number of basic science missions. The Transtage could satisfy all of the defense needs but did not have the potential for expanding into other types of missions. DOD concluded, “Shuttle payload limits (65,000 pounds) will limit both the IUS and Transtage growth such that these systems can never capture a significant portion of the long term defense needs.” [...]
The IOTV and Centaur were both powerful cryogenic rockets that DOD believed would not only handle all existing military needs, but could also provide tremendous potential for more difficult science missions. The problem with the IOTV was that it was a new stage involving a great deal of development complexity, as well as schedule and cost risks. As a result, NASA and the Department of Defense concluded, “The Centaur is the only vehicle capable of meeting near term NASA planetary requirements . . . [and] considerable enhancement of DOD mission capabilities.” ... [Taming Liquid Hydrogen, pages 188-189 (numbered as 171-172). Bolding added for emphasis.]
Some of the reasoning given, however, is surprising. Both the IUS and Transtage were limited by their capabilities (or, rather, lack thereof), but, while the IUS could still fulfill some science missions with only 5,000 pounds of lifting capacity, the Transtage allegedly had no ability to perform any science missions - only defence ones - despite having a 60%-better payload capability than the IUS.
Why would the Transtage have been unable to perform any science missions as a space-shuttle upper stage, when the much-less-capable IUS would still have been a possibility for at least some of those same science missions?
1: This changed following the loss of OV-099, when a suddenly-far-more-risk-averse NASA jumped ship for the IUS (despite its anemic ΔV budget), with unfortunate results for some of the spacecraft forced to use the latter upper stage.