The Inertial Upper Stage was a rocket upper stage that could boost payloads launched on the Space Shuttle or Titan rockets to higher orbits or to interplanetary missions.

What was inertial about this upper stage? (And couldn't whatever argument used also apply to other upper stages?)


1 Answer 1


I found a nice article about the evolution of the IUS in this issue of the Crosslink newsletter.

Originally, NASA intended to develop a reusable "space tug" that could deliver satellites from the relatively low orbits reachable by STS to geosynchronous and other high orbits. This was a big concept that was going to take years to develop, so NASA and DOD agreed to develop an expendable Interim Upper Stage, IUS, to perform those sorts of satellite deployments.

As the article describes:

At the end of 1977, NASA abandoned its plans for a space tug, so the IUS program name was formally changed from Interim Upper Stage to Inertial Upper Stage (because it used inertial navigation). The prime contractor was selected, and full-scale development began in April 1978.

Obviously a desire to retain the established TLA informed the choice of name; it's not unique in using inertial navigation. It's a pretty poor choice of name all around; it's two stages, not one.

  • 7
    $\begingroup$ Certainly it's not unique: among others, the Centaur is a current upper stage with inertial guidance. That is a nice benefit to the payload. Without inertial guidance in the upper stage, the spacecraft must carry 30-50 m/s of ∆V to make up for injection residuals, the 3-D components of velocity errors at sustainer shutdown. With inertial guidance that can be brought to less than 1-2 m/s. You mention TLA — much light has been made of that. In Charley Kohlhase's Voyager Trave Guides, instead of 4-letter and 5-letter, he had ETLA: Extended Three Letter Acronym and DETLA: Doubly Extended... $\endgroup$ Sep 23, 2020 at 5:18
  • 2
    $\begingroup$ Interesting, I never realized the name changed that early. There were still documents referring to "interim" in the 80s. I also thought it was called that because it was three-axis stabilized, not spinning like the PAMs. Live and learn. $\endgroup$ Sep 23, 2020 at 12:59
  • 3
    $\begingroup$ I suspect it's a fundamental law of the universe that "interim" projects will outlive their intended successors, which doesn't bode well for the SLS's EUS. $\endgroup$ Sep 23, 2020 at 16:14
  • 1
    $\begingroup$ @RussellBorogove That concept was expressed in my line of work with 'there's nothing more permanent than the temporary!' $\endgroup$
    – Neil_UK
    Sep 24, 2020 at 4:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.