 (U.S. Air Force) |
Evolution of the Inertial Upper Stage
W. Paul Dunn
Though initially conceived as a short-term program, the Inertial Upper Stage played a critical role in ensuring U.S. access to upper orbits and beyond.
The Inertial Upper Stage (IUS) is a highly redundant and ground-commandable launch vehicle used to insert payloads into higher orbits than would be possible with just a primary booster. Throughout the years, the IUS has evolved to become an integral part of America's access to space for both military and civilian sectors.
Initial Studies
The evolution of IUS began in 1969, when a presidential directive set in motion NASA and Air Force studies that led to development of the Space Transportation System (STS) and its principal vehicle, the space shuttle. As part of the early STS definition, NASA and the Air Force jointly studied several concepts for transporting payloads into higher operational orbits, especially the geosynchronous orbits used by most communications satellites. Physical limitations—such as the high propellant mass that would be required—prevent a standard booster from reaching these very high orbits. Rather than build a bigger booster, NASA and the Air Force focused on an upper-stage rocket as the most practical method available. Aerospace was integrally involved in these early studies, assessing the feasibility of cryogenically fueled orbit-to-orbit vehicles, piloted transfer vehicles, and modifications of existing upper stages such as the Centaur, Delta, Agena, and Transtage.
An Inertial Upper Stage during processing for an Air Force mission. The IUS began development in 1976. In 1982, it flew its first mission aboard a Titan 34D. (U.S. Air Force) |
Based on these studies, NASA decided on a "space tug"—a reusable transfer vehicle that would tow satellites from an orbiting space platform to their final operational orbits. Such a project, however, would take years to complete. In the meantime, NASA and the Department of Defense (DOD) agreed to develop what was then called the Interim Upper Stage. This decision fostered a series of additional studies—including many at Aerospace—to determine the requirements, capabilities, and ultimate configuration of this temporary upper stage. In 1975, DOD proposed an expendable solid-fueled rocket because it would cost less to produce than a liquid-fueled vehicle. Independently, NASA also settled on a solid-fueled stage after determining that a liquid-fueled stage interfered with STS designs and could compromise the safety of the flight crew.
Aerospace assisted the Air Force through its source-selection process before establishing a program office in 1976. Several contractors proposed modifications to their existing rocket designs to meet the requirements of the new upper stage. Shortly before commencement of the program-validation phase, however, the Air Force introduced a new requirement: The IUS must be compatible not only with the space shuttle, but with the Titan 34D booster as well. The Titan 34D was the largest available expendable booster and was capable of placing larger and heavier payloads into orbit (military payloads tend to be more massive than civilian payloads). This capacity was important because of the competitive position it offered for commercial access to space and the security it offered for national defense. The Air Force also wanted to use the upper-stage avionics to guide the booster through its powered flight phase to improve reliability.
 | ||
Components of the Inertial Upper Stage. The first-stage solid-rocket motor holds approximately 9700 kilograms of propellant and can maintain thrust for up to 150 seconds (View larger image). | ||
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.
Development Issues
Aerospace worked to overcome several immediate challenges during development. Key requirements included a payload capacity of 2268 kilograms and reliability of 96 percent or better, all with minimal impact to the STS program. Also, the necessary support equipment had to be designed, and unique configurations had to be produced for NASA planetary missions. Numerous technical difficulties nearly scuttled the entire program. Significant problems occurred in the propulsion subsystem (e.g., case burst, tacky liner, soft and cracked propellant, nozzle delaminations), in the software (sizing and timing, guidance, redundancy management, failure detection and correction), and in the avionics (space-rated parts, redundancy, testing). Evolving definitions of booster loads and environments also threatened development, as did problems arising in the qualification testing of the support equipment. Compounding matters, significant differences arose in interpretations of contract and specification requirements, and a serious weight-growth problem prompted a drastic weight-reduction program. A Titan-specific interstage and extendable exit cone had to be added to maintain performance, and a destruct system had to be added to ensure launch-range safety.
This array of technical problems—coupled with various programmatic changes—led to schedule delays, higher costs, and, subsequently, two program restructurings. These delays, in turn, affected other aspects of the program. For example, the Tracking and Data Relay Satellite (TDRS) was to be the first NASA payload for the IUS, and the DSCS II and III communication satellites were to be the first for DOD; however, delays and scheduling conflicts caused uncertainty as to whether STS or Titan 34D would be the first IUS booster. Both would present exceptional challenges for a first-time launch. Requirements continued to shift and evolve, and the Interface Requirements Documents, safety protocols, and other procedures had to be worked out for the first time—under considerable scheduling pressure. Qualification testing was still going on in many areas for the STS version, and Independent Readiness Review Team findings led to several "fix before launch" concerns. Also during this period, NASA requirements for planetary missions changed: The need for more accurate control dictated a three-axis stabilized configuration, which increased payload mass.
Workers in the vertical processing facility at Kennedy Space Center in Florida oversee the lowering of the IUS booster into a workstand for preflight processing. The IUS was attached to a Tracking and Data Relay Satellite deployed by the space shuttle Discovery in July 1995. (NASA) |
Though problematic at the time, these demands led to several modifications to the IUS that now reflect its uniqueness as an upper stage. For example, the IUS first-stage motor can maintain continuous thrust for as long as 150 seconds—longer than any other solid-fueled upper-stage rocket developed for space applications. IUS is also the most functionally redundant upper stage available, and it is the only one that can be commanded from the ground during flight. To support this ground-command capability, Aerospace helped develop an array of contingency procedures to guide rapid response to potential mission anomalies; moreover, the flight operations support team, which includes Aerospace personnel, conducts systematic simulation exercises to prepare for any eventuality during launch.
First Launches
The inaugural IUS launch, mated with a Titan 34D booster and a DSCS II/III spacecraft tandem, took place about five minutes after midnight on October 30, 1982, at Cape Canaveral Air Force Station in Florida. Aerospace provided technical support through its general offices and on-site launch personnel. Unfortunately, a loss of telemetry persisted for much of the flight—not surprising, perhaps, considering how many technical difficulties had to be overcome. Nonetheless, even though flying "blind" to Earth observers, the IUS completed its mission as planned because it was designed to fly autonomously by default, without commands from outside sources. After later analysis, the telemetry loss was attributed to a leak in the hermetic seal of a switch that routed radio-frequency signals to the IUS transmitting antennas. The leak allowed internal pressure in the switch to drop to a level where corona arcing occurred and caused switch failure.
System architecture of the Inertial Upper Stage, the most functionally redundant upper stage available and the only one that can be commanded from the ground during flight. Acronym key: TT&C—telemetry, tracking, and command; RIMU—redundant inertial measurement unit; TVC—thrust vector control; RCS—reaction control system; SRM—solid-rocket motor. |
Even with this problem out of the way, the second flight experienced difficulties that probably would have ended the mission for any other rocket. When a critical seal failed, the control system lost its ability to position the nozzle of the solid-rocket motor. The nozzle canted, causing the IUS to tumble through space along with the attached TDRS spacecraft. The lack of nozzle control was compounded by disruption of normal automatic mission sequencing as a result of unusually high cosmic radiation. Subsequent ground commands—possible only with the IUS—succeeded in separating the IUS from the TDRS, but the spacecraft was still tumbling. NASA was able to use the excess propellant on the TDRS to stabilize the spacecraft and eventually raise it to the desired geosynchronous orbit. Since then, the operational history of the IUS has been impressive, with 20 missions experiencing no significant anomalies. The lone exception occurred in April 1999, when the IUS stage-one component failed to separate normally from its stage-two component (see sidebar, From Lift to Release).
Aerospace also worked to make IUS as accurate as it is reliable, supporting a significant upgrade to the avionics for navigation, control, and guidance in 1999. To achieve this upgrade, the three original flight computer and inertial measurement units were redesigned into one chassis. This upgrade preserved the redundancy while modernizing the gyroscopic components from mechanical devices to more reliable ring laser devices and reducing overall mass by more than 45 kilograms. The flight software was also upgraded to employ more modern coding and other modifications. A single electronics unit, named the Flight Controller, incorporated all these changes. As a result, the IUS has since attained its highest level of orbit insertion accuracy (see flight history).
An IUS carrying a Tracking and Data Relay Satellite moves into a parking orbit following deployment from the space shuttle cargo bay before boosting the satellite to a geosynchronous orbit. (NASA) |
End of an Era
Though conceived only as a stopgap measure, IUS became an indispensable part of the U.S. space program, achieving several notable distinctions. For example, it was the first upper stage to be used on the Titan IVA and B vehicles and the space shuttle and the first to provide upper-stage guidance for the Titan 34D. A number of NASA and DOD spacecraft have been carried aloft by IUS, including the Galileo probe, the Chandra X-ray Observatory, and the latest Defense Support Program satellites (see Mechanisms).
In 1997, organizational restructuring folded IUS into the Titan program and ended its stand-alone status. The last IUS on the manifest is expected to launch sometime in 2003. This final mission will bring to a close an important chapter in the history of space launch.
Acknowledgement
The author would like to thank R. K. Luke, A. R. Shibata, H. Sokoloff, A. E. Goldstein, and G. D. Jensen for their contributions to this article.
To Winter 2003 Table of Contents
