In January 1986, astronaut Rick Hauck approached his STS-61F crew four months before their mission was scheduled to launch. The shuttle Challenger was set to deploy the Ulysses solar probe on a trajectory to Jupiter, utilizing a liquid-fueled Centaur G-Prime stage. While an upcoming launch should be an exciting time for any astronaut, Hauck's was anything but optimistic. As he spoke to his crew, his tone was grave. He couldn't recall the exact quote in a 2003 Johnson Space Center (JSC) oral history, but the message remained clear.

“NASA is doing business different from the way it has in the past. Safety is being compromised, and if any of you want to take yourself off this flight, I will support you.”

Hauck wasn’t just spooked by the lax approach that eventually led to the Challenger explosion. Layered on top of that concern was the planned method of sending Ulysses away from Earth. The Centaur was fueled by a combustible mix of liquid hydrogen and oxygen, and it would be carried to orbit inside the shuttle’s payload bay.

The unstoppable shuttle

Hauck's words may have seemed shocking, but they were prescient. In the early 1980s, the space shuttle seemed unstoppable. Technically called the US Space Transportation System program, the shuttle was on the verge of entering what was being called its “Golden Age” in 1984. The idea of disaster seemed remote. As experience with the craft grew, nothing seemed to have gone wrong (at least nothing the public was aware of). It seemed nothing could go wrong.

In 1985, the program enjoyed a record nine successful spaceflights, and NASA was expected to launch a staggering 15 missions in 1986. The manifest for 1986 was beyond ambitious, including but not limited to a Department of Defense mission into a polar orbit from Vandenberg Air Force Base, the deployment of the Hubble telescope to low Earth orbit, and the delivery of two craft destined for deep space: Galileo and Ulysses.

The space shuttle had been touted as part space vehicle and part “cargo bus,” something that would make traveling to orbit routine. The intense schedule suggested it would finally fulfill the promise that had faded during the wait for its long-delayed maiden flight in April 1981. As astronaut John Young, who commanded that historic first flight, stated in his book Forever Young, “When we finished STS-1, it was clear we had to make the space shuttle what we hoped it could be—a routine access-to-space vehicle.”

To meet strict deadlines, however, safety was starting to slide. Following the last test flight (STS-4, completed in July 1982), crews no longer wore pressure suits during launch and reentry, making shuttle flights look as “routine” as airplane rides. The shuttle had no ejection capability at the time, so its occupants were committed to the launch through the bitter end.

Yet by mid-1985, the space shuttle program had already experienced several near-disasters. Critics of the program had long fretted over the design of the system, which boasted two segmented solid rocket boosters and an external tank. The boosters were already noted to have experienced “blow by” in the O-rings of their joints, which could leak hot exhaust out the sides of the structure. It was an issue that would later come to the forefront in a horrific display during the Challenger disaster.

But there were other close calls that the public was largely unaware of. In late July 1985, the program had experienced an “Abort to Orbit” condition during the launch of STS-51F, commanded by Gordon Fullerton. A center engine had failed en route to space, which should normally call for the shuttle’s immediate return. Instead, a quick call was made by Booster Systems Engineer Jenny Howard to “inhibit main engine limits,” which may have prevented another engine from failing, possibly saving the orbiter Challenger and its seven-man crew. (The mission did reach orbit, but a lower one than planned.)

People who followed things closely recognized the problems. The “Space Shuttle” section of Jane's Spaceflight Directory 1986 (which was largely written the year before) underscored the risky nature of the early program: “The narrow safety margins and near disasters during the launch phase are already nearly forgotten, save by those responsible for averting actual disaster.”

The push for Shuttle-Centaur

All of those risks existed when the shuttle was simply carrying an inert cargo to orbit. Shuttle-Centaur, the high-energy solution intended to propel Galileo and Ulysses into space, was anything but inert.

Shuttle-Centaur was born from a desire to send heavier payloads on a direct trajectory to deep space targets from America's flagship space vehicles. The Centaur rocket was older than NASA itself. According to a 2012 NASA History article, the US Air Force teamed up with General Dynamics/Astronautics Corp. to develop a rocket stage that could be carried to orbit and then ignite to propel heavier loads into space. In 1958 the proposal was accepted by the government's Advanced Research Products Agency, and the upper stage that would become Centaur began its development.

The first successful flight of a Centaur (married to an Atlas booster) was made on November 27, 1963. While the launch vehicle carried no payload, it did demonstrate that a liquid hydrogen/liquid oxygen upper stage worked. In the years since, the Centaur has helped propel a wide variety of spacecraft to deep-space destinations. Both Voyagers 1 and 2 received a much-needed boost from their Centaur stages en route to the Solar System's outer planets and beyond.

General Dynamics was tasked with adapting the rocket stage so it could be taken to orbit on the shuttle. A Convair/General Dynamics poster from this period read enthusiastically, “In 1986, we're going to Jupiter...and we need your help.” The artwork on the poster appeared retro-futuristic, boasting a spacecraft propelled by a silvery rocket stage that looked like something out of a sci-fi fantasy novel or Omni magazine. In the distance, a space shuttle—payload bay doors open—hovered over an exquisite Earth-scape.

The verbiage from a 1984 paper titled “Shuttle Centaur Project Perspective,” written by Edwin T. Muckley of NASA's Lewis (now Glenn) Research Center, suggested that Jupiter would be the first of many deep-space destinations. Muckley optimistically announced the technology: “It's expected to meet the demands of a wide range of users including NASA, the DOD, private industry, and the European Space Agency (ESA).”

The paper went on to describe the two different versions of the liquid-fueled rocket, meant to be cradled inside the orbiters' payload bays. “The initial version, designated G-Prime, is the larger of the two, with a length of 9.1 m (30 ft.). This vehicle will be used to launch the Galileo and International Solar Polar Missions (ISPM) [later called Ulysses] to Jupiter in May 1986.”

According to Muckley, the shorter version, Centaur G, was to be used to launch DOD payloads, the Magellan spacecraft to Venus, and TDRSS [tracking and data relay satellite system] missions. He added optimistically, “...[It] is expected to provide launch services well into the 1990s.”

Dennis Jenkins' book Space Shuttle: The History of the National Space Transportation System, the First 100 Missions discussed why Centaur became seen as desirable for use on the shuttle in the 1970s and early 1980s. A booster designed specifically for the shuttle called the Inertial Upper Stage (developed by Boeing) did not have enough power to directly deliver deep-space payloads (this solid stage would be used for smaller satellites such as TDRSS hardware). As the author explained, “First and most important was that Centaur was more powerful and had the ability to propel a payload directly to another planet. Second, Centaur was 'gentler'—solid rockets had a harsh initial thrust that had the potential to damage the sensitive instruments aboard a planetary payload.”

However, the Centaur aboard the shuttle also had its drawbacks. First, it required changes in the way the shuttle operated. A crew needed to be reduced in size to four in order to fit a heavier payload and a precipitously thin-skinned, liquid-fueled rocket stage inside a space shuttle's payload bay. And the added weight meant that the shuttle could only be sent to its lowest possible orbit.

In addition, during launch, the space shuttles' main engines (SSMEs) would be taxed unlike any other time in program history. Even with smaller crews and a food-prep galley removed mid-deck, the shuttle’s main engines would have to be throttled up to an unheard-of 109-percent thrust level to deliver the shuttle, payload, and its crew to orbit. The previous “maximum” had been 104 percent.

But the risks of the shuttle launch were only a secondary concern. “The perceived advantage of the IUS [Inertial Upper Stage] over the Centaur was safety—LH2 [liquid hydrogen] presented a significant challenge," Jenkins noted. "Nevertheless, NASA decided to accept the risk and go with the Centaur.”

While a host of unknowns remained concerning launching a volatile, liquid-fueled rocket stage on the back of a space shuttle armed with a liquid-filled tank and two solid rocket boosters, NASA and its contractors galloped full speed toward a May 1986 launch deadline for both spacecraft. The project would be helmed by NASA's Lewis. It was decided that the orbiters Challenger and Discovery would be modified to carry Centaur (the then-new orbiter Atlantis was delivered with Centaur capability) with launch pad modifications taking place at the Kennedy Space Center and Vandenberg.