AEDC played integral role in readying NASA Cassini spacecraft for its launch

ARNOLD AIR FORCE BASE, TENN. -- On Sept. 15, NASA's Cassini spacecraft collided with the atmosphere of Saturn, thus ending its 13-year tour of the planet, as well as ending an historic era in the exploration of the solar system.

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. Jet Propulsion Laboratory (JPL), a division of Caltech in Pasadena, manages the mission for NASA's Science Mission Directorate in Washington. However, AEDC and its test engineers also played an important role in the successful launch of the Cassini.

According to Zak Mohyuddin, an engineer at Arnold Air Force Base, AEDC test teams conducted testing of the second stage engine of the Titan-IV rocket, known as an LR-91 engine, which launched the Cassini in October 1997.

“AEDC was first approached about the test of LR-91 in August 1995, and the tests had to be completed by the following summer, so it was a very tight schedule,” he said. “There also was a need for altitude testing and our J-4 Rocket Motor Test Facility had the capability for that.”

Mohyuddin explained that the program posed multiple challenges, including safety, environmental, logistics, contracting, procurement, design, fabrication and crew training, among the few.

“The highest concern was propellant safety,” he said. “The propellant for LR-91 engine consists of a blend of hydrazine and Unsymmetrical Dimethyl Hydrazine (UDMH) as fuel and nitrogen tetroxide as the oxidizer. The propellants are hypergolic, which means the fuel and oxidizer ignite upon contact without need for external ignition source. These propellants are also very toxic and environmentally hazardous.”

At the time of the LR-91 testing, these propellants had not been used at this scale, of several hundred gallons, at the J-4 Rocket Motor Test Facility or any other test facilities at Arnold AFB, since the 1960s.

“New storage and run tanks, and flare stack had to be designed, fabricated, installed and integrated into the existing J-4 infrastructure,” Mohyuddin said. “No one at AEDC had experience with handling these quantities of propellants, and the OSHA/EPA regulations did not exist in the 1960s.”

Many J-4 team members traveled to Lockheed-Martin facilities in Denver, Colorado, and Aerojet facility in Sacramento and to Vandenberg Air Force Base, California, for training on safe handling of these propellants.

“Because the propellant quantities exceeded the threshold for public notification under environmental laws, a public meeting was held at the Gossick Leadership Center,” Mohyuddin added.

All propellant deliveries, transfers and engine tests occurred only on weekends, and during these periods, access was restricted to essential crew for parts of the base beyond the warehouse and Model and Machine Shop. Each person entering the restricted area was given 5-minute escape masks for quick evacuation in case of a spill or leak.

“To mitigate the highly hazardous operations, the J-4 team employed a concept called ‘pinning procedures’ during the planning phase. It proved very decisive for a safe and successful test program. It involved the work instructions for propellant transfers, pressurization, drain and vent. A schematic drawing of the propellant system was placed on a wall. A multi-discipline team observed as each step of a work instruction was called out, and the step was then visually represented on the schematic drawing by pushing a color-coded tack pin onto the corresponding valve or device; for example, red for open valve. This method allowed the review team to immediately detect inadvertent hazards such as trapped fuel, locked pressure or open vent line. The work instructions could be changed or the hardware design modified to remove the unsafe condition.”

Despite the challenges and tight test schedule, AEDC teams were able to successfully complete the testing of the LR-91 in early July 1996.
Besides Mohyuddin, several other AEDC team members have mentioned that working on this project has been one of their most memorable while working for AEDC. They include J-4 engineers from that time Brent Bates and Joe Migliaccio, as well as Air Force project manager Randy Quinn.

“It’s quite an accomplishment that AEDC went from a no-notice first inquiry in August 1995, to completion of testing in July 1996 to meet the launch window,” Mohyuddin said.

And the memories came flooding back to Mohyuddin, and those who worked on the LR-91, with the recent news of the Cassini spacecraft mission’s end.

Data analyst Becky Combs remembers the tests well, and said it was “an honor to be a part of AEDC’s history at its greatest.”
In an announcement of the Cassini mission’s end, Thomas Zurbuchen, NASA’s Science Mission Directorate at NASA headquarters in Washington, commented on the significance of the information gained from the spacecraft’s journey.

"This is the final chapter of an amazing mission, but it’s also a new beginning,” Zurbuchen said. “Cassini’s discovery of ocean worlds at Titan and Enceladus changed everything, shaking our views to the core about surprising places to search for potential life beyond Earth."
Telemetry received during the plunge indicates that Cassini entered Saturn's atmosphere with its thrusters firing to maintain stability, as it sent back a final set of observations. Loss of contact with the Cassini spacecraft occurred at 7:55 a.m. EDT on Sept. 15, with the signal received by NASA's Deep Space Network antenna complex in Canberra, Australia.

"It's a bittersweet, but fond, farewell to a mission that leaves behind an incredible wealth of discoveries that have changed our view of Saturn and our solar system, and will continue to shape future missions and research," said Michael Watkins, director of NASA's JPL in Pasadena, California.

Cassini's plunge brought to a close a series of 22 weekly "Grand Finale" dives between Saturn and its rings, which had never been attempted by any other spacecraft.

"The Cassini operations team did an absolutely stellar job guiding the spacecraft to its noble end," said Earl Maize, Cassini project manager at JPL. "From designing the trajectory seven years ago, to navigating through the 22 nail-biting plunges between Saturn and its rings, this is a crack shot group of scientists and engineers that scripted a fitting end to a great mission. What a way to go. Truly a blaze of glory."
As planned, data from eight of Cassini's science instruments was emitted back to Earth. Mission scientists will examine the spacecraft's final observations in the coming weeks for new insights about Saturn, such as hints about the planet's formation and evolution, and processes occurring in its atmosphere.

"Things never will be quite the same for those of us on the Cassini team now that the spacecraft is no longer flying," said Linda Spilker, Cassini project scientist at JPL. "But, we take comfort knowing that every time we look up at Saturn in the night sky, part of Cassini will be there, too."

Cassini launched in 1997 from Cape Canaveral Air Force Station in Florida and arrived at Saturn in 2004. NASA extended its mission twice – first for two years, and then for seven more. The second mission extension provided dozens of flybys of the planet's icy moons, using the spacecraft's remaining rocket propellant along the way. Cassini finished its tour of the Saturn system with its Grand Finale, capped by the Sept. 15 intentional plunge into the planet to ensure Saturn's moons – particularly Enceladus, with its subsurface ocean and signs of hydrothermal activity – remain pristine for future exploration.
While the Cassini spacecraft is gone, its enormous collection of data about Saturn – the giant planet, its magnetosphere, rings and moons – will continue to yield new discoveries for decades to come.

"Cassini may be gone, but its scientific bounty will keep us occupied for many years,” Spilker said. “We've only scratched the surface of what we can learn from the mountain of data it has sent back over its lifetime."

Further information and resources for Cassini's Grand Finale is available at: https://saturn.jpl.nasa.gov/grandfinale

A portion of the above story was written using information from the NASA release, “NASA’s Cassini Spacecraft Ends Its Historic Exploration of Saturn.”