Unique propulsion system boosts Chandra's high-flying succes

says Marshall Space Flight Center

The challenge: Put the largest and one of the heaviest payloads ever into orbit.

Mission improbable: Get that huge, heavy payload known as NASA's Chandra X-ray Observatory into an orbit 200 times higher than the Hubble Telescope.

Mission accomplished: Thanks to the teamwork of scientists and engineers at the Marshall Center, The Boeing Company in Seattle, Wash., and TRW Space and Electronics group of Redondo Beach, Calif., who designed the integral propulsion system necessary to send Chandra one-third of the way to the Moon.

It was just over three years ago when the first amazing images from Chandra were seen. But when Marshall Center engineers began plans to propel the 45 feet and nearly 11,000 pounds of mirror and metal into space, they chose a system known as the inertial upper stage propulsion system to launch the X-ray telescope from the Space Shuttle cargo bay in to orbit. But, that was only half the battle.

Robert Sackheim, now assistant director and chief engineer of propulsion at the Marshall Center, was the propulsion manager for TRW when Marshall engineers decided to use the inertial upper stage system to boost the X-ray observatory to about 37,000 miles. But that was still far less than the 87,000-mile goal set for the Chandra mission.

That's when Marshall called on TRW to build a dual-mode, liquid propulsion system to work in tandem with the inertial upper stage system, Sackheim said. By using the two separate propulsion systems, the Chandra team was able to achieve the highly elliptical orbit mission goal, thus giving the X-ray telescope a better vantage point to view the universe.

"The higher orbit enables Chandra to collect more data per pass, increasing the science yield and making the program more economical. But, the high end -- or apogee -- of Chandra's orbit would not have been possible without the help of the TRW-designed high-performance system," which used the highest performance engine available at the time, Sackheim explained.

The high-end of Chandra's orbit helps scientists at the Center who manage the program; scientists at the Smithsonian Astrophysical Observatory in Cambridge, Mass., who manage the data and control the observatory, and other scientists all over the world learn more about the mysteries of our universe. There are some real scientific and technical advantages to getting a highly elliptical orbit, said Martin Weisskopf, Chandra project scientist at Marshall.

"You get a much more efficient program where there are more hours of viewable data, and less time when the view is obstructed," said Weisskopf.

Aside from obstructed views, another problem engineers faced when designing the Chandra mission was the radiation belts that encircle the Earth. While they pose no threat to life on Earth, they could damage the sensitive instruments of the X-ray telescope. Using the dual propulsion system enabled engineers to place Chandra well out of harm's way.

The Boeing Company designed and built the inertial upper stage propulsion system, the one that propelled Chandra from the Space Shuttle Columbia's cargo bay to its initial 37,000-mile orbit. Bob Hughes, Marshall's chief engineer for reusable launch vehicles, praised the Boeing/NASA upper stage team for their major role in the Chandra project.

"The Boeing/NASA team worked night and day to make sure the inertial upper stage was ready for the mission," he said. "They were exceptional."

The Chandra X-ray Observatory is an unqualified success by almost anyone's standard of measurement. Originally slated for a five-year mission, last year managers at NASA Headquarters in Washington, D.C., extended Chandra's mission to a 10-year tour of duty. Tony Lavoie, Marshall's Chandra Observatory program manager, said the data coming from the mission is groundbreaking.

"It's the first time we've been able to see new and exciting details of supernovas and black holes," Lavoie said. "In a lot of cases the assumptions and the hypotheses about what's going on have to be changed because now you have these new details and they don't make sense with current models. It's really revolutionary in X-ray astronomy."

Generations of future scientists will be studying data from Chandra. This sophisticated satellite is being used to target known celestial objects, and to perform deep-field surveys - where the same patch of sky is viewed for a long time. So far, scientists have spotted objects in these deep-field surveys they do not yet understand. In many cases, even when the data gathered from Chandra answers one question, that answer leads to more questions.

Sackheim speculates the current mission's success could lead to another Chandra-like mission.

"We've discovered the high probability of black holes, " Sackheim said. "When all this data gets analyzed, the scientists at Marshall and at the Smithsonian Astrophysical Observatory will want to ask more questions about peering into where these sources are, and mapping the heavens even better. I can see another mission that goes beyond Chandra."