WASHINGTON — The looming retirement of the U.S. space shuttle fleet has NASA scrambling to come up with a new approach to keeping the international space station (ISS) — and its hundreds of replaceable parts — in working order well beyond 2010.

For years, NASA’s ISS managers took for granted that they would have the shuttle’s capacious payload bay at their disposal for hauling up loads of spare parts and hauling worn out hardware back to Earth for refurbishment.

That particular supply line, however, shuts down once the shuttle flies its last mission in 2010. And while Europe, Japan and possibly the United States should join Russia by then with spacecraft capable of hauling spares to the ISS, none of these systems are designed to bring back much if anything in the way of spent hardware.

William Gerstenmaier, NASA’s human space flight chief, discussed the problem — and what the agency is doing about it — during a recent Senate hearing on NASA’s plans to transition from the space shuttle to its comparatively tiny replacement, the Orion Crew Exploration Vehicle.

“Our original plan for space station, essentially, was to bring spares down from station, repair them at a depot, and take that spare back up to station,” Gerstenmaier, a former ISS program manager, said during the March 28 hearing. “When we lose the shuttle, we lose the ability to return large amounts of cargo to the ground.”

To prepare for the loss of shuttle, Gerstenmaier said, NASA’s ISS logistics experts are busy designing a new “sparing philosophy” that takes into account the predominately one-way nature of future supply runs.

Gerstenmaier also said the new approach NASA is hatching would be a better fit for the kinds of long duration missions the agency plans to undertake in the future, since routinely sending hardware back from the Moon or Mars for repair or refurbishment will be even less practical then sending it back from the space station in the post-shuttle era.

The epicenter of this re-planning effort is the ISS Vehicle Office at Johnson Space Center in Houston. The manager of that office, Dan Hartmann, and his deputy for logistics and maintenance, Bill Robbins, told Space News in a recent interview that the approach taking shape could be described as “build and burn” since in the not too distant future most spent hardware will be jettisoned from the space station aboard Russian Progress spacecraft, European Automated Transfer Vehicles, Japanese H-2 Transfer Vehicles — all designed to burn up when they hit the Earth’s atmosphere — and possibly U.S. commercial alternatives.

While the two main U.S. companies in the running to sell NASA ISS re supply services — Oklahoma City-based Rocketplane Kistler and El Segundo, Calif.-based Space Exploration Technologies — plan to offer the capability to return at least some hardware safely to the ground. T he so-called recoverable down mass capacity of their planned reusable system is not dramatically greater than what Russia’s three-person Soyuz capsule can bring down today or what Orion will be able to bring back when its operational in 2015. In other words, only a tiny fraction of what the space shuttle is capable of hauling back today.

“We were previously looking at rotating about 5,000 pounds (2,267 kilograms) per year of spares up and failed hardware down,” Robbins said. “The entire 5,000 pounds coming down would be sent back to vendors to be repaired and your next spares on the shelf would go up.”

Since refurbishing and reflying a piece of hardware will not be viable once the shuttle is no longer around to haul failed hardware back from the space station, NASA is looking at placing new orders for additional quantities of pumps, processors and other parts that need replacing from time to time to keep the space station in good working order.

Those additional quantities will wait on the shelf until the time comes to load them onto a supply ship and send them to the station. Failed hardware will be packed away inside the unloaded supply ship along with all the trash that piles up on board the station and sent away to burn up in the atmosphere. Thus, Robbins said, “build and burn.”

Hartman said NASA expects to spend between $30 million and $80 million a year on spare parts for the space station between now and the end of the shuttle program. Up until now, NASA has spent as little as $15 million and as much as $90 million a year on ISS spares, he said.

While some of the money will be used to stock up on spares that come 2010 will have to be replaced rather than refurbished, Robbins said NASA also is budgeting for redesigning some space station hardware to make it easier for astronauts to service on orbit.

Most of the space station’s replaceable hardware, or Orbital Replacement Units (ORUs) in NASA parlance, were designed with an emphasis on minimizing the amount of time astronauts would have to spend fussing with it. If a piece of hardware failed, Robbins said, it would be pulled out, packed on the shuttle and sent back to Earth for repair in a proper workshop. “Now we’re having some people look at whether some of these [ORUs] can be repackaged so that high failure rate components can be more easily accessed by a crew member than they are currently,” Robbins said.

NASA already has had some success with astronauts repairing station hardware not really intended to be repaired on orbit. For example, Expedition 6 crew member Don Petit, working on the fly with the ground team, was able to get the space station’s ultra-low temperature freezer up and running.

If NASA determines that redesigning a given piece of hardware will make it easier for astronauts to make on the spot repairs such as changing out a failed component to get it back on line, the agency will do so, Robbins said.

But Hartman said such ORU redesign decisions would be made on a case-by-case basis.

“I’m not going to do this on an item that weighs 10 pounds, but I am going to do this on an ORU that weighs 300 pounds,” he said.

Hartman said Johnson Space Center is working with ISS prime contractor Boeing to study whether it makes more sense to order more of a given ORU or redesign it to make it easier to keep in service. Those trade studies, he said, are due to wrap up this summer and would inform NASA’s 2008 budget decisions.

While NASA combs through its inventory of hundreds of separate space station parts to decide what it should stock up on and what it should consider redesigning to facilitate on-orbit repairs, the agency also is evaluating what it should take up to the station while it still has the shuttle.

“There are certain ORUs we have to think about getting on board the station with the shuttle,” Robbins said. “Either we really want to understand the launch environment the ORU will go through on the way to station, or it’s a matter of pure size.”

In other words, some space station parts are just too big to fit on any current or planned launch vehicle besides the shuttle, while others were designed with the shuttle’s rather cushy ride in mind. Getting such spares ready to launch on another vehicle, Hartman and Robbins said, could require a substantial amount of new engineering.

A case in point is the space station’s control momentum gyroscopes. The 500-pound components, which are vital to maintaining the space station’s proper orientation in orbit, have proved troublesome over the years, and were replaced over a series of spacewalks in 2005.

Hartman said the station’s control momentum gryoscopes are heavier than most ORUs and also are “pretty sensitive to the ride,” into orbit, making them a strong candidate for launching aboard the shuttle while there is still opportunity.

“Could we clear it to launch on another vehicle? Yes. It’s just a lot of work. If we could get it up on the shuttle, that’s the preferred way,” Hartmann said.

A small number of other space station components simply will not fit on any existing or planned alternative to the shuttle. Robbins said one such component is the station’s Ku-band antenna. The nearly 2-meter wide antenna, the station’s main downlink for video and science data, has been in service for more than five years. Robbins said NASA would like to take up at least one spare while it still has the shuttle, and possibly two if it can get one built in time.

“We think one spare would get us through 2015 but a second would make us more comfortable,” Robbins said.

But with the shuttle slated to fly just 13 to 15 more missions between now and September 2010, it is not clear whether there is enough time to order a second Ku-band antenna and get it delivered by shuttle to the space station.

“Because it’s a very complex piece of hardware, we don’t know if we can get it built before the shuttle retires,” Robbins said.

Looking further ahead than the last shuttle mission, Hartman agreed with Gerstenmaier that the “sparing philosophy” NASA is being forced by shuttle’s looming retirement to come up with is a better fit for the kinds of missions the agency intends to undertake to the Moon, Mars and beyond. As the logistics tail stretches even further from Earth, commonality of parts and easy on-orbit repair, he said, will be the name of the game.

Space station officials are not waiting until their program is history to share the valuable lessons they have learned with their colleagues in the Exploration Systems Mission Directorate.

“We’re already doing it,” Hartman said. “They sit right across the lake from us here at [Johnson Space Center] . We’re debating if we go beyond 2016 with space station, should we come up with a different concept for our external battery and, if so, should we cooperate with our [Exploration Systems Mission Directorate] friends.”