WASHINGTON — Capping a week in which U.S. lawmakers introduced legislation that would keep the space shuttle flying through at least 2012, the head of NASA’s shuttle program said adding missions is still technically feasible but not necessarily advisable.

John Shannon, a 22-year-veteran of the space shuttle program he has led since 2008, touched off a flurry of media coverage when he told reporters March 9 that NASA contractors could restart production and deliver the parts needed to keep the fleet flying beyond its September retirement date. He said money, not safety, was the only stumbling block.

Shannon sought to clarify his position the next day, posting a lengthy comment on the SpacePolitics.com message board saying he opposes shuttle extension on the grounds that the vehicle is too big and expensive to keep in service just to fly crew-and-cargo runs to the international space station. He also expressed a reluctance to rehire laid-off workers just to fire them again a couple of years down the road.

In a March 11 interview with Space News, Shannon made many of the same points, but was more ambivalent about what he would like to see happen with the shuttle.

“The shuttle is really a highly capable vehicle, but it’s also a very difficult and expensive vehicle to go fly,” Shannon said. “But on the other hand we don’t have anything else that can provide that on the U.S. side. … The shuttle is probably too much, but if it’s all you have, then maybe you should think about retaining that capability.”

NASA plans to retire the space shuttle fleet after flying four more missions, the last of which is slated for mid-September — two weeks before the start of the U.S. government’s new budget year.

The spending proposal the White House sent Congress in February includes $600 million to cover three months of continued shuttle operations should any of the remaining flights slip beyond Oct. 1.

The remainder of the roughly $1 billion NASA is requesting for the shuttle beyond this year is for various activities related to shutting down the 30-year-old program.

Under President Barack Obama’s plan, NASA is preparing to shut down efforts to replace the shuttle with the Orion crew capsule and Ares 1 launcher being developed under the Moon-bound Constellation program and provide more support to private companies developing commercial spacecraft they say could be ready to launch astronauts to the space station sooner than Ares and Orion.

U.S. lawmakers worried about looming layoffs have expressed support for extending the shuttle program, with Sen. Kay Bailey Hutchison (R-Texas) introducing legislation March 3 seeking a reprieve for NASA’s space shuttle fleet. Reps. Suzanne Kosmas (D-Fla.) and Bill Posey (R-Fla.) introduced companion legislation March 10.

David Radzanowski, a former White House budget analyst now serving as NASA’s deputy associate administrator for program integration, said March 10 that money and a projected two-to-three-year gap between the last flight and the first new additional flight are the biggest stumbling blocks to extending shuttle operations by more than one mission.

“If we’re directed to do so, and the money actually shows up, and we bring the work force and suppliers on board that we need to move forward, there would still be a two-to-three-year gap between the last flight and a new additional flight,” Radzanowski said at the Goddard Memorial Symposium in Greenbelt, Md. “It takes us that long to build an external tank.”

Shannon said NASA has the external tank, twin solid-rocket boosters and other hardware it would need to conduct a fifth mission. But if Congress directs NASA to add more flights in order to keep the program going through 2012, Shannon said that would entail rehiring workers already laid off from their jobs at NASA’s Michoud Assembly Facility in New Orleans where the shuttle’s external tanks are built, and at Alliant TechSystems’ Utah manufacturing facilities, where the  solid-rocket boosters are cast.

“The layoffs have been very difficult,” Shannon said. “I think it would be very difficult to reconstitute that work force if it were just for a two- or three-year limited number of flights. To bring back folks after all these layoffs and then just lay them off again two years later is really tough.”

Shannon said rehiring would make more sense if NASA planned to develop a heavy-lift vehicle using shuttle-derived hardware.

“You could just use that work force and those facilities and that infrastructure that you’d made that second investment in … for your heavy-lift derived vehicle,” he said.

NASA’s budget scraps development of the partially shuttle-derived Ares 5 heavy-lift launcher in favor of investing $3.1 billion over the next five years in as-of-yet-unspecified heavy-lift and propulsion research that NASA says could include development of a new main-stage rocket engine that burns a mix of kerosene and oxygen and would be big enough to replace the Russian-built RD-180 engine that powers the Atlas 5 rocket.

The president’s proposal has come under bipartisan fire from lawmakers, some of whom want the administration to return to the drawing board. In a March 11 letter to NASA Administrator Charles Bolden, more than a dozen House lawmakers, including five Democrats, called on the agency’s human spaceflight experts to come up with a plan to develop and test a heavy-lift vehicle and spacecraft within the five-year budget proposed in the president’s 2011 spending request for NASA.

“Under this administration’s proposal, each of these critical elements is missing,” the letter states. “By the time commercial low-Earth orbit vehicles are cleared for flight, U.S. astronauts may have nowhere to go. NASA will no longer have a clear vision on its direction and ultimately the U.S. will no longer be a spacefaring nation.”

Amid widespread political opposition to the president’s proposed NASA overhaul, the White House said March 7 that Obama will visit Florida April 15 to “host a conference on the administration’s new vision for America’s future in space.” NASA Deputy Administrator Lori Garver said March 10  the conference would take place at or near the agency’s Kennedy Space Center, where several thousand people stand to lose their jobs when the shuttle stops flying.

Shannon says despite anticipated job losses, there is a bright side to shuttle retirement, especially for companies seeking to develop commercial space taxis.

“I see the tide rising for every other aerospace company trying to fly humans in space whenever the space shuttle ends,” he said. “Not only will there be more funding to go around, but there will be this incredibly talented work force that can go out and really help.”

In the meantime, Shannon said while it is technically possible to continue flying shuttle through 2012, it is doubtful that additional external tanks could be ready in that timeframe. As a result, NASA would have to spread its remaining flights over the next two-and-a-half years to keep the shuttle work force from standing idle.

“I think the congressional bills take that into account by slowing down the flight rate and reducing the cost,” he said.

While NASA currently spends about $200 million a month on the shuttle, Shannon said a study NASA completed in February showed the program could reduce its monthly burn rate to around $170 million by retiring two of its three remaining orbiters, flying just twice a year and shedding about 40 percent of the program’s 9,800-person contractor work force.