CAPE CANAVERAL, Fla. – With space shuttle retirement just months away, a senior NASA manager said Tuesday it wouldn't be hard to add more flights, provided the nation is willing to keep paying $200 million a month.

NASA's three shuttles are scheduled to retire in September, after four more trips to the International Space Station. Some in Congress, however, are pushing for additional missions to fill the gap between the end of the shuttle program and the nation's next manned spaceship, whatever and whenever that might be.

Last month, President Barack Obama killed NASA's Constellation program, which would have created a shuttle successor to ferry astronauts to the space station, and ultimately to the moon. Instead, Obama has directed NASA to turn to commercial companies for getting astronauts to orbit and, instead, focus on deep-space exploration.

Money is the key to keeping the shuttles flying, said program manager John Shannon.

"The shuttle program is fairly expensive. We burn at about a $200 million-a-month rate. So that gives you a base of about $2.4 billion per year ... almost irregardless of how many flights," Shannon told reporters.

He added: "Where that money comes from is the big question."