CAPE CANAVERAL, Fla. - A somewhat generational battle over NASA's future is escalating even as NASA is about to close the book on the space shuttle era.

Heroic former astronauts and some current top managers are stepping up their criticism of an agency they see ending its only way to get astronauts into space and going nowhere fast. NASA's chief counters that his agency is heading somewhere new for a change and dismisses critics as people who "must all be living on another planet."

The critics say NASA is ignoring its own long-standing advice: Have a backup plan.

Once shuttle Atlantis has completed its mission, NASA won't have a way to get into space for years except hitching a ride on the Russian Soyuz spacecraft. A new design will come from private developers, but that will take at least three years, probably longer, experts believe.

First moonwalker Neil Armstrong, first American in orbit John Glenn, Mission Control founder Chris Kraft, Apollo 13 commander Jim Lovell, first shuttle pilot Robert Crippen and others are pushing for a last minute reprieve for the about-to-be-retired space shuttle fleet. They're even urging a delay of Friday's final launch. They may get a delay of a day or two because of bad weather.

But the NASA veterans are looking for a pause of more than a year, until more shuttle parts are ready to keep flying and extend the 30-year program.

Back in June, as Atlantis headed to the launch pad, launch director Mike Leinbach on a live audio loop groused to his fellow workers "we're all victims of poor policy out of Washington, D.C.," for not having a new mission for the post-shuttle era.

Glenn, who returned to space at the age of 77 by flying on the shuttle Discovery in 1998, said: "I told the president, 'We're violating one of NASA's critical design criteria."'

That means there must be a backup system for getting into space and bringing astronauts home from the International Space Station.

Armstrong, Kraft and Lovell sent a letter June 30 to President Barack Obama and NASA chief Charles Bolden asking that they keep shuttles flying and delay this final launch. Glenn, who wasn't involved in the letter campaign, is also calling it a mistake to end the space shuttle program - planned since 2004.

Kraft said he considered a backup crucial as he ran Mission Control or oversaw the people who did - missions from the Mercury days of the 1960s through early space shuttle days. He said it is still possible at this late date to put Atlantis' final mission on hold while NASA builds new external fuel tanks and boosters for future shuttle flights - a process that would delay the launch about 18 months.

"It's a generational thing. It's a culture thing and mostly it's a political thing," said Kraft, 87. Nearly all the signees of the letter are in their 70s and 80s. Glenn, who didn't sign the letter, will turn 90 this month.

It's a fight Kraft has waged for at least three years, pulling in Armstrong, 80, and others. Armstrong, in an email to The Associated Press, wrote: "Chris is an exceptional engineer and manager who has always been reliable in the many cases where he held the success or failure of American human space flight in his hands." He wrote that if Kraft thinks this is too risky a plan, "I can readily accept that."

For his part, NASA Administrator Bolden, a former shuttle commander, defended the shuttle retirement and post-shuttle plans in a speech last week at the National Press Club.

"American leadership in space will continue at least for the next half century." Bolden said. "We need future generations to do more than what we can do today. When that final shuttle landing occurs and the cheers and tears subside, we'll keep on moving to where we want to go next."

More than a year ago, Obama and Bolden charted a new direction for NASA, sticking with President George W. Bush's decision to retire the space shuttles. But Obama canceled Bush's plan to return astronauts to the moon. Instead, NASA is designing a new rocket and capsule to take astronauts to an asteroid and eventually Mars. The Obama plan wants private companies to take over the job of shuttling astronauts to Earth orbit, and Bolden thinks that could happen by 2015.

But some of NASA's own managers aren't sitting quietly with the changes.

"Throughout the history of the manned spaceflight program we've always had another program to transition into. .. we had that and it got canceled and we don't have anything," launch manager Leinbach told his fellow workers at Kennedy Space Center. "Frankly as a senior NASA manager I would like to apologize that we don't have that."

Paul Hill, who oversees mission operations in Houston, was just as critical: "It isn't clear at all that we are going to transition to anything."

Lining up with Bolden and the president are the widows and widower of the astronauts who died in the 1986 Challenger accident. In a letter supporting the new NASA direction, they called the commercialization of space a 21st Century approach: "This is a century with new challenges and also new opportunities."

Scott Parazynski, a 49-year-old former astronaut who heads the educational center created by Challenger families, said in an email that he agrees with Kraft that NASA shouldn't be left without a backup to the Soyuz, but disagrees with the idea of delaying the shuttle retirement.

"The cards have been dealt, and even though we may not all like the cards we've gotten, we've got to play," Parazynski wrote. "I see a path forward that gives American industry (new enterprise as well as established aerospace) and NASA a bright future."

The American public apparently wants the U.S. to continue to be a space leader. According to a poll by the Pew Research Center released Tuesday, 58 percent of Americans think it's essential the nation continue as a leader in space.

For his part, Glenn said he doesn't disagree with Obama's plans, although he said he believes private spaceflight will take years longer than Bolden predicts. What Glenn objects to is the gap between the shuttle and a future spacecraft. While the Soyuz is reliable, Glenn said NASA should always want an alternative in case of a "hiccup" in the Soyuz plans.

"I think we should be keeping the shuttle going," Glenn said. "It's the most complicated vehicle ever put together by people."