NASA's first manned test flight of the Orion spacecraft that will replace the retiring U.S. space shuttles won't launch until 2014, a year later than the agency hoped, due to funding and technical concerns, program managers said Monday.

Jeff Hanley, manager of NASA's Constellation program overseeing the development of the multibillion-dollar Orion crew capsules and their Ares I rockets, told reporters that the agency remains on target for its March 2015 deadline to bring the new spacecraft online.

But the agency's internal target of launching astronauts aboard the new vehicles as early as September 2013 has proven untenable due to available funding resources.

"This new plan, September 2014, aligns our schedule to what we forecast will be the available resources," Hanley said in a teleconference. "We are slowing down the work to match and stay under our available funding, and to do that we had to go to a later date."

• Click here to visit FOXNews.com's Space Center.

NASA's three aging space shuttles — Discovery, Atlantis and Endeavour — are due to retire in 2010 once construction of the still unfinished International Space Station is complete.

The agency plans 10 more shuttle missions, two of which are set to fly this fall, to finish station assembly and overhaul the Hubble Space Telescope.

The agency is replacing its shuttle workhorse with Orion capsules that are designed to launch atop two-stage Ares I rockets.

The spacecraft are expected to be used initially for space station-bound missions and sit at the core of NASA's vision of returning astronauts to the moon by 2020 with the help of the planned Ares V heavy-lift rockets and Altair lunar landers.

NASA officials have repeatedly said that March 2015 is the official target to begin operational manned flights of Orion spacecraft, but the agency is hoping to fly the vehicles earlier to minimize the current five-year gap between their crewed launch debut and the shuttle fleet's retirement.

"We are adhering to our commitment date of March 2015 for initial operating capability," said Doug Cook, NASA's deputy associate administrator for exploration.

[Between 2010 and 2015, American astronauts will still be going into space — they'll just have to get there on Russian Soyuz craft. Dependence upon the Russians puts NASA in a tight situation, as Russian cooperation is subject to geopolitical events out of the space agency's control.]

The agency hopes to test Orion's launch abort system and fly the first Ares rocket test — Ares I-X — in the first half of 2009.

But subsequent unmanned abort system and Ares I launch tests before the first crewed flight may see additional schedule slips, program managers said.

"Our confidence that the gap will get no worse than five years has actually improved," Hanley added.

A separate report released Monday by the Aerospace Safety Advisory Panel (ASAP), a group that monitors NASA performance annually, also stressed the importance of adequate funding to the success of NASA's Constellation program.

"While there are still opportunities for improvement, the panel's finding concluded that NASA is making significant progress in improving safety issues during the past year," said ASAP chairman Joseph Dyer in a written statement.

The panel also expressed concern over what its report described as a lack of clear direction for the Constellation program, as well as an Orion design process that has focused on minimizing weight by weighing the benefits and drawbacks of each system.

"When safety elements have to "earn their way" onto a design that has already begun to take shape, objectivity and consistency in the decision-making could be compromised," the ASAP report stated.

But Hanley said the preliminary design process for Orion is still under way, with engineers giving every system for the spacecraft an immense amount of attention.

The goal, he said, is to refine the design to a final spacecraft that is robust, reliable and safe.

"We are not just blindly cutting out redundancy or robustness in this design process," Hanley said. "I could not be more pleased with the progress we are making."

Part of that progress, he added, includes a solution for excessive vibration issues afflicting Orion's Ares I booster. Engineers have drawn up plans for spring-like electromagnetic mass absorbers to dampen the vibrations.

"The Orion is headed where it needs to be," Hanley said.

Copyright © 2008 Imaginova Corp. All Rights Reserved. This material may not be published, broadcast, rewritten or redistributed.