The development of NASA's biggest, most powerful rocket yet is running ahead of schedule and on budget, its primary contractor said Wednesday (April 10).

The towering Space Launch System (SLS) is a 384-foot (117 meters) behemoth intended to launch astronauts beyond low-Earth orbit to deep-space asteroids and Mars. The vehicle is slated to make its first test flight in 2017, when it will launch an unmanned Orion capsule (also in development) beyond the moon. The first manned flight is pegged for 2021.

So far, NASA and The Boeing Co., which has been contracted to build the rocket's core stage, are on track to meet that date, officials said.

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"We're on budget, ahead of schedule," John Elbon, Boeing's vice president and general manager of space exploration, told reporters here at the 29th annual National Space Symposium. "There's incredible progress going on with that rocket."

At the end of December 2012 -- five months ahead of schedule -- the team passed a milestone called preliminary design review, which certified that the rocket design meets its requirements within acceptable risk parameters. Its final technical review, called critical design review, is scheduled for 2014.

The booster, in its initial configuration, uses solid rocket boosters based on the space shuttle's design, with an upper stage taken from United Launch Alliance's well-tried Delta 4 rocket.

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"The whole theory of it was to use existing hardware so we could design something relatively low-risk and get a capability soon," Elbon said.

Eventually, the SLS will have to be outfitted to carry heavier loads than its initial configuration can lift. It must carry the crew and equipment needed for a mission to Mars -- which will be a multistep, complex operation. What those steps will be, exactly, is yet to be settled by NASA.

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NASA's Monster Mars Rocket Ahead of Schedule