New York (CNN Business) After nearly a decade of funneling money into a project to build a gargantuan rocket that NASA hopes will return astronauts to the moon, the launch vehicle is finally assembled and ready for testing.

But the rocket, called SLS or Space Launch System, is still haunted by critiques of long delays and cost overruns.

NASA Administrator Jim Bridenstine on Monday revealed his latest guess at how much SLS will cost each time it launches: $800 million per rocket for a bulk order and $1.6 billion if NASA purchases just one, Bridenstine, told CNN Business' Rachel Crane.

Boeing BA That's just an estimate, Bridenstine noted, because the space agency "needs to sit down with its primary contractor,, and negotiate the best solution to getting the right mix of the number of rockets and the cost per rocket." Boeing declined to comment on those negotiations and referred questions to NASA.

SLS promises to be the mightiest rocket ever built, able to produce up to 20% more thrust than the Saturn V rocket that powered the Apollo missions 50-or-so years ago. But the project is controversial because Boeing has spent far more time and money building the rocket than it initially promised it would.

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