CAPE CANAVERAL, Fla. — 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.

That’s just an estimate, Bridenstine noted, because the space agency “needs to sit down with its primary contractor, Boeing, 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.

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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.

Critics have long pointed out that commercial rockets, like those built by SpaceX and United Launch Alliance, are a fraction of the price and can still accomplish many of the tasks lined up for SLS. SpaceX’s Falcon Heavy, for example, has a sticker price of about $90 million per launch and only slightly less lift capacity than SLS. United Launch Alliance — a joint venture between Lockheed Martin and Boeing — also builds a rocket called Delta IV Heavy that costs about $300 million.

But Bridenstine and a band of federal lawmakers have staunchly defended SLS. They point out those commercial rockets are not quite as powerful as SLS and neither is currently qualified to fly humans. The rocket will be able to in a single trip get Orion, the spacecraft that will carry astronauts, and large cargo to the moon. So SLS is essential to the Trump Administration’s goal of returning humans to the moon by 2024.

“Yes, it has taken a long time to develop, and of course the American taxpayer has invested a lot into it, but this is a key capability for America to lead in space,” Bridenstine told Crane. “And that’s what we intend to do.”

NASA’s lunar program, dubbed Artemis, has plenty of critics in Congress that Bridenstine will need to win over to secure funding. (He previously told CNN that NASA will need $20 billion to $30 billion over the next five years, on top of its standard annual budget, to fund the program).

Bridenstine’s remarks also come about one month after a letter from the White House’s Office of Management and Budget suggested SLS would cost more than $2 billion per launch.

Laura Forczyk, the founder of space industry analysis firm Astralytical, told CNN Business that Bridenstine likely shared an estimated price per launch for SLS on Monday to ease some of the sticker shock lawmakers may have felt after seeing the White House’s $2 billion-plus per launch estimate.

But, Forczyk added, it’s unrealistic to think NASA will be able to buy SLS rockets for as cheap as $800 million.

“Where is NASA going to get the money for 10 to 12 launches for [a bulk order] for SLS? NASA is struggling to get the money it needs for Artemis as it is,” Forczyk said.

She acknowledged that the Saturn V rocket launched 13 times last century. “But we were in the Cold War when NASA’s budget was much higher due to geopolitical motivations,” she said. “Congress doesn’t have incentive to provide NASA $8 billion to 9.6 billion to a bulk purchase” rockets.

Bridenstine did not say whether NASA will actually need 10 or more SLS rockets. The Artemis program will require three rockets — two for test flights and another for the 2024 lunar landing — and “maybe more after that.”

NASA has also frequently talked about SLS’s usefulness for launching robotic exploration missions deep into the solar system. But those tasks, critics say, can likely be accomplished by cheaper commercial rockets.

For example, Europa Clipper, a spacecraft that will study one of Jupiter’s Moons, is slated to fly on an SLS rocket in 2025. But the White House OMB, in its recent letter, asked Congress to use a commercial rocket instead. That would provide “over $1.5 billion in cost savings,” the letter states.

Boeing was contracted in 2012 to build SLS’s main components, and the rocket was expected to start flying in December 2017.

It’s now more than two years behind schedule and Congress has allocated $14.6 billion for SLS development and construction of the first two rockets — far more than originally predicted.

Boeing promised to get the project back on track after Bridenstine publicly threatened to sideline the SLS rocket earlier this year.

The administrator was at Boeing’s facility in New Orleans on Monday where the company just finished assembly of SLS’s core stage. The rocket will next move to a testing site in Mississippi.