NASA brought the media to one of the rarest rocket test sites in America Tuesday to show how close it is to a big squeeze test of critical parts of the Space Launch System (SLS).

The space agency has assembled the following at Marshall Space Flight Center in Huntsville, Ala.:

- A propulsion stage like the one that will power the Orion capsule toward the moon when it leaves SLS on the system's first test flight

- A large cone-shaped adapter where the propulsion stage will be inserted

- And two other large metal ring adapters that will join the propulsion stage to the big rocket.

Two of the big adapters are already in place and could be seen in the stand Tuesday.

In early January, Dee Vancleave and her team will begin a series of more than 50 tests to squeeze, jam and shift the stack to simulate the forces of a rocket launch. She said she'll be "very nervous" during Thursday's crane move, but eager to get started with the tests. Vancleave is one of NASA's test experts who left the world of rockets on computers because, "I like to get my hands on the test equipment. I like turning knobs."

The fuel tanks come next

Also next year, NASA will also ship the huge oxygen and fuel tanks for SLS to Marshall for more of the same - death by squeezing so the real rocket won't fail.

NASA is so sure the parts will pass these "qualification tests" it has already started building actual flight hardware just like them. That's the only way to stay on track for a 2018 launch, a NASA official said, and it's standard procedure.

"Before we started building both the qualification hardware and the flight hardware, we had our critical design review," said Steve Creech, deputy manager of the office at Marshall that manages the SLS program for NASA. "That gave us confidence we had a good design.

"What you typically do in space programs is you do go in parallel with building your flight units before your qualification units are fully tested," Creech said. "You have to. If you did it serially, it would just stretch out the time of the design so much. We've got confidence we've got a good design and it's low-risk to do that."

If NASA finds problem

If NASA finds a structural problem in the testing, Creech said, "We'll have to look at what the impact is. We may constrain the first few flights of the vehicle to lower the loads. There's things we can do other than going back and starting over."

Time is important to NASA as it works to keep its plan to launch SLS the first time at the end of 2018. The first launch will be uncrewed, but astronauts will be on top of the second, modified SLS.

"We believe we've handled the major challenges," said Boeing's Cataldo Mazzola, who is leading development of the cryogenic propulsion stage. "The problem is when you have something of this size, it's the details that get you. So, we had a minor detail (recently). We were able to recover. It took us about an hour to recover. Those things are going to keep on happening."

Alabama facility is 'rare'

Mazzola has been a part of big tests like this at Marshall on and off for 25 years. He says the Alabama facility is "rare."

Marshall's stands can not only apply big stress loads to really big rocket parts, they can do that even when the parts are tanks loaded with rocket fuel.

"We've done tests with (liquid) hydrogen," Mazzola said. "There aren't many facilities that will let you get these huge tanks, fill them up with hydrogen and squeeze them."