WASHINGTON — Boeing Space Exploration of Houston is about three months behind schedule on a giant welding tool to be used in the assembly of NASA’s Space Launch System heavy-lift rocket, but the first flight of the massive vehicle remains on track for 2018, a senior company official said.

Construction of the roughly 50-meter-tall Vertical Assembly Center, which will weld together the 8.4-meter-diameter structural components of SLS stages at NASA’s Michoud Assembly Facility near New Orleans, was completed in September. Shortly thereafter, however, Boeing discovered a problem with the machine during final alignment testing, Virginia Barnes, the company’s vice president and program manager for SLS, said in a Feb. 24 interview.

The machine is designed to lift pieces of the SLS stages up and down its great height using a large ring that moves along rails. However, Boeing discovered the rails were “slightly misaligned and would not allow complete translation over the entire height of the Vertical Assembly Center,” Barnes said.

In other words, the machine cannot lift the components as high as they need to be lifted.

Boeing now expects a fix will be in place by “midsummer,” about three months later than planned, Barnes said.

Boeing is supplying the SLS core stages under a six-and-a-half-year, $2.8 billion contract that was finalized in July. The deal runs through 2021 and calls for the company to deliver two SLS cores, including hydrogen and oxygen tanks, and avionics.

The Vertical Assembly Center uses a technique called friction stir welding to bond pieces of metal without melting them or using filler material to seal joints. The bonding process involves pressing the pieces of metal together at a carefully calculated pressure and applying a rolling pin that creates heat through friction to sufficiently soften the metal.

Boeing is investigating the exact source of the alignment issue. Possible culprits include a manufacturing defect with the tool itself, or a shift in the massive concrete foundation Boeing dug in the notoriously soft Louisiana soil to accommodate the tool, Barnes said.

“Louisiana’s not known for its hard soil,” Barnes said. “This foundation we laid for the Vertical Assembly Center would hold the largest building in downtown New Orleans, 1 Shell Square.”

The first SLS launch is on the slate for 2018. The rocket will send an uncrewed Orion capsule to the same distant lunar retrograde orbit to which NASA has proposed redirecting a small asteroid sometime next decade. NASA will repeat the mission in 2021, this time with a crew aboard Orion.

SLS stages are made up of large aluminum alloy segments — domes, rings and barrels — that are built elsewhere at Michoud and put together using the Vertical Assembly Center. The domes are joined via rings to the ends of the barrels. Barnes said Boeing has already put together all the rings needed for the first SLS mission in 2018.

There are barrels and domes in various stages of completion at Michoud, but those are qualification and testing units, not flight hardware, Barnes said.

Until the alignment issue is resolved, the Vertical Assembly Center is a bottleneck for Boeing’s entire SLS stages contract, Barnes said. Assuming Boeing comes up with a fix by midsummer, as it believes it will, SLS should still make its 2018 launch date.

About 250 Boeing employees are working on the SLS stages contract at Michoud now, an uptick of about 100 since NASA cut the ribbon on another giant friction stir welding tool at Michoud — the Vertical Weld Center — two years ago. The Vertical Weld Center puts together the individual SLS barrel and dome segments. Ring segments are made with the Segmented Ring Tool.