Listen to your experts, listen to your vehicle, and expect the unexpected.

That’s the advice a panel of human spaceflight veterans has for SpaceX as the company prepares for its landmark Demo-2 flight to the International Space Station (ISS) later this month, which will restore the U.S. human orbital spaceflight capability lost after the 2011 retirement of the space shuttle.

“Listen to the technical experts,” said Wayne Hale, a former NASA space shuttle program manager and director of human spaceflight for Special Aerospace Services. “Listen to the guys who are turning the wrenches ... because I think a lot of managers get wrapped up in the pressures to go fly.

“That’s what got us in trouble on the shuttle program—trying to rush to flight and not paying attention to what people with their finger on the hardware knew about it,” Hale continued. “So that would be my advice—listen to the people on the flight line.”

“Always listen to your vehicle,” said Peggy Whitson, the former NASA astronaut whose cumulative 665 days in space is the current record for an American. “Pay attention to the vehicle.”

Fellow NASA astronaut and former shuttle commander Pam Melroy, CEO of Melroy & Hollett Technology, recommends SpaceX be ready to respond flexibly if things don’t go entirely as planned.

“Anticipate the unexpected and be structured around it,” Melroy said during Aviation Week’s recent webinar on the return of U.S. human spaceflight. “Be ready for everything from putting tiger teams together to being able to talk about things if they’re not going well.”

On May 27, NASA astronauts Doug Hurley and Bob Behnken are scheduled to lift off from Kennedy Space Center, Florida, aboard a SpaceX Falcon 9/Crew Dragon vehicle, making them the first human space travelers to lift off from U.S. soil since the final shuttle launch in July 2011, ending the longest gap in indigenous U.S. human spaceflight capability since the 1960s.

Such a long gap was not planned. In 2010, NASA began funding the development of SpaceX’s Crew Dragon and Boeing’s CST-100 Starliner vehicles with the goal of establishing safe, reliable, and cost-effective commercial crew flights to and from the ISS by 2015.

“There were some funding issues” in Congress, Hale said of the delay. “I have to acknowledge that.” But ultimately, “it’s just very hard,” he said. “It’s taken longer because it’s a difficult technical problem to solve.”

Hurley and Behnken will remain aboard the ISS for as long as a few months before returning to Earth for an Atlantic Ocean splashdown. Melroy has a simple yardstick for measuring whether the mission will be considered a success.

“The same as it always is, which is that the crew gets home safely,” she said. “It’s an old joke in the aviation world—any landing you can walk away from is a good one.”

While the Commercial Crew program is an “experiment” by NASA, it already has informed the agency’s plans to return humans to the Moon by 2024, Hale said. NASA plans to spend nearly $1 billion over the next 10 months helping teams led by Blue Origin, Dynetics and SpaceX adapt their commercially developed technologies into human lunar lander systems.

“Part of the experiment is to see if industry, which is more nimble [than government], can design these systems and build these systems and operate them with an equivalent level of safety,” Hale said. “That’s an experiment.”

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