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Falcon 9 rocket
SpaceX’s Falcon 9 sits on Kennedy Space Center’s Launch Pad 39A. (NASA via YouTube)

SpaceX postponed the first launch from NASA’s Kennedy Space Center since the last shuttle flight in 2011, due to concerns about a control system on the Falcon 9 rocket’s second stage.

The Falcon 9 had been due to loft a robotic Dragon cargo capsule into orbit from the center’s Launch Pad 39A in Florida, delivering 5,500 pounds worth of supplies and experiments for the International Space Station.

But with less than 20 seconds left in today’s countdown, SpaceX’s mission managers decided they needed more time to work through a nagging technical issue with the controls for the second stage’s rocket engine nozzle. “They did not fully understand what they’re seeing in this second-stage thrust vector control system issue,” NASA spokesman George Diller said.

SpaceX’s billionaire founder, Elon Musk, addressed the issue in a series of tweets:

Liftoff was rescheduled for no earlier than 9:38 a.m. ET (6:38 a.m. PT) Sunday.

The highlight of this launch is the return of Launch Complex 39A, which served as the springboard for space missions ranging from the Apollo moonshots of the 1960s to the Atlantis flight that brought the space shuttle era to an end in 2011.

“It gives me a little bit of chills when I walk out there and see stuff that’s left over from Apollo,” Hans Koenigsmann, SpaceX’s vice president of flight reliability, said during a pre-launch briefing.

“I’ve got to admit, I’m a little bit partial to Pad A,” said Bob Cabana, a former astronaut who is now Kennedy Space Center’s director. “All four of my flights went off this pad. … This is absolutely the right thing to do, and SpaceX is a great partner in making this happen.”

SpaceX and Blue Origin, the space venture founded by Amazon billionaire Jeff Bezos, vied to inherit Pad 39A after the shuttle fleet’s retirement. After months of wrangling, NASA gave a 20-year lease to SpaceX in 2014. Since then, the California-based company has been refurbishing 39A and adapting it for the Falcon 9.

Kennedy Space Center’s other launch pad, 39B, has been reserved for NASA’s heavy-lift Space Launch System. Meanwhile, Blue Origin struck a deal to lease Spaceport Florida’s commercial Launch Complex 36 at neighboring Cape Canaveral Air Force Station.

SpaceX also has a pad at Cape Canaveral’s Launch Complex 40, but that facility sustained significant damage last September when a Falcon 9 blew up during launch preparations. The accident added to the pressure to get 39A up and running again.

“Every launch for me is a significant emotional event,” SpaceX’s president, Gwynne Shotwell, told reporters at the pad on Friday. “There’s not one launch that I feel comfortable and calm. They’re always nerve-wracking. I can tell you it’s an extra-special launch tomorrow, for sure. Maybe extra nerve-wracking.”

The cargo for this resupply mission is particularly heavy on scientific experiments, in part because the space station is well-stocked with food, water and other essentials.

One of the payloads – known as Stratospheric Aerosol and Gas Experiment, or SAGE III – will be attached to the station’s exterior to measure levels of ozone, aerosols and other chemicals in the upper atmosphere. It’s the latest step in a scientific investigation that dates back to 1979.

The Dragon will also carry the Raven experiment, an instrument that will test the sensors and avionics for future autonomous docking operations. Other experiments focus on growing crystalline antibodies in space, countering drug-resistant bacteria and accelerating stem cell growth.

Just minutes after launch, the Falcon 9’s first-stage booster will be programmed to fly itself back for touchdown and recovery at SpaceX’s Landing Zone 1, just miles from the launch site on the Florida coast.

SpaceX’s flight plan calls for a rendezvous with the space station two days after launch. Astronauts will use the station’s robotic arm to pull in the Dragon for its berthing. The cargo will be unloaded over the course of several weeks, and then the Dragon will be reloaded with payloads destined for a robotic return to Earth.

During Friday’s launch-pad news conference, Shotwell said she expected SpaceX to launch its first heavy-lift Falcon Heavy rocket and get Launch Complex 40 back in business this summer.

In response to a question, she said SpaceX’s plan to send a Red Dragon capsule to Mars would probably have to be delayed from 2018 to 2020, so that the company can concentrate on the Falcon Heavy program and the development of an advanced Dragon space taxi to ferry astronauts to and from the space station.

The crew-capable Dragon, as well as Boeing’s Starliner space taxi, are currently scheduled to start flying in 2018. But this week, the Government Accountability Office released a report saying that SpaceX and Boeing might not be certified for crewed flights until 2019 due to technical issues.

Shotwell insisted that SpaceX would stick to its schedule. “I’m confident we’ll fly in 2018,” she said. “The response to that report this morning was, ‘The hell we won’t fly before 2019,'” she said.

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