WASHINGTON — SpaceX successfully launched a Dragon cargo spacecraft to the International Space Station July 25 as the company plans a transition next year to a new contract and new cargo spacecraft.

The SpaceX Falcon 9 lifted off from Space Launch Complex 40 at 6:01 p.m. Eastern, placing the Dragon cargo spacecraft into orbit nine minutes later. The rocket’s first stage, which first flew on the previous Dragon cargo launch May 4, landed at the company’s Landing Zone 1 at Cape Canaveral Air Force Station.

The launch was previously scheduled for July 24, but scrubbed 30 seconds before liftoff because of poor weather. Weather was also unsettled during the second launch attempt, but cleared in time for the launch.

SpaceX and NASA originally scheduled the launch for July 21, but delayed the launch by three days when the company pushed back a standard pre-launch static-fire test from July 16 to July 19. Jessica Jensen, director of Dragon mission management at SpaceX, said at a pre-launch press conference July 24 that the static-fire test was delayed to correct a small liquid oxygen leak detected in the first stage.

The Dragon spacecraft is scheduled to arrive at the ISS July 27, bringing to the station 2,312 kilograms of cargo. That cargo includes nearly 1,200 kilograms of science investigations as well as the International Docking Adapter 3, which will be removed from the Dragon’s trunk and installed on the station to support dockings by future commercial crew and cargo spacecraft.

The Dragon making this flight, designated CRS-18, flew two previous missions to the station: CRS-6 in April 2015 and CRS-13 in December 2017. This mission is the first time a Dragon has been flown on three flights, and Jensen said the next two Dragon missions will also use spacecraft that made two previous flights.

The Dragon spacecraft is rated for three flights to and from orbit, with refurbishment and testing between missions. “What we do between the first flight and the second flight, and then the second flight to the third flight, is very similar,” she said.

The next two missions will mark the end of SpaceX’s original Commercial Resupply Services (CRS) contract with NASA to transport cargo to and from the ISS, awarded in 2008. SpaceX will then transition to a follow-on contract, CRS2, with the first mission under that contract planned for the third quarter of 2020, Jensen said.

The change in contract will also mark a shift in vehicles, with SpaceX retiring the original Dragon in favor of Dragon 2, a version of the Crew Dragon spacecraft the company has been developing for NASA’s commercial crew program. That vehicle is designed for at least five flights to and from the station, and will splash down in the Atlantic Ocean rather than the Pacific, allowing for faster turnaround between launches.

Jensen said she did not anticipate the ongoing investigation into the Crew Dragon test accident in April, linked to a problem with a valve in the spacecraft’s SuperDraco abort thrusters, would affect the use of the spacecraft for CRS2 missions. “The cargo Dragon 2 capsule does not have the SuperDraco high-flow abort system,” she said. “While it is largely the same vehicle, the system that was involved in our in-flight abort test anomaly is not on the cargo Dragon 2 vehicle.”

The company will incorporate any other safety-related changes resulting from the investigation into the vehicle, she said, “but as of this point in time, we do not anticipate that anomaly investigation to hold up cargo Dragon 2.”

That lack of SuperDraco thrusters means, she said, that cargo Dragon 2 spacecraft won’t be used for crewed missions, and vice versa. “They will be different vehicles. We won’t interchange between cargo and crew vehicles.”

However, other than the absence of the SuperDraco system, and well as differences in the interior to accommodate cargo racks rather than astronauts, the cargo Dragon 2 won’t be that different from the Crew Dragon spacecraft, using the same docking system and guidance and navigation sensors. “Anywhere where we could possibly make the subsystems the same, we did,” she said.