That Falcon 9 rocket launch delivered 10 satellites into low-Earth orbit for Iridium, a communications company.
Because of the secretive nature of the mission, not much is known about the NROL-47 payload. Once in orbit, the spacecraft will be operated by the U.S. National Reconnaissance Office.
The NRO is the U.S, Government agency in charge of designing, building, launching and maintaining America's fleet of intelligence satellites.
The recent launch of the Delta 4 rocket into orbit came days after SpaceX's launch of another secretive satellite codenamed Zuma. The main engine of the Delta IV is the RS-68A, manufactured by Aerojet Rocketdyne.More news: Prince Harry Hasn't Asked Prince William to Be His Best Man
United Launch Alliance's CEO and President, Tory Bruno expressed the excitement of seeing the final planned launch of a Delta IV Medium+ (5,2) soar through California's skies when he tweeted: "Delta is ripping the sky at incredible speed".
There's an 80 percent chance of favorable weather for the Friday afternoon launch, United Launch Alliance said on Twitter.
Colonel Greg Wood, 30th Space Wing vice commander, said: "We are ready and eager to take on this Delta launch".
Thursday's delays were due to an issue with a ground system valve, according to ULA, which had also earlier reported issues with swing arms, which connect parts of the rocket to the launchpad and typically move away from the vehicle between ignition and liftoff. "This will be the first of two launches scheduled for NRO in 2018".
"We are proud to provide this national defence capability and every Team V member involved has tirelessly worked to ensure the launch is safe and successful".