Enlarge / Liftoff of ULA's Atlas V rocket on the US Space Force's USSF-51 mission. (credit: United Launch Alliance)

United Launch Alliance delivered a classified US military payload to orbit Tuesday for the last time with an Atlas V rocket, ending the Pentagon's use of Russian rocket engines as national security missions transition to all-American launchers.

The Atlas V rocket lifted off from Cape Canaveral Space Force Station in Florida at 6:45 am EDT (10:45 UTC) Tuesday, propelled by a Russian-made RD-180 engine and five strap-on solid-fueled boosters in its most powerful configuration. This was the 101st launch of an Atlas V rocket since its debut in 2002, and the 58th and final Atlas V mission with a US national security payload since 2007.

The US Space Force's Space Systems Command confirmed a successful conclusion to the mission, code-named USSF-51, on Tuesday afternoon. The rocket's Centaur upper stage released the top secret USSF-51 payload about seven hours after liftoff, likely in a high-altitude geostationary orbit over the equator. The military did not publicize the exact specifications of the rocket's target orbit.

Enlarge / A top-down view of United Launch Alliance's first Vulcan rocket before its liftoff in January. (credit: United Launch Alliance)

United Launch Alliance is targeting September 16 for the second test flight of the new Vulcan rocket, and a flawless mission could finally set the stage for the first Vulcan launch for the US military by the end of the year.

The US Space Force has contracted ULA's Vulcan rocket to launch the majority of the military's space missions over the next few years. Pentagon officials are eager for Vulcan to get flying so they can start checking off a backlog of 25 military space missions the Space Force wants to launch by the end of 2027.

By any measure, the first Vulcan launch in January was a resounding success. On its debut flight, the new rocket delivered a commercial lunar lander to an on-target orbit. The next Vulcan mission, which ULA calls Cert-2, will be the rocket's second certification flight. The Space Force requires ULA to complete two successful flights of the Vulcan rocket before entrusting it to launch national security satellites.