BREMEN, Germany — As United Launch Alliance prepares for the maiden flight of its Vulcan Centaur rocket, the company no longer has a clear timeline for a major second-stage upgrade.

ULA is preparing to launch Vulcan Centaur in 2021. The rocket features a new first stage powered by BE-4 engines from Blue Origin and an improved version of the Centaur upper stage currently used on ULA’s Atlas 5.

ULA had planned as recently as 2018 to replace Centaur with ACES, short for the Advanced Cryogenic Evolved Stage, in 2023. ACES would have the ability to operate in space for weeks instead of hours, enabling transport between orbits and more missions beyond Earth, while also increasing how much Vulcan could lift.

ULA officials declined Nov. 20 to provide a date for Vulcan ACES.

Tiphaine Louradour, ULA’s president of global commercial sales, said the company is taking “iterative steps” to evolve Vulcan, but would not give a timeline for ACES when asked about it during a panel discussion at Space Tech Expo Europe here.

“The increased capability of the upper stage is somewhere in there in the future,” she said.

ULA spokeswoman Jessica Rye told SpaceNews by email that the company still plans to introduce an “advanced upper stage,” but only after Vulcan flies. Rye also declined to provide a specific timeline.

ULA presented a paper on its Atlas, Delta and future Vulcan rockets at the 2019 International Astronautical Congress in October that included no mentions of Vulcan ACES. In 2018, ULA gave multiple presentations that listed 2023 as the target debut for ACES.

ULA originally anticipated using Vulcan ACES to replace the Delta 4 Heavy, which retires in 2023 or 2024, but decided the faster route was to upgrade the Atlas 5 Centaur stage. With stretched fuel tanks and Aerojet Rocketdyne-supplied RL10C-X engines, Vulcan Centaur can carry 2.5 times as much second-stage propellant as the Atlas 5.

Louradour said ULA is still planning to eventually reuse Vulcan’s first-stage engines.

ULA’s reuse plan unveiled in 2015 involves using parafoils and helicopters to capture the engines midair. The company said in the paper it presented at the International Astronautical Congress last month that the first stage propulsion module accounts for around 65% of Vulcan Centaur’s costs.

ULA chose to start Vulcan Centaur as an expendable vehicle in order to shift as quickly as possible to Blue Origin’s BE-4 from the Russian-built RD-180 that powers Atlas 5, the paper said.

Louradour said ULA still believes reusability makes sense when saving the engines instead of the entire first-stage booster, as SpaceX does.

“It’s inherently reusable, so it makes sense, and with a minimal performance impact,” she said. “We think it is very important to not have 40+% impact to your performance and yet be able to do what makes sense, which is: bring back the most expensive part.”

She did not give a timeline for making Vulcan reusable.