The Advanced Cryogenic Evolved Stage (ACES) was a proposed liquid oxygen/liquid hydrogen upper-stage from 2005–2020 for use on a number of different launch vehicles produced by Boeing, Lockheed Martin, and after a late-2006 merger, United Launch Alliance (ULA). During the last five years of the program, ACES was proposed for eventual use on the Vulcan space launch vehicle designed by the U.S. company United Launch Alliance. The ACES concept had the objective to improve the on-orbit lifespan of current upper stages.
In 2015, ULA announced conceptual plans to transition the Vulcan rocket to the ACES second stage, also referred to as Centaur Heavy, after 2024. Vulcan will initially launch with the Centaur V upper stage. Both Boeing and Lockheed Martin had separate but similar ACES concepts prior to the formation of the jointly-owned (50% Boeing/50% Lockheed Martin) ULA in late 2006.
In the event, the long-duration upper stage concept was never able to obtain top-line attention from any of the companies as it never won full development funding from the US government, the primary source of funding for the Delta, Atlas and Vulcan rockets. However, some aspects of changes planned for ACES, such as the larger tank-diameter, were eventually used in the Centaur V upper stage.
By 2010, ULA had inherited the intellectual property of both proposals, and the ACES concept had evolved into a new high-performance upper stage to be used on both Atlas V and Delta IV/Delta IV Heavy launch vehicles. Now called the Advanced Common Evolved Stage, ACES was proposed to be a lower-cost, more-capable and more-flexible upper stage that would supplement, and perhaps replace, the existing ULA Centaur and Delta Cryogenic Second Stage (DCSS) upper stages. This upper stage was intended to incorporate improved insulation for improved cryogenic storage and longer coast durations.
In April 2015, the name of the stage was reverted to the original Advanced Cryogenic Evolved Stage name, as the new ULA Vulcan design would be the only first stage rocket intended to use ACES, beginning no earlier than 2023.
In September 2020, ULA stated they were no longer actively developing ACES.
As of April 2015[update], ACES was expected by ULA to debut on the Vulcan launch vehicle no earlier than 2023  but in July 2015 the timeframe was clarified to not likely fly until 2024–2025. In 2018, ULA gave multiple presentations that again showed an ACES debut in 2023. In 2019 however, ULA said that while they still planned to develop ACES, they no longer had a specific date for when that would be.
ACES was planned to use ULA's proprietary Integrated Vehicle Fluids (IVF) technology to significantly extend its lifetime in space.
ACES was planned to include common bulkhead propellant tanks with a diameter of 5.4 m (18 ft), capable of carrying 68,000 kg (150,000 lb) of propellant.
In late 2017, ULA decided to bring the 5.4 m (18 ft) diameter and advanced insulation elements of the ACES upper stage forward. Under the new plan, Vulcan's upper stage is the Centaur V, with two LH2/LOX RL10 engines and no IVF or other ACES extended-duration technology as had been planned for ACES. ACES was then expected to have the same tank diameter as Centaur V, but stretched (longer tank length), with the possible addition of two more RL10s and IVF.
The IVF technology uses a lightweight internal combustion engine to use hydrogen and oxygen propellant boil-off (normally wasted when boil-off gases are vented to space) to operate the stage. The design included producing power, maintaining stage attitude and keeping the propellant tanks autogenously pressurized. Using these fluids was designed to eliminate the need for hydrazine fuel, helium for pressurization,: 4, 5 and nearly all batteries in the vehicle.
IVF was designed by ULA to be optimal for depot operations, since only liquid hydrogen and liquid oxygen would need be transferred, and it could, if built, conceivably extend mission lifetimes from the present dozens of hours to multiple days.: 2–4 : 4
In August 2016, ULA's President and CEO Tory Bruno said both Vulcan and ACES were intended to be human rated.
One potential application for ACES was stated by ULA in 2010 to be the use of the longer endurance and the greater fuel capacity as propellant depot with in-space refueling capability to retrieve derelict objects for near-space clean up and deorbit. These new approaches offer the technical prospect of markedly reducing the costs of beyond-low Earth Orbit object capture and deorbit with the implementation of a one-up/one-down launch license regime to Earth orbits.
ACES design conceptualization has been underway at ULA for many years. It leverages design features of both the Centaur and Delta Cryogenic Second Stage (DCSS) upper stages and intends to supplement and perhaps replace these stages in the future. The baseline ACES will contain twice the Centaur or 4m DCSS propellant load, providing a significant performance boost compared to our existing upper stages. The baseline 41-mT propellant load is contained in a 5m diameter, common bulkhead stage that is about the same length as ULA's existing upper stages.
Peller described ACES as a concept that ULA is no longer actively pursuing. "We did a lot of studies, we invested in a lot of technology development to assess the feasibility of some of the innovative features of ACES", he said. "That has served us well, because a lot of that original ACES work has its fingerprints in our new version of Centaur, the Centaur 5 we're fielding with Vulcan. Those studies five, eight years ago certainly served us well, and it put us on a good path forward here for the evolution of our upper stages. We will continue to evolve our upper stage to meet the needs of the market going forward".
The increased capability of the upper stage is somewhere in there in the future
for disposing of these obsolete or derelict spacecraft all [approaches] involve the expenditure of substantially more delta V than what has been traditional. It may well be required that old spacecraft be removed at the same time new spacecraft are being emplaced. ... [this architecture] anticipates the task of removing derelict spacecraft by providing an infrastructure to permit these high ΔV missions and enables the likely new paradigm of removing a spacecraft for each one deployed.