The Advanced Cryogenic Evolved Stage (ACES) is a proposed liquid oxygen/liquid hydrogen upper-stage rocket for use on the Vulcan space launch vehicle designed by the U.S. company United Launch Alliance (ULA). The ACES concept is intended 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 approximately 2024. Vulcan will initially launch with the Centaur V upper stage.
By 2010, ULA had inherited the intellectual property of both proposals, and ACES 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 intended 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 Sept 2020 ULA stated they were no longer actively developing ACES.
As of April 2015[update], ACES was expected to debut on the Vulcan launch vehicle no earlier than 2023 but in July 2015 the timeframe was clarified to not likely fly until 2024–25. 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 have a specific date for when that will be.
ACES will use ULA's proprietary Integrated Vehicle Fluids (IVF) technology to significantly extend its lifetime in space.
ACES is planned to include common bulkhead propellant tanks with a diameter of 5.4 m (18 ft), capable of carrying 68 tonnes (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 RL-10 engines and no IVF. ACES is now expected to have the same tank diameter as Centaur V, but stretched, with the possible addition of two more RL-10s and IVF.
Bringing critical items from ACES into the Centaur V development workstream in 2017 was expected to increase the lift capacity of the first generation Vulcan, so it could carry planned high mass, high energy national security reference payloads. The Centaur V was projected to permit ULA to retire both the Atlas V and Delta IV earlier than planned.
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. This includes producing power, maintaining stage attitude and keeping the propellant tanks autogenously pressurized. Using these fluids eliminates the need for hydrazine fuel, helium for pressurization,:4, 5 and nearly all batteries in the vehicle.
In August 2016 ULA's President and CEO Tory Bruno said both Vulcan and ACES were intended to be human rated.
One possible application for ACES is 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-LEO 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.