The Advanced Cryogenic Evolved Stage is a proposed liquid oxygen/liquid hydrogen upper-stagerocket for use on the Vulcanspace launch vehicle designed by the U.S. company United Launch Alliance. The ACES concept is currently intended to improve the on-orbit lifespan of current upper stages, enabling a variety of applications. 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 Vupper stage.
History (Advanced ''Common'' Evolved Stage)
Two Advanced Cryogenic Evolved Stage concepts were originally proposed in 2006 by both Boeing and Lockheed Martin. 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 upper stages. This upper stage was intended to incorporate improved insulation for improved cryogenic storage and longer coast durations. In April 2015, the stage was reverted to the original Advanced Cryogenic Evolved Stage name, as the new Vulcan will be the only application, beginning no earlier than 2023.
Advanced Cryogenic Evolved Stage
, 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 technology to significantly extend its lifetime in space. ACES is planned to include common bulkhead propellant tanks with a diameter of 5.4 meters, capable of carrying of propellant.
In late 2017, ULA decided to bring the 5.4 m 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 energynational security reference payloads. The Centaur V was projected to permit ULA to retire both the Atlas V and Delta IV earlier than planned. On May 11, 2018 United Launch Alliance announced that the Aerojet Rocketdyne RL-10 engine was selected for Centaur V, following a competitive procurement process.
Integrated Vehicle Fluids
The IVF technology utilizes a lightweight internal combustion engine to use hydrogen and oxygen propellant boiloff to operate the stage including production of power, maintaining stage attitude, and keeping the propellant tanks autogenously pressurized, eliminating the need for hydrazine fuel and helium, and nearly all batteries from the vehicle. IVF is optimal for depot operations, since only LH2 and LO2 need be transferred, and it extends mission lifetimes from the present dozen hours to multiple days. , the internal combustion engine to be used to power the IVF system on ACES will be produced by Roush Racing. In August 2016 ULA's President and CEOTory Bruno said both Vulcan and ACES were intended to be human rated.
Possible applications
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 :Category:Derelict satellites|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.
