The Advanced Common Evolved Stage, or ACES, is a proposed upper stage rocket for use on space launch vehicles. The design concept is from the U.S. company United Launch Alliance (ULA).[1] ACES is intended to boost satellite payloads to geosynchronous orbit or, in the case of an interplanetary space probe, to or near to escape velocity. Other alternative uses include a proposal to provide in-space propellant depots in LEO or at L2 that could be used as way-stations for other rockets to stop and refuel on the way to beyond-LEO or interplanetary missions, and to provide the high-energy technical capacity for the cleanup of space debris.[1]
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The ACES is intended as 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 stage vehicles.[1]
The vehicle is "based on a simple modular design" where the "use of multiple barrel panels, similar to Centaur, provides a straightforward means to building multiple-length (propellant load) stages that are otherwise common. The common equipment shelf accommodates one, two, or four RL10 engines. While ACES can start with existing Centaur and Delta pneumatic, avionics and propulsion systems it is intended to transition to lower-cost and higher capability systems founded on our Integrated Vehicle Fluids (IVF) system concept. IVF eliminates all hydrazine, helium, and nearly all batteries from the vehicle. It consumes waste hydrogen and oxygen to produce power, generate settling and attitude control thrust, and autogenously pressurize the vehicle tanks. 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."[1]
With the addition of a solar power system, the vehicle can remain in space and operate indefinitely.[1]
As of 2009[update], the upper-stage versions of ACES were proposed to be powered by enhanced RL-10 engines.[2]
The ACES modular design supports the production of a number of standard propellant load stages, in a number of standard lengths, that are otherwise common, including the main propellant tank diameter of 5 metres (16 ft), "a size not seen since the 1970s":[1]
One explicit objective of the ACES design and the depot-based space architecture is to utilize the longer-stage endurance and the greater fuel capacity with in-space refueling capability to retrieve derelict objects for near-space clean up and deorbit. More specifically, it is an explicitly stated goal that the technical potential for derelict capture/deorbit will be enabled to provide the large delta-V required to deorbit even heavy objects from geosynchronous orbits. 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.[3]