Advanced Common Evolved Stage
The Advanced Common Evolved Stage, or ACES, is a proposed LOX/LH2 upper stage rocket for use on space launch vehicles. The design concept is from the U.S. company United Launch Alliance (ULA). 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.
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.
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 the 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."
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":
- ACES 41 upper stage — 41 tonnes (90,000 lb) propellant capacity
- ACES 73 upper stage — 73 tonnes (161,000 lb) propellant capacity
- ACES 41 tanker — no engine(s)
- ACES 73 tanker — no engine(s)
- ACES 121 depot — consists of an ACES 41 upper stage main vehicle (for L02 storage) and an ACES 73 tank (modified for LH2 storage) with 121 tonnes (267,000 lb) of long-term, in-space, propellant depot capacity
Space debris cleanup
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 (change in velocity) 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.
- Zegler, Frank; Bernard Kutter (2010-09-02). "Evolving to a Depot-Based Space Transportation Architecture". AIAA SPACE 2010 Conference & Exposition. AIAA. Retrieved 2011-01-25. "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. ACES will become the foundation for a modular system of stages to meet the launch requirements of a wide variety of users. A common variant is a stretched version containing 73t of propellant."
- Kutter, Bernard F.; Frank Zegler; Jon Barr; Tim Bulk; Brian Pitchford (2009). "Robust Lunar Exploration Using an Efficient Lunar Lander Derived from Existing Upper Stages". AIAA.
- Zegler, Frank; Bernard Kutter (2010-09-02). "Evolving to a Depot-Based Space Transportation Architecture". AIAA SPACE 2010 Conference & Exposition. AIAA. pp. 13–14. Retrieved 2011-01-25. "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."