Advanced Cryogenic Evolved Stage

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The Advanced Cryogenic Evolved Stage (ACES), formerly the Advanced Common Evolved Stage, is a proposed liquid oxygen/liquid hydrogen upper-stage rocket for use on the Vulcan space launch vehicle.

The design concept is from the American 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 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 delta-v technical capacity for the cleanup of space debris.[1]

In 2016, ULA announced conceptual plans to transition the Vulcan rocket to the ACES second stage after approximately 2024. Vulcan will initially launch with the Centaur upper stage, beginning with its first flight no earlier than mid-2020.[2]


The ACES concept was originally proposed as the Advanced Cryogenic Evolved Stage by Boeing in 2006 for use as a new Delta IV second stage[3]—and subsequently, the Advanced Common Evolved Stage by its corporate successor, United Launch Alliance by 2010—ACES was 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 included 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]

The late-2000s ACES proposal by ULA also had a predecessor at Lockheed Martin, prior to the merger of Boeing and Lockheed Martin launch vehicle manufacturing and operations to form ULA in 2006. Known then as the Lockheed Martin common-stage concept, the upper stage was intended to "provide efficient, robust in-space transportation, and take advantage of the high-mass fraction that is enabled by Centaur’s monocoque design and its common bulkhead to minimize combined LO2/LH2 boil off. ... application of long-duration LO2/LH2 in-space propulsion technology [would be expected to] result in significant launch cost savings for space exploration. ... passive long duration capability [would be achieved] by implementing cross cutting Cryogenic Operation for Long Duration (COLD) technologies, and improve cryogenic storage capability by more than two orders of magnitude compared to existing large-scale flight-proven systems."[4] A study funded by NASA led to the development of the Lockheed Martin concept, also known as ACES, under the original name of Advanced Cryogenic Evolved Stage in 2006.[5]

In April 2015, after ULA had announced the end of production of the Delta IV Medium in 2019 and the Delta IV Heavy in the mid-2020s, ULA renamed the stage the Advanced Cryogenic Evolved Stage, as ACES would in this case serve as the second stage on only a single launch vehicle, the Vulcan, beginning no earlier than 2023.[6]

Vulcan Centaur Upper Stage[edit]

In late 2017, ULA decided to bring some[which?] elements of the ACES upper stage forward and begin work on Centaur V. This upper stage would be expanded to the same 5.4-meter (18 ft)-diameter of Vulcan core, using a cluster of four LH2/LOX engines, but not include the Integrated Vehicle Fluids (IVF) feature expected with ACES.[citation needed]

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 national security[clarification needed] payloads. The Centaur V was projected to permit ULA to retire both the Atlas V and Delta IV earlier than planned.[citation needed]

On May 11, 2018 United Launch Alliance (ULA) announced that the Aerojet Rocketdyne RL-10 engine was selected for ULA’s next-generation Vulcan Centaur rocket, following a competitive procurement process.[7]

Advanced Common Evolved Stage[edit]

After the formation of ULA in 2006, the ACES concept became one that would provide a common stage that would be evolved from both Atlas and Delta rocket technology. and could be used on both launch vehicles—thus "common". The concept by 2010 was to utilize the new high-performance upper stage, if built, on both Atlas V and Delta IV/Delta IV Heavy launch vehicles.

As further refined in a 2010 conference paper, 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 stage vehicles.[1]

As of 2009, the upper-stage versions of ACES were proposed to be powered by enhanced RL10 engines produced by Pratt & Whitney Rocketdyne (which later became Aerojet Rocketdyne in 2013).[8]

The modular design of ACES supported 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." Several variants were proposed by ULA in 2010—[1] an ACES 41 upper stage with 41 tonnes (90,000 lb) propellant capacity; an ACES 73 upper stage with 73 tonnes (161,000 lb) propellant capacity; ACES 41 and ACES 73 tankers, which would have no engines; and an ACES 121 depot which would consist of an ACES 41 upper stage main vehicle (for LO2 storage) and an ACES 73 tank (modified for LH2 storage) with 121 tonnes (267,000 lb) of long-term, in-space, propellant depot capacity—but none of those options survived into the post-Vulcan ACES design concept by 2018.

Advanced Cryogenic Evolved Stage[edit]

In April 2015, ULA renamed the stage the Advanced Cryogenic Evolved Stage, and announced conceptual plans to complete development of the ACES technology for the Vulcan launch vehicle, flying no earlier than 2023,[6] but currently planned for 2024–25.[9] No plans to develop the stage for the Atlas V or Delta IV launch vehicle lines remain.

However, just like earlier ACES concept proposals, ACES would continue to blend legacy technical aspects of both Delta and Atlas technologies and manufacturing processes, as well as use ULA's proprietary Integrated Vehicle Fluids (IVF) technology to significantly extend the ability of the upper stage to operate in space long term.[10]

ACES is planned to include common bulkhead propellant tanks with a diameter of 5.4 meters, capable of carrying 68 tonnes (150,000 lb) of propellant.[11]


The ACES 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.

With the addition of a solar power system, the vehicle can remain in space and operate indefinitely.[1]

In August 2016 ULA's President and CEO Tory Bruno said they intend to human rate both the Vulcan and ACES.[12]

Integrated Vehicle Fluids[edit]

The IVF technology utilizes a lightweight internal combustion engine to use hydrogen and oxygen propellant boiloff (normally wasted when boiloff gasses are vented to space) to operate the stage including production of power, maintaining stage attitude,[10][13] and keeping the propellant tanks autogenously pressurized, eliminating the need for hydrazine fuel and helium,[6][14]:4, 5 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.[1][14]:2–4[15]:4

As of April 2015, the internal combustion engine to be used to power the IVF system on ACES will be produced by Roush Racing.[6]

Space debris cleanup[edit]

One explicit objective of the ACES design from the beginning, as part of the depot-based space architecture, has been to use the longer-stage 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. 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.[16]

XEUS lunar lander[edit]

In 2015 ULA suggested upgrading the proposed XEUS lunar lander to use ACES as a structural core instead of a Centaur. In this design, based on ULA's earlier Dual Thrust Axis Lander (DTAL[8]), the ACES stage would land on its side, with four short, simple legs supporting the lower curved surface just clear of the ground. The descent engines (also used for ascent) would be mounted on the sides of the stage, about three metres above the lunar surface when landed, and would use the same H2 and O2 fuel as all the other engines. "A lander fashioned as a 'kit' on an existing 2nd stage should be affordable for a commercial program like Golden Spike, recognizing [that] the habitat element would still be a significant development." [14]:5, 6

This version of XEUS could ferry something like 25 tonnes of supplies between the lunar surface and one of the Earth–Moon Lagrange points, making many round trips and needing only to be refilled with LH2 and LO2 (some of which could potentially be produced on the lunar surface). This would fit very well with ULA's depot-based system of space logistics.[1] The IVF generator would also make XEUS independent of sunlight, and provide ample electrical power even if it landed in a permanently shaded crater near one of the lunar poles, where there might be accessible water ice.[14]:6

See also[edit]


  1. ^ a b c d e f g h Zegler, Frank; Kutter, Bernard (2 September 2010). Evolving to a Depot-Based Space Transportation Architecture (PDF). AIAA SPACE 2010 Conference & Exposition. American Institute of Aeronautics and Astronautics. Retrieved 25 January 2011. 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.
  2. ^ @jeff_foust (18 January 2018). "Tom Tshudy, ULA: with Vulcan we plan to maintain reliability and on-time performance of our existing rockets, but at a very affordable price. First launch mid-2020" (Tweet) – via Twitter.
  3. ^ LeBar, JF; Cady, EC (2006). "The Advanced Cryogenic Evolved Stage (ACES) - A Low-Cost, Low-Risk Approach to Space Exploration Launch" (PDF). Retrieved 2016-01-02.
  4. ^ 2005: Atlas Centaur Extensibility to Long-Duration In-Space Applications, Bernard F. Kutter, Frank Zegler, et al, Lockheed Martin Space Systems Company, (AIAA 2005-6738), accessed 20 October 2015.
  5. ^ 2006: Centaur Extensibility For Long Duration, Gerard Szatkoski, et al, NASA/KSC and Lockheed Martin Space Systems Company, (AIAA Space 2006 Conference Paper no. 60196), accessed 20 October 2015.
  6. ^ a b c d Gruss, Mike (2015-04-13). "ULA's Vulcan Rocket To be Rolled out in Stages". SpaceNews. Retrieved 2015-04-18.
  7. ^ "United Launch Alliance Selects Aerojet Rocketdyne's RL10 Engine". ULA. May 11, 2018. Retrieved May 13, 2018.
  8. ^ a b 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" (PDF). AIAA. Archived (PDF) from the original on 2011-07-24.
  9. ^ Bruno, Tory (28 July 2015). "@MrMonster911 @PopSci @ulalaunch enabler will be ACES, our ultra long duration upper stage. Planned to fly in the 2024-5 time frame". Retrieved 11 August 2017.
  10. ^ a b Ray, Justin (14 April 2015). "ULA chief explains reusability and innovation of new rocket". Spaceflight Now. Retrieved 2015-04-18.
  11. ^ "2-1 Transportation & Propellant Resources in the Cislunar Economy-Kutter.pdf" (PDF). 12 June 2018. Retrieved 20 January 2019.
  12. ^ Tory Bruno. ""@A_M_Swallow @ULA_ACES We intend to human rate Vulcan/ACES"". Retrieved August 30, 2016.
  13. ^ Boyle, Alan (2015-04-13). "United Launch Alliance Boldly Names Its Next Rocket: Vulcan!". NBC. Retrieved 2015-04-18.
  14. ^ a b c d Barr, Jonathan (2015). ACES Stage Concept: Higher Performance, New Capabilities, at a Lower Recurring Cost (PDF). AIAA SPACE 2015 Conference & Exposition. American Institute of Aeronautics and Astronautics. Archived from the original (pdf) on 13 March 2016. Retrieved 18 March 2016.
  15. ^ Barr, Jonathan; Kutter, Bernard (2010). Phase 2 EELV - An Old Configuration Option with New Relevance to Future Heavy Lift Cargo (PDF). AIAA SPACE 2010 Conference & Exposition. American Institute of Aeronautics and Astronautics. Retrieved 17 April 2016.
  16. ^ Zegler, Frank; Bernard Kutter (2010-09-02). "Evolving to a Depot-Based Space Transportation Architecture" (PDF). AIAA SPACE 2010 Conference & Exposition. AIAA. pp. 13–14. Archived (PDF) from the original on 2011-10-20. 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.

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