Earth Departure Stage
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The Ares V Earth Departure Stage (EDS) is the high-energy cryogenic upper stage of NASA's proposed new Ares V launch vehicle. Its purpose is to provide propulsion for the Orion spacecraft as part of Project Constellation operations in Earth orbit, near-Earth space, and beyond. Unlike the S-IVB stage of the earlier Saturn V launcher, which had to propel the entire Apollo spacecraft (i.e. both the Apollo Command/Service Module and Lunar Module) into both a low Earth "parking" orbit and then again to the Moon, the EDS will boost only the Orion spacecraft's Lunar Surface Access Module (LSAM) into space; the Orion CSM stack will be launched separately atop an Ares I launch vehicle and will link up with the EDS/LSAM on orbit, a method of known as Earth Orbit Rendezvous (EOR), a technique considered by NASA and Dr. Wernher von Braun for the Apollo program in the early stages of planning, but was dropped in favor of the lunar orbit rendezvous approach. Once all vehicle components are properly configured, the EDS will restart and propel the complete Orion spacecraft to Earth escape velocity for transits to lunar orbit and other destinations.
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[edit] Description
Image:Earth Departure Stage with CEV and LSAM.jpg
The EDS will utilize an uprated version of the Apollo-era J-2 rocket engine, known as the J-2X. Like the original J-2, the J-2X is a liquid hydrogen (LH2)-fueled restartable rocket motor similar to the approach used on the Agena and Centaur upper stages (most rocket engines, including the Space Shuttle Main Engine (SSME), are fired only once; only the J-2X and the RL-10 rocket engines have restart capability). Originally, the EDS would have carried two J-2X engines, but NASA's decision to switch the Ares V's main liquid-fueled stage from five throwaway versions of the SSME to five RS-68 motors with increased impulse allowed NASA to go with the single-engine configuration for the launch vehicle's upper stage.
Control and telemetry for the EDS will be provided via a Saturn-derived Instrument Unit (IU) computer system built with modern upgraded and hardened off-the-shelf technology. The IU will capable of detecting and, even to some extent, correcting problems found with the vehicle during launch and trans-lunar burn activities (such as "pogo" oscillations, low thrust in an engine, etc.).
An onboard attitude control system, possibly derived from either the Hubble Space Telescope or the International Space Station, will stabilize the EDS/LSAM stack during vehicle operations.
[edit] Missions
Aside from its role in lunar missions, the EDS will also provide Orion propulsion for tasks such as ISS resupply, the rescue and evacuation of other spacecraft crews, long-duration satellite servicing and repair sorties, and manned expeditions to Mars. One non-lunar mission the EDS is almost certain to be used for is ferrying the Orion spacecraft to the Sun/Earth L2 Lagrange Points, the stable "harbor" that exists in space on the opposite side of earth from the sun where man-tended robotic spacecraft such as the future James Webb Space Telescope (JWST) (or a second-generation version of the Hubble Space Telescope) will be stationed. Other possible Orion/EDS mission destinations include the Earth-Moon Lagrange points, Earth's polar orbit (in a manner derived from the cancelled Apollo Applications Program, as there are currently no plans of launching Orion CSMs from Vandenberg Air Force Base like those originally planned for both the Manned Orbiting Laboratory and the Space Shuttle), and Near-Earth Objects (NEOs).
[edit] EDS Alternatives
Whilst NASA has indiciated that a J2X powered EDS is to be used in project Constellation, it can be argued that development of this rocket stage is not necessary and that the Lockheed Martin 'Wide Body Centaur' rocket stage, powered by up to six RL-10 rocket engines, would adequately fulfill the same role whilst providing significant benefits in cost, schedule, and operational experience.
[edit] Other uses
As part of the Ares V heavy lift system, the EDS can also be used in launching components of the International Space Station (ISS) that cannot be lifted by the current Russian Proton or future heavy-lift Angara booster (with four LOX/RP-1 strap-on boosters and high-energy LOX/LH2 upper stage). In addition to the ISS and launching unmanned LSAMs to the lunar surface in a cargo-carrying role like that of the Soviet/Russian Progress cargo craft, the EDS, along with a LSAM-based or Centaur "kick-motor," can propel heavy-weight space probes in the same class as either Galileo or Cassini-Huygens to the Outer Solar System. Such an arrangement would allow NASA to simplify a Mars sample return mission, as well as launching the JIMO Jupiter Moon probe or the Neptune Orbiter probe on direct trajectories without having to rely on flybys around either Venus or Earth – instead using Jupiter, Saturn or both planets as gravitational slingshots in a manner similar to that employed by Voyager 2.
