The Ares V Earth Departure Stage (EDS) is a rocket stage which will be designed by NASA at its Marshall Space Flight Center in Huntsville, Alabama as part of Project Constellation. The EDS will be propelled by a single J-2X main engine fueled with liquid oxygen (LOX) and liquid hydrogen (LH2), and will fly in a manner similar to that of the S-IVB rocket stage used on the Saturn V rockets that propelled the three-man Apollo missions to the Moon between 1968 and 1972.
When the Ares V was then redesigned around the use of five (later six) RS-68B rocket engines currently used on the Delta IV EELV family, the EDS was then redesigned using only a single J-2X engine and a common bulkhead, thus in its current design, the EDS now resembles an oversized S-IVB, but with the capability of on-site storage (using new propellant storage techniques along with a "loiter skirt" containing solar panels for electricity) for up to 30-days, something impossible with the old S-IVB.
Once the Orion is docked with the Altair and its systems are checked out, the crew will then jettison the "loiter skirt" and then fire the J-2X engine for a second time, this time at 80% rated thrust, for the trip out to the Moon. Unlike the S-IVB, which propelled the Apollo Spacecraft and its three-man crew in a forward-facing motion, the EDS will fire its onboard rocket with the crew facing the EDS. This "eyeballs out" type of flying, will be similar to the flight profile of the proposed, but never flown Manned Venus Flyby from the cancelled Apollo Applications Program of the late 1960's.
After the EDS is shutdown for the last time, it is jettisoned to fly into a heliocentric orbit around the Sun, or in a manner similar to that employed by NASA from Apollo 13 to Apollo 17, it may be deliberately crashed into the lunar surface to help scientists calibrate sensitive seismometers placed on the lunar surface by either astronauts on lunar sortie flight or by unmanned robotic probes.