Editing Space Shuttle external tank (section)

===As basis for Ares in Constellation===
With the retirement of the Space Shuttle in 2011,<ref name="NASAlaunchschedule">[http://www.nasa.gov/missions/highlights/schedule.html NASA launch schedule], accessed 2009/09/23</ref> NASA, with its canceled [[Constellation program]], which featured the [[Orion spacecraft]], would also have featured the debut of two Shuttle-derived launch vehicles, the [[Human-rating certification|human-rated]] [[Ares I]] crew-launch vehicle and the heavy-lift [[Ares V]] cargo-launch vehicle.

While both the Ares I and Ares V would have utilized a modified five-segment Solid Rocket Booster for its first stage, the ET would have served as a baseline technology for the first stage of the Ares V and the second stage of the Ares I; as a comparison, the Ares I second stage would have held approximately {{convert|26000|USgal|L|abbr=on}} of LOX, versus the ET holding {{convert|146000|USgal|L|abbr=on}}, more than 5 times that amount.{{Citation needed|date=February 2011}}

The Ares V first stage, which would have been fitted with five [[RS-68 (rocket engine)|RS-68]] rocket engines (the same engine used on the [[Delta IV rocket]]), would be {{convert|33|ft|m|0}} in diameter, as wide as the [[S-IC]] and [[S-II]] stages on the [[Saturn V]] rocket. It would have utilized the same internal ET configuration (separate LH<sub>2</sub> and LOX tanks separated with an intertank structure), but would have been configured to directly accept LH<sub>2</sub> and LOX fill and drain, along with LOX venting on a retractable arm like that used on the Shuttle for LH<sub>2</sub>.

[[Image:Rocket size comparison.png|thumb|300px|Comparison of the [[Saturn V]], [[Space Shuttle]], [[Ares I]], [[Ares IV]] and [[Ares V]]]]

The Ares I second stage, on the other hand, would have only used the spray-on insulation foam currently used on the current ET. Originally configured like that of the Ares V and the Shuttle ET, NASA, upon completing its design review in 2006, decided, in order to save weight and costs, to reconfigure the internal structure of the second stage by using a combined LH<sub>2</sub>/LOX tank with the propellants separated by a common bulkhead, a configuration successfully used on the S-II and [[S-IVB]] stages of the Saturn V rocket. Unlike the Ares V, which would have used the same fill/drain/vent configuration used on the Shuttle, the Ares I system would have utilized a traditional fill/drain/vent system used on the Saturn IB and Saturn V rockets, but with quick-retracting arms due to the "leap frog" speed the Ares I would expect upon SRB ignition.{{Citation needed|date=February 2011}}

As originally envisioned, both the Ares I and Ares V would have used a modified "throw away" version of the [[RS-25]] engine, but in due course, because of the need to keep R&D costs down and to maintain a schedule set by NASA Administration [[Michael D. Griffin]] to launch the Ares and Orion by 2011, NASA decided (after the 2006 review) to switch to the cheaper [[RS-68]] engine for the Ares V and to an uprated [[J-2 (rocket engine)|J-2]] engine for the Ares I. Because of the switch to the less efficient RS-68, the Ares V was widened from {{convert|28.6|to|33|ft|m|2}} to accommodate the extra propellants, while the Ares I was reconfigured to incorporate a fifth solid-rocket segment with the J-2X upper stage, as the new engine has less thrust than the original RS-25. Because of the trade-off, NASA would save an estimated [[USD]] $35 million by using simplified, higher thrust RS-68 engines (reconfigured to fire and perform like the SSME), while at the same time, eliminate the costly tests needed for an air-startable RS-25 for the Ares I.