Space Shuttle External Tank - Future Use

Future Use

With the retirement of the Space Shuttle in 2011, NASA, with its planned Project Constellation, which features the Apollo-derived Orion spacecraft, will also feature the debut of two Shuttle-derived launch vehicles, the man-rated Ares I crew-launch vehicle and the heavy-lift Ares V cargo-launch vehicle.

While both the Ares I and Ares V will utilize a modified five-segment Solid Rocket Booster for its first stage, the current ET will serve 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 will hold approximately 26,000 US gal (98,000 l) of LOX, versus the ET holding 146,000 US gal (550,000 l), more than 5 times that amount.

The Ares V first stage, which will be fitted with five RS-68 rocket engines (the same engine used on the Delta IV rocket), will be 33 feet (10 m) in diameter, as wide as the S-IC and S-II stages on the Saturn V rocket. It will utilize the same internal ET configuration (separate LH₂ and LOX tanks separated with an intertank structure), but will be configured to directly accept LH₂ and LOX fill and drain, along with LOX venting on a retractable arm like that used on the Shuttle for LH₂ (as the "beanie cap" would be useless due to the in-line design of the three-stage vehicle).

The Ares I second stage, on the other hand, will only use 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 LH₂/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 will use the same fill/drain/vent configuration used on the Shuttle, the Ares I system will utilize 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 will expect upon SRB ignition.

As originally envisioned, both the Ares I and Ares V would have used a modified "throw away" version of the SSME, 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 to switch to the RS-68 engine for the Ares V and to an uprated J-2 engine for the Ares I. Because of the switch to the RS-68, the Ares V was widened from 28.6 to 33 feet (8.72 to 10.06 m) to accommodate the extra propellants, while the Ares I was reconfigured to incorporate a fifth solid-rocket segment as the J-2X, as the rocket engine is known, has less thrust than the SSME. 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 SSME for the Ares I (as the J-2X and its predecessor were designed to be started in both mid-air and in a near vacuum).

The DIRECT project, a proposed alternative shuttle-derived vehicle, uses a modified, standard diameter, external tank with three SSMEs, with two standard SRBM, as a Crew Launch Vehicle. The same vehicle, with one extra SSME, and an EDS upper stage, serves as the Cargo Launch Vehicle. It is purported to save $16 billion, eliminate NASA job losses, and reduce the post-shuttle, manned spaceflight gap from five plus years to two or less.