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" Late last week, NASA announced that its future heavy-lift cargo rocket\u2014designed to haul lunar landers and exploration hardware to Earth orbit and beyond\u2014will not be powered by the Space Shuttle Main Engine (SSME), NASA&amp;#039;s workhorse rocket motor. The agency had originally planned to use five SSMEs to propel the core stage of the Cargo Launch Vehicle (CaLV), hoping to save substantially on development costs with an engine that has powered the shuttle for 25 years (through 114 shuttle missions, only one SSME has suffered an in-flight failure). NASA says the change will save money over the life of the the CaLV because newer RS-68 engines\u2014first flown in 2002 and now used on the Boeing Delta IV expendable booster\u2014have lower costs compared to the shuttle&amp;#039;s proven SSME when you project the cost over twenty or more years of cargo launches. The logic behind the change seems sound, but it further undercuts one of NASA&amp;#039;s key arguments for its particular approach to the Vision for Space Exploration: That using the space shuttle&amp;#039;s boosters, main engines, and external tank will reduce the cost of getting back to the Moon. Riding the SSME Although it was plagued by development problems in the 1970s, the SSME has amassed more than a million seconds (more than eleven days) of reliable run time during the shuttle&amp;#039;s three-decade-long career. I&amp;#039;ve ridden twelve SSMEs to orbit on four shuttle flights, and they are smooth-running marvels of engineering. Fully reusable and burning highly efficient liquid hydrogen as fuel, each SSME is as finely tuned as a Swiss watch. But that complexity and efficiency means that building an SSME\u2014even for just one throwaway use\u2014is more labor-intensive than turning out an RS-68, which was designed for low cost and streamlined manufacturing. The RS-68, which also burns liquid hydrogen and oxygen, doesn&amp;#039;t yet have the track record of the SSME, but it has proven reliable in three Delta IV launches. For an expendable, non-astronaut-carrying Cargo Launch Vehicle, NASA&amp;#039;s engineers evidently decided that the SSME&amp;#039;s high efficiency and reliability were not worth the extra premium. Behind the switch Shuttle heritage in the new programs has been slowly decreasing. The agency announced in March that it was dropping plans to use the SSME to power the upper stage of its Crew Launch Vehicle (CLV), designed to carry future astronauts to orbit. Instead, NASA argued that the Apollo-era J-2X engine would be cheaper to build and re-qualify for that second-stage role. That substitution and weight growth in the new Crew Exploration Vehicle forced NASA to add an extra solid-fuel segment to the CLV&amp;#039;s shuttle-derived boosters, requiring additional testing (and expense). The new decision to drop the SSME from the cargo vehicle means that it will now play no part in the next generation of American human spaceflight. By 2010, the SSME will retire with the shuttle orbiter. The downside of the RS-68 decision is that the Cargo Launch Vehicle&amp;#039;s core fuel tanks must grow from a planned width of 27.5 feet to 33 feet (as wide as the old Saturn V&amp;#039;s first stage). That&amp;#039;s because the RS-68, although more powerful than the SSME (650,000 lbs. of thrust vs. 420,000) is a heavier, less efficient engine, requiring more fuel to lift the same amount of payload to orbit. The larger stage diameter will prevent NASA from using its existing external tank production line at Michoud, LA for the new rocket; instead it will have to invest in new production tooling at the same site. Those &amp;quot;shuttle heritage&amp;quot; savings are dwindling fast. Is the Vision in trouble? The latest engine swap, along with other design changes and the evaporation of much shuttle heritage from the major return-to-the-Moon systems, worries those who doubt NASA has a firm handle on &amp;quot;the Vision thing.&amp;quot; The engine change reinforces suggestions that the NASA team that produced ESAS, the mid-2005 Exploration Systems Architecture Study, based some of its design choices on wobbly assumptions. Perhaps. But critics have had more time to second-guess the architecture than the ESAS team did to come up with the initial exploration concepts. Cost estimates inevitably get refined as planning goes from broad concepts to detailed specifications. The RS-68 decision shows that NASA, refreshingly, is not sticking with shuttle heritage when it doesn&amp;#039;t make economic sense. Engines are a lot more expensive than fuel tanks, so choosing a cheaper engine should pay for the extra metal-bending on the tank production line. And although I trusted the SSME with my life (and nervously hoped through four launches that each trio would hold together through eight-and-a-half minutes of run time), sentimentality can&amp;#039;t hold a candle to hard economics. NASA won&amp;#039;t have the luxury of wasting a penny on these new systems if it really wants to blast astronauts loose from Earth orbit once again. If Administrator Mike Griffin is responsible for this hard-eyed approach top NASA&amp;#039;s future, he is to be commended. But the constant tinkering with launchers and the cost and schedule difficulties still to be grappled with in getting the new Crew Exploration Vehicle off the ground will only exacerbate Congressional suspicions that NASA, faced with painful budget choices, is trying to low-ball the costs for its exploration plans. The agency tried the same tactic 15 years ago with the Space Station, and suffered a huge loss in credibility with the administration, Congress and the public. NASA knows it must not squander the Vision opportunity, but can it avoid a repetition of its past missteps? If America hopes to fly astronauts again once the shuttle retires, the answer must be yes. \u2014Tom Jones Tom Jones is a planetary scientist and a veteran astronaut of four Space Shuttle missions and the author of the new book Sky Walking: An Astronaut&amp;#039;s Memoir. Space Station DIY: Astronaut (and PM contributor) Tom Jones helps to assemble the ISS. NASA/JSC The RS-68 engine from the Delta IV rocket during testing. Boeing With NASA&amp;#039;s latest announcement, the Space Shuttle Main Engines will shut down - permanently - when the Orbiters retire. Boeing An artist&amp;#039;s conception of the Cargo Launch Vehicle. NASA "^^<http://www.w3.org/2001/XMLSchema#string>
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