Back to the drawing board

 作者:齐舷狙     |      日期:2019-03-07 03:03:01
By Jeff Hecht in Boston NASA’s boldest gamble to slash the costs of launching reusable spacecraft is looking riskier than ever. The future of its X-33 vehicle is in question after an innovative composite fuel tank failed on 3 November. The $1.3 billion X-33 is one of several NASA projects aimed at replacing its ageing space shuttle fleet. Most involve modest improvements in technology. But the X-33, which aims to cut the cost of space launches by a factor of ten, is on “the razor edge of feasibility”, according to John Pike, a space analyst with the Federation of American Scientists in Washington DC. The craft is produced by Lockheed Martin’s “Skunk Works” in Palmdale, California, which specialises in high-risk projects. The X-33 is designed to demonstrate two crucial technologies: a novel “aerospike” engine (New Scientist, 6 July 1996, p 36), and lightweight graphite-epoxy composite liquid-hydrogen fuel tanks (see Diagram). The craft is a half-scale prototype of VentureStar, a single-stage vehicle which NASA originally hoped would fly by 2004. Even before the latest setback, the fuel tanks were causing problems. “As originally designed, the composite tanks would save about 20 per cent weight,” says a Lockheed Martin spokesman. But modifications to strengthen the two 8.8-metre-long tanks had already made them about as heavy as metal designs. Now it seems even these changes may not be enough. The latest blow came after engineers at NASA’s Marshall Space Flight Center in Huntsville, Alabama, pumped 110 000 litres of liquid hydrogen into one tank to test whether it could support the liquid oxygen tank that would sit on top of it at launch. After the hydrogen was pumped out, engineers noticed that the outer graphite-epoxy skin and an insulating honeycomb structure had separated from the inner skin along a structural seam. Lockheed Martin is putting a brave face on the failure. “It’s what we’re in this programme to learn,” says a spokesman. However, the company last month announced that it plans to “aggressively pursue the reduction of capital expenditures”. VentureStar, which will require a large investment by Lockheed Martin,