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The GX rocket, a next-generation craft being developed by Japanese and U.S. concerns, is too heavy and needs to be slimmed down, sources familiar with its development said Saturday.

The second stage of the two-stage rocket, which is being manufactured by the Japanese side, is 800 kg heavier than originally planned mainly due to a change made to the fuel tanks, the sources said.

But if the 48-meter-long rocket is not made lighter, there will be little demand for it, the sources added.

The first-stage of the midsize rocket is being produced by Lockheed Martin Corp. of the United States using a Russian engine, while the second stage will be made by Japan’s Ishikawajima-Harima Heavy Industries Co. using an engine newly developed by the National Space Development Agency of Japan.

Ishikawajima and NASDA plan to cut the weight by simplifying the fuel injection system used in the rocket, they said.

The new engine uses liquefied natural gas as a propellant for the first time ever. To improve the power of the engine, they strengthened the tanks so the LNG can be expelled under more pressure and improve thrust, the sources said.

But as a result, the weight was about 500 kg above the original plan of 1.87 tons, they said. An extra 300 kg of fuel was also found to be necessary.

NASDA is expected to carry out two GX rocket tests at the end of fiscal 2005 and 2006 at Tanegashima Space Center in Kagoshima Prefecture.

But making the rocket lighter should have no influence on the launch schedule, an Ishikawajima official said.

The project is operated by Galaxy Express, which was financed and set up by Ishikawajima, NASDA and Lockheed. The firm is aiming to put a 2-ton satellite into orbit at an altitude of 800 km.