TOKYO — Technical problems continue to plague Japan’s Galaxy Express (GX) rocket development program, pushing the vehicle’s debut to no earlier than 2011, five years later than originally planned, according to a recently released report by the Japan Aerospace Exploration Agency, or JAXA.

The culprit continues to be the GX’s liquid natural gas-fueled second-stage engine, whose projected development cost has nearly tripled, to 34.7 billion yen ($292 million), JAXA said in a report submitted Sept. 29 to the Space Activities Commission, the government’s most senior decision-making body for space programs. That figure is more than half of what the entire GX development effort was expected to cost when it began in 2001.

Toshio Akimoto, JAXA project leader for development of the GX second-stage engine, said in an Oct. 11 interview that major design changes adopted in 2004 and 2005 did not work out as expected, thus require additional modifications. He said an engine featuring the latest design changes will not be ready for testing until April 2007. And even if there are no further problems, the resulting engine will be less capable than previously envisioned, he said.

The first stage of the GX rocket will be powered by the Russian-built RD-180 engine, which also serves as the main engine on Lockheed Martin Corp.’s Atlas 5 rocket. Lockheed Martin of Bethesda, Md., is a partner in Galaxy Express Corp. of Tokyo, an industrial consortium led by Japan’s Ishikawajima-Harim a Heavy Industries that was set up in March 2001 to manage and help fund the GX development project.

The GX is intended to fill the payload niche between the Japan’s large H-2A rocket and the smaller, solid-fueled M-5. But the program has suffered numerous funding and technical setbacks over the years.

The original 2006 inaugural launch date previously slipped to 2008 after cracks were discovered in the lightweight composite fuel tank for the second-stage engine. But a new design featuring a heavier aluminum tank and a turbopump adapted from the H-2A’s LE-5B second-stage engine has had problems of its own, Akimoto said. Specifically, he said, the revamped engine has experienced sharp fluctuations in combustion pressure during testing, requiring another redesign.

Meanwhile, the heavier tank will diminish the GX’s payload capacity, Akimoto said. The vehicle is advertised as being capable of lofting a 2-metric-ton satellite into sun-synchronous orbit with an altitude of 800 kilometers. But the vehicle now will have an initial capability to place a 1,500-kilogram payload into a 500-kilometer orbit, he said.

In an Oct. 13 response via e-mail to a request for comment, Yoshihisa Shibayama, general manager of general affairs and procurement at Galaxy Express, declined to elaborate on the vehicle’s problems, saying the company explained its position to the Space Activities Commission in the JAXA report. In the report, the company argued that Japan needs the GX rocket to launch medium-sized Earth observation and reconnaissance satellites.

JAXA’s Web site says the vehicle is intended primarily for the commercial market.

To date, no specific payloads for the vehicle have been identified.

The Space Activities Commission is evaluating the latest GX plan and will decide in late October or early November how or whether to proceed with the program, according to Toshihiro Matsui, director of the Space Development Cooperation Promotion Office in Japan’s Ministry of Education, Culture, Sports, Science and Technology, which oversees JAXA.

Kazuto Suzuki, assistant professor at the University of Tsukuba and an expert on Japanese space activities, said it is very unlikely that the GX program will be canceled outright, despite its problems. “This is a … program that can’t be stopped because [the Space Activities Commission] … JAXA and the Ministry of Finance and industry all agreed to do it and once everyone says ‘yes,’ it’s difficult to stop it,” he said in an Oct. 13 interview.