Japan plans to develop a more powerful version of its H-IIA rocket to increase its payload for flights to the International Space Station by attaching an extra main engine to the craft, sources in the government and the National Space Development Agency of Japan said Saturday.

The improvement was chosen because the cost will be nearly 10 percent less than that for enhancing the H-IIA's payload with a liquid rocket booster, an idea currently being considered by NASDA, they said.

Japan is likely to drop the LRB method plan, which was to enter the experimental phase in fiscal 2005, and formally adopt the new plan around June, after deliberations within the Space Activities Commission, they said.