Technicians finished loading propellant into a Japanese cargo ship this week, achieving a critical milestone before the first-of-a-kind spacecraft launches to the space station next month.

The HTV sits inside a clean room last month. Credit: JAXA

The H-2 Transfer Vehicle, or HTV, was filled with a noxious mix of hydrazine and nitrogen tetroxide during a week-long process on Tanegashima Island, home of Japan's spaceport at the southern end ot the country.

The fueling was completed Wednesday and tank pressures will be monitored during the next week to see if more propellant needs to be added, according to Hiro Uematsu, an engineering manager on the HTV project for the Japan Aerospace Exploration Agency.

Officials expect to top off the tanks by next Wednesday, Uematsu said.

The HTV carries more than 5,300 pounds of fuel and oxidizer to fine tune its approach to the International Space Station after launching on Sept. 10. Liftoff is set for 1704 GMT (1:04 p.m. EDT), or 2:04 a.m. Japanese time on Sept. 11.

After more than five days of engine firings and tests, the ship will reach the complex the morning of Sept. 16, U.S. time. Astronauts will use the station's robotic arm to grapple the free-flying spacecraft and berth it to an open port on the U.S. segment.

The barrel-shaped vehicle will be attached to the station until the middle of October, when it will depart and guide itself to a destructive finale in the Earth's atmosphere.

But first the HTV must pass one more functional test and be mated to the H-2B rocket, an upscaled version of Japan's main booster that has flown since 2001.

"Once we complete the fueling, before we deliver the vehicle for the integration with the fairing, we'll do a final functional checkout. That's the final checkout before it's delivered to the launch vehicle side," Uematsu said.

Making its first flight, the two-stage launcher is awaiting its payload in Tanegashima's Vehicle Assembly Building. The rocket underwent extensive testing at the launch site earlier this year, including two engine tests and a check of ground systems at its new launch pad.

The 36,000-pound resupply craft will be trucked to the rocket integration building, lifted and mated atop the launcher's second stage on Aug. 30, officials said.

Workers have already loaded two experiments into the HTV's unpressurized section. The ship will also deliver drinking water for the outpost's residents.

A few more late items, mostly crew supplies, will be put into the spacecraft's pressurized section in the final days before launch.

JAXA officials say they have until Sept. 30 to launch the mission before having to stand down for up to four months. The space agency can only launch during specific periods each year to allow the powerful Japanese fishing industry to operate in restricted waters offshore Tanegashima.

Uematsu said the flight may have to wait until February for another chance to launch, but officials say everything is on schedule for September.

Earlier this year, the launch was delayed from Sept. 1 to give engineers more time to prepare the spacecraft and rocket.

"That kind of gave us a week back, so we're ready to go from our side," Uematsu said.