For the second time this month, Japanese engineers rolled the nation's new H-2B rocket to the launch pad and fired its two main engines Wednesday, this time for a thundering two-and-a-half minutes.

Credit: JAXA

The 17-foot-diameter heavy-lifting rocket ignited its two LE-7A engines at 1 p.m. local time, or 0400 GMT.

The hydrogen-fueled engines each revved up to about 200,000 pounds of thrust and burned for 150 seconds.

Officials planned the test to measure vibrations the rocket will experience during the first stage burn. Engineers also used the demo to verify the pressure characteristics of the rocket, according to the Japan Aerospace Exploration Agency.

The test firing used the real first and second stages destined for flight on the H-2B rocket's first launch in September.

The launch team was also exercised during Wednesday's test, which included a mock countdown that included loading of liquid hydrogen and liquid oxygen into the first stage propellant tanks.

Wednesday's test was postponed from Monday because of poor weather on Tanegashima Island, home of Japan's spaceport near the southern tip of the country, according to JAXA.

The rocket waited out the delay on Launch Pad No. 2 at Tanegashima's Yoshinobu launch complex.

The H-2B went through another countdown rehearsal and engine test on April 2. That firing lasted only 10 seconds.

An artist's concept shows the future H-2B rocket lifting off. Credit: JAXA

JAXA and Mitsubishi Heavy Industries Ltd., the rocket's prime contractor, have already rolled the partially-integrated launcher back to Tanegashima's Vehicle Assembly Building to begin checking it for compatibility with ground infrastructure.

Technicians will remove two inert solid rocket boosters from the base of the first stage and replace them with four motors packed with flammable solid propellant. Engineers added the inert boosters for the long-duration engine test to provide extra weight to the vehicle for additional stability, a JAXA spokesperson said.

JAXA will also replace the engines tested Wednesday with new powerplants, officials said.

The launch team will also conduct a full-up practice countdown, including fueling of both stages with cryogenic propellants and standard pre-launch tests that were not part of this month's activities.

After the ground interface checks are completed this summer, workers will bolt the H-2 Transfer Vehicle atop the rocket during the final launch campaign.

The HTV will ferry experiments, spare parts and crew supplies to the international space station during the September mission. The cylindrical craft will be grappled by the complex's robot arm and attached to the station's Harmony module for about one month.

Components of the first HTV left their testing facility at the Tsukuba space center near Tokyo last week.

The equipment was shipped by boat from the Japanese mainland to Tanegashima, where it arrived a few days later, a JAXA spokesperson said.

An artist's concept of HTV nearing the space station. Credit: JAXA

Workers will spend the next few months double-checking the spacecraft's readiness for flight and putting together the vehicle's parts. Officials will also load the HTV with maneuvering fuel that will fine-tune the craft's approach to the station.

The $444 million H-2B rocket program was established to develop a new launcher with the lifting capacity to haul the 36,000-pound ship to the space station's orbit.

Officials reduced development costs by designing the new rocket around Japan's existing H-2A rocket, which has launched government satellites on 15 flights, amassing a 93 percent success record.

The H-2B uses a stretched first stage that is four feet wider than the H-2A rocket. The “widebody” first stage uses two LE-7A main engines, while the H-2A rocket is powered an identical single engine.

The H-2B's second stage and four solid rocket boosters are unchanged from the hardware used by the H-2A rocket.