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H-II Transfer Vehicle

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Design

HTV is about 9.2 m long (including maneuvering thrusters at one end) and 4.4 m in diameter. Empty, it weighs 10.5 tons. HTV is a larger and simpler vehicle than the Progress spacecraft currently used by Russia to bring supplies to the station, since it does not have a complex docking and approach system. Instead, it will be flown just close enough to the station to allow capture by Canadarm2, which will pull it to a berthing port on the ISS Harmony module.

HTV can carry supplies in a combination of two different "segments" that can be attached together. One is a pressurized hold with a capacity of 6,000 kg, which includes an optional docking adapter at one end to allow it to be unloaded in a shirt-sleeves environment. It is designed specifically to carry eight International Standard Payload Racks (ISPRs) in total. After the planned retirement of NASA's Space Shuttle in 2010, HTV will be the only vehicle which can carry ISPRs to the ISS. It will also have a tank to deliver up to 300 kg of water to the station. The other is a lighter and slightly longer unpressurized segment, which includes a hatch on the side to allow it to be unloaded remotely.

The baseline configuration, known as the "Mixed Logistics Carrier", uses one pressurized and one unpressurized segment and can carry 7,600 kg of cargo in total and is 9.2 m long. When two pressurized units are used together the cargo decreases slightly to about 7,000 kg, and the overall length is reduced to 7.4 m.^[2] These numbers are somewhat vague in the various sources, some suggesting that the pressurized/unpressurized combination carries only 6,000 kg in total, less than the pressurized/pressurized combination, which should be heavier. No sources suggest an unpressurized/unpressurized combination is planned, perhaps due to the overall length.

HTV propulsion is used to generate the torque to control the HTV attitude and the thrust to perform the orbital maneuvers such as rendezvous and re-entry. The HTV has four 500 N class main thrusters and twenty-eight 110 N class attitude control thrusters. Both are using bipropellant, namely monomethylhydrazine (MMH) as fuel and the mixed oxides of nitrogen (MON3) as oxidizer.^[3]

Both types of thrusters are manufactured by Aerojet, the 500 N being of the R-4D type of Apollo heritage and the 110 N being of the R-1E type (Shuttle vernier engine).^[3] The HTV carries about 2400 kg of propellant in four tanks.^[3]

After the unloading process is completed, the HTV will be loaded with waste and undocked. The vehicle will then be de-orbited and destroyed during re-entry, the debris falling into the Pacific ocean.^[4]

Possible usage by NASA

In July 2008, it was reported that the United States space agency NASA had begun unofficial negotiations with JAXA on the purchase of HTV spacecraft as the successor to the space shuttle fleet due to NASA's concerns about refueling and servicing the ISS after it retires the shuttle fleet in 2010.^[5] A day later, NASA released a press statement declaring that "NASA has not officially or unofficially been discussing the purchase of H-II Transfer Vehicles." The space agency remains committed to "domestic commercial cargo resupply to the space station."^[6] NASA has been working with private launch firms such as SpaceX and Orbital Sciences Corporation.

Flights

The first vehicle was launched on an H-IIB rocket, a more powerful version of the earlier H-IIA, at 17:01 GMT on September 10, 2009, from Launch Pad 2 of the Yoshinobu Launch Complex at the Tanegashima Space Center^[7]. Six subsequent missions are planned.