Development of the spacecraft began in the early 1990s and the HTV's first mission,HTV-1, was launched on 10September 2009 on anH-IIB launch vehicle.[4] The HTV was crucial for ISS resupply, especially after theretirement of the Space Shuttle, as it was the only vehicle capable of transporting largeInternational Standard Payload Racks (ISPR) and disposing of old ones within the ISS'sUS Orbital Segment. The final HTV mission,Kounotori 9, was launched on 20 May 2020. HTV's successor, theHTV-X, madeits maiden flight in October 2025.[5]
"a white stork carries an image of conveying an important thing (a baby, happiness, and other joyful things), therefore, it precisely expresses the HTV's mission to transport essential materials to the ISS".[6]
Structure of HTVInside view of the Pressurised Logistics Carrier section of HTV-1Canadarm2 removing unpressurised payload from HTV-2The four main thrusters. Smaller attitude control thrusters can be seen at the right side of this view of HTV-1.HTV-2 departingTanegashima spaceport bound for theInternational Space Station
The HTV was about 9.8 metres (32 ft) long (including maneuvering thrusters at one end) and 4.4 metres (14 ft) in diameter. Total mass when empty was 10,500 kilograms (23,100 lb), with a maximum total payload of 6,000 kilograms (13,000 lb), for a maximum launch weight of 16,500 kilograms (36,400 lb).[2] It consists of Pressurised Logistics Carrier (PLC), Unpressurised Logistics Carrier (ULC), and Main Body which containsavionics and propulsion system. The intent behind the HTV's modularized design was to use different module configurations to match different mission requirements.[7] However, to reduce the development cost it was decided to fly the mixed PLC/ULC configuration only.[7]
To control the HTV's attitude and perform the orbital maneuvers such as rendezvous and reentry, the craft had four 500-N-class main thrusters and twenty-eight 110-N-class attitude control thrusters. Both used bipropellant, namelymonomethylhydrazine (MMH) as fuel andmixed oxides of nitrogen (MON3) as oxidizer.[8] HTV-1, HTV-2, and HTV-4 usedAerojet's 110 N R-1E,Space Shuttle'svernier engine, and the 500 N based on theApollo spacecraft'sR-4D.[8] Later HTVs used 500 N classHBT-5 thrusters and 120 N class HBT-1 thrusters made by Japanese manufacturerIHI Aerospace Co., Ltd.[9] The HTV's four spherical propellant tanks usually carried about 2 tonnes of propellant (the maximum capacity was 2432 kg[8]).
The HTV was comparable in function to theRussianProgress, theESAATV, theSpaceXCargo Dragons, and theCygnus spacecraft, all of which brought supplies to the ISS. Like the ATV, the HTV carried more than twice the payload of the Progress, but was launched less than half as often. Unlike Progress spacecraft, Cargo Dragon 2's and ATV's which used thedocking ports automatically, HTVs and AmericanDragon 1 approached theISS in stages, and once they reached their closest parking orbit to the ISS, crewgrappled them using the robotic armCanadarm2 and berthed them to an openberthing port on theHarmony module.[10]
The HTV had an external payload bay which was accessed by the robotic arm after it had been berthed to the ISS. New payloads could be moved directly from the HTV toKibō's exposed facility. Internally, it had eightInternational Standard Payload Racks (ISPRs) in total which could be unloaded by the crew in ashirt-sleeve environment. After the retirement ofNASA'sSpace Shuttle in 2011, HTVs became the only spacecraft capable of transporting ISPRs to the ISS. TheSpaceX Dragon andNorthrop GrummanCygnus could carry resupply cargo bags but not ISPRs.[citation needed]
After the unloading process was completed, the HTV was loaded with waste and unberthed. The vehicle then deorbited and was destroyed during reentry, the debris felling into thePacific Ocean.[11]
Initially seven missions were planned between 2008–2015. With the extension of the ISS project through 2028, three more missions were added, with the tenth flight planned to debut an improved, cost-reduced version called theHTV-X.[12]
By May 2020, all nine missions planned for HTV had been successfully launched, and the spacecraft was retired. The improved HTV-X is planned to be first used for the tenth flight and will perform scheduled ISS resupply duties starting in 2025.[14]
In May 2015, Japan'sMinistry of Education, Culture, Sports, Science and Technology announced a proposal to replace the HTV with an improved, cost-reduced version preliminary calledHTV-X.[12][24] The HTV-X has a length of 6.2 m, or 10 m with the unpressurised cargo module fitted. The payload fairing adaptor and payload dispenser have been widened from 1.7 m to 4.4 m.[25] An evolved version of HTV-X called HTV-XG is being considered for transferring cargo to theLunar Gateway as part of theArtemis program.[26] The first flight of HTV-X took place on 26 October 2025.[5]
As of 2010[update], JAXA was planning to add a return capsule option. In this concept, HTV's pressurized cargo would be replaced by a reentry module capable of returning 1,600 kilograms (3,500 lb)cargo from ISS to Earth.[27][28] Further, conceptual plans in 2012 included a follow-on spacecraft design by 2022 which would accommodate a crew of three and carry up to 400 kilograms (880 lb) of cargo.[29]
As of 2014[update], both JAXA andMitsubishi conducted studies of a next generation HTV as a possible Japanese contribution to the proposedinternational crewed outpost atEarth-Moon L2.[30][31] This variant of HTV was to be launched byH-X Heavy and can carry 1800 kg of supplies to EML2.[30] Modifications from the current HTV included the addition of solar electric paddles and extension of the propellant tank.[30]
A proposal announced in June 2008, "Preliminary Study for Manned Spacecraft with Escape System and H-IIB Rocket" suggested combining HTV's propulsion module with ahuman-rated capsule for four people.[32]
A Japanese space station was proposed to be built up from HTV modules.[33] This method was similar to how the modules inMir, as well as many modules of theRussian Orbital Segment of theISS were based on theTKS cargo vehicle design.[citation needed]
^abMiki, Yoichiro; Abe, Naohiko; Matsuyama, Koichi; Masuda, Kazumi; Fukuda, Nobuhiko; Sasaki, Hiroshi (March 2010)."Development of the H-II Transfer Vehicle (HTV)"(PDF).Mitsubishi Heavy Industries Technical Review.47 (1). Mitsubishi Heavy Industries.Archived(PDF) from the original on 20 July 2015.
^Fujimoto, Nobuyoshi (23–26 November 2010).Kibo Utilization Status Update(PDF). 17th Session of the Asia Pacific Regional Space Agency Forum. Archived fromthe original(PDF) on 17 March 2012. Retrieved20 June 2019.
^JAXA (2007)."HTV Operations".Archived from the original on 26 January 2011. Retrieved2 January 2011.
^ab2016年～2020年のISS共通システム運用経費（次期CSOC）の我が国の負担方法の在り方について(PDF). Research and Development Division, Ministry of Education, Culture, Sports, Science and Technology. 20 May 2015.Archived(PDF) from the original on 5 June 2015. Retrieved4 June 2015.
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