H-II Transfer Vehicle (HTV-1) approaching the ISS
|Country of origin||Japan|
|Applications||Automated cargo spacecraft to resupply the ISS|
|Launch mass||16,500 kg (36,400 lb)|
|Dry mass||10,500 kg (23,100 lb)|
|Volume||Pressurized: 14 m3 (490 cu ft)|
|Length||~9.8 m (32 ft) (including thrusters)|
|Diameter||4.4 m (14 ft) |
|Payload to ISS|
|Mass||6,000–6,200 kg (13,200–13,700 lb)|
|Status||Retired (original model)|
In development (HTV-X)
|Maiden launch||10 September 2009|
|Last launch||20 May 2020|
The H-II Transfer Vehicle (HTV), also called Kounotori (こうのとり, Kōnotori, "Oriental stork" or "white stork"), is an expendable, automated cargo spacecraft used to resupply the Kibō Japanese Experiment Module (JEM) and the International Space Station (ISS). The Japan Aerospace Exploration Agency (JAXA) has been working on the design since the early 1990s. The first mission, HTV-1, was originally intended to be launched in 2001. It launched at 17:01 UTC on 10 September 2009 on an H-IIB launch vehicle. The name Kounotori was chosen for the HTV by JAXA because "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". The HTV is very important for resupplying the ISS because after the retirement of the Space Shuttle it is the only vehicle that can transfer new 41.3 in (105 cm) wide International Standard Payload Racks (ISPRs) and dispose old ISPRs that can fit the 51 in (130 cm) wide tunnels between modules in the US Orbital Segment.
The HTV is 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 is 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).
The HTV is comparable in function to the Russian Progress, ESA ATV, commercial Cargo Dragon 1, and Cargo Dragon 2 of SpaceX. Plus the Cygnus spacecraft, all of which bring supplies to the ISS. Like the ATV, the HTV carries more than twice the payload of the Progress, but is launched less than half as often. Unlike Progress spacecraft, Cargo Dragon 2's and ATV's which use the docking ports automatically, HTVs and American Dragon 1 approach the ISS in stages, and once they reach their closest parking orbit to the ISS, crew grapple them using the robotic arm Canadarm2 and berth them to an open berthing port on the Harmony module.
The HTV has an external payload bay which is accessed by the robotic arm after it has been berthed to the ISS. New payloads can be moved directly from the HTV to Kibō's exposed facility. Internally, it has eight International Standard Payload Racks (ISPRs) in total which can be unloaded by the crew in a shirt-sleeve environment. After the retirement of NASA's Space Shuttle in 2011, HTVs became the only spacecraft capable of transporting ISPRs to the ISS. The SpaceX Dragon and Northrop Grumman Cygnus can carry resupply cargo bags but not ISPRs.
The intent behind the HTV's modularized design was to use different module configurations to match different mission requirements. However, to reduce the development cost it was decided to fly the mixed PLC/ULC configuration only.
To control the HTV's attitude and perform the orbital maneuvers such as rendezvous and reentry, the craft has four 500-N-class main thrusters and twenty-eight 110-N-class attitude control thrusters. Both use bipropellant, namely monomethylhydrazine (MMH) as fuel and mixed oxides of nitrogen (MON3) as oxidizer. HTV-1, HTV-2, and HTV-4 use Aerojet's 110 N R-1E, Space Shuttle's vernier engine, and the 500 N based on the Apollo spacecraft's R-4D. Later HTVs use 500 N class HBT-5 thrusters and 120 N class HBT-1 thrusters made by Japanese manufacturer IHI Aerospace Co., Ltd. The HTV carries about 2400 kg of propellant in four tanks.
Initially seven missions were planned in 2008–2015. With the extension of the ISS project through 2028, three more missions were added, with the tenth flight seeing an improved, cost-reduced version called the HTV-X.
The first vehicle was launched on an H-IIB rocket, a more powerful version of the earlier H-IIA, at 17:01 UTC on 10 September 2009, from Launch Pad 2 of the Yoshinobu Launch Complex at the Tanegashima Space Center.
As of December 2020[update], a total of nine missions successfully launched — one each year for 2015–2019 (though there was no launch in 2017, pushing back the latest to 2020)  — one fewer total mission than had been planned in August 2013 at the time the fourth HTV mission was underway.
The improved version of the craft HTV-X is planned to be first used for the tenth flight and will perform scheduled ISS resupply duties for 2021-2024 (with a first launch scheduled for February 2022). In addition, JAXA has agreed to provide HTV-X logistic resupply flights to the Gateway mini-space station (launched by either Falcon Heavy or Ariane 6) as part of its Gateway contribution in addition to co-developing a habitation module with the ESA.
|HTV||Launch date/time (UTC)||Berth date/time (UTC)||Carrier rocket||Reentry date/time (UTC)||Outcome|
|HTV-1||10 September 2009, 17:01:56||17 September 2009, 22:12||H-IIB F1||1 November 2009, 21:26 ||Success|
|HTV-2||22 January 2011, 05:37:57||27 January 2011, 14:51||H-IIB F2||30 March 2011, 03:09 ||Success|
|HTV-3||21 July 2012, 02:06:18||27 July 2012, 14:34||H-IIB F3||14 September 2012, 05:27||Success|
|HTV-4||3 August 2013, 19:48:46||9 August 2013, 15:38||H-IIB F4 ||7 September 2013, 06:37 ||Success|
|HTV-5||19 August 2015, 11:50:49||24 August 2015, 17:28 ||H-IIB F5||29 September 2015, 20:33 ||Success|
|HTV-6||9 December 2016, 13:26:47||13 December 2016, 18:24||H-IIB F6||5 February 2017, 15:06 ||Success|
|HTV-7||22 September 2018, 17:52:27||27 September 2018, 18:08||H-IIB F7||10 November 2018, 21:38 ||Success|
|HTV-8||24 September 2019, 16:05:05||28 September 2019, 14:09||H-IIB F8||3 November 2019, 02:09||Success|
|HTV-9||20 May 2020, 17:31:00||25 May 2020, 12:13||H-IIB F9 (last)||20 August 2020, 07:07||Success|
|HTV-X1||February 2022 ||H3 F3||Planned|
In May 2015, Japan's Ministry of Education, Culture, Sports, Science and Technology announced a proposal to replace the HTV with an improved, cost-reduced version preliminary called HTV-X.
The proposal of HTV-X as of July 2015[update] is as follows:
Re-using the PLC design will allow minimizing the development cost and risk. Concentrating the reaction control system (RCS) and the solar panels on the Service Module will simplify the wiring and piping, to reduce the weight and manufacturing cost. Loading the unpressurized cargo outside the spacecraft allows larger cargo, only limited by the launch vehicle fairing. The aim is to cut the cost in half, while keeping or extending the capability of the existing HTV.
The simplification of the overall structure will allow the launch mass of HTV-X to be dropped to 15,500 kg from HTV's 16,500 kg, while the maximum weight of cargo will be increased to 7,200 kg (net weight 5,850 kg excluding support structure weight) from HTV's 6,000 kg (net 4,000 kg).
In December 2015, the plan to develop HTV-X was approved by the Strategic Headquarters for Space Policy of the Cabinet Office, targeting launch in fiscal year 2021 for the flight of HTV-X1 (Technical Demonstration Vehicle) by the H3 rocket. As of June 2019[update], new ISS plans from NASA's Flight Planning Integration Panel have set the launch of HTV-X1 for February 2022, which is on schedule.
With the Japan-US Open Platform Partnership Program (JP-US OP3) agreement in December 2015 to extend cooperation on ISS operations through 2024, Japan will provide its share of ISS operation costs with the form of transportation by HTV-X, and also be given an opportunity to develop a possible small return capsule.
The final form of the HTV-X consists of three modules: a lower, 3.5 m-long pressurised logistics module nearly identical to that of the HTV, elongated by 0.2 m and with a side access hatch added to allow late loading while mated to the rocket; a 2.7 m-long central Service Module capable of operating independently of the other modules, which contains two arrays of solar panels generating 1 kW of electrical power as opposed to the 200 W generated by the HTV, batteries capable of providing a peak output of 3 kW compared to the 2 kW of the original, and a 1 Mbit/s communication link in addition to the original 8 kbit/s link, though the main thrusters have been removed, so the HTV-X is purely reliant on Reaction Control System (RCS) motors mounted in a ring around the Service Module for propulsion, selected service module components have been mounted externally on the top for easy astronaut access. The last component is a 3.8 m long unpressurised cargo module, essentially a hollow cylinder with shelves that vastly expands the volume of unpressurised cargo.
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 to allow the pressurized cargo module to be swapped out for alternate modules, to add increased structural strength, and to accommodate the side hatch.
Other payloads being considered to replace the unpressurised cargo module while carrying out ISS resupply missions are an external sensor package, a technology trial of an IDSS airlock with automated station docking as used by the Progress and ATV craft, a trial of rendezvous and docking with a simulated satellite module, a smaller satellite piggybacking the launch to reach ISS orbit, a station return capsule, assembling a beyond Earth orbit mission such as lunar lander from smaller modules and acting as a space tug shuttling orbiting unpressurised cargo modules to the ISS allowing stuff such as recyclable materials, excess propellant and spare parts to be stored in orbit for future use rather than discarded.
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.
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.
As of 2014[update], both JAXA and Mitsubishi conducted studies of a next generation HTV as a possible Japanese contribution to the proposed international crewed outpost at Earth-Moon L2. This variant of HTV was to be launched by H-X Heavy and can carry 1800 kg of supplies to EML2. Modifications from the current HTV includes the addition of solar electric paddles and extension of the propellant tank.
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 a human-rated capsule for four people.
A Japanese space station has been proposed to be built up from HTV modules. This method is similar to how the modules in Mir, as well as many modules of the Russian Orbital Segment of the ISS are based on the TKS cargo vehicle design.
HTV-3 near ISS
Kounotori 5 (HTV-5) with Aurora australis
HTV-6 grappled to a robotic arm of ISS
The new HTV is known as HTV-X and is now planned to make its inaugural trip to the Station in February 2022.
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