HTV-X consists of three main modules, arranged from top to bottom on the launch pad:
A 3.8-metre-long (12 ft) unpressurised cargo module, which is a hollow cylinder with internal shelving. This module significantly increases the volume available for unpressurised cargo.
A 2.7-metre-long (8.9 ft) central service module, capable of independent operation. It features two solar panel arrays generating 1 kW of power (compared to 200 W on the original HTV) and batteries with a peak output of 3 kW (up from 2 kW). Communications are also upgraded, with a 1 Mbit/s link in addition to the previous 8 kbit/s link. The HTV-X lacks a main engine and instead uses a ring of reaction control system (RCS) thrusters for propulsion. Some components are mounted externally for easier astronaut access.[7]
A 3.5-metre-long (11 ft) pressurised logistics module, similar in design to that of the HTV but extended by 0.2 metres (7.9 in). This module is now located at the bottom of the stack rather than at the top. An initial design proposed a side access hatch for late cargo loading while the spacecraft was mated to the launch vehicle. However, the final 2021 design eliminated the side hatch. Instead, late access is provided through the same hatch used while docked to the ISS. This required the development of a special Payload Adapter Fitting (PAF) on top of the H3 rocket’s upper stage.[1]
The HTV-X has a length of 6.2 metres (20 ft), or 10 metres (33 ft) with the unpressurised cargo module fitted. Atop the H3 rocket, the payload fairing and payload dispenser have been widened from 1.7 to 4.4 metres (5 ft 7 in to 14 ft 5 in), allowing the pressurised logistics module to be swapped out for alternate modules, to add increased structural strength, and to accommodate the originally planned side hatch.[8]
Re-using the pressurised logistics module design of HTV allows for minimizing the development cost and risk. Concentrating the RCS and the solar panels on the service module will simplify the wiring and piping, reducing weight and manufacturing cost. Loading the unpressurised 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.[9] By the simplification of the overall structure it was expected the launch mass of HTV-X to be dropped to 15,500 kilograms (34,200 lb) from HTV's 16,500 kilograms (36,400 lb), while the maximum weight of cargo will be increased to 5,850 kilograms (12,900 lb) (net weight, excluding support structures) from HTV's 4,000 kilograms (8,800 lb).[8]
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 for resources such as recyclable materials, excess propellant and spare parts to be stored in orbit for future use rather than discarded.[8]
Comparison of uncrewed resupply spacecraft, the original HTV is third from the left, HTV-X is fourth from left
To re-use the design of HTV's Pressurised Logistics Carrier (PLC) as much as possible, except for adding a side hatch for late cargo access after the spacecraft-launch vehicle integration.
To replace the Unpressurised Logistics Carrier (ULC), Avionics Module, and Propulsion Module with a new Service Module.
To load the unpressurised cargo on top of the Service Module rather than inside the spacecraft.
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 Launch Vehicle.[12][8] As of June 2019[update], NASA's Flight Planning Integration Panel had set the launch of HTV-X1 for February 2022.[13]
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.[14]
A design change in 2021 removed the side hatch of the pressurised module.[1]
As of 2021[update], an evolved version of HTV-X called HTV-XG is being considered for transporting cargo to the Lunar Gateway as part of the Artemis program.[15]
^"新型宇宙ステーション補給機（HTV-X（仮称））プロジェクト移行審査の結果について" [New Space Station Resupply Vehicle (HTV-X (provisional name)) Project Progress Review] (PDF). JAXA (in Japanese). 6 December 2017. Retrieved 20 June 2019.
^ abcdJAXA (14 July 2016). ＨＴＶ‐Ｘの開発状況について(PDF) (in Japanese). Archived(PDF) from the original on 15 July 2016. Retrieved 18 July 2016.
^ abＨＴＶ－Ｘ（仮称）の開発（案）について(PDF) (in Japanese). 2 July 2015. Archived(PDF) from the original on 20 July 2015. Retrieved 17 July 2015.
^2016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. Retrieved 4 June 2015.
^宇宙基本計画工程表（平成27年度改訂）(PDF) (in Japanese). Strategic Headquarters for Space Policy. 8 December 2015. Archived(PDF) from the original on 20 October 2016. Retrieved 18 July 2016.
^JAXA (30 June 2021). アルテミス計画に関する各国の開発状況について(PDF) (in Japanese). Ministry of Education, Cultura, Sports and Technology. Retrieved 9 July 2021.
^"HTV-X1". Next Spaceflight. Retrieved 26 October 2024.
^ ab"宇宙基本計画⼯程表 (令和５年度改訂)" [Basic Plan on Space Policy (2023 Revision)] (PDF) (in Japanese). Cabinet Office. 22 December 2023. p. 45. Archived(PDF) from the original on 25 December 2023. Retrieved 26 December 2023.
^宇宙基本計画工程表 （令和６年度改訂）(PDF) (in Japanese). Cabinet Office, Japan. 24 December 2024. Retrieved 21 June 2025.
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