HTV-X consists of three main modules, arranged from top to bottom on the launch pad:
Unpressurized Cargo Support System (UPCSS): Unpressurized cargo is mounted externally on top of Service Module, rather than inside the spacecraft, allowing larger items limited only by the launch vehicle fairing and increasing capacity compared to the original HTV.
Service Module (SM): A 2.7-metre-long (8 ft 10 in) unit capable of independent operation. It includes two deployable solar arrays generating 1 kW of power (replacing the original HTV’s 200 W side-mounted panels), batteries with a 3 kW peak output (up from 2 kW), and upgraded communications with a 1 Mbit/s link alongside the previous 8 kbit/s channel. HTV-X omits a main engine, using a ring ofreaction control system (RCS) thrusters for propulsion, and mounts some components externally for easier astronaut access.[8]
Pressurised Module (PM): A 3.5-metre-long (11 ft 6 in) section with an internal volume of 39.6 cubic metres (1,400 cu ft)[9] and a payload capacity of 4,070 kilograms (8,970 lb).[2] Identical in design to the original HTV, it is positioned at the bottom of the stack to improve mass distribution.
The HTV-X measures 8 metres (26 ft 3 in) in length. It is launched by the H3 rocket with a payload fairing widened to 5.4 metres (18 ft) (in contrast to normal 5.2 metres (17 ft)), and Payload Adapter Fitting (PAF) widened to 4.4 metres (14 ft 5 in) to allow increased structural strength and accommodate the side hatch.[2][10]
Reusing the pressurised logistics module design from previous HTV reduces development cost and risk. Concentrating RCS thrusters and solar arrays on the service module simplifies wiring and piping, lowering weight and manufacturing cost. External loading of unpressurised cargo enables larger payloads, limited only by the fairing. The goal is to halve costs while maintaining or extending HTV capabilities.[11] Simplification was expected to reduce launch mass to 15,500 kilograms (34,200 lb) from HTV’s 16,500 kilograms (36,400 lb), while increasing maximum cargo mass to 5,850 kilograms (12,900 lb) from 4,000 kilograms (8,800 lb).[10]
Other proposed payloads for replacing the unpressurised cargo module during ISS resupply missions include external sensor packages, anIDSS connector for automated docking, satellite rendezvous trials, piggyback small satellites, station return capsules, assembly of lunar mission modules, and acting as a space tug to store resources such as recyclable materials, excess propellant, and spare parts in orbit for future use.[10]
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 theCabinet Office, targeting launch in fiscal year 2021 for the flight of HTV-X1 (Technical Demonstration Vehicle) by theH3 Launch Vehicle.[14][10] As of June 2019[update], NASA's Flight Planning Integration Panel had set the launch of HTV-X1 for February 2022.[15]
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 possible use of a small return capsule.[16]
An early concept included a side hatch of Pressurised Module for late cargo loading, but the final 2021 design removed it; late access now uses the ISS docking hatch, requiring a Payload Adapter Fitting (PAF) with access opening on the H3 rocket’s upper stage.[1]
Due to the delay ofH3 development, the launch of the first HTV-X slid to 2025.HTV-X1 was launched successfully on 26 October 2025.[4]
^"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. Retrieved20 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. Retrieved18 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. Retrieved17 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. Retrieved4 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. Retrieved18 July 2016.
^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. Retrieved26 December 2023.
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