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 (8ft 10in) unit capable of independent operation. It includes two deployable solar arrays generating 1kW of power (replacing the original HTV's 200W side-mounted panels), batteries with a 3kW peak output (up from 2kW), and upgraded communications with a 1Mbit/s link alongside the previous 8kbit/s channel. HTV-X omits a main engine, using a ring of reaction control system (RCS) thrusters for propulsion, and mounts some components externally for easier astronaut access.
Pressurised Module (PM): A 3.5-metre-long (11ft 6in) section with an internal volume of 39.6 cubic metres (1,400cuft) and a payload capacity of 4,070 kilograms (8,970lb). 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 (26ft 3in) in length. It is launched by the H3 rocket with a payload fairing widened to 5.4 metres (18ft) (in contrast to normal 5.2 metres (17ft)), and Payload Adapter Fitting (PAF) widened to 4.4 metres (14ft 5in) to allow increased structural strength and accommodate the side hatch.
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. Simplification was expected to reduce launch mass to 15,500 kilograms (34,200lb) from HTV's 16,500 kilograms (36,400lb), while increasing maximum cargo mass to 5,850 kilograms (12,900lb) from 4,000 kilograms (8,800lb).
Other proposed payloads for replacing the unpressurised cargo module during ISS resupply missions include external sensor packages, an IDSS 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.
Comparison of uncrewed resupply spacecraft, the original HTV is third from the left, HTV-X is fourth from left
History
In May 2015, Japan's Ministry of Education, Culture, Sports, Science and Technology announced a proposal to replace the HTV with a redesigned, lower-cost successor preliminarily designated HTV-X. The 2015 proposal included the following design choices:
Reuse of the design of the HTV's Pressurised Logistics Carrier (PLC) as much as possible
Replacement of the Avionics Module and Propulsion Module with a unified Service Module
Placement of unpressurised cargo on top of the Service Module rather than within an Unpressurised Logistics Carrier (ULC)
In December 2015, development of HTV-X was approved by the Strategic Headquarters for Space Policy of the Cabinet Office, with the first flight of HTV-X1 (Technical Demonstration Vehicle) targeted for fiscal year 2021 aboard an H3 launch vehicle. As of June2019[update], NASA's Flight Planning Integration Panel scheduled the launch of HTV-X1 for February 2022.
Under the Japan-US Open Platform Partnership Program (JP-US OP3) agreement signed in December 2015, which extended cooperation on ISS operations through 2024, Japan agreed to provide part of its contribution to ISS operating costs through HTV-X transportation services and the possible use of a small return capsule.
An early concept included a side hatch on the Pressurised Module to permit late cargo loading, but the final design adopted in 2021 omitted this feature. Late cargo loading remained possible through a Payload Adapter Fitting (PAF) with an opening in the H3 upper stage.
Due to delays in the development of the H3, the first HTV-X launch was delayed until 2025. HTV-X1 was successfully launched on 26 October 2025.
Flights
As of May2026[update], three flights are planned to resupply the ISS. All times are in UTC. Future events are listed as "no earlier than" (NET) the stated date. Some dates are given in Japanese fiscal year (JFY), which runs from 1 April to 31 March of the numerated year.
↑"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.
1234JAXA (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.
12ＨＴＶ－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.
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