HTV-X

The HTV-X is manufactured by Mitsubishi Heavy Industries with contributions from Mitsubishi Electric.^[4] IHI Aerospace provides the propulsion system^[5] and American company Sierra Nevada Corporation provides the Common Berthing Mechanism and hatch kit.^[6]

HTV-X consists of three main modules, arranged from top to bottom on the launch pad:

• Unpressurised Cargo Module: A 3.8-metre-long (12 ft 6 in) open-top cylinder with shelving for payload attachment. Cargo is mounted externally 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: 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 of reaction control system (RCS) thrusters for propulsion, and mounts some components externally for easier astronaut access.^[7]
• Pressurised Logistics Module: A 3.5-metre-long (11 ft 6 in) section with an internal volume of 39.6 cubic metres (1,400 cu ft)^[8] 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. An early concept included a side hatch for late cargo loading, but the final 2021 design removed it; late access now uses the ISS docking hatch, requiring a new Payload Adapter Fitting (PAF) on the H3 rocket’s upper stage.^[1]

The HTV-X measures 8 metres (26 ft 3 in) in length. The H3 rocket’s payload fairing and dispenser were widened to 4.4 metres (14 ft 5 in) to allow alternate modules, increase structural strength, and accommodate the originally planned side hatch.^[2][9]

Reusing the pressurised logistics module design 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.^[10] 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).^[9]

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.^[9]

Comparison of uncrewed resupply spacecraft, the original HTV is third from the left, HTV-X is fourth from left

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 preliminarily called HTV-X.^[11][12] The proposal of HTV-X in 2015 was as follows:^[10]

• 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.^[13][9] As of June 2019^[update], NASA's Flight Planning Integration Panel had set the launch of HTV-X1 for February 2022.^[14]

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.^[15]

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.^[16]

As of April 2025^[update], three flights are planned to resupply the ISS.^[1]

As of December 2024^[update], HTV-X4 and HTV-X5 are tentatively planned for launch in JFY2027–JFY2029.^[18]

• Cargo spacecraft
• Comparison of space station cargo vehicles