HTV-X reaches the ISS on maiden flight
On October 26th, at 00:00 UTC, the Japan Aerospace Exploration Agency (JAXA) launched an H3 rocket from the Tanegashima Space Center; inside the 5.4-m-wide fairing, atop the rocket, there was the first HTV-X vehicle (H-II Transfer Vehicle - X), marking the first launch of the next generation of HTV, which is a family of uncrewed, expandable, berthing resupply vehicles.
This launch comes more than five years after the final launch of HTV to the ISS, with delays caused by rocket development and the COVID pandemic.
HTV family - A look at what brought us here
HTV internal design, explained below. Credit: JAXA
JAXA started developing a resupply vehicle for the International Space Station in the early 2000s, coming up with the idea of the H-II Transfer Vehicle (HTV), also known as “Kounotori” (“white stork,” representing the analogy of this particular bird delivering babies around the world with HTV delivering vital cargo to the ISS).
After years of development, the maiden launch of the HTV occurred on September 10th, 2009, while the 9th and final launch occurred in May 2020.
Overall, the design of the HTV was divided into modules, from top to bottom:
Pressure Logistics Carrier (PLC): the PLC sat atop the vehicle, and it was designed to carry pressurized cargo to the ISS, including up to 8 International Standard Payload Racks (ISPRs) and a 300-kg water tank. Each ISPR had an internal volume of 1.6 m^3 and a payload capacity of 700 kg, so the pressurized payload carried would vary based on the internal configuration of the vehicle. After the Shuttle’s retirement in 2011, HTV became the only vehicle able to deliver ISPRs to the ISS for a long time.
The PLC also housed the Common Berthing Mechanism on the very top, which was the berthing point where the vehicle would be connected to the station; this was the only module the crew could access, through a vestibule, and it housed the majority of the cargo carried.
UnPressurized Logistics Carrier (UPLC): this module contained the minority of the payloads carried (up to 1500 kg maximum of unpressurized payload out of 6700 kg total) attached to the Exposed Pallet, which could only be reached with a robotic arm through a hatch, to then be installed on the outer part of the station.
Avionics module: this module sat just below the overall payload modules, and it contained all the avionics for the control of the vehicle, including flight computers, electrical and navigational equipment, 2-kW internal batteries, 0.2-kW solar panels laid flat on the vehicle’s body, and communication equipment able to transmit up to 8 kbit/s of data.
Propulsion Module: the lower and final module of the vehicle, this contained the propulsion system for the control of the spacecraft: it included 4 main 500-N R-4D bipropellant thrusters and 28 secondary RCS 110-N R-1E thrusters, all developed by Aerojet Rocketdyne; both kinds of thrusters used MMH and MON3 as fuel and oxidizer, respectively, feeding from 4 tanks containing a total of 2400 kg of propellant.
The vehicle’s overall size was 10 m long (3.3 m for the PLC, 3.5 for the UPLC, 1.2 for the avionics module, and 2 for the propulsion module) and 4.4 m wide, with a mass of 10.500 kg empty and a payload capacity of 6000 kg (4.5-5.2 tons of pressurized cargo, the rest unpressurized).
Thanks to its solar panels and battery power, it usually stayed berthed to the ISS for 45 days, plus an additional 2 weeks for the free-flying time between going and returning.
A usual mission included the launch and insertion into a 200X300 km X 51.6° orbit, followed by a journey lasting several days to reach the ISS; once within reach (literally a few meters), it would slow down until it stopped in order to be grappled by the Canadarm2 robotic arm on the ISS and be berthed onto a node, where it would stay for 45-60 days, during which the cargo would be unloaded and stored. Then, upon its departure, it would be loaded with trash and other disposable materials, and the Canadarm2 would grapple it again and release it far from critical ISS structures; now on its own, HTV would begin maneuvering away from the space station, reentering Earth’s atmosphere after a few days.
HTV-X
Comparison between HTV (left) and HTV-X (right). Credit: Ansur_nied/JAXA
The HTV-X vehicle was first proposed in 2015 as an improvement of the HTV, which already had several successful resupply flights under its belt, with a launch date set to be in 2021.
This was then moved to 2022, as it’s common in the aerospace industry for design-related delays; however, the vehicle’s development came to a major setback with the COVID pandemic and the subsequent delays in the development of the H3 rocket. These 2 factors combined cause the launch date to slip by 3 years, putting the first launch in late 2025.
The design has been greatly optimized and improved, so let’s go explore the new vehicle, which is now divided into 3 modules, from top to bottom:
Unpressurized cargo module: this hollow, 3.8-m-long, cylinder-like structure sits at the top of the vehicle and can accommodate up to 1750 kg of unpressurized payload, including technological demonstrators.
Service module: a 2.7-m-long module, it sits at the center and is the combination between the avionics and propulsion modules on the HTV; the structure is composed of an inner central cylinder, 1 m wide, and an outer octagonal cylinder. The combination of these 2 structures has been developed to sustain the loads of the payloads and hardware installed overall on the vehicle. The service module also contains all the avionics for the control of the vehicle, such as 3 Flight Computers (FCs) that are each able to conduct the 3 main operations needed for the control of the vehicle: GNC (Guidance, Navigation, and Control), data processing, and system management (so reading and reacting to the signals sent by the spacecraft); this means that even if 2 FCs failed, the third one would still be able to control the spacecraft on its own.
The service module also contains the communication system, which includes 2 kinds of antenna: the Tracking and Data Relay Satellite antenna (TDRS antenna) and the Proximity Communication System (PROX) antenna. The TDRS antenna is used when the vehicle is far away from the ISS, while the PROX antenna is used for direct communication to the ISS when the vehicle is near, as well as for ground communication. Both antennas send and receive data thanks to a transceiver and a telemetry and command processing unit, with a rate of 1 Mbit/s.
Another subsystem housed in the service module is the power subsystem, which is composed of 1-kW solar panels that are now extensible and can tilt by up to 30° to increase solar power intake based on the season; these solar panels are used to charge the secondary 3-kW batteries (the primary batteries aren’t rechargeable), which increase the power that the spacecraft has, allowing for longer missions.
The propulsion subsystem is also housed on the service module, and the main thrusters have been eliminated in favor of a ring of secondary RCS thrusters, optimized to give as much thrust and control as possible while limiting their number; the propellant capacity has also been increased by 30% compared to the HTV. There are also 3-4 navigation lights on this module: white, red, and green.
Pressurized cargo module: this 3.5-m-long module is designed to carry pressurized cargo, offering great improvements compared to the HTV’s PLC. The design initially included a side hatch for late payload installation on the rocket, but this was later eliminated. The pressurized module also houses the CCU (Cabin Control Unit), which includes a circulation fan, lights, smoke sensors, O2/CO2 sensors, pressure sensors, a vent relief valve, and temperature/humidity sensors. This module can hold 4069 kg of cargo, which crew members can access through a hatch on the bottom, where the CBM for berthing is.
So we can see many changes and improvements compared to HTV, including the easier berthing procedures, better flexibility for unpressurized cargo, better redundant systems, and more power. This power, in particular, allows HTV-X to remain docked to the ISS for up to 6 months, followed by a secondary mission after unberthing, something that wasn’t possible with HTV: after being unberthed, HTV-X will remain in space for up to 18 months to conduct a technology demonstration mission and release small satellites, even in orbits higher than the ISS (up to 500 km).
HTV-X1
HTV-X 1 approaching the ISS. Credit: NASA TV / Arstechnica
HTV-X1 represents the first HTV-X mission, and it launched on October 26th at 00:00 UTC aboard an H3 rocket. About 14 minutes after the launch, the spacecraft was deployed in the correct orbit, starting the journey on its own: during the following couple of days, it deployed the solar panels and started testing out its main systems.
Then, on October 29th, HTV-X1 began the final approach to the ISS, getting closer and closer; meanwhile, inside the space station, Japanese astronaut Kimiya Yui was preparing to grapple the spacecraft with Canadarm2, the station’s robotic arm; this was completed at approximately 15:58 UTC, followed by berthing at 11:10 UTC on October 30th, 2025.
The spacecraft will stay berthed to the ISS until January 2026, and then it will begin a 3-month period of technological demonstration missions.
Cargo for ISS
The vehicle is carrying about 4250 kg of total cargo, including 4000 pressurized and 250 unpressurized. Among the pressurized cargo, interesting payloads include crew supplies, some components for the Japanese Kibo module, more components for the ISS (like DRCS, Demonstration of Removing Carbon Dioxide, or NORS, Nitrogen/Oxygen Recharge System tank), 6 cubesats to be released from the ISS, and commercial payloads like private passports from Japan Airlines’ customers that will get delivered to them once they return to Earth on a future Dragon mission, and sensors to detect the air quality ahead of future use on commercial space stations.
The non-pressurized cargo only includes i-SEEP (IVA-replaceable Small Exposed Experiment Platform).
Technological demonstrations
These payloads will be tested during the 3-month phase after the unberthing from the ISS. Among those, there’s the Ten-Koh 2 cubesat, which will demonstrate the capability of HTV-X to release cubesats at altitudes higher than the ISS (it will be deployed at 500 km). There is also the Mt. FUJI experiment to determine the distance from the ground and the accuracy of the attitude determination of the spacecraft through SLR (Satellite Laser Ranging).
More technologies for the future construction of giant in-space structures will also be tested, including the mechanics for a deployable lightweight panel, the lightweight flat antenna DELIGHT, and the test of SDX next-generation solar cells.
References
JAXA | Next generation vehicle“HTV-X”
H-II Transfer Vehicle - Wikipedia
New unmanned cargo transfer spacecraft (HTV-X) | JAXA
