Mitsubishi Heavy Industries Technical Review Vol. 58 No. 4 (December 2021)
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Development Status and Future Plans of
Next Generation Cargo Transfer Spacecraft HTV-XDAISUKE TSUJITA*1SATOSHI FUJIWARA*1

TAKESHI UCHIDA*1

ATSUSHI NAKAJIMA*2

The development of next generation cargo transfer spacecraft HTV-X is steadily progressing
toward the launch of the first unit. Currently, the detailed design has been completed and thespacecraft manufacturing has begun. One of the key points in designing HTV-X is to address
diversifying cargo requirements. Mitsubishi Heavy Industries, Ltd. (MHI) has carried out design
invents by detailed design that meets the constraints of the spacecraft structure form, thenconducted step-by-step prototype tests, and confirmed that these requirements can be satisfied.
HTV-X will be used to deliver supplies to the International Space Station (ISS) and a plan to usean advanced version of this spacecraft to contribute to resupply of the lunar orbit crewed base
(Gateway), which is being promoted mainly by the United States, is also under consideration. This
report presents the development status of HTV-X, the results of the detailed design and efforts for a
future plan related to an advanced version of this spacecraft.

|1. Introduction
Since the operation of the International Space Station (ISS) has been extended up to 2024,
next generation cargo transfer HTV-X to ISS, is being developed as a successor of HTV. The first
HTV-X is planned to be launched with the H3 launch vehicle in JFY 2022(1).
Like HTV, HTV-X has a highly-reliable spacecraft and is intended to deliver supplies on aregular basis in order to share common system operation costs for ISS operation. However, thesurrounding situation has changed compared to the time when HTV was developed, and the cargorequirements have been diversified. For example, three private resupply spacecraft (Cygnus,
Dragon and Dream Chaser) selected for the CRS2 (commercial resupply service 2) program
launched in 2019 by NASA in the United States are capable of meeting demand for delivering
cargo payloads with various transport constraints such as small animals. In response to this, abenchmark analysis of the capacity of private resupply spacecraft was conducted and thedevelopment specifications of HTV-X were determined so that it can provide equal or better
service with the greatest exposed and pressurized cargo carrying capacity that exclusively enables
the transport and disposal of large inboard racks and other capabilities (Table 1).
Currently, the detailed design of HTV-X has been completed and it has been confirmed thatthe system requirements including the capacity of cargo transport, which is the most important roleof a resupply spacecraft, can be satisfied and the spacecraft manufacturing has begun. In this report,
Chapter 2 explains the outline and development status of the HTV-X, Chapter 3 describes theresults of the detailed design with respect to the cargo requirements and Chapter 4 reports the
future prospects for the use of HTV-X in international space exploration.

*1

Chief Staff Manager, Engineering Department, Integrated Defense & Space Systems, Mitsubishi Heavy Industries, Ltd.

*2

Chief Staff Manager, Space Division, Integrated Defense & Space Systems, Mitsubishi Heavy Industries, Ltd.

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Table 1Comparison of capabilities and services of transfer vehicle to ISS
Japanesetransfer vehicle

Private transfer vehicle selectedby U.S. commercial resupply service (CRS2)
Dream Chaser
DragonCygnus(produced(produced(produced byby SNC)by Space X)
Northrop Grumman)

Item
HTV-X

Appearance
Loading/disposal
capacity (total ofpressurized andexposed)Pressurized cargo
Loading
capacity/type

Exposed cargoloading
capacity/type

Pressurized cargorecovery capacity
Cargo service
Late-accesscargo(3) reception
timing andloading capacity-◎5 , 820kg [No.1]〇4 , 070kg◎Advantage:
Transport/disposalof large rack (1)[only1]◎1 , 750 kg, 4 cargo
payloads◎Advantage: Large
cargo capacity[No.1]△Available: Dealt
with by the same
small collection
capsule that ismounted on HTV7-

3 , 754kg

3 , 307kg

5 , 500kg-

3 , 754kg

2 , 507kg

5 , 000kg

2 , 500kg to
5 , 000kg

Disposal only

800kg

1 , 500kg

500kg to1 , 500kg(1 to 3 cargo
payloads)

Not available

Available

Available

1 , 500kg

-

-

-

-

24 hours before launch 24 hours before launch

24 hours before
launchReception
timing:Unknown

24 hours before
launch, 6CTB to
10CTB

Total amount:
Unknown (including
power receiving cargo
4CTB)
Loaded in the main
hatch area

Standard cargoaround 5CTB
Loaded in theside hatch area

Loading
capacity:
Unknown
Loaded in theside hatch area

75 W/ch, 4 ch
RS-422/RS-485
communication

28 V power
supply, 75 W/ch,
6 ch
1553B/Ethernet/
RS422
communication

Power supply
andcommunication:
Available(details
unknown)〇Power receiving
cargo equivalent to2CTB(2) + Cargo
3CTB Loaded in themain hatch area
Resource
provision to
power receiving
cargo(4)(Reference)
Requirementsfrom CRS2〇28 V power supply,
75 W/ch, 2 ch
Ethernet
communication(Legend)◎：No.1, Only 1 level, 〇：Equivalent to others, △：Inferior to others
Supplement 1: Large rack: International standard type rack for cargo mounting (2 m × 1.05 m ×
0.86 m). Can be transported/disposed of without assembling/disassembling in ISS
only by HTV-X.
Supplement 2: CTB: Cargo transfer bag
Supplement 3: Late-access cargo: Cargo to be loaded immediately before launch
Supplement 4: Power receiving cargo: Pressurized cargo that requires power supply

Source: JAXA

|2. Development status of HTV-X(1)

Outline of spacecraft
Figure 1 shows the changes from HTV to HTV-X. In contrast to HTV, which consists offour modules, HTV-X consists of two modules: a pressurized module (PM) that accommodates
inboard supplies (pressurized cargo) and a service module (SM) that integrates the exposedcargo loading function and the flight function. Unlike HTV, HTV-X locates the PM on thelower part (launch vehicle side) and the SM on the upper part, which makes the SM structure
lighter and improves the transport capacity(2).
Moreover, HTV-X has the ability to execute services to provide on-orbit demonstration
opportunities in addition to the ability to transport cargo payloads. Experimental equipment isbasically loaded on the upper part of the SM together with exposed cargo payloads, but it canbe flexibly loaded in other areas according to the mission requirements. HTV-X can, after thedelivery of supplies to ISS, provide experimental opportunities for up to 1.5 years from the timeMitsubishi Heavy Industries Technical Review Vol. 58 No. 4 (December 2021)
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of leaving ISS to the time of re-entry (Figure 2).

Figure 1 Outline of HTV-X spacecraft

Figure 2(2)

Outline of HTV-X operation

Outline of development status
Figure 3 illustrates the development organization. Under the Japan Aircraft Exploration
Agency (JAXA) development contract, MHI are in charge of the development of the system(system design, loading of cargo/experimental equipment, launch site work and operational
preparation) and the pressurized module (PM) of HTV-X. Mitsubishi Electric Corporation is incharge of the development of the service module (SM).
The detailed design that can satisfy the mission requirements and system specifications
has been completed and the maintenance design phase has commenced. PM and SM havestarted manufacturing. At the same time, as the entity in charge of the system, we areproceeding with system tests to conduct verification in an assembled form where the PM andSM have been combined, the planning of launch site operation and the refinement of each
operational document.
In response to the fact that in 2020 JAXA has selected the experimental equipment to beinstalled on the first HTV-X, we started the design of the installation of the equipment on
HTV-X. Since experimental requirements such as flight altitude and attitude differ dependingon the experimental equipment, it is planned that the experiments will be conducted in a certain
order after leaving ISS (refer to Figure 4). Currently, we have completed the design review for
the installation of the experimental equipment and have started manufacturing and arepreparing for testing the interface between the experimental equipment and HTV-X.

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In order to maintain the development plan despite the deterioration of parts availability
and restrictions on face-to-face meetings due to the COVID-19 disaster, we have arranged andchanged the processes and are using IT technology to conduct remote meetings.

Figure 3

HTV-X development organization

Figure 4

On-orbit demonstrative experiment plan for the first HTV-X|3. Results of detailed design for satisfying cargo requirements
3.1(1)

Cargo loading
Cargo loading plan
The PM has a pressurized space with a volume of 39 m3 and can accommodate up to 313
standard bag-sized cargo payloads and up to 2 cargo payloads that require power supply(hereinafter referred to as power receiving cargo) such as small animal transport devices. Dueto the large quantity, the work of loading all cargo payloads takes several weeks. Therefore, thecargo loading timing during launch site work is divided into three parts in order to deal withcargo payloads with time constraints including power receiving cargo payloads.
In the first cargo loading, the majority of cargo payloads will be loaded onto the lone PM
module as normal access. In the second and third cargo loading, cargo payloads with time
constraints will be loaded after all modules of HTV-X are assembled and mounted on the
launch vehicle. This is called late access and such cargo payloads are scheduled to be received
and loaded three days before and twenty-four hours before launch (Figure 5).
In the case of HTV-X, unlike HTV, the PM that accommodates pressurized cargo
payloads is located at the lower part. Therefore, openings are provided on the launch vehicle
fairing and the payload attach fitting (PAF) in order to allow access from the bottom side of the
PM for late access (Figure 5).

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At the time of late access twenty-four hours before launch, in addition to cargo loading,form finalizing for flight including leak inspection after closing the hatch, final inspection of
external equipment, etc., will also be carried out. In order to complete the work before thelaunch vehicle starts moving to the launch pad, we are trying to shorten the work time throughapproaches including changing the leak inspection method and reducing the amount of external
equipment. Furthermore, in order to reduce the risk of launch delays due to problems such asprocedural mistakes during late access, it is planned that a late access demonstration will be
conducted using the PM and ground support equipment.

Figure 5(2)Outline of HTV-X launch site work and loading work of late-access cargo

Design of loading layout
Late-access cargo payloads such as power receiving cargo payloads will undergo
check-out and packing work performed by the cargo user until immediately before launch. Inorder to meet the time constraints associated with this, late-access cargo payloads will be placed
closest to the hatch so that they can be loaded last among all cargo payloads (Figure 6). In
order to prevent the opening/closing of the hatch from being hindered, a new loading structure
was provided in the passage space in the center of the PM in the layout design. This structure
was designed to be detachable so that the passage space can be secured after the cargo payloads
are unloaded in orbit. In addition, a simple-to-operate lock pin system was adopted as the fixing
method of this structure instead of fixing with bolts and nuts in order to allow astronauts toperform on-orbit attaching/detaching operation easily and to shorten the work time at the timeof late access. This design allows astronauts to access late-access cargo payloads first after
arriving at ISS, minimizing the process from receiving on the ground to unloading in orbit.

Figure 6 Loading form of pressurized cargo payloads (Left: Perspective view of PM,
Center: PM hatch access work, Right: PM main structure of first HTV-X)

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 4 (December 2021)
6(3)Development of ground support equipment (GSE)
Late access includes entering through the opening of the fairing/PAF and operating in the
narrow space between the launch vehicle and the PM. In order to enable this work, the groundsupport equipment (GSE) as shown below is being developed.
- A scaffold that can be easily placed/removed in a short amount of time and canaccommodate multiple workers. since standing directly on the launch vehicle is notallowed.
- A cargo loading device for loading cargo payloads weighing up to 90 kg through asmall opening in the fairing.
- A lifter for lifting cargo payloads weighing up to 90 kg through the PM hatch (1 m x 1m) in the narrow space on the work scaffold.
In particular, we have no on-HTV experience of loading cargo by lifting, so we made a
prototype lifter and a full-scale mockup (model) of the PM of HTV-X and conducted a
prototype test with the participation of actual workers (Figure 7). As a result, items that would
be reflected in the design of GSE could be obtained, such as providing a fixing method toprevent the lifter from tipping over when pushing cargo on the lifter into the loading structure.

Figure 7 Cargo lifter prototype test (Left: Appearance of full-scale PM
mockup, Right: Photo of lifting work)

3.2

Power supply to cargo

Electric power for power receiving cargo payloads is supplied from the cabin control unit
(CCU) of the PM. NASA demands strict power quality for the power supply function in order toallow the function to be used for a wide variety of cargo payloads.
Ready-made DC/DC converters (commercially available components for space use) do notmeet the power quality characteristics (impedance, power noise, inrush current, etc.) requirements
for use inside the CCU, so we newly developed a DC/DC converter dedicated to HTV-X. In orderto prevent rework, the development was carried out in the following three stages.
(1) BBM (breadboard model): It was confirmed by using BBM that the impedance
requirement, which was particularly strict, had been satisfied by reviewing the feedback
circuit system of the secondary output of the DC/DC converter, tuning the circuit
constants, etc.
(2) EM (engineering model): An EM was manufactured based on the BBM and tested in
combination with a simulator that simulated cargo power supply characteristics. Based on
the data acquired in this test, the interface was adjusted with NASA and it was confirmedthat the design was valid.
(3) PFM (proto flight model): If by any chance an interface inconsistency occurs at the timeof late access, which is conducted immediately before launch, postponement of the launch
is inevitable. Therefore, a joint interface test using a power receiving cargo and a CCU,
both of which are flight models, is planned to be conducted.

3.3

Cargo thermal control

Power receiving cargo payloads generate heat of up to 150 W. Like HTV, HTV-X is basedon a passive thermal design that does not have a heat exhaust device in order to maximize the cargocapacity. Under such a design constraint, in order to prevent deviation from the permissible

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temperature due to the heat generated by the power receiving cargo itself, the design feasibility was
ensured by taking the following two measures.
(1) Lowering the initial temperature
Normally, since satellites do not have openings on the PAF, air circulation inside the
PAF and the entire fairing is limited, so the fairing air conditioning effect is limited. Focusing
on the fact that power receiving cargo payloads are loaded near the boundary between the PM
and PAF, the in-PAF circulation of air cooled by fairing air conditioning was simulated with ananalysis model that incorporated the PAF opening, which has a shape unique to HTV-X. By
increasing the degree of analysis simulation and pursuing the cooling effect, the temperature of
the power receiving cargo could be lowered and the design feasibility could be ensured(Figure 8).

Figure 8 Analysis of fairing air conditioning (Left: Overview of analysis
model, Right: Air conditioning analysis result)(2)

Suppression of temperature increase during flight
HTV had a body-mounted solar cell panel and was always oriented toward the earth witha constant attitude. However, HTV-X uses an expandable panel in order to improve the power
generation efficiency and takes two postures during flight: an attitude oriented toward the sun
for power generation and an attitude oriented toward the earth for communication. When
HTV-X takes an attitude oriented toward the sun, the PM may be exposed to sunlight, whichcauses the power receiving cargo temperature to increase. The simple conditions in the case of
an attitude always oriented toward the earth similar to HTV lead to overly harsh evaluation, so
the excessive margin was deleted by setting the thermal analysis conditions to be closer to theactual environment, reflecting the actual flight attitude sequence conditions. Furthermore,unlike HTV, the multi-layer insulation around the outside of the PM hatch near power receiving
cargo payloads has been removed to improve heat dissipation. As a result, the temperature
increase of power receiving cargo payloads could be suppressed and the temperature profile
could be maintained within the allowable temperature range.

|4. Future prospects for the use of HTV-X in deep space exploration
In order to expand the area of human activity, the United States is promoting the Artemis
program, which aims to realize sustainable activities on the moon, in cooperation with participating
countries. A plan to use an advanced version of HTV-X for the Japanese contribution to resupply tothe lunar orbit crewed base (Gateway), which is a base element of the architecture of that program,is being discussed by the Space Development and Utilization Subcommittee of the Ministry of
Education, Culture, Sports, Science and Technology (Figure 9).
At least in the early stages of construction of Gateway, the duration of an astronaut's stay islimited to up to 30 days and autonomous operation of resupply spacecraft is required compared toMitsubishi Heavy Industries Technical Review Vol. 58 No. 4 (December 2021)
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ISS, in which astronauts are always present. For this reason, it is necessary to adopt an autonomous
automatic docking method instead of a docking method using a robot arm that requires operation
by an astronaut like that of HTV-X. Therefore, it is planned that the second HTV-X will beequipped with an automatic docking demonstration device and technology acquisition will be madethrough a demonstrative experiment of automatic docking with ISS.

Figure 9 International space exploration initiatives discussed by the Space Development
and Utilization Subcommittee (upper side) and concept of resupply to Gateway using
advanced version of HTV-X (lower side)

For cargo resupply to Gateway, a pressurized cargo quantity equivalent to HTV-X and apressurized cargo power supply 10 times or more that of HTV-X are required. In order to realize
the former, we are working on weight reduction by applying a composite material to thepressurized structure to maximize cargo transport volume. In order to realize the latter, we areconsidering adding a heat exhaust function by providing a radiator, etc., in the pressurized area.
While the Artemis program is progressing, the possibility of extending the operation of ISS
is increasing. The NASA Authorization Act approved by the U.S. Senate on June 8, 2021 calls forMitsubishi Heavy Industries Technical Review Vol. 58 No. 4 (December 2021)9the extension of ISS operation from 2024 to 2030. Since resupply is essential for ISS operation, it
is quite possible that Japan will continue to use HTV-X for resupply. Based on these surrounding
conditions, we are considering to partially incorporate the aforementioned composite material
structure and heat exhaust function, and pre-demonstrating them by utilizing a chance of anadditional HTV-X.

|5. Conclusion
This report introduced the fact that we have completed the detailed design of HTV-X so thatthe cargo requirements that have been diversifying due to changes in the surrounding conditions are
met and that HTV-X has a more advanced spacecraft than the HTV. In the future, in order toachieve the mission of the first HTV-X, MHI will proceed with manufacture of the PM and as theentity in charge of the system, steadily implement the system tests to conduct verification in an
assembled form, the launch site operation including cargo loading and the preparation ofoperational documentation. In addition, aiming at contributing to resupply in the Artemis programand utilizing the opportunities to verify technologies with HTV-X, we will proceed with theexamination of necessary new technologies and continue to the further development andimprovement of transport technology from low earth orbit to the vicinity of the moon.

References(1)(2)

The Material 60-5, Space Development and Utilization Subcommittee, Subcommittee on Research
Planning and Evaluation of the Council for Science and Technology, (#60 February 9. 2021)
Daisuke Tsujita, et al., Development of HTV-X, Mitsubishi Heavy Industries Technical Review, Vol. 56,
No. 1 (2019)