HTV-8 departs ISS ahead of destructive re-entry

by William Graham

Japan’s Kounotori H-II Transfer Vehicle has completed its eighth mission at the International Space Station following launch aboard an H-IIB carrier rocket, berthed mission and departure.

The H-II Transfer Vehicle (HTV), also known as Kounotori, is the Japanese contribution to a fleet of uncrewed spacecraft that fly cargo and resupply missions to the International Space Station (ISS).

Along with the Russian Progress, US Dragon and Cygnus and formerly the European Automated Transfer Vehicle (ATV), it can deliver supplies, equipment and experiments to the astronauts aboard the outpost.

The Kounotori 8 (HTV-8) mission is the penultimate flight of the HTV, which is to be replaced with the enhanced HTV-X in the early 2020s.

Japan’s contributions to the ISS program are managed by the Japan Aerospace Exploration Agency, JAXA. The launch, JAXA’s eighth to the station, comes on the tenth anniversary of the first HTV launch. Launched on 11 September 2009 (10 September in UTC) the Kounotori 1, or HTV-1, spacecraft spent six weeks berthed at the ISS before ending its mission with a planned destructive reentry on 1 November.

The Kounotori 1 mission also marked the maiden flight of the H-IIB rocket and the first use of the second pad of the Tanegashima Space Centre’s Yoshinobu Launch Complex, both of which will be used for the launch.

Manufactured by Mitsubishi Electric, HTV measures about ten meters (33 feet) in length and 4.4 meters (14 feet) in diameter. It has a mass of up to 16,500 kilograms (36,400 lb), including up to 4,100 kg (9,000 lb) of pressurised and 1,900 kg (4,200 lb) unpressurised cargo.

HTV is designed for five days of free-flight either side of a 45-day stay at the International Space Station, with the ability to loiter on-orbit for seven days in the event of a problem during initial berthing attempts.

Primary propulsion for the HTV is provided by four IHI Corporation HBT-5 thrusters, fuelled by monomethylhydrazine and mixed oxides of nitrogen (MON-3, a mixture of three percent nitrogen monoxide and 97% dinitrogen tetroxide), while twenty-eight 120-Newton (27 pound) reaction control thrusters are used for attitude control and maneuvering. Surface-mounted solar cells on spacecraft’s exterior generate power for its systems.

Kounotori is designed to carry both pressurized and unpressurized cargo to the Space Station in two compartments. The Pressurised Logistics Carrier (PLC) is located at the nose of the spacecraft and incorporates the Common Berthing Mechanism that will secure it to the station. Once berthed, astronauts will enter the PLC and access its cargo.

Behind this, the Unpressurised Logistics Carrier (ULC) contains an Exposed Pallet with additional cargo to be accessed from outside of the station. For the Kounotori 8 mission, a Type III Exposed Pallet will be used, which is designed to be used in conjunction with the Mobile Base System on the US Orbital Segment. The other type of pallet, Type I, is designed instead to be mounted to the Exposed Facility of the Japanese Kibo module.

CanadArm2, the space station’s robotic arm, and its Dextre attachment are used to extract the pallet from Kounotori, and to reinstall it prior to departure.

The Exposed Pallet is loaded with six Orbital Replacement Units (ORUs) for batteries on the station’s Integrated Truss Structure (ITS). These consist of lithium-ion batteries which will replace the original nickel-hydrogen units that launched with the truss segments.

The EP for HTV-8 via JAXA

Once these have been installed – involving a series of spacewalks – three quarters of the truss batteries will have been replaced with a final batch of six replacements due to launch aboard the next Kounotori mission next year. The old batteries will be loaded into Kounotori 8 for disposal, burning up with the spacecraft when it reenters the atmosphere at the end of its mission.

The spacewalks will be completed next year.

The Pressurised Logistics Carrier incorporates a new racking system developed for the next-generation HTV-X spacecraft, which increases the number of Cargo Transfer Bags that can be carried from 248 to 316. Each bag measures 50.2 by 42.5 by 24.8 centimeters (19.8 by 16.7 by 9.76 inches), providing a volume of about 50 liters (3,050 cubic inches). The cargo includes provisions and fresh food for the space station’s crew, as well as experiments that will be conducted in the station’s Japanese Experiment Module (JEM), Kibo.

The Cell Biology Experiment Facility – Left (CBEF-L) contains a centrifuge to support biological and other experiments that require artificial gravity. CBEF-L will join the existing Cell Biology Experiment Facility (CBEF), providing new capabilities to simulate a greater range of gravity conditions and to facilitate experiments on larger animals than mice.

The Hourglass experiment, also known as Gravitational Dependence Research of Flexible Surface on a Planet is a materials research project which will use the CBEF centrifuge to investigate how powders and granular materials behave in microgravity and low-gravity conditions. Samples will be tested in cylindrical and hourglass-shaped containers, with the experiment aimed at giving better insights on how surface dust or sand might behave on planets and moons.

The Small Optical Link for International Space Station (SOLISS) will test optical communications with a gimballed laser and receiver assembly and engineering camera to be mounted outside the space station on the IVA-Replaceable Small Exposed Experiment Platform (i-SEEP). A partnership between JAXA and Sony, SOLISS can send and receive laser communications to and from the ground via a 1,550-nanometre beam. While the engineering camera’s primary purpose is to observe operation of the gimbal, its images can also be transmitted to the ground as part of the experiment.

Three small satellites are also being carried aboard Kounotori 8, for deployment from the International Space Station via the JEM Small Satellite Orbital Deployer (J-SSOD). Built to the CubeSat standard, these spacecraft will be released from the Kibo module’s airlock later this year.

The University of Tokyo’s Aqua Thruster Demonstrator (AQT-D) is a three-unit CubeSat which will test the Aqua Resistojet Propulsion System (Aquarius-1U) in orbit. This will eject water vapor from the satellite to generate an impulse, adjusting the satellite’s orbit. A water propulsion system has been proposed as a way of extending the lifespan of small satellites deployed from the space station without endangering the crew or outpost by carrying traditional propellants. AQT-D will attempt to validate this in space. The satellite also carries a UHF communications payload.
NARSScube-1 is a single-unit CubeSat built by Egypt’s National Authority for Remote Sensing and Space Sciences (NARSS). Equipped with a miniature camera with a 200-meter (650-foot) resolution, the satellite will record images of the Earth and transmit them back to its operators, while providing them with experience and demonstrating technologies ahead of future missions. It follows on from the identical NARSScube-2, which was deployed from a US Cygnus spacecraft in August.

The final CubeSat aboard HTV-8 is Rwanda Satellite 1, or RWASAT-1. Rwanda’s first satellite, RWASAT-1 carries a communications payload that will collect and forward data from remote monitoring stations on the ground. The satellite also carries two cameras for Earth observation and will serve as a technology demonstrator.

Mitsubishi Heavy Industries’ H-IIB rocket is used to launch the Kounotori spacecraft. The H-IIB is a modified version of Japan’s workhorse H-IIA rocket, featuring a wider first stage with two LE-7A engines instead of the single engine used on the H-IIA. Used only in conjunction with the HTV, this launch marks the H-IIB’s eighth and penultimate flight.

JAXA has taken lessons learned with both the H-IIA and H-IIB to develop a next-generation rocket, H-III, which will reduce the cost per launch of Japanese satellites. H-III is expected to make its first flight in late 2020 or 2021 and will take over HTV launches when the enhanced HTV-X is introduced.

The launch used the second pad of the Yoshinobu Launch Complex at JAXA’s Tanegashima Space Centre. The Yoshinobu Complex was constructed for the original H-II rocket in the 1990s, initially consisting of a single pad. This was subsequently converted for H-IIA missions, and in the early 2000s a backup pad was built for the H-IIA close to the original.

H-IIA never flew from the backup pad, which was later repurposed for the H-IIB. All of the H-IIB’s missions have been flown from this second pad, while the H-IIA continues to fly from its original pad.

Prior to launch, the H-IIB was integrated atop a mobile launch platform in Bay 2 of the Vehicle Assembly Building 350 meters (1,150 feet) northwest of the pad.

The platform, with the rocket atop, was then rolled into position ahead of liftoff. In the hours leading up to launch the rocket was fuelled: the rocket’s first and second stages both burn cryogenic propellant: liquid hydrogen and liquid oxygen, which was vented and topped off throughout the countdown as they are liable to boil off.

It was around this time during the first launch attempt two weeks ago that a fire was seen on the webcast. It was notably close to the rocket and has since classed as between two of the solid rocket motors.

The cause of the fire was later revealed to be due to an increase in the concentration of liquid oxygen used to cool the H2B rocket engine.

“Liquid oxygen usually diffuses in the wind, but at that time the wind was weak and the concentration increased, making it easier to ignite. After the liquid oxygen concentration became high, it may have been ignited by static electricity.”

The stack was rolled back off the pad for further checks before returning for the new launch attempt on Tuesday.

This time the count was trouble-free.

About three seconds before launch the first stage’s twin LE-7A engines ignited. At the zero mark in the countdown – termed X-0 for Japanese launches – four solid rocket motors attached to the first stage also ignited and the rocket began its ascent.

The solids are SRB-A3 motors, which provide additional thrust during the early stages of flight. These burned for 108 seconds before exhausting their propellant. The boosters separated in pairs, fifteen and eighteen seconds after burnout.

Three minutes and thirty-eight seconds into flight, with the rocket at an altitude of about 119 kilometers (74 miles, 64 nautical miles), the payload fairing separated from the nose of the H-IIB. This structure, designed to protect Kounotori 8 during its ascent through the atmosphere and to ensure the rocket has a consistent aerodynamic profile, is no longer needed once the rocket reaches space and can be discarded to save weight. The fairing split into two halves, which fall away from the vehicle.

H-IIB’s first stage continued to burn until Main Engine Cutoff (MECO), at the five-minute, 44-second mark in the mission. Having expended their supply of fuel the two first stage engines will shut down, with the spent stage separating eight seconds later. Eleven seconds after stage separation, H-IIB’s second stage ignited its LE-5B engine for an eight-minute, eleven-second burn.

HTV-8 heading into orbit, as envisioned by Mack Crawford for NSF/L2

This second-stage burn took Kounotori 8 directly into its initial orbit. JAXA has stated that this will be a 200 by 300 kilometer (124 by 186 mile, 108 by 162 nautical mile) orbit, although the actual apogee – or highest point – is likely to be slightly higher than this. Orbital inclination will be 51.6 degrees, matching that of the space station.

At fifteen minutes and five seconds mission elapsed time, HTV-8 separated from the upper stage of its carrier rocket. Eighty-four minutes after separation the second stage restarted for a short disposal burn, firing for about 64 seconds to remove itself from orbit, ensuring a safe reentry.

After separation, Kounotori 8 underwent initial activation and checkout, before spending the first few days of its mission making a series of maneuvers ahead of rendezvous with the International Space Station on the fourth day of the flight. Kounotori moved into position ten meters (33 feet) away from the station, where it was grappled by the CanadArm2 remote manipulator system (RMS) under the control of the outpost’s crew. After capture, the spacecraft was be berthed at the nadir, or Earth-facing, port of the Harmony module.

Kounotori remained berthed until November 1, as the crew unloaded its cargo and replaced it with material and hardware to be disposed of. At the end of its stay, CanadArm2 was used to remove Kounotori from its berth and released it away from the station.

HTV grabbed by the SSRMS – via NASA

After departure the spacecraft will be commanded to deorbit, leading to its destruction during re-entry over the Pacific Ocean. The HTV is not designed to be recovered, so this process ensures its safe destruction with any surviving debris falling harmlessly into an uninhabited region of the ocean.

The launch was the second in what has been a quiet year for Japan, following on from the successful deployment of a group of experimental satellites by a small Epsilon rocket in January.

Japan may launch up to two more rockets by the end of the year – with H-IIA rockets believed to be slated to carry a military communications satellite and an IGS reconnaissance satellite to orbit – although because of the secretive nature of these launches accurate dates are not currently available.

The next HTV mission, HTV-9, is currently expected to launch next May, marking the final flight of the current-generation HTV spacecraft and the H-IIB rocket.

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