With its HTV, the Land of the Rising Sun can now send equipment and provisions to the ISS.
HTV stands for H-II Transfer Vehicle as it is a cargo module that is launched by a Japanese H-IIB rocket, a more powerful version of the H-IIA. Although automatic, this spacecraft is of direct concern to manned space flights as it can be used for carrying 6 tonnes of equipment and provisions to the International Space Station.
Schéma du HTV Diagram of the HTV 1: docking port 2: pressurised section 3: solar array 4: unpressurised section 5: avionics module 6: propulsion module Lenght : 9,8 m - Diameter : 4,4 m - Mass at lift-off : 10,5 t + 6 t of cargo Credit: JAXA
A (temporary) Station module With the HTV, Japan is becoming even more involved in the orbital complex, where it already owns Kibo, the biggest of the laboratory modules. It is also joining the small club of nations, namely, the United States (with the shuttle), Russia (Progress cargo ships and Soyuz manned spaceships) and Europe (ATV cargo ships), capable of taking payloads up to the ISS on their own. Once docked to the Station (see the next paragraph), the HTV will become a temporary module of the orbital complex as the astronauts will have access to its pressurised section in order to remove the supplies it contains. The Japanese spacecraft is novel with regard to the European ATV and Russian Progress cargo ships, in that it is fitted with an unpressurised section, exposed to the space vacuum. Equipment intended to be automatically installed outside the ISS will, therefore, be directly grappled by the Station’s robotic arm, thus avoiding having to take it in through the orbital complex; an operation which can sometimes prove to be delicate. We would also point out that the Japanese Kibo laboratory has an external platform designed to take experiments which have to be exposed to the rigours of the space vacuum. The unpressurised section of the HTV will also enable experiments to be transferred to this platform without having to take up any of the Station’s living space.
An original docking procedure With the HTV, Japanese engineers found themselves confronted with a type of spacecraft that they had never developed. In addition, during the design phase, NASA asked JAXA to make more than 1,300 modifications before approving the Japanese cargo ship as a spaceship authorised to dock with the International Space Station. We would explain that we are talking about a total weight of 16 tonnes (6 tonnes of cargo + 10 tonnes for the spacecraft) launched at 27,000 km/h towards a group of modules housing 6 astronauts! And, even though the ISS is obviously orbiting the Earth at the same 27,000 km/h, the greatest caution is required to guarantee the safety of the crew. The HTV, equipped with its sophisticated avionics, will consequently join up with the Station in successive stages. A few days after its launch, it will make a first stop 23 km away. Then, the spaceship will be authorised to come within 7 km. If all goes according to plan, the automatic cargo ship will stop at just 500 m and then 300 m from the ISS. Following further checks, the final docking procedure will begin and the HTV will then move to less than 10 m from the orbital complex. During this phase, the astronauts in the Station will be able to command the spaceship to stop once again or to move away if necessary. But they should then capture the HTV using the ISS’ Canadarm 2 robotic arm in order to pull it into the docking port on the Harmony module (which communicates with the European Columbus, American Destiny and Japanese Kibo laboratories). This original procedure makes the final docking operation easier as the robotic arm can be used for grappling the HTV even if its final “parking” position varies by a few dozen centimetres. Whereas the standard docking procedure involving direct docking, such as that carried out by the American space shuttle, the Russian Soyuz rockets and Progress cargo ships as well as the European ATV cargo ships, requires a tolerance that must not exceed several centimetres. This clever Japanese scenario should therefore maximise the chances of success.
Below, a picture summary of a typical automatic HTV cargo ship mission (image credits: JAXA).
Launch via an H-IIB rocket from the Tanegashima base in Japan.
Having reached Earth orbit, the HTV separates from the second and final stage of its H-IIB launch vehicle.
Several steps later, the space cargo ship comes within reach of the Station’s Canadarm 2 robotic arm (made in Canada).
The arm, controlled by the crew of the orbital complex, grapples the HTV and docks it with the Harmony module. Once the airlock is open, the astronauts have access to the pressurised section of the Japanese cargo ship.
The elements transported in the non-pressurised section are taken by the Canadarm 2 arm and delivered directly to the outside of the Station.
After 30 days as a temporary Station module, the HTV is to be filled with elements that are no longer of any use to the orbital complex. Once detached, it is to be directed towards the upper layers of the atmosphere where it will be totally burnt up: the ISS’ “bins” will consequently be completely destroyed.
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