European space tug set to fly: the Automated Transfer Vehicle (ATV), due to blast off towards the ISS on an Ariane 5 launcher in November, is one of Europe's core contributions to the ISS programme.

In a fitting culmination to the year that has seen the official adoption of a European Space Policy, if all goes according to plan, the month of November should see Europe's Ariane 5 launcher lift off for the first time with a European spacecraft bound for the International Space Station (ISS). The Automated Transfer Vehicle (ATV), due to blast off towards the ISS on an Ariane 5 launcher, is one of two major European contributions to the ISS programme. The other is the Columbus orbital laboratory, currently scheduled to ride into space on the US Shuttle in December (see box). EADS Astrium Transportation is prime contractor for both the ATV and Columbus programmes.

The ATV represents a way for Europe to pay its share in ISS running costs by spending money within European industry rather than by cash transfers to its ISS partners. Beyond this, the development of ATV technology gives ESA the capacity for automatic rendez-vous between spacecraft--crucial for robotic sample return missions, assembling complex spacecraft, and future human planetary exploration. Because of its interfaces with Russian and US ISS partners, this spacecraft also represents a good example of cooperation with Russian and US space programmes.

ATV development was formally approved at an ESA ministerial council meeting in October 1995. The inaugural launch is set to occur three years later than initially planned, due to "small technical problems and failures late in the processing of Jules Verne" and the overall technical complexity of the programme--the ATV is the largest and heaviest spacecraft ever built in Europe, apart from launch vehicles. It measures 9.79m in length and 4.48m in diameter (22.28m with solar panels deployed). It will have a launch mass of 20.75t and will carry 7,667kg of cargo to the space station. Propulsion is provided by four main engines, generating 490N of thrust and 28 attitude control engines (220N thrust).

Its mission is to resupply the space station with food, water, oxygen, fuel, equipment and scientific experiments. It will also be used to raise the station's orbit and to remove up to 6.5t of waste to be burned up in the atmosphere.

Autonomous navigation

During the initial 8 minutes of Ariane 5 flight, the ATV operates just like an upper stage rocket and then separates from its launcher. About 100 minutes after lift-off, the ATV fires its thrusters, manoeuvring on its own. It has the capability of navigating in orbit as an automatic spaceship towards the ISS. Normally, after three days in orbit, this fully robotic spacecraft docks with the station with precision and safety without any crew or ISS active role. ATV's inaugural mission, with "Jules Verne", will take some extra days in orbit to test several back-up scenarios before docking.

Using its own energy and life support system, the ATV becomes a new 45[m.sup.3] pressurised module docked to the station. ISS crews can load and unload hardware. From time to time, the ATV will accelerate the ISS with its own propellant in order to raise the ISS altitude which naturally decreases, due to the residual atmospheric drag, up to 200m per day. After six months in space, the ATV, loaded with 6.5 tonnes of used material and hardware, separates from the ISS, breaks up and burns up during a guided and safe re-entry high over the Pacific. Because of the variety of ATV objectives, the vehicle is described as the most versatile ISS spacecraft.

Service module

The ATV service module, which is not pressurised, includes propulsion systems, electrical power, computers, communications and most of the avionics. The avionics bay, which looks like a cylindrical ring 1.36m high, is located in the upper part of the service module. The ATV propulsion system provides the spaceship with the orbit transfer capability and the ISS reboost support.

The ATV navigates, as a fully automatic spaceship, with four main engines (490N thrust) plus 28 smaller thrusters (220N) for attitude control. All valves and thrusters are controlled by four control units connected to the main ATV computers. For launch, the ATV service module is mounted on the Ariane 5 launcher using a cylindrical adapter which has a locking and separation system which is jettisoned 70 minutes after the lift-off.

After deployment 100 minutes after liftoff, the four solar arrays reach a total span of 22.3m, providing electrical power to rechargeable batteries for eclipse periods in orbit. Silicium solar cells--spread on four carbon fibre reinforced plastic sandwich panels per array with a total surface of 33.6[m.sup.2] (4 x 8.4[m.sup.2)]--are able to produce an average of 4800 Watts. Mounted on the ATV service module, the four sun tracking arrays are totally independent and can get the best orientation to the sun thanks to rotating mechanisms.

The ATV service module also accommodates several rechargeable and non-rechargeable batteries and some redundant items like a sun sensor and a Russian-made KURS antenna. The avionics bay, which is the brain of the ATV, is located in a non-pressurised module which accommodates critical items like computers, gyroscopes, navigation and control systems and communications equipment. All these items are mounted on 10 equipment carrier trays which are protected from the temperature variations by state-of-the-art variable conductance heat pipes.

All the propellant tanks for the spaceship propulsion are located in the ATV service module, between the main engines and the avionics bay: there are eight titanium propellant tanks and two high-pressure helium tanks. The tanks hold up to seven tonnes of MMH and [N.sub.2][O.sub.4] propellants, part of which will be used for station attitude and orbit control.

After docking, the ATV can perform ISS attitude control, debris avoidance manoeuvres and raise the 183t Station's orbit to overcome the effects of atmospheric drag. In order to perform this re-boost the ATV may use up to 4.7 tonnes of its own propellant at intervals of 10 to 45 days. With its own flight-control and propulsion systems, the ATV has a high level of autonomy which allows it to stay in free flight for long periods of time, as well as to dock even if the Station is unmanned.

Automated, not automatic

EADS programme leader Nicolas Chamussy explains that the ATV is referred to as "automated", not "automatic", because at each stage of the mission a green light from ground control is required before proceeding to the next stage. There will be a total of five hold points for missions to the ISS. On the first flight of the Jules Verne prototype, the ATV will initially approach to 3km from the ISS. It will then retreat to a parking orbit before returning to a distance of 12m the next day. The actual docking will be performed on the third day.

Fuel reserves will allow for up to three or four docking attempts. The ATV must tolerate one failure and still complete its mission. Even with any combination of two possible onboard failures, the craft must still be safe for the crew and for the Space Station. The ATV software package, comprising one million lines of code, is claimed to be the most complex ever developed in Europe. In the event of a major failure during the approach phase to the ISS, or if any manoeuvre endangers the Station, a dedicated backup computer will intervene, commanding a special "retrieve" manoeuvre--using separate computers, separate software, separate batteries, separate trajectory-monitoring sensors and separate thrusters--to take the vehicle into a safe trajectory.

Software testing

Testing of ATV software is currently nearing completion in the Functional Simulation Facility (FSF) at EADS Space Transportation's Les Mureaux plant. This facility comprises a mockup of the equipment bay with all the onboard avionics, plus the Russian docking mechanism and associated electronics (supplied by Energia). The last of 61 simulations was due to be completed by the end of May. Only the GPS navigation unit remains to be installed on the outside of the space station, during an EVA scheduled for the month of June.

The total cost of the ATV programme amounts to 1 billion [euro] for development, including the Jules Verne prototype, and 800 million [euro] for an additional six spacecraft, to be launched at 18-month intervals. The total of seven ATVs will cover operating needs from 2007 to 2018.

Jules Verne is currently scheduled to go into orbit on an Ariane 5/ES, a specially modified version featuring structural reinforcement of the Ariane equipment bay--the first ATV represents more than double the heaviest single payload ever lifted by Ariane 5--and a minor modification to the reignitable Aestus engine on the EPS storable-propellant upper stage. The engine has to be reignited twice to place the ATV in the required trajectory for rendez-vous with the ISS (300km circular orbit with 51.6[degrees] inclination). A third ignition is then required for de-orbiting. The Columbus module is set for launch in December. Built at a cost of 880 million [euro], this is the successor to the Spacelab module that flew 16 times between November 1983 (Spacelab-1) and April 1998 (Neurolab). Columbus offers a minimum design life of 10 years and should remain in service until 2018. According to Philippe Berthe, who is in charge of future concepts at EADS, once the station has been completed in 2010, the goal will be to maximise the return on investment by using the facility for as long as possible ... possibly through 2020.

It could prove necessary to add new features to the ATV, such as the PARES Payload Retrieval System; or to use it as a non-pressurised cargo transportation system. It could also contribute to the development of the Crew Space Transportation System (CSTS), the ESA programme to develop an autonomous solution for ferrying European astronauts to the space station, currently planned as a cooperative effort with Russia. Such a system could be in service during the period of development of the US Orion capsule, which is scheduled to replace the Shuttle around 2015. The concept could be based around an upgraded Soyuz or a new vehicle for ISS and space exploration missions.

Berthe cautions that it will not be possible to develop a manned variant of the ATV. However, the competencies and technologies developed for ATV could provide inputs for the development of such a vehicle. The problem is that the ATV only has a manned spaceflight qualification for the ISS rendez-vous phase, not for launch or return to Earth. Human spaceflight issues must be taken into account from the outset, as they were for the Russian Progress spacecraft, which was derived from the Soyuz, a vehicle designed to carry astronauts from the outset.

with contributions from Duncan Macrae

RELATED ARTICLE: Columbus set for December shuttle lunch.

Columbus has been in storage at Kennedy Space Centre since May 2007. Handover to NASA for Shuttle integration is scheduled for September.

Columbus is a multi-purpose science laboratory, which is scheduled to be delivered to the ISS by the US Space Shuttle. This highly flexible facility comprises a pressurised module and several unpressurised external payload platforms.

At the heart of the 4.5m-diameter cylindrical module will be 10 experiment racks, each of which can accommodate a variety of scientific equipment. These facilities will enable ground-based researchers to conduct scientific experiments in life and physical sciences in space.

The research equipment, which has to be compact enough to fit into a confined space and robust enough to withstand years of service, has been designed to accommodate multiple users. It is also largely automatic and fully controllable from ground stations, since astronauts will have a limited amount of time to supervise the operations.

Columbus also has four mounting points where external payloads can be exposed to the space environment. Experiments installed on these platforms can be used in a wide range of scientific and technological investigations, such as the ability of organisms to survive in space, measurements of radiation from the sun and the stars, and studies of the way materials behave in the environment. It is also intended to place an earth viewing camera on u platform outside Columbus.

Researchers across Europe will be able to use the laboratory, controlling their own experiments directly from specialist User Centres or their own work places. The results of these experiments will ultimately be brought to a wider audience through ESA's Erasmus Experiment Archive, which already contains records of European microgravity research dating back to the 1960s. Their efforts will be channelled through the Columbus Control Centre in Germany, which will interface with the laboratory itself and also the other international partners centres around the world.