Manufacturing for Space – the ISS
In use for more than a decade, the International Space Station will remain in orbit until at least 2020. E&T visited Thales-Alenia Space in Turin where ISS pressurised modules were built.
Large-scale, high-tech manufacturing projects are often newsworthy and leave a visible legacy. However, the International Space Station (ISS) will never be seen by more than a few hundred lucky spacefarers because of its location, some 400km above our heads.
Despite this, the ISS is a hugely impressive feat of engineering, not least because its 450t of modules, solar arrays and supporting structures all had to be transported from the Earth.
However, considering the leading role of the American space agency Nasa, it comes as a surprise to find that half the pressurised volume provided by its habitable modules was manufactured by Thales-Alenia Space (TAS) in Turin, Italy.
Dr Dino Brondolo, director of infrastructure programmes at TAS, is 'extremely proud' of the fact that, back in December 2009, the Space Station Processing Facility at Kennedy Space Center contained 'only Italian hardware'. Tourists peering down from the viewing gallery could observe no fewer than five key ISS modules: three multipurpose logistics modules (MPLMs), a connecting module (Node 3) and the Cupola, a seven-windowed observation platform. Had they visited Turin in the preceding months and years, they would have seen the same modules in their evolution from CAD renderings to space-qualified hardware.
Designing for Space
At the basic mechanical level, design requirements for space hardware are both challenging and contradictory: spacecraft must be capable of withstanding the forces, mechanical vibrations and acoustic pressures experienced at launch, while being as light as possible, because launch mass is roughly proportional to launch cost. Among other things, this leads to the specification of light-weight 'aerospace materials', from titanium and magnesium to fibre reinforced plastics.
Once in orbit, the challenge is to ensure that the materials maintain their finely honed specification and overall integrity in the aggressive radiation and micrometeoroid environment of low Earth orbit.
While bursts of solar or cosmic radiation can cause 'bit-flips' and other deleterious effects in electronic components, large doses of either can prove life-threatening to humans, which is why all manned modules incorporate a degree of radiation shielding. If a vigorous solar storm is forecast, astronauts retire to a more heavily shielded section, typically surrounded by water tanks, to ride out the storm.
Space station modules are also required to incorporate protection against the ever-present risk of penetration and depressurisation by natural micrometeoroids or manmade orbital debris.
One mass-efficient design solution is the sacrificial shield, a layer of material that absorbs and disperses the kinetic energy of incident particles, in effect by sacrificing its own structural >< integrity. Debris strikes the shield and often penetrates it, but the smaller particles resulting from the impact have insufficient energy to penetrate the pressurised volume itself.
Fundamentally important though this is, when it comes to producing space-station modules in an industrial environment the manufacturing challenges are decidedly more mundane. According to Guido Mazzoni, TAS's technologies and production engineering manager, 'the day-by-day challenge is to meet the customer's requirement to be more and more competitive in terms of schedule and costs while at the same time maintaining high levels of performance and quality.'
More specifically, he points to the need to meet 'quite stringent dimensional tolerances' for such large structures, which is achieved by careful tooling design and 'an optimised manufacturing sequence.'
The interface flange of the modules must mate precisely with all similar docking ports on the orbiting station if an airtight seal is to be maintained. Machining of a flange is completed only after it has been welded to the cone section of the module 'in order to recover any deformation that occurs during the welding process itself'.
In general, the decision to weld structural components, rather than develop a mechanical connection, depends on two parameters: allowable leakage and mass. For permanent modules such as the nodes, says Mazzoni, it is 'quite impossible' to meet Nasa's air leakage requirements, over the operational lifetime of the station, with any kind of seal, 'so the design solution is to have as many welded joints as possible for the pressurised structures'.
Usefully, this also has a positive impact on the mass budget, as a welded joint is much lighter than the alternative mechanical solution, summarised by Mazzoni as 'two flanges with a seal and a large number of bolts, washers and nuts.'
The total pressurised volume at the 'assembly complete' stage is specified at 935m3, described by Nasa rather ambiguously as 'larger than a five-bedroom house,' or 'about 1.5 Boeing 747s.'
Maintenance and supply
People often ask what the ISS has done for science in the years since its first (Russian-built) module gained orbit in November 1998, but they are missing the point, at least to some extent.
The idea that research aboard the ISS would produce new materials, new pharmaceuticals and that elusive 'cure for cancer' was the product of a Nasa PR machine intent on justifying the cost. What it seemed to ignore was the challenge – and the triumph – of manufacturing a laboratory in multiple earthbound factories, launching its components on different vehicles from different launch sites, and flawlessly fixing them together 400km above the Earth.
The figures speak for themselves. Some 40 flights of the US space shuttle and the Russian Proton were required to complete the assembly, while flights to deliver crews and supplies to the station boost the total to over 100. More than 150 spacewalks – close to 1,000 hours, have been conducted in support of assembly. At the tenth anniversary of continuous human occupation, celebrated on 2 November 2010, the station had been visited by 196 individuals from eight nations.
As well as building ESA's Columbus laboratory module, Nasa's Node 2 and 3, and the Cupola as fixed modules, TAS has supplied three MPLM logistics modules, which have so far conducted ten missions between them. The MPLMs – named Leonardo, Raffaello and Donatello – are pressurised cylinders 6.6m long and 4.2m in diameter with a cargo capacity of 9,000kg.
Transported to the ISS by the Shuttle, they are removed from its payload bay and berthed to the ISS using the station's remote manipulator arm. Once docked, they can be used by the crew for a typical 12-day mission, before being loaded with rubbish and other waste materials, undocked and sent to burn up in the Earth's atmosphere.
Another non-permanent addition to the station for which TAS supplies the primary pressurised structure is ESA's automated transfer vehicle (ATV), which is also designed to deliver supplies to the ISS before conducting a destructive re-entry. Launched by Europe's Ariane 5, the ATV locates and meets with the ISS entirely autonomously. As well as being able to deliver about nine tonnes of cargo, the ATV serves as a 'space tug', using its own propulsion system to correct the station's attitude and boost its orbital altitude when necessary. One ATV, the Jules Verne, flew in 2008; a second, the Johannes Kepler, successfully docked with the ISS last month; three others are in preparation.
With the retirement of Nasa's shuttle due later this year, the agency has restricted its options for station crew delivery to the extent that the only vehicle currently qualified for the task is the Russian Soyuz. However, with a limited amount of seed-funding under its Commercial Orbital Transportation Services (COTS) programme, it is attempting to pass the baton to the US commercial sector, initially as suppliers of cargo delivery spacecraft, and later crew capsules.
This potential new market is led by SpaceX with its Dragon capsule, which performed a successful first mission in December 2010, and Orbital Sciences, which is fielding a spacecraft known as Cygnus.
Thales-Alenia's experience with space agency contracts has allowed the company to offer its modules in the commercial market and, in 2009, it signed a $245m contract with Orbital Sciences for nine Cygnus pressurised cargo modules (PCMs).
Under contract to Nasa, Orbital's first Cygnus is due to deliver cargo to the ISS in October 2011. 'This type of programme is really strategic for our company', says Luigi Maria Quaglino, TAS senior VP for space infrastructure and transportation. 'It allows us to demonstrate to ASI and ESA that we can export what we learned from [space agency] programmes to the commercial market'.
Nasa's recent extension of the space station's operational lifetime from 2015 to 2020 has allowed TAS to consider further re-supply opportunities. In addition to any further orders for Cygnus and the ATV, the company is already providing an enhanced version of its MPLM supply module as a permanent addition to the station. Having delivered its cargo, this renamed PMM (permanent multipurpose module) will remain docked to Node 1, explains Dino Brondolo, effectively providing 'an extra 70m3 free of charge'.
As ever, weight-saving measures are a fundamental part of spacecraft upgrades. For example, by constructing the shell of the Cygnus PCM from 3mm-thick aluminium, as opposed to the 3.2mm used for the ATV 'cargo carrier', some 50kg is saved.
Likewise, replacing the ATV's own meteoroid/debris protection shield with Kevlar (similar to that used in bullet-proof vests) and replacing internal payload racks with lighter versions makes the vehicle some 300kg lighter. 'When you consider that the cost of putting a kilogram of payload into orbit is between $30,000 and $40,000, this may be viewed as substantial,' says Brondolo.
As the ISS itself makes a transition from the construction phase to the utilisation phase the space manufacturing industry is beginning a transition from government to commercial contracts for station supply. For the moment, it is confined to the 'service entrance' application of cargo delivery, but in the next few years it seems likely to execute a transition to the more glamorous function of crew transport and, if necessary, rescue.
To most of us, looking up at the night sky, the International Space Station looks like a bright star, moving slowly through the constellations. To an astronaut, watching the sun glint off its metallic cylinders, set against the impenetrable blackness of space, it must look more like home.
As with other manufactured products, several types of welding are available for space-station modules. Thales-Alenia's earliest creations included the Spacelab and SpaceHab modules which operated from within the Space Shuttle payload bay and were welded using the conventional tungsten inert gas (TIG) process. However, according to Thales-Alenia's Guido Mazzoni, for the 'free-flying' ISS modules the company decided to graduate to a plasma welding process known as VPPA (variable polarity plasma arc).
'Our VPPA process has a significantly lower occurrence of defects,' explains Mazzoni, which results in a 'reduction of non-conformances and subsequent repair'.
TAS has also pioneered friction stir welding (FSW) for spacecraft components, and has applied the process, under contract to the Italian Space Agency, to a full-scale prototype of a cryogenic tank using an aluminium-lithium alloy. Mazzoni says the technique is 'really promising' in comparison with 'traditional fusion welding techniques,' because of the low defect occurrence, good repeatability and enhanced mechanical properties of the joint. However, the advantages are outweighed by the high cost of tooling. 'I know several applications in the train and ship manufacturing industries,' explains Mazzoni, 'but for the typical production rate of our modules, the traditional welding techniques are still the best compromise between quality, performance and cost.'
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