Clean water in space

As the UK provides ISS crew with clean-water tank linings, NASA develops a liquid waste recovery system. Siobhan Wagner reports


Astronauts on the International Space Station are now able to enjoy a clean drink of water thanks to tanks lined with a specialist polymer formed in the UK.

Fluorinated ethylene propylene (FEP) flexible bladders, manufactured by Holscot, are now replacing the rubber that once lined water tanks used on the space station since its inception.

The first set of new tanks was integrated into the European space ferry Automated Transfer Vehicle (ATV) and launched this summer by an Ariane 5 Heavy Lift Rocket as part of the EU’s contribution to the continuation of the ISS.

Martin Daff, sales director of Holscot, said the bladders are made from two vacuum formings, each 900mm in diameter and 300mm deep. The bladders are used in tanks that hold 300l of drinking water.

‘They’re the biggest vacuum formings ever done in FEP,’ claimed Daff.

Getting the new bladders on board the space station was a long and drawn-out process. Daff said it was in 2000 when Man Technologie Germany (now MT Aerospace) first approached Holscot to develop a unique FEP-lined tank.

FEP, which is inert, was to be used instead of the rubber liner. The rubber tended to leach into the water and NASA decided to replace it with a material that would not contaminate the potable water.

Daff said Holscot designed the new tanks, first using half-size models, which they tested by filling up and emptying many times over.

The fabricated liners were thermoformed from Holscot’s own extruded FEP film. Daff said the moulds used to make the vacuum formings were made with ‘high-quality aluminium’ by a company that specialises in supplying mouldings for Formula One applications.

The film was vacuum formed in two parts and welded to form a circular tank construction. The top half of the tank was designed to pull down and expel the water under zero-gravity conditions. The lower half of the tank is bonded on to its inner shell. For this, the FEP had to be externally treated using Holscot’s own proprietary etching process. The stainless steel tank was then over-wrapped with filament-wound carbon fibre.

Daff said over time Holscot’s team came up with the ideal way to vary the wall thickness of the bladder membrane from 0.5mm to 1.5mm.

The reason for this, explained Daff, was that NASA wanted the membrane as thin as possible for the weight factor. ‘This,’ he said, ‘is all right for the bottom part of the lining because it is bonded to the tank and doesn’t have to move. But the top part gets flexed and pulled, and so has to be a little bit thicker and a little more resilient.’

After Holscot performed tests on the bladders, NASA carried out its own trials. Daff said everything was finally given the green light this year for installation onboard the space station. ‘It’s been a long old process,’ he said.

Once the astronauts use up the water from the bladders, said Daff, they are back-loaded with waste and released to fall back into the earth’s atmosphere and burn up.

Water is an important resource on board the space station, but the cost and logistics involved in transporting it there are becoming increasingly burdensome. The issue will be further compounded next month when the crew of the station will double in size from three to six — meaning more water will be required.

To help ease this problem, NASA has been working on a new ‘Water Recovery System’ that will recycle liquid wastes — meaning urine, sweat, or leftover water used for bathing or food preparation — by filtering it through a series of chemical processes, turning it into safe drinking water.

The agency estimates that the recycling system will reduce the dependence on Earth re-supply by cutting the amount of water needed to be launched by 6,800kg a year.

NASA’s water recovery system will be taken to the space station on the STS-126 mission, scheduled to launch next month.