WATER PURIFICATION
Thirst 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.
40
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 thermo-
formed 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
The new tank
linings are
replacing the
existing rubber,
which tends to
leach into 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.
the EnGIneeR 13–26 OCTOBER 2008
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