AEROSPACE – SATELLITES
performing breast stroke, rising and
falling in the water as she inhales and
exhales. In fact, a class of submersible
craft called gliders propel themselves
through the water using exactly this
technique. In the first part of the stroke,
a rubber bladder toward the rear of
the glider inflates with compressed
air, buoying it up and increasing its
displacement (size; this also has a
lifting effect). In the second part of the
stroke, the system recompresses the air,
deflating the bladder and forcing the craft
down and forwards.
In that case, the craft changes
displacement, while its mass remains
the same. In the case of the Phoenix,
displacement remains the same, while
mass increases as air is pumped
in. That’s mass, rather than weight,
because it – not weight – contributes
to momentum, which determines how
hard it will crash-land, and so the risks
it poses to earth-dwellers. That was also
a key aspect of gaining airworthiness
approval. (In contrast, weight is the force
exerted on an object due to gravity). Total
mass includes not only the airframe and
payload, but also the volume of air in the
bladder and in the wings, plus even the
helium in its 135m3 volume. The test
vessel’s mass was limited to 150kg,
because that was the maximum that
would be approved by the UK aviation
authorities; more massive aircraft require
flight approval at a European level, which
adds time and cost.
So unlike an underwater glider,
Phoenix’s mass increases but its volume
stays the same when it breathes in.
That means necessarily its pressure
increases, slightly (operational pressure
ranges from 5-20 mbar). That its
shape does not change is important,
aerodynamically speaking. For any kind
of inflatable craft, a sphere has the
lowest, and therefore most efficient,
ratio of surface area to volume, explains
Andrew Rae, professor of experimental
and applied aerodynamics at the
University of the Highlands and Islands
(UHI). But, since spheres are not easy to
manoeuvre, air ships have tended to be
made in a tapered, teardrop-shaped form,
symmetrical along the long axis, and
resembling a wing in profile. The Phoenix
fuselage is close to a standard NACA
(National Committee for Aeronautics)
0030 airfoil.
That did not mean that engineering the
outer envelope was straightforward. As it
was extremely lightweight, the 172m2 of
woven, coated fabric consumed two-fifths
of the craft’s mass budget. In addition
to weight, another vital parameter of
the woven Vectran fabric chosen was
its resistance to stretch under tension.
Once given a polyurethane coating, the
fabric needed to be helium-proof; and any
stretch would tear the coating, creating
pinprick holes through which the helium
would escape. With an atomic number
of two, this gas has one of the smallest
nuclei of any material known, making it
difficult to contain.
The airship’s skin sits at the core of
a fundamental physical constraint of the
shape. From a design perspective, the
success of the airship rests on finding the
best balance between volume and surface
area. The volume of helium determines
the vessel’s maximum carrying capacity.
Although the vessel was a success at
its size, building a version that would
navigate the stratosphere will require
another design, according to Rae. The
team cannot simply scale up the trial
craft’s existing dimensions, because the
vessel needs more helium to be able
to reach 60,000 ft and carry a larger
payload. The team estimates that its 10kg
design payload may need to be increased
by a factor of 10 to be most useful in
stratospheric missions.
However, a bigger ship of the same
design could not reach operating pressure
before its fabric would be torn apart.
Bulking up the fabric to withstand the
so-called hoop stresses imposed by the
contained gases would add extra weight,
reducing its carrying capacity. And even if
the same fabric were used, it would also
require additional treatment to resist UV,
which would further add weight.
Instead, potential ideas for a larger
craft include tandem inflated sections
joined together, or inserting spherical
helium bags inside a larger rigid teardrop
vessel skin. Rae says that the team
is hopeful of assembling funding from
multiple sources for another trial, this
time to take 100kg to 60,000ft. The
first step toward that would be another
conceptual design.
SEAM DISTRESS
David Banks is director of Banks Sails, the
industry partner responsible for buoyancy
and the airship structure. He recalls that
once the fabric for the ship was chosen and
coated, the cut pieces had to be attached
to one another to form the teardrop-shaped
airframe. Normally, polyurethane adheres well
to itself, so seams of material coated with it
can be permanently joined together by using
a high-temperature ‘welding’ machine.
The Vectran fabric, chemically speaking
an aromatic polyester spun from a liquid
crystal polymer did not weld well. To get a
good seal, technicians had to use higherthan
recommended temperatures during the
welding process. Unfortunately, doing that
melted the fabric’s polyurethane coating,
rendering the seam permeable.
The team eventually got around the
problem by inserting a piece of solid PU film
in the weld area between the two layers of
coated Vectran. Although doing so generated
a small amount of additional weight, it
ultimately saved more than it cost, as that
prevented the need to apply a heavier, meltresistant
PU coating over the entire ship
surface.
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