IEEE Spectrum October, 2010 - 47

need to be made of extremely durable, yet
lightweight materials-materials that may
not yet exist. In addition, its power system
will need advanced photovoltaics and fuel
cells capable of generating enough power
to operate the radars, navigation system,
communications gear, and the electric
motors that will turn the airship's giant
propellers. A lot of extreme engineering
is going into today's airship designs.

Emily CoopEr

h i l e t h e u p c o m i n g s t r at o s p h e r i c
surveillance airships will carry relatively small payloads, some airships
now in development will lift a great deal
more-payloads of hundreds of tons,
albeit at lower altitudes. That presents
an entirely different set of challenges.
An airship designed to carry 50 metric
tons of cargo would be hundreds of meters
long and weigh tens of tons lying empty of
helium on the factory floor. The sheer size
would make its assembly a daunting task.
These new vehicles would likely be built
in smaller subsections that would later be
joined together in immense hangars.
A more critical issue is how to compensate for the sudden increase in the airship's static lift that occurs when a heavy
payload is unloaded. The most straightforward remedy is to add onto the airship
an amount of weight equal to the payload
as the payload is removed.
Some heavy-lift designers are also
developing hybrid vehicles. These incorporate the static lift of helium along
with some form of dynamic lift, such
as helicopter-style rotors or airplanelike wings. In most of these designs, the
helium is sufficient to lift the vehicle's
weight, while the dynamic lift is devoted
to the payload's weight. This produces
an aircraft that is slightly heavier than
air and so is much less buoyant during
cargo unloading.
Lockheed's Skunk Works first testflew its P-791 proof-of-concept hybrid airship in 2006. The aircraft has two propulsion motors on the exterior of its envelope
and two attached to its tail. This generates
about 20 percent of the dynamic lift when
the vehicle is flying forward. Other hybrid
airships under development include
Hybrid Air Vehicles' SkyCat, which will
be the basis for the U.S. Army's LEMV;
the Worldwide Aeros Corp.'s Aeroscraft,
which was recently submitted to the FAA
for design certification; and the proof-ofconcept Dynalifter, being readied for test
flight by Ohio Airships.
spectrum.ieee.org

airS h ipS in Space
Earth isn't the only place where airships
could find a home. A number of offworld destinations have enough of an
atmosphere to support airships, including
mars, Venus, and Saturn's largest moon,
titan. compared with robotic rovers,
airships would be able to survey far more
ground. And they don't require much
power, which is always at a premium
on planetary missions. So far, the only
successful extraterrestrial deployment
was the Soviet union's 1984 Vega mission
to Venus, which sent two balloons to float
54 kilometers above the planet's surface
for nearly two days.
more recently, NASA has funded a
number of balloon and airship projects for
solar system exploration. researchers at
the Jet propulsion Laboratory in pasadena,
calif., have developed several lighter-thanair designs for use on a possible follow-up
mission to titan and Venus. they've also
conducted extensive flight tests over
the mojave Desert with an 11-meter-long
autonomous airship. there's even talk of
sending entire fleets of rovers and airships
to scout out new planetary frontiers.

While these hybrids hold promise, they
also have some inherent technical challenges. For one, the additional dynamic lift
increases aerodynamic drag. To help with
generating dynamic lift, they also typically
have a flatter profile than conventional airships, but this shape gives them a higher
ratio of envelope fabric to gas volume,
increasing the airship's empty weight.
Higher weight and drag, of course, mean
more propulsive power and more fuel,
both of which make the ship even heavier.
And some hybrids employ multiple lobes
in their design, which can create problems
as the gases inside heat up from the sun's
rays. Helium conducts heat six times as

efficiently as air, so a multi-lobed hybrid
may tend to list toward the side that's not
exposed to the sun.
Perhaps the biggest issue, though, is
the hybrid's potential to pitch nose up or
down and to roll from side to side. A conventional, single-hulled airship avoids
this problem because the majority of its
gas volume is positioned well above its
center of gravity, imparting what's known
as pendulum stability. The higher up the
center of lift is, the more stable the airship
is; conversely, the closer the center of lift is
to the center of gravity, the greater the tendency of pitching from wind gusts.
To get around these problems, Boeing
and the Canadian company SkyHook
International are collaborating on a different approach: a rotary-airship hybrid.
It combines a conventional ellipsoidal
envelope with four powerful helicopter
rotor units, which are installed below the
helium envelope. The helium is sufficient
to support the weight of the vehicle itself,
leaving the full power of the rotors to lift
a 36-metric-ton payload. One of the first
applications of the SkyHook is moving
equipment and supplies for oil-drilling
operations in northern Canada.
To spur further progress in heavy-lift
designs, I and several other airship enthusiasts are setting up an international contest to promote the development of airships as a green, low-carbon form of cargo
transport for commercial operations. The
Zero Emissions Transport Airship Prize,
or Z-Prize, similar to the more familiar
X-Prize, will offer a large cash award for
the successful development and flight test
of a heavy-lift airship that meets the competition's criteria. We hope to entice airship developers to focus their efforts on
designing cost-effective cargo airships
that will have their greatest applications in
developing regions-places where moving
freight by conventional transport is difficult and hugely expensive or subject to disruption by criminals or terrorists. And by
emphasizing airship designs with low carbon emissions, we hope also to encourage
the creation of the first environmentally
sustainable air-transport system.
It's an exciting time to be an airship
engineer. These vehicles represent both
the oldest and now the latest forms of aircraft. They're also an aviation technology
that has yet to be fully exploited. While
some naysayers may think the time of
these leviathans is long past, in fact their
day is just dawning.
o
octobEr 2010 * iEEE SpEctrum * NA

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Table of Contents for the Digital Edition of IEEE Spectrum October, 2010