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A sophisticated mechanism to unroll the mast in microgravity. In the integration
hall in Bremen, the researchers test the smooth operation of this technique.
On the suction table, the team cut and glue the delicate film to create the sails. A
thread in the outer edge then ensures that the deployed sail will have the correct
tension.
The Gossamer team: (left to right) Patric Seefeldt, Siebo Reershemius, Peter
Spietz and Tom Spröwitz want to send their first solar sail into space in 2015.
These masts need to withstand a great deal before they
even begin to support the unfurled sails on their flight through
space. Rolled up on electrical spooling mechanisms, the launch
constitutes the first serious test, as they are subjected to forces
equivalent to 50 times the force of gravity. An aluminium struc-
ture would perhaps be more robust, but it would weigh too
much and would greatly limit the acceleration and speed of the
solar yacht. “The heavier the structure is, the less performance
the thrust from solar radiation can deliver. The alternative would
be to make the sails even bigger to compensate for the
increased mass.”
Test in microgravity
The mast and unrolling mechanism experienced micro-
gravity for the first time during a parabolic flight in 2009.
Straubel starts up the video. The scientists tested a variety of
methods for unrolling the masts over the Bay of Biscay. Uncon-
trolled deployment, with the assistance of air pressure and
finally the newly-developed unwinding mechanism. As though
drawn by a tiny thread, the electric motor deploys and reels
out the mast behind it. After a few seconds the motor disen-
gages from its mast as intended. Smiles all round for the
researchers – the test could not have gone any better. Eight
metres of mast unreeled within five or six seconds. “During
the parabolic flight, we only had a limited time for the experi-
ment. Up in space, the masts are deployed more slowly – that
is much safer.”
In their Integration Hall in Bremen, the scientists are, of
course, unable to escape gravity during testing. However, they
have developed their own gravity compensation system that
enables them to support the deployment mechanism, cancel-
ling out its weight. Problems that can be detected and
resolved on the ground should not pose a hazard once in
space. “For example, we tested how the masts behave when
being unrolled quickly or slowly,” says Bremen-based scientist
Tom Spröwitz. The scientists then went on to test sails and
masts separately. The next step is already clear: “We want to
unroll a mast and, at the same time, deploy two sails.” This all
helps to bring the researchers one step closer to their goal –
the first space mission.
the sails need to have? How must they be folded and stowed
for transport? How is it possible to ensure that they will deploy
correctly after launch and that they will deliver propulsion
throughout missions of extended duration? On the suction
table, the first thing required is craftsmanship. The initial step is
to stick one-metre wide strips of polyimide film together to
make a giant sail. After this, the edges of the fine material are
folded over and stuck down, and a reinforcing thread is inserted
in the edges of the sail. As the sails deploy down the length of
the mast, it is this thread that will impart the necessary stability
to the structure. “Then the sail needs to form a kind of
package,” explains Spröwitz – a space-saving package that can
be rolled up on two spools. Without the suction table to restrain
the light material for the team headed by Project Manager Peter
Spietz, this would be a very difficult task.
Eventually, the job is done; on carriages, the two spools
deploy outwards along two rails arranged at right angles to one
another. The scientists keep a careful eye on the sail as it unfurls
evenly. The slightest malfunction could cause the carriages to
jam on their rails. Up in space, however, no malfunctions are
allowed; the speed of the carriages must match and the spools
must not snag. Only then can the space yacht tap into the
power of the Sun and set sail. After launch, several cameras on
board the craft will transmit images to Earth – of the successful
deployment of sails, as well as any problems that may arise
during this premiere in space.
Feather-light mast
The mast test bench is located just a few metres from the
test facility that scientists are using to put the deployment of
their solar sail through its paces. The masts are also lightweight:
“A one-metre mast weighs just 40 grams, not even as much as
half a bar of chocolate,” says Marco Straubel from the DLR Insti-
tute of Composite Structures and Adaptive Systems, whose
team developed the masts and the unrolling mechanisms. To
ensure that the material remains stable and flexible, carbon
fibres are impregnated in liquid resin. Once the resin hardens, a
mast is created with walls just one tenth of a millimetre thick
and incredible bending properties. “Just like a spring, they
always return to their original shape.”
A roadmap for outer space
Once Gossamer 1, with its 25 square metres of sail and
its four masts, orbits Earth and continues on to plot its course
into space successfully, the bar will have been raised for the
ensuing missions; two years later, there are plans for
Gossamer 2, with a sail area of 20 by 20 metres, to set sail
500 kilometres above Earth. Cameras on board will document
how this craft can be steered. To accomplish this, scientists
will use cables to alter the position of weights inside the
hollow masts. This, in turn, will relocate the centre of gravity
of the spacecraft and alter its flight direction. On this mission,
the craft will weigh less than 60 kilograms. Gossamer 3 will
follow in 2019, this time with 50 by 50 metres of sail, in an
orbit more than 10,000 kilometres from Earth. “None of
these flights will carry a scientific payload – Gossamer is solely
a technology demonstration mission,” says Spröwitz.
Before any next step can be contemplated, the
gleaming silver sail will have to survive yet one more gruelling
test – in a facility in Bremen, it has to withstand space-like
conditions. The Complex Irradiation Facility (Komplexe
Bestrahlungseinrichtung; KOBE), bombards the aluminium-
coated film with a mixture of protons, electrons, ultraviolet
and vacuum-ultraviolet light, as well as sunlight. “We want,
of course, to use Gossamer on long-duration missions, which
means we need to know precisely how the material will
change in the process,” says Spröwitz. The extremely thin
material from which the sails are made needs to demonstrate
that it does not degrade over extended periods of time and is
ready for a sailing trip into the vast expanses of space.
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More information:
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Project leader Peter Spietz examines the delicate edges of the solar sail. The
material is just 0.007 millimeters thick. This feather-weight must be handled with
caution.
The masts that support the sails during flight must be both strong
and flexible. Tom Spröwitz, Head of the System Conditioning
Department at the DLR Institute of Space Systems knows that
carbon fibres impregnated with liquid resin are suitable.
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