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HONOLULU, HAWAII - THURSDAY
JUNE 26 2003
An
remotely piloted plane that set an altitude record two years ago
broke apart during a test flight today and crashed
into the Pacific Ocean, according to NASA officials:
The
Helios Prototype solar electric plane crashed some
distance off Kauai inside the test area of the US Navy's Pacific
Missile Range Facility at Barking Sands. The news
was released by the National
Aeronautics and Space Administration.
Helios
was a $15 million dollar, solar-electric project.
She was propeller-driven and had a
wingspan of 247 feet. Described by some as more like a flying wing
than a conventional plane. Helios
reached an altitude of 96,500 feet during a flight in 2001
also from Barking Sands. The roughly 18 mile altitude, was
considered by NASA to be a record for a propeller
powered
winged aircraft. It was designed for atmospheric
science and imaging missions as well as relaying telecommunications
up to 100,000 feet.
The $15 million dollar solar-electric, Helios
airplane
This
flight was to test the plane's fuel cell system.
According to Alan Brown, a spokesman for NASA's Dryden
Flight research Centre, Edwards, California, USA, Helios
was 30 minutes into the flight at around 8,000 feet west
of Kauai, when the aircaraft broke up over water. The cause
of the crash is unknown. NASA is said to be forming an
accident investigation team.
Helios
had been flying under the guidance of ground-based
mission controllers for Aero Vironment Inc. of Monrovia,
California USA, the plane's builder and operator. It was one of
several remotely piloted aircraft whose technological
development NASA has sponsored.
The
prototype, powered by solar cells during the day and by
fuel cells at night. This technology is similar to
that employed in Solar Navigator A spokesman
said: NASA intends to develop another Helios aircraft,
calling it "technology worth pursuing."
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The
Helios Prototype is the latest and largest example of a
slow-flying ultralight flying wing designed for
high-altitude, long-duration Earth science or
telecommunications relay missions. A follow-on to the
Pathfinder and Pathfinder-Plus solar aircraft, the
Helios Prototype soared to 96,863 feet altitude in
August 2001, setting a new world record for sustained
altitude by winged aircraft, powered only by energy from
the sun.
Developed
by AeroVironment, Inc., of Monrovia, Calif., under
NASA's Environmental Research Aircraft and Sensor
Technology (ERAST) project, the unique craft was
designed to demonstrate two key missions: the ability to
reach and sustain horizontal flight near 100,000 feet
altitude on a single-day flight, and to maintain flight
above 50,000 feet altitude for almost two days, the
latter mission with the aid of an experimental fuel
cell-based supplemental electrical system now in
development.
The
Helios Prototype is an enlarged version of the Centurion
flying wing that flew a series of test flights at Dryden
in late 1998. The craft has a wingspan of 247 feet, 41
feet greater than the Centurion, 2 1/2 times that of the
Pathfinder flying wing, and longer than the wingspans of
either the Boeing 747 jetliner or Lockheed C-5 transport
aircraft.
The
remotely piloted Helios Prototype first flew during a
series of low-altitude checkout and development flights
on battery power in late 1999 over Rogers Dry Lake
adjacent to NASA's Dryden Flight Research Center in the
Southern California desert.
In
upgrading the Centurion to the Helios Prototype
configuration, AeroVironment added a sixth wing section,
a fifth landing gear pod and a differential Global
Positioning Satellite (GPS) system to improve
navigation, among other improvements. The additional
wingspan increased the area available for installation
of solar cells and improved aerodynamic efficiency,
allowing the Helios Prototype to fly higher, longer and
with a larger payload than the smaller craft.
During
2000, more than 62,000 bi-facial silicon solar cells
were mounted on the upper surface of Helios' wing.
Produced by SunPower, Inc., these solar arrays convert
about 19 percent of the solar energy they receive into
electrical current and can produce up to 35 kw at high
noon on a summer day.
The
second milestone established by NASA for its development
Š a long-endurance demonstration flight of almost two
days and nights Š required development of a
supplemental electrical power system to provide power at
night when the solar arrays are unable to produce
electricity. AeroVironment developed an experimental
fuel cell-based electrical energy system combining
advanced automotive fuel cell components with
proprietary control technology designed for the harsh
environment above 50,000 feet altitude.
The
first version of this system combines gaseous hydrogen
from two pressurized tanks mounted on Helios' outboard
wing sections with compressed oxygen from the atmosphere
via a series of proton-exchange membrane fuel cell
"stacks" mounted in the central landing gear
pod. The system produces more than 15 kW of
direct-current electricity to power Helios' motors and
operating systems, with the only by-product being water
vapor and heat. The system will increase the Helios
Prototype's flight weight by about 800 lb to about 2,400
lb.
Two
other versions of the system are contemplated: One,
employing liquid hydrogen, would enable the Helios to
fly for up to two weeks in the stratosphere anywhere
around the Earth, not limited to temperate or equatorial
latitudes. Another version, a closed or
"regenerative" system, uses water, a fuel
cell, and an electrolyzer to form a system similar in
function to a rechargeable or "secondary"
battery, but with much greater efficiency than the best
rechargeable battery systems.
A
production version of the Helios with the regenerative
fuel cell system is of interest to NASA for
environmental science, the military and AeroVironment
for various roles, primarily as a stratospheric
telecommunications relay platform. With other system
reliability improvements, production versions of the
Helios are expected to fly missions lasting months at a
time, becoming true "atmospheric satellites."
The
Economist: Projects UAVs Like Helios May Soon
Be Doing Work of Satellites 07-17-2003
HYPERSONIC
drone aircraft that could bomb any target in the world
from a base in America recently grabbed headlines in the
Guardian and other newspapers. But fast and flashy is no
substitute for slow and steady. UAVs (unmanned aerial
vehicles) that can loiter over the same spot for months
are likely to be of more lasting military and commercial
significance. They
are also hard to build. The most advanced prototype of
such a UAV, a solar-powered craft called Helios, was
destroyed on June 26th when it crashed into the Pacific.
The cause of the crash is still unknown, although
turbulence is thought to have been a factor.
Helios
was built by AeroVironment, a Californian company that
specialises in innovative engineering. (Its founder,
Paul MacCready, built the first human-powered aeroplane.)
Unfortunately, the recent bad luck of NASA, which
manages the programme, seems to have rubbed off—and
the speed with which an accident investigation team was
assembled is testimony to NASA's current investigative
fervour. Unlike the ill-fated space shuttle Columbia,
however, Helios was exactly the sort of programme that
NASA should be funding—an unmanned craft that is
pushing technology to its limits.
The
ultimate aim is to create a pilotless aircraft that can
loiter over a particular spot on the ground, at an
altitude of around 20km, for up to six months. This
would allow the craft to serve as a "geostationary"
communications relay, substituting for satellites that
now do the job. Being closer to the ground than such
satellites (which are in orbit almost 36,000km away),
means that the transmitters and receivers involved could
be less powerful, and more information could be
transmitted at far less cost. Hovering UAVs could also
serve as sentries, watching over a country's coastline
for smugglers and terrorists, and as military
reconnaissance platforms. And both communications and
reconnaissance UAVs would be significantly cheaper to
build and launch than their satellite equivalents.
But
there are still many hurdles to be jumped before
solar-powered UAVs can loiter for months at a time. Most
observers estimate that such flight durations are at
least a decade away. When it crashed, Helios was
preparing for a flight of only 40 hours. Nonetheless,
AeroVironment is planning to start a commercial, UAV-based
communications relay service within three years,
according to Stuart Hindle, a vice-president of both
AeroVironment and its subsidiary, SkyTower, which is
devoted to the communications project.
By
that time, Mr Hindle says, SkyTower should be able to
make flights of several weeks' duration over the wealthy
markets of North America and Europe. (Longer flights
will be possible over countries near the equator,
because they are sunnier and there is less wind.) This,
he says, is long enough to make it possible to provide a
broadband service that would be cost-competitive with
today's land and satellite-based systems. A single UAV
could provide connections of at least five gigabits per
second to around 200,000 subscribers, and a rotating
fleet of them would provide continuous coverage.
Although
broadband communications may be the first UAV technology
to go commercial, the craft are also appealing as
"virtual" mobile-phone towers, which could
provide extra capacity when a crowd migrates to a
normally under-populated area (as in the recent
Glastonbury music festival, when more than 100,000
people flocked to a field in the British countryside).
Indeed, SkyTower successfully tested such a system in
2002, in co-operation with Japan's Ministry of
Telecommunications. Armed forces around the world are
also interested. UAV-based systems could provide a
battlefield with temporary communications coverage.
Long-duration
UAVs depend on sunlight. All the existing models are
pulled around the sky by electric motors, and the
electricity to do the pulling has to come from
somewhere. However, blanketing a craft's wings with
photo-cells that convert sunlight into electricity is
not sufficient, as night inevitably follows day. The
most important trick is to find an electricity-storage
system that is light and efficient enough to power a
craft through the hours of darkness. If that system can
be fully re-charged during the day, then a UAV could fly
indefinitely.
The
problem is that re-chargeable systems tend to be too
heavy. To solve this, AeroVironment has turned to fuel
cells, which work by combining hydrogen and oxygen to
produce water and electricity. The most appropriate sort
of fuel cell, called a closed-cycle cell, uses
electricity from solar panels to break its water up into
hydrogen and oxygen during the day—in effect,
re-charging itself. But closed-cycle systems are, for
the moment, too heavy to be feasible in windy, sunless
high latitudes. So Helios and its immediate successors
will rely on open-cycle systems that bring hydrogen with
them and take oxygen from the air. Such fuel cells can,
however, power a UAV for only a couple of weeks, as the
hydrogen they carry eventually runs out.
According
to Chris Kelleher, a project manager at QinetiQ, a
British defence contractor that is building a UAV called
the Zephyr 3, another option worth examining is
super-efficient batteries, probably derivatives of the
lithium-based batteries used in mobile phones. As Mr
Kelleher points out, the distinction between batteries
and fuel cells is a bit blurred: both rely on combining
chemicals to produce electricity.
One
way to avoid the problem altogether, says Dyke
Weatherington, an official responsible for UAV planning
at America's Department of Defence, is to turn to
nuclear power. Like that on satellites, this would rely
not on nuclear fission, but on heat generated by
radioactive decay, which would be converted into
electricity by devices called thermocouples.
Some
people in the American defence establishment see this as
the ideal solution, he says, though he recognises that
it is fraught with environmental and political risks.
And unlike Mr Hindle and Mr Kelleher, who seem convinced
that propellers are the best way to pull UAVs through
the thin air at high altitudes, Mr Weatherington reckons
that the next five years may also see the development of
new sorts of low-fuel-consumption jet engine.
The
main competition to fixed-wing UAVs will come from
unmanned airships. Although airships would be less
useful for reconnaissance missions that require
mobility, they might serve as communications relays.
Indeed, QinetiQ is combining the two, after a fashion.
In a few weeks' time Zephyr 3 will be lifted to a height
of 9km by a helium-filled balloon, before being launched
on a mission intended to break Helios's record for
high-altitude flight, which stands at 29km. (The balloon
will then follow Zephyr 3 up, so the mission could also
break the altitude record for a manned balloon.)
As
Mr Kelleher points out, airship technology is less
mature than that of UAVs. Although the buoyancy of a bag
full of helium allows an airship to carry a larger
payload, that bag is also buffeted by the winds of the
stratosphere, which means that an airship has to have a
more powerful propulsion system. Leland Wight, a
programme manager for high-altitude airships at Boeing,
says that the biggest engineering challenge is to get
the balloon, which is only partially inflated at ground
level, up to its operational altitude without its
getting tangled up.
One
benefit of the greater carrying capacity of airships is
that they will be able to use less exotic power sources
than fixed-wing UAVs. Mr Wight says that Boeing is
planning to employ an internal-combustion engine powered
by a special fuel to supplement the solar cells. In
competition with Lockheed Martin and a third team led by
Aeros, an airship company based in California, Boeing is
developing a prototype for America's Missile Defence
Agency. This should fly in 2006. Mr Wight says Boeing is
especially interested in the possibility of using
airships as communications relays. But, if SkyTower's
plans go well, UAVs will enter the market a lot sooner
than airships.
SCIENTISTS
are searching for cleaner ways to power vehicles and to
make better use of domestic energy resources. The fuel
cell, an electrochemical device that converts the
chemical energy of a fuel directly to usable energy
without combustion, is one of the most promising of
these new technologies. Running on hydrogen fuel and
oxygen from the air, a 50-kilowatt fuel cell can power a
lightweight car without creating any undesirable
tailpipe emissions.
If the fuel cell is designed to operate also in reverse
as an electrolyzer, then electricity can be used to
convert the water back into hydrogen and oxygen. (See
Figure 1.) This dual-function system is known as a
reversible or unitized regenerative fuel cell (URFC).
Lighter than a separate electrolyzer and generator, a
URFC is an excellent energy source in situations where
weight is a concern.
Weight
was a critical issue in 1991 when scientists at Lawrence
Livermore National Laboratory and AeroVironment of
Monrovia, California, began looking at energy storage
options for an unmanned, solar-powered aircraft to be
used for high-altitude surveillance, communications, and
atmospheric sensing as part of the Strategic Defense
Initiative. Called Pathfinder, the aircraft set an
altitude record for solar-powered flight in 1995, flying
to 15,400 meters (50,500 feet) and remaining aloft for
about 11 hours. Pathfinder's successor, Helios, will
remain aloft for many days and nights. For that
aircraft, storage devices were studied that would
provide the most energy at the lowest weight, i.e., the
highest energy density. The team looked at flywheels,
supercapacitors, various chemical batteries, and
hydrogen- oxygen regenerative fuel cells. The
regenerative fuel cell, coupled with lightweight
hydrogen storage, had by far the highest energy
density--about 450 watt-hours per kilogram--ten times
that of lead-acid batteries and more than twice that
forecast for any chemical batteries.
The
Prototype
Fuel
cells have been used since the 1960s when they supplied
on-board power for the Gemini and Apollo spacecraft.
Today, fuel cells are being used for Space Shuttle
on-board power, power plants, and a variety of
experimental vehicles. However, none of these
applications uses the URFC because early experience did
not uncover the usefulness of the reversible technology,
and little research had been funded. Recent results of
Livermore research indicate otherwise, based on more
thorough systems engineering and improved membrane
technology.
Challenged
by a lack of information on the technology, Livermore
physicist Fred Mitlitsky was determined to uncover just
how to make the combination of technologies work.
Mitlitsky continued in 1994 with a little funding from
NASA for development of Helios and from the Department
of Energy for leveling peak and intermittent power usage
with sources such as solar cells or wind turbines. (See
Figure 2.)
Figure
2. Unitized regenerative fuel cells will someday find a
multitude of applications. URFCs are ideal for
cars, solar powered aircraft, energy storage, propulsion
in satelites and micro spacecraft and load leveling at
remote power sources such as wind turbines and solar
cells.
The
50-watt prototype that Mitlitsky's team developed is a
single proton-exchange membrane cell (a polymer that
passes protons) modified to operate reversibly as a URFC.
It uses bifunctional electrodes (oxidation and reduction
electrodes that reverse roles when switching from charge
to discharge, as with a rechargeable battery) and
cathode-feed electrolysis (water is fed from the
hydrogen side of the cell). By November 1996, the
prototype had operated for 1,700 ten-minute
charge-discharge cycles, and degradation was less than a
few percent at the highest current densities.1
Testing will continue in a variety of forms. Larger,
more powerful prototypes will be created by increasing
the size of the membrane and by stacking multiple fuel
cells. For use on Helios, a prototype will likely
provide 2 to 5 kilowatts running on a 24-hour
charge-discharge cycle. As funding becomes available,
prototypes may also be tested for other uses. A lunar
rover, for example, would require cycles of about 29
days.
URFC-Powered
Electrical Vehicles
In a 1994 study for automotive applications, Livermore
and the Hamilton Standard Division of United
Technologies studied URFCs. They found that compared
with battery-powered systems, the URFC is lighter and
provides a driving range comparable to gasoline-powered
vehicles. Over the life of a vehicle, they found the
URFC would be more cost effective because it does not
require replacement.2
In
the electrolysis (charging) mode, electrical power from
a residential or commercial charging station supplies
energy to produce hydrogen by electrolyzing water. The
URFC-powered car can also recoup hydrogen and oxygen
when the driver brakes or descends a hill. This
regenerative braking feature increases the vehicle's
range by about 10% and could replenish a low-pressure
(1.4-megapascal or 200-psi) oxygen tank about the size
of a football.
In the fuel-cell (discharge) mode, stored hydrogen is
combined with air to generate electrical power. The URFC
can also be supercharged by operating from an oxygen
tank instead of atmospheric oxygen to accommodate peak
power demands such as entering a freeway. Supercharging
allows the driver to accelerate the vehicle at a rate
comparable to that of a vehicle powered by an
internal-combustion engine.
The URFC in an automobile must produce ten times the
power of the Helios prototype, or about 50 kilowatts. A
car idling requires just a few kilowatts, highway
cruising about 10 kilowatts, and hill climbing about 40
kilowatts. But acceleration onto a highway or passing
another vehicle demands short bursts of 60 to 100
kilowatts. For this, the URFC's supercharging feature
supplies the additional power. A URFC-powered car must
be able to store hydrogen fuel on board, but existing
tank systems are relatively heavy, reducing the car's
efficiency or range. Under the Partnership for a New
Generation of Vehicles, a government-industry consortium
dedicated to developing high-mileage cars, the Ford
Corporation provided funding to LLNL, EDO Corporation,
and Aero Tec Laboratories for development of a
lightweight hydrogen storage tank (a pressure
vessel).
The
team combined a carbon fiber tank with a laminated,
metalized, polymeric bladder (much like the ones that
hold beverages sold in boxes) to produce a hydrogen
pressure vessel that is lighter and less expensive than
conventional hydrogen tanks. Equally important, its
performance factor--a function of burst pressure,
internal volume, and tank weight--is about 30% higher
than that of comparable carbon-fiber hydrogen storage
tanks. In tests where cars with pressurized carbon-fiber
storage tanks were dropped from heights or crashed at
high speeds, the cars generally were demolished while
the tanks still held all of their pressure - an
effective indicator of tank safety. Unlike other
hydrogen-fueled vehicles whose refueling needs depend
entirely on commercial suppliers, the URFC-powered
vehicle carries most of its hydrogen infrastructure on
board.3 But even a highly efficient URFC-powered vehicle
needs periodic refueling.
Until
a network of commercial hydrogen suppliers is developed,
an overnight recharge of a small car at home would
generate enough energy for about a 240-kilometer
(150-mile) driving range, exceeding the range of
recently released electrical vehicles. With the
infrastructure in place, a 5-minute fill up of a
35-megapascal (5,000-psi) hydrogen tank would give a
580-kilometer (360-mile) range. Commercial development
of unitized regenerative fuel cells for use in
automobiles is perhaps 5 to 10 years away. With their
long life, low maintenance requirements, and good
performance, URFCs hold the promise of someday supplying
clean, quiet, efficient energy for many uses.
Figure
1. The electrochemistry of a unitized regenerative fuel
cell. In the fuel cell mode, a proton-exchange
mebrane combines oxgen and hydrogen to create water.
When the cell reverses operation to act as an
electrolyzer, electricity and water are combined to
create oxygen and hydrogen.
Fuel
Cell Control
For
years, fuel cell technology has been touted as the power
technology of the future. It seems, however, that the
future is still ahead of us as regards this technology.
Fuel cells are, according to many futurists, going to
replace the internal combustion engine, and sharply
reduce our dependence on petroleum and natural gas as
fuels. This will, in turn, help us to preserve our
environment and work to improve our quality of life.
Anybody who lives in the Los Angeles basin will
certainly be awaiting this development.
General
Motors, Ballard Power, General Electric and others are
developing fuel cell technology for earthbound
applications. Ballard has announced their first
practical fuel cell product, and General Motors,
according to Ian Jakupca from Analex, plans to have
the first production version of a fuel cell powered
automobile in the market by the end of 2004. Also for
2004, Ford has scheduled a fleet version of the Focus
to be powered by a fuel cell. Giner Electrochemical
Systems (Newton, MA) is working for NASA at Glenn
Research Center in Ohio to develop fuel cell
technology for flightline and space applications.
NASA
is working on regenerative fuel cells for the
Environmental Research and Atmospheric Sensor
Technology (ERAST) project. The goal of the project is
to be able to produce an unmanned aerial vehicle
capable of continuous flight for six months at
altitudes beyond 60,000 feet, carrying a payload. Very
clearly, ERAST vehicles could replace many
telecommunications satellites at an incredibly
economical cost, since they can be flown down for
repairs and flown back to the station.
Regenerative
fuel cells are integrated energy systems that
incorporate a fuel cell (electrochemical device to
convert hydrogen and oxygen into electricity and
water) and an electrolyzer (electrochemical device to
convert electricity and water into hydrogen and
oxygen). In earthbound systems, oxygen can be
delivered from the air, but in ERAST systems and in
future space vehicles, oxygen must come from the
electrolyzed water.
Fuel
Cell Basics
Simply, how does a Proton Exchange Membrane (PEM)
fuel cell work? Hydrogen is passed over an anode, and
split into H+ ions and electrons. The electrons are
forced into a circuit, and made to do work, while the
H+ ions migrate through a proton exchange membrane. At
the cathode, O2 is combined with the H+ and
the electrons to form water. The electrolyzer operates
in a similar fashion. Water passes an anode and splits
into H+ and O- ions, and electrons. The O- ions make O2
gas and bubble out into storage. The H+ ions migrate
through the membrane, and at the cathode, the H+ ions
and electrons combine to form H2 gas and it
bubbles out into storage, where the fuel cell can use
it.
The
points of danger in a regenerative fuel cell system
are the hot, wet gas coming out of the electrolyzer
and going into the fuel cell, and the coolant going
into the fuel cell stack. The fuel cell must be kept
cool enough so that the components aren’t destroyed,
yet warm enough to sustain the electrochemical
reaction. The coolant of choice is water. The water
must be deionized, and have very low conductivity.
Higher conductivity water indicates the presence of
dissolved solids, which may plate out on the cooling
coils inside the fuel cell, degrading performance and
eventually causing failure.
Flow
must be measured, too. Water flow in the cooling
system must be adjusted to maintain the proper fuel
cell stack temperature, and any fluctuation needs to
be alarmed. Gas flow throughout the system must also
be measured, since stagnant flow conditions indicate
real problems. Temperature and pressure must also be
measured, both in the coolant and in the fuel cell
itself. Humidity must be controlled in the gas feed to
the fuel cell. And, of course, the output of the fuel
cell (current and especially voltages) must also be
measured.
So,
what looks like a simple problem requires a fairly
long list of sensors, and a control system to monitor
those sensors and control the process. conductivity,
flow, pressure, temperature, humidity, voltage, and
current, and derived values for efficiency and
contamination, are required to control the process.
Off
the Shelf
NASA tries to use commercial off-the-shelf (COTS)
products whenever they can, so all of the sensors NASA
is using for the ERAST fuel cell projects are just
that, from vendors such as Thornton, Vaisala, Dynaload,
Lambda, Hastings, Setra, Tescom and Omega, as well as
a PC-based data acquisition and control system from
National Instruments. In August of 2001, the Helios
Unmanned Aerial Vehicle broke the record for sustained
horizontal flight at altitude by flying at 96,500
feet. This is the type of flight profile needed for
flying in the Martian atmosphere. The wings of Helios
are covered with solar cells, which operate the plane,
and the electrolyzer (storing hydrogen and oxygen
during the day) and fuel cell operates the plane at
night. So
measuring flow and a few other variables may someday
allow us to replace telecommunications satellites
inexpensively, and even fly across the face of Mars.
References
1. F. Mitlitsky, B. Myers, and A. H. Weisberg,
Lightweight Pressure Vessels and Unitized Regenerative
Fuel Cells, LLNL, Livermore, California, UCRL-JC-125220
(November 1966). Presented at the 1996 Fuel Cell
Seminar, San Diego, California, November 17-20, 1996.
2. F. Mitlitsky, N. J. Colella, and B. Myers, Unitized
Regenerative Fuel Cells for Solar Rechargeable Aircraft
and Zero Emission Vehicles, LLNL, Livermore, California,
UCRL-JC-117130 (September 1994). Presented at the 1994
Fuel Cell Seminar, Orlando, Florida, November
28-December 1, 1994.
3. "Getting along without Gasoline--The Move to
Hydrogen Fuel," Science & Technology Review,
UCRL-52000-96-3 (March
1996), pp. 28-31.
Group:
Hydrogen Fuel Cells May Hurt Ozone
Fuel
Cells May Redistribute Power
Freeing
Gases For Cheap Fuel Cells With Orbiting Laser Cannons
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