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The thin Mars atmosphere, composed mostly of carbon
dioxide and lacking oxygen for combustion, provides an
inhospitable environment for conventional aircraft and
helicopters. Compounding the challenge are size
constraints imposed by the spacecraft delivering air
vehicles to Mars.

But the flapping wing "Entomopter," a patented
mechanical insect capable of both flying and crawling,
may be ideal for meeting the demanding requirements of
Mars aerial exploration.

With support from NASA's Institute for Advanced
Concepts, a team of researchers that includes Georgia
Institute of Technology engineers is conducting a
comprehensive feasibility study designed to show whether
a fleet of scaled-up Entomopters could one day help
explore the Red Planet.

"Mars is a nasty place to fly a conventional air vehicle
because almost everything there is working against you,"
said Anthony Colozza, who coordinates the Entomopter
study as the principal investigator for the Ohio
Aerospace Institute (OAI), the project facilitator. "The
Entomopter concept is really a breath of fresh air
because it makes the environment of Mars our friend."

He envisions exploration by a fleet of Entomopters
landing and taking off, perhaps from a rover able to
refuel and support them as it crawls across the Mars
surface gathering scientific information.

In that scenario, the Entomopters could study the
surface from an altitude of less than 100 feet, sample
the atmosphere, look for minerals and collect surface
samples, while guiding the rover to the most interesting
locations for study. Though limited in range to one or
two kilometers on either side of the rover, the
Entomopters could nevertheless cross canyons, large
rocks and other features that would stop the rover.

"The trouble with the rovers is that they land in one
spot and are very limited in the extent to which they
can explore," says Robert Michelson, principal research
engineer at the Georgia Tech Research Institute (GTRI)
and lead developer of the Entomopter design. "It's
frustrating to be looking through the camera of a rover
and wonder what might be on the other side of the next
ridge. If we could get a vehicle that could fly over
that ridge, we could do surveys much more efficiently."

The Entomopter concept originated at GTRI with U.S.
military interest in palm-sized "micro air vehicles"
that could surreptitiously explore underground bunkers
and other structures. For that mission, a 50-gram
Entomopter with a 15-centimeter wingspan could fly
through ventilation ducts and using insect-like legs,
crawl through narrow passageways or half-open doors.
Development of that version continues in parallel with
the Mars version.

Flying on Mars involves overcoming a series of
obstacles, Michelson and Colozza said. The Mars
atmosphere is 95 percent carbon dioxide, with slightly
more than a tenth of 1 percent oxygen. That rules out
oxygen-breathing motors and forces flying machines to
rely on chemical or electrical propulsion. The Mars
atmosphere is very thin, similar to the Earth's
atmosphere at 100,000 feet.

"Nothing flies at that altitude with any regularity,"
Michelson said. "You must fly very fast and are on the
ragged edge of control." Because the Mars atmosphere is
so thin, a conventional aircraft would have to fly at
least 250 miles an hour to generate enough lift to stay
aloft. At that speed, landing or taking off from the
rocky terrain would be impossible, limiting a
conventional aircraft to a single flight.

A wide turning radius would also make it difficult to
come back for a closer look at an object of interest.
Temperatures swing wildly from 20 degrees Celsius to
minus 140 degrees Celsius, creating materials and fuel
challenges. Because the speed of sound is 20 percent
lower in carbon dioxide, propellers or rotors can't spin
as fast as they could on Earth without creating
destructive shock waves. That limits the lifting power
of rotorcraft, or forces them to use less efficient
multiple rotor systems.

In the past decade, scientists have begun to understand
how insects use their flapping wings to generate lift.
It's a complicated phenomenon believed to involve the
formation of wing vortices that multiply the lifting
power.

Flapping wings also give insects the unique ability to
land and take off, quickly change directions and hover.
Unlike aircraft, which must move the entire vehicle
rapidly to generate lift, insects can move only their
wings rapidly - while the body flies slowly. That could
be as useful for exploring Mars as it is for spotting
nectar in flowers.

One scientist who has contributed to the understanding
of insect flight is Charles Ellington, a professor at
the University of Cambridge in England. Ellington met
Michelson four years ago during a conference on micro
air vehicles, and he has since become part of the team
developing the terrestrial version of the Entomopter.

To control their direction, insects use a complex system
to vary the beating of each wing and alter how they
encounter the air. Rather than replicate that system,
Michelson and GTRI collaborator Robert Englar are
adapting an active flow-control technique originally
developed for fixed-wing aircraft.

On aircraft, the system uses compressed air released by
valves to control direction and augment lift over the
wings. On the Entomopter, waste gases produced by its
power source - a reciprocating chemical muscle - would
substitute for the compressed air in multiplying lift
and providing control.

"This allows us to have a much simpler wing-beating
mechanism," Michelson explains. "It makes the Entomopter
manufacturable and helps keep the costs down."
The term Entomopter combines the concept of an insect
(ento) with segmented wings (mopter). The multi-modal
design concept - combining wings for flight, legs for
ground locomotion and a chemical muscle for power -
received patent protection in July 2000.

Operating on a variety of fuels, the chemical muscle
needs no oxygen to produce the motion required for
flapping wings. Michelson and his team have advanced the
muscle - for which they are seeking a patent - through
three different prototypes and can now generate motion
at 70 cycles a second with enough power to fly.

"It's a simple device that can generate the fairly high
levels of power that are essential to flight," he said.

"Our liquid fuel has a higher energy density than a
battery. We can extract enough of that energy to be able
to create the force necessary to flap the wings, fly and
still have some energy left over for other
applications."

Like real muscles, Michelson's chemical muscle generates
wastes - heat and gases. On the Entomopter, the heat
could be used to create electricity through a
thermoelectric process. Beyond augmenting lift and
providing control, the gases can also operate an
acoustic ranging system to help the machine navigate and
avoid obstacles.

Since electrical energy is essential for the machines'
autonomous navigation system, science package, radio
transmitter and control systems, the team is also
exploring the use of flexible solar panels on the wings.
A safe tritium-powered generator could keep critical
electrical systems alive between flights or during times
the Entomopter may have to "hibernate" during a Mars
dust storm.

Though the Mars environment provides mostly challenges
to overcome, it does offer one important advantage.
Gravity there is only one-third that on Earth, meaning
the Entomopter's size can be scaled up without incurring
the same weight penalty it would on Earth. Michelson
said the larger size, perhaps a meter across, would
enable it to carry a sufficient payload without
sacrificing the attractive aerodynamics.

Following an initial Phase I review completed in
November 2000, Georgia Tech researchers, Colozza and
scientists from the Ohio Aerospace Institute have now
launched a 12-month Phase II study. The goals are to
develop data to support the concept and to recommend the
best choices for options such as fuel, electrical
generation sources, size and range.

The Mars Entomopter builds on five years of previous
work supported by GTRI, the Defense Advanced Research
Projects Agency (DARPA) and the U.S. Air Force's
Revolutionary Technology Program.

Over that time, development of the chemical muscle has
advanced dramatically, the aerodynamics of the
circulation control system have been studied, an
acoustic ranging system has been tested in GTRI's
aeroacoustics facilities, manufacturing processes have
been developed to build the wing structures directly
from computer models, and a tissue-and-wood model has
made hundreds of brief flights.

"We have demonstrated a lot of the pieces of it,"
Michelson said, "but what we need now is one big program
to pull it all together.

"One of the major challenges that faces us is working
out the wing aerodynamics," Michelson said. "That is a
major issue. Steady aerodynamics over fixed wings is
well understood, and even the active flow control of
wings already has a good body of knowledge. But we are
talking about pneumatic control of unsteady airflow over
a flapping wing. No work has been done on that."

The problems of autonomous navigation and flight also
loom large, though significant progress has been made
over the past decade - much of it through the aerial
robotics competition Michelson created for the
Association for Unmanned Vehicle Systems, International.

If the feasibility study turns out as positive as
Michelson hopes, the next step will be to convince one
of NASA's research centers to pick up the project and
invest the resources needed to develop the technology.

If all goes well, Entomopters could be flying on Mars
within a decade, giving scientists a unique new
capability. "Combining Entomopters with a rover would
give us a very nice integrated solution," Colozza said.

Source: Georgia Institute of Technology
Cosmiverse Staff Writer