ONE OF THE OLDEST forms of flight – the flapping wings of insects – may support a revolutionary new class of robotic flying machine uniquely suited for exploring a brave new world: the planet Mars.

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," says 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.

Another World, Another Environment

Flying on Mars involves overcoming a series of obstacles, Michelson and Colozza agree. Among them:

• 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 says. "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.

Learning from Insects

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.

"They are intelligent, autonomous aerial robots that do more than just fly," he adds.

An Artificial Chemical Muscle

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 says. "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 believes the larger size, perhaps a meter across, would enable it to carry a sufficient payload without sacrificing the attractive aerodynamics.

The Next Step

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," says Michelson, "but what we need now is one big program to pull it all together."

The complex wing aerodynamics now pose the greatest challenge to the future of the Entomopter.

"One of the major challenges that faces us is working out the wing aerodynamics," Michelson explains. "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 adds. "They will give us a unique capability not available any other way."