Close encounters of the winged kind

Ordinary aircraft can't fly in the thin air of Mars, so engineers have turned to insects for inspiration, says Ian Brown. Enter the entomopter
Click to follow
The Independent Online

It sounds like something on the cover of a Ray Bradbury novel. The idea of a metre-long mechanical insect flitting across the rust-red canyons of Mars can surely only belong to the pages of science fiction?

Not if Robert Michelson realises his concept of the "entomopter", a robotic drone capable of both flying by flapping its wings and crawling like an insect. Michelson says it would be perfectly suited for the low-level aerial exploration of Mars, which he believes could occur within the next decade or so.

Flying through the thin Martian atmosphere, which is 95 per cent carbon dioxide and 0.1 per cent oxygen, would be virtually impossible for conventional planes or helicopters. Mars's atmosphere is similar to Earth's stratosphere, explains Michelson, principal research engineer at the Georgia Tech Research Institute in Smyrna, Georgia. "Nothing flies at that altitude with any regularity. You must fly very fast and are on the ragged edge of control."

A conventional aircraft would need to fly at 250mph or more to stay aloft, requiring a wide turning radius to return for a closer look at anything of interest. Conventional take-off and landing on the rocky Martian landscape would be impossible. And because the speed of sound is 20 per cent slower in a carbon dioxide atmosphere, propellers or rotors can't spin as fast as they can on Earth without creating destructive shock waves.

So Michelson, the lead developer of his team of designers and engineers, looked to nature for inspiration and devised the wing-flapping entomopter. The name is a combination of insect (ento) and segmented wings (mopter) and Michelson says these intelligent, autonomous aerial robots will do more than just fly.

Backed by Nasa's Institute for Advanced Concepts, Michelson's team is examining whether a fleet of larger entomopters could one day explore the red planet. "Mars is a nasty place to fly a conventional air vehicle because almost everything is working against you," says Anthony Colozza, who co-ordinates the entomopter study for the Ohio Aerospace Institute, the project's facilitator. "The entomopter concept is really a breath of fresh air because it makes the environment of Mars our friend."

With gravity only one-third as strong as Earth's, Michelson believes a metre-long craft could carry a sufficient payload of scientific instruments. A fleet of entomopters could be launched from a rover vehicle that would refuel them while it trundles across the surface. The robots could fly ahead at altitudes of up to 100ft, testing the atmosphere, scanning the geology and guiding the rover to the most interesting spots for study. Although limited in range to one or two kilometres, the entomopters, unlike the rovers, could easily traverse canyons, gullies and large rocks.

"The trouble with the rovers is that they land at one spot and are very limited in the extent to which they can explore," Michelson says. "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."

Just as intercontinental ballistic missiles begat space rockets, the entomopter concept began when the American military expressed interest in developing a palm-sized "micro air vehicle" for surreptitious reconnaissance. The Pentagon envisaged a 50g device with a 15cm wingspan that could fly through ventilation shafts and crawl on insect legs through narrow passageways. Development continues in parallel with the Mars version.

Why look to insects? Long baffled by the unaerodynamic body-form of insects, scientists now know more about how insect wings create vortices to generate lift. This phenomenon also allows them to land and take off, change direction quickly and hover. Unlike aircraft, which must fly rapidly to generate lift, insects need only flap their wings rapidly while the body flies slowly.

To control direction, insects use a complex system to vary the beat of each wing. Rather than replicate that, Michelson is adapting a technique developed for fixed-wing aircraft. This active flow-control system uses compressed air released by valves to control direction and augment lift over the wings. On the entomopter, waste gases would substitute for compressed air. "This allows a much simpler wing-beating mechanism," says Michelson. "It makes the entomopter manufacturable and helps to keep the costs down."

The main power source is a reciprocating "chemical muscle" which, using a variety of fuels, doesn't need oxygen. Michelson's team has developed the muscle through three prototypes – for which they are seeking a patent – and claim they 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 essential to flight," Michelson 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 left over for other applications."

Like real muscles, Michelson's chemical one generates waste in the form of heat and gases. On the entomopter, this heat could create electricity through a thermoelectric process. Solar panels on the wings would be another way to keep systems alive between flights, or when the entomopter may have to "hibernate" during a Martian storm. Meanwhile, waste gases could be used to operate an acoustic ranging system to help the machine steer clear of obstacles.

The entomopter project has almost completed a year-long study to gather enough data to support the insect-flight concept and recommend options for fuel, electrical generation, size and range. The development of the chemical muscle has advanced, the acoustic ranging system has been tested, manufacturing processes have been developed to build the wing structures 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 says, "but what we need is one big programme to pull it all together. One of the major challenges facing us is working out the wing aerodynamics. Steady aerodynamics over fixed wings is well understood, and even the active flow-control of wings 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."

But if the study turns out as positive as he hopes, Michelson is optimistic he can convince Nasa to invest the resources. If all goes well, he insists, Mars could be buzzing with busy bees within a decade. Didn't Ray Bradbury always say that science fact would prove just as wonderful as science fiction?