Flying Underwater

April 24, 2008
The same principles that keep aircraft aloft are being used to let submersibles travel underwater.

It’s taken Graham Hawkes more than 20 years to realize a lifetime goal — to build and “fly” an underwater vehicle, and possibly make some money doing it.

He and his company, Malibu-based Hawkes Ocean Technologies, are now selling their fourthgeneration submersible, the Super Falcon. It’s a far cry from their first sub, Deep Flight I, but they both rely on the same principles to maneuver through and under the water.

Downward lift

Hawkes’ submersibles, like airplanes, have wings, ailerons, and rudders, but the wings are upside down compared to a plane’s. With electric motors and propellers providing thrust, the wings travel through the water generating downward “lift,” keeping the slightly buoyant craft beneath the surface. To go deeper, you push the joystick control forward, the nose angles down, and the craft descends. The newest craft, for example, the Super Falcon, can dive at 320 fpm and climb at 600 fpm. In the hands of a trained pilot, the craft can bank through turns, snap rolls, and loops. And if the craft slows down too much, the wings stall, lose lift, and the sub rises to the surface. Hawkes even coined a new word for this type of underwater flight: hydrobatics.

“Conventional subs are like underwater balloons, adjusting their buoyancy in the medium, which is water not air, to ascend and descend,” says Hawkes. “But we do underwater flight. We’re more like planes than balloons.”

Heck of a Hull

One of the most startling innovations in the Super Falcon is its hull. Pressure hulls in conventional submarines and bathyspheres have round cross sections because that shape evenly distributes the hoop stresses generated by outside water pressure. But if the hull’s circular shape deforms by 1 or 2%, water pressure instantly crushes the hull if it is deep enough.

The Aviator, the predecessor to the Super Falcon, has a cast-aluminum hull that conforms roughly to the pilot’s shape, but still has circular cross sections. “It’s a curved and tapered cylinder,” says Hawkes. “Sort of a banana with round cross sections.”

Super Falcon’s hull, a prebuckled pressure hull according to Hawkes, is made of a proprietary isotropic composite that is incredibly strong and relatively light. “We used the strength of the material to break the rule about round cross sections,” says Hawkes. “It let us build a shape that better fits a recumbent pilot. And compared to the Aviator, there’s more shoulder room and a few extra inches everywhere it matters for pilot comfort. And even if the hull deforms under pressure or impact, it won’t collapse.”

The pilot and passenger ride in a hull pressurized to within about 1% of normal atmospheric conditions and breath normal air. Outside metal panels make up the body or skin and they are attached to the hull. The body provides the aerodynamic form and covers most of the equipment, which is also housed outside the hull. But the outside skin is not watertight; rather it lets water in, keeping pressure equalized inside and outside the body. Of course, this means components have to withstand enormous pressures. (The Super Falcon is rated for 1,000 ft, where water pressure is 460 psi, but has a safety factor of two built in to the rating. So the hull should not crush until it gets below 2,000 ft, where pressure is 905 psi.)

The electric motors that power the propellers, for example, are filled with oil, making them immune to pressure. And in the Aviator, the leadacid batteries were similarly filled with oil. “The lead-acid chemistry doesn’t care about pressure,” says Hawkes. “Our pressure-compensated versions will work all the way down to the bottom of the ocean. But if you take the submersible inverted, in effect flying upside down, the acid and oil switch places and the battery doesn’t work anymore.”

The newer Super Falcon uses lithium batteries that rely on lithium- iron-phosphate chemistry instead of the older lithium cobalt. “The newer batteries are safer because they are not as much at risk of thermal runaway,” says Hawkes. “They also have a better life expectancy that is rated in thousands of cycles. The batteries may well outlive the rest of the submersible.” The batteries are also solid and in a sealed housing, so they are also ideally suited to the pressures of the deep.

Keeping it safe . . .

Another difference between Aviator and Super Falcon is the flight system. In the Aviator, flight controls were mechanical with direct linkages, Super Falcon uses lighter, morecompact fly-by-wire technology and a three-axis joystick. Other instruments include a compass and depth gage, pitch-and-roll indicators, speedometer, battery voltage, current draw and electrical leakage status, and critical life-support parameters including cabin pressure and air quality. And life support, mainly air, is also microprocessor controlled in the newer craft. “It’s relatively easy and inexpensive to get hardware and software that monitors air quality 1,000 times per minute and keeps pressure inside the hull to within 0.5% of that at sea level,” notes Hawkes.

For communication, Hawkes uses UQC, a two-way communication system developed by the Navy for use by scuba divers and submarine crews. It uses a high-frequency acoustic carrier modulated by the voice signal. The high-frequency carrier gives better voice quality but limits range. “Range depends on power, so for the Challenger, which was supposed to go 37,000 ft down, we had a custom UQC built which has the power to span that distance,” says Hawkes.

Inside, the pilot and crew are in a shirt-sleeve environment with no heater or air conditioning. But Hawkes admits that in the Bahamas where his underwater flight school is located, the tropical waters, coupled with sunlight hitting the canopy, can make it uncomfortably warm inside the sealed submersibles. But there are no heaters or air conditioners in his designs. There are no fire extinguishers either, so far.

“Fire in small enclosed spaces is a terrifying idea,” says Hawkes, “So to minimize the risk and reduce complexity, we just don’t let anything in the sub that resembles a fuel or ignition source. For example, we limit electrical current to 4 A for inside the hull and put all high-power circuits outside the hull.”

The craft also lacks an emergency exit. “Not having emergency-egress procedures is common on deep submersibles,” explains Hawkes. “The pressure differences from inside to outside the hull are just too large.”

Instead Hawkes relies on his ultimate safety feature: the fact the his craft surfaces if power is lost, all motors fail, or the propellers fall off. Of course, this also means the craft cannot be stopped to observe life and geology outside, nor can it “rest” on the bottom of the sea.

But Hawkes still takes safety seriously. When it comes to batteries, for example, he carries a full-sized redundant backup pack outside the hull with the normal battery pack. “Internal backup batteries are often as small as possible to maximize available room and minimize weight,” notes Hawkes. “But if you are in an emergency and need power for life support or communications, it’s best to have a backup with enough power for everything you might need.”

. . . But exciting

Hawkes realizes he is pioneering a new form of underwater travel and he has a bit of the daredevil in him. He seems intent on making sure the experience of taking one of his creations out for a cruise is more like riding a performance motorcycle than a station wagon, more like flying a jet fighter than an airliner.

“One of my fears with the Aviator when we were testing it was that it would be so docile as a flyer that it would be dull to drive,” says Hawkes. “So I wanted it to stall with a bite, so you actually had to fly the sub. And the first time I stalled an Aviator, got going too slow to counter the buoyancy, the sub rocketed up to surface out of control. I was quite delighted with that. But I’ve always knows that this technology, the birth of underwater aviation, barnstorming beneath the waves, would have an interesting future. Should be a heck of a lot of fun.”

Challenger was built for adventurer Steve Fosset who was going to pilot it to the bottom of the Marianas Trench (about 37,000 ft deep) to set the world depth record for a one-man vehicle. It was planned to have a quartz canopy rather than acrylic which would not have withstood pressures at that depth. Since his accidental death last year, Hawkes and his team have shelved Challenger out of respect. “No one who knew Steve well believes he would want anybody else to climb into his machine and go down there,” says Hawkes. “And though he wouldn’t want progress and exploration to stop, I don’t think you’ll see a rush to get that machine ready to go.”

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