How Aerial Spycams Are Driving Engine Technology

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If it sometimes seems that the entire aerospace industry is given over to developing ways to plant cameras in the sky to spy on people and things, it’s probably because the entire aerospace industry is given over to developing ways to plant cameras in the sky to spy on people and things. Although the actual number of aircraft is relatively small, the number of developmental projects related to surveillance is, as far as I can tell, quite broad. I’m constantly running into people working in this field and my reaction is always the same: “You have a UAS project?”

The latest I encountered by happenstance was in tiny Moriarty, New Mexico, a dust-blown little airport east of Albuquerque. When I was out there in early April shooting the SubSonex jet, we drove by a huge, obviously new hangar complex. When I inquired about it, my host, Bob Carlton, said, “Oh, that’s Google’s.” What the hell is Google doing out here in a windowless hangar in the middle of the desert? Building high-altitude, long-duration solar-powered drones not to loft cameras (or so we’re told) but to provide internet service to remote parts of the world. These are being called “atmospheric satellites.”

The airport rumor had it that the Google project was about to fly and that proved correct. It did fly. And as our news story reported, it also crashed on May 1, although the details are sketchy.This is a big aircraft, by the way, with a wingspan of more than 100 feet, with the wing’s surface area apparently covered with high-efficiency solar cells, probably of the sort made by Alta Devices.

The intent is to keep these things on station for years at a time, but the technology may itself be years from delivering on that. As with everything else related to electric flight, the lack of high-energy density, lightweight batteries continues to limit endurance.

So as attractive as electric propulsion is, designers are still leaning heavily on hydrocarbon engines to get cameras high and keep them up there for as long as possible. The Holy Grail continues to be persistent presence; the camera as an unblinking eye that’s always there peering down on everything and everyone 24/7/365. The industry’s not quite there yet, but it’s not for lack of trying.

For a brief time and maybe still, this has resulted in resurgence of piston engine development. Going back some 15 years, when the then Thielert Aircraft Engines was developing what became the Centurion line of four-cylinder diesels, General Atomics picked that engine to power the YMQ-1C Warrior, a variant of the Predator drone. Prior to that, all the way back to the mid-1990s when few of us knew this technology existed, General Atomics was using Rotax 912 and 914 engines.

To some in the military, reverting back to anachronistic pistons from jets must have been a little hard to swallow. But there was really no choice if the services wanted long-duration, medium-size drones. Jet engines were and remain too inefficient to carry the fuel, the payload and keep it at altitude for long duration. If these guys are asked how much duration these drones should have, the answer is more likely to be weeks and months than hours and minutes. Diesels fit the design brief, but they’re heavy and altitude-limited.

I’m sure the services never gave up on small turbine-powered drones because for some missions, it appears that the altitude/endurance tradeoff favors altitude. Weran this story last month describing an Air Force design contest to create a high-efficiency 100-hp turboshaft engine to operate at an SFC of 0.55 lbs/hp/hr. The lucky winner will get a $2 million prize. I wish the contestants luck on that one. Not only will it be a challenge to design and produce such an engine, doing it on $2 million sounds like trying to finance a new Airbus on a Visa card. In case the fuel efficiency isn’t challenge enough, the engine has to weigh no more than 50 pounds, since there’s a two-horsepower-per-pound requirement.

A BSFC of 0.55, by the way, is efficient only in relative terms. Typical turboshaft engines run in the mid 0.6s, although some of the larger ones do better than that. Four-cylinder diesels of the sort Thielert developed from automotive antecedents get down around 0.36, which explains why, despite their weight, they found favor for MALE-class drones. (Medium-altitude long-endurance.)

Turboshafts don’t scale down well, if efficiency is a goal. The Allison—now Rolls—250-C20S used in Cessna 185 and 206 conversions runs at about 0.65 BSFC, while the Garrett TPE331 engines used in the Mitsubishi MU-2, at twice the horsepower, operate at 0.55. Some do even better than that. The 2100-hp PW121 gets just below 0.5—almost to piston efficiency, but not quite.

Diesels have the same scale issues. The larger they are, generally, the more efficient. The most efficient diesel known is the giant Wartsila-Sulzer RTA96C meant for marine use. With a three-foot bore and an eight-foot stroke, it runs at 0.278 BSFC, which is more than 50 percent thermal efficiency. Eat your heart out, General Atomics.

Because it’s a contest, the Air Force isn’t exactly saying how such an efficient engine should be designed. But Lt. Col Aaron Tucker, deputy chief of the service’s turbine engines division at Wright-Patterson, thinks 3-D printing—additive manufacturing—may help. We’ll see.

Not to be left out, the Navy has its own efficient turboshaft project underway at the Naval Research Lab. But rather than a contest, the Navy is doing its own in-house development to serve as a technological base for contractors to produce such an engine. It may be a while before anyone does. The Navy declined a request to provide more details on the project, but it has been underway since 2004. It has also yielded at least one patent.

This is a small engine—four horsepower to start—intended for what’s obviously a UAV application. One of its design goals is to meet the defense department “single-fuel forward” requirement that allows any vehicle to burn any fuel. The patent indicates that the Navy is pursuing what’s called recuperative technology to improve thermal efficiency. Basically, that involves using engine exhaust to heat the compressor output air via a heat exchanger. The heat added to the combustion process reduces the amount of energy that has to come from the fuel and thus reduces fuel consumption, typically by as much as 20 percent. That’s the difference between 0.65 and 0.52 BSFC and it’s a considerable efficiency improvement, although still a far cry from piston efficiency.

Inevitably, there’s a tradeoff for the recuperative cycle and that’s weight and complexity. A plain Brayton cycle is elegant in its simplicity, which is why jet engines are so attractively reliable at the expense of high fuel burn. According to the patent, the Navy developmental work favors lightweight ceramics for the heat exchanger and other components. I can imagine that would be quite a tiny little—and expensive—jewel for a four-horsepower engine. That thought illuminates the economics of these engines, which are quite likely to be low volume and high priced, unless they find broad civil applications.And there are always spinoffs into civil aerospace. These projects rarely stay in their silos forever. Maybe as useful experience is gained, the recuperative cycle could prove practical for other turboshafts of all sizes, especially for a hybrid-drive electric aircraft.

As we all learned in history class, World War II rapidly accelerated aeronautical development, for both the good and the bad. We got more powerful piston engines and jets and more efficient airframes. We also got ever more effective ways to kill each other from, with and by airplane.

It’s fair to say the same thing about UAS technology; it’s pushing aeronautical science ever forward in propulsion, aerodynamics, avionics and structure. The creativity being applied to these challenges is inspiring. On the other hand, welcome to an age when you won’t be able to walk to your mailbox without a camera recording it. The potential for abuse of such surveillance technology may loom large as the next generation’s most pressing challenge, even ahead of terrorism. It worries me. But not so much that I’m not happy to see this intriguing research underway. To think otherwise would be to wear a permanent set of blinders. The Navy researcher overseeing the NRL project, Rita Manak, told me that part of what propels the lab is to try to nibble away at the edges of the unknown. “You don’t have to worry about the inevitable,” she said, “that never happens. The unexpected always does.”

As the world shrinks into little more than a subject for a lens, I’d say that’s a timely thought.

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