Drone Engines: Unsung Heroes

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In the very near future unmanned aerial vehicles (UAVs) will be able to refuel in midair, flying next to a tanker aircraft, before continuing their mission. They’ll effectively be able to remain flying above their target indefinitely.

This isn’t an imaginary scenario: Northrop Grumman already conducted aerial refueling experiments with the Global Hawk UAV. The idea is to grant heavy, high altitude and long-range UAVs an almost limitless endurance. This is the main advantage an unmanned system has over manned aircraft: The ability to remain airborne for prolonged periods of time, perhaps for days or even more. There aren’t any operational aerial refueling systems for UAVs, although some successful experiments have been conducted and automation systems allow for continued development. The ability to extend operational ranges by aerial refueling is especially important for the U.S. Navy and Marine Corps, as their aircraft-carrier-operated planes have strict weight limitations.

During aerial refueling the fuel flows from the tanker aircraft to the UAV. The question is which fuel to use, and which engine. This are the pressing issues facing the world of unmanned vehicles today. Companies and organizations invest billions in order to find the best engine suitable for every class of UAV.

Emanuel Liban, a well known UAV engines expert, points to the fact that while the UAV itself and its payload are being developed very rapidly all over the world, engines lag behind: “The business world hasn’t developed engines for tactical and small UAVS, since engine development requires a massive investment while the number of UAVs manufactured around the world is relatively small. Profit considerations demand the manufacturing of tens of thousands of UAVs, and the world still isn’t up to the task in this area. As a result there are no internal combustion engines for UAVs, and aircraft manufacturers have to develop their own engines – unlike passenger plane manufacturers such as Boeing or Airbus, who leave engine development to Rolls Royce, Pratt and Whitney, General Electric and others.”

Technically there’s no difference between the engines of manned and unmanned aircraft, although the latter have special requirements: “In UAVs all the command and control systems must be autonomous, or at least automatic – human pilots aren’t there to monitor the situation. There has to be an automatic mechanism for real time malfunction reports, since there’s no pilot to look at the different gauges. UAVs have to be very, very reliable. If a manned plane malfunctions the pilot can attempt to land it. In case of UAVs operating in enemy territory, though, you don’t want them landing if there’s a malfunction. They also have to be very quiet, to avoid being detected. This requires relatively quiet engines, and that’s why small tactical UAVs fly at low altitudes using quiet electric engines.”

There are several types of UAV engines: Electric, combustion, jet and hybrid. The latter are defined as engines that utilize a combination of at least two energy sources – such as electricity and internal combustion, fuel cells or solar power.

Electric engines, like electric cars, get their energy from batteries. There are ongoing, immense efforts to develop a small, light weight, powerful and reliable battery that will be able to provide enough power to UAVs, vehicles or even soldiers in the modern battlefield: The latter carry batteries for radios, night vision systems and more. “Battery development is a slow process,” said Liban. “No one found the right formula as far as energy density (capacity and weight) and price are concerned, so usually UAVs are powered by hybrid, fuel and electric, engines. When taking off the electric engine provides quick boosts of power, switching to fuel when the aircraft reaches its cruising altitude.”

“Fuel cells are a recent development, aimed at overcoming the weight problem. Fuel cell engines are hydrogen- and oxygen-based. Oxygen is taken from the air, while hydrogen is stored in special high-pressure bottles, composed of light metals reinforced by carbon fibers. The two gases pass through a membrane array to produce electricity. Some small aircraft already use fuel cell engines, but in order to power larger, heavier aircraft you have to use hydrogen bottles that are as heavy as heavy batteries. Fuel cells can supply two to three times as much energy compared to the most advanced batteries, so the automotive world invests a lot of effort in developing this technology. In two years we’ll see vehicles powered by fuel cells.”

iHLS – Israel Homeland Security

Manufacturers are also considers using solar energy to power high-altitude UAVs, in order to achieve unlimited endurance. This will be a hybrid engine: The UAV will take off using electricity, switching to solar power when it reaches its cruising altitude. During the day its systems will store enough energy to power it for the night. Liban says that solar power technology is still in its infancy, with engines of this type used so far only for short demonstration flights. Unlimited endurance using current technology requires a wing span similar to that of a 747 Boeing, and that’s just one limitation.

Another issue is UAV fuel types. The current trend is gasoline out, heavy fuels in. Gasoline, as it turns out, is too combustible, while logistic requirements call for using uniform fuels for multiple platforms rather than having multiple fuel types. The U.S. Army doesn’t acquire gasoline-powered UAVs, for example – they must be powered by heavy fuels like diesel fuel or jet fuel – the latter being basically processed oil free of particles that are harmful to the engine.

A new type of fuel is biofuel: Biological fuels, produced from plants such as sugar canes, potatoes and even seaweed. Some airlines already use passenger planes powered by an experimental mixture of jet fuel and bio fuel, with the latter being much cleaner and environment-friendly compared to other fuel types. Liban: “You produce bio fuel by artificially creating its molecules, so you can leave out harmful materials that exist in natural fuels, materials which cause corrosion and otherwise harm the aircraft’s engine. For that reason an engine using biofuel can power UAVs. Another option is turning natural gas into clean jet fuel or diesel fuel.”

What do the various UAV manufacturers do in the meantime? An Israeli example: Elbit systems manufactures its own UAV engines. In the U.S. engines from civilian aircraft are used to power large, heavy UAVs – the Predator, for example. Since there are no pilots there’s no need to maintain a pressurized cabin, and high-altitude flight saves fuel.

UAV experts describe the perfect UAV: An aircraft that can take off quickly, flying at high speeds and at high altitudes (to save fuel) to its target, where it can remain for prolonged periods of time. To complete its intelligence gathering mission it descends to a lower, optimal altitude. This means engine and aircraft must fit together perfectly. Liban explains: “The ideal situation is taking an engine and designing the aircraft around it. You are better off with a proven, mature engine than with a new one still suffering from unexpected issues and malfunctions.”

Aviation literature frequently mentions the fact that the most advanced technologies for aircraft – manned or unmanned – are developed for the engines. Engine development is also very risky financially, so engine development processes tend to be much more conservative than avionics, electronics, hull and wing development. It requires huge investments, expensive infrastructure, special metals, high temperatures: That’s why engine development rates determine how quickly the entire aviation industry progresses.

Liban adds that engines of civilian/business jet planes have shown a lot of improvement over the last few years. Engines we use today on passenger planes are much more efficient (due to rising fuel prices), quiet (due to noise requirements) and simpler to maintain, lasting tens of thousands of flight-hours. An average advanced engine can remain installed on a plane’s wings for 6-8 years (!) before having to be taken down for maintenance.

Turbofan engines are also currently in development around the world. This revolutionary concept might allow aircraft to efficiently achieve very high speeds when closing in on their target area, later slowing down and patrolling the skies with very low fuel consumption rates. Engines of this type are aimed at the next generation of jet fighters, manned or unmanned.

These technological advancements influence military and civilian UAV development. In the future the manned/unmanned ratio will continue to lean toward unmanned aircraft. Even today the U.S. Air Force launches more unmanned missions than manned. These UAVs are getting more and more popular, and they’re waiting for a new generation of powerful, efficient and silent engines, in addition to a new generation of miniaturized, high capacity batteries and fuels based on plants, natural resources or the power of human ingenuity.