How small drones can revolutionise warfare once battery life is improved. An extract
David Hamling
There is an old, pre-smartphone-era joke about a man who meets his inventor friend carrying two suitcases down the street. The inventor shows off the wristwatch he has built, which as well as indicating the time anywhere in the world, also shows the tides, phases of the moon, and position of the stars. His friend is duly impressed. The only problem, says the inventor picking up the suitcases, is the size of the batteries.
That about describes the present situation of small drones. What they do is great; the only problem is the batteries. Whatever small drones can do, they cannot do it for long because of their limited battery life. The smallest only fly for ten minutes or so. Quadrotors may manage half an hour, and the Raven, our gold standard for capability, keeps flying for about ninety minutes.
The situation is better the larger the drone. The RQ-20 Puma, the Raven’s big brother with twice the wingspan and three times the weight, has an endurance of three hours. Scaling means that, other things being equal, bigger drones carry proportionately more batteries or fuel.
Switching from battery power to liquid fuel extends the range of small drones dramatically. In 2003, a drone weighing eleven pounds with a wingspan of six feet flew across the Atlantic, from Cape Spear in Newfoundland to Mannin Beach in Ireland. The drone, known as TAM 5, took thirty-eight hours to make a crossing of almost two thousand miles, at a steady speed of around fifty miles an hour. Small drones can cross oceans and continents.
Unfortunately, the internal combustion engine has disadvantages. For one thing, it is very noisy; this was one of the flaws that sunk the RQ-16 T-Hawk. It cannot sneak up and take pictures without being spotted.
A drone with a combustion engine is more vulnerable to being shot at, and when it crashes, there is a safety hazard. Those falling apart landings, or running into a wall, become less hilarious. And the logistical issues—having to keep stores of drone fuel and drone oil, servicing and maintaining the engine—make it more complex and less attractive than the battery-powered alternative.
On the face of it, small drones are doomed to perpetual disadvantage compared to their larger cousins. The Raven cannot match a Predator’s ability to orbit over a target area for twenty four hours or more, and the Raven is confined to a small area. With current technology, a Raven could in theory fly out thirty miles, take a picture, and then return to base before the batteries gave out, but that is literally as far as it goes. In practice, because a battery reserve is needed and there is likely to be wind, the maximum range for a Raven is much less.
One way of dealing with this is a drone carrier. A transport aircraft, manned or unmanned, acts as a mothership, carrying a fleet of drones to the battle area. It then releases them to fly the last few miles under their own steam. For expendable, Switchblade style drones this would be a one-way trip, as they are effectively miniature cruise missiles. Reconnaissance drones could return and be picked up by the mothership via a net, just like the old Firebees. The drone carrier, like the aircraft carrier, could become a powerful tool for force projection.
However, it would be wrong to assume that small drones could never carry out strategic reconnaissance or strike missions. In the near future, small drones will stay on the wing 24/7, with mission times measured in days, months, or longer. A clutch of different technologies are contributing to this transformation.
Batteries That Are Stronger, Longer
Over the years we have seen batteries progress from lead-acid to nickel-cadmium (NiCad), nickel metal hydride, lithium-ion, and the related lithium phosphate and lithium polymer cells.
Years of intensive research have pushed these types of cells close to the theoretical maximum for the battery chemistry involved. The current state of the art are the batteries in the Tesla Model S, packing in about 240 watt-hours per kilogram; each pound of battery holds enough energy to boil about a third of a pint of water. That is less than a tenth of the energy density of gasoline or other liquid fuel, but several times greater than NiCads.
Drones like the Raven have the same rechargeable lithium-ion battery technology as laptops, phones, and electric cars. Any improvement will mean a change in battery technology.
Lithium-air batteries look like an attractive alternative to lithium-ion. They take oxygen from the air and have many times as much power per pound. In fact, the energy density of lithium-air batteries is close to that of gasoline. The problem is making them safe, as lithium-air cells produce a lot of heat during operation. This is hardly surprising; the chemical reaction involved is similar to burning highly reactive metal. The designers also need to find a way of avoiding contact between the metallic lithium and water in the air, otherwise spontaneous fires can result.
IBM gave up on lithium-air battery development in 2014 after five years of work. The Joint Center for Energy Storage Research set up by the Department of Energy, did the same after two years. Both concluded that while lithium-air might be the future in the long-term, it was not yet feasible.
A more promising variation is the “molten air battery” created by researchers at George Washington University with support from the National Science Foundation. This features a molten electrolyte of special materials such as vanadium boride, which can handle more electrons per molecule than lithium. Researchers estimate that this technology will be able to store twenty to fifty times as much energy as lithium ion, but it is likely to take at least a decade to develop.
There is a gentler and more immediate alternative in the form of lithium-sulfur (Li-S) chemistry. This stores less energy than lithium-air but far more than lithium-ion, and it is also significantly cheaper. The main component, sulfur, is a by-product of the oil industry. Oil refineries produce millions of tons of sulfur every year, whereas Li-ion batteries require expensive metals like manganese, nickel, and cobalt. Lithium-sulfur batteries are also safer and more environmentally friendly than lithium-ion.
Researchers around the world have been working on Li-S for years, but have struggled with a couple of basic problems. When the battery is discharged, a layer of lithium sulphide tends to build up around the anode, degrading the cell capacity after a few cycles. The other problem is that the conversion to lithium sulphide and back involves swelling and contraction—the bulk can change by more than half—putting mechanical strain on the cell.
However, researchers now claim to have overcome these problems. The British company OXIS Energy aims to be first to market in 2016 with a lithium-sulfur battery. The Li-S cell consists of a lithium metal anode and a sulfur-based cathode in a polymer binder, separated by an electrolyte.
Lithium batteries carry an element of risk because of the growth of root-like metallic lithium “dendrites” within the cell. These can cause short circuits resulting in overheating and fires. However, the sulfur electroyte in Li-S creates an insulating layer of lithium sulphide that nullifies this risk. Further, OXIS’ electrolyte is non-flammable. OXIS batteries have passed a wide range of abuse tests including overcharging and short-circuiting. The British Ministry of Defence has tested safety by firing bullets through them, and even then the cells continued to function safely, though at reduced capacity.
The initial commercial version already stores almost 50% more energy per pound than lithium-ion, and that will continue to improve. OXIS has a roadmap that includes reaching 500 Wh/Kg by 2018 (more than double the Tesla battery), and they are well on the way with prototype cells. Looking further ahead this should double again as the full potential of lithium-sulfur is developed as Li-ion has been. They also benefit from an unlimited shelf life, as lithium-sulfur cells do not require periodic recharging like Li-ion.
Prototype lithium-sulfur cells have already been used in experimental drones. Simply upgrading the batteries to Li-S could increase the flying time of the Raven to around six hours in a few years’ time. As other battery technology starts this could be extended further.
SWARM TROOPER: HOW SMALL DRONES WILL CONQUER THE WORLD
David Hamling
Natraj Publishers, Pg 338, Rs 699