Drone Technology's Quiet Revolution
Exploring the impact of drones from warehouse to battlefield
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In today’s newsletter we discuss drones. From their humble beginnings as balloon bombers, drones have evolved to have a huge impact in many areas, from making Amazon deliveries more efficient to turning the tide of the war in Ukraine. Read on to see what today’s drone tech looks like, and to also see technologies like ion thrusters that will advance drones even further.
Unless you’ve been living under a rock, you probably know about the Russia-Ukraine War and its devastating consequences. What you’ve probably not heard about is the Bayraktar TB2 drone, which has been one of the most important reasons behind Ukraine’s stubborn defense against the Russian military colossus. The Bayraktar, now on the purchase lists of armies all over the world , is just one example of the incredible impact drone technology is already having from warehouses to battlefields.
Drones, or to be a bit more precise unmanned aerial vehicles (UAVs), have been in use for around 100-150 years. As with many technologies, drones first found widespread use in military contexts. The first recorded use of a UAV was on August 22, 1849 when Austria attacked Venice with a fleet of pilotless balloons carrying explosives. The idea was that the wind would carry the balloons in the general direction towards the target. By estimating the speed with which the wind would move the balloons, the Austrians could time the explosives to explode at the target. Some balloons also apparently had fuses triggered by electrical signals which had to be carried by long copper wires. As you might guess, unlike the Bayraktar these balloons weren’t too accurate. Leaving drone propulsion to the whims of the wind meant that explosives usually didn’t reach their target and sometimes even threatened the Austrians.
Despite this less than stellar debut, the fundamental value proposition of drones was clear — they could inflict damage without putting humans at risk. In non war contexts, they could do useful work without the need for human labour. Over the next century or so, drone design advanced exponentially thanks to numerous advances in science and technology such as the discovery of radio in 1880.
Instead of being a cute nuisance like the Austrian balloons, today’s drones are highly effective precision instruments. Their impact on the battlefield, the original theatre for which they were invented, has revolutionised modern warfare. The Bayraktar drone’s ability to launch quick and precise airstrikes at low cost against lumbering Russian military convoys has played a huge role in bolstering Ukrainian resistance against its much more powerful neighbour. In the 2020 Armenia Azerbaijan war, Azerbaijan’s use of Bayraktar drones against Armenian soldiers and ammunition depots decisively turned the tide of that war too. It even prompted even US military planners to study Azeri tactics. Thanks to cheap and effective drones, any country can now buy an automated air force off the shelf to bolster its land troops. In this sense, drones have had a huge impact by equalising the asymmetric advantage traditional large military powers have against smaller opponents. As drones become cheaper, better, and more widespread, their impact will also stretch beyond the battlefield into ordinary commercial applications.
What are Drones Like Today?
Today, there are two main categories of drones: fixed wing and rotor drones. Each of these comes with its own tradeoffs of advantages and disadvantages which make them suited for different applications.
Fixed Wing Drones
Fixed wing drones have rigid wings and look similar to airplanes. As with airplanes, the wings provide the lift to let them take off. Fixed-wing drones also tend to be the largest types of drones. For example, the Bayraktar TB2 deployed by Ukraine against Russia has a wingspan of 12 m and a length of 6.5 m.
The greatest advantage of fixed-wing drones is that they have long flight times as their large sizes allow them to carry a larger amount of fuel/larger batteries than other smaller types of drones. For example, the Bayraktar TB2 has a flight time of 27 hours. Long flight times mean that fixed wing drones are ideal for applications which take time such as mapping out large areas of land. The Northrop Grumman RQ-4 surveillance drone, another fixed wing UAV with a mammoth 39.8m wingspan and 34 hour flight time, can survey about 100,000 sq. km — roughly the size of South Korea — per day.
Another advantage is that fixed wing drones can carry large payloads. The TB2 supports a 700kg payload, enough for four sets of laser guided munitions. In addition to fuel, the RQ-4 can carry a payload of 1360 kg. They also tend to have the highest flight altitudes. While the TB2 can fly at a maximum height of 7620m — 86% the height of Mount Everest — the RQ-4 can reach almost 20,000m. High altitudes once again help with surveillance and precision targeting across large areas, and make the UAVs more difficult to detect. The combination of long flight time, high payload size, high altitude and lower maneuverability make fixed-wing drones ideal for military applications.
However the same properties which help fixed-wing drones’ flight time and payload size also hinder their flexibility and ease of use. One disadvantage of fixed-wing drones is that since they are like smaller remote airplanes, they must also have gradual takeoffs and landings like airplanes. Fixed-wing drones require a large open area since they can’t takeoff and land vertically. Unlike smaller types of drones, they also cannot make very sharp turns.
Rotor Drones
Rotor drones are characterised by multiple rotors that generate vertical lift as opposed to the large wings of fixed-wing drones. Many drones for commercial applications, like the quadcopter that many people imagine when they hear about drones, are rotor based UAVs.
While fixed-wings take their inspiration from airplanes, rotor drones are inspired by helicopters. This means that, like helicopters, rotor drones can change their speed, altitude, and direction more quickly than fixed-wings. Rotor drones can also take off vertically from almost anywhere, whether it’s a small rooftop or an aircraft carrier deep in the ocean. They also tend to be much smaller and cheaper than fixed-wings, which is what allows them to be used in commercial applications.
The exact characteristics of rotor drones depend on whether they have single or multiple rotors. Multi rotor drones, like the quadcopter, have much better maneuverability because they have multiple rotors to adjust variables like pitch, angle etc. This means that multi rotors can be more stable in the air because they can more precisely account for small disturbances from wind/other factors. Single rotor drones are less maneuverable but tend to be larger and so can support larger payloads. Single rotor drones are ideal if you want your drone to carry some heavy sensing equipment like LIDAR sensors or magnetometers for large surveys.
On the other hand, rotor drones have much lower flight times than fixed-wings since rotors require a lot of energy to maintain lift. The Austrian S-100 CAMCOPTER single rotor drone has a flight time of 6-10 hours. The DJI Inspire 2 drone, a commercially available multi rotor drone for capturing HD video, has a flight time of only 20-30 minutes. Finally, the noise from the rotors may attract unwanted attention which is a disadvantage when stealth is required.
Fixed Wing Hybrids
Recently, engineers have tried to achieve a middle ground in the tradeoff between fixed-wings and rotor drones by trying to make fixed-wing hybrid drones capable of vertical takeoff and landing. This is a relatively newer field with many startups competing with giants like Amazon. The goal is to make drones that are maneuverable yet can carry heavy payloads, perfect for applications like delivering goods. Amazon’s Prime Air drone service, is set to go into test in Lockeford, CA very soon, while startups such as Aergility are innovating with new hybrid drones that can carry payloads of 600 lbs (270 kg) due to lift force provided by both rotors and wings simultaneously.
From Combustion to Ion Engine
One of the most important features of a drone is its propulsion or how it generates the thrust needed to move itself. The traditional mechanisms of propulsion in drones are either rotors for rotor based drones or mechanical engines much like those which power commercial airliners such as the turbofan engine.
These longstanding propulsion methods obviously work well, but there is still plenty of space for innovation in this area. One of the most exciting prospects is the ion thruster engine, first used with a drone by a team from MIT in 2018. Although it sounds like something straight out of Star Wars, ion engines have already been used for years in satellites floating in empty outer space. The real challenge lies in making ion thruster technology more effective in the messy friction filled conditions of our atmosphere.
Like any thrust generating engine, ion engines rely on Newton’s Third Law: if A exerts a force on B, B exerts an equal force in the opposite direction on A.1 For example, as commercial turbofan engines exert a force to push out air behind them, the air in turn exerts a force forward on the engine and whatever it is attached to. This is what creates the thrust.
In ion engines, we use beams of ions to create thrust. An ion is simply an atom or molecule that has an electrical charge because it has lost or gained an electron. We can use electrical fields to exert forces on ions and push them backwards out of the engine, much like air is pushed out of a combustion engine. This makes them accelerate and generate forward thrust through Newton’s Third Law.
For a concrete example, let’s look at how a Gridded Ion Thruster — one of the most widespread types of ion engine — works. The diagram below shows the main parts:
First, we have our green neutral propellant atoms injected into the engine on the left. In addition, there are also extra electrons that are injected from the electron gun. When the electrons collide with the atoms, they knock out electrons and turn the atoms into positive ions. There is a strong electric field between the positive and negative grid, and when the positive ions enter this space, they are repelled from the positive grid and attracted to the negative grid. This electrical force ejects the ions out into space. By Newton’s Third Law, the ejected ions exert an equal force on the engine but in the forwards direction, creating thrust. The strength of the force on the ions, and hence of their acceleration, is proportional to how strong the electric field between the grids is.
That is a somewhat simplified explanation and an actual ion thrust engine is a little more complicated. One issue is that the negative electrons are also attracted to the positive grid. To manage this, we put magnetic fields around the engine. The key fact here is that electric charges are also affected by magnetic forces, but only if the charges are in motion. When the electrons are streaming towards the positive grid, the magnetic fields can affect them and are oriented such that the electrons actually travel in a DNA shaped helix pattern instead of a straight line. This means the electrons are forced to travel a much longer path on their way to the grid, increasing the chance they collide with a neutral atom and hence create an ion. Another complication is that if too much positive charge is released, a negative charge will build up on the engine. Not only will this cause problems with the ion thruster but it could damage other electronics as well. To solve this, a secondary electron gun is used at the exhaust is to release an equal amount of negative charge so that the overall charge of the engine stays neutral.
Ion thrusters have the huge advantage of not having moving mechanical parts, which has numerous downstream consequences. Parts do not have to be designed to be strong enough to withstand the large stress forces that come from motion, which could result in lower costs. Moving parts are also noisy, and silent engines would be valuable especially in military applications like reconnaissance where stealth is important. Another advantage is that ion engines are more efficient than chemical engines as they generate more thrust per unit of propellant.
Overall however, ion engines produce much less thrust than chemical engines. The NEXT ion engine used on satellites generates 0.237 N of thrust. If the Bayraktar’s combustion engines were replaced with 100 NEXT engines, it would give an acceleration only 0.3% as strong as the acceleration with which objects naturally fall on Earth due to gravity. In space, where there’s no air resistance to work against the thrust, this isn’t a problem because you can just keep applying this small force for a long time and get an impressive acceleration. On Earth, you’d need much larger forces to generate the same acceleration, which would requires very large electric fields to generate adequate thrust.
This is the challenge that the MIT team was able to overcome with their ion thrust engine. The final fixed-wing design of the MIT team is a careful optimisation subject to numerous constraints, like the mass of the overall drone, the number of ion thrusters, and the type of batteries needed to generate the large electric fields. To generate the required fields, the MIT drone’s Lithium Ion batteries needed to supply electricity at an enormous 40,000 volts. This huge electric field ionises the Nitrogen found naturally in the atmosphere, which is what gives the drone its thrust and allowed it to demonstrate sustained flight for 60 m.
The MIT team’s demonstration has led to a flurry of research and development into ion engine tech. In particular, a lot more thrust will probably be needed to achieve vertical takeoff and landing, and solving that problem is much more difficult than simply tacking on more thrusters. Many startups such as Undefined Technologies have already begun tackling these problems and developing the next generation of ion engine tech.
How To Power Drones
As the previous discussion on propulsion was hinting at, another crucial technical challenge of designing a drone is designing its power supply. Many large military level fixed wing drones like the Bayraktar rely on tried and tested internal combustion engines. Smaller drones, such as rotor drones, use electrical power supplies like batteries. One of the most common batteries used is the Lithium ion battery, which was used by the MIT ion engine team as well. These, which I explored in detail in my earlier post on batteries, are easy to use but tend to have low energy density so they can only be used by small sized drones. A newer alternative that might be promising is a hydrogen fuel cell.
Hydrogen fuel cells deserve their own post sometime in the future, but they work somewhat similarly to lithium ion batteries. Instead of lithium ions however, these use hydrogen and oxygen. In the chemical reaction, hydrogen combines with oxygen to produce electrical energy. Unlike lithium batteries, which produce environmentally hazardous waste when they are used up, hydrogen fuel cells produce water which is a great advantage. Hydrogen and oxygen are also abundantly available. As the chart below shows, hydrogen fuel cells are also more energy dense than lithium batteries so they could be used to power larger drones.
Hydrogen fuel cells are a newer technology but many manufacturers are already making notable progress with them. In 2016, Singapore’s ST Aerospace used a hydrogen fuel cell to power its 9 kg, 3 m wingspan Skyblade 360 UAV for distance of 300 km, showing that hydrogen could definitely be viable for drones. With hydrogen, smaller rotor drones could fly for much longer, meaning that we might see them replace fixed wing drones in certain contexts. Perhaps a future Bayraktar will be a more nimble rotor drone powered by hydrogen fuel cells that is capable of precision strikes on both not only slow and large tank convoys but also small and agile targets.
Navigating with Drones
The final challenge for drones is navigation and this becomes a real problem in areas where GPS is not accessible, like environments where tall obstacles block GPS signals. Examples include jungles or big urban cities full of huge skyscrapers. In situations like these, navigation systems like GPS which rely on external signals are difficult to use, so drones typically tend to use what’s known as an inertial navigation system, which can calculate a drone’s position from internal data.
The basic idea is fairly simple. If a drone can measure its own motion in all three directions of space, it can calculate its trajectory. This trajectory, combined with the knowledge of the starting point, can pinpoint the drone’s current location. To collect the relevant information on its motion, inertial navigation systems use three accelerometers and three gyroscopes. These accelerometers and gyroscopes are rigidly attached so that their orientation relative to each other is fixed. This orientation defines the axes of motion relative to which the trajectory is measured. The accelerometers are used to determine the speed in all three directions (up-down, left-right, forward-backward), while the gyroscopes keep track of rotational motion. This data allows estimation of the drone’s velocity. The velocity information, combined with knowledge of the starting point, gives the drone’s current position.
Conclusion
Drone technology is undergoing a quiet renaissance. From their humble beginnings as ineffective balloon bombers in 1849, drones today like the Bayraktar TB2 are revolutionising warfare. Today’s drones come in various shapes and forms, such as the fixed-wing behemoths that dominate military applications and smaller more nimble rotor drones which are prevalent in civilian applications such as Amazon’s upcoming autonomous delivery service. Drones are also an exciting testbed for innovations with technology like the ion engine and new types of batteries finding promising applications in powering autonomous aerial vehicles. As the underlying technologies continue to improve and costs continue to decline, expect drones to become even more common in the future.
Unnecessary but fun technical point: If you’re curious why Newton’s Third Law is true, it’s because momentum (the product of a thing’s mass and its velocity) is conserved. This means the total momentum of all objects before and after an interaction (like a collision) is the same. Why should such a random thing be conserved? It’s because of an incredible theorem known as Noether’s theorem combined with the fact that physics is translation invariant — the laws of physics remain the same whether you’re in America or in India or Pluto or the Andromeda Galaxy. Noether’s theorem says that because physics is translation invariant, every physical interaction must conserve momentum.