The valveless pulsejet has a complete lack of moving parts in its construction1. An obvious benefit to this is its reliability, requiring almost no maintenance to operate2. They can also operate in extreme conditions, with cheap materials and even cheaper methods of construction3. These use cases make it ideal for being the powerplant in a medical drone in LEDCs, as they must operate continuously, all day, in harsh conditions and in impoverished areas4.
The use of UAVs for medical purposes has long been prevalent in countries that lack the infrastructure for other means of transportation, like roads for emergency vehicles in general5. Zipline, (https://www.zipline.com/), is currently the largest drone delivery service in the world. It has integrated deeply into local public health systems, driving economic growth and proving that drone delivery is both practical and beneficial on small and large scales6.
Zipline currently uses an all-electric powertrain7, so the switch to oil and gas-powered drones seems like a step in the wrong direction, until investigating the pros and cons of battery powered planes. A major factor in why no commercial plane has been made to use batteries is the sheer weight of even the most advanced cells. Solid state batteries are still largely in development, but even the most promising candidate to replace conventional Lithium-ion batteries, silicon-anode lithium-ion cells, still have an energy density of 1080 Wh L−18. Compared to the energy density of aviation fuel such as Jet A, Jet B, JP-4, JP-5, and JP-8, which average 11833.33 Wh L−1 9 , even the best batteries average around 9% the energy density of fuels that are cheaper and more readily available anyway.
Electric power systems have their benefits, namely in efficiency, quietness, and renewable integration.10 In more urban areas, especially those in HEDCs, these benefits might outweigh the costs of having such a low energy density power system. In environments that do not match these requirements, a fossil-fuelled powered contraption makes a lot more sense.
In the context of medical drones however, the need to have immediate access to individuals in many different financial or geographical situations has priority over the push for the greener initiative. If an area cannot implement an electrically based emergency drone delivery system due to the upfront or continuous cost of batteries and electrical power, the fuel powered alternative will prevail.
The engine I have chosen for the powerplant of this medical drone is the valveless pulsejet, the focus of this EPQ. This is mostly due to its sheer reliability and indifference for the fuel it is supplied with, whether that be jet fuel or even general fuels like gasoline. The famous V-1 pulsejet powered bomb, used by the Nazis in WW2, was an unmanned bomb weighing more than two tons, including 80-octane gasoline and an 850 kg warhead.’11 Each V1 rocket had a range of 150 miles, capable of cruising at around 360mph with a profile of just 8 meters long 12.
I threw together a mock promotional video in Unreal Engine 5 with an early design of how I thought the drone should look. In this video, the pulsejets are mounted under each wing, akin to how turbojets are mounted on commercial airliners, though I will discuss in the later sections of this text why this is suboptimal.
For any medical purpose regarding an emergency with a reasonable number of humans, the carrying capacity of the drone does not need to exceed around 4kg to effectively carry all the supplies necessary; the max payload capacity of the current generation Zipline drones is 3.6kg. If a pulsejet powered rocket has been documented to reliably carry 850kg across 150 miles, then the propulsion system has already proved itself, and just needs to be implemented into a much smaller package.
By observing the side profiles of both the V1 (above) and the current generation Zipline drone (below), there are striking similarities in how their body shapes and propulsion systems compare. Both have a high centre of thrust that is above their centre of gravity and centre of lift. This means that by removing the T-pylon that would normally house the electric motors in the Zipline, and replacing it with a pulsejet engine, we would not be left with very different flight characteristics.
To test this theory before creating the actual engine, I 3D-printed out this scale model with PLA and tested its performance in both CFD and scale wind tunnel testing. I tested the engine without any nosecone initially, using Sim Scale to model the regions of high pressure.
The large red centre of pressure represents an area of high drag, which would be detrimental for the aerodynamic efficiency of the engine. It would
also deflect air around the engine itself, drastically reducing the
boundary air layer that would otherwise help cool the engine to maintain thermal stability.13
I therefore chose to add a nosecone to the engine that would ideally be hollow and made from low grade, temperature resistant steel. The purpose of this cone would purely be to prevent the high-pressure region of air and boundary diversion mentioned above for better thermal performance and reduced drag.
This plan, using the Zipline model, could then be modified heavily in CAD to bring it towards my design:
To make the manufacturing of the exhaust more feasible, I decided to straighten it, forming more of a traditional Chinese style pulsejet14. This would also increase thrust by allowing the high-pressure waves of exhaust gas to leave with minimal skin friction and therefore minimal induced vortices that would suck away kinetic energy.15
