Aircraft engines for unmanned aerial vehicles

As is well known, small unmanned aerial vehicles (UAVs) are now widely used in most of the world's leading militaries. Their designs have reached unprecedented levels of sophistication in recent years—for example, the flight endurance of some models used for aerial reconnaissance is now approaching 24 hours of continuous flight. UAVs used for missile guidance provide previously unimaginable missile accuracy—up to 1 meter — and the resolution of their video equipment allows for the detection of even the smallest objects on the ground. Detecting and destroying such aircraft is very difficult, and sometimes downright impossible — due to their small size, UAVs are virtually invisible and inaudible from the ground, even with the most sophisticated radar systems. There are also numerous other tasks solved by this type of aircraft, and not just military ones.

The experience of wars and local conflicts in recent years has already demonstrated that conventional combat aircraft (fighters, attack aircraft, and bombers) alone cannot solve a number of serious combat missions. This already likely lead (and will be more in the future) to the widespread introduction and use of unmanned aerial vehicles (UAVs) in the near future.

The development of UAVs of various designs and types began over 35 years ago, and work in this area had already reached significant intensity by the early 1980s. This work was carried out not only abroad, but also in the USSR, where a significant impetus for these developments was given after the First Lebanon War, where Israeli UAVs played a significant role in the defeat of Arab-Soviet forces in the Bekaa Valley. This work continued successfully internationally, while the economic collapse of the USSR seriously hindered the developments that had been completed by that time.

However, by the late 1970s, work on developing small reconnaissance UAVs equipped with highly advanced electronic and video equipment was already well underway in the USSR. These UAVs were not only created on paper and in metal, but also tested at test sites, where the performance of all systems was assessed and even the detection and engagement capabilities of these aircraft by the most modern air defense systems (which, incidentally, had already proven ineffective against this type of aircraft).

Small-sized turbojet engines

How does a small UAV differ from a regular full-size aircraft? Obviously, it's not just the absence of a pilot and the need for complex electronic control systems. One of the main challenges of such an aircraft is the engine. Because an aircraft weighing several hundred kilograms, and especially several dozen, doesn't require tons of thrust — it requires a very, very miniature engine with a thrust of only 10 or 20 kilograms. Or a power output of 10 or 20 horsepower.

Despite its apparent simplicity (well, what's so difficult about making an engine smaller or larger?), the task of creating any small engine based on existing experience with full-size designs is extremely difficult to solve in practice. If we're talking about a power plant with an internal combustion engine and a propeller, there are simply no equivalents with the required power suitable for aviation use. On the other hand, trying to reduce the size of full-size aircraft engines by a factor of 10 is simply a lost cause. In the years when work on this topic was underway (including at the Moscow Aviation Institute), aircraft model engines seemed the closest and most suitable option — they offered the highest performance and had a low power-to-weight ratio, which was precisely what was needed for small-scale unmanned aircraft.

Unfortunately, attempts to scale up such well-developed microdesigns even by 5-10 times proved futile — those proven design solutions that worked well at microscale proved completely ineffective at macroscale. Ultimately, the opposed 2-stroke design gained some popularity — four-stroke engines had not yet been developed, and the opposed 2-stroke engine provided the required 20-30 hp. And, all other things being equal, not the highest level of vibration, which was extremely undesirable for the onboard television cameras used at the time.

Jet engines were also being developed for a wide range of potential applications. But here, the situation was even worse — there were no analogues or prototypes at all. Today, there are micro-turbojet engines for model aircraft, but back then, the smallest turbojet engine could be made, for example, from a turbostarter (used to start larger turbojet engines). But that produces about 50 kg of thrust, which is too much for aircraft weighing less than 200-250 kg. And what kind of auxiliary unit is an engine anyway? It can start other engines, but it's too heavy to operate independently...

And what's the biggest problem with micro-turbojet engines? It's the clearances between the rotor blades and the housing. When the engine is large, the clearances are relatively small, and this determines good compressor and turbine efficiency, and therefore engine thrust. But as soon as you start reducing the dimensions, the gaps take their toll — you can't make them smaller, otherwise the blades will rub against the housing as RPMs increase and heat rises. And if you make them normal, the inevitable leaks through such gaps will seriously reduce efficiency. And there's no thrust...

So an idea arose — how could we get around these gaps altogether? That is, make the housing and impellers one piece? Thus arose the idea of ??a rotary turbojet engine. The idea proved quite original — nothing comparable existed anywhere in the world at the time. The essence of the concept is extremely simple (see diagram): the rotor rests on two bearings, but the compressor and turbine wheels are connected not by a shaft along the inner diameter, but by a drum along the outer diameter. A cavity is formed between the wheels — the combustion chamber. Air exiting the compressor moves in a spiral motion; during combustion, it maintains this spiral motion and enters the turbine.

The advantages of this design, given its small size, are undeniable: not only are there no gaps in the rotor blades, but a compressor straightener and turbine nozzle are also eliminated. That is, there are no fixed blades that first straighten the swirling flow after the compressor and then swirl it again before the turbine. The combustion chamber itself is also unique: with the air spiraling, mixing it with hot gases to achieve an acceptable gas temperature before the turbine requires no special devices. Simply separating a section of the chamber near the axis from the main flow with a cone and injecting the required amount of fuel into it is sufficient.

Invention certificates were obtained for the described engine design and its combustion chamber. Interestingly, one of the most renowned engine design bureaus of the time defended its priority for this engine design for a long time, but ultimately lost. Unfortunately, for various reasons, the actual production of a prototype never took place, although the documentation was prepared. Rather than arguing with the design bureau, it would probably have been better to team up for a joint project, but the opportunity and time for that were missed...

Small-sized pulse jet engines

Another trend in those years was the use of jet engines of other designs, including the infamous pulsejet engine. This type of engine gained infamy because it powered the V-1 (V-1) flying bombs used by the Germans to bomb London in 1944-45. The German unmanned "V-1" (cruise missile) Fieseler Fi-103 was effectively the world's first combat unmanned aerial vehicle, used en masse during World War II.

The full-scale pulsejet was first developed in Germany in the late 1930s by the German aerodynamicist Paul Schmidt, based on a design proposed as early as 1913 by the French designer Lorin. The industrial prototype of this AS109-014 engine was created by the Argus company in 1938. It was a near-perfect fit for the primary powerplant of a disposable unmanned aerial vehicle: its simple and technologically advanced design ensured low production costs, and it could use regular aviation gasoline as fuel. The AS109-014 engine consisted of a variable-section sheet steel tube consisting of a nose cone with a diffuser and inlet valve grid, a combustion chamber, and an exhaust pipe (nozzle). This engine experienced 40-45 pulsating fuel combustion cycles per second, generating up to 300 kgf of thrust, sufficient to propel the 2,160 kg V-1 aerial vehicle at 600-650 km/h.

After the war, captured engine and aircraft samples were brought to Moscow and carefully studied. Aircraft Factory No. 51 began producing copies, and the Chelomey Design Bureau developed a number of improved models. Soviet scientists B. Stechkin and E. Shchetinkov even developed a theory and the fundamentals of gas-dynamic calculations for this type of engine. However, the project never progressed beyond pilot batches — the military was dissatisfied with the target-hitting accuracy of the developed systems. There were also complaints about the engine — its increased fuel consumption prevented it from achieving long range and flight endurance. Another problem was the limited flight speed, although the latest prototypes of the Chelomey Design Bureau's improved aircraft with twin pulsejets were able to reach 960 km/h. The intense noise and vibration of a pulsejet engine also hindered its adoption in aviation.

Similar problems were encountered in the United States, where captured V-1 prototypes were also being studied and improved versions developed. As a result, by the early 1950s, all work on this type of engine was gradually curtailed in both the USSR and the United States.

However, this engine type did not fade into oblivion. Numerous engineers and researchers continued, and continue to do, to study the processes and develop various pulsejet designs. And there are reasons for this. If we consider the potential applications of a given engine, it is easy to see that they depend on the tasks the aircraft must perform. For example, in applications where very high speed and flight endurance are not required, a pulsejet is quite capable of achieving both. Therefore, interest in this type of engine has periodically resurfaced. This was also the case by the late 1970s, when work on small-scale unmanned aircraft began.

Obviously, a full-scale pulsejet was too large for the tasks of the 1970s and 1980s. However, scale was not an issue here – the engine itself is extremely simple in design and has no rotating parts. Incidentally, aircraft modelers had long since tested the German invention and mastered such engines with a thrust of just a few kilograms. A similar prototype was used as the basis – it differed from the German prototype not only in its many times smaller size and almost 100 times lower thrust, but also in its cycle frequency, which, on the contrary, reached 150 Hz.

During the course of the work, it was gradually possible to significantly refine the theory of the operating process and develop a calculation program to optimize the parameters of this type of engine (at that time, stationary computers with parameter input from punch cards were used). As a result of theoretical and experimental testing, the optimal chamber and inlet geometry was selected, which served as the basis for the prototype design.

However, it wasn't all that simple — the specific task involved not only creating the engine itself, but also its starting system, one that was autonomous and capable of starting the engine automatically in flight. Accordingly, the design included a small air cylinder compressed to 100 bar, an air valve driven by the servo, a fuel tank with a solenoid valve, a gas cylinder, and a boost pressure regulator, as well as an ignition system with a special ultra-miniature coil and a surface-discharge spark plug.

For simplicity and compactness, liquefied propane was used as the gas used to pressurize the fuel tank. However, this necessitated dividing the tank into gasoline and gas chambers with a sealed rubber membrane (to prevent propane from dissolving in the gasoline). When the start command was given, the servo released the locking pin on the air valve stem. The compressed air pressure in the tank opened the supply to the combustion chamber. The stem then pressed a limit switch, which activated the electromagnetic fuel valve and ignition system — this was sufficient for a reliable start.

The resulting design was very compact, with good specific parameters (relative to its weight), allowing further development. To this end, several prototypes of the power plant were built, after which they underwent numerous bench and flight tests. The engine was assembled with all its components (see photo) and installed on a specially designed folding aircraft. The aircraft was loaded into a cassette (a variant of the attack UAV system, with four engines per cassette — approximately 30 years before similar DAPRA projects). After being released from a high altitude, it exited the cassette, deployed, and automatically transitioned to horizontal flight. The engine then automatically started and the aircraft flew according to a preset program.

During system development at the MAI airfield in Alferyevo in 1982, test drops of the aircraft were conducted from an altitude of approximately 100 meters, which was achieved by a meteorological balloon. Based on the test results, the power plant demonstrated exceptional reliability — 100% of the engine starts in the air were successful.

It's difficult to say today what all this work might have resulted in if it hadn't stalled in the mid-1980s. But it's quite possible we would now look at some modern foreign drones with somewhat different eyes. For us, this work was a stepping stone to the internal combustion engine — if we replace the combustion chamber of a turbojet with a cylinder and piston, all the processes would be very similar...

Current state of the problem

At various times, we conducted several research projects, the results of which have been published and presented in our articles on UAVs. These are:

1. The most complete and detailed description of our mathematical model and online simulation program for a pulsejet engine.
Khrulev A., Muntyan V. (2025). Mathematical model and computer program development for online modeling of pulse jet engine working cycle, parameters and characteristics. Drone Systems and Applications, 2025, Just-In, 48 p. DOI: https://doi.org/10.1139/dsa-2025-0027

2. A brief description of our mathematical model and simulation program for a pulsejet engine.
Khrulev O., Muntyan V. (2025). Development of universal mathematical model and computer program for online modeling of valved and valveless pulse jet engines. Aerospace Technic and Technology No.4, sup.2, 2025, pp. 6-29. DOI: https://doi.org/10.32620/aktt.2025.4sup2

3. Using dimensionless similarity criteria and the piston analogy method for modeling of an engine with a periodic working process .
Khrulev A. (2025). Modeling of engine with periodic workflow using dimensionless similarity criteria and piston analogy method. World of scientific research, 2023, Issue 23, Opole, Poland, 24 October 2023. Aviliable at: https://www.economy-confer.com.ua/full-article/4861/

4. Using the piston analogy method for modeling of gas flow in resonant tubes and channels of engines with a periodic working process .
Khrulev, A. (2023). Determination of gas parameters in resonant pipes and channels of engines with a periodic workflow using the piston analogy method. Eastern-European Journal of Enterprise Technologies, 5 (7 (125)), 50–59. DOI: https://doi.org/10.15587/1729-4061.2023.288520

5. Mathematical modeling of a reed valve in intake systems of engines with a periodic working process.
Khrulev, A. (2024). Technical condition assessment and modelling of reed valves in vehicle engine intake systems. Communications. University of Zilina, 27 (1), B41-B52. DOI: https://doi.org/10.26552/com.C.2025.006

6. How to use commercial microturbojet engines for high-speed small-sized operational-tactical UAVs.
Khrulev A. (2023) Analysis of possibility of using commercial micro turbojet engines for high-speed small-sized operational-tactical UAVs. Aerospace Engineering and Technology, No. 4, special issue 2 (190), pp. 5-18. DOI: https://doi.org/10.32620/aktt.2023.4sup2.01

7. How to choose and modeling the parameters of the pneumatic catapult launch system taking into account the characteristics of the engine and the UAV
Khrulev, A. (2023). Analysis of pneumatic catapult launch system parameters, taking into account engine and UAV characteristics. Advanced UAV, 3 (1), pp. 10-24. Aviliable at: https://ab-engine.com/smi_paper.html

So, what came out of all this swarm?

All this work allowed us to stretch the legs a bit and... create a serious program Pulsejet-sim for online modeling of the parameters and characteristics of pulse jet engines.

In fact, this program is designed so that the engine's operating process is calculated on the server, which doesn't require a high-end computer or downloading the program — the drive simply transfers data to the server and receives the finished results. And the server isn't a home computer; it's 128 parallel processors!

Using such resources allows, on the one hand, to perform the most complex calculations (tens of thousands of time points) in no more than 2 seconds, and on the other, to do it easily, even on the phone! All the science is in your smartphone, 100%!

What can be modeled, what kind of engine? Any engine, valved or valveless. Any size. The modeling results in characteristics that no one else has. For example, diagrams of changes in instantaneous pressure, temperature, and gas velocity in the cycle. And even valve lobe oscillations. You can also view an indicator diagram of a pulsating engine (know where else you can get one?). And even altitude-speed characteristics (where else can you get those?). And conduct parametric studies on the influence of individual dimensions on parameters. And also assemble any diagrams together.

Don't believe? Well, if you want to try it yourself... :

THEN YOU NEED TO BE HERE >>>

Our other work in aviation...