Do-it-yourself jet engine
The valveless PuVRD is an amazing design. It has no moving parts, compressor, turbine, valves. The simplest PuVRD can do without even an ignition system. This engine can work on just about anything: replace the propane tank with a gas canister and it will continue to pulsate and create traction. Unfortunately, PuVRD proved to be insolvent in aviation, but recently they have been seriously considered as a source of heat in the production of biofuels. And in this case, the engine runs on graphite dust, that is, on solid fuel.
Finally, the elementary principle of operation of a pulsating motor makes it relatively indifferent to manufacturing accuracy. Therefore, the production of PuVRD has become a favorite pastime for people who are not indifferent to technical hobbies, including aircraft modelers and novice welders.
Despite all its simplicity, the PuVRD is still a jet engine. It is very difficult to assemble it in a home workshop, and in this process there are a lot of nuances and pitfalls. Therefore, we decided to make our workshop a multi-part one: in this article we will talk about the principles of operation of the PuVRD and tell how to make an engine case. The material in the next issue will be devoted to the ignition system and the starting procedure. Finally, in one of the following issues, we will definitely mount our motor on a self-propelled chassis to demonstrate that it is really capable of creating serious traction.
From the Russian idea to the German rocket
It is especially pleasant to assemble a pulsating jet engine, knowing that for the first time the principle of operation of the PuVRD was patented by the Russian inventor Nikolai Teleshov back in 1864. The authorship of the first operating engine is also attributed to the Russian - Vladimir Karavodin. The highest point of development of the PuVRD is rightfully considered to be the famous Fau-1 cruise missile, which was in service with the German army during World War II.
Of course, we are talking about pulsating valve motors, the principle of operation of which is clear from the figure. The valve at the entrance to the combustion chamber freely passes air into it. Fuel is supplied to the chamber, a combustible mixture is formed. When the spark plug ignites the mixture, overpressure in the combustion chamber closes the valve. Expanding gases are directed into the nozzle, creating jet thrust. The movement of the combustion products creates a technical vacuum in the chamber, due to which the valve opens and air is sucked into the chamber.
Unlike a turbojet engine, in the PuVRD the mixture burns not continuously, but in a pulsed mode. This explains the characteristic low-frequency noise of pulsating motors, which makes them inapplicable in civil aviation. From the point of view of economy, PUVRDs also lose to turbojet engines: despite the impressive thrust-to-mass ratio (after all, PuVRDs have a minimum of parts), the compression ratio in them reaches 1.2: 1 from the force, so the fuel burns inefficiently.
But PuVRD are priceless as a hobby: after all, they can do without valves at all. Fundamentally, the design of such an engine is a combustion chamber with inlet and outlet pipes connected to it. The inlet pipe is much shorter than the outlet. The valve in such an engine is nothing but the front of chemical transformations.
The combustible mixture in the PuVRD burns at subsonic speed. Such combustion is called deflagration (in contrast to supersonic detonation). When the mixture is ignited, combustible gases escape from both pipes. That is why both the inlet and outlet pipes are directed in one direction and together participate in the creation of jet thrust. But due to the difference in lengths at the moment when the pressure in the inlet pipe drops, exhaust gases still move along the outlet. They create a vacuum in the combustion chamber, and air is drawn in through the inlet pipe into it. Part of the gases from the exhaust pipe is also sent to the combustion chamber under the action of vacuum. They compress a new portion of the combustible mixture and set it on fire.
The valveless pulsating engine is unpretentious and stable. To maintain operation, he does not need an ignition system. Due to rarefaction, it absorbs atmospheric air, without requiring additional boost. If you build a liquid fuel engine (for simplicity we preferred propane gas), the inlet pipe regularly functions as a carburetor, spraying a mixture of gasoline and air into the combustion chamber. The only time you need an ignition system and forced boost is the start.
Chinese design, Russian assembly
There are several common designs for pulsating jet engines. In addition to the classic “U-shaped pipe”, which is very difficult to manufacture, the “Chinese engine” is often found with a conical combustion chamber, to which a small inlet pipe is welded at an angle, and the “Russian engine”, which resembles a car muffler in design.
Before experimenting with our own PuVRD designs, it is strongly recommended that you build the engine according to ready-made drawings: after all, the cross sections and volumes of the combustion chamber, inlet and outlet pipes completely determine the frequency of resonant pulsations. If proportions are not respected, the engine may not start. A variety of PuVRD drawings are available on the Internet. We chose a model called "Giant Chinese Engine", the dimensions of which are given in the sidebar.
Amateur PuVRD are made of sheet metal. It is permissible to use finished pipes in construction, but it is not recommended for several reasons. Firstly, it is almost impossible to select pipes of exactly the required diameter. It is even more difficult to find the necessary conical sections.
Secondly, pipes, as a rule, have thick walls and corresponding weight. For an engine that should have a good thrust to weight ratio, this is unacceptable. Finally, during operation, the engine is red-hot. If pipes and fittings made of different metals with different expansion coefficients are used in the design, the motor will not live long.
So, we have chosen the path that most fans of PuVRD choose - to make a body of sheet metal. And then they faced a dilemma: turn to professionals with special equipment (CNC water-abrasive cutting machines, pipe rollers, special welding) or, armed with the simplest tools and the most common welding machine, go through the difficult path of a novice engine builder from start to finish the end. We preferred the second option.
Back to school
The first thing to do is draw a scan of future details. To do this, you need to remember school geometry and quite a bit of university drawing. To make reamers of cylindrical pipes as simple as possible — these are rectangles, one side of which is equal to the length of the pipe, and the second to the diameter times pi. Calculating the development of a truncated cone or a truncated cylinder is a slightly more complicated task, for the solution of which we had to look into the drawing textbook.
The choice of metal is a very delicate matter. From the point of view of heat resistance, stainless steel is best suited for our purposes, but for the first time it is better to use black low-carbon steel: it is easier to mold and weld. The minimum thickness of a sheet that can withstand the temperature of combustion of fuel is 0.6 mm. The thinner the steel, the easier it is to mold and the harder it is to cook. We chose a sheet with a thickness of 1 mm and, it seems, did not fail.
Even if your welding machine can operate in plasma cutting mode, do not use it to cut out sweeps: the edges of parts processed in this way do not weld well. Manual scissors for metal are also not the best choice, as they bend the edges of the workpieces. An ideal tool is electric scissors that cut a millimeter sheet like clockwork.
To bend a sheet into a pipe there is a special tool - rollers, or listogib. It belongs to professional production equipment and therefore is unlikely to be in your garage. A vise will help bend a decent pipe.
The process of welding millimeter metal with a full-size welding machine requires some experience. Having slightly overexposed the electrode in one place, it is easy to burn a hole in the workpiece. During welding, air bubbles can enter the seam, which will then leak. Therefore, it makes sense to grind the seam with a grinder to a minimum thickness so that the bubbles do not remain inside the seam, but become visible.
In the next series
Unfortunately, within the framework of one article it is impossible to describe all the nuances of the work. It is generally accepted that these works require professional qualifications, but with due diligence, they are all accessible to the amateur. It was interesting for us, journalists, to learn new specialties for ourselves, and for this we read textbooks, consulted with professionals, and made mistakes.
We liked the case we welded. It's nice to look at him, it's nice to hold in his hands. So we sincerely advise you to take up such a thing. In the next issue of the magazine, we will tell you how to make an ignition system and start a valveless pulsating jet engine.The article "Do-it-yourself jet engine" was published in the journal Popular Mechanics (No. 8, August 2013).