Once you’ve calculated the dimensions for a pulsejet, how do you then go about building one?
Of course It really helps if you’ve got access to a workshop or some basic metalworking tools such as a hacksaw, drill, welder, etc. but don’t be put off if your resources are a little more modest.
You’d be surprised how helpful your local welder or engineer can be when you explain that you’re building a jet engine and would be happy to demonstrate it to them when it’s done.
It’s also amazing what you can do with a minimum of tools – if you’ve got enough patience.
The Engine Body/Tailpipe
Most commercial pulsejets are made from thin stainless steel sheet that is rolled or otherwise formed into tubes and cones before being welded together.
This results in a durable engine that is light enough to be practical for such uses as powering model airplanes.
These cones and tubes are formed using a device known as a slip-roll which looks rather like an old washing-machine wringer and consists of three rollers that can be adjusted to both grip and curve the metal sheet as it’s wound through.
A hand-operated slip-roll like this one is limited to rolling stainless steel that is no more than 1mm thick – and even then it’s damned hard work if you’re rolling a piece the full 600mm long which is the maximum this set of rolls can handle.
For larger engines it really pays to find someone who has a set of motorized rolls that can handle the thicker material and longer lengths you’ll need to use.
Unfortunately, stainless steel is not only expensive but can also be very difficult to weld when it’s very thin. For this reason, many enthusiasts prefer to make their engines from cheaper and more easily worked materials
Providing weight isn’t a problem you can save a lot of time and significantly simplify construction by using mild steel pipe for the body of your engine. The best stuff is exhaust tubing which is usually protected from rusting by a thin layer of aluminum on the surface.
This stuff is relatively cheap, available in a wide range of sizes and can be cut and welded easily using MIG, arc or oxy-acetylene equipment.
Unfortunately it’s also quite heavy, but that’s usually unimportant to the eager enthusiast who simply wants to build an engine and get it running ASAP.
Here’s a picture of my gokart with an engine built from exhaust tubing. It only produced just enough thrust to get the kart moving on a smooth surface but it was an interesting experiment none the less.
As you can see, this pipe is quite thick-walled which accounts for its weight and easy welding characteristics.
The Valving System
Exactly how you construct this depends on the type of valving system you plan to use. If you have access to a lathe then you can easily make a petal-valve system with a nicely turned aluminum head. If you’re not lucky enough to have a lathe at your disposal, all is not lost.
Instead of making the front of the engine from a single, large piece of aluminum rod, you can cut a circle from a sheet of plate. Even on small engines it pays to use a piece at least 3/8”
(8mm) thick for this.
Because aluminum is such a soft metal, you can actually do a pretty good job of cutting a circle from a flat sheet by using a jig-saw. You can even use a small coping saw or jeweler’s saw to cut the valve holes (after drilling a starting hole first).
To create a circular valve-plate from a flat piece of aluminum, simply mark out your circle using a compass then cut it slightly over-sized. It can then be filed down to a precise fit into the front of your engine.
However, before you cut out the circle, it pays to mark out and drill the valve holes. By doing this before you cut the circle to shape, you have a larger piece of metal to hold onto when drilling the valve holes and this makes the job much easier.
Here’s a valve-plate that was made from flat-sheet, cut to shape with a jigsaw and with valve-holes that were cut with a jeweler’s saw. In this case, a lathe was used to finish the surface of the disk and it was then anodized to provide a hard, corrosion resistant layer and a pleasing gold color. See the chapter on
anodizing for details of how to perform the anodizing process.
Note that there’s no reason why the valve-holes should be circular and a trapezium shape as in the picture actually allows a greater valve area for a given size of valve-plate.
Having made the valve plate a snug fit in the front of your engine, you can then drill 4-6 small holes around the circumference of the pipe so that they also go through into the edge of your valve plate. By choosing the correctly sized drill, you can then fit self-tapping screws to hold the valve-plate firmly in place.
Any leaks in this area can be fixed by the liberal application of some muffler-sealant – the type that comes in a small tube and is designed to block-up gaps in exhaust systems. This stuff will expand slightly as the engine heats and seal any small gaps between the valve plate and the engine body.
If you don’t have a lathe then the other option is to build a V-valve system instead of a petal valve one.
The V-valve can be created from flat pieces of steel as in this drawing.
Making Reed Valves
Since the reed-valves are the heart of a conventional pulsejet engine, it’s important that they are well made and that the right materials are used.
Most valves are made from high-carbon spring steel of between 0.006” and 0.012” thickness.
If you can’t find a source for this material locally then I suggest you take a look at some of the online mail-order metal supply companies.
Once you’ve got the right material, the next problem is cutting it to the required shape.
Spring steel sheet is incredibly brittle and will split or crack very easily if you try to cut complex shapes with regular metal snips.
While it’s simple enough to cut the rectangular shaped valves used in a V-valve system, creating the intricate shape of a petal valve represents more of a challenge.
There are two methods you can use to fabricate a petal valve from spring-steel sheet.
The first involves using a Dremel or similar tool fitted with a cut-off tool as shown in this picture.
This technique requires a bit of practice and is most suitable for smaller valves.
The preferred method for making petal valves involves the use of a process known as electrochemical etching.
Full details of how to make reed valves using this process are provided later in this book.
Welding
There are really only two welding options when it comes to joining al the pieces together:
• TIG (tungsten inert gas)
• MIG (metal inert gas)
The ideal welding process is TIG, since this allows total control over the amount of heat used and how much (if any) extra filler metal is added to the weld seam.
However, since TIG welders are more expensive than simple MIG units, and most people find MIG welding much easier than TIG, I’ll describe both processes.
Welding with MIG
I won’t turn this into a welding tutorial so I assume you’ll already be moderately competent with a MIG welder. Instead, I’ll focus on the points specific to welding the various parts of these engines together.
It is important to use the right filler-wire. If you’ve cut and rolled the parts out of 304 stainless steel then you can use 308 or 316 stainless filler wire. If you’ve cut your parts from 316 stainless then use 316 filler rather than 308.
I’ve found that 0.8mm (0.032”) wire is about the right thickness and you should only need a small spool to build an entire engine.
While you can use plain steel wire, the resulting weld will rust very quickly and become weak, so it’s not recommended. Stainless steel also has a much higher rate of thermal expansion than plain steel so you’ll get additional stresses set up as the engine heats and cools.
When welding, adjust the current, wire-speed and stickout used to try and get the flattest weld bead you can without burning through.
I find when working with
It also helps to have a “chill bar” behind the weld seam whenever possible. A length of 25mmx25mm (1”x1”) aluminum bar does a good
job here, with a length of 12.5mmx25mm used along the top surface to ensure intimate contact between the two sides. A pair of two C-clamps can be used to hold everything together as in this picture.
Welding with TIG
TIG welding thin stainless is not as easy as you might think.
The use of a chill-bar (as described in the MIG welding section above) is absolutely essential. Without this, the stainless will tend to sag and form ugly black “danglers” on the back-side of the weld. This not only produces a weaker weld but those danglers will also interfere with the gases that flow at very high speed through these tubes.
Unfortunately, unless you want to go to the bother of turning up conical chill-bars to match the radius of each
circumferential weld, you’ll have to just do your best without one. Backpurging the weld with argon will certainly help here, as will making sure that the fit-up of the pieces is as near to perfect as you can get it.
Another option is to use a special high-temperature flux paste designed for use with stainless welding. This usually comes as a powder that is mixed with methyl alcohol then spread on the backside of the weld to stop oxidation during the welding process. The brand I’ve used with some success is “Solar”.
Unless you’ve turned up those conical chill-bars, you’re probably going to have to overlap the cones slightly to get a good fit-up, since the rolling process combined with the stresses produced when welding the length-wise seam, will almost certainly mean that the cones themselves are not perfectly circular in cross-section.
When welding, try at all times to keep the weld bead the same thickness as the sheets being joined. It’s obvious why you would not want the weld to be too thin – but read the piece on heat-stress later in this bookto find out why a thick weld bead is just as bad.
Getting a Good Fit-up
A good fit-up is essential to the success of any welding job and there are some simple methods of getting a good fit-up when assembling these engines.
Butting a cone up to a matching tube in such a way that the edges match perfectly so as to ensure a good weld is a very difficult (some would say impossible) task when using very thin material – but fortunately there are alternatives.
I always flare or flange edges of my cones to ensure the maximum contact area between the two parts and to ensure that the springiness of the stainless steel actually works to keep the two surfaces in intimate contact for welding.
There are two ways of flaring and flanging – you can use a rotary machine (sometimes called a
“jenny”) or you can just beat the snot out of it with a hammer and dolly.
If you’ve got a jenny then you’re probably already aware of the techniques used to create flares and flanges – but if you don’t, here’s how you do things the manual way:
First you’ll need a good hammer (or two) and a dolly. Although I use the term “dolly”, I use nothing more than a 100mm (4 inch) long piece of 25mm (1 inch) steel bar. I’ve ground a fairly large (6mm - ¼ inch) radius onto one end and a small recess onto the other [PICTURE].
This piece of metal is then used as a backstop as you beat the stainless into submission, carefully working around the edge of the cone until the desired flare/flange is obtained.
If you do a good job, the cones and tubes should stay together before they’re welded without the need for clamps or other mechanical devices.