PULSO JET

Copyright 2004 Bruce Simpson. http://aardvark.co.nz/pjet

Second revision (May, 2004)

Copyright Notice:

This book is copyright 2004 to Bruce Simpson and all commercial rights are reserved.

Anyone discovering a violation of these terms is requested to contact the author through thewebpage at http://aardvark.co.nz/contact/


ForewordModern jet engines, like the ones found on large passenger aircraft or military fighters areincredibly complex and expensive to make.

Built from thousands of individual parts, many of which are made from exotic alloys liketitanium and Inconel, these engines are a masterpiece of modern engineering.

But what if I was to tell you that there is at least one type of jet engine that has been aroundfor almost 100 years, can be built out of plain old steel using simple tools, and in some caseshas no moving parts at all?

Well its true and I am, of course, talking about the pulsejet.

In this book Ill do my best to explain how these engines work, how to build them, how toimprove on the basic designs and how they can be used to power all manner of vehicles frommodel airplanes to gokarts.

In an attempt to make the information contained in this book accessible to the widest range ofpeople, Ive taken a few liberties in explaining some of the more complex concepts. Im surethere will be physicists, engineers and mathematicians who will throw up their arms in disgustwhen they look at how Ive presented some of this information.

My justification for this is that the majority of readers are probably not equipped nor interestedin wading through pages of complex mathematical formulas in order to understand someaspect of a pulsejets operation. In such cases, I believe, its better to replace all thiscomplexity with a simple analogy or basic calculation that hopefully anyone can follow.

Another are of contention is the very explanation of how a pulsejet works.

Although there is some consensus on the basic mechanisms behind the operation of a pulsejet,much of the detail is a topic of hot debate and disagreement. Wherever possible Ive tried topresent all sides to an argument but obviously I favor my own opinions which are based onyears of empirical observation and experimentation.


A Note About The AuthorWho is Bruce Simpson?

Well Im a middle-aged guy who has always had astrong interest in technology and things that go bang.From an early age youd find me out in the garageplaying with my chemistry set, building all manner ofweird and wonderful devices from old discardedradios, or just reading books about science.

Since the age of about seven, Ive also been an avidbuilder of model airplanes, mostly of my own design.Over the years Ive created all manner of odd-ballflying creations including flying wings, flying saucers,flying lawnmowers, flying carpets, and many others.

It was only natural therefore that eventually myfascination with things that go bang, chemistry, physicsand aerodynamics would collide and produce a stronginterest in jet engine technology.

It was also inevitable that, rather than focus on currently fashionable small gas turbinetechnology, Id instead concentrate my efforts on the almost forgotten pulsejet.

Over the past couple of years Ive built dozens of different pulsejet designs, mainly to test myown ideas. As a result of this experimentation Ive developed a reputation for being at theleading edge of this almost forgotten technology and have come up with a number ofinnovations such as the blast ring and a novel fuel-injection system that significantly extendsthe valve life of small pulsejet engines.

For about a year I was actively involved in the commercial manufacture of several of mypulsejet designs but unfortunately I rapidly found myself extremely embarrassed at beingunable to keep up with the unexpected demand. As a result, I have sold the manufacturingrights for these engines and am again focused on pure research and development in this area,working on several projects including some commissioned by clients in the aerospace anddefense industries.

I am also performing some design work on a new generation of ultra-low-cost high-speedpulsejet-powered UAVs designed for reconnaissance and other applications.


Contents:How do pulsejets work? 5The worlds simplest pulsejet 8Pulsejets for models 10How to design a pulsejet 12Comparing intake valving systems 20Making reed valves last 23Fuel systems 29Constructional techniques 37Powering things with pulsejets 43Schmidts contributions 46Ignition systems 48How to start a pulsejet 51Valveless pulsejets 54The design of valveless pulsejets 59Improving pulsejet performance 66Accidents and failures 69A simple guide to anodizing 76Making reed valves with electrochemical etching 82Newtons third law 88The Reynolds effect 92The Bernoulli effect 91The Coanda effect 93Plans 95Wacky ideas 97Afterburning augmentors 99A Little History and a Few Important People 103The future of pulsejet technology 105


How Do Pulsejets Work?The honest answer to this question is that nobodys really 100 percent sure of all themechanisms that drive a typical pulsejet engine.

While most of the basic principles of operation are understood fairly well, there are many smalldetails that are still the subject of debate amongst engineers and experimenters to this very day.

It is safe to say however, that the primary effect behind the function of a pulsejet is the factthat gases are compressible and tend to act like a spring.

This springiness is crucial to the way a pulsejet draws in a fresh mixture of air and fuel thenexpels the hot burning gasses that are generated when that fuel is ignited.

The Kadenacy EffectThe effect of this springiness has been labeled the Kadenacy effect and heres how it works:

Take a regular 12 inch rule and lay it over the edge of a table so that just two or three inchesare held firmly against that table.

Pull the free end down an inch or so and release it.

Did you see what happened?

The ruler, acting like a simple spring, quickly flicked back, away from your hand but it didntstop once it became fully straightened it continued to move and actually bent the other wayvery briefly.

This flexing back and forth probably continued for a second or so withthe magnitude of each swing being slightly smaller than the previousone.

Now something very similar happens when we take a sealed containerand fill it with a compressed gas such as air.

If we suddenly release that pressure by popping thecork, the compressed air will rush out but (andheres the surprising bit) even once the pressure inside falls to match thepressure outside, the air will continue to flow out.

Its pretty easy to see that this will cause thepressure inside the container to fall below the

pressure outside and then the gas will flow back inwards. This cycleof increasing and decreasing pressure will repeat a number of times,decreasing in magnitude each time.

Thats the Kadenacy effect in action driven by the springiness of air.


For a practical demonstration, find an empty bottle that has an opening about the size of yourfinger or thumb.

Wet your finger or thumb and slide it into the neck of the bottle, allowing the air to escape asyou do.

Now remove your finger quickly.

Hear that sound? Thats the air rapidly bouncing in and out just like the springy rulervibrated when you let the free end go.

See how easy this jet engine stuff is?

Now that weve seen how Kadeancy works its time to explain how it becomes the drivingforce behind a pulsejet engine.

The Pulsejet Engine Operating CycleLets assume that a mixture of finely atomized fuel and air has just been ignited inside ourpulsejet.

The rapidly burning fuel generates gases such as carbon dioxide, carbon monoxide and water-vapor. These gases take up far more room than the air and fuel alone did so pressure build upinside the engine.

The heat generated by the combustion causes those gases to expand so the pressure isincreased even more.

Our pulsejet has become a container filled with pressurized gases just like the one describedpreviously.

Those gases now rush out the opening at the end of the engines tailpipe and in doing so, theycreate thrust which pushes the engine (and whatever its attached to) in the other direction.

It was Benjamin Newton who first described this effect when he said for each and everyaction there is an equal and opposite reaction.

A fraction of a second after the air/fuel is ignited and the hot gases have started flowing outthe tailpipe, the pressure inside the engine has dropped to match that of the outside air.However, thanks to the Kadenacy effect, the gases keep flowing down the tailpipe and a partialvacuum is created inside the engine.

At this point, two very important things start happening.

Firstly, the valves at the front of the engine open. Theyre pushed open because the air outsidethe engine is at a higher pressure than that inside the engine. This pressure difference pusheson the valve and causes them to move aside, thus allowing fresh air and fuel to enter.


At the same time, those hot burning gases that were travelling down the tailpipe stop for aninstant then start travelling back towards the front of the engine driven by the air outsidewhich is at a higher pressure than that inside.

You can see that at this point, we have fresh air and fuel coming in the front and still-burninggases coming back down the tailpipe.

Can you guess what happens when the two meet?

Thats right the whole cycle starts all over again when the flames and hot gases from thetailpipe ignite the highly flammable mixture of air and fuel that has been sucked in through thevalves in the front.

And of course, as soon as that fuel ignites, the pressures generated push the valves at the frontof the engine closed, leaving the hot gases only one way to go out the back.

When a pulsejet is running, this whole process is repeated many times per second and it is thisrepeated blasting of hot gases out the tailpipe that gives the engine its characteristic noisy bark.

Wasnt that simple?


The Worlds Simplest PulsejetHeres a chance to make your own ultra-simple pulsejet using nothing more than a hammer, ascrewdriver, and a few readily available materials.

This simple design was first dreamt up by one of the grandfathers of pulsejet engines, aDutchman by the name of Francois Henri Reynst back in the first part of last century.

Although this pulsejet wont hurt your ears or produce massive amounts of thrust, its still agood idea to include a few warnings at this point.

SafetyPulsejets use explosive mixtures of air and fuel to create power. They also produce lots ofburning hot gases when running.

For these reasons, you should never attempt to run a pulsejet (not even this very simple one)indoors or near anything that could catch fire.

Also be aware that because a pulsejet generates pressurized gases, theres always a small riskthat part of the engine could fly off and strike anyone standing nearby. This is particularly trueif youre using a glass jar in the following experiment. The glass can (and will eventually) crackand break due to the heat and pressures involved.

Here are some basic rules that will help keep you safe:

1. Always wear eye protection2. Keep a safe distance from a pulsejet when it is running.3. Keep a garden hose and/or bucket of water nearby at all times4. Use hearing protection

Now on with the fun.

MaterialsIn order to build our simple demonstration pulsejet youll need to find the following items:

1. A small jar with a screw-top metal lid (75mm or 3 diameter)2. A screwdriver or nail.3. Some methanol or model airplane fuel

Heres how we go about building our engine.

With the nail or screwdriver, make a hole in the center of the removable metal lid. You canthen enlarge this hole to around a half-inch (13 millimeters) in diameter.

Now pour some methanol or model airplane fuel into the bottom of the jar to a depth ofabout a quarter inch (5 millimeters).


Replace the lid and swirl the liquid around in the bottom of the jar a few times.

Remember that you need to run this little engine well away from anything that could catch fireand one suggestion I have is to dig a small hole in the ground so that the jar can be inserted,leaving the lid slightly above ground-level. This will protect you and reduce the fire risk if thejar should crack.

Now bring a lighted match or flame near the hole in the jars lid keeping your face and handswell away from the area directly above that hole because a large flame may come shootingout with a whooshing noise that can give you a bit of a fright.

It is highly recommended that you wear eye-protection and at least a long-sleeved cotton shirtto protect yourself when performing this experiment.

If youre lucky, your simple little jam jar engine should start puffing away producing aseries of little pulses of hot gas and perhaps a gentle purring noise.

IMPORTANT: do not let this simple jam-jar engine run for more than 5-6 seconds at a timeor the glass will crack, possibly spilling burning fuel. You can stop it by covering the hole inthe lid with a small piece of wood or even a suitably sized coin.

How Does It Work?Now the more observant reader will have noticed that this pulsejet has no valves so howdoes it work?

The answer is simple when you ignited the air/fuel mixture that was originally in the jar itburnt, expelling that large jet of flame and making that whooshing noise.

Because the hot gases rush out of the jar with great speed, the pressure inside the jar quicklydrops below normal atmospheric pressure and a weak vacuum is created just as described inthe previous chapter.

When this happens, a fresh gulp of air is sucked in through the hole in the lid and that air thenmixes with highly flamable methanol vapor that is rising from the pool of fuel still sitting in thebottom of the jar. The jar once again contains a combustible mixture of air and fuel but howdoes it ignite? After all, we had to use a match to ignite it the first time didnt we?

Well, also inside the jar are some remnants of the hot gases generated from the lastcombustion cycle. Eventually the hot gases and the air/fuel mixture run into each other whereupon ignition occurs, pushing its hot gases out the hole starting the whole cycle all overagain.

Hows that? Were not even a quarter way through this book and youve already built yourown pulsejet engine!


Pulsejets for ModelsForty or fifty years ago there were a number of manufacturers producing small pulsejet enginesdesigned specifically for use on model aircraft.

Most of these engines are very similar in design and construction, consisting of a lightweightstainless-steel body and tailpipe, with a machined aluminum head and thin spring-steel valves.

Quite a few of these engines were made in Eastern-bloc countries and at least one was madeby the OS model engine company in Japan.

However, perhaps the most recognized model pulsejet of all time is the Dynajet.

The DynajetThousands of avid model airplane enthusiasts have owned, seen or lusted after this icon of thepulsejet era.

The Dynajet was so popular, and so many were sold, that it rapidly became the benchmarkagainst which all other small pulsejets were compared.

Its simple design and lightweight construction made it perfect for use on model airplanes andwell suited to the U-control speed models of the era.

Even today the Dynajet is a popular item on auction website such as eBay, often producingbids of several hundred dollars or more.

This picture is of an earlymodel Dynajet that had thespark plug located right at therear of the combustionchamber and didnt have an anodized head. In later models the sparkplug was moved forwardand the aluminum head was anodized a rich red color which resulted in the engine beingknown as the Dynajet Red-head.

The body of these engines is made from two pressed stainless steel shells that are welded alongan upright seam. This technique results in a nice smooth contour between the combustionchamber and the tailpipe, which probably helps the performance somewhat.

This picture shows the front of a later-modelDynajet with a more deeply finned valve-headsection. You can clearly see the effects of thered anodizing on the aluminum and the moreforward location of the sparkplug.

The company that used to make theseengines, Dynafog, is still in business but nowfocuses on industrial fogging machines whichuse the pulsejet principle to atomize and spray insecticide and other chemicals.


Another company, Bailey Machine Service, is still making a clone of the Dynajet engine whichit sells for about US$250 although the supply is said to be somewhat erratic.

Many of the other pulsejet designs youll see from this era are very similar to the Dynajet insize, dimensions and performance, although there are still plenty of weird and wonderfulvariations on the basic theme.

The OS pulsejetThis engine was manufactured in the 1950s and 1960s by the Japanese company OS.

There were apparentlytwo slightly differentversions of this engine,one being a little larger and more powerful than the other but they were both obviously verysimilar to the Dynajet in both form and function.

However, unlike the Dynajet which used a single machined piece of aluminum for its valve-head and venturi, the OS unit consists of two separateparts which screw together.

There was also a myriad of other pulsejet designsmanufactured about the same time and sold under araft of different names such as TigerJet, etc.

An even wider range of designs and ideas werepublished as plans for a generation of enthusiastswho, in a post-war era, were eager to build one ofthese magical jet engines for themselves.

As a result, many magazines such as Popular Science and Popular Mechanics were littered withadvertisements for such plans.

It is unlikely that many of those who purchased these plans ever managed to construct aworking engine and at least a few of the designs were so fatally flawed that the publisher wasobviously just trying to cash in on a craze.


How to design a pulsejetOne of the more interesting and more readable texts on pulsejet design and theory was writtenby a C.E. Tharratt while he was a staff scientist at the Chrysler Space Division in the late1950s.

Titled The Propulsive Duct, this paper condensed much of the known pulsejet theory into afew simple formulas and constants.

Using these calculations Tharratt claimed that we are in the surprising position of beingable to determine the dimensions of a duct capable of developing a given thrust literally onthe back of an envelope and without knowing anything about its gas dynamics!

Sounds great doesnt it?

Unfortunately, while Tharrats formulas have stood the test of time and his understanding ofthe mechanisms behind the pulsejet (or propulsive duct as he called it) remains valid, when itcomes to designing a powerful, reliable pulsejet engine, the devil is in the detail.

However, here is the simple formula that Tharratt proposed to be the core of pulsejet design(note: Tharratts papers and constants are presented in the imperial measurement system sothats what Ive used in this chapter. Ill update with metric versions in the next release of thisbook):

V/L = 0.00316F

Where:V = engine volume (cu ft)L = effective acoustic length of the engine (ft)F = thrust (lb)

The validity of this formula has been verified against a wide number of different and provenpulsejet designs including the Argus V1 and Dynajet.

Lets take a look at what this formula actually means in terms of the way that the dimensionsof a pulsejet affect its power output.

We can see that if we kept the volume of the engine (V) constant but increased its effectiveacoustic length (L) then the power would reduce. Note that in order to do this, the diameter(and cross-sectional area) of the engine would need to be reduced so it would appear thatthere is a definite relationship between cross-sectional area and power.

Now, if we keep the length (L) constant but increase the volume (V), the power wouldincrease. To accomplish this however, wed have to increase the diameter (and cross-sectionalarea) of the engine. This confirms that relationship between cross-sectional area and poweroutput.


If we manipulate that simple formula a little more, we come up with a new formula containinga very interesting constant:

F = 2.2A

Where:F = thrust (lbs)A = mean cross-sectional area (square inches)

Lets just make an important point here this 2.2lbs/sq-in constant is derived from a formulathat includes the engines total volume as a factor. This is why its not just the cross-sectionalarea of the tailpipe that is important (as many would have you believe), but the mean (average)cross-sectional area of the entire engine along its total length.

However, it should be added that Tharratt didnt expect engines built to his formulas to have ahuge bulbous combustion area at the front so dont expect that such a feature will significantlyincrease an engines performance over a straight pipe.

So now we can plug in our required power output of 10lbs thrust and get this:

10=2.2A

which simplifies to:

A=10/2.2A = 4.545 square inches

From this we can calculate the mean diameter of our engine as follows:

D = 2(4.545/)D = 2.4 inches

Now we need to decide on a length for the engine. Remember that making the engine longeror shorter wont actually increase or decrease its power only a change to the cross-sectionalarea will do that.

However, the length does have a bearing on the frequency at which our engine will operate.

The only reference I can find from Tharratt in respect to the suggested length of a pulsejetengine is that it be at least eight times the mean diameter.

This length to diameter ratio is usually expressed as L/D and Tharratt notes that withgeometrical ratios of L/D < 7 the development problems become particularly challengingand that in an engine with an L/D < 7 combustion with chemical fuels is difficult tosustain, let alone develop maximum thrust


It makes sense therefore, to use an L/D somewhat greater than 7 and its been my experiencethat a figure of about 14 is a good place to start for relatively small engines like this.

By way of comparison, the Dynajet has an L/D of 15 and the Argus V1 has an L/D of 9.6.As a rule of thumb, the smaller the engine, the higher the LD needs to be in order to getreliable operation and good power levels.

So now we can calculate the length of our engine as follows:

L = 14DL = 14 x 2.4L = 33.6 inches

Okay, so now we know that to create a 10lb-thrust pulsejet engine well need a piece of pipethat is 33.6 inches long and 2.4 inches in diameter -- but wait, theres more!

Another key formula presented by Tharratt was one for calculating the valve area for an engineof a given size and power:

Valve area = 0.23 x mean cross-sectional areaOr

Valve area = 0.1045F sq in

It is interesting to note that this 0.23 (or 23 percent) figure contrasts sharply with thatproposed by other pulsejet experts of the era who suggest a figure of 0.4-0.5 is better.

Lets use Tharratts formula and constant to work out the size of the valve area we need forour 10lb-thrust engine.

Using the first formula we get:

Valve area = 0.23 x mean cross-sectional areaValve area = 0.23 x 4.545Valve area = 1.045 sq in

Using the second formual we get:

Valve area = 0.1045FValve area = 0.1045 x 10Valve area = 1.045 square inches

Yep, we get the same answer both times so we now know that the effective valved area for ourl0lbs-thrust engine is just over 1 square inch.

Now Tharratts formulas assume that the intake is an open hole, with no losses due to thepresence of valves. Unfortunately, the presence of spring-steel reed valves will significantlyimpact the flow of gas so we need to take those losses into account when designing our intakevalving system.


The precise efficiency of a valving system depends on many factors and I suggest you read thechapter on intake valving for more information but in the case of our little design, lets use asimple petal valve and assume that it is just 50 percent efficient.

To get the actual area of the valve required to achieve an effective area of 1.045 squareinches at 50 percent efficiency we simply divide by 0.5 to get a figure of 2.090.

So lets see how our pulsejet is looking like so far:

Thrust 10lbsLength (from valves to end of tailpipe) 33.6 inchesMean diameter 2.4 inchesTotal valve area (assuming 50% efficiency) 2.090 square inches

So what about those valves then?

A petal valve system consists of a ring of holes over which a spring steel valve, consisting of amatching number of petals, is laid.

If we were to use 10 holes, spaced at 36 degree intervals, each hole would need to have an areaof:

2.090/10 = 0.209 square inches

which requires a diameter of 0.516 inches.

Alternatively, if we used a ring of 12 holes spaced at 30 degree intervals, each hole would needto have an area of:

2.090/12 = 0.174 square inches

which requires a diameter of 0.470 inches

Its been my experience that a half-inch hole is the upper limit for petal valve holes. Once yougo beyond this size the pressure on the valves themselves cause them to be bent so that theybegin to dish into the hole and this adversely affects their operation. For this reason, well gowith the 12-hole option.

Youve probably already noticed that most small pulsejets have a much larger diameter sectionat the front, from where they funnel down to a narrower tailpipe. Many people mistakenlyassume that this bulbous front is a combustion chamber, designed to contain the burningair/fuel mixture. While it may be true that much of the air/fuel is burnt inside this bulbousfront section, the reason small engines are shaped this way is actually quite different and wellsee why when we do the next set of calculations.


Weve decided to use a petal valve system consisting of 12 holes, each of 0.47 inches indiameter. If we assume that the holes will be placed in a ring around the edge of the pipe, andthat we allow a certain amount of space between the holes so that theres room for the spring-steel valves to rest, then we have a problem.

Lets assume we need a inch gap between the holes that means the total circumference of acircle drawn through the center of each hole will be:

NumOfHoles x diameter + NumOfGaps x GapSize

And when we plug in our numbers we get:

12 x 0.47 + 11 x 0.25 = 8.39 inches

That represents a circle with a diameter of:8.39/ = 2.67 inches

Whats more, that figure is the diameter of a circle that runs through the center of each hole sowe need to add two times the radius of the holes to get the size of a circle that will run aroundthe outer edge of the ring of holes.

2.67 + 0.47 = 3.14 inches

Clearly, the absolute minimum diameter of our valve system (3.14 inches) is larger than thecalculated diameter of our engines pipe (2.4 inches)

This disparity grows even further when we build in a bit of extra space so that the valves dontscrape against the side of the engine when they swing open and closed. Its been myexperience that you should allow a space between the outer edge of the valve holes and theside of the engine which represents an area equal to the total area of our valve holes.

Or in other words, we need to leave 2.090 square inches of space around our ring of holes.That can be calculated as follows:

The area of a single circle covering our ring of valve holes would be:

R2 or 3.1415 x 1.57 x 1.57 = 7.74 square inches

Add 2 .090 square inches to get the size of the larger circle and we get 9.830 square inches

From this we can calculate the diameter needed to provide that extra 2.090 square inches ofspace around the edge of the valve-ring:

Diameter = 2(9.830/)Diameter = 3.53 inches

Is your head sore from all these calculations yet? Dont worry, were nearly done.


Now that we know the diameter of our combustion chamber we need to calculate how longthis section of the engine should be.

Since Tharratt didnt use petal valves, he didnt need this bulbous front section so has noadvice (that I can find) for calculating this dimension.

However, we can look to the work performed by Schmidt (the guy who designed the Argus V1engine) and my own empirical research that indicates the following:

During the intake phase of a pulsejets operation it will draw in a fresh charge of air equalto 15%-20% of the total engine volume.

I see no reason why we shouldnt design the engine so that this front section is just largeenough to hold this fresh charge of air/fuel. In that case we need to do some morecalculations to determine its length:

If our engine were just a straight pipe of 33.6 x 2.4 inches then it would have a volume of152.7 cubic inches. Such an engine would suck in 152.7 x 0.2 = 30.54 cubic inches of airduring each cycle so our front section needs to have a volume of 30.54 cubic inches.

Weve already calculated the area of this section as being 9.83 square inches we can calculatethe required length as follows:

30.54 / 9.83 = 3.11 inches.

Of course more alert readers will notice that this 30.54 cubic inches no longer represents 20percent of the engines total volume. This is because by making this front section widerwithout reducing the overall length of the engine, weve actually increased its total volume byan additional 16.4 cubic inches.

The total volume of our engine is now nearer 169 cubic inches so the 30.54 cubic inch frontsection only represents 18 percent of the total volume but this difference is so small as to beunimportant.

The only thing remaining now is to join the front section of the engine to the tailpipe using acone with a diameter of 3.53 inches at one end and 2.4 inches at the other.

How long (ie: what angle) should this cone be?

Well if we look at the Dynajet we can see that its hardly a cone at all more of an abrupttransition. By comparison, the Argus V1 engine uses a very long, shallow angled cone to jointhe two sections. So how do we decide which is best?

Lets look at the effects that the angle of this cone might have on an engines operation.

Coming up with a suitable angle for this cone requires balancing a number of factors. Toexamine them, lets look at extreme examples:


1. If we simply used a flat plate to join the two sections of the engine then the hot exhaustgases would have a rather torturous path to follow. Some of those gases would have totravel around two 90 degree bends to get from the combustion chamber to the tailpipe andthat would potentially reduce the speed at which they were able to exit from the engine.Remember, the speed at which the gases leave the engine affects the thrust we wantthose gases leaving as fast as possible. For this reason, a flat plate is obviously not such agood idea.

However, this configuration is not quite as bad as you might think, after all, the Dynajethas a very steeply angled cone that must constrict the flow of exhaust gases right? The veryfact that it is so hard for the combustion gases to get into the tailpipe means thatimmediately after ignition, pressure will build up inside the combustion chamber as allthose gases try to rush around a tight bend and down the tailpipe.. Those higher pressurescan improve combustion efficiency and actually increase the speed of the gases in thetailpipe. This is called post-ignition confinement (PIC).

Its also worth noting that in the case of a flat plate, the hot gases that return from theengine's tailpipe and ignite the fresh air-fuel charge may do so far more efficiently. This isdue to an interesting effect that occurs in the way they form a narrow jet that reaches deepinto the chamber rather than a larger diffuse front that ignites the fuel more slowly.

Remember that the faster the fuel burns, the more power our pulsejet will develop becauseit will have less time to expand as it burns thus producing the higher internal pressuresthat will, in turn, result in higher tailpipe gas velocities.

This diagram shows how ignition differs based on the angle of the cone betweencombustion chamber and tailpipe. Note that in the second diagram, the distance betweenthe hot gases and the engine body is far less than in the first this is important.

The speed at which the combustion flame-front travels through the fresh air/fuel mixtureis relatively slow (just a few tens of feet per second) in a low-compression engine like thepulsejet. Because of this, the mixture in the second diagram will be burnt far more quicklythan that in the first, since the flame-front will be wider with a much shorter distance totravel.

2. If we used a very long cone that had a shallow taper all the way to the end of the engine (ie:no tailpipe as such) --then it would obviously be much easier for the combustion gases to


flow out under pressure. However, wed also be significantly reducing the ability of theengine to create a vacuum after combustion is completed because a much smallerpercentage of the exhaust mass would be travelling at maximum velocity inside the engine.Remember that the force exerted by the escaping gases is equal to their mass times thevelocity to which theyre accelerated (F=MA). For a given size of engine engine, the masswill always be the same but the velocity to which those gases are accelerated will dependvery much on the design of the tailpipe. We need plenty of velocity to get the forcerequired to establish a strong Kadenacy effect. In fact, tests conducted by the NACAduring the 1950s indicated that an engine designed with just a long convergent coneinstead of a straight tailpipe was very difficult to get running at all.

Once again it seems that a compromise is in order so well chose an angle of 30 degrees for thesection between the combustion chamber and the tailpipe. This will provide some post-combustion confinement to increase the internal operating pressures while ensuring that theengine still has good internal mass-flow speeds to provide maximum Kadenacy effect.

A 30-degree cone will be a fairly short section just 1.84 inches long.

So here are the final dimensions of our pulsejet engine, wasnt that simple?

It should be noted that this is a very basic engine and there are still a few tricks we can use toimprove its performance but more of this later.


Comparing Intake Valving SystemsOne of the most critical components of a traditional pulsejet engine is the intake valvingsystem.

The demands placed on the intake valves are amazing.

They have to open and close several hundred times a second while being exposed to thethermal stresses associated with being alternately blasted by searing hot combustion gases andcold incoming air. At the same time, these thin strips of spring steel must resist metal fatigueand fracture resulting from the high mechanical stresses imposed.

Whats more, they have to do all this while providing a 100 percent seal against combustiongases when closed, and allowing the smooth, unimpeded flow of fresh air when open.

To make life even harder, the only power available to open them is the tiny difference inpressure between the outside air and the small vacuum created inside the engine by thekadenacy effect of escaping exhaust gases down the tailpipe. (just a few psi).

Its no wonder therefore, that no aspect of pulsejet design and construction has caused moresleepless nights, scratched heads and frustration than the valving.

Lets examine the alternatives:

Petal ValvesSmall engines almost always use a petal-valve. These valves offer the following benefits:

1. simplicity. The valve can be etched or cut from a single piece of spring-steel.

2. Low cost. As a side effect of their simplicity, petal valves can also be very economical tomanufacture especially when you consider that the valve plate consists of a simple pieceof aluminum with a ring of holes drilled in it.

Unfortunately, the petal valve also has a number of disadvantages:

1. poor aerodynamic performance. Since the air passing through a petal valve must negotiatetwo near-90 degree bends on its way into the engine, the efficiency of such a system is notparticularly high.

2. low durability. Because the tips of the petals are directlyexposed to the hot combustion gases, petal valves oftensuffer from premature tip cracking or fracture.

3. High maintenance. Since petal valves are usually made asa single piece, the failure of individual petal requires thereplacement of the entire spring-steel valve.


Despite their drawbacks, petal valves are generally the best option for small pulsejet engines,although I wouldnt recommend them for any engine larger than about 20lbs of thrust.

The V or multi-V valveGenerally only seen on larger engines, these valves aregenerally more efficient than petal valves because theyproduce less deflection of the airflow when theyre inan open position.

There are two basic methods of constructing such avalve system one involves the use of two or more flatmetal plates with holes in them, joined at an angle (45degrees is a good starting point).

The other method of forming a V valve is the one usedin the Argus V1 where a cast or machined spacer withmultiple ribs is used to hold the valves in position and limit their movement as in the diagrambelow:

V valves provide the following benefits:

1. Higher efficiency than a simple petal valve. Since the incoming air has a far straighterpathway into the engine, more air is able to flow for a given size of valve opening whencompared to a petal-valve.

2. Lower maintenance costs. Since the individual spring steel valves in a V-valve system canbe replaced as/when they fail, maintaining the engine becomes a less expensive task and allvalves can be used to the full extent of their lifespan.

3. Scalability. Unlike the petal-valve, a V-valve can be easily scaled to create the required valvearea by simply increasing the length or number of V-valves in the array.


Of course there are downsides too:

1. Greater complexity. A V-valve generally requires more machining steps and a highercomponent count than a petal-valve setup.

2. Increased expense. As a side effect of this complexity, the production cost for a V-valvesystem is significantly higher than for a petal-valve. This is another reason why most cheapmodel engines dont use V-valving.

Less commonly used valving systemsPetal and V-valves are not the only systems that have been used on pulsejets but they are byfar the most common.

Perhaps the only other practical valving system for a pulsejet is:

The Rotary ValveThese generally consist of either a spinning disk containing a hole that controls the flow of gasby covering and uncovering a matching hole in the front of the engine, or a spinning butterfly-type valve that alternately blocks and allows the flow of gas.

Rotary valves can be made very robust and thus have the potential to create very reliable, long-lived pulsejets. Unfortunately however, they are fraught with hidden complexities, not theleast of which includes the issue of timing.

In a conventional pulsejet valving system, the valve timing is automatically controlled by thechanging pressure inside the engine. When the internal pressure goes up (because the fuel hasignited) then the valves close. When the pressure falls (due to the Kadenacy effect) then thevalves open). This results in a very simple and quite reliable system that automaticallycompensates for any fluctuations in the engines operating frequency or phase.

Rotary valves on the other hand, have no such intrinsic timing control and therefore require avery sophisticated system to control their rotational speed and phase relationship to theengines basic operating cycle. This immediately negates the pulsejets two single mostendearing qualities simplicity and low cost.

Research done in the USA during the 1940s cited engines using the rotary valve as offeringvery long useful operating periods along with good thrust and specific fuelconsumption but also mentioned the complexity associated with driving such a valve in asynchronous manner.

Never the less, rotary valves are being considered as a viable option for the new generation ofpulse detonation engines (PDEs) currently under development. Since these PDEs alreadyrequire a significant number of ancillary control systems anyway, the overhead of the rotaryvalve adds little to the cost or complexity of these engines.


Making Reed Valves Last LongerThe thin spring-steel valves normally used to control the flow of air into a pulsejet and stopthe hot combustion gasses from escaping out the front are the most highly stressed part of aconventional pulsejet engine.

The life of the reed valves in most pulsejet designs is measured in minutes rather than hoursand they must be considered a consumable part of any conventional pulsejet engine.

It's not hard to understand why these fragile little pieces of metal don't last long. They'reslammed back and forth between the intake and retainer plates with great force, severalhundred times per second. What's more, they're usually exposed to extremely hot combustiongasses

There are three factors that contribute to reed-valve failure:

heat-damage impact damage fatigue due to flexing

A well designed valve system attempts to minimize all these factors so as to provide maximumvalve life but (woudnt you know it) there are always compromises involved.

Lets look at the simplest and easiest to solve issue first:

Fatigue due to flexingThis picture shows the effect of valve failure due to metalfatigue brought about by the flexing motion of a petal valve.Note that one of the petals has completely broken near theroot. Close inspection of this valve showed that stress crackswere starting to appear at the root of the other petals. In thiscase, failure was due to a poorly designed valve-retainer whichhad an uneven radius of curvature.

Obviously, reed valves must flex in order to operate properly.The key to avoiding premature failure however, is to limit thisflexing to a bare minimum and try to keep the flex radius aslarge as possible.

To this end, the conventional petal-valve arrangement with a curved valve-retainer is quitegood providing the curvature of the retainer is of a constant radius.

If the valve retainer doesnt have an even, large radius curve to it, most of the valve flexingwill be concentrated over a small area near the root of the petal. In a fairly short space of time,the stresses caused by this flexing will cause the spring steel to crack and fracture.


This diagram shows the right and wrong way todesign the valve retainer for a petal valve system.Note that when a small radius is used near thebase of the valve retainer, most of the reed valveremains unbent so all the stress is concentratedat the root.

There are several ways to make a valve retainerthat has a large, even radius but because its somuch easier to make a straight-sided retainer,people often make the mistake of creating whatamounts to a shallow cone instead withpredictably bad results.

Impact DamageMost small pulsejets run at somewhere between 180 and 250 cycles per second. This meansthat the valves must open and close as often as 15,000 times per minute.

Each time the valves close, they slam into the valve-plate at quite high speed and thereforewith significant force. All the energy that is contained in these fast-moving valves has to gosomewhere and some of it is absorbed by the valve material itself.

This constant hammering eventually causes minute cracks toform at the tips of the valves after which they begin to frayand small fragments will eventually flake off. If yourerunning your pulsejet at night, these small fragments can beseen as impressive sparks flying out the tailpipe.

This picture shows a badly frayed petal valve that has certainlyreached the end of its useful life. It is a very good idea toreplace valves long before they get to this state because thesharp, ragged ends will soon damage the comparatively softmaterial of the aluminum valve-plate against which theyimpact.

There are a few techniques that can be used to reduce impact damage to reed valves.

Reduce the amount of valve travel. If the valves can open too far then they will reach amuch greater velocity when theyre closing and this will increase the forces applied tothem as they impact the valve seat. Of course limiting the valve-opening will also tend toreduce an engines power as it means that less air can be drawn in during the intake phase.

Use a softer material for the valve-plate. Most small pulsejets already use an aluminumalloy for the valve plate so theres not a lot of room for improvement here. However, theNACA did conduct tests on engines which had a think coating of neoprene on the valve-plates. This was said to almost double valve-life by reducing the impact shockexperienced by the closing valves.


Note that although it might seem like a good idea to simply increase the amount that a valveoverlaps the hole it covers so that the air trapped between the valve and the plate acts as acushion to soften the impact, this is actually a bad idea.

If the overlap area is too great, the air cant get out of the way quickly enough and the tips ofthe valves are actually bent backwards by this trapped air. As a result, tip fraying isdramatically increased because any cracks that form grow very quickly due to this additionalstress.

Determining the ideal overlap area is something best done by trial and error. If the overlap istoo great then youll get premature fraying and poor engine performance (due to late closingof the valves). If the overlap is too small then the valve plate will be damaged by the highpressure loadings and this will ultimately affect engine performance because the valves will nolonger seal properly.

Heat DamageReed valves are usually made from hardened, tempered spring steel because its strong and willreturn to its original position after flexing.

The problem with spring steel is that the hotter it gets, the softer it gets. If it gets too hotthen it will lose much of its strength and some of its springiness.

Unfortunately, the inside of a pulsejet engine is a very hot place and it is the pressuregenerated by extremely hot (1,500 deg C) combustion gasses that actually cause the valves tobe closed.

So why dont the valves simply get red hot and go all floppy?

Well fortunately, the valves are only exposed to the hot combustion gasses for part of theoperating cycle. During the intake cycle theyre cooled by the incoming charge of fresh air.

In a petal valve setup, the valve retainer also provides a measure of protection from the heatof combustion by shielding most of the valve from direct exposure to the hot gases.

However, the tips of the reed valves will still get hot and, as a result, they will become softerthan the rest of the valve. Whats more, the valve retainer will itself heat up once the engine isrunning and some of this heat will be transferred to the valves when theyre open.

A number of solutions have been proposed to the problem of valve heating but most of themwill reduce an engines performance to some degree.

One of the simplest solutions is to place a flame-trap in the engine directly behind the valves.This flame trap consists of little more than a mesh of stainless steel or some similar heat-resistant metal.

Its well known that a metal mesh with suitably sized holes will not allow a flame to passthrough it but it will allow air and other gases to do so. This was the principle behind the


old miners safety lamps. Before the days of battery-operated flashlights, miners still neededsome form of illumination while working underground. A naked flame such as that from acandle or oil lamp would pose a very real danger as it risked igniting underground pockets ofexplosive gases such as methane.

By enclosing the flame in a metal mesh, the flame could not extend beyond that mesh so thesafety lamp could provide light and still be used without risk of sparking an explosion.

Now, the problem with a using a flame-trap mesh in a pulsejet is that in order to be effective,the size of the holes in the mesh must be quite small. As a result, the mesh represents asignificant obstacle to the flow of the incoming fresh air charge. This means less air is drawnin during each intake cycle so less power is produced..

Never the less, a flame-trap mesh is one way of making a relatively low-powered engine thatwill run for far longer between valve-changes than a regular pulsejet.

Over the past two years Ivegiven quite a bit of thought tothe issue of extending valve lifeand have come up with twoideas of my own.

The first is the Blast Ringconcept which works byproviding a physical shieldbetween the hot exhaust gasesand the valve tips.

By blocking the direct path of the combustion flame to the valve-tips, the operatingtemperature of the valves is significantly reduced. However, because the Blast Ring has a verylarge hole in the middle it doesnt restrict the flow of the incoming charge of fresh air to thesame degree as a flame-trap mesh.

This picture shows my PJ15 design running with aBlast Ring in place.

Youll notice that the ring itself is glowing red-hot, anindication that it is indeed absorbing much of the heatthat would otherwise be reaching the valves.

However, even this system imposes about a 15%-20%performance penalty on the power levels that wouldotherwise be obtained from an engine.

Another method for reducing the valve heating is to design the engine so that theres a bufferof cold, dense air between the valves and the combustion gases. The easiest way to do this isto inject the fuel into the combustion chamber some distance from the front.


In such an engine, the air infront of the injection point willnot contain any fuel thus willnot actually take part in thecombustion process. It willhowever, act as an insulatingbuffer between the hotcombustion gases and thevalves.

Unfortunately, as with the other methods mentioned so far, theres a performance penaltyassociated with this method.

Since a pulsejet normally only draws in a fresh charge of air equal to about 15-20 percent of itstotal volume, creating a buffer zone which contains no fuel leaves less available for thecombustion process. That means less fuel can be added and, as a result, the engine producesless power.

There are very few free lunches in the world of pulsejet design.

More recently however, I have come up with what appears to be a system that imposes noperformance penalty, yet significantly improves valve-life.

This system works by creating a two-layer valve retainer that iscooled by the incoming fuel.

As you can see in this diagram, the fuel (purple) passes between thetwo thin dished valve-retainer plates before mixing with theincoming air.

As you can see, this method allows virtually all the air in the combustion chamber to be mixedwith fuel and therefore provides good power.

This provides multiple other benefits over the traditional system.

1. The fuel is pre-heated and/or vaporized before it mixes with the incoming air. Thisprovides a much better (and more combustible) mixture than is normally achieved eitherby direct injection or atomization.

2. As it atomizes, the fuel absorbs a tremendous amount of heat from the two dished plates,this cooling them to a much lower temperature than they would otherwise run at.

3. The two disks, with the small gap between them, act as a far more efficient heat shieldthan does the normal one-piece valve retainer.

4. The small gap between the retainer disks tends to absorb some of the hot combustiongases that would otherwise reach the valves.


Heres how Ive implemented this design concept onmy PJ15 engine. The two disks are formed from thin0.020 (0.5mm) stainless steel which is spun to shapeon a lathe.

Ive actually observed a small power increase afterchanging from a conventional one-piece valveretainer to this new concept and valve-life has beenalmost doubled.

Unfortunately, building a valve system like this takes more time and more time and skill thanthe traditional one-piece valve-retainer remember what I was saying about free lunches?


Fuel Systems

Next on the list of critical elements of a pulsejet must surely be the fuel system.

AtomizationSmaller engines such as the Dynajet have traditionally used a very crude form of carburetorthat using the incoming air to create a spray of rather coarsely atomized fuel droplets.

This atomizing process occurs right at the front of the engine when the incoming air is forcedthrough a slight venturi.

An Italian by the name of Bernoulli discovered that the faster air flows, the lower its pressurebecomes. This observation was promptly labeled (wait for it) the Bernoulli Effect.

The atomizer on these small pulsejets uses a venturi to squeeze the incoming air through anarrowing in the intake. As it squeezes through, it has to speed up. As it speeds up thepressure drops.

Now, if we stick a pipe carrying some fuel into the middle of this low-pressure area, the fuel isliterally sucked out and turned into a fine spray of droplets.

What could be simpler?

Unfortunately, although this system does work, the magnitude of the low-pressure area createdin the pulsejets venturi is quite small and this means that theres not much energy available tosuck that fuel through.

A Note About Atomization and VaporizationAnother problem with the simple atomizer is that the fuel droplets created tend to be verylarge and therefore do not vaporize particularly well. It should be remembered that liquid fuelsthemselves dont actually burn only the vapors that they emit will ignite. In order to obtaingood vaporization, the goal should be to create the smallest possible droplets because thisresults in the largest surface area (from which vapor is emitted) for a given volume of liquid.

Fortunately however, the inside of a pulsejet engine is a very hot place so, despite the fact thatthe simple atomizer does a poor job of converting liquid fuel into a nice fine spray, the highinternal temperatures of the engine greatly assist the conversion of those large droplets of fuelinto vapor.[endnote]

The end result is that most of these small pulsejets are extremely sensitive to just where thefuel tank is placed relative to the atomizer assembly.

If you place the tank too low then the engine wont have enough suck to pull the fuel up tothe atomizer nozzle.

Place the tank too high and gravity will draw the fuel through effectively flooding the engine.


Whats worse, even if you do get the engine running nicely, moving the fuel tank up or downby even an inch or two can cause it to stop because the fuel flow is affected.

There are ways to reduce this sensitivity to fuel-head however and perhaps the simplest is touse a pressurized fuel tank.

By delivering the fuel under pressure, the effect of a changing fuel-level is dramaticallyreduced. The big problem is how do we generate this pressure?

One option is to simply pump some compressed gas into the fuel tank then seal it up. Inorder for this to work, the tank should only be filled with fuel to only about 25 percent of itscapacity otherwise the pressure inside will drop significantly as the fuel is drawn off.

Alternatively, the compressed gas can be stored in a separate container and fed into the fueltank through a regulator. This is how the fuel system for the Argus engine that powered theV1 flying bomb was configured and is illustrated in the diagram above.

Rather than rely on a large reservoir of compressed airinside the tank, it is possible to tap into the pressureproduced by the combustion of the pulsejet itself.

This diagram shows how some of that pressure can bedirected into the tank to keep it pressurized. Note thesmall reed valve that stops the pressure from leakingback into the engine during the intake phase.


In practice, the reed valve should be placed in the pipe that leads from the engine to the tankrather than in the tank itself. Surprisingly, theres little risk that the hot gases from inside theengine will ignite the fuel in the tank. This system can be used with both atomized andinjected fuel systems.

Another simple way to achieve fuel pressurization is to use something like a small balloon for afuel tank. This configuration is called a bladder tank. The elasticity of the balloon willautomatically pressurize its contents but be aware that some fuels will quickly break downthe rubber from which normal balloons are made and if it goes pop, youll have a very realfire danger.

Some of those using pulsejets in model airplanes often use these bladder tanks to ensure goodpressurization and reliable fuel feed under varying G-forces. Its worth noting however, thatthe rubber tank is usually contained inside another leak-proof container such as a plastic sodabottle. This way, if the bladder bursts, the fuel remains contained

Most flyers of pulse-jet powered model airplanes also use a device calleda Cline regulator to ensure not only that the fuel pressure remainsconstant but also to automatically shut off the fuel flow if the enginestops unexpectedly.

You should also be aware that any leak in a fully pressurized fuel systemcan result in large amounts of flammable liquid being dumped onto theground or in the general area of the engine. This is an obvious fire risk. Whats even worse isthat if the engine stops for any reason, the flow of fuel will continue to flood into what is nowa red-hot steel tube. That can result in a very impressive fireball that could also be verydangerous.

InjectionVirtually all engines over 20lbs of thrust use direct fuel injection rather than atomization.

In such a system, the fuel is squirted directly into the engines combustion chamber undersome form of pressure.

This makes the engines operation far more reliable and adds the additional benefit that byvarying the amount of fuel being injected, the engines power can be varied. Yes, a throttleablepulsejet!

The Argus V1 engine used direct injection but, to the best of my knowledge, no attempt wasmade to provide any form of throttle control not that it would have been of any use on aflying-bomb anyway.

The downside of fuel injection is that you need some method of pressurizing the fuel to forceit into the engine in a fine spray.

There are really only two options use a fuel pump or pressurize the entire fuel tank.


The V1-flying bombs used the latter option and the fuel tank was pressurized using the samecompressed-air source which drove the missiles gyroscopes and other onboard systems.

Most of my injected engines use propane as a fuel because this has the advantage of being self-pressurizing. Your common BBQ tank has around 100psi of pressure in it so you can use thisfor direct injection without the need for a supply of compressed air or a fuel pump.

Using such a system, the pulsejet remains a stand-alone engine that requires no extra bits andpieces to keep it running.

The simplest injection system for a petal-valved enginesimply involves locating a cross-drilled injection nozzledirectly behind the valve-retainer plate.

This nozzle is drilled so that the incoming fuel is sprayedout directly towards the side of the combustionchamber. This ensures optimum mixing with the air and(in the case of liquid fuels) means that any droplets offuel that arent vaporized by the incoming air will beinstantly flashed into vapor when they hit the hotcombustion chamber walls.

A more recent innovation Ive come up with howeverinvolves placing an additional disk behind the valveretainer, separated by just a small space.

By injecting the fuel in thesame radial pattern aswith the previous system

but between the two disks, the fuel is not only vaporized moreeffectively but also serves to cool down the valve retainer disk(and the valves). Building a system like this does however, requireaccess to a lathe in order to turn up the key component which isthis radial injector nozzle.

Using this double-disk setup Ive been able to double the life of the reed valves used in a petalvalve engine while also slightly increasing the engines performance and throttle range.

Timed InjectionOne disadvantage of direct fuel injection is that simple systems such as the one used in theArgus V1 engine tend to spray fuel throughout the engines operating cycle.

Fuel will only burn efficiently (or at all) when mixed with exactly the right amount of air. Thiscombustible mixture of air to fuel is referred to as the stoichiometric ratio and it variesdepending on the type of fuel being used.


It makes little sense therefore, to waste fuel by injecting it when there is no incoming air to mixwith it as that fuel will be unable to burn inside the engine thus contributes nothing to thethrust being generated.

Back in 1947, the guys at Princeton University came to this same conclusion and suggestedthat using timed fuel injection would be a way to improve the fuel-efficiency of pulsejetengines.

Now there are two ways in which timed fuel injection could be done: the simple way and thecomplex way.

Given that the simplicity of a pulsejet is its single greatest virtue, Im all in favor of keeping atimed fuel injection system simple too.

I regularly get email from people who think it would be a good idea to use an electricallydriven fuel injector like the ones used in modern car engines but I disagree.

In order to make one of these injectors work youd need a rather complex system that involveda battery to drive the injector, sensors to measure the pressure inside the combustion chamberfor timing, and some electronics to tie the whole thing together.

This setup, although Im sure it could be made to work, would be costly, complex and offeronly minimal benefits over the system I use to obtain timed fuel injection.

Fortunately it is a simple job to synchronize the injection of fuel into the engine with the intakeof a fresh air charge. This is because the pressure inside the engine falls to below 1atmosphere (14.7psi at sea-level) during the intake phase and rises to as much as twiceatmospheric (30psi+) during combustion and exhaust phases.

A valve placed over the fuel jet is sufficient to provide a degree ofinjection timing and the addition of this mechanism can provide anoticeable improvement in the fuel-efficiency of a large pulsejet.

Ive experimented with a number of different valved injectors rangingfrom a simple bolt drilled length-wise with a flap of spring-steel over theend like the one illustrated here

To this carefully machined injector made from stainless steel andnickel-plated steel components I fitted to the 100lbs-thrust engine onmy gokart. I noted a very definite improvement in the fuel-efficiencyof this engine after fitting the timed injector system.

What Fuel is Best?One of the great advantages of pulsejet engines is that they can, at least in theory, be made torun on almost any type of combustible liquid or gas.


Pulsejets arent limited to liquid or gas fuels however on at least two occasions, coal dust hasbeen used as a fuel. It is rumored that the Germans attempted to run the Argus V1 engine oncoal dust when liquid fuel supplies became almost unobtainable near the end of WW2 andsome of Reynsts pulsed combustors were designed specifically to use this unusual fuel.

Before you start worrying too much about what is the best fuel, its worth citing part of areport published by Princeton University in 1947 that summarized a large amount of theresearch done into pulsejet engines up to that time. It said the pulsating jet engine ofcontemporary design ran on almost any common fuel with negligible variations inperformance.

The only caveat the report included was that principal [sic] differences were in the degreeof body heating and the rapidity of valve destruction.

They found that even the use of exotic fuels such as nitropropane or nitromethane offeredonly a slight power increase at the expense of doubling an engines fuel consumption.

It makes sense therefore to choose your fuel on the basis of whatevers cheapest or mostconvenient to use.

For most of us however, the choice of fuels is fairly simple and boils down to one of these:

GasolineThis has the advantage that its relatively cheap, very easy to obtain, and is pretty clean burning.Its also quite volatile so atomizes easily to promote easy starting.

Note that, contrary to what you might think, higher-octane gasoline is not going to produceany more power than regular gasoline. In fact (in theory) it may produce slightly less power. Ifyou plan to use gasoline, just use whatevers cheapest.

Propane (LPG)Thanks to the popularity of gas-fired BBQs, propane has also become quite easy to obtain andsuitable 20lb refillable tanks can be bought for well under $50.

In some countries, propane is even cheaper than gasoline and it burns very cleanly indeed leaving no smell and very little residue at all. Despite the fact that its stored under pressure, itis actually quite a bit safer to use than gasoline because its vapors dissipate very quickly in theopen air.

Since the boiling point of propane is well below normal room temperature, it either comes outof the tank as a gas (thus avoiding the need for vaporization) or, when drawn off as a liquid,instantly boils into a vapor. This makes a propane-powered pulsejet one of the easiest to start.

Note that bigger engines will almost certainly demand to be fed with liquid propane because anaverage BBQ tank simply cant provide gas at a sufficiently high rate to keep up.


If youre planning to use a BBQ tank of propane as a fuel, youll have get rid of the regulatorthat is normally used to limit the flow of gas. This regulator reduces the pressure of the gas tojust a few psi, far too low for a pulsejets needs.

To give you an idea of just how much gas is needed to run a pulsejet, my own 15-lbs-thrustengines (PJ15) will drink all the propane gas you can feed them with the regulator removed.The Lockwood valveless engines will drink all the liquid propane you can feed them withoutany regulator in place.

If you try to use propane without removing the regulator then all youll get from a pulsejet is afew bangs and pops it wont run.

However, you will still need some form of control over the flow of gas into the engine and forthis I recommend buying a cheap propane/air torch of the type often used for soldering orbrazing.

These torches are available from almost any hardwarestore and cost just $25-$30. Note that depending onthe exact make/model of torch you buy, you mayneed to purchase an additional adapter fitting so thatit can be screwed directly onto a 10lb or 20lb propanetank.

To use a torch like this as the gas-control valve foryour pulsejet, simply unscrew the burner fitting on the end and slide your propane-certifiedplastic fuel pipe over the end, securing it with a small hose-clip.

The gas-flow knob on the torch will now enable you to control the amount of gas that isdelivered to your engine. If you invert your BBQ tank of propane, the torch will still serve as avery simple way to control the flow of liquid propane to larger engines. Very simple, veryinexpensive, and very effective.

Another method of controlling the flow of propaneto your engine is to simply use a device called aneedle-valve. These valves are readily available froma number of sources and, just like the gas-torch, offera very fine degree of control over fuel-flow.

ButaneIt should be noted that although it is also often sold for use on small camp stoves, butane isnot a good substitute for propane. It contains less energy and doesnt produce as muchpressure as propane at room temperature. In short dont waste your time or money trying touse butane as a fuel for pulsejet engines.

White Spirit/Coleman fluidThis is simply a very low octane unleaded form of gasoline which has no fancy anti-knock orcombustion improvemnet additives included. Its actually a better fuel than high-octane


gasoline for pulsejet use. Many of the early small pulsejet engines such as the Dynajet run beston this fuel.

MethanolThis is my second-favorite pulsejet fuel. It has the advantage that it will burn over a very widerange of rich/lean mixture settings making an engine less sensitive to fuel head or startingconditions.

It also burns very cleanly with no smelly or oily residue and creating little more than watervapor and some C02 as combustion byproducts.

On the downside, methanol is more expensive than gasoline, your engine will burn more of itfor a given amount of power, and it can be very dangerous if spilled because it burns with analmost invisible flame. Many people have been burnt because theyve walked straight into amethanol fire without seeing it.

Despite the downsides, I prefer to use methanol for all my aspirated engines because itgenerates a little more power, allows the valves to run cooler, and doesnt leave my handsstinking of gasoline.

Note that you shouldnt use pre-mixed model airplane fuel instead of straight methanol.Model airplane fuel contains up to 20% oil that will leave significant deposits inside yourpulsejet and also affects the vaporization of the mixture. Its also a lot more expensive thanplain old methanol so youll be wasting money.

Your local hot-rod or drag-racing club ought to be able to help you find a source of methanolbut if all else fails, try one of the major oil companies like Mobil they sell me 5-gallon drumsof the stuff when I want it.

Another thing to watch when using methanol as a fuel is that it is very hydroscopic which isto say that it tends to absorb moisture out of the air. If you leave a can of methanol uncappedthen it may well absorb so much moisture that its combustibility is affected and this can resultin hard-starting.

Also be aware that when you use methanol as a fuel, one of the combustion byproducts iswater (albeit as water vapor). This means that the spring-steel reed valves used in an enginerun on methanol are prone to rusting unless you oil them lightly before storing your engineafter each run.


Constructional TechniquesOnce youve calculated the dimensions for a pulsejet, how do you then go about building one?

Of course It really helps if youve got access to a workshop or some basic metalworking toolssuch as a hacksaw, drill, welder, etc. but dont be put off if your resources are a little moremodest.

Youd be surprised how helpful your local welder or engineer can be when you explain thatyoure building a jet engine and would be happy to demonstrate it to them when its done.

Its also amazing what you can do with a minimum of tools if youve got enough patience.

The Engine Body/TailpipeMost commercial pulsejets are made from thin stainless steel sheet that is rolled or otherwiseformed 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 poweringmodel airplanes.

These cones and tubes are formed using a device known as a slip-roll which looks rather likean old washing-machine wringer and consists of three rollers that can be adjusted to both gripand curve the metal sheet as its wound through.

A hand-operated slip-roll like this one is limited to rolling stainless steel that is no more than1mm thick and even then its damned hard work if youre rolling a piece the full 600mm longwhich 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 canhandle the thicker material and longer lengths youll need to use.


Unfortunately, stainless steel is not only expensive but can also be very difficult to weld whenits very thin. For this reason, many enthusiasts prefer to make their engines from cheaper andmore easily worked materials

Providing weight isnt a problem you can save a lot of time and significantly simplifyconstruction by using mild steel pipe for the body of your engine. The best stuff is exhausttubing which is usually protected from rusting by a thin layer of aluminum on the surface.

This stuff is relatively cheap, available in a widerange of sizes and can be cut and welded easilyusing MIG, arc or oxy-acetylene equipment.Unfortunately its also quite heavy, but thats usuallyunimportant to the eager enthusiast who simplywants to build an engine and get it running ASAP.

Heres a picture of my gokart with an engine builtfrom exhaust tubing. It only produced just enoughthrust to get the kart moving on a smooth surfacebut 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 weldingcharacteristics.

The Valving SystemExactly how you construct this depends on the type of valving system you plan to use. If youhave access to a lathe then you can easily make a petal-valve system with a nicely turnedaluminum head. If youre 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 cancut 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 circlefrom a flat sheet by using a jig-saw. You can even use a small coping saw or jewelers saw tocut 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 circleusing a compass then cut it slightly over-sized. It can then be filed down to a precise fit intothe front of your engine.

However, before you cut out the circle, it pays to mark out and drill the valve holes. By doingthis before you cut the circle to shape, you have a larger piece of metal to hold onto whendrilling the valve holes and this makes the job much easier.


Heres a valve-plate that was made from flat-sheet,cut to shape with a jigsaw and with valve-holes thatwere cut with a jewelers saw. In this case, a lathe wasused to finish the surface of the disk and it was thenanodized to provide a hard, corrosion resistant layerand a pleasing gold color. See the chapter onanodizing for details of how to perform the anodizingprocess.

Note that theres no reason why the valve-holesshould be circular and a trapezium shape as in thepicture actually allows a greater valve area for a givensize of valve-plate.

Having made the valve plate a snug fit in the front of your engine, you can then drill 4-6 smallholes around the circumference of the pipe so that they also go through into the edge of yourvalve plate. By choosing the correctly sized drill, you can then fit self-tapping screws to holdthe valve-plate firmly in place.

Any leaks in this area can be fixed by the liberalapplication of some muffler-sealant the type thatcomes in a small tube and is designed to block-upgaps in exhaust systems. This stuff will expandslightly as the engine heats and seal any small gapsbetween the valve plate and the engine body.

If you dont have a lathe then the other option is tobuild a V-valve system instead of a petal valve one.

The V-valve can be created from flat pieces of steelas in this drawing.

Making Reed ValvesSince the reed-valves are the heart of a conventional pulsejet engine, its important that theyare 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 cant find a source for this material locally then I suggest you take a look at some of theonline mail-order metal supply companies.

Once youve 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 complexshapes with regular metal snips.


While its 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 valvefrom spring-steel sheet.

The first involves using a Dremel or similar tool fitted with acut-off tool as shown in this picture.

This technique requires a bit of practice and is most suitablefor smaller valves.

The preferred method for making petal valves involves the useof a process known as electrochemical etching.

Full details of how to make reed valves using this process are provided later in this book.

WeldingThere 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 usedand 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 findMIG welding much easier than TIG, Ill describe both processes.

Welding with MIGI wont turn this into a welding tutorial so I assumeyoull already be moderately competent with a MIGwelder. Instead, Ill focus on the points specific towelding the various parts of these engines together.

It is important to use the right filler-wire. If youvecut and rolled the parts out of 304 stainless steelthen you can use 308 or 316 stainless filler wire. Ifyouve cut your parts from 316 stainless then use316 filler rather than 308.

Ive found that 0.8mm (0.032) wire is about the right thickness and you should only need asmall spool to build an entire engine.

While you can use plain steel wire, the resulting weld will rust very quickly and become weak,so its not recommended. Stainless steel also has a much higher rate of thermal expansion thanplain steel so youll 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 weldbead you can without burning through.

I find when working withvery thin material (

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