Valveless Pulsejet Engines 1.5
-- a historical review of valveless pulsejet designs --
by Bruno Ogorelec
The idea that the simplest engine an enthusiast can make at home is a jet engine will
sound strange to most people -- we perceive jet engines as big complex contraptions pushing
multi-million dollar aircraft through the skies. Yet, this is completely true. In its most basic
form – the valveless pulsejet -- the jet engine can be just an empty metal tube shaped in a
proper way. Everyone able to cut sheet metal and join metal parts can build one in a garage
or basement workshop.
Due to peculiar historical circumstances, this interesting fact has escaped popular
attention. It is not familiar even to enthusiasts of jet propulsion. You are not very likely to see
or hear jet engines roaring in people’s back yards on Sunday afternoon. Few if any people
can be seen flying aircraft powered by jet engines they have built themselves.
This document aims to help change that.
However, it is not a how-to primer. It is an attempt to describe and explain the valveless
pulsejet in principle. It also offers a rough sketch of the amazing variety of layouts the
inventors and developers have tried during the long but obscure history of this device.
My aim is to inspire, rather than teach. My goal is to demonstrate that jet power is
accessible to everyone in a great variety of simple ways. Should you find the inspiration,
plenty of information on the practical steps towards jet power will be available elsewhere.
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HOW DOES A VALVELESS PULSEJET WORK?
The picture below shows one of the many possible layouts of a valveless pulsejet engine.
It has a chamber with two tubular ports of unequal length and diameter. The port on the right,
curved backwards, is the intake pipe. The bigger, flared one on the left is the exhaust, or
tailpipe. In some other engines, it is the exhaust pipe that is bent into the U-shape, but the
important thing is that the ends of both ports point in the same direction.
When the fuel-air mixture combusts in the chamber, the process generates a great amount
of hot gas very quickly. This happens so fast that it resembles an explosion. The immediate,
explosive rise in internal pressure first compresses the gas inside and then pushes it
forcefully out of the chamber.
Two powerful spurts of hot expanding gas are created – a big one that blows through the
tailpipe and a smaller one blowing through the intake. Leaving the engine, the two jets exert
a pulse of thrust – they push the engine in the opposite direction.
As the gas expands and the combustion chamber empties, the pressure inside the engine
drops. Due to inertia of the moving gas, this drop continues for some time even after the
pressure falls back to atmospheric. The expansion stops only when the momentum of the
gas pulse is completely spent. At that point, there is a partial vacuum inside the engine.
The process now reverses itself. The outside (atmospheric) pressure is now higher than
the pressure inside the engine and fresh air starts rushing into the ends of the two ports. At
the intake side, it quickly passes through the short tube, enters the chamber and mixes with
fuel. The tailpipe, however, is rather longer, so that the incoming air does not even get as far
as the chamber before the engine is refilled and the pressure peaks.
One of the prime reasons for the extra length of the tailpipe is to retain enough of the hot
exhaust gas within the engine at the moment the suction starts. This gas is greatly rarified by
the expansion, but the outside pressure will push it back and increase its density again. Back
in the chamber, this remnant of previous combustion mixes vigorously with the fresh fuel/air
mixture that enters from the other side. The heat of the chamber and the free radicals in the
retained gas will cause ignition and the process will repeat itself.
The spark plug shown on the picture is needed only at start-up. Once the engine fires, the
retained hot gas provides self-ignition and the spark plug becomes unnecessary. Indeed, if
spark ignition is left on, it can interfere with the normal functioning of the engine.
It took me more than 300 words to describe it, but this cycle is actually very brief. In a
small (flying model-sized) pulsejet, it happens more than 250 times a second.
The cycle is similar to that of a conventional flap-valve pulsejet engine, like the big Argus
(which powered the V-1 flying bomb) or the small Dynajet used to power flying models.
There, the rising pressure makes the valve flaps snap shut, leaving only one way for the hot
gas to go -- into the exhaust tube. In the J-shaped and U-shaped valveless engines, gas
spews out of two ports. It does not matter, because they both face in the same direction.
Some valveless pulsejet designers have developed engines that are not bent backwards,
but employ various tricks that work in a similar fashion to valves -- i.e. they allow fresh air to
come in but prevent the hot gas from getting out through the intake. We shall describe some
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of those tricks at a later point.
You may wonder about the sharp transition from the intake tract into the chamber. It is
necessary to generate strong turbulence in the incoming air, so that it mixes with injected fuel
properly. A gentler, more gradual entry would not generate the necessary swirling of gases.
In addition, turbulence increases the intensity of combustion and the rate of the heat release.
THE BEGINNINGS
The idea of using the elastic properties of air to generate power pulses is very old. The
first pulsejet engines were built in France at the very beginning of the 20th century. They
found only very limited use at the time and were soon forgotten for all practical purposes.
In the 1930s, however, German engineer Paul Schmidt rediscovered the principle by
accident while trying to develop a detonation engine. He built a series of impressive pulsejets
with valves. At roughly the same time and in the same country, engineers at the Argus
engine company were working on a valveless device that used compressed air.
The circumstances were much more propitious now. The world was preparing for a big
war and the war machines were gearing up. The German War Ministry brought Schmidt and
Argus together, which resulted in the development of the first mass produced jet engine. Like
the Schmidt engines, it used valves and natural aspiration, but its mechanisms were greatly
modified by Argus.
Thus, while the opposed sides
in World War II were still trying to
put together their first jet-powered
fighter aircraft in 1944, the
Vergeltungswaffe 1 (or V-1 for
short) was regularly buzzing its
way to England with a 1,870-lb
load of explosives. Its Fieseler
airframe was powered by the
Argus As 109-014 pulsejet engine.
You can see one flying over the
English countryside on the photo
on the right.
The utter simplicity, low cost
and demonstrated effectiveness of
the pulsejet impressed the Allies
so much that they badly wanted to
have something similar. It looked
amazing to everyone that a device
that simple could power a serious
flying machine. Captured
examples of the Argus were
carefully studied and copies built
and tested.
It soon became obvious that the
pulsejet had certain drawbacks
and limitations, but the basic
principle still looked very attractive
and ideas for improvement
abounded. Various uses for the
device were contemplated. Ford Motor Company built a proper assembly line to manufacture
Argus copies. With the end of the war, some of the projects were scuttled, but the Cold War
started soon and the quest for a better pulsejet continued.
Unfortunately, progress was very slow and purely incremental. In the mid 1950s, after a
decade of effort, developers were not that much better off than their wartime German
predecessors. In total contrast, the advances in turbojet design over the same period were
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tremendous. By that time, turbojet-powered fighters already had the Korean War behind
them. Turbojet strategic bombers were carrying nuclear weapons in their bomb bays and
turbojet airliners were getting ready to earn their money carrying businessmen and the idle
rich from continent to continent.
It was becoming completely clear to everyone that the turbojet was the jet engine of the
future. Engineers were still excited by the promise of the pulsejet, but the reality was not to
be denied. During the 1950s and 1960s, most pulsejet researchers gradually abandoned
their efforts and turned to other things.
THE ADVANTAGES
What originally attracted and excited the researchers and developers most of all about the
pulsejet engine was a peculiar property of pulsating combustion – it can be self-compressing.
In the pulsejet, the fuel-air mixture does not burn steadily, at a constant pressure, as it does in
the other jet engines. It burns intermittently, in a quick succession of explosive pulses. In
each pulse, the gaseous products of combustion are generated too fast to escape from the
combustor at once. This raises the pressure inside the combustor steeply, which increases
combustion efficiency.
The pulsejet is the only jet engine combustor that shows a net pressure gain between the
intake and the exhaust. All the others have to have their highest pressure created at the
intake end of the chamber. From that station on, the pressure falls off. Such a decreasing
pressure gradient serves to prevent the hot gas generated in the combustor from forcing its
way out through the intake. This way, the gas moves only towards the exhaust nozzle in
which pressure is converted to speed.
The great intake pressure is usually provided by some kind of compressor, which is a
complex and expensive bit of machinery and consumes a great amount of power. Much of
the energy generated in the turbojet engine goes to drive a compressor and only the
remainder provides thrust.
The pulsejet is different. Here, the exhaust pressure is higher than the intake pressure.
There is pressure gain across the
combustor, rather than loss. Moreover,
the pulsejet does it without wasting the
power generated by combustion. This
is very important. According to some
rough figures, a 5-percent gain in
combustion pressure achieved by this
method gives about the same
improvement in overall efficiency as the
85-percent gain produced by a
compressor, all other things being
equal. Now, that’s rather impressive.
Personally, I am interested in the
pulsejet for another reason -- because it
brings the jet engine back to the people.
It is a back-to-basics kind of machine,
so simple to be accessible even to
enthusiasts with rudimentary skills and
simple tools. Turbojets and fanjets are
at the opposite end of the complexity
scale. In most cases they employ
inaccessible, cutting-edge technology.
Just look at the collection of
pulsejets on the picture on the right.
They were built by Stephen Bukowsky,
a high-school student, purely out of fun.
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If I remember it right, the three valveless engines (second, third and fifth from left) each took
him about a couple of days to make. This is just a part of Steve’s collection!
Cost is another advantage. Pulsejets are cheaper than even the simplest piston engines
of comparable output. In contrast, turbojets are frighteningly expensive.
THE DISADVANTAGES
So, given the advantages, why did the pulsejet disappear from view? There are several
reasons.
A big problem is that the gain in efficiency offered by pulsating combustion is not at all
easy to utilize for propulsion. Paradoxically, the central problem here is the same as the
source of the benefit – namely, pulsation. The very means of increasing combustion
efficiency makes it difficult to take advantage of the result.
The real potential for the pulsejet has always been in its use as the combustor for a turbine
engine, rather than as an engine in itself. Its ability to generate pressure gain is greatly
multiplied in a high-pressure environment. Compared to the more usual constant-pressure
combustor, it can either give the same power with much smaller mechanical loss and lower
fuel consumption, or much greater power for the same amount of fuel.
Alas, a turbine demands steady flows to function efficiently. Unsteadiness generates loss.
Also, pulsations are dangerous for the brittle axial turbine blades. Radial turbines are tougher
in that respect, but they are less efficient, especially so with intermittent flow. They are mostly
used to exploit waste heat, as in a turbocharger, rather than as prime movers. Researchers
have toyed with converting pulsations into a steady flow, but most methods proved inefficient.
But, how about simplicity? In a manner of speaking, a pulsejet is what remains when you
remove all the complex and expensive parts from a turbojet and leave only the simple and
cheap combustor that is hidden in the middle.
Well, yes, simplicity is attractive, but it also has its disadvantages. The promise of the
pulsejet on its own, outside a turbojet, is less significant. The pressure gain is still there, but
in the atmospheric pressure environment, without the multiplication offered by the
compressor, it does not amount to very much. The average pressure in the working cycle is
low, the specific power unimpressive and fuel efficiency poor. The power ‘density’ is much
lower too. For the same engine bulk, you get less thrust than with the competing jet engines.
Pushing the pulsejet further down the scale of desirability in the postwar era was the fact
that even with the improvements arrived at in the 1950s and 60s; the pulsations still produced
horrible noise and mad vibration. Pulsejets depending on reed valves were also short-lived
and unreliable. OK, they were cheap, but in the Cold War era that was certainly not a prime
consideration.
Finally, there was little that pulsejets were really good for. For a while, it looked like they
would power small helicopters. Some spectacular-looking prototypes were built, especially in
France. In the end, however, they never made the grade, mostly for aerodynamic reasons.
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The French briefly used pulsejet power on motor gliders and flying drones, too. Cheap
flying drones and missiles were built in several countries, including the US, Russia and China.
The picture above shows the French Arsenal 501 target drone, powered by a valved engine.
The color picture on the first page of this document shows a Chinese target drone with a
valveless engine.
That was about it. Given the ample defense budgets, most of the real-life applications that
required a jet engine were better satisfied with a turbojet or with rocket power.
Civilian industry did not look upon the pulsejet with any greater kindness. Turbojet
development was intense and engineers had little time for the exotic pulsating things that few
people understood properly anyway. The difficulty of defining the processes inside the
pulsejet mathematically was a major problem for most researchers and engineers. Modeling
the semi-chaotic pulsating combustion was far too much for the computing abilities of the
time. It meant that pulsejet design was unpredictable -- part science and part black art.
Industry tries hard to avoid such tricky propositions.
By the mid-1960s only a few isolated enthusiasts still considered the pulsejet as a potential
aircraft powerplant. The noisy tube was in a blind alley and relegated to the role of model
aircraft engine and such humdrum applications as an efficient combustor for central heating
systems, a power unit for agricultural spray dusters and a blower and shaker for industrial
slurry drying machinery.
CHANGE OF CIRCUMSTANCES
So, why look at pulsejets now? Well, my reason is the change of circumstances.
Sometime in the early 1980s, ultralight fun flying started getting increasingly popular due to
the availability of good, simple and affordable flying platforms – hang gliders and paragliders.
When provided with motor power, these machines offered unprecedented freedom of flight to
anyone interested. In addition, with the fantastic development of modern electronics, a whole
new class of unmanned flying machines appeared, designed as utility platforms for a variety
of telecommunications, surveillance, measuring and sensing devices.
All those new flying machines, whether designed for fun or utility, are powered by piston
engines that drive propellers. Jet engines only appear at the very top end of the price scale –
on machines costing several hundred thousand dollars apiece.
All the piston engines currently used in ultralight flying are relatively heavy and
cumbersome, even in their simplest form. They also require much ancillary equipment, like
reductors, prop shafts, propellers etc. etc. Having all that gear mounted on a lightweight
flying machine almost defeats the original purpose. A simple lightweight pulsejet seems
much more appropriate.
Turbojets, on the other hand, are terribly
expensive – far out of enthusiasts’ reach. Things
are not likely to get much better in the near
future, either. Because of the very high
technological requirements, the cost of turbojet
engines has always remained high. Only the
small turbojets based on old turbocharger parts
are relatively inexpensive, because their most
precious parts are taken off scrapped truck
engines, but even their prices are not pleasant.
In contrast, the humble low-technology
pulsejet is laughingly cheap by any standard.
Besides, in the engine sizes likely to be used
by enthusiasts, the best pulsejets can compete in
performance with the other jet engines,
especially in the power-to-weight stakes.
I am often told that a jet engine will never be good for recreational purposes. Jet
propulsion is really efficient only at relatively high airspeeds, seemingly making it unsuitable
for low-speed devices such as hang gliders. However, maybe a niche for a simple jet engine
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can be found at the top end of hang-glider performance – possibly with rigid wings.
Also, the rule does not seem to be very strict. For instance, a British Doodlebug harness
powered by a Microjet turbojet engine has been tested with delightful results with a regular
foot-launched hang glider (see the picture).
This bodes well for pulsejets. When equipped with a thrust augmenter, a good pulsejet
can be optimized for speeds much lower than those of other jet engines. It can hardly fail to
perform at least as well as the Microjet in a similar application. In terms of thrust to weight it
is already superior.
Tote up those points and the lightweight, simple, cheap low-speed pulsejet engine
suddenly starts making a lot of sense. Its admittedly high fuel consumption, noise and
vibration need not be of major importance for the applications I have in mind -- or may
perhaps be alleviated or designed out of the concept.
The enormous advances in computing power over the past few decades have made
modeling of pulsating combustion more realistic, too. It is still not easy even for the
supercomputers, but it can now be done. This can cut down development time drastically and
make it much more straightforward.
Finally, our understanding of pulsating combustion has advanced to the point where these
engines can be designed on paper with performance predictability much closer to that of the
other engine types.
It is perhaps time to blow the dust off the old tube.
WHY VALVELESS?
The ordinary pulsejet is already a very simple engine. It is just a piece of tube cut to the
required dimensions, with a few small flaps and a fuel jet at one end. So, one might ask, why
go that one small step further and eliminate the valves?
The prime reason is that the use of flap valves limits the reliability and longevity of the
engine. The valves of the As 109-014 lasted for only about 30 minutes of continuous use.
Given that its role was to destroy itself in the end anyway, this was not a big fault, but today
you might have a flying model that is your pride and joy up in the air, or you may even want to
fly yourself. You really need your engine to last a bit longer.
Admittedly, development has improved the design in many ways and stretched its working
life from minutes into hours, but the fundamental problem remains. In fact, it looks well nigh
insoluble, given that the valves are supposed to satisfy conflicting demands.
In the interest of combustion efficiency, they should not impose their own timing on the
flows. This is very important, as the combustion process is not only intermittent but also
somewhat erratic and highly dependent on feedback. If we want to avoid disturbing the
natural progress of the pulsation as much as possible, the valves must respond to changes of
pressure almost instantaneously. To do that, they have to be as light as possible.
At the same time, however, they have to endure great mechanical stress (bending open
and slamming shut at high-speed) and do it in a high-temperature environment. They have to
be very tough. If something has to be light, yet exposed to great abuse, it either spells short
life or exotic technology. The former is impractical and the latter is expensive.
Finally, there is a question of elegance. I find the idea of a jet engine that is actually just a
cheap empty metal tube without moving parts very appealing. Making the various gases jump
through hoops and produce useful tricks without resorting to any mechanical complexity is a
nifty thing that will be appreciated by all lovers of simplicity and elegance. (I am talking of
elegance in the mathematical sense -- desired result achieved with minimal complication.)
KADENACY OSCILLATION, THERMAL BREATHING AND ACOUSTIC RESONANCE
Before getting into details of actual engine designs, let’s get some important theory out of
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the way. People who hate theory may skip this part, but my advice is to skip it only if you are
already reasonably familiar with the laws of acoustics and fluid mechanics and aware of how
they pertain to pulsejets. On the other hand, people who like theory should be warned that
the following is a greatly simplified description of very complex mechanisms.
Kadenacy Effect
In the explanation of the working cycle, I described how inertia keeps driving the
expanding gas out of the engine all the way until the pressure in the chamber falls below
atmospheric. The opposite thing happens in the next part of the cycle, when the outside air
pushes its way in to fill the vacuum. The combined momentum of the gases rushing in
through the two opposed ports causes the chamber briefly to be pressurized above
atmospheric before ignition.
There is thus an oscillation of pressure in the engine caused by inertia. The gases
involved in the process (air and gaseous products of combustion) are stretched and
compressed between the inside and outside pressures. In effect, those fluids behave like an
elastic medium, like a piece of rubber. This is called the Kadenacy Effect.
The elastic character of gas is used to store some of the energy created in one
combustion cycle and use it in the next. The energy stored in the pressure differential (partial
vacuum) makes the aspiration (replacement of the burned gas with fresh fuel-air mixture)
possible. Without it, pulsejets would not work.
Some observers have noticed another, additional facet of the process, akin to breathing.
Swiss pulsating combustion wizard Francois H. Reynst called it ‘thermal breathing’ – heating
the gas causes it to expand (and the engine to ‘exhale’) while the cooling of the gas due to
convection of heat to the cooler chamber walls leads to contraction, and the engine ‘inhales’.
Acoustics
Other people studying the process came up with the acoustic explanation of the same
process. They detected acoustic resonance behind the pressure swings.
Namely, the explosion in the chamber generates a pressure wave that strikes the engine
tube and the air within it, making them ‘ring’ like a bell hit by a hammer. The pressure wave
travels up and down the tube. When the wave front reaches an end of the tube, part of it
reflects back. Reflections from opposed ends meet and form the so-called ‘standing wave’.
Everyone who has heard a pulsejet roar knows that it is a sound generator. The fact
needs no amplification – the noise is… well, not just deafening; it is an über-sound that
shakes all things around you seriously. What the establishment of the standing wave means
is that this ‘sound’, just like its lesser brethren, will obey the laws of resonance.
Graphically, the standing wave is best represented by a double sine curve. The same is
true for the pulsejet cycle. The undulations of a single sine curve depict the changes of gas
pressure and gas speed inside a pulsejet engine very well. The doubling of the curve – the
addition of a mirror image, so to say – shows that the places where the pressure and speed
are the highest in one part of the cycle will be the places where they are the lowest in the
opposite part.
The changes of pressure and the changes of gas speed do not coincide. They follow the
same curve but are offset from each other. One trails (or leads) the other by a quarter of the
cycle. If the whole cycle is depicted as a circle – 360 degrees – the speed curve will be offset
from the pressure curve by 90 degrees.
The resonance establishes a pattern of gas pressures and speeds in the engine duct that
is peculiar to the pulsejet and not found in the other jet engines. In some ways it resembles a
2-stroke piston engine resonant exhaust system more than in does a conventional jet engine.
Understanding this pattern is very important, for it helps determine the way the events in the
engine unfold.
When considering a pulsejet design, it is always good to remember that those machines
are governed by a complex interaction of fluid thermodynamics and acoustics.
Elements of Resonance
In acoustic terms, the combustion chamber is the place of the greatest impedance,
meaning that the movement of gas is the most restricted. However, the pressure swings are
the greatest. The chamber is thus a speed node but a pressure antinode.
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The outer ends of the intake and exhaust ports are the places of the lowest impedance.
They are the places where the gas movement is at the maximum and the speed changes are
the greatest – in other words, they are speed antinodes. The pressure swings are minimal,
so that the port ends are pressure nodes.
The pressure outside the engine is constant (atmospheric). The pressure in the
combustion chamber seesaws regularly above and below atmospheric. The pressure
changes make the gases accelerate through the ports in one direction or another, depending
on whether the pressure in the chamber is above or below atmospheric.
The distance between a node and an antinode is a quarter of the wavelength. This is the
smallest section of a standing wave that a resonating vessel can accommodate. In a
valveless pulsejet, this is the distance between the combustion chamber (pressure antinode)
and the end of the tailpipe (pressure node). This length will determine the fundamental
wavelength of the standing wave that will govern the engine operation.
The distance between the chamber and the end of the intake is rather shorter. It will
accommodate a quarter of a wave of a shorter wavelength. This secondary wavelength must
be an odd harmonic of the fundamental.
Given that a valveless pulsejet is a tube open at both ends, you may wonder at the above
statements. Namely, an open tube is not a quarter-wave resonator. It normally has a
pressure antinode in the center and a node at each end – which comprises half a wavelength.
Nevertheless, it is much closer to reality to look at a valveless engine as two different quarterwave
oscillators mounted back to back than as a single half-wave oscillator. The underlying
half-wave character of the resonance of the entire duct is still there, of course, but its effects
are completely drowned by everything else that is happening inside.
So, the tailpipe length must be an odd multiple of intake pipe lengths for the engine to
work properly. However, please note that we are talking of acoustic length. The required
physical length is somewhat different. It changes with the temperature (which changes the
local speed of sound). Thus, it will not be the same in all parts of the engine. It will not be the
same with the engine cold (e.g. at the startup) and when it is hot, either. This is the source of
much frustration for experimenters and the reason why a new pulsejet invariably requires
some tuning and fiddling to achieve proper working resonance.
Waves and Flows
Both the ‘Kadenacy’ and the ‘acoustic’ approaches to the definition of the pulsejet cycle
are correct. In a roundabout way, both may be considered just different manifestations of the
same thing. However, they are not the same thing. This should not be forgotten.
The classical acoustical phenomena take place at small pressure changes, low gas
velocities and little gas displacement. Sound waves are vibrations -- roughly speaking,
elastic, reversible disturbances in the medium. In pulsejets, we see great pressure variations,
high gas velocities and great gas displacement. The forces involved are stronger than the
elastic forces keeping the molecules of the medium together, meaning that the medium (gas)
is not just made to vibrate, but is irreversibly displaced. It is made to flow.
It is difficult to see the difference between the wave and the flow, but it can be done. A
wave is not a material phenomenon, but an energy phenomenon. It is a moving disturbance
in a force field. That is why it will easily turn any corner, including doubling back 180 degrees.
A fluid flow, which has mass and inertia, will not. So, the two can be made to separate, which
demonstrates that they are in fact two, rather than one.
You can see pressure waves separated from flow in the valveless pulsejet designs that
feature ports with irreversible flows (e.g. an intake that does not also serve as an auxiliary
exhaust). In such ports, pressure waves will move with the flow in one direction and without
the flow in the opposite direction.
To recapitulate, pulsejets follow their own, distinctive, Kadenacy-like cycle of compression
and rarefaction powered by the self-excited explosive combustion process and helped along
by the heat convection pattern. The genesis of the cycle has nothing to do with acoustics and
everything to do with thermodynamics. There is no doubt, however, that the scenario of
events resembles acoustical phenomena very closely. As a consequence, the laws of
acoustics can and do apply. They superimpose themselves over the thermodynamic events
and modify the inflow and outflow of gas, often significantly so.
Because of that, one should watch out for acoustic resonance, knowing that the regular
pressure impulses will inevitably set up standing waves, which will influence the timing and
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distribution of gas pressure, the speed and intensity of combustion, the speed and intensity of
gas flows etc. The negative influence of resonance must be avoided and – if possible – the
positive influence harnessed to help the engine along.
This is a very complex task and some designs do this better than others. Some have been
brilliant at the task, using a hugely complex concatenation of wave reflections, reversals,
mergers and collisions to boost the efficiency of aspiration and combustion appreciably.
Others have taken only the roughest note of the possibility. I cannot deal with the issue in
great detail because of inadequate knowledge, and will mention it as we go along only in a
superficial manner.
What I am really interested in are the practical results of pulsejet design.
ENGINE DESIGNS
Marconnet
The world may have been shocked into awareness of the pulsejet by the German flying
bomb in the 1940s, but the history of that curious engine goes much further back, to the very
beginning of the 20th century and the efforts of French engineers to develop a gas turbine.
Steam turbine was a fine machine but needed a huge burner, boiler and condenser
apparatus to handle the water vapor cycle. It looked to the innovative French as if hot gas
generated by combustion of fuel might power the turbine wheel just as well as steam did, but
with much less complication, bulk and cost. Pulsating combustion occurred to them because
it provided automatic aspiration. The pulses not only drove hot gas forward to power the
turbine, but also sucked in fresh charge in the condensation part of the cycle. No special
machinery was needed, just a couple of spring-loaded poppet valves.
In 1909, Georges Marconnet went a step further and developed the first pulsating
combustor without valves. It was the grandfather of all valveless pulsejets.
Marconnet figured that a blast inside a chamber would prefer to go through a bigger
exhaust opening, rather than squeezing through a relatively narrow intake. In addition, a
longish diffuser between the intake and the combustion chamber proper would direct the
charge strongly towards the exhaust, the way a trumpet directs sound. He tolerated what hot
gas did escape from the intake.
In their descriptions of the Marconnet engine, F. H. Reynst and J. G. Foa (each in his time
a noted expert on pulsating combustion) agreed that it could not have worked very well, really
requiring forced air at the intake (by a fan or a similar device) if the blowback was to be
avoided. Foa actually called the Marconnet “a bad ramjet” on account of the need for some
ram pressure at the intake. In principle, it does resemble ramjets of a few decades later
rather closely.
To my eyes, the combustion chamber of the Marconnet ‘engine’ lacks a notable means of
creating turbulence in the incoming flow, meaning that the mixing of fuel with air may have
been problematic and the combustion was of a relatively low intensity. Later practitioners of
the art introduced much more pronounced cut-offs between the intake and the combustion
chamber.
While it may not have been very practical either as a jet engine or as a turbine combustor,
the basic idea behind the Marconnet design was good. It just needed development.
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However, it was not destined to receive it. Even in France, the valveless combustor soon
became just a footnote in history. Outside France, few people were even aware of the idea.
Instead of developing valveless pulsating combustors, most experimenters of Marconnet’s era
concentrated on various layouts incorporating poppet valves.
[A historical note: One of the earliest valve-equipped pulsating combustors to be devised actually
made commercial history. The first gas turbine ever to be marketed commercially was designed by
Hans Holzwarth in 1905 and developed for practical applications by the Swiss Brown Boveri
Corporation. It enjoyed some commercial success between 1908 and 1938, the bulky drum-shaped
devices reportedly operating faultlessly (if not particularly economically) for ages. There are indications
that the Holzwarth combustor, which features intake and exhaust valves, is being re-evaluated for
modern use in low cost turbine engines.]
Schubert
The principle of the valveless pulsating combustor was rediscovered -- by all accounts
independently from Marconnet – by Lt. William Schubert of the US Navy in the early 1940s.
(It was patented in 1944.) His design, called the “resojet” at the time, on account of its
dependence on resonance, is one of the simplest successful valveless designs of all.
The most probable reason for the scant interest in valveless combustors early in the 20th
century was the lack of good means to prevent the wasteful and unpleasant blowback through
the intake. At first sight, the Schubert engine does not look better in that respect than the
Marconnet, just more angular. However, the appearance is deceiving.
First, Schubert’s sharp cutoff at the entrance of the intake port into the chamber provided
strong turbulence for better mixing of fuel and air, as well as more vigorous combustion.
Second, and more interesting, Schubert carefully calculated the geometry of the intake tube
so that the exhaust gas could not exit by the time the pressure inside fell below atmospheric.
The resistance of a tube to the passage of gas depends steeply on the gas temperature.
Thus, the same tube will offer a much greater resistance to outgoing hot gas than to the
incoming cold air. The impedance is inversely proportional to the square root of the gas
temperature. This degree of irreversibility seems to offer the possibility for the cool air
necessary for combustion to get in during the intake part of the cycle, but for the hot gas to
encounter too much resistance to get out during the expansion part.
In practice, it worked less well. For a number of reasons ignored by the simple general
theory, the Schubert engine still displayed a bit of blowback when stationary. It needed to
move forward at some speed (or to have air blown in by a fan) to prevent it. The intake tract
long enough to prevent the blowback completely would choke the air supply too much for
good performance. Nevertheless, the Schubert was a notable step forward from the
Marconnet.
Trick Intakes, Baffles, Serrated Tubes, Convoluted Passages…
After Schubert, a great number of developers tried to come up with other ways of making
the combustor tube irreversible, to have gases moving through the pulsejet in one direction
only. It is not easy to do without a mechanical non-return valve, but the inventors have
nevertheless come up with a variety of tricks supposed to do the job. Some, like Schubert,
introduced ways to make the resistance to the passage of gas unsymmetrical. Others came
up with ways to deflect gases in different directions.
Paul Schmidt and Jean Henri Bertin (among others) tested a number of designs featuring
concave ring baffles in the intake tract, which offered great resistance to back flow but let
fresh air in easily. A simple version of the Bertin baffle intake is pictured below. Fresh air
coming in from the left encounters a series baffles, but flows easily past them. The baffles
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have increasingly broader openings, forming a diffuser.
In the opposite direction, however, the story is different. Hot exhaust gas will be trying to
expand as it travels forward (towards the left in the picture) and increasing amounts will be
trapped in the pockets between the baffles. Only a relatively small amount will ever be likely
to escape. At least, that was what the designers hoped would happen.
However, all the configurations they had tried produced lower thrust and consumed more
fuel than the equivalent engines with mechanical valves. Most also displayed at least some
blowback, no matter how hard the designer tried to prevent it.
Alas, that is the sad story of almost every valveless pulsejet that employs some kind of
asymmetry of resistance. Such devices never work as well as their designers hope for. They
mostly pitch in only at high gas speeds, meaning that the engine will suffer from at least some
blowback at the beginning of each cycle.
Numerous versions of tubes with similarly serrated walls have been tried, sometimes with
baffles/serrations awaiting exhaust gas on more than one side. The next picture shows a
typical design of that family, from the pen of a man better known for pulsejets with valves.
The problem with most serrated designs is that the return flow is not impeded as much as
their inventors would like because the exhaust gas quickly fills the small concave ‘pockets’ in
the tube sides and forms cushions of pressurized dead air or small trapped vortices, which
offer little resistance to the passing stream. Under some conditions, the flow of gas in one
direction will actually be very similar to the flow in the opposite direction.
Few people will be surprised to hear that the amazingly prolific and spectacularly inventive
researcher of things electrical, Nikola Tesla, also turned his mind to the problem of pulsating
combustion. He wanted to have a good gas generator for his neat smooth-disk rotor turbine
that used the viscosity of the working fluid to transfer energy to a rotating shaft. He
immediately saw that mechanical valves would not offer the simplicity and reliability he had
sought. So, he studied the ways to rectify the gas flow aerodynamically. Eventually he came
up with arguably the best aerodynamic ‘valve’ ever. Its cross section is shown below.
At first glance, it looks like another serrated passage, but if you take a closer look, you can
see that it does not really employ either baffles or dead air pockets. Instead, it just changes
the direction of the gas and turns it upon itself. At each turn, a side blast of gas will push the
main flow towards the side passage that eventually turns backwards. The harder you blow
into that tube, the harder it will resist.
While undoubtedly ingenious, the ‘valvular conduit’, as Tesla called it, never found
practical application to the best of my knowledge. Tesla himself probably did not have time or
13
inclination to pursue its development after applying for patent, being busy with his
experiments in electromagnetism, and the patent was mostly forgotten. As the inventor has
recently become the center of a cult following, his modern disciples have revived the idea. A
few have been built, but I have not been able to find data on their performance.
Escopette
In 1950, dissatisfied with the baffled intake designs, Bertin and his fellow engineers at the
French SNECMA (Societe Nationale d'Etude et de Construction de Moteurs d'Aviation)
corporation simply turned the intake tract backwards. That way, blowback contributed to
thrust. To its designers, the machine looked like one of the old-fashioned musket guns and
they called it Escopette (which is French for musket).
It looks very similar to the picture we used in the introduction. However, the intake does
not curve backwards directly from the combustion chamber. Its first part points straight
ahead. What turns the hot exhaust gases backwards is a separate curved tube mounted at
some distance from the mouth of the intake proper. So, the engine breathes through the gap
between the intake and the 'recuperator', as the designers called the curved tubular deflector.
This neat design deftly exploits the resonance in several different ways.
The functioning of the split intake is subject to some controversy, but simply put, it may be
said that it allows the engine to behave as if its length were variable – long during the
expansion part of the cycle and short during the suction part. During expansion, it treats the
recuperator as a part of the effective length of the engine and uses it to turn the escaping gas
around and increase thrust. In the intake part of the cycle, however, the effective front end is
at the gap between the intake and the recuperator. This reduces the effective length of the
intake and lets the Escopette inhale more easily.
Next, the tailpipe, instead of being just a straight pipe, is in fact a series of steps of
increasing section. Each transition from a straight section into a diffusing section (flaring
cone) represents a point from which the pressure waves traveling up and down the tube will
reflect in the opposite direction and with the opposed sign. A compression (high pressure)
wave passing a step will reflect back as a rarefaction (low pressure) wave and vice versa.
Just glancing at the picture will give you an idea of how many different compression and
rarefaction waves are generated by each blast of the charge in the chamber as it tries to exit
the engine. Remember also that each step of the pipe works at a different temperature from
the preceding or succeeding steps, meaning that the wave will travel at different speeds.
The tracing of the events is not for the faint of heart and is certainly too complex for me to
attempt to describe here. Bertin et al. harnessed all those waves very carefully, tuning them
to produce the maximum possible aid to aspiration. What seems to be happening is that the
engine ‘skips a beat’, so to say. It inhales twice for each expansion cycle, with the second
inhalation topping up the first.
The next trick Bertin employed on the Escopette – for the first time ever in a pulsejet -- was
the utilization of surplus heat in the exhaust stream to increase thrust. The effect is
sometimes called ‘primary thrust augmentation’. It requires some explanation.
The problem starts with the amount of air available for propulsion. Basically, a jet engine
is a device that employs heat to accelerate air. Ambient air is made to pass through the
engine duct and absorb the heat generated inside by combustion. However, air does not flow
through the pulsejet duct the way it does through other jet engines.
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In a turbojet, a little hot gas pushes along a great quantity of cool air. In pulsejets, a small
amount of air is sucked into the combustion chamber and used for combustion and a slightly
larger amount is sucked back into the exhaust tube between explosions. That is all. There is
no through-flow.
At the same time, the exhaust gas produced by a pulsejet is much hotter than in a turbojet.
Combustion takes place at similar temperatures -- between 2000 and 25000C -- but the
exhaust gas in a turbojet is immediately mixed with a lot of cool air, so that the temperature is
lowered to between 800 and 12000C before it enters the turbine. The main reason is to keep
the turbine from damage. In the pulsejet, which has no moving parts, the exhaust gas does
not need to be cooled. It travels towards the end of the engine at very close to its initial
temperature – two to three times hotter than in a turbojet.
Because there is no through-flow of air, however, there is very little propulsion mass for
this considerable thermal energy to act on. This generates problems. The small mass of
exhaust gas and fresh air is propelled to the maximum speed possible under the
circumstances (the local speed of sound) and no further. The sonic choking of the duct
prevents the gas speed from rising further, despite the fact that there is sufficient energy for
further acceleration. A ‘de Laval’ nozzle would probably push the speed beyond the Mach
barrier, but it does not work at all well with a pulsating flow, so pulsejet designers avoid them.
So, only a small part of the heat liberated by the process of combustion is converted into
useful kinetic energy. Much of the available energy has nowhere to go. At sonic speed, gas
is not capable of absorbing more heat. This generates compression waves that travel up and
down the engine, disrupting the cycle.
In other words, the energy-mass transfer ratio is low and the resulting thrust is lower than it
could be. So, the super-hot exhaust of the pulsejet simply cries out for additional propulsion
mass to heat up and accelerate.
Enter Bertin. He made the exhaust tube on the Escopette progressively larger towards the
end, so that the final section is a veritable bustle. This increased the volume of the exhaust
duct considerably as well as gave the duct a shape that promoted the intake of fresh air
during the suction part of the cycle. The result was an exhaust filled with a large amount of
fresh air, which the engine could use as additional propulsion mass.
Each blast from the combustion chamber pushes the fresh air ‘plug’ mechanically, but the
engine also transfers a lot of its heat to the air, both from the hot tube walls and from the
pushing hot gas. Heating increases static pressure, which increases the speed at which fresh
air is expelled backwards. Much additional thrust is exerted.
The version intended for commercial applications, model 3340, developed static thrust of
about 22 lbs, operating at the frequency of between 90 and 100 Hz. The weight with the
ancillaries was about 11 lbs. At the widest part, the Escopette was only about 4 inches wide,
but the length was a somewhat unwieldy 9 feet plus.
The Escopette is one of the few pulsejets to have carried people aloft. It was extensively
tested on the French Emouchet SA 104 sailplane in various configurations. The first trials
were with a single engine under each wing, but later models carried two and three engines on
each underwing pylon. While the test results were positive – the auxiliary power enabled the
15
pilot to take off and achieve soaring altitude without a tow plane or a winch, it seems that it
was never offered commercially.
Kentfield’s Recuperator
The idea of a recuperator or deflector found several adherents who produced variations on
the theme, some simple and others complex. J. A. C. Kentfield, one of the most recent
scientific researchers in the pulsejet field, tried to make up for the energy lost in turning the
gas flow around by introducing thrust augmentation to the recuperator.
Instead of a simple tube bent backwards, he employed a gently flaring curved cone, which
let fresh air be sucked in by the hot gas stream as extra reaction mass. (This is often called
secondary thrust augmentation, to make it distinct from the kind used in the Escopette. I will
talk about it in more detail in a later chapter.) According to Kentfield, who patented the idea,
the gain more than offset the drag and turbulence losses incurred by the 180-degree turn.
He experimented with variations on the theme a lot. Most recuperators were symmetrical
and used internal vanes to help control the flow and lower the turbulence. Two such designs
are shown on the next two pictures.
The one on the left looks more ambitious. It attempts to harness ram pressure of the
incoming air. Ram pressure would seem to give a welcome boost to the power output at no
16
cost. The J-shaped and U-shaped engines as well as most engines with recuperators up
front must forgo that advantage, as their intake ports are either turned in the wrong direction
or masked by the recuperator structure.
This one has an almost straight path for fresh air from the front intake to the combustion
chamber (see the lower half of the picture). The trick that prevents the exhaust gas from
escaping through the same route may have been borrowed from Tesla, but similar methods
are also used in various other pneumatic flow control devices.
Note the two small airfoil-section vanes in the central passage, right behind the intake
wedge. When the hot exhaust gas is pushed forward by the blast, the part blowing into the
gap between the vanes is divided into two flows, one going upwards and the other going
downwards. Each flow forms a kind of a gas curtain that cuts across the path of the main flow
(see the upper half of the picture). The curtain deflects exhaust gas flow towards the curved
passages that turn the flow around and eventually eject it backwards. As a result, almost all
the exhaust gas that would normally be blown out of the intake port gets deflected and
contributes to the thrust.
According to Kentfield, the simple one on the right outperformed the more complex one on
the left in laboratory testing. I am not surprised. Due to intermittent operation, a pulsejet is
not very good at tapping ram pressure, most of which goes to waste. Providing a straight
path for fresh air is simply not as important as in the other jet engines. A pulsejet will happily
suck air from the side or even from the rear, because it has to accelerate it from standstill
anyway. Its direction matters very little. The only thing that matters is pressure.
Messerschmitt
One of the best practical recuperators I
have seen is the one developed by the
German Messerschmitt company in the early
1970s. The intention of its engineers was to
build an engine that would segue from
pulsating combustion at low speeds at which
ram pressure is poor, to constant combustion
at high speeds, at which the ram pressure is
sufficient to contain combustion. The task
required a deflector that would be efficient in
redirecting the reverse flow coming from the
intake, but would not represent too much of
an obstacle to the entry of fresh air.
I will disregard the ramjet part or the Messerschmitt engine in this explanation, as it is not
the subject of this paper. However, the recuperator is a different story, as it is eminently
usable on ‘ordinary’ pulsejets. It is simple and elegant -- and easy to make even for an
enthusiast of average skill. As you can see, it consists of a simple sharp cone whose rear is
shaped to deflect the blast from behind at the right angle to the engine axis. When stationary,
this did not do much for thrust, but even at relatively gentle forward speed the deflected gas
stream bent backwards, around the engine, helped by the Coanda effect.
For low speeds, Messerschmitt designers provided a nose cowling to help entrain the flow.
As the speed rose, the cowling became less necessary. At a considerable fraction of the
speed of sound, the incoming air stream is so strong that even the deflector is not strictly
necessary anymore, as the hot gas is entrained tightly between the air stream and the outer
engine surface.
Capped Tubes
Gas deflectors need not be ancillaries tacked onto the engine. They can be an integral
part of its structure. For instance, if you put a loose cap over the end of a tube, so that a gap
17
is left between the cap and the tube, you get a kind of deflector that will turn your exhaust
gases backwards. Unlike a separate recuperator, however, this one also serves as the intake
tract. Arguably, this is the simplest valveless pulsejet design of all.
Possibly the most prominent among pulsejet developers to tackle a capped tube design
were none other than the Argus engine company, best known for their reed-valve engine that
powered the V-1 flying bomb. They tested a number of layouts, some of which appear to be
useful only for stationary applications.
The sketch below shows the central part of their valveless engine. The ‘combustion
chamber’ is formed between the bottle shape we know from many other pulsejet designs and
a cap with hemispherical top. Fuel is injected through a nozzle situated on the tip of the cap
and protected from the chamber by a metal grid. The grid functions as a heat sink and
prevents gas from burning at the nozzle itself.
In the first tests, this central core engine was enveloped in a plenum chamber into which
air was forced at pressure by a compressor. Only the exhaust was sticking out. This meant
that the pressure of forced air prevented hot gas from getting out into the plenum chamber
and almost all of it went into the exhaust. (If it were designed to work without the pressurized
shroud, the chamber and the exhaust passage would probably have to be longer than on the
above picture, to provide the necessary resonant properties.)
Argus engineers were apparently delighted with their valveless machine and were about to
develop it for aircraft purposes, but were ordered to halt the work and concentrate on the
valve-equipped engine inspired by Paul Schmidt’s ideas.
One can only wonder how far they would have gone if not rudely interrupted by the
authorities. The next sketch shows the layout they developed to work without forced air. The
outer streamlining is the most obvious difference, but a subtler one is the annular chamber
through which fresh air must pass before it is drawn into the combustion chamber. Curved
arrows on the sketch show the gas paths inside the engine.
It is obvious that the incoming air will swirl in a toroidal vortex that will allow outside layers
of the vortex to detach and slip into the combustion chamber. However, the hot gas exiting
between the chamber and the mantle will also swirl in a vortex – one whose direction of
rotation will be opposite to the direction of the exit from the annular chamber. In addition, the
18
slit through which the annular chamber communicates with the outside is very narrow. It
might let sufficient fresh air in, but will choke when hot gas tries to pass.
The engineers at Argus were almost certainly unaware of the work of F. H. Reynst, whose
pulsating combustors will be described later in this review. Reynst’s machine also utilizes the
special properties of the toroidal vortex. It was first patented in 1933, while Argus developed
its jet almost a decade later. A decade was hardly enough for the relatively obscure Dutch
patent to percolate to German engineers dabbling in jet propulsion. One can only wonder
what influence, if any, Reynst’s idea would have had on engine development had the contact
between the two been made.
As it is, the toroidal vortices must have helped the Argus engine work well, but they are not
central to its operation. The Argus is not essentially different from the bent intake engine
whose sketch – the very first in this paper -- we used to explain the workings of a valveless
pulsejet. The only difference is that a narrow annular gap between the combustion chamber
and the cap is used for intake instead of a bent tube.
Saunders Roe
The next to exploit a similar configuration was C. E. Tharratt, a British researcher working
on pulsejets for the Saunders Roe aircraft company in the early 1950s. I have no idea
whether he was familiar with the Argus layout or not, but the principle he employed is similar.
Here is the simplified longitudinal cross-section of one of his valveless engines.
The ‘cap’ is no longer a simple cap but a mantle that curves inwards and backwards,
following the curvature of the front part of the combustion chamber. Again, the annular gap
between the mantle and the chamber wall serves
as the auxiliary exhaust. During expansion, most
of the hot gas escapes through the exhaust proper,
at the rear end of the engine, but a portion is driven
forward, into this auxiliary exhaust. The device in
the center of the front plate is the fuel injector. Its
fuel jets are disposed radially, injecting fuel at the
right angle to the fresh air flow.
Tharratt apparently cared little for the
effectiveness of the cap as the exhaust, only really
devoting attention to its properties as the intake port. There have been several versions, with
rather different internal arrangements, but they all offered a smooth passage to fresh air
towards the chamber and an indifferently shaped path for the hot gas out.
On the first two diagrams, the engine looks like having a bluff front end, but that is because
it is shown without the streamlined nose. With it in place, the engine looked like this:
Like Hiller and some others, Saunders Roe was trying to develop a small helicopter with
pulsejets at rotor tips. I have seen no records on the success of the project, but it could not
have been great, given that most helicopter history books neglect to mention it even in
19
passing. Of course, it does not necessarily mean that Tharratt’s engine was not good. The
problem here lies with the helicopter concept, rather than the engine. Rotor-tip jet helicopters
are just not a very sound idea, regardless of the kind of engine that is mounted on the rotor.
A detail that makes Tharratt’s designs particularly interesting is the move away from the
chamber-and-tube configuration found in most pulsejets, valved or valveless. It does not
have the traditional ‘tulip’ or ‘bottle’ shape but tapers gently down, somewhat like a baseball
bat or a bowling pin, having no pronounced bulge where the combustion chamber is situated.
Its narrowest point is all the way back, just before the end of the tailpipe.
The tulip shape has been inherited from the petal-valve designs like the popular Dynajet,
and has created a lot of misdirection among enthusiasts. Namely, according to a number of
experts, it does not necessarily have great relevance to what is going on in the engine. The
broad front part is not the combustion chamber and the narrower pipe is not a tailpipe.
In a well-designed pulsejet, Tharratt says, combustion will be going on through most of the
tube interior and there will be no functional difference between the ‘combustion chamber’ and
the ‘tailpipe’. Look at the engines that Paul Schmidt, the father of the modern valved pulsejet,
designed for his own purposes and you will see either a straight, constant-section tube or the
one that broadens (flares) gently towards the exhaust end. The latter layout is shown in the
next picture. The objective is to provide the least possible obstruction to the gas flow and the
propagation of pressure waves.
The reason Dynajet-style pulsejets are broad up front is because the petal valves and air
passages are inefficiently packaged and take up a lot of room. So, to provide an adequate
amount of valve area, the valve head must be disproportionately broad. The tulip ‘waist’ is
nothing but a gentle transition from broad section to narrower.
That may well be true in theory, but in practice, the tulip shape is difficult to do without.
Kadenacy breathing will not take place unless (a) the pressure differential between the
chamber and the ambient is high enough and (b) the differential can be maintained over a
sufficient minimum period. Gas must be pushed forward by sufficient pressure and given
sufficient time to accelerate to a great enough speed. It must gain at least the minimum
momentum necessary for the Kadenacy effect. This requires a certain amount of
confinement.
Generally speaking, the openings through which the combustion chamber is evacuated
and refilled must not be too big, or gas speeds will be too low, the gas momentum will be too
small, and there will be no Kadenacy effect. The engine will not work.
We know that Paul Schmidt achieved exceptional filling ratios of his pulse tubes and quite
probably exceptional rates of flame propagation due to very careful design. That gave him
pressure peaks so high that he could get away with relatively poor confinement of the burning
gases in the tube. Few designers after him have managed to duplicate his results reliably.
Those results simply cannot be taken as a reasonable yardstick.
Tharratt tried to combine the conflicting demands -- providing confinement to the gases,
yet producing the least amount of harmful obstruction -- by having the chamber narrow down
towards the exhaust in a very gentle slope, so that the narrowest part was situated almost at
the very end of the engine. He argued that the shape conformed to the natural shape of
accelerating gas flow. Only the very end of the pipe exhibited a short outward flare, to enable
the fresh air sucked back into the exhaust to enter more easily.
Interestingly, F. Schultz-Grunow, a noted German pulsejet researcher, pronounced this
shape highly unsatisfactory in his landmark comparative study of pulsejet duct shapes. Yet, it
20
obviously worked for Saunders Roe, as more or less the same shape was employed on their
engines with valves, which saw greater use than the valveless ones.
The design remains unusual. No other designer I know of has chosen to emulate it. Yet,
a very basic pulsejet of this kind would be extremely easy for an enthusiast to build. It would
consist just of a kind of a tin can capping the chamber and exhaust pipe of a conventional
pulsejet, like a Bailey, or Atom jet, or any other of a score of small flying-model pulsejets.
Spacers – possibly just simple screws – could hold the cap centered, keep it at a set distance
from the chamber wall and prevent its fore-and-aft movement. An advantage of the design is
the relatively easy altering of the cap placement. Moving the position of the cap slightly
forward or backward will give the builder a way to fine tune the configuration and find the
‘sweet spot’ at which it works best.
The problem will be the same as in any annular design – how to provide the fuel supply
that will allow good mixing. Looking at the Argus engine, perhaps having the propane supply
right at the center of the chamber front plate would work well.
Foa
Joseph G. Foa, another well-known pulsejet researcher, investigated a capped-tube layout
with a very interesting twist. He added the front entry of fresh air. It is shown on the next
diagram.
Such a direct entry reduces the pumping loss and allows the Foa to benefit from ram
pressure (to the extent that a pulsejet can benefit). This is a very elegant flow rectifier and
has reportedly worked well. Perhaps worth noting here is the fact – not mentioned in the
literature that I have seen -- that it is no different in principle from the Escopette ‘recuperator’,
even though its shape looks very different.
Given the simplicity of the layout, I am surprised that something similar has not gained
popularity among amateur enthusiasts. One is truly tempted to consider such an engine as a
DIY project, as the only real difficulty seems to be presented by the semi-toroidal surfaces.
However, as pulsejet enthusiast Mike Kunz has noted, those can be made relatively easily
by cutting sections of bent tube lengthwise to get semicircular-section channels. Such
channel segments (say, four 90-degree ones) are the joined by welding the ends together and
form a nice half-torus. Some people I know have looked at old torque converter casings with
a spark in their eyes, too, but I have not heard of anyone actually using it for this purpose.
[Please note that most diagrams in this book are not at all useful as engineering drawings. Some of
them have been constructed from bits and pieces of information and represent only a rough
representation of what the actual engine layouts were like. Dimensions shown on the sketches are
approximate at the very best. If you build an engine based on any of t