ABSTRACT

A pulse detonation engine, or "PDE", is a type of propulsion system that has the potential

to be both light and powerful and can operate from a standstill up to supersonic speeds.

To date no practical PDE engine has been put into production, but several testbed engines

have been built, proving the basic concept to some extent at least. In theory the design

can produce an engine with an efficiency far surpassing more complex gas turbine

Brayton cycle engines, but with almost no moving parts.

All regular jet engines and most rocket engines operate on the deflagration of fuel, that is,

the rapid but subsonic combustion of fuel. The pulse detonation engine is a concept

currently in active development to create a jet engine that operates on the supersonic

detonation of fuel.

The basic operation of the PDE is similar to that of the pulse jet engine; air is mixed with

fuel to create a flammable mixture that is then ignited. The resulting combustion greatly

increases the pressure of the mixture to approximately 100 atmospheres, which then

expands through a nozzle for thrust. To ensure that the mixture exits to the rear, thereby

pushing the aircraft forward, a series of shutters are used with careful tuning of the inlet

to force the air to travel in one direction only through the engine.

The main difference between a PDE and a traditional pulsejet is that the mixture does not

undergo subsonic combustion but instead, supersonic detonation. In the PDE, the oxygen

and fuel combination process is supersonic, effectively an explosion instead of burning.

The other difference is that the shutters are replaced by more sophisticated valves.

CONTENTS

1) INTRODUCTION…………………………...………………………………………4

1.1 CONCEPT………………………………………………………………..4

1.2 COMBUSTION…….…………………………………………………….5

1.3 DEFLAGRATION………………………………………………….……7

(i) FLAME PHYSICS………………………………………………….8

1.4 DETONATION ………………..

……………………………………….10

(i) APPLICATIONS………………………………………………….12

2) BASIC PDE CYCLE……………………….………………....……………………12

3) DETONATION INITIATION IN PDE…………………………………………….14

3.1 DIRECT INITIATION………………………………………………….14

3.2 DEFLAGRATION TO DETONATION TRANSITION……….………15

(i) FACTORS INFLUENCING DDT………………………………..18

(ii) TIME VS POSITION GRAPH…………….……………………..20

4) BLOCK DIAGRAM OF PDE……………………………..………………………..20

5) WORKING OF PDE………………………………………………………………..21

5.1 T-S DIAGRAM………………………………………………………..25

6) PRACTICAL PROBLEMS INVOLVED IN PDE……….…………………………26

7) CURRENT STATE OF DEVELOPMENT…………………………………………27

8) THE FUTURE OF PDE…………………………………...………………………..27

9) REFERENCES……………………………………………..………………………28

1

1.INTRODUCTION

1.1 CONCEPT

All regular jet engines and most rocket engines operate on the deflagration of fuel, that is,

the rapid but subsonic combustion of fuel. The pulse detonation engine is a concept

currently in active development to create a jet engine that operates on the supersonic

detonation of fuel.

The basic operation of the PDE is similar to that of the pulse jet engine; air is mixed with

fuel to create a flammable mixture that is then ignited. The resulting combustion greatly

increases the pressure of the mixture to approximately 100 atmospheres, which then

expands through a nozzle for thrust. To ensure that the mixture exits to the rear, thereby

pushing the aircraft forward, a series of shutters are used with careful tuning of the inlet

to force the air to travel in one direction only through the engine.

The main difference between a PDE and a traditional pulsejet is that the mixture does not

undergo subsonic combustion but instead, supersonic detonation. In the PDE, the oxygen

and fuel combination process is supersonic, effectively an explosion instead of burning.

The other difference is that the shutters are replaced by more sophisticated valves. In

some PDE designs from General Electric, the shutters are even removed because the

process can be controlled by timing on the periodic sudden pressure drops that occur after

each shock wave when the "combustion" products have been ejected in one shot.

2

The main side effect of the change in cycle is that the PDE is considerably more efficient.

In the pulsejet the combustion pushes a considerable amount of the fuel/air mix (the

charge) out the rear of the engine before it has had a chance to burn (thus the trail of

flame seen on the V-1 flying bomb), and even while inside the engine the mixture's

volume is continually changing, an inefficient way to burn fuel. In contrast the PDE

deliberately uses a high-speed combustion process that burns all of the charge while it is

still inside the engine at a constant volume. The maximum energy efficiency of most

types of jet engines is around 30% a PDE can attain an efficiency theoretically near 50%.

Another side effect, not yet demonstrated in practical use, is the cycle time. A traditional

pulsejet tops out at about 250 pulses per second, but the aim of the PDE is thousands of

pulses per second, so fast that it is basically continual from an engineering perspective.

This should help smooth out the otherwise highly vibrational pulsejet engine -- many

small pulses will create less volume than a smaller number of larger ones for the same net

thrust. Unfortunately, detonations are many times louder than deflagrations. To know

more about deflagration and detonation, we need to learn about combustion.

1.2 COMBUSTION

Combustion or burning is a complex sequence of exothermic chemical reactions between

a fuel and an oxidant accompanied by the production of heat or both heat and light in the

form of either a glow or flames. It is a chemical process in which a substance reacts

3

rapidly with oxygen and gives off heat. The original substance is called the fuel, and the

source of oxygen is called the oxidizer. The fuel can be a solid, liquid, or gas, although

for airplane propulsion the fuel is usually a liquid. The oxidizer, likewise, could be a

solid, liquid, or gas, but is usually a gas (air) for airplanes.

Fig.1 Combustion [Ref no: 9]

During combustion, new chemical substances are created from the fuel and the oxidizer.

These substances are called exhaust. Most of the exhaust comes from chemical

combinations of the fuel and oxygen. When a hydrogen-carbon-based fuel (like gasoline)

burns, the exhaust includes water (hydrogen + oxygen) and carbon dioxide (carbon +

oxygen). But the exhaust can also include chemical combinations from the oxidizer

alone. If the gasoline is burned in air, which contains 21% oxygen and 78% nitrogen, the

exhaust can also include nitrous oxides (NOX, nitrogen + oxygen). The temperature of

the exhaust is high because of the heat that is transferred to the exhaust during

combustion. Because of the high temperatures, exhaust usually occurs as a gas, but there

can be liquid or solid exhaust products as well. Soot, for example, is a form of solid

exhaust that occurs in some combustion processes.

4

During the combustion process, as the fuel and oxidizer are turned into exhaust products,

heat is generated. Interestingly, some source of heat is also necessary to start combustion.

Gasoline and air are both present in an automobile fuel tank; but combustion does not

occur because there is no source of heat. Heat is both required to start combustion and is

itself a product of combustion. Also, once combustion gets started, heat source need not

be provided because the heat of combustion will keep things going.

There are different types of combustion like rapid combustion, slow combustion,

complete combustion, turbulent combustion etc. But in pulse detonation engine, primary

interest is on two types: deflagration and detonation.

1.3 DEFLAGRATION

Deflagration (Lat: de + flagrare, "to burn") is a technical term describing subsonic

combustion that usually propagates through thermal conductivity (hot burning material

heats the next layer of cold material and ignites it). Most "fire" found in daily life, from

flames to explosions, is technically deflagration. Deflagration is different from detonation

which is supersonic and propagates through shock compression

It is the set of phenomena accompanying the rapid passage of a reaction front, e.g., the

front of a flame (combustion of a gas or a vapor, more rarely of a solid). In a

homogeneous mixture of air and a combustible gas or vapor, a flame propagates at a

constant velocity that is high but remains of the same order of magnitude of many

familiar phenomena. It is of the order of 1 to 10 feet/second, hence comparable to that of

a walker or a runner (as opposed to detonation that propagates several times faster than

5

sound in air). A deflagration takes place during the rapid inflammation of the mixture of

air and gas above a burner in a stove. If the amount of gas is small, this is uneventful; if

the amount is important, the result may be an explosion. The flame of a gas burner is a

deflagration moving at a constant velocity, in the direction opposed to that of the gas

flux.

In a deflagration, the combustion products move at a subsonic velocity, in the direction

opposed to that of the flame.

In engineering applications, deflagrations are easier to control than detonations.

Consequently, they are better suited when the goal is to move an object (a bullet in a gun,

or a piston in an internal combustion engine) with the force of the expanding gas. Typical

examples of deflagrations are combustion of a gas-air mixture in a gas stove or a fuel-air

mixture in an internal combustion engine, a rapid burning of a gunpowder in a firearm or

pyrotechnic mixtures in fireworks.

(i) FLAME PHYSICS

We can better understand the underlying flame physics by constructing an idealized

model consisting of a uniform one-dimensional tube of unburnt and burned gaseous fuel,

separated by a thin transitional region of width in which the burning occurs. The

burning region is commonly referred to as the flame or flame front. In equilibrium,

thermal diffusion across the flame front is balanced by the heat supplied by burning.

6

There are two characteristic timescales which are important here. The first is the thermal

diffusion timescale τd, which is approximately equal to where is the

conductivity. The second is the burning timescale τb, which is approximately equal to

where ε is the total energy released by burning per unit mass, and is the

burn rate (i.e., the rate of increase of specific thermal energy).

In equilibrium, these two rates are equal: The heat generated by burning is equal to the

heat carried away by heat transfer. This lets us find the characteristic width δ of the flame

front:

Now, the thermal flame front propagates at a characteristic speed Sl, which is simply

equal to the flame width divided by the burn time:

This simplified one-dimensional model neglects the possible influence of turbulence. As

a result, this derivation gives the laminar flame speed -- hence the designation Sl.

In free-air deflagrations, there is a continuous variation in deflagration effects relative to

maximum flame velocity. When flame velocities are low, the effect of a deflagration is

the release of heat. Some authors use the term flash fire to describe these low-speed

deflagrations. At flame velocities near the speed of sound, the energy released is in the

7

form of pressure and the results resemble a detonation. Between these extremes both heat

and pressure are released.

When a low-speed deflagration occurs within a closed vessel or structure,

pressure effects can produce damage due to expansion of gases, as a secondary effect.

The heat released by the deflagration causes the combustion gases and excess air to try to

expand thermally as well. The net result is that the volume of the vessel or structure

needs to either expand/fail to accommodate the hot combustion gases, or build internal

pressure to contain them. The risks of deflagration inside waste storage drums is a

growing concern among storage facilities

1.4 DETONATION

Detonation is a process of supersonic combustion in which a shock wave is propagated

forward due to energy release in a reaction zone behind it. It is the more powerful of the

two general classes of combustion, the other one being deflagration. In a detonation, the

shock compresses the material thus increasing the temperature to the point of ignition.

The ignited material burns behind the shock and releases energy that supports the shock

propagation. This self-sustained detonation wave is different from a deflagration, which

propagates at a subsonic speed (i.e., slower than the sound speed of the explosive

material itself), and without a shock or any significant pressure change. Because

detonations generate high pressures, they are usually much more destructive than

deflagrations.

8

The simplest theory to predict the behavior of detonations in gases is known as Chapman-

Jouguet (CJ) theory, developed around the turn of the 20th century. This theory,

described by a relatively simple set of algebraic equations, models the detonation as a

propagating shock wave accompanied by exothermic heat release. Such a theory confines

the chemistry and diffusive transport processes to an infinitely thin zone.

A more complex theory was advanced during World War II independently by Zel'dovich,

von Neumann, and Doering. This theory, now known as ZND (explosion) theory, admits

finite-rate chemical reactions and thus describes a detonation as an infinitely thin shock

wave followed by a zone of exothermic chemical reaction. In the reference frame in

which the shock is stationary, the flow following the shock is subsonic. Because of this,

energy release behind the shock is able to be transported acoustically to the shock for its

support. For a self-propagating detonation, the shock relaxes to a speed given by the

Chapman-Jouguet condition, which induces the material at the end of the reaction zone to

have a locally sonic speed in the reference frame in which the shock is stationary. In

effect, all of the chemical energy is harnessed to propagate the shock wave forward.

Both CJ and ZND theories are one-dimensional and steady. However, in the 1960s

experiments revealed that gas-phase detonations were most often characterized by

unsteady, three-dimensional structures, which can only in an averaged sense be predicted

by one-dimensional steady theories. Modern computations are presently making progress

in predicting these complex flow fields. Many features can be qualitatively predicted, but

the multi-scale nature of the problem makes detailed quantitative predictions very

difficult.

9

Detonations can be produced by high explosives, reactive gaseous mixtures, certain dusts

and aerosols.

(i) APPLICATIONS

Detonations are hard to control and are used primarily for demolition and in warfare. A

great deal of research is conducted on achieving or preventing detonation in various

materials to improve the performance of explosives and engines. An experimental form

of jet propulsion, the pulse detonation engine, uses a series of well-timed detonations to

generate thrust.

Detonation in reciprocating engines is the uncontrolled supersonic explosion of the fuel-

air charge, and is caused by excessively high combustion chamber temperatures.

Increasing the temperature of the fuel-air charge increases the speed of combustion until

the flame propagates at supersonic speeds, resulting in a pressure shockwave. This force

is extremely destructive to common piston engines, and often results in holes blown

through the top of pistons or cracks in cylinder heads.

2.BASIC PDE CYCLE

10

Fig.2 Basic PDE cycle [Ref no: 10]

1- A detonation is initiated in a detonation tube filled with reactants.

2- The detonation propagates through the detonation tube and exits at the open end.

3- The combustion products exhaust through a blowdown process.

4- At the end of the exhaust process, the tube contains expanded combustion products.

5- The valve opens and reactants flow into the tube, pushing the combustion products out

of the tube.

6- When the tube is filled with reactants, the valve closes and the cycle repeats.

At first the air is allowed inside the combustion chamber. Fuel, controlled by a solenoid

valve in the head end of the tube, is allowed to enter through a mixing element for a

specified amount of time to fill a pre-defined percentage of the combustion chamber

11

volume based on the velocity of air through the system. The fuel valve is then closed and

the mixture is initiated using a spark located just downstream of the mixing element. The

initial flame kernel grows as it begins to propagate down the tube. The flame speed

increases as it propagates down the tube and encounters turbulence, eventually

transitioning to a supersonic detonation wave. The steady-state detonation wave travels in

excess of 1960 m/s (for hydrogen-air mixtures) or 1800 m/s (for ethylene-air mixtures)

burning the remaining reactants and pushing the gases out of the open end of the tube,

resulting in a thrust. The tube is then purged of the combustion products by the

continuously flowing air and refilled with fuel to repeat the cycle

3. DETONATION INITIATION IN PDEs

Basically there are two methods in which detonation is initiated in a pulse detonation

engine:

1- Direct initiation.

2- Deflagration to detonation transition.

12

Fig.3 PDE Engine Schematic [Ref no: 2]

3.1 DIRECT INITIATION

A detonation may form via direct initiation or deflagration-to-detonation transition

(DDT). The former mode is dependent upon an ignition source driving a blast wave of

sufficient strength such that the igniter is directly responsible for initiating the detonation.

The latter case begins with a deflagration initiated by some relatively weak energy source

which accelerates through interactions with its surroundings into a coupled shock wave-

reaction zone structure characteristic of a detonation. Direct initiation by a concentrated

source requires an extremely large energy deposition relative to deflagrative ignition. A

deflagration can be ignited in a typical hydrocarbon mixture such as 1 bar stoichiometric

propane-air with a 1 mJ spark, whereas direct initiation of a detonation in the same

mixture requires an energy deposition of over 100 kJ. This six order of magnitude

difference in ignition source energy is indicative of the general difficulty associated with

employing direct initiation techniques in pulse detonation engines. On the other hand,

after a small spark has created a deflagration, the transition process can take several

meters or longer and a corresponding large amount of time. The key to detonation

initiation schemes applicable to pulse detonation engines is to significantly shorten the

distance and time required for deflagration-to-detonation transition.

13

3.2 DEFLAGRATION TO DETONATION TRANSITION (DDT)

A detonation, formed in a tube that is ignited at a closed end, begins with a combustion

wave that accelerates due to heating of the unburned gases ahead of the wave. This

heating occurs from successive compressive waves formed from the expansion of the

burned gas products, which have a specific volume that is 10-15 times greater than the

unburned gases ahead of the flame. The higher temperature of the unburned gases causes

the sound velocity to increase enabling the succeeding waves to catch up to the initial

wave. The higher temperature in the unburned gases also contributes to increasing the

flame speed, accelerating the unburned mixture. Turbulence in the flow initiates due to

this unburned gas acceleration, the natural instability of high Reynolds number flows and

the vortices associated with flow over defects or obstructions in the tube. This turbulence

causes a distortion of the flame front. As flow turbulence in the tube increases, additional

increases in the velocity and acceleration cause the formation of additional compression

waves in addition to further distorting the flame front separating it into distinct sheets.

The positive feedback between the flame and the flow ahead of it progresses to a point

where the flame breaks into a distributed reaction zone with strong straining motions and

large fluctuations in the temperature and species concentrations at the characteristic flame

scale length. Some portions of the flame front are extinguished due to the locally high

strain rates. As parts of the previously extinguished mixture re-ignite in the form of

exploding eddies; weak shock waves are formed ahead of the front. At this point the

burning rate increases slightly and the interaction of reaction waves, hotspots, and the

amplification of weak shock waves results in the reactants exploding close behind the

14

shock. This energy release is sufficient to maintain the shock’s strength, thus forming a

detonation.

Fig.4 DDT tube schematic [Ref no: 11]

The DDT process can be divided into four phases:

Deflagration initiation: A relatively weak energy source such as an electric spark

is used to create a flame. The energy release from the initiator device along with

radical production and energy release from the mixture compete with loss

processes including expansion of the reacting flow field and thermal conduction

and species transport away from the flame front. Flammability limits which result

from this competition have been extensively researched.

Flame acceleration: Increasing energy release rate and the formation of strong

shock waves are caused by flame acceleration. The observed mechanisms for

flame acceleration are outlined above.

Formation and amplification of explosion centers: One or more localized

explosion centers form as pockets of reactants reach critical ignition conditions

15

(the so-called explosion within the explosion). Critical temperatures are typically

around 1100 K and 1500 K for fuel-oxygen and fuel-air mixtures, respectively.

The explosion centers create small blast waves which rapidly amplify in the

surrounding mixture.

Formation of a detonation wave: The amplified blast waves and existing shock-

reaction zone complex merge into a supersonic detonation front which is self-

sustaining.

16

Fig.5 Deflagration to Detonation Transition [Ref no: 5]

(i) FACTORS INFLUENCING DDT:

The following list of factors was deemed to be important to DDT behavior in a pulsed

detonation engine.

1. Obstacle geometry and spacing

17

a. Blockage ratio

b. Spacing

c. Length

2. Placement of obstacle section in tube

3. Fuel/Air equivalence ratio

4. Fuel/Air mixing

5. Fuel Valve open/close time

6. Inlet air velocity

7. Initial pressure/temperature

8. Mixture characteristics

(LT/δ, σ = ρreactant/ρproduct, SL/Csr, SL/Csp, γr, Le, β, d/λ

where LT: Integral turbulence length scale

ρ: Density

SL: Laminar flame speed

Csr: Speed of sound in reactants

Csp: Speed of sound in products

γr: Ratio of specific heats of reactants

Le: Lewis number

d/λ: Minimum tube diameter criteria

δ: Laminar flame thickness)

18

(ii) TIME VS POSITION GRAPH:

Fig.6 Time Position graph [Ref no: 11]

The time required for a directly initiated wave trajectory is much less than that for DDT

wave trajectory which is even less than that for a laminar flame trajectory.

4. BLOCK DIAGRAM OF PDE

Fig.7 Block diagram of PDE [Ref no: 12]

19

5. WORKING OF A PDE

Fig.8 Schematic cross-sectional view of PDE [Ref no: 12]

Fig. 9 Cross-sectional view along sections [Ref no: 12]

20

Fig.10 Cylindrical rotary valve [Ref no: 12]

The pulse detonation engine provides an intermittently opening and closing valve in the

open end of the combustor outlet referred to as an "exit valve". In addition to a large flow

of combustion gas at high pressure and high temperature passing through this exit valve,

it is also required to rapidly open and close, and conventional butterfly solenoid valves

and mushroom valves therefore cannot be employed. A cylindrical rotary valve has been

invented for the present invention to achieve the necessary function and durability. It is

able to handle large flows of operating fluid at high pressure and high temperature in a

pulse detonation engine, and employs a valve mechanism for rapid operation. FIG. 8 is a

schematic cross-section drawing of the pulse detonation engine.

It comprises primarily an intake receiving ram-compressed air, an air cooler, a

precombustor, a fuel injector, an igniter, a supply valve, a combustor, an exit valve, and a

21

nozzle. In the pulse detonation engine, ram-compressed air received at the intake is

cooled by heat-exchange with a cryogenic coolant and increased in density, and supplied

to the combustor (combustion tube) via the supply valve. It is desirable that liquid

hydrogen fuel be employed as the coolant of the air cooler. As a result, the fuel is heated

in the air cooler, and subsequently supplied to the combustor via the fuel injector and

supply valve. A plurality of combustors are positioned axially on the circumference

(sixteen combustors shown in FIG. 9), combustion start timing differing for each

combustor. The sixteen combustors are labeled sequentially "a" through "p" (in FIG. 9).

As shown in FIGS. 10, the supply valve is of tubular shape with the end on the inlet side

closed and the other end open, and a plurality of supply holes (two holes spaced at 180°)

are provided in the tube surface such that the gaseous mixture flows into the

precombustor and combustors only when the phases of the supply holes in the supply

valve and the holes in the combustors match. In the condition shown in sectionin FIG. 9,

air and fuel (air and fuel are referred to as a "gaseous mixture") are supplied to the

combustors a, b, i, and j facing the supply hole. At this time, the combustors c, d, k, and 1

previously filled with the gaseous mixture are ignited with the igniter from the closed end

of the precombustor, and combustion occurs with the detonation wave. Initial combustion

immediately after ignited forms a deflagration wave, however since the diameter of the

precombustor is less than the combustor, the transition from deflagration to detonation is

readily achieved, and a stable detonation wave is propagated in the combustor.

The exit valve is provided at the outlet of the combustors, and a plurality of outlet holes

are provided in the tube surface at positions at which the phase differs from the supply

22

valve such that the exit valve is closed when the gaseous mixture is supplied to the

combustors. Thus, the supply pressure of the gaseous mixture may be equalized with the

outlet pressure of the intake. As shown in FIG. 9, at the time the detonation wave reaches

the exit valve, the outlet holes in the exit valves rotate to overlap with the combustor

outlet, and the detonation wave reaches the exterior of the engine (combustors e, f, m,

and n) via the exit valve and the nozzle. After the detonation wave reaches the nozzle

outlet, the high-pressure combustion gas is exhausted (combustors g, h, o, and p) to

atmosphere via the nozzle generating thrust. The supply valve and exit valve rotate

rapidly, and supply, combustion, and exhaust are repeated at a frequency of between 10

Hz and 100 Hz to match the phase of rotation. Since satisfactory thrust is not generated if

the timing of opening and closing of the supply valve and the exit valve is disturbed, the

supply valve and exit valve are connected by a central shaft and rotated by an electric

motor or fuel turbine as described below. .

FIGS. 10 show perspective views of the cylindrical rotary valve wherein the supply valve

and the exit valve are integrally configured by connection via a central shaft. As shown in

FIG. 8, the central shafts are connected with an appropriate driving actuator such as a

motor and the like via a transmission mechanism, and rotated with the prescribed timing.

The supply valve and exit valve are always rotated in the same phase relationship.

Realization of the pulse detonation engine requires that the timing of supply, ignition, and

exhaust for each combustor be maintained in the same phase without disturbance,

however by mechanically connecting the supply valve and exit valve, it is possible to

prevent disturbance of the phases of the cylindrical rotary, and to always maintain the

same phase in operation. Furthermore, by controlling the ignition timing of the igniter

23

with the position signal of the drive mechanism, disturbance of the ignition timing is

reduced. Since the vicinity of the supply valve and the exit valve are periodically subject

to high temperature and high pressure due to detonation, there is the possibility of

deterioration in strength and brake due to metal fatigue and thermal fatigue, and a robust

structure is required. The cylindrical rotary valve of the present invention ensures that the

processes of supply, combustion, and exhaust always occur at phases differing by 180°,

and by ensuring that high-pressure air acts on the outside of the valve, suppresses metal

fatigue and thermal fatigue. In other words, the pressure due to combustion always acts in

the direction of the central axis of the tube, and by constantly generating an equal force in

the opposite phase, cancels out the force acting on the entire valve. Thus, it is possible to

improve reliability of the supply valve and exit valve, reduce the valve drive force, and

reduce the size and weight of the drive mechanism. Furthermore, the life of the bearing

mechanism supporting the valve and shaft can be increased and its size reduced.

5.1 T-S DIAGRAM

Fig.11 T-S Diagram [Ref no: 5]

24

6. PRACTICAL PROBLEMS INVOLVED IN PDE

There are quite a number of practical problems to be overcome before a working PDE

can be built.

Firstly there's the issue of valving. The effective life of a traditional pulsejet tends to be

measured in minutes rather than hours -- and that's even though they're only called on to

handle the relatively low pressures generated by deflagration. If the same fragile valves

are used when detonating an air/fuel mixture they would instantly be destroyed.

To get around this problem, some of the existing PDE designs appear to use robust rotary

valves -- but this often requires a sophisticated synchronization system to ensure that the

externally driven valves open and close at exactly the right times.

Another alternative is to use a valve less setup and rely on a careful synchronization of

the shockwaves produced to control the gas flows.

Other problems with PDEs at this stage of their development include being able to inject

and detonate the trigger charge at exactly the right moment to produce detonation of the

main air-fuel charge. Too early and there won't be enough air/fuel to provide a good blast

-- to late and the air/fuel will have already started leaving the tailpipe.

Then there is the problem of structural integrity. What you're effectively doing with a

PDE is repeatedly setting off a small charge of hi-explosive inside a metal tube. This

obviously requires that a PDE be massively stronger than a pulsejet. It also means that the

levels of noise and vibration are similarly far higher.

25

7. CURRENT STATE OF DEVELOPMENT

PDE is yet to be developed to the point of being a practical propulsion device.

The main focus is currently being placed on researching and improving the

detonation process.

The current generation of PDEs don't seem capable of continuous running for any

length of time.

They are more or less just single-shot devices requiring several seconds to

recharge between detonations.

8. THE FUTURE OF PDEs

Many developers have high hopes that the PDE will ultimately become the most cost-

effective method of propelling supersonic sub-orbital craft. The ultra-high compressions

obtained by detonation offer the potential for much better fuel-efficiency than even the

best turbojet, and the fact that they are an air-breathing engine reduces the fuel-load and

increases safety when compared to rocket motors.

Unfortunately there are still a number of negative issues that will need to be addressed.

Firstly there's the noise.Then there's the issue of vibration. Although multiple engines

could possibly be synchronized to fire in a manner that reduces vibration levels, they will

still be significantly greater than those generated by turbojet or rocket motors. High levels

of vibration place incredible demands on the materials from which motors and airframes

are constructed.

26

9. REFERENCES

1) http://en.wikipedia.org/wiki/Pulse_detonation_engine

2) http://aardvark.co.nz/pjet/pde.shtml

3) (Martin J L Turner ) ROCKET and SPACECRAFT PROPULSION

PRINCIPLES, PRACTICE, and NEW DEVELOPEMENT

4) M. Cooper, S. Jackson, J. E. Shepherd, Effect of Deflagration-to-Detonation

Transition on Pulse Detonation Engine Impulse, pg 5 – 10.

5) Pulse Detonation Engine Simulations with Alternative Geometries and

Reaction Kinetics, He, X. and Karagozian, A. R., Journal of Propulsion and

Power, Vol. 22, No. 4, pp. 852-861, 2006

6) http://www.seas.ucla.edu/combustion/projects/pulsed_detonation_wave.html

7) Numerical Simulation of Pulse Detonation Engine Phenomena, He, X. and

Karagozian, A. R., Journal of Scientific Computing, Vol. 19, Nos. 1-3, pp.201-

224, December, 2003.

8) http://www.grc.nasa.gov/WWW/K-12/airplane/combst1.html

9) David M Chapin (December, 2005), A Study of Deflagration to Detonation

Transition in a Pulse Detonation Engine, pg : 9

10) E. Schultz, E. Wintenberger, J. Shepherd, California Institute of

Technology, Pasadena, CA 91125 USA, Investigation of Deflagration to

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Detonation Transition for Application to Pulse Detonation Engine Ignition

Systems, pg 8-10.

11) Takayuki Kojima, Tetsuya Sato, Hiroaki Kobayashi, Pulse Detonation Engine

and valve, pg 1-9

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