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DEPARTMENT OF MECHANICAL AND INDUSTRIAL ENGINEERING

CONCORDIA UNIVERSITY

FALL 2013

Gas Dynamics

MECH 6111

Dr. Pierre Q. Gauthier

Scramjets

Prepared by:

Alejandro Guerrero Zamora 6477607

Eusebio Olalde 6708307

2013-12-20

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Tabla de contenido

1 Scope ........................................................................................................................... 1

2 Technical Overview ...................................................................................................... 2

2.1 Hypersonic machines ............................................................................................. 3

2.2 Principle of ramjets ................................................................................................ 5

2.3 Scramjets ............................................................................................................... 7

2.3.1 Scramjet fuel injection [7].................................................................................. 9

2.3.2 Wind tunnel problematic ................................................................................ 11

2.3.3 Fuels .............................................................................................................. 12

2.3.4 Thermal protection systems (TPS)[1]

............................................................. 13

3 International Scramjet Developments ......................................................................... 15

3.1 International Chronicle of Scramjet Development ................................................ 15

United States ....................................................................................................... 15

France................................................................................................................. 16

Russia................................................................................................................. 17

Germany.............................................................................................................. 18

Japan ................................................................................................................... 19

Australia.............................................................................................................. 19

3.2 Recent Scramjet Developments ....................................................................... 22

4 Future Developments ................................................................................................. 24

5 Conclusions and Remarks ......................................................................................... 25

6 References ................................................................................................................. 26

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1 Scope

This work deals with an introductory overview of the Supersonic Combustion RAM-jet

(SCRAMjet) for Hypersonic air-breathing propulsion applications, which has been considered the

most promising technology for powering vehicles at high Mach number speeds, since its

establishment as a solution for more efficient hypersonic propulsion during the 1950s [5].

International efforts on Research and Development (R&D) projects of Scramjet are summarized

in order to provide a global picture of the most important contributions to this technology in

different countries. For the last 60 years, hypersonic air-breathing propulsion has been of big

priority to the most important national aerospace programs in the world; the most significant

developments and results of the scramjet projects carried in Russia, USA, Australia, France,

Germany, Japan and England are discussed.

In an air-breathing engine, when flight speeds reach the hypersonic regime, the total pressure

and temperature contained inside are very large, so large that in order to maintain the static

pressures within the structural limits of a suitable mass engine, the working fluid must flow

through the engines core at supersonic speeds. This flow characteristic becomes a problem in

the combustion part of the cycle, for two main reasons [6]:

Fuel ignition becomes a difficult task when operating in this regime.

Flow of air inside the engine is so fast, that even a close to instantaneous reaction like

combustion becomes hard to be completely achieved before the flow leaves the

combustor.

The RAM-jet engine has been utilized in missiles and other hi-speed applications for a long time

now [7], but its operation speed limits have been restricted below the hypersonic regime by the

combustors capability of overcoming these problems when burning fuel at such speeds. The

present work covers mainly the research and developments of the Supersonic Combustion part

of the scramjet engine, such as combustor performance and combustor-inlet interactions.

The present scramjet engines development situation is assessedto give a base ground on the

operation cycle and performance of this hypersonic engine, as well as the critical gaps that still

remain unfilled in order to put the scramjet into operation in the following years.

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2 Technical Overview

After World War II several countries including the United States, Russia, China, Japan, Australia

among other realize that hypersonic flights were a necessity to achieve better war gear, less

expensive commercial flights and means to bring load to outer space. Since then, there have

been a lot of sole and combined efforts from all these countries to achieve hypersonic propulsion.

However, the list of technical design difficulties related to hypersonic flights is really long, just to

mention some; aerothermodynamics environment, propulsion systems, structure, flight control

systems. Moreover there are decisive factors to consider as weight, complexity, variability,

longevity, cost, handling, stowability, etc. [1] Some of these factors and difficulties will be

addressed in the present work with the most resent research available.

But, what is hypersonic flight? To answer this question, the dimensionless relation known as

Mach number should be presented. It relates the local velocity of the free stream (V) with the

local sonic velocity (a) as,

1.1

The Mach number yields to 4 different speeds of flight, also known as regimes of flight: subsonic

(M1) and hypersonic (M>5). To have an idea about the

meaning of the number; speed of sound at sea level is 768 miles per hour or 1,236 kilometers

per hour, thus an hypersonic flight at sea level will require to achieve a speed of at least 3840

miles per hour or 6180 kilometers per hour. As mentioned by H. K. Beckmann Mach number is

like aborigine counting: one, two, three, four, many. Once you reach many, the flow is

hypersonic.[2]

It is possible to see that it is difficult to achieve that speed; furthermore there is a lot of

information that remains unknown or unaware, as presented in the Knowledge Management

Space developed by Matsch and McMasters

[1]

. The graph is divided into 4 quadrants, each onerepresenting the level of knowledge available; the first quadrant is information known that we are

aware what we know we know. The second quadrant is information unknown but we are aware

what we know we dont know. The third quadrant is the most dangerous one, the information

that we do not know and we are not aware what we dont know we dont know . This is the main

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reason a lot of tests developed either on wind tunnels or with actual prototypes result in damage

of equipment, unexpected behavior of the system, wrong data or, as it has happened before, in

lost of lives. And finally the last quadrant is information known but we are not aware what

someone knows, but that we havent found yet. Figure 1.1 shows all 4 quadrants.

Figure 1.1 Knowledge Management Domain [2]

Hypersonic flights research is full of gaps related to the second and third quadrant, however

scientist and researchers should focus their energies to solve second quadrant as it represent

capabilities we have but we have not fully developed or they are further from our current

technology. Usually they try to compensate them with over protection, over design, adding

structural or thermal protection or by restricting certain flight conditions.

2.1 Hypersonic machines

Any propulsion machine is capable to work efficiently throughout different flight regimes. So the

use of combine engine cycles is required. It means that in order to an aircraft achieves a speed

over Mach 5, it must have a propulsion system from take off to a low Mach number, perhaps 2 or

3, and after another propulsion system will accelerate it up to the final desired speed. Hence, the

aircraft will have to carry more than one propulsion system, increasing the price, complexity and

weight of the overall aircraft. Furthermore, fuel is one of the biggest limitations each propulsion

system will have. With the concern about greener technologies and the imminent shortage of

fossil fuels, there have been developments in this field. However, with the current state of the art,

most engines work with some kind of fossil fuel. As shown in Figure 1.2 and 1.3 different

propulsion engines with different regimes can be added to each other to have a wider speed

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spectrum as well as the use of hydrogen as fuel. Also in Table 1.1 a comparison of some

hypersonic applications with the technology related to them is presented.

Figure 1.2 Specific impulse and achievable Mach numberfor different propulsion machines[2]

Figure 1.3 Engine options for different Mach numbers [6]

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MissilesManeuvering reentry

vehiclesGlobal reach

aircraft systemsSpace launch

Mach number 1-6 0-20 0-18 0-25

Aerodynamictechnology

High lift/dragratio

Low drag

High lift/drag ratio

Minimal aero heating

Flow modification

High lift/drag ratio

Low drag

Airframe-propintegration

Low aero heating

Low drag

Airframe-prop integration

Propulsion

Rocket

Ramjet /Scramjet

RocketRocket

Ramjet / Scramjet

Rocket

Ramjet / Scramjet

Fuel Hydrocarbon -Hydrocarbon

Hydrogen

Hydrocarbon

Hydrogen

StructureHeat sink

Ablatives

Thermal protection

Radiation cooled

Fuel cooled

Radiation cooled

Long life structure

Fuel cooled

Radiation cooled

Low structural weightfraction

Table 1.1 Comparison of some hypersonic application [2]

So, from the information provided above, it is possible to see that there are 3 propulsion

machines capable to achieve Mach 5 or greater. However the ones in our interest are ramjets

and scramjet. They are really similar, but with some different fundamental principles that turn

them into completely different machines. Both of them are capable to achieve hypersonic speed,

however scramjets can reach (theoretically) speeds up to Mach 15 as ramjets only up to Mach 6.

2.2 Principle of ramjets

The ramjet can be credited to the French Rene Lorin, who patented in 1908. Although it was a

subsonic ramjet, it gave the general idea of the functioning. He never could build such a machine

because in that time there was no aircraft capable to achieve the speed required to make the

ramjet functions properly. It is a simple machine without any moving parts as it compresses the

airflow through the change of internal geometry. However, as mentioned before, it is not suitable

for a full range of speeds, especially during take-off, as dynamic pressure caused by subsonic

airflow inside the chamber of the ramjet is really low, making it inefficient.[6]Also, as shown in

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table 1.1, on the section of propulsion, usually the ramjet (scramjet) is not implemented by itself;

rather it is used with a rocket. The reason is that compression depends on velocity and

decreases dramatically with vehicle speed.[3] So, during take-off rocket provides the required

initial acceleration. The operation principle is that air enters to the compression area, an inlet and

a diffuser, after it will go to the combustion chamber, where is mixed with injected fuel and ignited.

The combustion of the oxygen and fuel produces energy, which is transmitted to the gas in form

of high speed expansion, directed to the outer nozzle. The speed of the output air is higher than

the input, thus there will be production of thrust. In comparison with normal turbomachinery,

where air compression is accomplished by means of moving parts, with a speed of Mach 3 or

higher, there is no sustain gain in the pressure efficiency, making machines with changes in

geometry more suitable and efficient for these regimes.

Since its conceptions, according to Ronal S. Fry There are key enabling technologies,

components, or events that had significant impact on the maturation of ramjet propulsion and

engine designs. All these key matters are classified into10 different sections[6]:

1. High speed aerodynamics analysis: Study of flows with the aid of computational tools as CFD

and the improvement in design tools and techniques.

2. Air induction system technology: Improved designs of fixed and variable geometries,

subsonic, supersonic and dual-flow paths, integration with airframes, mixed cycle flow path

development and improved materials.

3. Combustor technology: Study of fuel mapping and heat transfer distribution, improved

insulators and structural materials and combustion ignition, piloting and flame holding.

4. Ramjet/scramjet fuels: Study of low temperature, high energy and endothermic solid and

liquid fuels.

5. Fuel management systems: Improved injectors, spray distribution, feed systems and

feedback control systems.

6. Propulsion/airframe integration, materials and thermal management: Improved metal alloys,

structures, passive and active cooling devices and fiber reinforced composites.

7. Solid propellant booster technology: Development in tandem, integral rocket-ramjet boosters

and self-boosted ramjet

8. Ejectable and nonejectable technology: Development in inlets, port covers and nozzles.

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9. Thermochemical modeling and simulation development: Development in thermochemical

tables, cycle analysis and performance modeling.

10. Ground test methodologies: Improved airflow quality, instrumentation and flight-test

correlation.

2.3 Scramjets

In summary, the ramjet operation is based on air entering to the inlet at a supersonic speed and

low pressure, decelerated to subsonic speed and high pressure, two conditions that provide

more convenient combustion conditions. After it is exhaust to the nozzle to provide thrust. But,

when the incoming air is travelling too fast, around Mach 5, the static pressure and temperature

produced from decelerating the flow to subsonic speed are really high, causing molecular

dissociation of the incoming flow and excessive material stress.[7]

To overcome this limitation, it

was proposed to decelerate the air but keeping it still supersonic; this was called supersonic

ramjet or scramjet.

As shown in figure 1.4, due to the condition in the combustion chamber for the ramjet a physical

nozzle is installed to maintain the desired inlet operational conditions. However, for the

supersonic counterpart an area increase is expected to provide the right heat conditions during

the combustion gas release.[3]

Figure 1.4 Schematic of ramjet and scramjet engines [3]

More differences between them are:[3]

1. Due to the absence of terminal normal shock in scramjet system, the stagnation pressure

recovery, a parameter used to measure the amount of pressure loss in the inlet and diffuser

system, is about 30 times larger. Thus, making scramjets more efficient as the engine thrust

loses 1% for each 1% of loss in pressure recovery.[3]

2. As temperature difference among the entrance of the ramjet combustion chamber and further

downstream in the nozzle are relatively high and chemical equilibrium needed to convert

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recovered heat into kinetic energy require long distances, heavier and longer nozzles have to

be designed.[3]

3. A limiting design factor for ramjets is the ratio of the nozzle throat and capture area, if this

ratio is increase then the thrust will be increased as well. But, producing a heavier nozzle.

Whereas, for scramjets, during deceleration of the flow the physical throat is substitute by a

thermal throat.[3]

4. A lighter construction and increased system efficiency is accomplish with scramjets by the

lower static pressure which reduces the structural load of engines ducts. [3]

It is possible to say that all these facts are the bright side of scramjet. However when it comes

to its operational characteristics and its integration with the aircraft, a lot of considerable design

difficulties have to be taken into account. [3]

1. The time air remains inside the scramjet is in the order of millisecond, hence to provide an

optimal mix between oxygen and fuel is complicated, as air remains in the combustion

chamber very few time. Usually, mechanism to accelerate the mix are used, however they

create momentum losses, sacrificing the overall efficiency of the engine.[3]

2. The fact that air travels so fast create the problem of maintaining the flame on, so flame

holders must be placed. [3]

3. The high speeds aircraft achieves will create a large temperature around itself and the

engine. Usually, fuel is used as coolant. Most common fuels are liquid hydrogen and

hydrocarbons. Liquid hydrogen is a good choice to cool down the entire engine; however it

has a very low density, thus larger amounts of fuel have to be carried for each flight. Whereas,

hydrocarbons have higher density but its cooling capacity is smaller and sometimes not

enough to absorb all the heat created around the engine and the aircraft. [3]

4. A recurrent problem, already mentioned in previous sections, is the fact that scramjets

cannot operate from takeoff and during low speed flight they are very inefficient. Hence, more

propulsion devices are required. But as speed increases, the integration between vehicle

aerodynamics and engine performance become more coupled. Also the integration of

several components in the aircraft will add more weight and aerodynamics problems, related

mostly to creation of drag.

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5. Testing new concept in ground represents a challenge itself, because the difficulty to achieve

high Mach numbers and the noise that the tunnel creates, which interferes with the boundary

layer transition.[1]

Further explanation of some of these issues will be provided.

2.3.1 Scramjet fuel injection [7]

In order to have efficient combustion inside the scramjet the mix between air and fuel must be

almost in stoichiometric proportions. But, as mentioned in problem number 1 of section 2.2, the

speed of the air passing in the combustion chamber is really high, providing a really small

amount of time to produce the mixture and ignite it. Also, one of the main fuels used;

hydrocarbons, have a long ignition delay, compared with liquid hydrogen, requiring more

advanced mixing techniques. This is why injectors have a huge impact in the overall

performance of the engine. They have to provide a quick mixing and combustion of air and fuel,

providing a smaller and shorter combustor. Besides, they must interfere as less as possible with

the total pressure losses since these losses reduce the thrust. Finally the fuel distribution inside

the engine should be really uniform providing an efficient nozzle expansion process. Some of the

most common injectors used in scramjet application will be described below.

1. Parallel injection. It consists of flow of air and fuel travelling parallel to each other and

separated by a splitter plate. At the end of the plate they will mix by a shear layer created by

the difference of velocities both fluids have.

Figure 1.5 Schematic of a parallel fuel injector

2. Normal injection. It consists of a blast of fuel in a direction perpendicular to the air through a

port on the wall. The problem is that a detached normal shock is created upstream of the

injector, causing flow separation and losses in total pressure, thus affecting the efficiency of

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the engine. However, separation can be used downstream as a flame holder. In overall is not

a very efficient method as the mixing rate is not so high.

Figure 1.6 Schematic of a normal injector

3. Ramp injectors. This method is an improvement of the parallel injection. It consists of ramps

that have an injector on the trailing edge adding velocity to the fuel and increasing fuel mixing.

These ramps also create normal shocks and expansion fans, hence pressure gradients

increase mixing as well. There are two kinds of ramps, expansion and compression ramps.

As compressing ramps increase the fuel air mixing, expansion ramps have higher

combustion efficiency. However, the way ramps are placed inside the combustion chamber

provides low fuel penetration throughout the flow field.

Figure 1.7 Schematic of ramp injectors

4. Strut injector. It consists of a strut with a wedge leading edge placed all along the vertical axis

of the combustion chamber, injectors are placed on the trailing edge of the strut, this provides

the fuel to be added in all the flow field. It can be added either parallel or normal to the free

stream direction. However, it produces high pressures losses, decreasing its performance.

Recently research suggests that with a different shape of strut, pressure losses can be

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decreased, some suggestions are to change the shape of the trailing edge or to make a strut

in a diamond shape.

Figure 1.8 Schematic of a strut injector

5. Pylon injection: It is very similar to the parallel injection method but instead of having just a

splitter plate, there is a tall narrow in-stream body. Injection could be either axial, normal or at

some angle. Research has showed that flow penetration and mixing are high as pressure

losses low, in comparison with the previous methods.

Figure 1.9 Schematic of a central pylon fuel injector

Certainly, there are many more injection systems. Just to mention some; plasma igniter,

upstream injector, barbitage injection system, pulsed injector, cavity-pylon flame holder,

micro-flame holder, conventional scale bluff body flame holder, cantilever fuel injector, however it

is not the purpose of this work to go in depth of all these, rather is to present the basic idea and

reference of all of them.

2.3.2 Wind tunnel problematic

Definitely it is not easy to perform ground test in wind tunnels for very high Mach number. It is

easy to figure it, because the means to achieve a Mach number where the local velocity of the

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free stream is 5 or more time larger than the local sonic velocity are complicated. Basically, and

according to equation 1.1, there are two solutions to this problem, one is to provide a really high

free stream velocity and the other one is to make sonic velocity really small. In order to make a

decision the equation to get the speed of sound is shown:

1.2

Where is the heat capacity ratio, R the universal gas constant and T is the local temperature in

kelvin degrees.

So, from the two solution proposed, the first one provides a really challenging situation as the

free stream velocity of Mach 5 or higher is difficult to accomplish. Hence, the second solution will

bring an easierway to achieve wind tunnel testing. From equation 1.2, it is possible to see that

and R are gas constants; therefore the only parameter that could be varied is the temperature.

Hence, test gas is expanded to provide a static temperature just above liquefaction temperature.

This is a safe path to achieve a high Mach number and keeping safe the surrounding equipment.

As total enthalpies related to hypersonic flights are really high, simulation of these conditions are

not possible to recreate in ground without causing equipment damage or just for a very small

period of time.

Another wind tunnel problem is related to the prediction of boundary layer transition is difficult to

understand and predict. A variety of experiments have been performed to produce a

semi-empirical model to predict transition at different flight conditions as well as attempts to use

linear stability theory, but all of them have inaccurate results. Recent research points that maybe,

one of the problems to get accurate results is the noise produce by the wind tunnel. If it is true, all

the information gotten so far will have to be discarded. [1]And a quiet hypersonic wind tunnel

must be constructed; however difficulties for this task are still in the second quadrant of

Figure 1.1.

2.3.3 Fuels

As mentioned before, there are two different fuels used for scramjets application, both of them

having advantages and disadvantages. Table 1.1 shows a comparison of them.

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Advantages Limitations

Liquid

hydrogen

It can be used as a coolant of theengine as well as the external

surfaces.

Faster combustion and mixing

rates

Small density, hence larger fuel

tanks are required, creating a

larger cross section and increasing

the drag, surface heating and

vehicle weight.

Boiling point of hydrogen is low

(20 K), hence it is required to carry

cryogenic tanks.

Hydrocarbons

Larger boiling point, facilitating

logistics.

Large density, hence smaller fuel

tanks are required.

Lower cooling capacity

Specific impulse is near one third

of hydrogens.

Table 1.2 Comparison of scramjet fuels [1]

2.3.4 Thermal protection systems (TPS)[1]

Environment for vehicles travelling at a hypersonic speed is severe, so, thermal protection

systems are installed to provide insolation to critical parts of the aircraft. This protection is divided

into 3 categories; passive, semi-passive and active. The main difference among them is the

system to extract the heat. Passive uses no fluid to remove the heat. Semi-passive uses fluid,

however it does not have an external pumping or circulation system. Finally active uses fluid and

an external pumping system. As different parts of the vehicle may be exposed to different kind of

flight conditions, it may be possible to use more than one protection system.

Passive systems are mainly thermal insulations made of different materials; organic composites

as carbon-carbon composites, metal matrix composites as titanium-aluminide alloys or ceramic

matrix composites as silicon nitride or silicon carbide.

Semi-passive systems are heat pipes and ablators. Heat pipe is a heat transfer device

composed of a container, a wick and coolant. It is useful when there is an area of localized heat

just beside an area of low heat. The heat transfer is perform by the coolant, which vaporizes as it

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is heated, then the hot gas moves to the cool side of the heat pipe to discharge the heat and after

it comes back through the wick to start the process again. Ablators are used where really high

temperatures are reached. It is a layer of a chemical product that will pyrolyze as it absorbs heat,

leaving a layer of char between the outer surface and the ablator.

Finally, the concept of active systems is using pipes and pumps to circulate coolant through the

high heat zones, absorbing it and then bringing it to a low heat area. Where it is discharged and

the process starts again. In comparison, this is the best method to keep the key areas cool,

however, it is expensive and makes the design more complex. Also, the weight of the vehicle is

increased from the pipes, pumps and coolants. Convective cooling is the only one that does not

add an extra weight from the coolant, as it uses the fuel to cool the structure and then it is

brought to the combustion chamber to be burn. It does not only have a beneficial impact in the

temperature of the cooled area, but it also benefits combustion, as it brings the fuel already hot,

facilitating burning and cutting the time it has to mix with oxygen and burn.

Figure 1.10 Examples of TPS

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Figure 3.1 First known freejet-tested scramjet by Ferri 1963 [7]

Among the hypersonic flight programs worldwide, probably the most famous and prolific is

NASAs Hyper-X program, launched in 1996 with the purpose of demonstrating in flight the first

hypersonic airframe-scramjet integration, the X-43A.

The Hyper-X program had the objective of verifying and demonstrating experimental,

computational and design advances in the development of the air-breathing hypersonic craft [7]

required to develop confidence in future hypersonic vehicle designs. The first flight of the X-43A

was attempted in 2001 but the mission was canceled just 13.5 s before launch due to a booster

failure. [8]

France

After the first American studies on scramjet engines, the world research community showed

great interest in hypersonic flights beyond Mach 6-7. The French started the ESCOPE program

in 1966, inspired by the HRE program, focused on demonstrating dual-mode scramjet in a

flight-test program at Mach 7. [7] In a series of tests conducted in 1970 and 1972, complete

supersonic ignition was not achieved, going only until transonic combustion at Mach 5. [4]

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Figure 3.2 One of the first supersonic combustion models, US AIM [7]

The development of the scramjet in France gained momentum again with the launch of a joint

government-academia-industry Program of Research and Technology for Air-breathing

Hypersonic Aircraft (PREPHA) in 1987-1999. PREPHA was oriented to prepare a new

generation of engineers with an inside experience in hypersonic air-breathing propulsion

technology for the development of future long-term applications. [4,7] The Promethe project

started in 1999, concerning an air-to-surface missile with flight speeds of Mach 2-8 at high

altitudes.

Russia

Scramjet R&D in Russia has been on the map since the late 1950s, having an important

increment in the 1980s and 1990s. [7] Because of the Cold War, all the important research

conducted in Russia remained classified until its end in late 1980s, when the war ended; due to

the restricted access to the Russian literature, it was until Curran [4] collected the most important

contributions of the soviets in English edition that they became accessible to the worldwide

scientific community.

Russia had a lot of activity in the early 1990s, performing the very first hypersonic flight test

(Mach 5.35) in 1991, and two combined Russian-French launches in 1992 and 1995. In 1994,

they joint-ventured with NASA to study the operating of the scramjet at full supersonic

combustion.[7]American tests of the Russian engine were planned, but never done. The program

ORYOL was established by the Russian Space Agency to carry a comprehensive research and

development of combined propulsion systems for reusable space transportation. [4]

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One of the most promising concepts of hypersonic vehicles started its conception in the mid

1990s. [4] The AJAX incorporates three of the most current advancements in supersonic

combustion, Magnetic Hypersonic Deceleration (MHD), Endothermic Fuel Technologies (EFT)

and magnetoplasmadynamic (MPD) technologies.

Figure 3.3 The First Russian scramjet model tested 1964 [4]

Germany

Germany entered the hypersonic air-breathing propulsion development efforts first in the 1980s

with the German Hypersonic Technology Program (HTP), which had the purpose to develop the

Snger II concept, a two-stage, fully reusable, future space transportation vehicle. [4,7]

Cooperating with France, Germany also participated in the development of JAPHAR in 1997,

concerning hypersonic air-breathing propulsion for reusable launch vehicles between Mach 48,

which ended in 2001. [4,7]

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Table3.1

ScramjetDevelopments1950

19

90[7]

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Table3.2

Scram

jetDevelopments1990

2003[7]

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3.2 Recent Scramjet Developments

The principal ongoing scramjet research program is NASAs Hyper-X program. Having its first

successful flight test in March 2004 reaching Mach 6.8 at approximately 5 000 mph, on

November the same year, the X-43A broke the Guinness World record as the fastest

air-breathing vehicle going up to Mach 9.6, 7 000 mph approx. [10]For 10 seconds, the scramjet

engine burned successfully and produced thrust, giving the results needed to keep pushing for

higher hypersonic speeds. This was the ending of the Hyper-X program, setting a invaluable

ground base for air-breathing engines in hypersonic applications. [11]

Figure 3.6 First flight of the X-43A 2001 [11] The X-43A is the black nose seen in the picture, the

rest is a Pegasus rocket, used to bring the air-breathing vehicle to the flight test conditions

Figure 3.7 X-43 A project test vehicle [12]

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NASA has been simultaneously developing other hypersonic test vehicles, with new and more

complex air-breathing propulsion systems, scramjet-airframe integration, and other technologies

required for Mach 15 cruise operation (X-43C). The X-43 B, or Reusable Combined Cycle Flight

Demonstration (RCCFD), has the objective of demonstrating the latest developments on

combined cycle propulsion technologies, as well as airframe-engine integration technologies. [8]

It has been known for a while now, that the space shuttle era is finished and that the new space

transportation required for manned reentry missions is expected for 2025 [4,11] and scramjet

propulsion seems to be the launching approach intended for this and many other space

applications.

Figure 3.8 X-43 A project test vehicle [12]

Figure 3.9 X-43 A flight test mission [12]

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4 Future Developments

The aerospace field is going to be busy with air-breathing hypersonic propulsion for more years

to come. Curran [4], in is broad scramjet research compilation, anticipated that the topics of

greater interest for researchers was in the technologies that would improve the combustion

efficiencies and thus lower the costs of future flight testing as well as the operation costs of the

final developed engines.

Detonation Wave Ramjetsare expected to have a better performance than the typical scramjet

at higher Mach numbers. As the air flows into the engine a set of shock waves are intentionally

generated in order to improve air-fuel mixing and igniting the mixture. The detonation wave

connotation was given because when the fuel ignites close to a normal or oblique shock, the

combustion reaction couples with the shock and generates a detonation wave.

The application of Electro-Magnetic Fields to control the flow either around or inside the

hypersonic aircraft shows another interesting concept that can help to achieve higher speeds in a

more efficient way. This technology is based on the release of partially-ionized working

propulsion fluids into the flow and control them with electro-magnetic fields to decelerate the flow

through the engine and facilitating the combustion; or also can be applied to external flow in

order to reduce drag and aerodynamic

heating.

As mentioned in the earlier chapter, both

Russian and American agencies are currently

developing the new class of launching space

vehicle. So we can still expect in several years

to come research on the integration of

airframe and hypersonic engine will be of

great proportion and importance.

Figure 3.10 X-43A flight test mission [12]

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5 Conclusions and Remarks

The supersonic combustion ramjet has been of major global interest for the past 50 years and

keeps growing as an operational hypersonic vehicle comes closer to reality. From the beginning

of the Cold War until today, the task of developing a hypersonic air-breathing propulsion system

has been of enormous difficulty for engineers worldwide, and the progress in the scramjet engine

was slow due to the complexity and technology requirements of this kind of application.

Hypersonic propulsion programs have been very active recently as numerous tests and

experiments on scramjet technologies were conducted by the Hyper-X and SCRAMSPACE [13]

programs, and at the NAL in Japan. The first stage of the X- programs by NASA, the Hyper-X,

finished in 2004 with the successful 10-second flight of a hypersonic air-breathing propelled

vehicle at almost Mach 10. The following programs are going to be dealing with two other

test-flight vehicles, the X-43C, very similar to the first version but larger and oriented towards

longer steady flights of up to 10 min. The X-43B is a lot different than the previous two, oriented

to reusability demonstration and its going to incorporate the latest advancements in combined

cycle propulsion systems going from Mach 0.77.

The development of a completely tested and efficient scramjet still has a couple of decades to

come, but with the most recent achievements of NASA, and all the programed tests to come in

the next years, there is a very optimistic atmosphere among the R&D teams confronting thisconcept development.

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6 References

[1] John J. Bertin, Russell M. Cummings. Fifty years of hypersonics: where weve been, where

were going,Department of Aeronautics, United States Air Force Academy, 2003.

[2] Bertin JJ. Hypersonic aerothermodynamics, The American Institute of Aeronautics and

Astronautics; 1994.

[3] Corin Segal. The scramjet engine. Process and characteristics. Cambridge University

press, 2009.

[4] Curran, E. T., & Murthy, S. N. B. (2000). Scramjet propulsion. Reston, Va: American Institute

of Aeronautics and Astronautics.

[5] Swithenbank, J. (January 01, 1967). Hypersonic air-breathing propulsion. Progress in

Aerospace Sciences, 8, 229-294.

[6] Townend, L. H. (November 01, 2001). Domain of the Scramjet. Journal of Propulsion and

Power, 17, 6, 1205-1213.

[7] Fry, R. S. (January 01, 2004). A Century of Ramjet Propulsion Technology Evolution.

Journal of Propulsion and Power, 20, 1, 27-58.

[8] L, M. P., L, R. V., T, N. L., & R, H. J. (January 01, 2004). NASA hypersonic flight

demonstrators-overview, status, and future plans. Acta Astronautica, 55, 619-630.

[9] Stalker, R. J., Paull, A., Mee, D. J., Morgan, R. G., & Jacobs, P. A. (January 01, 2005).

Scramjets and shock tunnelsThe Queensland experience. Progress in Aerospace

Sciences, 41, 6, 471-513.

[10] Thompson, E., Henry, K. & Williams, L. (2005). Faster Than a Speeding Bullet: Guinness

Recognizes NASA Scramjet. NASA News, [online] 20th June. Retrieved from:

http://www.nasa.gov/home/hqnews/2005/jun/HQ_05_156_X43A_Guinness.html [Accessed:

10 Dec 2013].

[11] McClinton, C. R., Rausch, V. L., Shaw, R. J., Metha, U., & Naftel, C. (January 01, 2005).

Hyper-X: Foundation for future hypersonic launch vehicles. Acta Astronautica, 57, 614-622.

[12] NASA Facts (2004). NASA Hyper-X Program Demonstrates Scramjet Technologies, X-43A

Flight Makes Aviation History. [press release] October 20.

[13] Muller, D. (2013). SCRAMSPACE. [video online] Available at:

http://www.abc.net.au/catalyst/stories/3785345.htm [Accessed: 15 Dec 2013].