A Hypersonic Hybrid Vehicle
ABSTRACT:
An
aircraft with Scramjet flies at speed greater than Mach 5. Turbojet.s
efficiency is less at higher Mach speed.
We propose the use of Turbojet with Scramjet
to start the plane from rest and to fly it at a speed
greater than Mach 5. Due to less density of air at higher altitudes,
efficiency of scramjet reduces. For increasing its efficiency, we propose the
use of Plasma and Double
delta wings. Plasma reduces friction by deviating streamlines
of air in scramjet and double delta wings give good lift.
INTRODUCTION:
One of the greatest aeronautical research challenges
is flying at hypersonic speed -NASA
The question arises, why should one fly that fast?
Four reasons can be enumerated.
1.
For faster intercontinental
travel.
2.
To allow seamless entry to
space.
3.
For invincible spy planes and
strategic bombers and
4.
The last, but probably the most
important is the human being.s desire to constantly push the limits.
Mach is the unit to express the
speed. A regular passenger plane flies at the speed of 0.8 Mach. The military
plane flies at Mach 2, Concord
Conventional turbojet engines,
which are now a days used in most commercial aircrafts and military jets,
cannot be used at flying speeds above Mach 3.This is where the RAMJET engines
come in. These can be used far in to the hypersonic area, i.e. at speeds of
Mach 5 and over. They combine a greater range with largest possible weight and
more compact construction than systems currently in use. They utilize
atmosphere.s available oxygen to burn fuel. Its fuel consumption decreases with
flight speed and approaches reasonable values between the speed Mach 2 and Mach
4.
The Ramjet engines could easily
operate in the Mach 7 -9 regions. Above this speed the engines known as
SCRAMJETS come in picture. The engines with subsonic combustion are called
Ramjet; those with supersonic combustion are Scramjets (Supersonic Combustion
Ramjets). Scramjets open speed regime up to Mach 20.
CONSTRUCTION AND WORKING OF SCRAMJET AND
TURBOJET:
TURBOJET:
A gas turbine engine or a
turbojet consists of five major sections: an inlet duct, a compressor, a combustion chamber, a turbine
wheel, and an exhaust duct. In addition to the five major sections, each gas
turbine is equipped with an accessory section, a fuel system, a lubrication
system, and an ignition system. Some engines might also incorporate a water
injecting system, an afterburner system, a variable-area exhaust nozzle and
system, a variable-geometry compressor, a fan, a free-power turbine, a
propeller-reduction gearbox, and other additional systems and components to
improve or change engine operation, performance, and usage.
Construction
and Working of Turbojet:
The front, or inlet, duct is
almost entirely open to permit outside air to enter the front of the engine.
The compressor works on the incoming air and delivers it to the combustion
section. The compressor is basically a cone-shaped cylinder with small fan
blades attached in rows. As the air is forced through the compression stage its
pressure rises significantly. In some engines, the pressure of the air can rise
by a factor of 30. This high-pressure air then enters the combustion area,
where a ring of fuel injectors injects a steady stream of fuel. The fuel is
generally kerosene, jet fuel, propane or natural gas. The air entering in the
combustion area is highly pressurized and moving at hundreds of miles per hour.
A flame has to be kept burning continuously in that environment. The piece that
solves this problem is called a "flame holder," or sometimes a
"can." (Fig.1). The can is a hollow, perforated piece
of heavy metal. Half of the can in cross-section is shown below:
After leaving the turbine,
there is still enough pressure remaining to force the hot gases through the
exhaust duct and jet nozzle at the rear of the engine at very high speeds. The
engines thrust comes from taking a large mass of air in at the front end and
expelling it from the tailpipe at a much higher speed than it had when it
entered the compressor. Thrust, then, is equal to mass flow rate times change
in velocity.
Limitations of
Turbojet:
In conventional jet engines
thrust is created in three stages: Intake (through flight build up) and
compressor provide a building up of pressure, in the combustion chamber air is
enriched with energy as fuel is burnt. In the turbine and the nozzle the air
expands, while the inner energy of the gas is changed into kinetic energy and thrust.
As flight speeds increase, the
quality of the engine process deteriorates. This can be demonstrated with the
help of fuel specific impulses. This thermodynamic quantity describes the
created pressure per fuel mass. This value decreases with increasing speed.
In other words: At higher Mach
numbers the fuel consumption increases much more than thrust can be generated.
Above Mach 3 the fuel specific impulse of a scramjet engine is better than that
of a turbojet engine. The compressor is the main reason for this. This turbojet
component, which has several stages, rotors and stators, causes losses.
Furthermore turning parts wheels do not contribute to engine processes at high
Mach speeds. With the flight speed the pressure, created in the air intake
through flight build up rises considerably. The share the compressor
contributes to the entire compression sinks accordingly: At Mach 1 the value is
about 50 percent, at Mach 2 just 15 percent and at Mach 3 less than four
percent. From about three times the speed of sound the compression created by
the speed is enough to keep the engine process going. The compressor is really
not needed at higher speeds. Additionally the rise in temperature caused by the
build up is considerable. The build up temperature at Mach 8 - depending on the
altitude - is between 3,000 and 4,000 degrees Kelvin (between 2,727 and 3,727 °C), at Mach 12 about 8,000
degrees Kelvin. Conventional compressors cannot be used at such high
temperatures, because the compressor blades cannot be cooled and materials,
which are able to withstand these temperatures, do not exist.
SCRAMJET ENGINE:
A scramjet engine is an air
breathing engine system which does not have any moving parts. At high flying
speeds air in the intake is compressed so much just by the forward movement
that a compressor, which is needed in turbojet engines, is not necessary. This
is the main difference between Scramjets and conventional turbojet engines,
which are basically made up of five components: intake, compressor, combustion
chamber, turbine and nozzle If in the logical sequence compressor is omitted,
turbine becomes superfluous, the sole purpose of which is to drive compressor.
This is how the scramjet engine is far simpler in construction, consisting only
of intake, combustion chamber and nozzle.
Construction
and working of scramjet
Scramjets
(supersonic-combustion ramjets) are those in which the airflow through the whole engine remains
supersonic. It is mechanically simple, but
vastly more complex aerodynamically than a jet engine. The engine consists of a
supersonic diffuser, a subsonic diffuser section, a combustion chamber and a
discharge nozzle section (fig. 3).
The function of supersonic and
subsonic diffusers is to convert the kinetic energy of the entering air into a
pressure rise. The scramjet engine operates as follows. Air from the atmosphere
enters the engine, and, after its velocity has been reduced and its static
pressure increased by supersonic diffuser, the air enters the subsonic diffuser
wherein it is compressed further. The air then flows into the combustion
chamber, wherein the fuel burners are located, and here it is heated to a high
temperature (1500°C to 2000°C) by the continuous combustion of fuel. The highly
heated products of combustion are then allowed to expand with a speed exceeding
that of the entering air. Because of the rate of increase in the momentum of
the working fluid through the engine, a thrust is developed in the direction of
flight.
With the scramjet the mixing of
air and fuel is considerably bad because of high speeds at the entrance to the
combustion chamber between Mach 2 and Mach 3, at the exist between Mach 1.2 and
Mach 1.6. This makes combustion not very effective. For this reason the
combustion chamber has to be elongated to guarantee satisfactory mixing. In a
scramjet powered aircraft, there must be tight integration between the airframe
and the engine. Scramjet technology is challenging because only limited testing
can be performed in ground facilities. Long duration, full-scale testing
requires flight test speeds above Mach 8. X-43 Hyper-X, NASA's tested for the
scramjet, serves this purpose. To get the engine to that speed, some other
power has to be used. In the Hyper-X, this will be provided by OSC's Pegasus
booster. It must be noted here that scramjets are good only for sustaining hypersonic speeds, not for
achieving them from zero.
Limitations of
scramjet:
Aircrafts flying at speeds greater than Mach 3 have
to fly at a great height. This is done
because of several reasons. Few of them
can be enumerated below:
i. At higher altitude density of air is less so the resistance
of air experienced by aircraft is less.
ii. To avoid radar waves
iii. To avoid bullets of antiaircraft guns.
An aircraft with scramjet will
have less efficiency at greater heights.
This is because the scramjet is an air breathing engine. As the density of air is less, less
compression is created at the intake as compared to that at lower
altitude. And as less compression is
created, efficiency directly reduces.
Apart from this there is one
more difficulty in the operation of Scramjet and that is, neither ramjet nor
scramjet can operate at speeds below Mach 2 or 3. If the vehicle is to start of
its own a combination of the scramjet engine with other engine systems becomes
necessary. Since the compressor is lacking in a scramjet engine there is pressure
built-up and airflow in the engine when the vehicle is not moving. This means:
There is no air breathing engine that can cover the full operational range from
take-off to hypersonic speeds. But, a hypersonic aircraft cannot just take off
at Mach 3. It has to take off and cover the whole speed range. This makes it
necessary to accelerate the vehicle by other means until the ramjet/scramjet
engine can kick in at around Mach 3. This can be done with the help of a
booster rocket or by mounting this plane blow other plane. When this plane
reaches speed of Mach 2 or 3, scramjets ignite and the plane with scramjet is
released which is free to fly at speeds between Mach 15 -20. But this complete
process is very expensive and time consuming.
Following two challenges have emerged by now.
1. At higher altitudes, the density of air is less so mass of air
entering the scramjet would be less thereby reducing its efficiency.
2. Scramjets cannot start when the plane is at rest i.e. at speed
0 Mach.
INNOVATION:
1st challenge:
As we know by now that Scramjet
has the capacity to fly at the speed of Mach 15 to Mach 20. But the efficiency
of Scramjet greatly depends on the amount of air intake .moreover at higher
altitudes density of air is less .So the total mass of air entering the
scramjet would also be less and hence less compression is created which reduces
its efficiency but still it is more than turbojet engine.
So to enhance the capability of Scramjet engine, it
is desirable to increase air intake at higher altitudes. This paper proposes
the use of
-Plasma
-Double Delta Wings
Use of plasma
To increase the working or
efficiency of a scramjet, the fourth state of matter plasma can be used. Plasma
being an ionised state of matter, can be radiated and possess the properties of
friction or resistance to air.
Plasma consists of a collection
of free-moving electrons and ions - atoms that have lost electrons. Energy is
needed to strip electrons from atoms to make plasma. The energy can be of
various origins: thermal, electrical, or light (ultraviolet light or intense
visible light from a laser). With insufficient sustaining power, plasmas
recombine into neutral gas.
Plasma can be accelerated and
steered by electric and magnetic fields which allows it to be controlled and
applied. Plasma research is yielding a greater understanding of the universe.
Plasma temperatures and densities range from relatively cool and tenuous to
very hot and dense. (Fig.4) Ordinary solids, liquids, and gases are both
electrically neutral and too cool or dense to be in a plasma state (Fig 5).
Due to these properties of
plasma, it can be utilized to increase the working efficiency of scramjet. An
aircraft which does not use plasma radiation creates more friction at its nose
thereby increasing drag to a great extent. (Fig. 6) Consider that at the nose
of the plane on which scramjet is placed; we place a device which generates and
radiates plasma in the direction of motion of plane. This radiation should take
place up to certain specified distance. Now at the tip of the plasma radiation,
the friction takes place and the air is cut by this plasma tip. As air is being cut by plasma tip and not by
the nose of the plane the resistance of air or drag reduces to a great extent.
Moreover this plasma can regulate the stream lines of air is such a way that
the body of plane has minimum friction. But along with this, plasma regulates
the streamlines in such a way that maximum amount of air gets compressed and
enters the scramjet at relatively high velocity. Thus this phenomenon can help
the working of scramjet as well as reduce the friction.
Delta or double delta wings are
used in aircrafts flying at speeds more than Mach 2. Here we use double delta
wings which give lift to the plane as well as it is used to guide the
streamlines of air. The scramjets are placed at the junction of the two deltas.
The first delta is used to divert the streamlines and the second delta gives
lift and balance.
Steps to increase the mass of
air entering the Scramjet.
In following steps it can be
seen that how plasma and double delta wings increase the amount of air entering
the scramjet. There are mainly three streamlines of air which are shown by red,
green and purple lines in the diagram (Fig.7). Moreover it can be clearly seen
that how do they deviate in the following steps.
1. When the plasma is radiated it cuts through the air and causes
disturbance in form of shock waves. These waves in turn cause change in path of
the nearby streamlines and thus the streamlines of air deviate from straight
path. These deviated streamlines are shown by red arrows.
2. We use a double delta wing in this plane. This is a very
important part because of the following reasons:
- When the air reaches the first delta, it deviates and its
motion is parallel to the sides of the first delta. (.lines are shown in
purple ink.)
- Now the scramjets are located at the point where the first
delta and the second delta meet.
- This is done because the stream lines which are deviated by
the first delta which are running parallel to its sides are directly
introduced in the scramjet.
3. Moreover certain streamlines are not deviated from the path
because they are far away from the point of impact of plasma with air (These
are shown by .green arrows..)
4. Now scramjets are located at such a place below the wing that
these two types of treamlines .deviated. and .non-deviated. meet.
Due to this maximum amount of
air will be entering in the scramjet with large amount of pressure.
Thus the scramjet can attain
greater speed and can ignite at lesser speed.
Due to radiation of plasma, the
streamlines are deviated as discussed earlier and so they become parallel to
the surface of the body of the plane. ( Fig.7). As their motion becomes
parallel to the fuselage of the plane, very less amount of resistance or force
of friction due to air is experienced. Hence the plane can move smoothly
without much resistance of air.
2nd challenge:
The basic limitation of
scramjet engine is that it cannot start from rest. Moreover the engine will
stop when the plane comes below the speed of Mach 2. So there is a need of gas
turbine engine to be used in the range of Mach 0 to Mach 2 or 3. The gas
turbine engine should be located in such a place that it does not hinder the
function of Scramjet. So the most suitable place for it is in the
fuselage.
• Location
of Gas Turbine Engine:
i. There is one gas turbine engine in the fuselage behind the
cockpit. (Fig8.)
ii. There are three ports below the cockpit for inlet of air to the gas turbine engine. The
air is sucked by the turbojet engine through these ports.
iii. When the scramjet starts, the flap on the
ports close so that no air enters gas turbine engine and is diverted in desired
way.
• Sequence of operations :
i. The
gas turbine engine starts the plane from rest and takes the plane up to Mach 2.
ii. Now
at Mach 2, the scramjet ignites and covers the range of speed up to Mach 15-20.
iii. Simultaneously plasma is radiated and
efficiency of scramjet is increased so that speed of plane goes beyond Mach 20.
iv. Similarly while landing when the plane reaches Mach 3 or Mach
2, gas turbine engine is ignited and plane lands on its support. At this time
the scramjets are not working.
CONCLUSION:
Reviewing the functioning of
turbojet engine, and scramjet engine, the combination of both is very useful in
flying the plane at hypersonic speed from rest. With the use of plasma and
double delta wings, scramjet.s efficiency can be highly enhanced. Thus the
challenge of flying at hypersonic speed at higher altitude and space traveling
can be met.
REFERENCE:
1.
Trinklein, F.E. and Huffer,
C.M., Modern Space Science, New York
2.
Anderson
3.
Alasu, Valan, A, Turbo
Machines, Vikas publishing house (Pvt) LTD.
4.
Khajuria, P.R. and Dubey, P.R.,
Gas turbines and propulsive system, Dhanpat Rai
Publications (P) LTD. (1992, 97)
5.
Yahya, S.M, Turbines,
compressors and fans, New Delhi, Tata McGraw Hill Publishing Company limited.
(1983), reprint (1997)
6.
Miles, E.R.C., Supersonic
Aerodynamics (A theoretical introduction), New York, Dover Publications, Inc.
(1950).
WEBSITES:
www.nasa.org ,
www.jetpropulsion.com ,
www.plasma.com
www.aviationhistory.com/engines/ramjet.htm,
www.space.com/missionlaunches/hyshot_020816.html,
www.physics.uq.edu.au/lp/lasdiag/scram.html,
oea.larc.nasa.gov/PAIS/Hyper-X.html,
www.mech.uq.edu.au/hyper/hyshot,
www.time.com/time/2002/inventions/tra_scramjet.html,
news.bbc.co.uk/hi/english/sci/tech/newsid_1629000/1629739.stm.
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