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Hypersonic Speeds to Scramjets
One thing that everybody loves to be is to be fast. The faster we do things, the more things we get done,
the more time we spare, and obviously the more time we waste. Scientists and even common people all
around the world have been trying for decades to make products that make life of humans faster. There
are small things that we make such as toasters or washing machines or even big things that we observe
like high speed formula one cars, or even spaceships that speed up things a bit. We are in Aerospace
Engineering and we really don’t care about those small products like toasters or washing machines. We
work on high speed vehicles that are basis of the next generation space travel.
Today, I will talk about a few records of speeds achieved by mankind over the past few decades and the
classification of speed. Then, we will look at certain aircraft engines that reach those speeds, and then
the spacecraft modules. These might just be a review to some but you do still need the fundamentals of
basic engines once you are in Aerospace Engineering.
So, Hypersonic Speeds to Scramjets is the title. The main term that reaches our mind when we say speed
in terms of
space vehicles is
“Mach”. 1 Mach
is equal to the
speed of sound
as many of you
guys know. 1
Mach at the
ground level on
Earth is 333m/s
or roughly 1200
kmph. At an
altitude of 10
km, the speed
changes to
around 300 m/s
or 1000 kmph. This is the general area where our aircrafts fly at.The following graph shows us how the
temperature changes with altitude. This graph is spread into layers which represent the layers of our
atmosphere. At an altitude of about 10 km the temperatures drop and soar high again until an altitude
of 40 km. There is then a constant drop of temperature past close to 50 km of altitude. Another graph
below states the Speed of sound relation with the altitude. As you go higher on the altitude, the speed
of sound also decreases, then increases, and continues to decrease. You can see that both the graphs of
Temperature and the Speed of Sound behave the same way. Temperature is also a condition that affects
the speed of sound. Heat, like sound is a form of kinetic energy where molecules at higher temperatures
have more energy, thus having the ability to vibrate faster. Hence, the sound waves travel more quickly.
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Hypersonic Speeds occur at Mach speeds of 5+. The molecules of air at these speeds behave in a much
different way compared to the subsonic and lower supersonic speeds, that they deserve an entire new
classification called the hypersonic speeds. In these speeds, the process of matter dissociation and
ionization occurs and also where normal Ramjets don’t produce net thrust.
How do we get to these speeds? To understand that question, we must start from the base. We classify
speeds here into three: land, air, and space. A normal daily use car usually hits a maximum speed of 100
kmph or roughly 0.08 Mach, whereas, a Formula One car is capable of reaching a speed of 400 kmph or
around 1/3 of the speed of sound. A land record of the highest speed was set by ThrustSSC by Andy
Green reaching a speed of close to 1 Mach which is the speed of sound in the year 1997. Speaking of Air
vehicles, a normal light aircraft like the CESNA travels at 300 kmph or 0.25 Mach. A long haul passenger
aircraft travels at around 1000 kmph or 0.85 kmph. Bell X-1 was one of the first aircrafts of the NASA to
reach a speed of 1050 kmph or 1.06 Mach. This aircraft is partially rocket powered for propulsion.
Concorde currently holds the position of the fastest passenger aircraft at a speed of 2000 kmph or
roughly 2 Mach. This aircraft was a result of a British-French alliance in their Aeronautical Industries. A
military fighter F-14 which is among the toughest aircrafts on this planet travels at a speed of 2500
kmph or 2.5 Mach whereas some military interceptors reach 3 Mach. One of the best military aircrafts is
the Lockheed SR-71 or the Blackbird flying at around 3.5 Mach. This aircraft is a combination of Ramjet
and a Turbojet where the turbojet is used for takeoff and a ramjet is used to continue the increase in the
speed from 3 Mach. The first ever hypersonic aircraft launched was the X-15 reaching speeds up to 7500
kmph or 6.7 Mach. The NASA X-43A is a scramjet engine aircraft that was able to reach a high speed of
10 Mach holding the position of the fastest aircraft to date. Considering the space vehicles, another set
of machines arise. The SpaceShip One is the first ever privately funded spacecraft reaching a speed of 3
Mach. Freedom – 7 was the first ever spaceflight that reached a speed of 8 Mach. The fastest ever
suborbital spacecraft was the HyShot launched by the University of Queensland, Australia. This aircraft
has reached a speed of 8 Mach and was launched at an altitude taken by another long haul aircraft. The
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first ever human spacecraft was the Vostok – 1 which reached a speed of 25 Mach reentry speed. A
space shuttle from the ISS reaches a speed of 28 Mach when reaching back to the base and it usually has
to travel twice around the world to reach a speed of landing as it is potentially hazardous to land at a
high speed. The Apollo mission had reached a reentry speed of 35 Mach. It was protected by a heat
shield when reentry which usually melts away as it reaches the ground. The Mars Curiosity had reached
a speed of 36 Mach. The fastest ever machine made by man was the Jupiter Galeleo which reached the
speed of 170000 kmph or 50 Mach.
To generate speed in an aircraft, you need an engine. The basic of all types of engines is the IC Engine
which has 4 strokes. These four processes are inlet, compression, ignition, and exhaust. These four
components are responsible in rotating a shaft and further a wheel. A similar mechanism can be seen on
a Turbojet Engine. The turbojet Engine also has four processes occurring in it similar to an IC Engine. The
intake, compression, combustion, and exhaust make up the working of the turbojet. The intake is
mechanism is where the air gets drawn into the engine and enters the compressors. The compressors
are made of both rotary and stator blades which pushes the air into the combustors. The fuel is injected
in the combustion chamber and is sent through the turbine and finally the exhaust nozzle. A limitation
on turbojets is that they operate at subsonic speeds in the combustion chamber. The engine flying at
supersonic speeds and the air intake being supersonic is required to be converted to subsonic before
reaching the combustion chamber. As the aircraft speed increases, the more subsonic the speeds need
to reach to the combustion chamber. The compressors increase the temperature and pressure. This is a
limitation as it slows the aircraft down. The main advantage of the turbojet engine is that it can be used
to take off from the land. It operates at a speed of 0 to 3.5 Mach or 3500 kmph.
The two main parts of the turbojet are the compressor and the turbine. The turbine rotates the
compressor through a shaft running axially through the engine. Removing the turbine and the
compressor will give us a Ramjet. A Ramjet is an engine with just three systems: the intake, combustion
chamber, and the exhaust. The only goal for an aircraft engine is to create a higher momentum at the
exhaust than the intake to produce thrust. The main disadvantage of a ramjet is that it cannot be used
to take off from land. It only operates at high speed intakes of air which can be achieved at Mach speed
of 3 to 6. This means that the ramjets can practically be operated when they are launched by another
system into the air. The ram effect is responsible for forcing he air into the engine and raising the
pressure and temperature required for good combustion. After the intake, the fuel is injected directly
into the entering high speed and high temperature air in order to cause a heavy flow leading to thrust.
This means that the compressors which were required to turn high speed to subsonic for entering into
the combustion chamber will no longer be required and it can be achieved by just reducing the area.
Executing a little bit slower speed can potentially lead to a Scramjet which stands of Supersonic
Combustion Ramjet. The Scramjet is an engine which also raises the pressure and temperature but at
supersonic speeds and burn fuel in the air which enters the combustion chamber from the intake. The
Flow is supersonic in the combustion chamber and the entire mechanism has no moving parts. The main
drawback is that it cannot be operated at low speeds or from ground during lift. It can only be operated
from a Mach speeds six and the limit of them has not yet been achieved. The other drawback is that
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heating of the surface occurs causing damage onto the outer layer of the engine. The main purpose of
any engine is to create thrust by maintaining a higher exhaust momentum than the intake.
One may wonder where Rockets would fit into picture in the classification described above. The Rockets
don’t follow the classification because they are “non-air-breathing” engines which carry their own fuel
and oxidizer. They are further classified into single and two stage rockets each varying in loads of both
fuel and body weight. The main advantage of using a rocket is that it can be operated at any
environment. But the disadvantage would be the high mass which means that more thrust needs to be
generated for a lift.
In conclusion, engines which run from the runway to a speed of 3 Mach would be considered as turbojet
engines, and Mach 3 to Mach 6 engines are ramjets, and hypersonic Mach 6+ engines are usually
scramjets. Since rockets don’t fall into any of these categories, they are considered to be “non-airbreathing” engines. They can start off from land and go to unknown speeds.
The bibliography is given on the presentation.