Propulsion, Types of Rocket Engines, Propellants and their classification, SPRE, LPRE, HPRE, Staging, Space Flights

1. Unit-5: Rocket Propulsion
Prepared by:
Ankur Sachdeva
Assistant Professor, ME

2. Introduction to Propulsion
• Propulsion is a method by which an object is
propelled in a particular direction.
• The word “propulsion” stems from the Latin word
propellere, where pro means forward or backward
and pellere means drive or push.
• Spacecraft must produce thrust which must be equal
to the drag force caused due to the fluid motion over
the body of this spacecraft and the gravitational
force.
• For accelerating the spacecraft, one needs to supply
higher thrust than that of drag forces and
gravitational force acting on it.
Ankur Sachdeva, ME, KIET Group of Institutions

3. Aeropile by Hero
It is really an interesting device for
demonstrating the principle of
reactive thrust, which is the basis of
rocket propulsion.
Ankur Sachdeva, ME, KIET Group of Institutions

4. History of Rocket Engines
• The real rocket was invented by the Chinese around the tenth century AD while experimenting
with gunpowder and bamboo.
• The gunpowder was discovered in the ninth century AD by a Taoist alchemist.
• Subsequently, Feng Jishen managed to fire a rocket using gunpowder and bamboo
• A Chinese scholar, Wan Hu, had developed a rocket sled that comprised of a series of rockets
attached to the seat.
Ankur Sachdeva, ME, KIET Group of Institutions

5. Classification of Propulsive Devices
Ankur Sachdeva, ME, KIET Group of Institutions

6. Comparison Between Air-Breathing and
Rocket Engines
Ankur Sachdeva, ME, KIET Group of Institutions

7. Types of Rocket Engines
• On the basis of application:
• Space Rockets
• Military Rockets
• Weather Rockets
• Aircraft propulsion
• On the basis of no. of stages:
• Single stage
• Multi stage
• On the basis of size and range:
• Small-range small rockets
• Large-range large rockets
Ankur Sachdeva, ME, KIET Group of Institutions

8. Chemical Rocket Engines
• In case of chemical rocket engines, chemical energy released during the burning of fuel and
oxidizer is used to raise the temperature and pressure of the gas which is expanded in a CD nozzle
to produce thrust.
• Generally, the hot gases at high pressure are accelerated to high supersonic velocities in the range
of 1500–4000 m/s for producing thrust.
• both fuel and oxidizer are being carried along with the engine unlike in air-breathing engines.
• Based on the physical state of the propellant (fuel and oxidizer), chemical rocket engines can be
broadly divided into three categories:
(1) solid propellant,
(2) liquid propellant,
(3) hybrid propellant.
Ankur Sachdeva, ME, KIET Group of Institutions

9. Solid Propellant Rocket Engines
• Solid-propellant rocket engine (SPRE) is one of the oldest non-air-breathing
engines.
• The solid propellant composition, which was initially black powder, underwent
a series of changes with time.
• Propellant, which mainly consists of fuel, oxidizers, and various additives, is
entirely stored within the combustion chamber in the form of blocks of definite
shape called grain and is supported by the walls.
• Grain contributes to around 80%– 95% of the total mass of an SPRE.
• The igniter initiates the combustion process on the surface of the propellant
when actuated with the help of an electrical switch.
• As a result, the propellant grains will start burning and filling the empty
combustion chamber, hence building up the chamber pressure.
• Subsequently, the high-temperature and high-pressure gases are expanded in the
supersonic nozzle to produce the requisite thrust.
• Solid rocket engine is considered to be a non-air-breathing vehicle without any
moving parts
Ankur Sachdeva, ME, KIET Group of Institutions

10. Solid Propellant Rocket Engines
Advantages
• It is simple to design and develop.
• It is easier to handle and store unlike liquid
propellant.
• Detonation hazards of many modern SPREs are
negligible.
• Better reliability than Liquid Propellant Rocket
Engine (LPRE) (>99%).
• Development and production cost of SPREs is
much smaller than that of LPREs, especially in the
high-thrust bracket
Disadvantages
• It has lower specific impulse compared to
LPREs and hybrid propellant rocket engines
(HPREs).
• It is difficult to turn off its operation unlike in
an LPRE.
• Transport and handling of solid propellants are
quite cumbersome.
• The cracks on the propellant can cause an
explosion.
Ankur Sachdeva, ME, KIET Group of Institutions

11. Liquid Propellant Rocket Engines
• Around 1927, an American professor, Robert Goddard, had designed and developed an LPRE.
• In addition to having a liquid form, this propellant can be stored in a separate tank and can be
controlled easily, and hence thrust can be varied easily unlike in an SPRE.
• As LPREs are stored in separate tanks unlike SPRE, one can achieve a higher level of thrust and is
thus considered to be more powerful than an SPRE. Therefore, it is preferred for large spacecraft
and ballistic missiles.
• Both fuel and oxidizer propellants are stored separately in special tanks at high pressure.
• The pressurized liquid propellants are converted into spray consisting of arrays of droplets with the
help of atomizers.
• An igniter is used to initiate the combustion process on the surface of the propellant.
• As a result, the propellant will start burning and fill up the empty thrust chamber, thereby building
up pressure in the chamber
• High-temperature and high-pressure gases are expanded in a CD nozzle to produce the requisite
thrust.
Ankur Sachdeva, ME, KIET Group of Institutions

12. Liquid Propellant Rocket Engines
Advantages
• An LPRE can be reused.
• It provides greater control over thrust.
• It can have higher values of specific impulse.
• It can be used for long-duration applications.
• It is easy to control this engine as one can vary the
propellant flow rate easily.
• The heat loss from the combustion gas can be
utilized for heating the incoming propellant.
Disadvantages
• This engine is quite complex compared to the
SPRE.
• It is less reliable as there is a possibility of
malfunctioning of the turbopump injectors and
valves.
• Certain liquid propellants require additional safety
precaution.
• It takes much longer to design and develop.
• It becomes heavy, particularly for short-range
application.
Ankur Sachdeva, ME, KIET Group of Institutions

13. Hybrid Propellant Rocket Engines
• This engine can use both solid and liquid types of propellants.
• Most widely used propellant combination is a liquid oxidizer along with a solid
propellant.
• Only the oxidizer propellant in the present example is stored in a special tank under
high pressure.
• The pressurized propellants are converted into spray consisting of arrays of droplets
with the help of atomizers.
• It consists of major components, namely, a propellant feed system, a combustion
chamber, a solid fuel grain, an igniter system, and a nozzle.
• Some of the propellant evaporates due to the recirculation of hot gases and comes into
contact with the gaseous fuel that emanates from the solid fuel grains due to pyrolysis
• The combustion products start burning and fill the empty thrust chamber, thereby
building up pressure inside the chamber.
Ankur Sachdeva, ME, KIET Group of Institutions

14. Hybrid Propellant Rocket Engines
Advantages
• An HPRE can be reused.
• It provides greater control over thrust.
• It has relatively lower system cost compared
to the LPRE.
• It can have higher values of average specific
impulse compared to the SPRE.
Disadvantages
• This engine is quite complex compared to the
LPRE.
• It takes much longer to design and develop.
• It becomes heavy, particularly for short-range
application.
• Certain liquid propellants require additional
safety precaution
Ankur Sachdeva, ME, KIET Group of Institutions

15. What is a Propellant
• A propellant consists of all the chemical materials, including fuel and oxidizer,
along with certain additives necessary for sustaining the combustion process to
produce high-pressure hot gases, that which are expanded in a nozzle to produce
thrust.
• Principal ingredients of a propellant are the fuel and the oxidizer.
• Fuel is a chemical substance that reacts with an oxidizer while releasing thermal
energy
Ankur Sachdeva, ME, KIET Group of Institutions

16. Classification of Propellants
Ankur Sachdeva, ME, KIET Group of Institutions

17. Classification of Propellants
• Homogeneous propellant:
• fuel and oxidizer are contained in the same molecule of the propellant.
• Heterogenous propellant:
• solid fuel and oxidizer retain their respective physical identities.
• Monopropellants:
• A liquid propellant that contains both the fuel and the oxidizer in a single chemical is called a
monopropellant.
• Example:
• Hydrogen Peroxide, Hydrazine, Nitroglycerine, and Nitromethane
• Bipropellants:
• A liquid propellant in which an oxidizer and a fuel are stored separately in the tanks and mixed in the
combustion chamber.
• Example:
• Liquid oxygen and liquid hydrogen, Liquid oxygen and kerosene.
• Hypergolic propellants:
• Liquid fuel and oxidizer react spontaneously without external ignition energy
• Nonhypergolic propellants:
• Suitable amount of ignition energy is provided to ignite the liquid fuel and oxidizer for combustion to take place
Ankur Sachdeva, ME, KIET Group of Institutions

18. GENERAL CHARACTERISTICS OF PROPELLANTS
• Propellant must have high chemical energy release so that it can have higher combustion temperature
leading to high characteristic velocity C*.
• It can have low molecular weight of combustion product leading to high exhaust velocity Ve and thus can
have high specific impulse Isp.
• It can have a high density such that large amount of chemical energy can be stored in the smallest volume
and thus can have a compact design.
• Easy to ignite even under low-pressure condition.
• Physically and chemically stable with respect to time.
• Smoke-free and nontoxic in nature.
• Easy and expensive to manufacture and handle during operation.
• Easily available and low price.
• Less prone to explosion hazard.
• Low emission level.
Ankur Sachdeva, ME, KIET Group of Institutions

19. Propellant Feed System
• The main function of the propellant feed
system is to supply the requisite amount of
propellant by transferring it from the
propellant tank to the thrust chamber at a
higher desired pressure by which a spray of
liquid propellant can be formed.
• For this purpose, the pressure of the
propellant in the feed system must be raised,
which can be accomplished by supplying
energy.
Ankur Sachdeva, ME, KIET Group of Institutions

20. Gas Pressure Feed System
• This gas pressure feed system is one of the simplest methods of pressurizing the propellant in a rocket engine in which
high-pressure gas is being used to force the liquid propellants in a very controlled manner from their respective tanks.
• It consists of a high-pressure gas tank, an on-off valve, a pressure regulator, propellant tanks, feed lines
• Propellant tanks are filled in the beginning followed by high-pressure gas tanks.
• Subsequently, a high-pressure gas valve is actuated to allow high-pressure gas to enter the propellant tank in a regulated
manner at constant pressure through check valves.
• Once desired pressure is established in the propellant tanks, the propellants can be fed through injectors into the
combustion chamber by actuating the propellant valves.
• Commonly, the pressurized gas is allowed to pass through, even after complete consumption of propellant, to scavenge
and clean the feed lines, particularly for reusable rocket engines, namely, space-maneuver rockets.
Ankur Sachdeva, ME, KIET Group of Institutions

21. Turbo Feed Pump System
• Turbopumps help rockets achieve a high power-to-weight
ratio by feeding pressurized propellant to the rocket’s
combustion chamber.
• The pump-fed system uses a turbopump to pressurize and
feed the propellants into the thrust chamber at relatively high
pressures.
• The turbopump typically consists of one or more pumping
elements driven by a turbine.
• The energy to power the turbine itself is provided by the
expansion of high-pressure gases, which are usually
mixtures of the propellants being pumped.
Ankur Sachdeva, ME, KIET Group of Institutions

22. Ignition in Solid Rocket Motors
• Solid Rocket Motors (SRMs) require an efficient ignition
system to start functioning.
• A separate ignition system, called an igniter, is assembled
in the rocket motor to achieve the task.
• Igniters for SRMs are basically of two types, viz.,
Pyrogen igniters used for large rocket motors of ballistic
missiles, and Pyrotechnic Igniters used for small rocket
motors.
• The propulsive force of a solid propellant motor is derived
from the combustion of solid propellant at high
temperature and pressure.
• The igniter induces the combustion reaction in a
controlled and predictable manner by generating heat flux
in the form of hot, dense gases that rapidly ignite the
propellant surface.
• The igniter also contributes towards the generation of a
certain minimum pressure inside the motor that is
adequate for stable and sustained combustion of the
propellant
Ankur Sachdeva, ME, KIET Group of Institutions

23. Staging in Rockets
• All rockets use the thrust generated by a propulsion system to overcome
the weight of the rocket.
• For full-scale satellite launchers, the weight of the payload is only a small
portion of the lift-off weight.
• Most of the weight of the rocket is the weight of the propellants.
• As the propellants are burned off during powered ascent, a larger
proportion of the weight of the vehicle becomes the near-empty tankage and
structure that was required when the vehicle was fully loaded.
• In order to lighten the weight of the vehicle to achieve orbital velocity, most
launchers discard a portion of the vehicle in a process called staging.
Ankur Sachdeva, ME, KIET Group of Institutions

24. Series Staging
• In serial staging, there is a small,
second-stage rocket that is placed on
top of a larger first-stage rocket.
• The first stage is ignited at launch and
burns through the powered ascent until
its propellants are exhausted.
• The first stage engine is then
extinguished, the second stage
separates from the first stage, and the
second stage engine is ignited.
• The payload is carried atop the second
stage into orbit.
• Serial staging was used on the Saturn
V moon rockets.
Ankur Sachdeva, ME, KIET Group of Institutions

25. Parallel Staging
• In parallel staging, several small first stages
are strapped onto to a central sustainer
rocket.
• At launch, all of the engines are ignited.
• When the propellants in the strap-ons are
extinguished, the strap-on rockets are
discarded.
• The sustainer engine continues burning and
the payload is carried atop the sustainer
rocket into orbit.
• Parallel staging is used on the Space Shuttle.
• The discarded solid rocket boosters are
retrieved from the ocean, re-filled with
propellant, and used again on the Shuttle.
Ankur Sachdeva, ME, KIET Group of Institutions

26. Terminal velocity
• An object which is falling through the atmosphere is subjected to two
external forces.
• One force is the gravitational force, expressed as the weight of the object. The
other force is the air resistance or drag of the object.
• The net external force (F) is equal to the difference between the weight and the
drag forces (W - D).
• When drag is equal to weight, there is no net external force on the object and the
object will fall at a constant velocity as described by Newton's first law of motion.
• The constant velocity is called the terminal velocity
Ankur Sachdeva, ME, KIET Group of Institutions

27. Terminal velocity
Ankur Sachdeva, ME, KIET Group of Institutions

28. Space Flights
• Rocket engines are employed to launch a spacecraft from the surface of the earth.
• The spacecraft moves around an orbit of the earth or any other planet governed by the local
gravitational field and momentum of the spacecraft.
• This orbit can be either circular or elliptic in shape.
• For the motion of the spacecraft in a circular orbit, the gravitational force Fg holding the spacecraft
can be determined by using Newton’s law of gravitation:
• Where M is the mass of the planet (earth), m is the mass of the space vehicle
• R is the distance between the two masses, and G is the universal gravity constant (G = 6.67 x 10−11 m3/kg s2)
• ω is the angular velocity of the mass m
Ankur Sachdeva, ME, KIET Group of Institutions

29. Space Flights
• The gravitational force Fg is balanced by the pseudo-centrifugal force mω2R.
• The angular velocity ω and orbital velocity Vo from can be evaluated easily as follows:
• The orbital velocity decreases nonlinearly with the radius of the orbit.
• The time period required to revolve around this orbit can be determined as follows:
• The time period per revolution increases with the radius of the orbit at a higher rate compared to
the orbital velocity.
Ankur Sachdeva, ME, KIET Group of Institutions