1. Development and Principles of Rocketry
Rocket Motors
Lecture Three

2. Action and Reaction Remember Newton’s 3rd Law “TO EVERY ACTION,
THERE IS AN EQUAL
AND OPPOSITE REACTION”
This applies to rocket motors . . .

3. Action and Reaction The gas in a balloon is under pressure
and can only escape in one direction:
through the neck of the balloon (called the “throat”)
The escaping gas produces an “action”
The balloon moves as a “reaction”
Reaction
The balloon moves in
the opposite direction
Action
Air leaves the balloon
at high velocity
Compressed
Gas

4. Action and Reaction Balloons are inefficient as rocket motors.
They run out of gas very quickly.
They don’t provide much “action” (thrust).
We can improve performance by:
A continuous gas supply to provide thrust for longer.
Heating the gas to raise its pressure.
Make it leave at higher speeds to increase thrust.
All of which are used in rocket motors.

5. The Cold Gas Motor Very Inefficient
The simplest form of rocket propulsion
When thrust is needed a valve is opened.
The gas escapes through a nozzle
where it is accelerated.
Cold gas motors are used in NASA’s
Man Manoeuvring Unit (MMU)
Valve
Nozzle
Compressed Gas
Tank
Advantages
Cheap, Simple & Safe
Useful where small
amounts of thrust are
needed for short periods
Disadvantages
Low thrust
Very Inefficient

6. Monopropellant Engines
Uses a single chemical that spontaneously ignites
in the presence of a catalyst.
Hydrogen Peroxide and Hydrazine are monopropellants.
When hydrogen peroxide contacts a platinum catalyst
it spontaneously decomposes to water and oxygen,
creating superheated steam and oxygen.
Valve
Nozzle
Propellant
Tank
Catalyst
Advantages
Simplicity
Relatively high efficiency
Quick response
Easy to control
Disadvantages
Chemical can be very
dangerous to manufacture,
transport and store.

7. Bi-Propellant Engines
Are a form of liquid propellant engine
which require no ignition system.
Two chemical reagents spontaneously ignite when they
come into contact with each other – a Hypergolic Reaction
Nitrogen tetroxide and hydrazine are useful reagents
in bi-propellant engines.
Combustion Chamber
Reagent Tank
Valve
Nozzle
Advantages
Simple
Reliable
Well proven technology
Disadvantages
Chemical can be very
dangerous to manufacture,
transport and store.

8. Hybrid Motors Hybrid motors use a pressurised oxidiser
in liquid or gaseous form and a solid propellant
The solid propellant burns while the oxidiser is flowing,
but stops when the valve closes.
Plastics are commonly used as the propellant,
and LOX or Nitrous Oxide as the gaseous oxidiser.
Solid Propellant
Liquid Oxidiser
LOX
Combustion Chamber
Nozzle
Valve
Solid propellant
Advantages
Simple construction
Low cost to build & operate
Materials are safe to
manufacture and transport
Disadvantages
Some residual propellant
is usually left
Difficult to relight

9. Solid Fuel Rocket Motor
Is the most common type of rocket motor.
The igniter sets fire to the surface of the propellant.
Hot gas fills the hollow core
and accelerates through the throat and nozzle.
The propellant burns outwards from the core to the case
As more gas is produced the mass flow rate increases,
so the thrust increases with time.
This is called a Progressive Burn.
Case
Throat
Igniter
Propellant Grain
Nozzle

10. Solid Fuel Rocket Motor
Is the most common type of rocket motor.
Some common applications are:
Missiles, Launch vehicles, Model rockets,
Safety systems, Ejector seats, Flares & Fireworks.
Case
Throat
Igniter
Propellant Grain
Nozzle
Advantages
Easy to manufacture
Simple to use
Propellant is relatively safe
to store and transport
Disadvantages
Once ignited the propellant
cannot be controlled
or extinguished.

11. Solid Fuel Rocket Motor
The shape of the propellant
gives specific thrust profiles.
Changing the propellant shape creates thrust profiles that
Increase with the burn - Progressive
Remain constant with the burn - Neutral
Decrease with time - Regressive
Thrust with a regressive burn
reduces as the propellant burns
Core
Slotted
Star
Rod
Progressive Burn
Neutral Burn
Neutral Burn
Regressive Burn
Thrust
Time
Thrust
Time
Thrust
Time
Thrust
Time

12. Liquid Engines “Regenerative Cooling” DO NOT have Propellant grain
but DO HAVE a Turbine, Injectors and Pumps.
Fuel and Liquid Oxygen (LOX)
are pumped into the combustion chamber.
To prevent the throat and nozzle from melting,
cold, high pressure, Kerosene fuel is pumped
through narrow pipes to cool the steel.
This is called
“Regenerative Cooling”
Turbine
Injectors
LOX
Combustion
Chamber
Pumps
Throat
Nozzle
Fuel

13. Liquid Engines “Film Cooling” DO NOT have Propellant grain
but DO HAVE a Turbine, Injectors and Pumps.
Another cooling technique is to
inject a thin layer of turbine exhaust gasses
into the bell nozzle
This forms a film of cool gas between the nozzle walls and
the exhaust gas, preventing the nozzle wall from melting.
This is called
“Film Cooling”
Turbine
Injectors
LOX
Combustion
Chamber
Pumps
Throat
Nozzle
Fuel

14. Liquid Engines DO NOT have Propellant grain
but DO HAVE a Turbine, Injectors and Pumps.
Cooling does NOT affect the efficiency
of a liquid propellant engine
Turbine
Injectors
LOX
Combustion
Chamber
Pumps
Throat
Nozzle
Fuel
Advantages
Very efficient
High thrust is possible
Disadvantages
Extremely complex
Difficult to design
Expensive to design
and manufacture.

15. Electric Propulsion are not a type of chemical rocket motor.
Ion thrusters use electric and magnetic fields
to accelerate gas ions to very high velocity.
Ion thrusters therefore
are not a type of chemical rocket motor.
They only work in the vacuum of space,
not the atmosphere.
They are ideal as manoeuvring thrusters for satellites
where low thrust and high efficiency are important.
Ionization Chamber
Magnetic Field Coil
Anode Grid
Xenon
Electrons
Cathode
Beam

16. Electric Propulsion Ion thrusters use electric and magnetic fields
to accelerate gas ions to very high velocity.
Ionization Chamber
Magnetic Field Coil
Anode Grid
Xenon
Electrons
Cathode
Beam
Advantages
Simple
Light weight
High velocity.
Disadvantages
Low thrust
Need a lot of electrical power
Can’t work in the atmosphere

17. Check of Understanding
On the diagram of a solid-fuel rocket
which arrow represents
the solid propellant charge? D C B A

18. Check of Understanding
What is the burn profile of
a core burning solid motor
Regressive
Neutral
Progressive
Constant

19. Check of Understanding
Which of the following statements
about a solid propellant motor
with a regressive burn is true?
The thrust can be controlled
as the propellant burns
The thrust increases
as the propellant burns
The thrust remains constant
as the propellant burns
The thrust reduces
as the propellant burns

20. Check of Understanding
Kerosene rocket fuel
is pumped round the exhaust nozzles to
Cool the nozzles
Cool the nozzle and the fuel
Cool the fuel and heat the nozzle
Heat the fuel and the nozzle

21. Check of Understanding
On the diagram of a solid fuel rocket
what is indicated by arrow C?
Motor Case
Igniter
Nozzle
Charge

22. Check of Understanding
Which of the following components
is not found in a liquid propellant engine?
Turbine
Propellant Grain
Pump
Injector

23. Check of Understanding
The main disadvantage of monopropellant
and bipropellant engines is?
They are very inefficient
compared to solid rocket motors
The engines are very complex to build
The chemicals are dangerous
to transport and store
The motors are very heavy
relative to the thrust they produce

24. Check of Understanding
Regenerative cooling is used to:
Increase the pressure
of the propellant
Improve the efficiency
of the injectors
Prevent the combustion chamber
and nozzle from melting
Raise the temperature
of the LOX

25. Check of Understanding
Which of these statements is NOT true?
Cold gas motors are cheap,
simple and safe
Cold gas motors are
very efficient
Cold gas motors are
used on small satellites
Cold gas motors do not
provide much thrust
and are very inefficient

26. Check of Understanding
Which of the following sentences describes
a hybrid motor?
It uses a pressurised oxidiser
in liquid or gaseous form
and a solid propellant
It uses pressurised gaseous
propellant and oxidser
It uses two chemicals
which spontaneously react
It uses a solid oxidiser
and a liquid propellant

27. Check of Understanding
Which of the following does NOT
affect the efficiency of a liquid propellant engine:
The cooling system
The choice of propellant
The temperature of combustion
How well the propellant
and LOX are mixed

28. Check of Understanding
Which of the following is NOT
a type of chemical rocket motor?
Ion thrusters
Cold gas motors
Monopropellant engines
Solid propellant motors

29. Check of Understanding
In which part(s) of a rocket motor
are the exhaust gases accelerated?
The nozzle
The throat
The throat & the nozzle
None of the above

30. Development and Principles of Rocketry
End of Presentation