Rocket Basic And Space Propulsion

ROCKET AND SPACE PROPULSION
Topics Covered
Operating principle – specific impulse of a rocket – internal ballistics –
performance considerations of rockets – types of igniters- preliminary
concepts in nozzle-less propulsion – air augmented rockets – pulse
rocket motors – static testing of rockets & instrumentation –safety
considerations
7/22/2019 www.sagarbhandari.com.np 1

Operating principle of Rockets
A rocket in its simplest form is a chamber
enclosing a gas under pressure. A small opening at
one end of the chamber allows the gas to escape,
and in doing so provides a thrust that propels the
rocket in the opposite direction.
A good example of this is a balloon. Air inside a
balloon is compressed by the balloon's rubber
walls. The air pushes back so that the inward and
outward pressing forces are balanced. When the
nozzle is released, air escapes through it and the
balloon is propelled in the opposite direction

Newton's Laws of Motion.
1. Objects at rest will stay at rest and objects in motion will
stay in motion in a straight line unless acted upon by an
unbalanced force.
2. Force is equal to mass times acceleration.
3. For every action there is always an opposite and equal
reaction.

A rocket can lift off from a launch pad only when it expels gas out
of its engine. The rocket pushes on the gas, and the gas in turn
pushes on the rocket. The whole process is very similar to riding a
skateboard
An unbalanced force must be exerted for a rocket to lift off from a
launch pad or for a craft in space to change speed or direction
(first law). The amount of thrust (force) produced by a rocket
engine will be determined by the mass of rocket fuel that is
burned and how fast the gas escapes the rocket (second law). The
reaction, or motion, of the rocket is equal to and in the opposite
direction of the action, or thrust, from the engine (third law).

HOW DO ROCKETS WORK?

Specific Impulse Of A Rocket

From Newton's second law of motion, we can define a force to be the change in
momentum of an object with a change in time. Momentum is the object's mass
times the velocity. When dealing with a gas, the basic thrust equation is given as:
F = mdot e * Ve - mdot 0 * V0 + (pe - p0) * Ae
Thrust F is equal to the exit mass flow rate mdot e times the exit velocity Ve minus
the free stream mass flow rate mdot 0 times the free stream velocity V0 plus the
pressure difference across the engine pe - p0 times the engine area Ae.
For liquid or solid rocket engines, the propellants, fuel and oxidizer, are carried on
board. There is no free stream air brought into the propulsion system, so the thrust
equation simplifies to:
F = mdot * Ve + (pe - p0) * Ae
where we have dropped the exit designation on the mass flow rate.

Using algebra, let us divide by mdot:
F / modt = Ve + (pe - p0) * Ae / mdot
We define a new velocity called the equivalent velocity Veq to be the velocity on
the right hand side of the above equation:
Veq = Ve + (pe - p0) * Ae / mdot
Then the rocket thrust equation becomes:
F = mdot * Veq

The total impulse (I) of a rocket is defined as the average thrust times
the total time of firing. On the slide we show the total time as "delta t".
(delta is the Greek symbol that looks like a triangle):
I = F * delta t
Since the thrust may change with time, we can also define an integral
equation for the total impulse. Using the symbol (Sdt) for the integral,
we have:
I = S F dt
Substituting the equation for thrust given above:
I = S (mdot * Veq) dt
Remember that mdot is the mass flow rate; it is the amount of exhaust
mass per time that comes out of the rocket. Assuming the equivalent
velocity remains constant with time, we can integrate the equation to
get:
I = m * Veq

where m is the total mass of the propellant. We can divide this equation by
the weight of the propellants to define the specific impulse. The word
"specific" just means "divided by weight". The specific impulse Isp is given
by:
Isp = Veq / g0
where g0 is the gravitational acceleration constant (32.2 ft/sec^2 in English
units, 9.8 m/sec^2 in metric units). Now, if we substitute for the equivalent
velocity in terms of the thrust:
Isp = F / (mdot * g0)
Mathematically, the Isp is a ratio of the thrust produced to the weight flow of
the propellants. A quick check of the units for Isp shows that:
Isp = m/sec / m/sec^2 = sec

Internal ballistics
• is the study of the propulsion of a projectile.
• For rocket-propelled projectiles, internal ballistics covers the
period during which a rocket motor is providing thrust

Performance Considerations Of Rockets
• Chamber pressure
• Ambient pressure, altitude
• Nozzle expansion area ratio
• Nozzle shape and exit angle
• Propellant and their composition
• Initial ambient temperature

Rocket Igniters
• Igniters are defined as devices or
assemblies that release heat and
thereby initiate reaction of main
propellants.
• Igniters derive power from external
source or from a limited quantity of
stored energy.
• Igniters are mostly triggered by an
electrical signal

Types of Igniters
• Solid Propellant igniters
• Pyrogen Igniters
• Pyrotechnic Igniters
• Liquid Propellant igniters
• Hypergolic
• Non hypergolic
• Gaseous Igniters

Air Augmented Rockets

• air augmented rockets denotes mixing of air with rocket exhaust.
• Two propulsion system with a common combustion chamber operate
in sequence.
• Also known as Ducted rocket

Pulsed rocket motor
A pulsed rocket motor is typically defined as a multiple pulse solid-fuel
rocket motor.
This design overcomes the limitation of solid propellant motors that
they cannot be easily shut down and reignited.
The pulse rocket motor allows the motor to be burned in segments (or
pulses) that burn until completion of that segment. The next segment
(or pulse) can be ignited on command by either an onboard algorithm
or in pre-planned phase. All of the segments are contained in a single
rocket motor case as opposed to staged rocket motors

Needs for Pulsed rocket
• The pulsed rocket motor is made by pouring each segment of
propellant separately. Between each segment is a barrier that
prevents the other segments from burning until ignited. At ignition of
a second pulse the burning of the propellant generally destroys the
barrier.
• The benefit of the pulse rocket motor is that by the command ignition
of the subsequent pulses, near optimal energy management of the
propellant burn can be accomplished. Each pulse can have different
thrust level, burn time, and achieved specific impulse depending on
the type of propellant used, its burn rate, its grain design, and the
current nozzle throat diameter.

Micro Pulsed Rocket

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Rocket Basic And Space Propulsion

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