NASA’S SPACE LAUNCH SYSTEM:A HEAVY-LIFT PLATFORM FOR ENTIRELY NEW MISSIONS

GOKUL LAKSHMANAN

M.TECH THERMAL AND FLUID ENGINEERING

Contents

• Cryogenic Engine

• Construction

• One-dimensional analysis of gas flow in rocket engine

nozzles

• Rocket cycles

• Liquefaction and storage of cryogenic fuel

• Combustion Zones in Thrust Chamber

• Regenerative Cooling

Cryogenic Engine

• 4 engines

• Used in core stage

• Initial flights will use engines left over from the Space

Shuttle program

• Later flights use cheaper version of the engine not

intended for reuse

• Use LH / LOX

• LH at -2530c and LOX at -1830c

• Provides 7440KN thrust

Construction

• Combustion chamber

• Pyrotechnic initiator (igniter): zirconium – potassium

perchlorate mixture

• Fuel injector and fuel pumps

• Oxidizer pumps

• Gas turbine

• Fuel tanks

• Rocket engine nozzle

ROCKET ENGINE NOZZLE

ONE-DIMENSIONAL ANALYSIS OF GAS FLOW IN ROCKET

ENGINE NOZZLES

The analysis of gas flow through de Laval nozzles involves a number of assumptions:

1. The combustion gas is assumed to be an ideal gas.

2. The gas flow is isentropic i.e., at constant entropy, as the result of the assumption of non-viscous fluid, and adiabatic process.

3. The gas flow is constant during the period of the propellant burn.

4. The gas flow is non-turbulent

5. The flow behavior is compressible since the fluid is a gas.

When there is no external work and heat transfer, the

energy equation becomes

Differentiation of continuity equation,

and dividing by the continuity equation

We have;

For isentropic process ds=0 and combining equations

Differentiation of the equation and dividing the

results by the equation

Obtaining an expression for dU/U from the mass balance

equation

Rearranging equation

Recalling that

or

So the final relation becomes

Flow characteristics

• At entry into the nozzle flow is subsonic

• As the throat constricts, the gas is forced to accelerate

until at the nozzle throat

• At throat velocity becomes sonic

• From the throat the cross-sectional area increases

• The gas expands and the linear velocity becomes

progressively more supersonic.

The linear velocity of the exiting exhaust gases can be

calculated using the following equation

M = the gas molecular mass, kg/kmol

ɣ = cp/cv (isentropic expansion factor)

Pe = absolute pressure of exhaust gas at nozzle exit

p = absolute pressure of inlet gas

Types of Expansion

1. Under expanded

2. Ambient

3. Over expanded

4. Grossly over expanded

Expansion varies with altitude

If the exit pressure is too low jet separate from the nozzle.

This is often unstable, and the jet will generally cause large

off-axis thrusts and may mechanically damage the nozzle.

Specific Impulse

The thrust of a rocket engine nozzle can be defined as

Or;

The specific impulse Isp is the ratio of the of thrust produced

to the weight flow of the propellants.

Rocket cycles

• Pressure-fed engine cycle

• Expander cycle

• Gas-generator cycle

• Staged combustion cycle

Pressure-fed engine

• Propellant tanks are

pressurized to supply fuel

and oxidizer to the engine,

eliminating the need for

turbo pumps.

• Pressurized Helium is often

used

Expander cycle

• Heat from the nozzle and

combustion chamber

powers the fuel and oxidizer

pumps.

• Uses a gas generator of

some kind to start the

turbine and run the engine

until the heat input from the

thrust chamber and nozzle

skirt in sufficient enough to

run the turbine

Gas-generator cycle

• Some of the fuel and

oxidizer is burned

separately to power the

pumps and then discarded.

Most gas-generator engines

use the fuel for nozzle

cooling.

• Some of the fuel used to

cool the nozzle and

combustion chamber.

Staged combustion cycle

• All of the fuel and a portion

of the oxidizer are fed

through the pre-burner,

generating fuel-rich gas.

After being run through a

turbine the gas is injected

into the combustion

chamber and burned.

• Advantage: No loss of heat

compared to gas generator

cycle

• USED IN SPACE LAUNCH

SYSTEM

How to Liquefy cryogenic fuel

Critical temperature for hydrogen -2530c

1. At first gaseous hydrogen is compressed to 180 atm.

2. Compressed gas is cooled by allowing it to expand

rapidly.

3. The cooled expanded gas then passes through a heat

exchanger where it cools the incoming compressed gas

4. The cycle is repeated until hydrogen liquefies

How to store cryogenic fuel

• Cryogenic Dewar wall: Vacuum flask used for storing cryogenic fuels

• Have walls constructed in two or more layers of silver with a high vacuum maintained between the layers.

• Provides very good thermal insulation

• Reduces the rate at which the contents boils away

• Dewar allow the gas to escape through an open top to avoid risk of explosion

• More sophisticated Dewar trap the gas above the liquid, and hold it at high pressure

• This increases the boiling point of the liquid, allowing it to be stored for extended periods

The cryogenic Engine

• Fuel and oxidizer from the rocket's core stage will flow

directly into the fuel lines

• The fuel and oxidizer each branch out into separate paths

to each engines

• In the engine fuel and oxidizer follows various paths to

reach into the combustion chamber

Fuel Injection

• Injector introduce and meter the flow of liquid propellants

to the combustion chamber

• Hole pattern of the injector is closely related to feed

passages within the injector

• A large manifold allows low passage velocities and good

distribution.

• A small manifold allows for a lighter weight injector and

reduces the amount of "dribble" flow after the main valves

are shut.

• Dribbling results in afterburning, which is an inefficient

irregular combustion that gives a little "cutoff" thrust after

valve closing.

• For accurate vehicle velocity requirements, the cutoff

impulse has to be very small.

Types

1. Impinging-stream-type

2. Non-impinging

3. Coaxial injector

1. Impinging-stream-type : Fuel and oxidizer streams

impinge upon each other. Impingement aids atomization

of the liquids into droplets also its distribution.

2. Non-impinging type: relies on turbulence and diffusion

to achieve mixing

3. Coaxial injector It has been used for liquid oxygen and

gaseous hydrogen. This type of injector is used by SLS

Combustion Zones in Thrust Chamber

1. Injection/Atomization Zone

2. Rapid Combustion Zone

3. Stream Tube Combustion Zone

Injection/Atomization Zone

1. Injection, atomization and

vaporization occurs here

2. Fuel and Oxidizing agent are

introduced in this zone at

velocities between 7 and 60

m/sec

3. The individual jets break up

into droplets by impingement of

one jet with another

4. Heat is transferred to the

droplets by radiation from rapid

combustion zone and by

convection from moderately hot

gases in the first zone.

5. Chemical reactions occur in

this zone, but the rate of heat

generation is relatively low

• If one of the propellants is a gas:

• this occurs with liquid oxygen and gaseous hydrogen

propellant from thrust chambers or pre-combustion

chambers

• The gas usually has a much higher injection velocity

(above 120 m/sec) than the liquid propellant

• This cause shear forces to be imposed on the liquid jets

• Thus Coaxial injectors are used in such cases

Rapid Combustion Zone

1. Intensive and rapid chemical

reactions occur at increasingly

higher temperature

2. The mixing is aided by local

turbulence and diffusion of the

gas species

3. The rate of heat release

increases greatly and this

causes the specific volume of

the gas mixture to increase

4. Axial velocity increase by a

factor of 100 or more.

5. Gas flows from hot sites to

colder sites.

6. Rapid fluctuations in pressure,

temperature, density and

radiation emissions occurs

Stream Tube Combustion Zone

1. Axial velocities are high (200 to

600 m/sec)

2. Streamlines are formed and

there is relatively little

turbulence

3. Residence time in this zone is

very short

4. Usually less than 10

milliseconds

5. Volumetric heat release being

approximately 370 MJ/m3sec

6. The higher temperature in the

chamber causes chemical

reaction rates to be several

times faster

Regenerative Cooling

• It is a configuration in which

some or all of the propellant is

passed through tubes around

the nozzle to cool the engine

• The heated propellant is then

fed into a special gas

generator or injected directly

into the main combustion

chamber

• This is done because the

nozzle material cannot

withstand the heat produced

by combustion

• So the fuel itself is used to

take away the heat