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Jet Propulsion

Lecture - 27

Ujjwal K Saha, Ph. D.Department of Mechanical Engineering

Indian Institute of Technology Guwahati

Prepared underQIP-CD Cell Project

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High specific impulse high gas temperature and/or low molecular mass.

For minimum variation in thrust (or chamber pressure), the pressure or burning rate exponent and the temperature coefficient should be small.

Simple, reproducible, safe, low-cost, controllable, low-hazard manufacturing.

High density allowing a small-volume motor.

Low absorption of moisture, which often cause chemical deterioration.

Non-toxic exhaust gases.

Propellant Properties

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Grain Manufacture:

Extrusion: Ingredients are mechanically mixed and pushed through a die under high pressure. Limit to the grain size.

Casting: Ingredients are mechanically mixed, cast and cured (solidified). Large sizes can be made.

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Burning Rate is expressed for 294 K (prior to ignition) at a reference 6.9=cp MPa

( )==

cnc

r f pap

Burning Rate: Classical relations are only helpful in preliminary design, data extrapolation, and understanding the phenomena; however, supportive research has yet to predict the burning rate of a new propellant in a new motor.

Empirical relationEmpirical relation

Within certain limitsWithin certain limits

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where temp. coefficient. (empirical constant)(influenced by ambient grain temperature)combustion index (burning rate exponent)(describe the influence of on )

a =

n=cp

DBPCP

CDBP

r

= ncr ap

from 0.05 mm/s to 75 mm/s

High values of burning rate is difficult to achieve even with more catalyst, embedded wire or higher pressure (above 14 Mpa)

r

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Observation:is sensitive to

For stable operation, 0<n<1High value of n implies a rapid change of burning rate with chamber pressure.

Usually, 0.2< n <0.6when and becomes sensitive to

one another.burning becomes unstable & can extinguish.

Also, zero change in burning rate over the wide range of pressure.

1,n→

0,n→

0,=n

n

cp

r

r

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Grain Holding/Loading

Cartridge loaded grain (Free-standing grain)Manufactured separately by casting or extrusion and loaded.

Easily replaceable Low cost Used in small missiles/medium sized motors.

Case-bonded grain Propellant is directly cast in the motor case and bonded to the case.

Lower inert mass (no holding devices/pads) Difficult to manufacture Batter PerformanceTactical Missiles/Large motors.

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Burning Rates– The Burn Rate increases as both the pressure

and temperature rise.

– Classification by variation in burn rate:

– Regressive: As it burns, the burning surface area decreases.

– Neutral: The burning surface area remains constant

– Progressive: Burning surface area increases as it burns.

10A solid-fuel rocket immediately before and after ignition

The shape of the fuel block for a rocket is chosen for the particular type of mission it will perform. Since the combustion of the block progresses from its free surface, as this surface grows, geometrical considerations determine whether the thrust increases, decreases or stays constant.

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Fuel blocks with a cylindrical channel (1) develop their thrust progressively. Those with a channel and also a central cylinder of fuel (2) produce a relatively constant thrust, which reduces to zero very quickly when the fuel is used up. The five pointed star profile (3) develops a relatively constant thrust which decreases slowly to zero as the last of the fuel is consumed. The 'cruciform' profile (4) produces progressively less thrust. Fuel in a block with a 'double anchor' profile (5) produces a decreasing thrust which drops off quickly near the end of the burn. The 'cog' profile (6) produces a strong initial thrust, followed by an almost constant lower thrust.

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The idea of using 11-point star-shaped perforation is to increase the surface area of the channel, thereby increasing the burn area and therefore the thrust.

As the fuel burns the shape evens out into a circle. In the case of the SRBs, it gives the engine high initial thrust and lower thrust in the middle of the flight.

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Diagram of successive burning surface contours of a central cylindrical cavity with five slots. The length of these contour lines are roughly the same (within ± 15 %) indicating that the burning area is roughly constant.

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Web thickness (b): Maximum thickness of the grain from the initial burning surface to the case.

Web fraction (bf):

Volumetric Loading fraction

2radius diameterf

b bb = =

Propellant volumeChamber volume

bf

c

VVV

= =

b

γ

bm Vρ∴ =

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Nozzles: Fixed NozzleMovable NozzleSubmerged Nozzle Extendible Nozzle Blast-tube Nozzle

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Mounting Options of Igniters:

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Pyrotechnic Igniter:Uses solid explosives or energetic propellant as heat producing material. The common pallet basket design is a typical pyrotechnic igniter. Ignition of the main charge consisting of boron (24%), KClO4 (71 %), binder (5 %) is done by stages.

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Pyrotechnic Igniter:

First, on receipt of an electrical signal the initiator releases the energy of a small amount of sensitive powdered pyrotechnic housed within the initiator (known as the squib or the primer charge). Next, the booster charge is is ignited by heat released from the squib; and finally, the main propellant is ignited.

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Pyrogen Igniter:

This is basically a small rocket motor used to ignite a larger rocket motor. For pyrogen igniters, the initiator and the booster charge are similar to that of pyrotechnic igniters. Reaction products from the main charge impinge on the surface of the rocket motor grain, producing motor ignition.

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Solid Rocket Features

• High propellant density (volume-limited designs).

• Long-lasting chemical stability.

• Readily available, tried and trusted, proven in service.

• No field servicing equipment & straightforward handling.

• Cheap, reliable, easy firing and simple electrical circuits.

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Solid Rocket Features

• Lower specific impulses (compared with liquid rockets).

• Difficult to vary thrust on demand.

• Smokey exhausts.

• Performance affected by ambient temperature.

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Solid Propellant Rocket for GW

• Jet velocity: 1500-2600m/s

• Most widely used in Guided Weapons

• Short, medium range (< 50 km)

• Simple, reliable, easy storage, high T/W

Rapier

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References1. Hill, P.G., and Peterson, C.R., (1992), Mechanics and

Thermodynamics of Propulsion, Addison Wesley.2. Saravanamuttoo, H.I.H, Rogers, G.F.C, and. Cohen, H, (2001), Gas

Turbine Theory, Pearson Education.3. Oates, G.C., (1988), Aerothermodynamics of Gas Turbine and Rocket

Propulsion, AIAA, New York.4. Mattingly, J.D., (1996), Elements of Gas Turbine Propulsion, McGraw

Hill.5. Cumpsty, N.A., (2000), Jet Propulsion, Cambridge University Press.6. Bathie, W.W., (1996), Fundamentals of Gas Turbines, John Wiley.7. Treager, I.E., (1997), Aircraft Gas Turbine Engine Technology, Tata

McGraw Hill. 8. Anderson, J. D. Jr., (2000), Introduction to Flight, 4th Edition, McGraw

Hill. 9. M.J.L.Turner, (2000), Rocket and Spacecraft Propulsion, Springer.10. Sutton, G.P. and Biblarz, O., (2001), Rocket Propulsion Elements,

John Wiley & Sons.11. Zucrow, M.J., (1958), Aircraft and Missile Propulsion, Vol. II, John

Wiley.12. Barrere, M., Jaumotte, A., Veubeke, B., and Vandenkerckhove, J.,

(1960), Rocket Propulsion, Elsevier.

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