Jet Propulsion. Lecture Ujjwal K Saha, Ph. D. Department of Mechanical Engineering Indian Institute of Technology Guwahati

Transcription

1 Lecture - 27 Prepared under QIP-CD Cell Project Jet Propulsion Ujjwal K Saha, Ph. D. Department of Mechanical Engineering Indian Institute of Technology Guwahati 1

2 Propellant Properties 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, lowhazard manufacturing. High density allowing a small-volume motor. Low absorption of moisture, which often cause chemical deterioration. Non-toxic exhaust gases. 2

3 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. 3

4 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. Burning Rate is expressed for 294 K (prior to ignition) at a reference pc = 6.9 MPa r = = ( c ) f p ap n c Within certain limits Empirical relation 4

5 where a = n = temp. coefficient. (empirical constant) (influenced by ambient grain temperature) combustion index (burning rate exponent) (describe the influence of p c on ) r r = ap n c r DBP CP CDBP 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) 5

6 Observation: is sensitive to For stable operation, 0<n<1 High value of n implies a rapid change of burning rate with chamber pressure. Usually, 0.2< n <0.6 when n 1, r and p c becomes sensitive to one another. n 0, Also, r burning becomes unstable & can extinguish. zero change in burning rate over the wide range of pressure. n = 0, n 6

7 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 Performance Tactical Missiles/Large motors. 7

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9 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. 9

10 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. A solid-fuel rocket immediately before and after ignition 10

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14 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. 14

15 The idea of using 11-point starshaped 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. 15

16 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. 16

17 b γ Web thickness (b): Maximum thickness of the grain from the initial burning surface to the case. Web fraction (b f ): b b f = = radius Volumetric Loading fraction 2b diameter V f V = b = V c Propellant volume Chamber volume m = ρv b 17

18 Nozzles: Fixed Nozzle Movable Nozzle Submerged Nozzle Extendible Nozzle Blast-tube Nozzle 18

19 Mounting Options of Igniters: 19

20 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%), KClO 4 (71 %), binder (5 %) is done by stages. 20

21 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. 21

22 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. 22

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24 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. 24

25 Solid Rocket Features Lower specific impulses (compared with liquid rockets). Difficult to vary thrust on demand. Smokey exhausts. Performance affected by ambient temperature. 25

26 Solid Propellant Rocket for GW Rapier Jet velocity: m/s Most widely used in Guided Weapons Short, medium range (< 50 km) Simple, reliable, easy storage, high T/W 26

27 References 1. 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, 4 th 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. 27

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