lecture_17_18

lecture_17_18 - 16.512, Rocket Propulsion Prof. Manuel...

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16.512, Rocket Propulsion Prof. Manuel Martinez-Sanchez Lecture 17-18: Solid Propellants: Other Topics Combustion of Solid Propellants For a general discussion, read Sutton, Chapter 13. A detailed model of combustion of composite propellants is presented next. Combustion of Composite Propellant (Ref: Guy Lengellé, Jean-Robert Duterque, Jean-Claude Godon, Jean-Francois Trubert, ONERA, “Solid Propellant Steady Combustion – Physical Aspects”. In AGARD-LS-180-Combustion of Solid Proplellants,1991 (TL507.N867, no. 180)) Composite propellants are heterogenous mixtures of oxidizer grains and powdered aluminum fuel, both embedded in a rubber-like binder, which is also a fuel. The most common oxidizer by far is Ammonium Perchlorate (AP), ( C O 4 NH 4 ), a crystalline substance with ρ = 1.95 g / cm 3 , c p = 0.31 cal / g / K , thermal diffusivity = cm () 36 2.5x10 4.55x10 −− =− p d 0 T C 2 / sec , and an estimated m.p of 835K. AP is ground to sizes from a few to around 100 m µ . The finer grades are dangerous, so grinding is done just prior to fabrication. AP has M = 116.5 g / mole and 55% by mass is oxygen. The aluminum is also ground to similar sizes at the last minute. Al is a very exothermic fuel, producing A 2 O 3 which is liquid at the flame temperature ( 3500K), and condenses later to a solid. The binder is often polybutadiene (synthetic rubber), either Carboxyl Terminated (CTPB) or Hydroxyl Terminated (HTPB). The composition of CTPB is C 7 H 11.24 O 0.2 , with thermal conductivity 4 / 3.6x10 ( ) λ= cal sec cm K p , 3 0.97 / = g cm ρ , 0.39 / / = p cc a l g K Best performance is obtained with very high percentage of AP, although mechanical properties require a minimum of binder, and AP concentration ranges from 70% by mass when there is A ( 16%), the balance being binder (14%), to about 80%-85%, with no A (as in “smoke-less” compositions), the balance then being all binder. Overview of Combustion Mechanism – The burning of AP-binder propellants (no A ) is a complex series of phenomena, and the detailed geometry of the grains does matter (size, particularly). At P 20 atm , AP itself can deflagrate exothermically, and it decomposes partly in a thin liquid layer on the surface of a grain, partly in an “AP flame” a few above it. The heat of decomposition raises T to m µ 1205K by itself; heat from the outer flame (more below) can raise the AP flame temperature well above this, however. Around the AP grains, the heat from the main flame decomposes the binder, which generates a mixture of short-chain hydrocarbons, while absorbing about 360 cal/g, plus the energy to heat it to the surface temperature T s 1000 K -1100 K . 16.512, Rocket Propulsion Lecture 17-18 Prof. Manuel Martinez-Sanchez Page 1 of 15
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The O 2 -rich gas generated in the AP flame co-flows outwards with the binder decomposition products, with interdiffusion along the way. This is a “diffusion flame”, and the final combustion takes place in it, raising T to about 3540K.
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lecture_17_18 - 16.512, Rocket Propulsion Prof. Manuel...

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