lecture_9

lecture_9 - 16.512, Rocket Propulsion Prof. Manuel...

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16.512, Rocket Propulsion Prof. Manuel Martinez-Sanchez Lecture 9: Liquid Cooling Cooling of Liquid Propellant Rockets We consider only bi-propellant liquid rockets, since monopropellants tend to be small and operate at lower temperatures. In a bi-propellant rocket, both the oxidizer and the fuel streams are in principle available for cooling the most exposed parts of the chamber and nozzle prior to being injected. This is called “regenerative cooling ”, because the heat loss from the gas is recovered (“regenerated”) into the liquid, so no heat escapes. This is not to say no thermodynamic loss is incurred, though (heat is transferred from very hot gas to cool liquid, which implies irreversibility and loss of work potential). Of the two streams, the fuel is normally used for cooling. This is for two reasons: (a) Fuels tend to have higher specific heats, so more heat is removed for a given of the coolant, and T (b) Leakage from an oxidizer stream into the normally fuel-rich combustion gas can produce a local flame that can be catastrophic, whereas leakage from a fuel line into the same fuel-rich gas is inert. In addition, exposing hot metal to oxygen or strong oxidants always carries some risk of accelerated chemical attack, or even ignition. Some exceptions do exist where oxidizers are used for cooling, though. A typical arrangement is as shown below (Figure 1). The fuel at high pressure from the fuel pump (FP) is sent through a series of narrow passages carved into the nozzle and chamber walls, picks up the wall heat flux from the gas, and is delivered eventually to the injector manifold. Since the nozzle region 16.512, Rocket Propulsion Lecture 9 Prof. Manuel Martinez-Sanchez Page 1 of 12
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is the most thermally loaded one, often the coolant flow is split, with one part entering at the nozzle exit and another providing extra cool fluid by entering just downstream of the throat. A typical construction for the cooling channels is shown in Fig. 2. The load-bearing part of the structure is milled longitudinally with channels of varying depth and width (to obtain varying liquid velocity), and a high thermal conductivity thin layer of a Copper alloy is then brazed on the inside. Design Considerations Two aspects need to be verified in the design of the cooling system: (a) The coolant should have sufficient thermal capacity to absorb the heat load without exceeding some critical temperature, which may be a chemical decomposition limit (thermal cracking for hydrocarbons) or the boiling point (although, with care, boiling can be sometimes tolerated or exploited for its strong heat absorption properties). Suppose is the calculated total heat loss from the gas. As seen in a previous lecture, this amounts to 1-3% of , more for the smaller engines. Suppose also the fuel only is used as coolant, with a flow rate .
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This note was uploaded on 11/07/2011 for the course AERO 16.512 taught by Professor Manuelmartinez-sanchez during the Fall '05 term at MIT.

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lecture_9 - 16.512, Rocket Propulsion Prof. Manuel...

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