I finished the discussion of rockets. About half the lecture was devoted to showing a bit of the algebra required to connect rocket performance to typical design parameters. this is described in detail in the notes. You will also have the opportunity to do this for one of the homeowrk problems. The other half of the lecture involved a discussion of typical performance trades made in designing a rocket nozzle. In this context we talked about ideally-expanded, underexpanded and over-expanded nozzles. For each we talked about the impacts on momentum flux and pressure forces in the thrust equation and the impacts on nozzle weight. We did two PRS questions (PRS #1, PRS#2)Next lecture we will go back to aircraft engines and develop relations that connect principal design paramenters (like compressure pressure ratio, turbine inlet temperature, and flight Mach number) to the principal figures of merit for air-breathing engine performance(thrust and efficiency). Please review Chapter VIII of the notes.
Responses to 'Muddiest Part of the Lecture Cards'
(3 respondents, 49 students in class)
1) This ties in with fluids... for large values of Ae/A*, the exit velocity is large. I see why the equations say so, but why conceptually? I still have problems seeing this conceptually. (1 student) I am not sure if this helps, but let me give it a try--- The mass flow through a nozzle is density*velocity*area, and with no leaks it is constant at each point in the nozzle. For ideal gases at low speeds, the pressure changes do not lead to large density changes so reducing the area means the velocity must increase. However, for isentropic flows of ideal gases at very high speeds (M>1), changes in area produce large changes in the density of the gas. Indeed, the changes are so large that for a given reduction in area, the velocity actually increases. This is a feature of the thermodynamic properties of the gases that we often deal with.
2) No mud (2 students).