Lecture P1: Introduction and Integral Momentum Equation


General comments

I started the lecture with a discussion of the learning objectives. These cover several areas (integral momentum theorem, gas turbines, rockets, mission analysis, cycle analysis, energy exchange with moving blades). We will cover each of these in only limited depth, the primary goal is to connect concepts you have already been introduced to in 8.01, Unified fluids and Unified thermodynamics, and to show how these concepts can be used to analyze devices designed to MAKE THINGS MOVE. After discussing the learning objectives, I provided a brief overview of the workings of rocket motors and gas turbine engines. Much of the latter was in reference to the CFM56-3 that we rolled into class.

The lecture then focused on material related to the first learning objective. The goal was to firmly implant the concept of propulsive forces as arising largely from changes in the net flux of momentum. The two PRS questions (Q1 and Q2) were designed to emphasize this from an intuitive perspective. During the next lecture we will look at this more formally and do several examples. Please read the notes!


Responses to 'Muddiest Part of the Lecture Cards'

(23 mud cards, 72 students attended lecture)

1) Doesn't the weight change on the boat affect the force on the boat? I don't see how that was taken into account. (1 student) Very good. Yes it does. We will discuss this when we more formally discuss the integral momentum equation and especially when we get to rockets. I didn't want to delve into these details in the first lecture because they can overwhelm the basic concept: propulsive forces are related to a net flux of momentum across a control volume.

2) Remember, we haven't had dynamics yet. (1 student) All the dynamics we will need is covered in 8.01, but if there are items that are fuzzy let me know and I will go over them in more detail.

3) Does the air not going through the combustor produce any thrust? (1 student) Yes. In fact for a high bypass ratio engine, most of the thrust comes from the bypass air.

4) How do rocket fuels differ from jet engine fuels? (1 student) Liquid propellant rockets are fueled by a range of oxidizers (e.g. liquid oxygen, hydrogen peroxide, nitric acid, nitrogen tetroxide) and fuels (RP-1, liquid hydrogen, hydrazine, umsymmetrical dimethylhydrazine, monomethylhydrazine, etc.). RP-1 is a kerosine-like fuel which is similar to what is used for aircraft propulsion. The kerosine-like fuels are themselves mixtures of many different hydrocarbons (olefins, parafins, and aromatics). Of course the oxidizer for aircraft engines is the oxygen carried by the air.

5) How important is our ability to use complicated integral equations for this class? (1 student) It is important. But I hope I can make these a little less complicated for you. The ones that you will use most for propulsion are derived from the full fluids equations, but are specialized for the situations we see most and thus have a few less terms in them.

6) What part of fluids should we review? (1 student) For the first part of the material, review the integral momentum theorem and control volume analysis. For rocket nozzles, review isentropic, compressible flow.

7) How does an engine first start up? Do they use an electrical system? (1 student) There are several ways to start an engine. Most engines use high pressure air to blow on an air turbine which turns a gear/shaft which turns the engine until it is spinning fast enough to put fuel in and ignite it. The high pressure air typically comes from an A.P.U. (auxiliary power unit). The A.P.U. is just a small gas turbine. Which begs the question -- how is the A.P.U. started? A small mouse does this. No, the A.P.U. is typically started using a high speed electric motor.

8) Can you explain again the advantages & disadvantages of external mixing or air around the engine? (1 student) Mixing of the two streams (core and bypass) before they are accelerated in the nozzle produces a higher propulsive efficiency, but requires a longer, heavier nacelle. You can learn much more about this in either Kerrebrock, Aircraft Engines and Gas Turbines, or Hill and Peterson, Mechanics and Thermodynamics of Propulsion, both available in the library.

9) No mud (15 students).