Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!att!cbnews!military From: sman@prandtl.nas.nasa.gov (Sekong Man) Newsgroups: sci.military Subject: Turbo{jet,fan}s Message-ID: <14308@cbnews.ATT.COM> Date: 25 Feb 90 06:12:26 GMT Sender: military@cbnews.ATT.COM Organization: NASA Ames Research Center Lines: 47 Approved: military@att.att.com From: sman@prandtl.nas.nasa.gov (Sekong Man) In <14242@cbnews.ATT.COM> Ted Kim asked about the difference between turbofan and turbojet. He was right. The only difference is By-pass. (Although this has a bearing on the choice of thermodynamic cycle, design arrangement and structure etc.) By-pass is good only in certain operation condition. By moving a larger air mass, the propulsion efficiency (which depends on the Froude efficiency which in turn is a function of the jet exit velocity) is increased. It is like the difference between throwing a small stone fast or a big stone slow. You can get the same reaction force as long as the product of mass and velocity give you the same value of momentum. However the energy carried away by the small stone is higher, in proportion to its speed. So for the same thrust, by-pass reduces fuel consumption. All modern airliners use turbofans, some with by-pass ratio as high as 5. The fans get to be like shrouded propellers. Increase by-pass increases engine thurst at LOW speed. So if you look at the static thrust of F-100, it may seem so much more powerful than the J79. However, the thrust from the by-pass falls off quickly with aircraft speed since it is limited by the amount of energy transferrable to the fan air flow and thrust times velocity is energy. In other words, the by-pass air velocity has to go down at high speed. Meanwhile the core part of the exhaust derives its velocity from expansion at the expense of the heat from combustion and it is high at any speed. So supersonic performance of the re-engined Super Phantom is not that much better. Too much by-pass can incur aircraft performance penalty, mainly from the increase of engine(core + by-pass) air flow and frontal area. Larger air flow means larger intake and hence drag. Larger frontal area implies larger fuselage cross-section and also drag, especially at supersonic speed. Of course, larger air flow give more for reheat boost. But this is at the expense of fuel burn. The F-100 (for F15) with 0.5 by-pass gives like 50% reheat thrust and fuel burn goes up like 5 times. Supersonic fighters rarely have engines with by-pass larger than unity and the higher the Vmax usually the lower the by-pass. But with low by-pass, turbofan is very much like turbojet, except perhaps a little bit heavier, more complicated, slower in throttle response, more prone to surge and overheat, more maintenance problems, and the host of things you hear about the F-15 engine troubles. Turbofan will probably not be used above Mach 2.5 while turbojet is good for Mach 3+.