From: skunk-works-digest-owner@mail.orst.edu To: skunk-works-digest@mail.orst.edu Subject: Skunk Works Digest V5 #580 Reply-To: skunk-works-digest@mail.orst.edu Errors-To: skunk-works-digest-owner@mail.orst.edu Precedence: bulk Skunk Works Digest Sunday, 24 December 1995 Volume 05 : Number 580 In this issue: Re: Hodefoneaminit Happy Holidays See the end of the digest for information on subscribing to the skunk-works or skunk-works-digest mailing lists and on how to retrieve back issues. ---------------------------------------------------------------------- From: Brad Hitch Date: Sat, 23 Dec 1995 05:16:39 -0700 (MST) Subject: Re: Hodefoneaminit On Thu, 21 Dec 1995 Charles_E._Smith.wbst200@xerox.com wrote: > Brad Hitch writes in a well written letter ......... > > > Now add fuel to bring the combustor exit temperature (T4) up to 5500 R > (T4), where dissociation of the combustion products will be underway and > assume that you are flying at the altitude required to obtain a 2 > atmosphere combustor pressure for good chemical reaction rates. > > > > The engine > flow velocity will not change much with the heat addition. > > > > > > > > I guess that statement throws me a little. I was under the assumption > that the change in enthalpy would slow the flow. It depends on what the flow area is doing and the heat release rate. > In my paper and model the the combustion chamber exit velocity is limited by > thermal choking. (At Mach numbers higher than about 7, you > really can`t enough heat to do any good, without the flow getting > to Mach 1. What paper? If I recall correctly, a post of yours mentioned something about a scramjet model with the design point at Mach 8/60K. This is running at a freestream dynamic pressure over 6500 psf. Most aircraft can't take more than about 2000 psf. 6500 is ridiculous. So did you publish this somewhere? >The thing you kind of glossed over was > > Thrust = mdot (Uexit-Uintake). I was speaking of the combustor. The exhaust nozzle is easy if you can do the combustor. All the combustor does is convert chemical energy into heat energy. It doesn't make much thrust in general. Uexit occurs at the end of the exhaust nozzle. > The whole idea is to get propulsion efficiency. You need to add as much > heat as possible. The overall engine efficiency depends on both the engine thermodynamic cycle and the propulsion efficiency. The propulsion efficiencies of most hypersonic engines are close to 1 since Uexit is close to Uintake. The engine cycle efficiencies are much less than 1. The overall engine efficiency is therefore dominated by the engine thermodynamic cycle efficiency. What you really need is thrust to be able to fly at hypersonic speeds. The engine efficiency just tells you how far you can go with your fuel load. > But you get stuck with H2(l) since the > adiabatic flame temperature of hydro-carbons dictates that > the Ksp of the reaction will drive you away from stoichiometric. > (You get CO and N02 and all kinds of nasties the "tree huggers" > bitch about, along with a loss in thrust -instead of CO2 +H2O. You may have a loss of thrust with respect to what you would have obtained if the fuel products were undissociated, but adding more fuel increases thrust even beyond stoichiometric due to the additional mass flow and declining exhaust molecular weight. It kills your thrust specific fuel consumption though. Exhaust emissions for hypersonic aircraft will not be for the faint of heart, it is true. You can probably get a reading on how close a system is to realization by the effort spent on reducing emissions. > For the real world, you start with the chemical reaction, develop > the combustion chamber relationships, and then design the diffuser > and nozzle to match the conditions. Try Oates` AIAA book. > It gets into this. It doesn`t work just right fo hypersonics but > graduate-type rocket scientists should be able to make the > nessecary changes. Better yet, use his basics and re-derive. > You`ll find the FA ratio that avoids choking is so lean you can`t > get enough enthalpy gain. Just open up the flow area! Geez, you don't HAVE to have a constant area combustor. In fact, the huge pressure rise due to heat release in a supersonic constant area flow will likely separate the boundary layer, giving you a constant static pressure on the combustor wall for some distance (as Billig found out), which is really a variable area combustor due to the variable blockage of the separated flow region. > Also, just for grins you might want to check the area-ratio required > to get from M10 at U1 to M5 at U2. You might end up with > a pretty big intake! It is a well-known fact that hypersonic inlet and exhaust systems are very large. The engine dominates the whole aircraft (its an "engineframe" not an airframe). > If you have screw around with H2(l) you might as well bring the > oxygen along too. Now you have an engine that gives the same > thrust at any Mach number, and can accelerate you for takeoff. > (obviously, a M10 engine is useless for takeoff!) What I just > desribed was -Rocket Motors. > > Other than keeping the flow expanded at the exit > (which the aerospike and other designs claim to handle) > you get the same thrust all the time. Problem is you get > a big weight fraction for the T/O and cruise segments > of the flight. Does anyone have a picture of an aerospike nozzle? > Along the idea of area ratios, SCRAMjets are VERY design point > sensitive. I don`t see any off-design application. So, you need a > secondary propulsive source anyway(another engine with a > different design point?) > > And you are absolutely right, the TOTAL temps are a barriers, > to say the least. > > Better yet, look at the total pressures! Now you get feeling for > what thermal AND aero stresses such an engine would have. Yes, it doesn't make any sense to be heroic to obtain worse performance than a rocket. > About Anderson...... > A brilliant visionary, a heckofalot smarter than me, but - > he clings to Navier-Stokes in hypersonics. Be careful. Who said anything about Anderson? I said check out Heiser and Pratt's book - "Hypersonic Airbreathing Propulsion". But since you brought it up, what would you use instead of Navier-Stokes? The Burnett equation? ------------------------------ From: jstone@iglou.com (John Stone) Date: Sat, 23 Dec 95 17:17 EST Subject: Happy Holidays Hi Skunkers, Just a quick note wishing everyone a Happy Holiday.... Best, John Stone | / ^ \ ___|___ -(.)==<.>==(.)- --------o---((.))---o-------- SR-71 Blackbird U-2 Dragon Lady John Stone jstone@iglou.com U-2 and SR-71 Web Page http://wl.iglou.com/blackbird/ ------------------------------ End of Skunk Works Digest V5 #580 ********************************* To subscribe to skunk-works-digest, send the command: subscribe skunk-works-digest in the body of a message to "majordomo@mail.orst.edu". 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