Air breathing engines question

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craig42

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Does anyone know. Is Oxygen separated out of the intake some how or is the oxidiser just normal air passed in to the combustion chamber?
 
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propforce

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Just normal air that goes into combustion chamber, that's why you have the NOx pollution problems. <div class="Discussion_UserSignature"> </div>
 
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vogon13

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Seperating out the O2 for the combustion cycle would be 'the big enchilada'.<br /><br />Just don't think there is a way to seperate tons of O2 per minute from 3X as many tons of N2 while the engine is running.<br /><br />Oh that it could be done.<br /><br />The N2 dilutes the reactants. Decelerating the N2, running it through the combustion cycle, and then dumping it out the back all hurts the performance of the process.<br /><br />Selecting a fuel that would combust O2 and N2 might be something to look into. Potassium hyperhydride ?<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
 
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mlorrey

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vogon said: "The N2 dilutes the reactants. Decelerating the N2, running it through the combustion cycle, and then dumping it out the back all hurts the performance of the process. "<br /><br />Which must be why air breathing engines have Isp rated in the thousands while rocket engines can't seem to get above 450 seconds... <img src="/images/icons/wink.gif" />
 
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propforce

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<font color="yellow">Which must be why air breathing engines have Isp rated in the thousands while rocket engines can't seem to get above 450 seconds..</font><br /><br />That's not why.<br /><br />The reason it is why is because air is FREE !! If you're able to separate O2 out of air and run through an A/B engine, the O2 is still considered FREE !! <div class="Discussion_UserSignature"> </div>
 
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vogon13

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Think of the Isp you would have if earth had a 100% O2 atmosphere.<br /><br />It doesn't, so you have to deal with an oxidizer flow that is />75% inert gas.<br /><br />If there was a simple, light weight filter to put on the intake flow that was only porous for O2, you would really have something.<br /><br />Or burn something that reacts with N2. Then you would have an oxidizer (for lack of a better term) that is 99% (air is 1% Ar and CO2) potent.<br /><br /><br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
 
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mlorrey

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Of course it is free, but separating the O2 isn't free. That takes energy, and energy takes fuel, so you have to carry more fuel than just needed to get into orbit, so you can power the process of separating O2.<br /><br />Now, I happen to like the idea of using such a process for a launcher to fill its O2 tank once it is already in the air, but to generate O2 just to burn it again while flying in useful atmosphere, that is a waste of fuel. <br /><br />An alternative to using fuel to generate O2 is to use Liquid Nitrogen. Nitrogen is 1/8th less dense than O2, but has a significantly lower boiling point. If you carried a tank of liquid nitrogen with you at launch, and used air breathing engines, you could just funnel air through the tank. The liquid nitrogen would liquify oxygen out of the air as the nitrogen boiled, so in a little while, you'd wind up with a tank full of LOX and all the nitrogen would be gone.<br /><br />If the atmosphere were pure O2, it wouldn't be for long. As soon as someone lit a match, the whole ecosystem would burn around the world. Then you would have an O2 and CO2 atmosphere.
 
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propforce

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<font color="yellow">Of course it is free, but separating the O2 isn't free.</font><br /><br />Yes. Even the "dry weight" of using "free air" (less dense, hence bigger diameter and heavier) is not "free". That's what people miss when they are in love with a pure Isp number.<br /><br /><br /><font color="yellow">An alternative to using fuel to generate O2 is to use Liquid Nitrogen......The liquid nitrogen would liquify oxygen out of the air as the nitrogen boiled, so in a little while, you'd wind up with a tank full of LOX and all the nitrogen would be gone. </font><br /><br />This way you're complicating the aircraft and process instead of just carrying pure LOX in the first place. By carrying LN2, instead of using air for propulsion, you've significantly increase the aircraft gross take-off weight, thus requiring higher engine power, bigger wings, heavier landing gears, etc.<br /><br />Better yet, I'd just take off with the LOX tank empty. Pull up behind a KC-135 tanker and filled up my LOX before I fired up my rocket and take off to space.<br /><br />Oh wait, someone else has already proposed this idea <DOH!! /> <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> </div>
 
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rocketman5000

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There is such an engine that seperates the O2 from the air. It is called the Liquid Air Cycle Engine. (LACE) A complicated solution to the problem of supersonic combustion. A leading proponent is a British project which I can't remember the name of. I'll have to do some research.<br /><br />Basically the engine uses the rocket fuel (LH2) to liquify the air and burns it like a conventional rocket engine. claims to be a more versatile engine allowing a larger design freedoms for your spacecraft
 
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propforce

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<font color="yellow">A leading proponent is a British project which I can't remember the name of. I'll have to do some research. </font><br /><br />The British? Listen buddy, this thing was developed in the good 'ol U.S. of A. back in the '60s. <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> </div>
 
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sorehed

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Just use a great big turbofan and bypass 90% of the air. Then you don't have to worry about it. <img src="/images/icons/wink.gif" />
 
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mikeemmert

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I used to dream about a partially airbreathing engine, a mixed air/LOX oxidizer. The idea here is that at "low" (35,000 feet) altitudes, there's plenty enough air to run the engine, but as the vehicle climbs, there's less air available. So you add LOX to make up. You lose Isp, of course, but you keep thrust at the same level. LOX also cools and compresses the air, accelerating it to more like the velocity of the vehicle before it enters the combustion zone.<br /><br />When air is suddenly compressed, it's temperature goes up. If it's compressed enough, nitrogen starts combining with oxygen to form NO, a chemical reaction that absorbs energy. So the temperature stays at that level until all the oxygen has combined with the nitrogen, then the temperature of the NO/N2 starts rising again. If more oxygen is available, then more heat can be absorbed by this chemical reaction. This energy is released by combustion, so it's not lost.<br /><br />It's chemically cooling the air, which is in addition to the evaporation of the LOX.<br /><br />mlorrey, do you know anything about such engines? I think you mentioned one in a post some time back. Something about an exisiting jet engine modified for this. I would think you would need a clean sheet of paper; this partially airbreathing engine is neither a jet nor a rocket, it's a whole different animal.
 
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propforce

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<font color="yellow">mlorrey, do you know anything about such engines? I think you mentioned one in a post some time back.</font><br /><br />I am not mlorrey but I can answer the question. Yes, the concept you've described is called Mass Injection PreCompression Cooling (MIPCC), a concept that has been studied and published since the '70s. If you go to NASA GRC website, you'll find lots of papers in this area. You can do it with either LOX or H2O.<br /><br />Also, DARPA is funding a project called RASCAL which its first stage reusualbe aircraft will use this concet to gain altitude and speed upto Mach 5. <div class="Discussion_UserSignature"> </div>
 
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mlorrey

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You can do it with peroxide as well.<br /><br />Yes, injecting these ahead of the compressor causes the local airstream to become colder and denser, so the engine acts like it is operating at a slower speed at a lower altitude. This allows you to operate your engine at speed significantly above its normal operating range.<br /><br />The mechanicals of doing this with a turbine engine are pretty simple, you don't need to modify the turbine engine at all, the injectors can be bolted into the intake.<br /><br />The one instance I know of where this was used was by the F-4 Phantom to set its speed record (which is kinda cheating), beating the previous record set by a stock F-106. <br /><br />Here is a good graphic:<br />
 
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mikeemmert

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Thank you, propforce and mlorrey.<blockquote><font class="small">In reply to:</font><hr /><p>...MIPCC...<p><hr /></p></p></blockquote><img src="/images/icons/tongue.gif" />How do you pronounce that? That might be the biggest problem with this project...<br /><br /><i><font color="yellow">So I made me a vow to the moon and the stars<br />I'd search the *****-tonks and bars<br />And kill that man that gimme that awful name-</font></i><br /><font color="white">Johnny Cash.<br /><br />Peroxide releases energy when it decomposes, so in my humble opinion LOX would work better.</font>
 
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annodomini2

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<blockquote><font class="small">In reply to:</font><hr /><p>There is such an engine that seperates the O2 from the air. It is called the Liquid Air Cycle Engine. (LACE) A complicated solution to the problem of supersonic combustion. A leading proponent is a British project which I can't remember the name of. I'll have to do some research. <br /><br />Basically the engine uses the rocket fuel (LH2) to liquify the air and burns it like a conventional rocket engine. claims to be a more versatile engine allowing a larger design freedoms for your spacecraft <p><hr /></p></p></blockquote><br /><br />The project was called SABRE:<br /><br />http://en.wikipedia.org/wiki/Liquid_air_cycle_engine<br /><br />It was originally a US concept, but I believe the british have had more success.<br /><br />I am curious if the concept could work with a PDE engine? <div class="Discussion_UserSignature"> </div>
 
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tomnackid

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Andrews Space has been doing work under a DARPA contract to liquify atmospheric O2 in flight using a type of compressor called a "vortex tube". The idea has been around since the 1930s. A vortex tube can use the forward motion of the aircraft to compress and cool incoming gasses without any refrigerants or moving parts (other than moving the entire tube through the air that is!). They seem to think they have models light and efficient enough for flight testing. <br /><br />Here is a link to the project: <br /><br />http://www.andrews-space.com/content-main.php?subsection=MTAz
 
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mlorrey

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Yes, LOX would probably work better, though MIPCC systems normally use water AND LOX together. I figured that peroxide has a similar thermal aborption as water, and the heat of decomposition into H2O and O2 is slightly less than LOX to O2, so peroxide is a suitable sub for a vehicle short on space for cryogenic pressurized tanks, since you can store peroxide in plain-jane tanks. In the case of the X-106, I use the wet wing tanks as peroxide tanks.<br /><br />For a private effort, it is an option for a simpler installation.
 
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craig42

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I was asking about any air-breathing technology in general. Ramjet/Scramjet RBCC or one's I haven't heard of.<br /><br />vogon 13 <br /><br />What do you mean by 'the big enchilada'?
 
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mlorrey

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Then the answer is no, only LACE-type engines separate the oxygen from the incoming air.
 
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vogon13

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The 'brass ring'.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
 
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propforce

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<font color="yellow">I was asking about any air-breathing technology in general. Ramjet/Scramjet RBCC or one's I haven't heard of. </font><br /><br />Actually there's a class of engines called "combined cycle", which is the "CC" in RBCC, and have incorporated LACE into its overall propulsion system. <br /><br />As one recognizes each engine works best in some regime for space access, e.g., turbofan from Mach 0 to Mach 2, ramjet from Mach 2 to Mach 6, scramjet from mach 6 to 8~16, then the pure rocket mode from there on to insert oneself into orbit. One would propose a combination of one or more of propulsion "mode" into a propulsion "system" in order to make accomplish the overall mission objective.<br /><br />For example, instead of using turbofan engines (big & bulky - but very operable), one could use little rockets inside of a duct, kinda like JETO bottles, acting as ejector for take-off from Mach 0 to Mach 2. Ejector entrains additional air into the duct where you can inject some fuel down stream for additional combustion (like an afterburner) for more thrusts. This "duct" could be a scramjet flowpath, or a ramjet flowpath, so one could call them either a Ejector-scramjet or a Ejector-ramjet combined cycle propulsion system. One could go further by addition a compressor fan stage in front of the ejector hence "super-charging" the incoming air pressure for improved peformance, then you would have a "supercharged ejector ramjet" (SERJ). Complicated enough for ya? <img src="/images/icons/smile.gif" /><br /><br />The SR-71 use "turbo-ramjet" combining a high by-pass ratio turbojet with an aft-burning ramjet propulsion 'system'. This is an example of TBCC, turbine-based combined cycle system.<br /><br />A LACE system is a mean to collect LOX to be used for the rocket mode in the later part of trajectory (where there's no air!!). So depending on what you believe will work for a 'system', you could propose one of the following combined cycle propulsion syst <div class="Discussion_UserSignature"> </div>
 
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mikeemmert

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I have figured out that you, me, and propforce are talking past each other.<br /><br />Peroxide sounds like a pretty good idea from takeoff up to maybe (guessing a lot) mach 5 and (?again) 80,000 feet. And that fits really, really well with the X-106 project. It might also fit with a White Knight style launch plane (I prefer a 747 or AN-225, why reinvent the wheel?).<br /><br />I'm thinking mach 0.85 to mach 1.2 for LOX initially, to get the ramjet past the "sound barrier". Then LOX is conserved until the craft reaches mach 2.7, where LOX is once again used to get past the supersonic/hypersonic barrier. After that, you'd set the rate of climb such that at first, increasing speed increases airflow faster than increasing altitude decreases it. Peak airflow & therefore minimum LOX flow would be at about mach 4.2, then LOX flow is increased to make up for thinner, higher altitude air.<br /><br />(I actually did mathematical models of this! but they were very, very crude. I was using a hand calculator. The vehicle had no wings, because I knew how to do ballistic trajectories and am a little ignorant on wings.)<br /><br />That's why I mentioned the N2 + O2 /> 2 NO chemical reaction. This takes place at very high speeds. I get the impression that you were talking about a phase change (evaporation) which should cool intake air at lower speeds.<br /><br />That was late '80's or so while I was between taxi trips. Something to do with my hands, pushing calculator buttons.<br /><br />Good luck with your X-106 project! This sounds like something you might <b>actually be able to do</b>!
 
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mlorrey

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Thanks Mike!<br /><br />You have a good point about hypersonic shock waves and other hypersonic issues creating nitrous oxide. I was under the impression that that was primarily a transient state, at least for combustion products in fuel combustion streams, as hydrogen has a higher affinity for the oxygen than nitrogen does. As most combustion systems tend to run slightly off stoichometric ratios for thermal reasons, running slightly fuel rich minimizes NO production. Another idea is embedding catalytics in combustion chamber walls.<br /><br />Anyways, the hypersonic speeds would be 80k-150k altitudes, so given atmospheric stratification, I don't see this as affecting ground level smog interactions.<br /><br />Anyways, I don't see us flying more than 50-100 sorties a year at best, so I'd point the protesters toward the major airlines corporate offices first. I highly suspect we'd generate a lot more value per pound of pollution than they do on a daily basis.
 
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